All | Node.js v26.7.0 Documentation

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- Events
  - Passing arguments and `this` to listeners
  - Asynchronous vs. synchronous
  - Handling events only once
  - Error events
  - Capture rejections of promises
  - Class: `EventEmitter`
    - Event: `'newListener'`
    - Event: `'removeListener'`
    - `emitter.addListener(eventName, listener)`
    - `emitter.emit(eventName[, ...args])`
    - `emitter.eventNames()`
    - `emitter.getMaxListeners()`
    - `emitter.listenerCount(eventName[, listener])`
    - `emitter.listeners(eventName)`
    - `emitter.off(eventName, listener)`
    - `emitter.on(eventName, listener)`
    - `emitter.once(eventName, listener)`
    - `emitter.prependListener(eventName, listener)`
    - `emitter.prependOnceListener(eventName, listener)`
    - `emitter.removeAllListeners([eventName])`
    - `emitter.removeListener(eventName, listener)`
    - `emitter.setMaxListeners(n)`
    - `emitter.rawListeners(eventName)`
    - `emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])`
  - `events.defaultMaxListeners`
  - `events.errorMonitor`
  - `events.getEventListeners(emitterOrTarget, eventName)`
  - `events.getMaxListeners(emitterOrTarget)`
  - `events.once(emitter, name[, options])`
    - Caveats when awaiting multiple events
  - `events.captureRejections`
  - `events.captureRejectionSymbol`
  - `events.listenerCount(emitterOrTarget, eventName)`
  - `events.on(emitter, eventName[, options])`
  - `events.setMaxListeners(n[, ...eventTargets])`
  - `events.addAbortListener(signal, listener)`
  - Class: `events.EventEmitterAsyncResource extends EventEmitter`
    - `new events.EventEmitterAsyncResource([options])`
    - `eventemitterasyncresource.asyncId`
    - `eventemitterasyncresource.asyncResource`
    - `eventemitterasyncresource.emitDestroy()`
    - `eventemitterasyncresource.triggerAsyncId`
  - `EventTarget` and `Event` API
    - Node.js `EventTarget` vs. DOM `EventTarget`
    - `NodeEventTarget` vs. `EventEmitter`
    - Event listener
    - `EventTarget` error handling
    - Class: `Event`
      - `event.bubbles`
      - `event.cancelBubble`
      - `event.cancelable`
      - `event.composed`
      - `event.composedPath()`
      - `event.currentTarget`
      - `event.defaultPrevented`
      - `event.eventPhase`
      - `event.initEvent(type[, bubbles[, cancelable]])`
      - `event.isTrusted`
      - `event.preventDefault()`
      - `event.returnValue`
      - `event.srcElement`
      - `event.stopImmediatePropagation()`
      - `event.stopPropagation()`
      - `event.target`
      - `event.timeStamp`
      - `event.type`
    - Class: `EventTarget`
      - `eventTarget.addEventListener(type, listener[, options])`
      - `eventTarget.dispatchEvent(event)`
      - `eventTarget.removeEventListener(type, listener[, options])`
    - Class: `CustomEvent`
      - `event.detail`
    - Class: `NodeEventTarget`
      - `nodeEventTarget.addListener(type, listener)`
      - `nodeEventTarget.emit(type, arg)`
      - `nodeEventTarget.eventNames()`
      - `nodeEventTarget.listenerCount(type)`
      - `nodeEventTarget.setMaxListeners(n)`
      - `nodeEventTarget.getMaxListeners()`
      - `nodeEventTarget.off(type, listener[, options])`
      - `nodeEventTarget.on(type, listener)`
      - `nodeEventTarget.once(type, listener)`
      - `nodeEventTarget.removeAllListeners([type])`
      - `nodeEventTarget.removeListener(type, listener[, options])`

- FFI
  - Overview
  - Type names
  - Signature objects
  - `ffi.suffix`
  - `ffi.dlopen(path[, definitions])`
  - `ffi.dlclose(handle)`
  - `ffi.dlsym(handle, symbol)`
  - Class: `DynamicLibrary`
    - `new DynamicLibrary(path)`
    - `library.path`
    - `library.functions`
    - `library.symbols`
    - `library.close()`
    - `library[Symbol.dispose]()`
    - `library.getFunction(name, signature)`
    - `library.getFunctions([definitions])`
    - `library.getSymbol(name)`
    - `library.getSymbols()`
    - `library.registerCallback([signature,] callback)`
    - `library.unregisterCallback(pointer)`
    - `library.refCallback(pointer)`
    - `library.unrefCallback(pointer)`
  - Calling native functions
  - Primitive memory access helpers
  - `ffi.toString(pointer)`
  - `ffi.toBuffer(pointer, length[, copy])`
  - `ffi.toArrayBuffer(pointer, length[, copy])`
  - `ffi.exportString(string, pointer, length[, encoding])`
  - `ffi.exportBuffer(buffer, pointer, length)`
  - `ffi.exportArrayBuffer(arrayBuffer, pointer, length)`
  - `ffi.exportArrayBufferView(arrayBufferView, pointer, length)`
  - `ffi.getRawPointer(source)`
  - `ffi.getCurrentEventLoop()`
  - Safety notes

- File system
  - Promise example
  - Callback example
  - Synchronous example
  - Promises API
    - Class: `FileHandle`
      - Event: `'close'`
      - `filehandle.appendFile(data[, options])`
      - `filehandle.chmod(mode)`
      - `filehandle.chown(uid, gid)`
      - `filehandle.close()`
      - `filehandle.createReadStream([options])`
      - `filehandle.createWriteStream([options])`
      - `filehandle.datasync()`
      - `filehandle.fd`
      - `filehandle.pull([...transforms][, options])`
      - `filehandle.pullSync([...transforms][, options])`
      - `filehandle.read(buffer, offset, length, position)`
      - `filehandle.read([options])`
      - `filehandle.read(buffer[, options])`
      - `filehandle.readableWebStream([options])`
      - `filehandle.readFile(options)`
      - `filehandle.readLines([options])`
      - `filehandle.readv(buffers[, position])`
      - `filehandle.stat([options])`
      - `filehandle.sync()`
      - `filehandle.truncate(len)`
      - `filehandle.utimes(atime, mtime)`
      - `filehandle.write(buffer, offset[, length[, position]])`
      - `filehandle.write(buffer[, options])`
      - `filehandle.write(string[, position[, encoding]])`
      - `filehandle.writeFile(data, options)`
      - `filehandle.writev(buffers[, position])`
      - `filehandle.writer([options])`
      - `filehandle[Symbol.asyncDispose]()`
    - `fsPromises.access(path[, mode])`
    - `fsPromises.appendFile(path, data[, options])`
    - `fsPromises.chmod(path, mode)`
    - `fsPromises.chown(path, uid, gid)`
    - `fsPromises.copyFile(src, dest[, mode])`
    - `fsPromises.cp(src, dest[, options])`
    - `fsPromises.glob(pattern[, options])`
    - `fsPromises.lchmod(path, mode)`
    - `fsPromises.lchown(path, uid, gid)`
    - `fsPromises.lutimes(path, atime, mtime)`
    - `fsPromises.link(existingPath, newPath)`
    - `fsPromises.lstat(path[, options])`
    - `fsPromises.mkdir(path[, options])`
    - `fsPromises.mkdtemp(prefix[, options])`
    - `fsPromises.mkdtempDisposable(prefix[, options])`
    - `fsPromises.open(path, flags[, mode])`
    - `fsPromises.opendir(path[, options])`
    - `fsPromises.readdir(path[, options])`
    - `fsPromises.readFile(path[, options])`
    - `fsPromises.readlink(path[, options])`
    - `fsPromises.realpath(path[, options])`
    - `fsPromises.rename(oldPath, newPath)`
    - `fsPromises.rmdir(path[, options])`
    - `fsPromises.rm(path[, options])`
    - `fsPromises.stat(path[, options])`
    - `fsPromises.statfs(path[, options])`
    - `fsPromises.symlink(target, path[, type])`
    - `fsPromises.truncate(path[, len])`
    - `fsPromises.unlink(path)`
    - `fsPromises.utimes(path, atime, mtime)`
    - `fsPromises.watch(filename[, options])`
    - `fsPromises.writeFile(file, data[, options])`
    - `fsPromises.constants`
  - Callback API
    - `fs.access(path[, mode], callback)`
    - `fs.appendFile(path, data[, options], callback)`
    - `fs.chmod(path, mode, callback)`
      - File modes
    - `fs.chown(path, uid, gid, callback)`
    - `fs.close(fd[, callback])`
    - `fs.copyFile(src, dest[, mode], callback)`
    - `fs.cp(src, dest[, options], callback)`
    - `fs.createReadStream(path[, options])`
    - `fs.createWriteStream(path[, options])`
    - `fs.exists(path, callback)`
    - `fs.fchmod(fd, mode, callback)`
    - `fs.fchown(fd, uid, gid, callback)`
    - `fs.fdatasync(fd, callback)`
    - `fs.fstat(fd[, options], callback)`
    - `fs.fsync(fd, callback)`
    - `fs.ftruncate(fd[, len], callback)`
    - `fs.futimes(fd, atime, mtime, callback)`
    - `fs.glob(pattern[, options], callback)`
    - `fs.lchmod(path, mode, callback)`
    - `fs.lchown(path, uid, gid, callback)`
    - `fs.lutimes(path, atime, mtime, callback)`
    - `fs.link(existingPath, newPath, callback)`
    - `fs.lstat(path[, options], callback)`
    - `fs.mkdir(path[, options], callback)`
    - `fs.mkdtemp(prefix[, options], callback)`
    - `fs.open(path[, flags[, mode]], callback)`
    - `fs.openAsBlob(path[, options])`
    - `fs.opendir(path[, options], callback)`
    - `fs.read(fd, buffer, offset, length, position, callback)`
    - `fs.read(fd[, options], callback)`
    - `fs.read(fd, buffer[, options], callback)`
    - `fs.readdir(path[, options], callback)`
    - `fs.readFile(path[, options], callback)`
      - File descriptors
      - Performance Considerations
    - `fs.readlink(path[, options], callback)`
    - `fs.readv(fd, buffers[, position], callback)`
    - `fs.realpath(path[, options], callback)`
    - `fs.realpath.native(path[, options], callback)`
    - `fs.rename(oldPath, newPath, callback)`
    - `fs.rmdir(path[, options], callback)`
    - `fs.rm(path[, options], callback)`
    - `fs.stat(path[, options], callback)`
    - `fs.statfs(path[, options], callback)`
    - `fs.symlink(target, path[, type], callback)`
    - `fs.truncate(path[, len], callback)`
    - `fs.unlink(path, callback)`
    - `fs.unwatchFile(filename[, listener])`
    - `fs.utimes(path, atime, mtime, callback)`
    - `fs.watch(filename[, options][, listener])`
      - Caveats
        - Availability
        - Inodes
        - Filename argument
    - `fs.watchFile(filename[, options], listener)`
    - `fs.write(fd, buffer, offset[, length[, position]], callback)`
    - `fs.write(fd, buffer[, options], callback)`
    - `fs.write(fd, string[, position[, encoding]], callback)`
    - `fs.writeFile(file, data[, options], callback)`
      - Using `fs.writeFile()` with file descriptors
    - `fs.writev(fd, buffers[, position], callback)`
  - Synchronous API
    - `fs.accessSync(path[, mode])`
    - `fs.appendFileSync(path, data[, options])`
    - `fs.chmodSync(path, mode)`
    - `fs.chownSync(path, uid, gid)`
    - `fs.closeSync(fd)`
    - `fs.copyFileSync(src, dest[, mode])`
    - `fs.cpSync(src, dest[, options])`
    - `fs.existsSync(path)`
    - `fs.fchmodSync(fd, mode)`
    - `fs.fchownSync(fd, uid, gid)`
    - `fs.fdatasyncSync(fd)`
    - `fs.fstatSync(fd[, options])`
    - `fs.fsyncSync(fd)`
    - `fs.ftruncateSync(fd[, len])`
    - `fs.futimesSync(fd, atime, mtime)`
    - `fs.globSync(pattern[, options])`
    - `fs.lchmodSync(path, mode)`
    - `fs.lchownSync(path, uid, gid)`
    - `fs.lutimesSync(path, atime, mtime)`
    - `fs.linkSync(existingPath, newPath)`
    - `fs.lstatSync(path[, options])`
    - `fs.mkdirSync(path[, options])`
    - `fs.mkdtempSync(prefix[, options])`
    - `fs.mkdtempDisposableSync(prefix[, options])`
    - `fs.opendirSync(path[, options])`
    - `fs.openSync(path[, flags[, mode]])`
    - `fs.readdirSync(path[, options])`
    - `fs.readFileSync(path[, options])`
    - `fs.readlinkSync(path[, options])`
    - `fs.readSync(fd, buffer, offset, length[, position])`
    - `fs.readSync(fd, buffer[, options])`
    - `fs.readvSync(fd, buffers[, position])`
    - `fs.realpathSync(path[, options])`
    - `fs.realpathSync.native(path[, options])`
    - `fs.renameSync(oldPath, newPath)`
    - `fs.rmdirSync(path[, options])`
    - `fs.rmSync(path[, options])`
    - `fs.statSync(path[, options])`
    - `fs.statfsSync(path[, options])`
    - `fs.symlinkSync(target, path[, type])`
    - `fs.truncateSync(path[, len])`
    - `fs.unlinkSync(path)`
    - `fs.utimesSync(path, atime, mtime)`
    - `fs.writeFileSync(file, data[, options])`
    - `fs.writeSync(fd, buffer, offset[, length[, position]])`
    - `fs.writeSync(fd, buffer[, options])`
    - `fs.writeSync(fd, string[, position[, encoding]])`
    - `fs.writevSync(fd, buffers[, position])`
  - Common Objects
    - Class: `fs.Dir`
      - `dir.close()`
      - `dir.close(callback)`
      - `dir.closeSync()`
      - `dir.path`
      - `dir.read()`
      - `dir.read(callback)`
      - `dir.readSync()`
      - `dir[Symbol.asyncIterator]()`
      - `dir[Symbol.asyncDispose]()`
      - `dir[Symbol.dispose]()`
    - Class: `fs.Dirent`
      - `dirent.isBlockDevice()`
      - `dirent.isCharacterDevice()`
      - `dirent.isDirectory()`
      - `dirent.isFIFO()`
      - `dirent.isFile()`
      - `dirent.isSocket()`
      - `dirent.isSymbolicLink()`
      - `dirent.name`
      - `dirent.parentPath`
    - Class: `fs.FSWatcher`
      - Event: `'change'`
      - Event: `'close'`
      - Event: `'error'`
      - `watcher.close()`
      - `watcher.ref()`
      - `watcher.unref()`
    - Class: `fs.StatWatcher`
      - `watcher.ref()`
      - `watcher.unref()`
    - Class: `fs.ReadStream`
      - Event: `'close'`
      - Event: `'open'`
      - Event: `'ready'`
      - `readStream.bytesRead`
      - `readStream.path`
      - `readStream.pending`
    - Class: `fs.Stats`
      - `stats.isBlockDevice()`
      - `stats.isCharacterDevice()`
      - `stats.isDirectory()`
      - `stats.isFIFO()`
      - `stats.isFile()`
      - `stats.isSocket()`
      - `stats.isSymbolicLink()`
      - `stats.dev`
      - `stats.ino`
      - `stats.mode`
      - `stats.nlink`
      - `stats.uid`
      - `stats.gid`
      - `stats.rdev`
      - `stats.size`
      - `stats.blksize`
      - `stats.blocks`
      - `stats.atimeMs`
      - `stats.mtimeMs`
      - `stats.ctimeMs`
      - `stats.birthtimeMs`
      - `stats.atimeNs`
      - `stats.mtimeNs`
      - `stats.ctimeNs`
      - `stats.birthtimeNs`
      - `stats.atime`
      - `stats.mtime`
      - `stats.ctime`
      - `stats.birthtime`
      - Stat time values
    - Class: `fs.StatFs`
      - `statfs.bavail`
      - `statfs.bfree`
      - `statfs.blocks`
      - `statfs.bsize`
      - `statfs.frsize`
      - `statfs.ffree`
      - `statfs.files`
      - `statfs.type`
    - Class: `fs.Utf8Stream`
      - Event: `'close'`
      - Event: `'drain'`
      - Event: `'drop'`
      - Event: `'error'`
      - Event: `'finish'`
      - Event: `'ready'`
      - Event: `'write'`
      - `new fs.Utf8Stream([options])`
      - `utf8Stream.append`
      - `utf8Stream.contentMode`
      - `utf8Stream.destroy()`
      - `utf8Stream.end()`
      - `utf8Stream.fd`
      - `utf8Stream.file`
      - `utf8Stream.flush(callback)`
      - `utf8Stream.flushSync()`
      - `utf8Stream.fsync`
      - `utf8Stream.maxLength`
      - `utf8Stream.minLength`
      - `utf8Stream.mkdir`
      - `utf8Stream.mode`
      - `utf8Stream.periodicFlush`
      - `utf8Stream.reopen(file)`
      - `utf8Stream.sync`
      - `utf8Stream.write(data)`
      - `utf8Stream.writing`
      - `utf8Stream[Symbol.dispose]()`
    - Class: `fs.WriteStream`
      - Event: `'close'`
      - Event: `'open'`
      - Event: `'ready'`
      - `writeStream.bytesWritten`
      - `writeStream.close([callback])`
      - `writeStream.path`
      - `writeStream.pending`
    - `fs.constants`
      - FS constants
        - File access constants
        - File copy constants
        - File open constants
        - File type constants
        - File mode constants
  - Notes
    - Ordering of callback and promise-based operations
    - File paths
      - String paths
      - File URL paths
        - Platform-specific considerations
      - Buffer paths
      - Per-drive working directories on Windows
    - File descriptors
    - Threadpool usage
    - File system flags

- Global objects
  - `__dirname`
  - `__filename`
  - Class: `AbortController`
    - `abortController.abort([reason])`
    - `abortController.signal`
  - Class: `AbortSignal`
    - Static method: `AbortSignal.abort([reason])`
    - Static method: `AbortSignal.timeout(delay)`
    - Static method: `AbortSignal.any(signals)`
    - Event: `'abort'`
    - `abortSignal.aborted`
    - `abortSignal.onabort`
    - `abortSignal.reason`
    - `abortSignal.throwIfAborted()`
  - `atob(data)`
  - Class: `Blob`
  - Class: `BroadcastChannel`
  - `btoa(data)`
  - Class: `Buffer`
  - Class: `ByteLengthQueuingStrategy`
  - `clearImmediate(immediateObject)`
  - `clearInterval(intervalObject)`
  - `clearTimeout(timeoutObject)`
  - Class: `CloseEvent`
  - Class: `CompressionStream`
  - `console`
  - Class: `CountQueuingStrategy`
  - Class: `Crypto`
  - `crypto`
  - Class: `CryptoKey`
  - Class: `CustomEvent`
  - Class: `DecompressionStream`
  - Class: `DOMException`
  - `ErrorEvent`
  - Class: `Event`
  - Class: `EventSource`
  - Class: `EventTarget`
  - `exports`
  - `fetch`
    - Custom dispatcher
    - Related classes
  - Class: `File`
  - Class: `FormData`
  - `global`
  - Class: `Headers`
  - `localStorage`
  - Class: `MessageChannel`
  - Class: `MessageEvent`
  - Class: `MessagePort`
  - `module`
  - Class: `Navigator`
  - `navigator`
    - `navigator.hardwareConcurrency`
    - `navigator.language`
    - `navigator.languages`
    - `navigator.locks`
    - `navigator.platform`
    - `navigator.userAgent`
  - `performance`
  - Class: `PerformanceEntry`
  - Class: `PerformanceMark`
  - Class: `PerformanceMeasure`
  - Class: `PerformanceObserver`
  - Class: `PerformanceObserverEntryList`
  - Class: `PerformanceResourceTiming`
  - `process`
  - `queueMicrotask(callback)`
  - Class: `QuotaExceededError`
  - Class: `ReadableByteStreamController`
  - Class: `ReadableStream`
  - Class: `ReadableStreamBYOBReader`
  - Class: `ReadableStreamBYOBRequest`
  - Class: `ReadableStreamDefaultController`
  - Class: `ReadableStreamDefaultReader`
  - Class: `Request`
  - `require()`
  - Class: `Response`
  - `sessionStorage`
  - `setImmediate(callback[, ...args])`
  - `setInterval(callback, delay[, ...args])`
  - `setTimeout(callback, delay[, ...args])`
  - Class: `Storage`
  - `structuredClone(value[, options])`
  - Class: `SubtleCrypto`
  - Class: `TextDecoder`
  - Class: `TextDecoderStream`
  - Class: `TextEncoder`
  - Class: `TextEncoderStream`
  - Class: `TransformStream`
  - Class: `TransformStreamDefaultController`
  - Class: `URL`
  - Class: `URLPattern`
  - Class: `URLSearchParams`
  - Class: `WebAssembly`
  - Class: `WebSocket`
  - Class: `WritableStream`
  - Class: `WritableStreamDefaultController`
  - Class: `WritableStreamDefaultWriter`

- HTTP
  - Class: `http.Agent`
    - Response ordering with connection reuse
    - `new Agent([options])`
    - `agent.createConnection(options[, callback])`
    - `agent.keepSocketAlive(socket)`
    - `agent.reuseSocket(socket, request)`
    - `agent.destroy()`
    - `agent.freeSockets`
    - `agent.getName([options])`
    - `agent.maxFreeSockets`
    - `agent.maxSockets`
    - `agent.maxTotalSockets`
    - `agent.requests`
    - `agent.sockets`
  - Class: `http.ClientRequest`
    - Event: `'abort'`
    - Event: `'close'`
    - Event: `'connect'`
    - Event: `'continue'`
    - Event: `'finish'`
    - Event: `'information'`
    - Event: `'response'`
    - Event: `'socket'`
    - Event: `'timeout'`
    - Event: `'upgrade'`
    - `request.abort()`
    - `request.aborted`
    - `request.connection`
    - `request.cork()`
    - `request.end([data[, encoding]][, callback])`
    - `request.destroy([error])`
      - `request.destroyed`
    - `request.finished`
    - `request.flushHeaders()`
    - `request.getHeader(name)`
    - `request.getHeaderNames()`
    - `request.getHeaders()`
    - `request.getRawHeaderNames()`
    - `request.hasHeader(name)`
    - `request.maxHeadersCount`
    - `request.path`
    - `request.method`
    - `request.host`
    - `request.protocol`
    - `request.removeHeader(name)`
    - `request.reusedSocket`
    - `request.setHeader(name, value)`
    - `request.setNoDelay([noDelay])`
    - `request.setSocketKeepAlive([enable][, initialDelay])`
    - `request.setTimeout(timeout[, callback])`
    - `request.socket`
    - `request.uncork()`
    - `request.writableEnded`
    - `request.writableFinished`
    - `request.write(chunk[, encoding][, callback])`
  - Class: `http.Server`
    - Event: `'checkContinue'`
    - Event: `'checkExpectation'`
    - Event: `'clientError'`
    - Event: `'close'`
    - Event: `'connect'`
    - Event: `'connection'`
    - Event: `'dropRequest'`
    - Event: `'request'`
    - Event: `'upgrade'`
    - `server.close([callback])`
    - `server.closeAllConnections()`
    - `server.closeIdleConnections()`
    - `server.headersTimeout`
    - `server.listen()`
    - `server.listening`
    - `server.maxHeadersCount`
    - `server.requestTimeout`
    - `server.setTimeout([msecs][, callback])`
    - `server.maxRequestsPerSocket`
    - `server.timeout`
    - `server.keepAliveTimeout`
    - `server.keepAliveTimeoutBuffer`
    - `server[Symbol.asyncDispose]()`
  - Class: `http.ServerResponse`
    - Event: `'close'`
    - Event: `'finish'`
    - `response.addTrailers(headers)`
    - `response.connection`
    - `response.cork()`
    - `response.end([data[, encoding]][, callback])`
    - `response.finished`
    - `response.flushHeaders()`
    - `response.getHeader(name)`
    - `response.getHeaderNames()`
    - `response.getHeaders()`
    - `response.hasHeader(name)`
    - `response.headersSent`
    - `response.removeHeader(name)`
    - `response.req`
    - `response.sendDate`
    - `response.setHeader(name, value)`
    - `response.setTimeout(msecs[, callback])`
    - `response.socket`
    - `response.statusCode`
    - `response.statusMessage`
    - `response.strictContentLength`
    - `response.uncork()`
    - `response.writableEnded`
    - `response.writableFinished`
    - `response.write(chunk[, encoding][, callback])`
    - `response.writeContinue()`
    - `response.writeEarlyHints(hints[, callback])`
    - `response.writeHead(statusCode[, statusMessage][, headers])`
    - `response.writeInformation(statusCode[, headers][, callback])`
    - `response.writeProcessing()`
  - Class: `http.IncomingMessage`
    - Event: `'aborted'`
    - Event: `'close'`
    - `message.aborted`
    - `message.complete`
    - `message.connection`
    - `message.destroy([error])`
    - `message.headers`
    - `message.headersDistinct`
    - `message.httpVersion`
    - `message.method`
    - `message.rawHeaders`
    - `message.rawTrailers`
    - `message.setTimeout(msecs[, callback])`
    - `message.signal`
    - `message.socket`
    - `message.statusCode`
    - `message.statusMessage`
    - `message.trailers`
    - `message.trailersDistinct`
    - `message.url`
  - Class: `http.OutgoingMessage`
    - Event: `'drain'`
    - Event: `'finish'`
    - Event: `'prefinish'`
    - `outgoingMessage.addTrailers(headers)`
    - `outgoingMessage.appendHeader(name, value)`
    - `outgoingMessage.connection`
    - `outgoingMessage.cork()`
    - `outgoingMessage.destroy([error])`
    - `outgoingMessage.end(chunk[, encoding][, callback])`
    - `outgoingMessage.flushHeaders()`
    - `outgoingMessage.getHeader(name)`
    - `outgoingMessage.getHeaderNames()`
    - `outgoingMessage.getHeaders()`
    - `outgoingMessage.hasHeader(name)`
    - `outgoingMessage.headersSent`
    - `outgoingMessage.pipe()`
    - `outgoingMessage.removeHeader(name)`
    - `outgoingMessage.setHeader(name, value)`
    - `outgoingMessage.setHeaders(headers)`
    - `outgoingMessage.setTimeout(msecs[, callback])`
    - `outgoingMessage.socket`
    - `outgoingMessage.uncork()`
    - `outgoingMessage.writableCorked`
    - `outgoingMessage.writableEnded`
    - `outgoingMessage.writableFinished`
    - `outgoingMessage.writableHighWaterMark`
    - `outgoingMessage.writableLength`
    - `outgoingMessage.writableObjectMode`
    - `outgoingMessage.write(chunk[, encoding][, callback])`
  - `http.METHODS`
  - `http.STATUS_CODES`
  - `http.createServer([options][, requestListener])`
  - `http.get(options[, callback])`
  - `http.get(url[, options][, callback])`
  - `http.globalAgent`
  - `http.maxHeaderSize`
  - `http.request(options[, callback])`
  - `http.request(url[, options][, callback])`
  - `http.validateHeaderName(name[, label])`
  - `http.validateHeaderValue(name, value)`
  - `http.setMaxIdleHTTPParsers(max)`
  - `http.setGlobalProxyFromEnv([proxyEnv])`
  - Class: `WebSocket`
  - Built-in Proxy Support
    - Proxy security considerations
    - Proxy URL Format
    - `NO_PROXY` Format
    - Example

- HTTP/2
  - Determining if crypto support is unavailable
  - Core API
    - Server-side example
    - Client-side example
    - Class: `Http2Session`
      - `Http2Session` and sockets
      - Event: `'close'`
      - Event: `'connect'`
      - Event: `'error'`
      - Event: `'frameError'`
      - Event: `'goaway'`
      - Event: `'localSettings'`
      - Event: `'ping'`
      - Event: `'remoteSettings'`
      - Event: `'stream'`
      - Event: `'timeout'`
      - `http2session.alpnProtocol`
      - `http2session.close([callback])`
      - `http2session.closed`
      - `http2session.connecting`
      - `http2session.destroy([error][, code])`
      - `http2session.destroyed`
      - `http2session.encrypted`
      - `http2session.goaway([code[, lastStreamID[, opaqueData]]])`
      - `http2session.localSettings`
      - `http2session.originSet`
      - `http2session.pendingSettingsAck`
      - `http2session.ping([payload, ]callback)`
      - `http2session.ref()`
      - `http2session.remoteSettings`
      - `http2session.setLocalWindowSize(windowSize)`
      - `http2session.setTimeout(msecs, callback)`
      - `http2session.socket`
      - `http2session.state`
      - `http2session.settings([settings][, callback])`
      - `http2session.type`
      - `http2session.unref()`
    - Class: `ServerHttp2Session`
      - `serverhttp2session.altsvc(alt, originOrStream)`
      - Specifying alternative services
      - `serverhttp2session.origin(...origins)`
    - Class: `ClientHttp2Session`
      - Event: `'altsvc'`
      - Event: `'origin'`
      - `clienthttp2session.request(headers[, options])`
    - Class: `Http2Stream`
      - `Http2Stream` Lifecycle
        - Creation
        - Destruction
      - Event: `'aborted'`
      - Event: `'close'`
      - Event: `'error'`
      - Event: `'frameError'`
      - Event: `'ready'`
      - Event: `'timeout'`
      - Event: `'trailers'`
      - Event: `'wantTrailers'`
      - `http2stream.aborted`
      - `http2stream.bufferSize`
      - `http2stream.close(code[, callback])`
      - `http2stream.closed`
      - `http2stream.destroyed`
      - `http2stream.endAfterHeaders`
      - `http2stream.id`
      - `http2stream.pending`
      - `http2stream.priority(options)`
      - `http2stream.rstCode`
      - `http2stream.sentHeaders`
      - `http2stream.sentInfoHeaders`
      - `http2stream.sentTrailers`
      - `http2stream.session`
      - `http2stream.setTimeout(msecs, callback)`
      - `http2stream.state`
      - `http2stream.sendTrailers(headers)`
    - Class: `ClientHttp2Stream`
      - Event: `'continue'`
      - Event: `'headers'`
      - Event: `'push'`
      - Event: `'response'`
    - Class: `ServerHttp2Stream`
      - `http2stream.additionalHeaders(headers)`
      - `http2stream.headersSent`
      - `http2stream.pushAllowed`
      - `http2stream.pushStream(headers[, options], callback)`
      - `http2stream.respond([headers[, options]])`
      - `http2stream.respondWithFD(fd[, headers[, options]])`
      - `http2stream.respondWithFile(path[, headers[, options]])`
    - Class: `Http2Server`
      - Event: `'checkContinue'`
      - Event: `'connection'`
      - Event: `'request'`
      - Event: `'session'`
      - Event: `'sessionError'`
      - Event: `'stream'`
      - Event: `'timeout'`
      - `server.close([callback])`
      - `server[Symbol.asyncDispose]()`
      - `server.setTimeout([msecs][, callback])`
      - `server.timeout`
      - `server.updateSettings([settings])`
    - Class: `Http2SecureServer`
      - Event: `'checkContinue'`
      - Event: `'connection'`
      - Event: `'request'`
      - Event: `'session'`
      - Event: `'sessionError'`
      - Event: `'stream'`
      - Event: `'timeout'`
      - Event: `'unknownProtocol'`
      - `server.close([callback])`
      - `server.setTimeout([msecs][, callback])`
      - `server.timeout`
      - `server.updateSettings([settings])`
    - `http2.createServer([options][, onRequestHandler])`
    - `http2.createSecureServer(options[, onRequestHandler])`
    - `http2.connect(authority[, options][, listener])`
    - `http2.constants`
      - Error codes for `RST_STREAM` and `GOAWAY`
    - `http2.getDefaultSettings()`
    - `http2.getPackedSettings([settings])`
    - `http2.getUnpackedSettings(buf)`
    - `http2.performServerHandshake(socket[, options])`
    - `http2.sensitiveHeaders`
    - Headers object
      - Raw headers
      - Sensitive headers
    - Settings object
    - Error handling
    - Invalid character handling in header names and values
    - Push streams on the client
    - Supporting the `CONNECT` method
    - The extended `CONNECT` protocol
  - Compatibility API
    - ALPN negotiation
    - Class: `http2.Http2ServerRequest`
      - Event: `'aborted'`
      - Event: `'close'`
      - `request.aborted`
      - `request.authority`
      - `request.complete`
      - `request.connection`
      - `request.destroy([error])`
      - `request.headers`
      - `request.httpVersion`
      - `request.method`
      - `request.rawHeaders`
      - `request.rawTrailers`
      - `request.scheme`
      - `request.setTimeout(msecs, callback)`
      - `request.socket`
      - `request.stream`
      - `request.trailers`
      - `request.url`
    - Class: `http2.Http2ServerResponse`
      - Event: `'close'`
      - Event: `'finish'`
      - `response.addTrailers(headers)`
      - `response.appendHeader(name, value)`
      - `response.connection`
      - `response.createPushResponse(headers, callback)`
      - `response.end([data[, encoding]][, callback])`
      - `response.finished`
      - `response.getHeader(name)`
      - `response.getHeaderNames()`
      - `response.getHeaders()`
      - `response.hasHeader(name)`
      - `response.headersSent`
      - `response.removeHeader(name)`
      - `response.req`
      - `response.sendDate`
      - `response.setHeader(name, value)`
      - `response.setTimeout(msecs[, callback])`
      - `response.socket`
      - `response.statusCode`
      - `response.statusMessage`
      - `response.stream`
      - `response.writableEnded`
      - `response.write(chunk[, encoding][, callback])`
      - `response.writeContinue()`
      - `response.writeEarlyHints(hints)`
      - `response.writeInformation(statusCode[, headers])`
      - `response.writeHead(statusCode[, statusMessage][, headers])`
  - Collecting HTTP/2 performance metrics
  - Note on `:authority` and `host`

- HTTPS
  - Determining if crypto support is unavailable
  - Class: `https.Agent`
    - `new Agent([options])`
      - Event: `'keylog'`
  - Class: `https.Server`
    - `server.close([callback])`
    - `server[Symbol.asyncDispose]()`
    - `server.closeAllConnections()`
    - `server.closeIdleConnections()`
    - `server.headersTimeout`
    - `server.listen()`
    - `server.maxHeadersCount`
    - `server.requestTimeout`
    - `server.setTimeout([msecs][, callback])`
    - `server.timeout`
    - `server.keepAliveTimeout`
  - `https.createServer([options][, requestListener])`
  - `https.get(options[, callback])`
  - `https.get(url[, options][, callback])`
  - `https.globalAgent`
  - `https.request(options[, callback])`
  - `https.request(url[, options][, callback])`

- Inspector
  - Promises API
    - Class: `inspector.Session`
      - `new inspector.Session()`
      - Event: `'inspectorNotification'`
      - Event: `<inspector-protocol-method>`
      - `session.connect()`
      - `session.connectToMainThread()`
      - `session.disconnect()`
      - `session.post(method[, params])`
      - Example usage
        - CPU profiler
        - Heap profiler
  - Callback API
    - Class: `inspector.Session`
      - `new inspector.Session()`
      - Event: `'inspectorNotification'`
      - Event: `<inspector-protocol-method>`;
      - `session.connect()`
      - `session.connectToMainThread()`
      - `session.disconnect()`
      - `session.post(method[, params][, callback])`
      - Example usage
        - CPU profiler
        - Heap profiler
  - Common Objects
    - `inspector.close()`
    - `inspector.console`
    - `inspector.open([port[, host[, wait]]])`
    - `inspector.url()`
    - `inspector.waitForDebugger()`
  - Integration with DevTools
    - `inspector.Network.dataReceived([params])`
    - `inspector.Network.dataSent([params])`
    - `inspector.Network.requestWillBeSent([params])`
    - `inspector.Network.responseReceived([params])`
    - `inspector.Network.loadingFinished([params])`
    - `inspector.Network.loadingFailed([params])`
    - `inspector.Network.webSocketCreated([params])`
    - `inspector.Network.webSocketHandshakeResponseReceived([params])`
    - `inspector.Network.webSocketClosed([params])`
    - `inspector.NetworkResources.put`
    - `inspector.DOMStorage.domStorageItemAdded`
    - `inspector.DOMStorage.domStorageItemRemoved`
    - `inspector.DOMStorage.domStorageItemUpdated`
    - `inspector.DOMStorage.domStorageItemsCleared`
    - `inspector.DOMStorage.registerStorage`
  - Support of breakpoints

- Internationalization support
  - Options for building Node.js
    - Disable all internationalization features (`none`)
    - Build with a pre-installed ICU (`system-icu`)
    - Embed a limited set of ICU data (`small-icu`)
      - Providing ICU data at runtime
    - Embed the entire ICU (`full-icu`)
  - Detecting internationalization support

- About this documentation
  - Contributing
  - Stability index
  - Stability overview
  - JSON output
  - System calls and man pages

- Assert
  - Strict assertion mode
    - Message parameter semantics
  - Legacy assertion mode
  - Class: `assert.AssertionError`
    - `new assert.AssertionError(options)`
  - Class: `assert.Assert`
    - `new assert.Assert([options])`
  - `assert(value[, message])`
  - `assert.deepEqual(actual, expected[, message])`
    - Comparison details
  - `assert.deepStrictEqual(actual, expected[, message])`
    - Comparison details
  - `assert.doesNotMatch(string, regexp[, message])`
  - `assert.doesNotReject(asyncFn[, error][, message])`
  - `assert.doesNotThrow(fn[, error][, message])`
  - `assert.equal(actual, expected[, message])`
  - `assert.fail([message])`
  - `assert.ifError(value)`
  - `assert.match(string, regexp[, message])`
  - `assert.notDeepEqual(actual, expected[, message])`
  - `assert.notDeepStrictEqual(actual, expected[, message])`
  - `assert.notEqual(actual, expected[, message])`
  - `assert.notStrictEqual(actual, expected[, message])`
  - `assert.ok(value[, message])`
  - `assert.rejects(asyncFn[, error][, message])`
  - `assert.strictEqual(actual, expected[, message])`
  - `assert.throws(fn[, error][, message])`
  - `assert.partialDeepStrictEqual(actual, expected[, message])`
    - Comparison details

- Async hooks
  - Terminology
  - Overview
  - `async_hooks.createHook(options)`
    - Error handling
    - Printing in `AsyncHook` callbacks
  - Class: `AsyncHook`
    - `asyncHook.enable()`
    - `asyncHook.disable()`
    - Hook callbacks
      - `init(asyncId, type, triggerAsyncId, resource)`
        - `type`
        - `triggerAsyncId`
        - `resource`
        - Asynchronous context example
      - `before(asyncId)`
      - `after(asyncId)`
      - `destroy(asyncId)`
      - `promiseResolve(asyncId)`
    - `async_hooks.executionAsyncResource()`
    - `async_hooks.executionAsyncId()`
    - `async_hooks.triggerAsyncId()`
    - `async_hooks.asyncWrapProviders`
  - Promise execution tracking
    - Disabling promise execution tracking
  - JavaScript embedder API
    - Class: `AsyncResource`
  - Class: `AsyncLocalStorage`

- Asynchronous context tracking
  - Introduction
  - Class: `AsyncLocalStorage`
    - `new AsyncLocalStorage([options])`
    - Static method: `AsyncLocalStorage.bind(fn)`
    - Static method: `AsyncLocalStorage.snapshot()`
    - `asyncLocalStorage.disable()`
    - `asyncLocalStorage.getStore()`
    - `asyncLocalStorage.enterWith(store)`
    - `asyncLocalStorage.name`
    - `asyncLocalStorage.run(store, callback[, ...args])`
    - `asyncLocalStorage.exit(callback[, ...args])`
    - `asyncLocalStorage.withScope(store)`
    - Usage with `async/await`
    - Troubleshooting: Context loss
  - Class: `RunScope`
    - `scope.dispose()`
  - Class: `AsyncResource`
    - `new AsyncResource(type[, options])`
    - Static method: `AsyncResource.bind(fn[, type[, thisArg]])`
    - `asyncResource.bind(fn[, thisArg])`
    - `asyncResource.runInAsyncScope(fn[, thisArg, ...args])`
    - `asyncResource.emitDestroy()`
    - `asyncResource.asyncId()`
    - `asyncResource.triggerAsyncId()`
    - Using `AsyncResource` for a `Worker` thread pool
    - Integrating `AsyncResource` with `EventEmitter`

- Buffer
  - Buffers and character encodings
  - Buffers and TypedArrays
    - Buffer methods are callable with `Uint8Array` instances
  - Buffers and iteration
  - Class: `Blob`
    - `new buffer.Blob([sources[, options]])`
    - `blob.arrayBuffer()`
    - `blob.bytes()`
    - `blob.size`
    - `blob.slice([start[, end[, type]]])`
    - `blob.stream()`
    - `blob.text()`
    - `blob.textStream()`
    - `blob.type`
    - `Blob` objects and `MessageChannel`
  - Class: `Buffer`
    - Static method: `Buffer.alloc(size[, fill[, encoding]])`
    - Static method: `Buffer.allocUnsafe(size)`
    - Static method: `Buffer.allocUnsafeSlow(size)`
    - Static method: `Buffer.byteLength(string[, encoding])`
    - Static method: `Buffer.compare(buf1, buf2)`
    - Static method: `Buffer.concat(list[, totalLength])`
    - Static method: `Buffer.copyBytesFrom(view[, offset[, length]])`
    - Static method: `Buffer.from(array)`
    - Static method: `Buffer.from(arrayBuffer[, byteOffset[, length]])`
    - Static method: `Buffer.from(buffer)`
    - Static method: `Buffer.from(object[, offsetOrEncoding[, length]])`
    - Static method: `Buffer.from(string[, encoding])`
    - Static method: `Buffer.isBuffer(obj)`
    - Static method: `Buffer.isEncoding(encoding)`
    - `Buffer.poolSize`
    - `buf[index]`
    - `buf.buffer`
    - `buf.byteOffset`
    - `buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])`
    - `buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])`
    - `buf.entries()`
    - `buf.equals(otherBuffer)`
    - `buf.fill(value[, offset[, end]][, encoding])`
    - `buf.includes(value[, start[, end]][, encoding])`
    - `buf.indexOf(value[, start[, end]][, encoding])`
    - `buf.keys()`
    - `buf.lastIndexOf(value[, start[, end]][, encoding])`
    - `buf.length`
    - `buf.parent`
    - `buf.readBigInt64BE([offset])`
    - `buf.readBigInt64LE([offset])`
    - `buf.readBigUInt64BE([offset])`
    - `buf.readBigUInt64LE([offset])`
    - `buf.readDoubleBE([offset])`
    - `buf.readDoubleLE([offset])`
    - `buf.readFloatBE([offset])`
    - `buf.readFloatLE([offset])`
    - `buf.readInt8([offset])`
    - `buf.readInt16BE([offset])`
    - `buf.readInt16LE([offset])`
    - `buf.readInt32BE([offset])`
    - `buf.readInt32LE([offset])`
    - `buf.readIntBE(offset, byteLength)`
    - `buf.readIntLE(offset, byteLength)`
    - `buf.readUInt8([offset])`
    - `buf.readUInt16BE([offset])`
    - `buf.readUInt16LE([offset])`
    - `buf.readUInt32BE([offset])`
    - `buf.readUInt32LE([offset])`
    - `buf.readUIntBE(offset, byteLength)`
    - `buf.readUIntLE(offset, byteLength)`
    - `buf.subarray([start[, end]])`
    - `buf.slice([start[, end]])`
    - `buf.swap16()`
    - `buf.swap32()`
    - `buf.swap64()`
    - `buf.toJSON()`
    - `buf.toString([encoding[, start[, end]]])`
    - `buf.values()`
    - `buf.write(string[, offset[, length]][, encoding])`
    - `buf.writeBigInt64BE(value[, offset])`
    - `buf.writeBigInt64LE(value[, offset])`
    - `buf.writeBigUInt64BE(value[, offset])`
    - `buf.writeBigUInt64LE(value[, offset])`
    - `buf.writeDoubleBE(value[, offset])`
    - `buf.writeDoubleLE(value[, offset])`
    - `buf.writeFloatBE(value[, offset])`
    - `buf.writeFloatLE(value[, offset])`
    - `buf.writeInt8(value[, offset])`
    - `buf.writeInt16BE(value[, offset])`
    - `buf.writeInt16LE(value[, offset])`
    - `buf.writeInt32BE(value[, offset])`
    - `buf.writeInt32LE(value[, offset])`
    - `buf.writeIntBE(value, offset, byteLength)`
    - `buf.writeIntLE(value, offset, byteLength)`
    - `buf.writeUInt8(value[, offset])`
    - `buf.writeUInt16BE(value[, offset])`
    - `buf.writeUInt16LE(value[, offset])`
    - `buf.writeUInt32BE(value[, offset])`
    - `buf.writeUInt32LE(value[, offset])`
    - `buf.writeUIntBE(value, offset, byteLength)`
    - `buf.writeUIntLE(value, offset, byteLength)`
    - `new Buffer(array)`
    - `new Buffer(arrayBuffer[, byteOffset[, length]])`
    - `new Buffer(buffer)`
    - `new Buffer(size)`
    - `new Buffer(string[, encoding])`
  - Class: `File`
    - `new buffer.File(sources, fileName[, options])`
    - `file.name`
    - `file.lastModified`
  - `node:buffer` module APIs
    - `buffer.atob(data)`
    - `buffer.btoa(data)`
    - `buffer.isAscii(input)`
    - `buffer.isUtf8(input)`
    - `buffer.INSPECT_MAX_BYTES`
    - `buffer.kMaxLength`
    - `buffer.kStringMaxLength`
    - `buffer.resolveObjectURL(id)`
    - `buffer.transcode(source, fromEnc, toEnc)`
    - Buffer constants
      - `buffer.constants.MAX_LENGTH`
      - `buffer.constants.MAX_STRING_LENGTH`
  - `Buffer.from()`, `Buffer.alloc()`, and `Buffer.allocUnsafe()`
    - The `--zero-fill-buffers` command-line option
    - What makes `Buffer.allocUnsafe()` and `Buffer.allocUnsafeSlow()` "unsafe"?

- C++ addons
  - Hello world
    - Context-aware addons
      - Worker support
    - Building
    - Linking to libraries included with Node.js
    - Loading addons using `require()`
    - Loading addons using `import`
  - Native abstractions for Node.js
  - Node-API
  - Addon examples
    - Function arguments
    - Callbacks
    - Object factory
    - Function factory
    - Wrapping C++ objects
    - Factory of wrapped objects
    - Passing wrapped objects around

- C++ embedder API
  - Example embedding application
    - Setting up a per-process state
    - Setting up a per-instance state

- Child process
  - Asynchronous process creation
    - Spawning `.bat` and `.cmd` files on Windows
    - `child_process.exec(command[, options][, callback])`
    - `child_process.execFile(file[, args][, options][, callback])`
    - `child_process.fork(modulePath[, args][, options])`
    - `child_process.spawn(command[, args][, options])`
      - `options.detached`
      - `options.stdio`
  - Synchronous process creation
    - `child_process.execFileSync(file[, args][, options])`
    - `child_process.execSync(command[, options])`
    - `child_process.spawnSync(command[, args][, options])`
  - Class: `ChildProcess`
    - Event: `'close'`
    - Event: `'disconnect'`
    - Event: `'error'`
    - Event: `'exit'`
    - Event: `'message'`
    - Event: `'spawn'`
    - `subprocess.channel`
      - `subprocess.channel.ref()`
      - `subprocess.channel.unref()`
    - `subprocess.connected`
    - `subprocess.disconnect()`
    - `subprocess.exitCode`
    - `subprocess.kill([signal])`
    - `subprocess[Symbol.dispose]()`
    - `subprocess.killed`
    - `subprocess.pid`
    - `subprocess.ref()`
    - `subprocess.send(message[, sendHandle[, options]][, callback])`
      - Example: sending a server object
      - Example: sending a socket object
    - `subprocess.signalCode`
    - `subprocess.spawnargs`
    - `subprocess.spawnfile`
    - `subprocess.stderr`
    - `subprocess.stdin`
    - `subprocess.stdio`
    - `subprocess.stdout`
    - `subprocess.unref()`
  - `maxBuffer` and Unicode
  - Shell requirements
  - Default Windows shell
  - Advanced serialization

- Cluster
  - How it works
  - Class: `Worker`
    - Event: `'disconnect'`
    - Event: `'error'`
    - Event: `'exit'`
    - Event: `'listening'`
    - Event: `'message'`
    - Event: `'online'`
    - `worker.disconnect()`
    - `worker.exitedAfterDisconnect`
    - `worker.id`
    - `worker.isConnected()`
    - `worker.isDead()`
    - `worker.kill([signal])`
    - `worker.process`
    - `worker.send(message[, sendHandle[, options]][, callback])`
  - Event: `'disconnect'`
  - Event: `'exit'`
  - Event: `'fork'`
  - Event: `'listening'`
  - Event: `'message'`
  - Event: `'online'`
  - Event: `'setup'`
  - `cluster.disconnect([callback])`
  - `cluster.fork([env])`
  - `cluster.isMaster`
  - `cluster.isPrimary`
  - `cluster.isWorker`
  - `cluster.schedulingPolicy`
  - `cluster.settings`
  - `cluster.setupMaster([settings])`
  - `cluster.setupPrimary([settings])`
  - `cluster.worker`
  - `cluster.workers`

- Command-line API
  - Synopsis
  - Program entry point
  - Options
    - `-`
    - `--`
    - `--abort-on-uncaught-exception`
    - `--allow-addons`
    - `--allow-child-process`
    - `--allow-ffi`
    - `--allow-fs-read`
    - `--allow-fs-write`
    - `--allow-inspector`
    - `--allow-net`
    - `--allow-openssl-store`
    - `--allow-wasi`
    - `--allow-worker`
    - `--build-sea=config`
    - `--build-snapshot`
    - `--build-snapshot-config`
    - `-c`, `--check`
    - `--completion-bash`
    - `-C condition`, `--conditions=condition`
    - `--cpu-prof`
    - `--cpu-prof-dir`
    - `--cpu-prof-interval`
    - `--cpu-prof-name`
    - `--diagnostic-dir=directory`
    - `--disable-proto=mode`
    - `--disable-sigusr1`
    - `--disable-warning=code-or-type`
    - `--disable-wasm-trap-handler`
    - `--disallow-code-generation-from-strings`
    - `--dns-result-order=order`
    - `--enable-fips`
    - `--enable-source-maps`
    - `--entry-url`
    - `--env-file-if-exists=file`
    - `--env-file=file`
    - `-e`, `--eval "script"`
    - `--experimental-addon-modules`
    - `--experimental-config-file=path`, `--experimental-config-file`
    - `--experimental-default-config-file`
    - `--experimental-eventsource`
    - `--experimental-ffi`
    - `--experimental-import-meta-resolve`
    - `--experimental-import-text`
    - `--experimental-inspector-network-resource`
    - `--experimental-loader=module`
    - `--experimental-network-inspection`
    - `--experimental-package-map=<path>`
    - `--experimental-print-required-tla`
    - `--experimental-quic`
    - `--experimental-sea-config`
    - `--experimental-shadow-realm`
    - `--experimental-storage-inspection`
    - `--experimental-stream-iter`
    - `--experimental-test-coverage`
    - `--experimental-test-module-mocks`
    - `--experimental-test-tag-filter=<tag>`
    - `--experimental-vfs`
    - `--experimental-vm-modules`
    - `--experimental-wasi-unstable-preview1`
    - `--experimental-worker-inspection`
    - `--expose-gc`
    - `--force-context-aware`
    - `--force-fips`
    - `--force-node-api-uncaught-exceptions-policy`
    - `--frozen-intrinsics`
    - `--heap-prof`
    - `--heap-prof-dir`
    - `--heap-prof-interval`
    - `--heap-prof-name`
    - `--heapsnapshot-near-heap-limit=max_count`
    - `--heapsnapshot-signal=signal`
    - `-h`, `--help`
    - `--icu-data-dir=file`
    - `--import=module`
    - `--input-type=type`
    - `--insecure-http-parser`
    - `--inspect-brk[=[host:]port]`
    - `--inspect-port=[host:]port`
    - `--inspect-publish-uid=stderr,http`
    - `--inspect-wait[=[host:]port]`
    - `--inspect[=[host:]port]`
      - Warning: binding inspector to a public IP:port combination is insecure
    - `-i`, `--interactive`
    - `--jitless`
    - `--localstorage-file=file`
    - `--max-http-header-size=size`
    - `--max-old-space-size-percentage=percentage`
    - `--network-family-autoselection-attempt-timeout`
    - `--no-addons`
    - `--no-async-context-frame`
    - `--no-deprecation`
    - `--no-experimental-detect-module`
    - `--no-experimental-global-navigator`
    - `--no-experimental-repl-await`
    - `--no-experimental-require-module`
    - `--no-experimental-sqlite`
    - `--no-experimental-websocket`
    - `--no-experimental-webstorage`
    - `--no-extra-info-on-fatal-exception`
    - `--no-force-async-hooks-checks`
    - `--no-global-search-paths`
    - `--no-network-family-autoselection`
    - `--no-require-module`
    - `--no-strip-types`
    - `--no-warnings`
    - `--node-memory-debug`
    - `--openssl-config=file`
    - `--openssl-legacy-provider`
    - `--openssl-shared-config`
    - `--pending-deprecation`
    - `--permission`
    - `--permission-audit`
    - `--preserve-symlinks`
    - `--preserve-symlinks-main`
    - `-p`, `--print "script"`
    - `--prof`
    - `--prof-process`
    - `--redirect-warnings=file`
    - `--report-compact`
    - `--report-dir=directory`, `--report-directory=directory`
    - `--report-exclude-env`
    - `--report-exclude-network`
    - `--report-filename=filename`
    - `--report-on-fatalerror`
    - `--report-on-signal`
    - `--report-signal=signal`
    - `--report-uncaught-exception`
    - `-r`, `--require module`
    - `--run`
      - Intentional limitations
      - Environment variables
    - `--secure-heap-min=n`
    - `--secure-heap=n`
    - `--snapshot-blob=path`
    - `--test`
    - `--test-concurrency`
    - `--test-coverage-branches=threshold`
    - `--test-coverage-exclude`
    - `--test-coverage-functions=threshold`
    - `--test-coverage-include`
    - `--test-coverage-include-all`
    - `--test-coverage-lines=threshold`
    - `--test-force-exit`
    - `--test-global-setup=module`
    - `--test-isolation=mode`
    - `--test-name-pattern`
    - `--test-only`
    - `--test-random-seed`
    - `--test-randomize`
    - `--test-reporter`
    - `--test-reporter-destination`
    - `--test-rerun-failures`
    - `--test-shard`
    - `--test-skip-pattern`
    - `--test-timeout`
    - `--test-update-snapshots`
    - `--throw-deprecation`
    - `--title=title`
    - `--tls-cipher-list=list`
    - `--tls-keylog=file`
    - `--tls-max-v1.2`
    - `--tls-max-v1.3`
    - `--tls-min-v1.0`
    - `--tls-min-v1.1`
    - `--tls-min-v1.2`
    - `--tls-min-v1.3`
    - `--trace-deprecation`
    - `--trace-env`
    - `--trace-env-js-stack`
    - `--trace-env-native-stack`
    - `--trace-event-categories`
    - `--trace-event-file-pattern`
    - `--trace-events-enabled`
    - `--trace-exit`
    - `--trace-require-module=mode`
    - `--trace-sigint`
    - `--trace-sync-io`
    - `--trace-tls`
    - `--trace-uncaught`
    - `--trace-warnings`
    - `--track-heap-objects`
    - `--unhandled-rejections=mode`
    - `--use-bundled-ca`, `--use-openssl-ca`
    - `--use-env-proxy`
    - `--use-largepages=mode`
    - `--use-system-ca`
    - `--v8-options`
    - `--v8-pool-size=num`
    - `-v`, `--version`
    - `--watch`
    - `--watch-kill-signal`
    - `--watch-path`
    - `--watch-preserve-output`
    - `--zero-fill-buffers`
  - Environment variables
    - `FORCE_COLOR=[1, 2, 3]`
    - `NODE_COMPILE_CACHE=dir`
    - `NODE_COMPILE_CACHE_PORTABLE=1`
    - `NODE_DEBUG=module[,…]`
    - `NODE_DEBUG_NATIVE=module[,…]`
    - `NODE_DISABLE_COLORS=1`
    - `NODE_DISABLE_COMPILE_CACHE=1`
    - `NODE_EXTRA_CA_CERTS=file`
    - `NODE_ICU_DATA=file`
    - `NODE_NO_WARNINGS=1`
    - `NODE_OPTIONS=options...`
    - `NODE_PATH=path[:…]`
    - `NODE_PENDING_DEPRECATION=1`
    - `NODE_PENDING_PIPE_INSTANCES=instances`
    - `NODE_PRESERVE_SYMLINKS=1`
    - `NODE_REDIRECT_WARNINGS=file`
    - `NODE_REPL_EXTERNAL_MODULE=file`
    - `NODE_REPL_HISTORY=file`
    - `NODE_SKIP_PLATFORM_CHECK=value`
    - `NODE_TEST_CONTEXT=value`
    - `NODE_TLS_REJECT_UNAUTHORIZED=value`
    - `NODE_USE_ENV_PROXY=1`
    - `NODE_USE_SYSTEM_CA=1`
    - `NODE_V8_COVERAGE=dir`
      - Coverage output
      - Source map cache
    - `NO_COLOR=<any>`
    - `OPENSSL_CONF=file`
    - `SSL_CERT_DIR=dir`
    - `SSL_CERT_FILE=file`
    - `TZ`
    - `UV_THREADPOOL_SIZE=size`
  - Useful V8 options
    - `--abort-on-uncaught-exception`
    - `--disallow-code-generation-from-strings`
    - `--enable-etw-stack-walking`
    - `--expose-gc`
    - `--harmony-shadow-realm`
    - `--heap-snapshot-on-oom`
    - `--interpreted-frames-native-stack`
    - `--jitless`
    - `--max-heap-size`
    - `--max-old-space-size=SIZE` (in MiB)
    - `--max-semi-space-size=SIZE` (in MiB)
    - `--perf-basic-prof`
    - `--perf-basic-prof-only-functions`
    - `--perf-prof`
    - `--perf-prof-unwinding-info`
    - `--prof`
    - `--security-revert`
    - `--stack-trace-limit=limit`

- V8
  - `v8.cachedDataVersionTag()`
  - `v8.getHeapCodeStatistics()`
  - `v8.getHeapSnapshot([options])`
  - `v8.getHeapSpaceStatistics()`
  - `v8.getHeapStatistics()`
  - `v8.getCppHeapStatistics([detailLevel])`
  - `v8.queryObjects(ctor[, options])`
  - `v8.setFlagsFromString(flags)`
  - `v8.stopCoverage()`
  - `v8.takeCoverage()`
  - `v8.writeHeapSnapshot([filename[,options]])`
  - `v8.setHeapSnapshotNearHeapLimit(limit)`
  - Serialization API
    - `v8.serialize(value)`
    - `v8.deserialize(buffer)`
    - Class: `v8.Serializer`
      - `new Serializer()`
      - `serializer.writeHeader()`
      - `serializer.writeValue(value)`
      - `serializer.releaseBuffer()`
      - `serializer.transferArrayBuffer(id, arrayBuffer)`
      - `serializer.writeUint32(value)`
      - `serializer.writeUint64(hi, lo)`
      - `serializer.writeDouble(value)`
      - `serializer.writeRawBytes(buffer)`
      - `serializer._writeHostObject(object)`
      - `serializer._getDataCloneError(message)`
      - `serializer._getSharedArrayBufferId(sharedArrayBuffer)`
      - `serializer._setTreatArrayBufferViewsAsHostObjects(flag)`
    - Class: `v8.Deserializer`
      - `new Deserializer(buffer)`
      - `deserializer.readHeader()`
      - `deserializer.readValue()`
      - `deserializer.transferArrayBuffer(id, arrayBuffer)`
      - `deserializer.getWireFormatVersion()`
      - `deserializer.readUint32()`
      - `deserializer.readUint64()`
      - `deserializer.readDouble()`
      - `deserializer.readRawBytes(length)`
      - `deserializer._readHostObject()`
    - Class: `v8.DefaultSerializer`
    - Class: `v8.DefaultDeserializer`
  - Promise hooks
    - `promiseHooks.onInit(init)`
    - `promiseHooks.onSettled(settled)`
    - `promiseHooks.onBefore(before)`
    - `promiseHooks.onAfter(after)`
    - `promiseHooks.createHook(callbacks)`
    - Hook callbacks
      - `init(promise, parent)`
      - `before(promise)`
      - `after(promise)`
      - `settled(promise)`
  - Startup Snapshot API
    - `v8.startupSnapshot.addSerializeCallback(callback[, data])`
    - `v8.startupSnapshot.addDeserializeCallback(callback[, data])`
    - `v8.startupSnapshot.setDeserializeMainFunction(callback[, data])`
    - `v8.startupSnapshot.isBuildingSnapshot()`
  - Class: `v8.GCProfiler`
    - `new v8.GCProfiler()`
    - `profiler.start()`
    - `profiler.stop()`
    - `profiler[Symbol.dispose]()`
  - Class: `SyncCPUProfileHandle`
    - `syncCpuProfileHandle.stop()`
    - `syncCpuProfileHandle[Symbol.dispose]()`
  - Class: `SyncHeapProfileHandle`
    - `syncHeapProfileHandle.stop()`
    - `syncHeapProfileHandle[Symbol.dispose]()`
  - Class: `CPUProfileHandle`
    - `cpuProfileHandle.stop()`
    - `cpuProfileHandle[Symbol.asyncDispose]()`
  - Class: `HeapProfileHandle`
    - `heapProfileHandle.stop()`
    - `heapProfileHandle[Symbol.asyncDispose]()`
  - `v8.isStringOneByteRepresentation(content)`
  - `v8.startCpuProfile([options])`
  - `v8.startHeapProfile([options])`

- Virtual File System
  - Security
  - Basic usage
  - `vfs.create([provider][, options])`
  - Class: `VirtualFileSystem`
    - `new VirtualFileSystem([provider][, options])`
    - `vfs.provider`
    - `vfs.readonly`
    - APIs
      - Synchronous API
      - Callback API
      - Promise API
  - Class: `VirtualProvider`
    - Capability flags
    - Creating custom providers
  - Class: `MemoryProvider`
    - `memoryProvider.setReadOnly()`
  - Class: `RealFSProvider`
    - `new RealFSProvider(rootPath)`
    - `realFSProvider.rootPath`
  - Implementation details
    - `Stats` objects

- VM (executing JavaScript)
  - Class: `vm.Script`
    - `new vm.Script(code[, options])`
    - `script.cachedDataRejected`
    - `script.createCachedData()`
    - `script.runInContext(contextifiedObject[, options])`
    - `script.runInNewContext([contextObject[, options]])`
    - `script.runInThisContext([options])`
    - `script.sourceMapURL`
  - Class: `vm.Module`
    - `module.error`
    - `module.evaluate([options])`
    - `module.identifier`
    - `module.link(linker)`
    - `module.namespace`
    - `module.status`
  - Class: `vm.SourceTextModule`
    - `new vm.SourceTextModule(code[, options])`
    - `sourceTextModule.createCachedData()`
    - `sourceTextModule.dependencySpecifiers`
    - `sourceTextModule.hasAsyncGraph()`
    - `sourceTextModule.hasTopLevelAwait()`
    - `sourceTextModule.instantiate()`
    - `sourceTextModule.linkRequests(modules)`
    - `sourceTextModule.moduleRequests`
  - Class: `vm.SyntheticModule`
    - `new vm.SyntheticModule(exportNames, evaluateCallback[, options])`
    - `syntheticModule.setExport(name, value)`
  - Type: `ModuleRequest`
  - `vm.compileFunction(code[, params[, options]])`
  - `vm.constants`
    - `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`
  - `vm.createContext([contextObject[, options]])`
  - `vm.isContext(object)`
  - `vm.measureMemory([options])`
  - `vm.runInContext(code, contextifiedObject[, options])`
  - `vm.runInNewContext(code[, contextObject[, options]])`
  - `vm.runInThisContext(code[, options])`
  - Example: Running an HTTP server within a VM
  - What does it mean to "contextify" an object?
    - `vm.constants.DONT_CONTEXTIFY`
  - Timeout interactions with asynchronous tasks and Promises
    - When `microtaskMode` is `'afterEvaluate'`, beware sharing Promises between Contexts
  - Support of dynamic `import()` in compilation APIs
    - When the `importModuleDynamically` option is not specified or undefined
    - When `importModuleDynamically` is `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`
    - When `importModuleDynamically` is a function

- Web Crypto API
  - Modern Algorithms in the Web Cryptography API
  - Secure Curves in the Web Cryptography API
  - Examples
    - Generating keys
      - AES keys
      - ECDSA key pairs
      - Ed25519/X25519 key pairs
      - HMAC keys
      - RSA key pairs
    - Encryption and decryption
    - Exporting and importing keys
    - Wrapping and unwrapping keys
    - Sign and verify
    - Deriving bits and keys
    - Digest
    - Checking for runtime algorithm support
  - Algorithm support
    - Key Management APIs
    - Crypto Operation APIs
    - Key Formats
  - Class: `Crypto`
    - `crypto.subtle`
    - `crypto.getRandomValues(typedArray)`
    - `crypto.randomUUID()`
  - Class: `CryptoKey`
    - `cryptoKey.algorithm`
    - `cryptoKey.extractable`
    - `cryptoKey.type`
    - `cryptoKey.usages`
  - Class: `CryptoKeyPair`
    - `cryptoKeyPair.privateKey`
    - `cryptoKeyPair.publicKey`
  - Class: `SubtleCrypto`
    - Static method: `SubtleCrypto.supports(operation, algorithm[, lengthOrAdditionalAlgorithm])`
    - `subtle.decapsulateBits(decapsulationAlgorithm, decapsulationKey, ciphertext)`
    - `subtle.decapsulateKey(decapsulationAlgorithm, decapsulationKey, ciphertext, sharedKeyAlgorithm, extractable, keyUsages)`
    - `subtle.decrypt(algorithm, key, data)`
    - `subtle.deriveBits(algorithm, baseKey[, length])`
    - `subtle.deriveKey(algorithm, baseKey, derivedKeyType, extractable, keyUsages)`
    - `subtle.digest(algorithm, data)`
    - `subtle.encapsulateBits(encapsulationAlgorithm, encapsulationKey)`
    - `subtle.encapsulateKey(encapsulationAlgorithm, encapsulationKey, sharedKeyAlgorithm, extractable, keyUsages)`
    - `subtle.encrypt(algorithm, key, data)`
    - `subtle.exportKey(format, key)`
    - `subtle.getPublicKey(key, keyUsages)`
    - `subtle.generateKey(algorithm, extractable, keyUsages)`
    - `subtle.importKey(format, keyData, algorithm, extractable, keyUsages)`
    - `subtle.sign(algorithm, key, data)`
    - `subtle.unwrapKey(format, wrappedKey, unwrappingKey, unwrapAlgorithm, unwrappedKeyAlgorithm, extractable, keyUsages)`
    - `subtle.verify(algorithm, key, signature, data)`
    - `subtle.wrapKey(format, key, wrappingKey, wrapAlgorithm)`
  - Algorithm parameters
    - Class: `Algorithm`
      - `Algorithm.name`
    - Class: `AeadParams`
      - `aeadParams.additionalData`
      - `aeadParams.iv`
      - `aeadParams.name`
      - `aeadParams.tagLength`
    - Class: `AesDerivedKeyParams`
      - `aesDerivedKeyParams.name`
      - `aesDerivedKeyParams.length`
    - Class: `AesCbcParams`
      - `aesCbcParams.iv`
      - `aesCbcParams.name`
    - Class: `AesCtrParams`
      - `aesCtrParams.counter`
      - `aesCtrParams.length`
      - `aesCtrParams.name`
    - Class: `AesKeyAlgorithm`
      - `aesKeyAlgorithm.length`
      - `aesKeyAlgorithm.name`
    - Class: `AesKeyGenParams`
      - `aesKeyGenParams.length`
      - `aesKeyGenParams.name`
    - Class: `Argon2Params`
      - `argon2Params.associatedData`
      - `argon2Params.memory`
      - `argon2Params.name`
      - `argon2Params.nonce`
      - `argon2Params.parallelism`
      - `argon2Params.passes`
      - `argon2Params.secretValue`
      - `argon2Params.version`
    - Class: `ContextParams`
      - `contextParams.name`
      - `contextParams.context`
    - Class: `CShakeParams`
      - `cShakeParams.name`
      - `cShakeParams.outputLength`
      - `cShakeParams.functionName`
      - `cShakeParams.customization`
    - Class: `EcdhKeyDeriveParams`
      - `ecdhKeyDeriveParams.name`
      - `ecdhKeyDeriveParams.public`
    - Class: `EcdsaParams`
      - `ecdsaParams.hash`
      - `ecdsaParams.name`
    - Class: `EcKeyAlgorithm`
      - `ecKeyAlgorithm.name`
      - `ecKeyAlgorithm.namedCurve`
    - Class: `EcKeyGenParams`
      - `ecKeyGenParams.name`
      - `ecKeyGenParams.namedCurve`
    - Class: `EcKeyImportParams`
      - `ecKeyImportParams.name`
      - `ecKeyImportParams.namedCurve`
    - Class: `EncapsulatedBits`
      - `encapsulatedBits.ciphertext`
      - `encapsulatedBits.sharedKey`
    - Class: `EncapsulatedKey`
      - `encapsulatedKey.ciphertext`
      - `encapsulatedKey.sharedKey`
    - Class: `HkdfParams`
      - `hkdfParams.hash`
      - `hkdfParams.info`
      - `hkdfParams.name`
      - `hkdfParams.salt`
    - Class: `HmacImportParams`
      - `hmacImportParams.hash`
      - `hmacImportParams.length`
      - `hmacImportParams.name`
    - Class: `HmacKeyAlgorithm`
      - `hmacKeyAlgorithm.hash`
      - `hmacKeyAlgorithm.length`
      - `hmacKeyAlgorithm.name`
    - Class: `HmacKeyGenParams`
      - `hmacKeyGenParams.hash`
      - `hmacKeyGenParams.length`
      - `hmacKeyGenParams.name`
    - Class: `KeyAlgorithm`
      - `keyAlgorithm.name`
    - Class: `KangarooTwelveParams`
      - `kangarooTwelveParams.customization`
      - `kangarooTwelveParams.name`
      - `kangarooTwelveParams.outputLength`
    - Class: `KmacImportParams`
      - `kmacImportParams.length`
      - `kmacImportParams.name`
    - Class: `KmacKeyAlgorithm`
      - `kmacKeyAlgorithm.length`
      - `kmacKeyAlgorithm.name`
    - Class: `KmacKeyGenParams`
      - `kmacKeyGenParams.length`
      - `kmacKeyGenParams.name`
    - Class: `KmacParams`
      - `kmacParams.algorithm`
      - `kmacParams.outputLength`
      - `kmacParams.customization`
    - Class: `Pbkdf2Params`
      - `pbkdf2Params.hash`
      - `pbkdf2Params.iterations`
      - `pbkdf2Params.name`
      - `pbkdf2Params.salt`
    - Class: `RsaHashedImportParams`
      - `rsaHashedImportParams.hash`
      - `rsaHashedImportParams.name`
    - Class: `RsaHashedKeyAlgorithm`
      - `rsaHashedKeyAlgorithm.hash`
      - `rsaHashedKeyAlgorithm.modulusLength`
      - `rsaHashedKeyAlgorithm.name`
      - `rsaHashedKeyAlgorithm.publicExponent`
    - Class: `RsaHashedKeyGenParams`
      - `rsaHashedKeyGenParams.hash`
      - `rsaHashedKeyGenParams.modulusLength`
      - `rsaHashedKeyGenParams.name`
      - `rsaHashedKeyGenParams.publicExponent`
    - Class: `RsaOaepParams`
      - `rsaOaepParams.label`
      - `rsaOaepParams.name`
    - Class: `RsaPssParams`
      - `rsaPssParams.name`
      - `rsaPssParams.saltLength`
    - Class: `TurboShakeParams`
      - `turboShakeParams.domainSeparation`
      - `turboShakeParams.name`
      - `turboShakeParams.outputLength`

- Web Streams API
  - Overview
    - Example `ReadableStream`
    - Node.js streams interoperability
  - API
    - `ReadableStreamTee(stream[, cloneForBranch2])`
    - Class: `ReadableStream`
      - `new ReadableStream([underlyingSource [, strategy]])`
      - `readableStream.locked`
      - `readableStream.cancel([reason])`
      - `readableStream.getReader([options])`
      - `readableStream.pipeThrough(transform[, options])`
      - `readableStream.pipeTo(destination[, options])`
      - `readableStream.tee()`
      - `readableStream.values([options])`
      - Async Iteration
      - Transferring with `postMessage()`
    - `ReadableStream.from(iterable)`
    - Class: `ReadableStreamDefaultReader`
      - `new ReadableStreamDefaultReader(stream)`
      - `readableStreamDefaultReader.cancel([reason])`
      - `readableStreamDefaultReader.closed`
      - `readableStreamDefaultReader.read()`
      - `readableStreamDefaultReader.releaseLock()`
    - Class: `ReadableStreamBYOBReader`
      - `new ReadableStreamBYOBReader(stream)`
      - `readableStreamBYOBReader.cancel([reason])`
      - `readableStreamBYOBReader.closed`
      - `readableStreamBYOBReader.read(view[, options])`
      - `readableStreamBYOBReader.releaseLock()`
    - Class: `ReadableStreamDefaultController`
      - `readableStreamDefaultController.close()`
      - `readableStreamDefaultController.desiredSize`
      - `readableStreamDefaultController.enqueue([chunk])`
      - `readableStreamDefaultController.error([error])`
    - Class: `ReadableByteStreamController`
      - `readableByteStreamController.byobRequest`
      - `readableByteStreamController.close()`
      - `readableByteStreamController.desiredSize`
      - `readableByteStreamController.enqueue(chunk)`
      - `readableByteStreamController.error([error])`
    - Class: `ReadableStreamBYOBRequest`
      - `readableStreamBYOBRequest.respond(bytesWritten)`
      - `readableStreamBYOBRequest.respondWithNewView(view)`
      - `readableStreamBYOBRequest.view`
    - Class: `WritableStream`
      - `new WritableStream([underlyingSink[, strategy]])`
      - `writableStream.abort([reason])`
      - `writableStream.close()`
      - `writableStream.getWriter()`
      - `writableStream.locked`
      - Transferring with postMessage()
    - Class: `WritableStreamDefaultWriter`
      - `new WritableStreamDefaultWriter(stream)`
      - `writableStreamDefaultWriter.abort([reason])`
      - `writableStreamDefaultWriter.close()`
      - `writableStreamDefaultWriter.closed`
      - `writableStreamDefaultWriter.desiredSize`
      - `writableStreamDefaultWriter.ready`
      - `writableStreamDefaultWriter.releaseLock()`
      - `writableStreamDefaultWriter.write([chunk])`
    - Class: `WritableStreamDefaultController`
      - `writableStreamDefaultController.error([error])`
      - `writableStreamDefaultController.signal`
    - Class: `TransformStream`
      - `new TransformStream([transformer[, writableStrategy[, readableStrategy]]])`
      - `transformStream.readable`
      - `transformStream.writable`
      - Transferring with postMessage()
    - Class: `TransformStreamDefaultController`
      - `transformStreamDefaultController.desiredSize`
      - `transformStreamDefaultController.enqueue([chunk])`
      - `transformStreamDefaultController.error([reason])`
      - `transformStreamDefaultController.terminate()`
    - Class: `ByteLengthQueuingStrategy`
      - `new ByteLengthQueuingStrategy(init)`
      - `byteLengthQueuingStrategy.highWaterMark`
      - `byteLengthQueuingStrategy.size`
    - Class: `CountQueuingStrategy`
      - `new CountQueuingStrategy(init)`
      - `countQueuingStrategy.highWaterMark`
      - `countQueuingStrategy.size`
    - Class: `TextEncoderStream`
      - `new TextEncoderStream()`
      - `textEncoderStream.encoding`
      - `textEncoderStream.readable`
      - `textEncoderStream.writable`
    - Class: `TextDecoderStream`
      - `new TextDecoderStream([encoding[, options]])`
      - `textDecoderStream.encoding`
      - `textDecoderStream.fatal`
      - `textDecoderStream.ignoreBOM`
      - `textDecoderStream.readable`
      - `textDecoderStream.writable`
    - Class: `CompressionStream`
      - `new CompressionStream(format)`
      - `compressionStream.readable`
      - `compressionStream.writable`
    - Class: `DecompressionStream`
      - `new DecompressionStream(format)`
      - `decompressionStream.readable`
      - `decompressionStream.writable`
    - Utility Consumers
      - `streamConsumers.arrayBuffer(stream)`
      - `streamConsumers.blob(stream)`
      - `streamConsumers.buffer(stream)`
      - `streamConsumers.bytes(stream)`
      - `streamConsumers.json(stream)`
      - `streamConsumers.text(stream)`

- WebAssembly System Interface (WASI)
  - Security
  - Class: `WASI`
    - `new WASI([options])`
    - `wasi.getImportObject()`
    - `wasi.start(instance)`
    - `wasi.initialize(instance)`
    - `wasi.finalizeBindings(instance[, options])`
    - `wasi.wasiImport`

- Worker threads
  - `worker_threads.getEnvironmentData(key)`
  - `worker_threads.isInternalThread`
  - `worker_threads.isMainThread`
  - `worker_threads.markAsUntransferable(object)`
  - `worker_threads.isMarkedAsUntransferable(object)`
  - `worker_threads.markAsUncloneable(object)`
  - `worker_threads.moveMessagePortToContext(port, contextifiedSandbox)`
  - `worker_threads.parentPort`
  - `worker_threads.postMessageToThread(threadId, value[, transferList][, timeout])`
  - `worker_threads.receiveMessageOnPort(port)`
  - `worker_threads.resourceLimits`
  - `worker_threads.SHARE_ENV`
  - `worker_threads.setEnvironmentData(key[, value])`
  - `worker_threads.threadId`
  - `worker_threads.threadName`
  - `worker_threads.workerData`
  - `worker_threads.locks`
    - Class: `Lock`
      - `lock.name`
      - `lock.mode`
    - Class: `LockManager`
      - `locks.request(name[, options], callback)`
      - `locks.query()`
  - Class: `BroadcastChannel extends EventTarget`
    - `new BroadcastChannel(name)`
    - `broadcastChannel.close()`
    - `broadcastChannel.onmessage`
    - `broadcastChannel.onmessageerror`
    - `broadcastChannel.postMessage(message)`
    - `broadcastChannel.ref()`
    - `broadcastChannel.unref()`
  - Class: `MessageChannel`
  - Class: `MessagePort`
    - Event: `'close'`
    - Event: `'message'`
    - Event: `'messageerror'`
    - `port.close()`
    - `port.postMessage(value[, transferList])`
      - Considerations when transferring TypedArrays and Buffers
      - Considerations when cloning objects with prototypes, classes, and accessors
    - `port.hasRef()`
    - `port.ref()`
    - `port.start()`
    - `port.unref()`
  - Class: `Worker`
    - `new Worker(filename[, options])`
    - Event: `'error'`
    - Event: `'exit'`
    - Event: `'message'`
    - Event: `'messageerror'`
    - Event: `'online'`
    - `worker.cpuUsage([prev])`
    - `worker.getHeapSnapshot([options])`
    - `worker.getHeapStatistics()`
    - `worker.performance`
      - `performance.eventLoopUtilization([utilization1[, utilization2]])`
    - `worker.postMessage(value[, transferList])`
    - `worker.ref()`
    - `worker.resourceLimits`
    - `worker.startCpuProfile([options])`
    - `worker.startHeapProfile([options])`
    - `worker.stderr`
    - `worker.stdin`
    - `worker.stdout`
    - `worker.terminate()`
    - `worker.threadId`
    - `worker.threadName`
    - `worker.unref()`
    - `worker[Symbol.asyncDispose]()`
  - Notes
    - Synchronous blocking of stdio
    - Launching worker threads from preload scripts

- Zlib
  - Threadpool usage and performance considerations
  - Compressing HTTP requests and responses
  - Memory usage tuning
    - For zlib-based streams
    - For Brotli-based streams
    - For Zstd-based streams
  - Flushing
  - Constants
    - zlib constants
    - Brotli constants
      - Flush operations
      - Compressor options
      - Decompressor options
    - Zstd constants
      - Flush operations
      - Compressor options
      - Strategy options
      - Pledged Source Size
      - Decompressor options
  - Class: `Options`
  - Class: `BrotliOptions`
  - Class: `zlib.BrotliCompress`
  - Class: `zlib.BrotliDecompress`
  - Class: `zlib.Deflate`
  - Class: `zlib.DeflateRaw`
  - Class: `zlib.Gunzip`
  - Class: `zlib.Gzip`
  - Class: `zlib.Inflate`
  - Class: `zlib.InflateRaw`
  - Class: `zlib.Unzip`
  - Class: `zlib.ZlibBase`
    - `zlib.bytesWritten`
    - `zlib.close([callback])`
    - `zlib.flush([kind, ]callback)`
    - `zlib.params(level, strategy, callback)`
    - `zlib.reset()`
  - Class: `ZstdOptions`
  - Class: `zlib.ZstdCompress`
  - Class: `zlib.ZstdDecompress`
  - `zlib.constants`
  - `zlib.crc32(data[, value])`
  - `zlib.createBrotliCompress([options])`
  - `zlib.createBrotliDecompress([options])`
  - `zlib.createDeflate([options])`
  - `zlib.createDeflateRaw([options])`
  - `zlib.createGunzip([options])`
  - `zlib.createGzip([options])`
  - `zlib.createInflate([options])`
  - `zlib.createInflateRaw([options])`
  - `zlib.createUnzip([options])`
  - `zlib.createZstdCompress([options])`
  - `zlib.createZstdDecompress([options])`
  - Convenience methods
    - `zlib.brotliCompress(buffer[, options], callback)`
    - `zlib.brotliCompressSync(buffer[, options])`
    - `zlib.brotliDecompress(buffer[, options], callback)`
    - `zlib.brotliDecompressSync(buffer[, options])`
    - `zlib.deflate(buffer[, options], callback)`
    - `zlib.deflateSync(buffer[, options])`
    - `zlib.deflateRaw(buffer[, options], callback)`
    - `zlib.deflateRawSync(buffer[, options])`
    - `zlib.gunzip(buffer[, options], callback)`
    - `zlib.gunzipSync(buffer[, options])`
    - `zlib.gzip(buffer[, options], callback)`
    - `zlib.gzipSync(buffer[, options])`
    - `zlib.inflate(buffer[, options], callback)`
    - `zlib.inflateSync(buffer[, options])`
    - `zlib.inflateRaw(buffer[, options], callback)`
    - `zlib.inflateRawSync(buffer[, options])`
    - `zlib.unzip(buffer[, options], callback)`
    - `zlib.unzipSync(buffer[, options])`
    - `zlib.zstdCompress(buffer[, options], callback)`
    - `zlib.zstdCompressSync(buffer[, options])`
    - `zlib.zstdDecompress(buffer[, options], callback)`
    - `zlib.zstdDecompressSync(buffer[, options])`
  - Iterable Compression
    - `compressBrotli([options])`
    - `compressBrotliSync([options])`
    - `compressDeflate([options])`
    - `compressDeflateSync([options])`
    - `compressGzip([options])`
    - `compressGzipSync([options])`
    - `compressZstd([options])`
    - `compressZstdSync([options])`
    - `decompressBrotli([options])`
    - `decompressBrotliSync([options])`
    - `decompressDeflate([options])`
    - `decompressDeflateSync([options])`
    - `decompressGzip([options])`
    - `decompressGzipSync([options])`
    - `decompressZstd([options])`
    - `decompressZstdSync([options])`

- Performance measurement APIs
  - `perf_hooks.performance`
    - `performance.clearMarks([name])`
    - `performance.clearMeasures([name])`
    - `performance.clearResourceTimings([name])`
    - `performance.eventLoopUtilization([utilization1[, utilization2]])`
    - `performance.getEntries()`
    - `performance.getEntriesByName(name[, type])`
    - `performance.getEntriesByType(type)`
    - `performance.mark(name[, options])`
    - `performance.markResourceTiming(timingInfo, requestedUrl, initiatorType, global, cacheMode, bodyInfo, responseStatus[, deliveryType])`
    - `performance.measure(name[, startMarkOrOptions[, endMark]])`
    - `performance.nodeTiming`
    - `performance.now()`
    - `performance.setResourceTimingBufferSize(maxSize)`
    - `performance.timeOrigin`
    - `performance.timerify(fn[, options])`
    - `performance.toJSON()`
      - Event: `'resourcetimingbufferfull'`
  - Class: `PerformanceEntry`
    - `performanceEntry.duration`
    - `performanceEntry.entryType`
    - `performanceEntry.name`
    - `performanceEntry.startTime`
  - Class: `PerformanceMark`
    - `performanceMark.detail`
  - Class: `PerformanceMeasure`
    - `performanceMeasure.detail`
  - Class: `PerformanceNodeEntry`
    - `performanceNodeEntry.detail`
    - `performanceNodeEntry.flags`
    - `performanceNodeEntry.kind`
    - Garbage Collection ('gc') Details
    - HTTP ('http') Details
    - HTTP/2 ('http2') Details
    - Timerify ('function') Details
    - Net ('net') Details
    - DNS ('dns') Details
  - Class: `PerformanceNodeTiming`
    - `performanceNodeTiming.bootstrapComplete`
    - `performanceNodeTiming.environment`
    - `performanceNodeTiming.idleTime`
    - `performanceNodeTiming.loopExit`
    - `performanceNodeTiming.loopStart`
    - `performanceNodeTiming.nodeStart`
    - `performanceNodeTiming.uvMetricsInfo`
    - `performanceNodeTiming.v8Start`
  - Class: `PerformanceResourceTiming`
    - `performanceResourceTiming.workerStart`
    - `performanceResourceTiming.redirectStart`
    - `performanceResourceTiming.redirectEnd`
    - `performanceResourceTiming.fetchStart`
    - `performanceResourceTiming.domainLookupStart`
    - `performanceResourceTiming.domainLookupEnd`
    - `performanceResourceTiming.connectStart`
    - `performanceResourceTiming.connectEnd`
    - `performanceResourceTiming.secureConnectionStart`
    - `performanceResourceTiming.requestStart`
    - `performanceResourceTiming.responseEnd`
    - `performanceResourceTiming.transferSize`
    - `performanceResourceTiming.encodedBodySize`
    - `performanceResourceTiming.decodedBodySize`
    - `performanceResourceTiming.toJSON()`
  - Class: `PerformanceObserver`
    - `PerformanceObserver.supportedEntryTypes`
    - `new PerformanceObserver(callback)`
    - `performanceObserver.disconnect()`
    - `performanceObserver.observe(options)`
    - `performanceObserver.takeRecords()`
  - Class: `PerformanceObserverEntryList`
    - `performanceObserverEntryList.getEntries()`
    - `performanceObserverEntryList.getEntriesByName(name[, type])`
    - `performanceObserverEntryList.getEntriesByType(type)`
  - `perf_hooks.createHistogram([options])`
  - `perf_hooks.eventLoopUtilization([utilization1[, utilization2]])`
  - `perf_hooks.monitorEventLoopDelay([options])`
  - `perf_hooks.timerify(fn[, options])`
  - Class: `Histogram`
    - `histogram.count`
    - `histogram.countBigInt`
    - `histogram.exceeds`
    - `histogram.exceedsBigInt`
    - `histogram.max`
    - `histogram.maxBigInt`
    - `histogram.mean`
    - `histogram.min`
    - `histogram.minBigInt`
    - `histogram.percentile(percentile)`
    - `histogram.percentileBigInt(percentile)`
    - `histogram.percentiles`
    - `histogram.percentilesBigInt`
    - `histogram.reset()`
    - `histogram.stddev`
  - Class: `ELDHistogram extends Histogram`
    - `histogram.disable()`
    - `histogram.enable()`
    - `histogram[Symbol.dispose]()`
    - Cloning an `ELDHistogram`
  - Class: `RecordableHistogram extends Histogram`
    - `histogram.add(other)`
    - `histogram.record(val)`
    - `histogram.recordDelta()`
  - Examples
    - Measuring the duration of async operations
    - Measuring how long it takes to load dependencies
    - Measuring how long one HTTP round-trip takes
    - Measuring how long the `net.connect` (only for TCP) takes when the connection is successful
    - Measuring how long the DNS takes when the request is successful

- Permissions
  - Process-based permissions
    - Permission Model
      - Runtime API
        - `permission.has(scope[, reference])`
        - `permission.drop(scope[, reference])`
      - Audit Mode
      - File System Permissions
      - Configuration file support
      - Using the Permission Model with `npx`
      - Permission Model constraints
      - process._debugProcess() and cross-process Inspector activation
      - Limitations and Known Issues

- Process
  - Process events
    - Event: `'beforeExit'`
    - Event: `'disconnect'`
    - Event: `'exit'`
    - Event: `'message'`
    - Event: `'rejectionHandled'`
    - Event: `'workerMessage'`
    - Event: `'uncaughtException'`
      - Warning: Using `'uncaughtException'` correctly
    - Event: `'uncaughtExceptionMonitor'`
    - Event: `'unhandledRejection'`
    - Event: `'warning'`
      - Emitting custom warnings
      - Node.js warning names
    - Event: `'worker'`
    - Signal events
  - `process.abort()`
  - `process.addUncaughtExceptionCaptureCallback(fn)`
  - `process.allowedNodeEnvironmentFlags`
  - `process.arch`
  - `process.argv`
  - `process.argv0`
  - `process.availableMemory()`
  - `process.channel`
    - `process.channel.ref()`
    - `process.channel.unref()`
  - `process.chdir(directory)`
  - `process.config`
  - `process.connected`
  - `process.constrainedMemory()`
  - `process.cpuUsage([previousValue])`
  - `process.cwd()`
  - `process.debugPort`
  - `process.disconnect()`
  - `process.dlopen(module, filename[, flags])`
  - `process.emitWarning(warning[, options])`
  - `process.emitWarning(warning[, type[, code]][, ctor])`
    - Avoiding duplicate warnings
  - `process.env`
  - `process.execArgv`
  - `process.execPath`
  - `process.execve(file[, args[, env]])`
  - `process.exit([code])`
  - `process.exitCode`
  - `process.features.cached_builtins`
  - `process.features.debug`
  - `process.features.inspector`
  - `process.features.ipv6`
  - `process.features.require_module`
  - `process.features.tls`
  - `process.features.tls_alpn`
  - `process.features.tls_ocsp`
  - `process.features.tls_sni`
  - `process.features.typescript`
  - `process.features.uv`
  - `process.finalization.register(ref, callback)`
  - `process.finalization.registerBeforeExit(ref, callback)`
  - `process.finalization.unregister(ref)`
  - `process.getActiveResourcesInfo()`
  - `process.getBuiltinModule(id)`
  - `process.getegid()`
  - `process.geteuid()`
  - `process.getgid()`
  - `process.getgroups()`
  - `process.getuid()`
  - `process.hasUncaughtExceptionCaptureCallback()`
  - `process.hrtime([time])`
  - `process.hrtime.bigint()`
  - `process.initgroups(user, extraGroup)`
  - `process.kill(pid[, signal])`
  - `process.loadEnvFile(path)`
  - `process.mainModule`
  - `process.memoryUsage()`
    - A note on process memoryUsage
  - `process.memoryUsage.rss()`
  - `process.nextTick(callback[, ...args])`
    - When to use `queueMicrotask()` vs. `process.nextTick()`
  - `process.noDeprecation`
  - `process.permission`
    - `process.permission.has(scope[, reference])`
    - `process.permission.drop(scope[, reference])`
  - `process.pid`
  - `process.platform`
  - `process.ppid`
  - `process.ref(maybeRefable)`
  - `process.release`
  - `process.report`
    - `process.report.compact`
    - `process.report.directory`
    - `process.report.filename`
    - `process.report.getReport([err])`
    - `process.report.reportOnFatalError`
    - `process.report.reportOnSignal`
    - `process.report.reportOnUncaughtException`
    - `process.report.excludeEnv`
    - `process.report.signal`
    - `process.report.writeReport([filename][, err])`
  - `process.resourceUsage()`
  - `process.send(message[, sendHandle[, options]][, callback])`
  - `process.setegid(id)`
  - `process.seteuid(id)`
  - `process.setgid(id)`
  - `process.setgroups(groups)`
  - `process.setuid(id)`
  - `process.setSourceMapsEnabled(val)`
  - `process.setUncaughtExceptionCaptureCallback(fn)`
  - `process.sourceMapsEnabled`
  - `process.stderr`
    - `process.stderr.fd`
  - `process.stdin`
    - `process.stdin.fd`
  - `process.stdout`
    - `process.stdout.fd`
    - A note on process I/O
  - `process.throwDeprecation`
  - `process.threadCpuUsage([previousValue])`
  - `process.title`
  - `process.traceDeprecation`
  - `process.traceProcessWarnings`
  - `process.umask()`
  - `process.umask(mask)`
  - `process.unref(maybeRefable)`
  - `process.uptime()`
  - `process.version`
  - `process.versions`
  - Exit codes

- Punycode
  - `punycode.decode(string)`
  - `punycode.encode(string)`
  - `punycode.toASCII(domain)`
  - `punycode.toUnicode(domain)`
  - `punycode.ucs2`
    - `punycode.ucs2.decode(string)`
    - `punycode.ucs2.encode(codePoints)`
  - `punycode.version`

- Query string
  - `querystring.decode()`
  - `querystring.encode()`
  - `querystring.escape(str)`
  - `querystring.parse(str[, sep[, eq[, options]]])`
  - `querystring.stringify(obj[, sep[, eq[, options]]])`
  - `querystring.unescape(str)`

- Readline
  - Class: `InterfaceConstructor`
    - Event: `'close'`
    - Event: `'error'`
    - Event: `'line'`
    - Event: `'history'`
    - Event: `'pause'`
    - Event: `'resume'`
    - Event: `'SIGCONT'`
    - Event: `'SIGINT'`
    - Event: `'SIGTSTP'`
    - `rl.close()`
    - `rl[Symbol.dispose]()`
    - `rl.pause()`
    - `rl.prompt([preserveCursor])`
    - `rl.resume()`
    - `rl.setPrompt(prompt)`
    - `rl.getPrompt()`
    - `rl.write(data[, key])`
    - `rl[Symbol.asyncIterator]()`
    - `rl.line`
    - `rl.cursor`
    - `rl.getCursorPos()`
  - Promises API
    - Class: `readlinePromises.Interface`
      - `rl.question(query[, options])`
    - Class: `readlinePromises.Readline`
      - `new readlinePromises.Readline(stream[, options])`
      - `rl.clearLine(dir)`
      - `rl.clearScreenDown()`
      - `rl.commit()`
      - `rl.cursorTo(x[, y])`
      - `rl.moveCursor(dx, dy)`
      - `rl.rollback()`
    - `readlinePromises.createInterface(options)`
      - Use of the `completer` function
  - Callback API
    - Class: `readline.Interface`
      - `rl.question(query[, options], callback)`
    - `readline.clearLine(stream, dir[, callback])`
    - `readline.clearScreenDown(stream[, callback])`
    - `readline.createInterface(options)`
      - Use of the `completer` function
    - `readline.cursorTo(stream, x[, y][, callback])`
    - `readline.moveCursor(stream, dx, dy[, callback])`
  - `readline.emitKeypressEvents(stream[, interface])`
  - Example: Tiny CLI
  - Example: Read file stream line-by-Line
  - TTY keybindings

- REPL
  - Design and features
    - Commands and special keys
    - Default evaluation
      - JavaScript expressions
      - Global and local scope
      - Accessing core Node.js modules
      - Global uncaught exceptions
      - Assignment of the `_` (underscore) variable
      - `await` keyword
    - Reverse-i-search
    - Custom evaluation functions
      - Recoverable errors
    - Customizing REPL output
  - Class: `REPLServer`
    - Event: `'exit'`
    - Event: `'reset'`
    - `replServer.defineCommand(keyword, cmd)`
    - `replServer.displayPrompt([preserveCursor])`
    - `replServer.clearBufferedCommand()`
    - `replServer.setupHistory(historyConfig, callback)`
  - `repl.builtinModules`
  - `repl.start([options])`
  - The Node.js REPL
    - Environment variable options
    - Persistent history
    - Using the Node.js REPL with advanced line-editors
    - Starting multiple REPL instances in the same process
    - Examples
      - Full-featured "terminal" REPL over `net.Server` and `net.Socket`
      - REPL over `curl`

- Single executable applications
  - Generating single executable applications with `--build-sea`
    - Assets
    - Startup snapshot support
    - V8 code cache support
    - Execution arguments
    - Execution argument extension
  - Single-executable application API
    - `sea.isSea()`
    - `sea.getAsset(key[, encoding])`
    - `sea.getAssetAsBlob(key[, options])`
    - `sea.getRawAsset(key)`
    - `sea.getAssetKeys()`
  - In the injected main script
    - Module format of the injected main script
    - Module loading in the injected main script
    - `require()` in the injected main script
    - `__filename` and `module.filename` in the injected main script
    - `__dirname` in the injected main script
    - `import.meta` in the injected main script
    - `import()` in the injected main script
    - Using native addons in the injected main script
  - Notes
    - Single executable application creation process
      - 1. Generating single executable preparation blobs
        - Dumping the preparation blob to disk
      - 2. Injecting the preparation blob into the `node` binary
        - Injecting the preparation blob manually
    - Platform support

- SQLite
  - Type conversion between JavaScript and SQLite
  - Class: `DatabaseSync`
    - `new DatabaseSync(path[, options])`
    - `database.aggregate(name, options)`
    - `database.close()`
    - `database.loadExtension(path[, entryPoint])`
    - `database.enableLoadExtension(allow)`
    - `database.enableDefensive(active)`
    - `database.location([dbName])`
    - `database.exec(sql)`
    - `database.function(name[, options], fn)`
    - `database.setAuthorizer(callback)`
    - `database.isOpen`
    - `database.isTransaction`
    - `database.limits`
    - `database.open()`
    - `database.serialize([dbName])`
    - `database.deserialize(buffer[, options])`
    - `database.prepare(sql[, options])`
    - `database.createTagStore([maxSize])`
    - `database.createSession([options])`
    - `database.applyChangeset(changeset[, options])`
    - `database[Symbol.dispose]()`
  - Class: `Session`
    - `session.changeset()`
    - `session.patchset()`
    - `session.close()`
    - `session[Symbol.dispose]()`
  - Class: `StatementSync`
    - `statement.all([namedParameters][, ...anonymousParameters])`
    - `statement.columns()`
    - `statement.expandedSQL`
    - `statement.get([namedParameters][, ...anonymousParameters])`
    - `statement.iterate([namedParameters][, ...anonymousParameters])`
    - `statement.run([namedParameters][, ...anonymousParameters])`
    - `statement.setAllowBareNamedParameters(enabled)`
    - `statement.setAllowUnknownNamedParameters(enabled)`
    - `statement.setReturnArrays(enabled)`
    - `statement.setReadBigInts(enabled)`
    - `statement.sourceSQL`
  - Class: `SQLTagStore`
    - `sqlTagStore.all(stringElements[, ...boundParameters])`
    - `sqlTagStore.get(stringElements[, ...boundParameters])`
    - `sqlTagStore.iterate(stringElements[, ...boundParameters])`
    - `sqlTagStore.run(stringElements[, ...boundParameters])`
    - `sqlTagStore.size`
    - `sqlTagStore.capacity`
    - `sqlTagStore.db`
    - `sqlTagStore.clear()`
  - `sqlite.backup(sourceDb, path[, options])`
  - `sqlite.constants`
    - SQLite constants
      - Conflict resolution constants
      - Authorization constants
        - Authorization result codes
        - Authorization action codes

- Stream
  - Organization of this document
  - Types of streams
    - Streams Promises API
    - `stream.pipeline(streams[, options])`
    - `stream.pipeline(source[, ...transforms], destination[, options])`
    - `stream.finished(stream[, options])`
    - Object mode
    - Buffering
  - API for stream consumers
    - Writable streams
      - Class: `stream.Writable`
        - Event: `'close'`
        - Event: `'drain'`
        - Event: `'error'`
        - Event: `'finish'`
        - Event: `'pipe'`
        - Event: `'unpipe'`
        - `writable.cork()`
        - `writable.destroy([error])`
        - `writable.closed`
        - `writable.destroyed`
        - `writable.end([chunk[, encoding]][, callback])`
        - `writable.setDefaultEncoding(encoding)`
        - `writable.uncork()`
        - `writable.writable`
        - `writable.writableAborted`
        - `writable.writableEnded`
        - `writable.writableCorked`
        - `writable.errored`
        - `writable.writableFinished`
        - `writable.writableHighWaterMark`
        - `writable.writableLength`
        - `writable.writableNeedDrain`
        - `writable.writableObjectMode`
        - `writable[Symbol.asyncDispose]()`
        - `writable.write(chunk[, encoding][, callback])`
    - Readable streams
      - Two reading modes
      - Three states
      - Choose one API style
      - Class: `stream.Readable`
        - Event: `'close'`
        - Event: `'data'`
        - Event: `'end'`
        - Event: `'error'`
        - Event: `'pause'`
        - Event: `'readable'`
        - Event: `'resume'`
        - `readable.destroy([error])`
        - `readable.closed`
        - `readable.destroyed`
        - `readable.isPaused()`
        - `readable.pause()`
        - `readable.pipe(destination[, options])`
        - `readable.read([size])`
        - `readable.readable`
        - `readable.readableAborted`
        - `readable.readableDidRead`
        - `readable.readableEncoding`
        - `readable.readableEnded`
        - `readable.errored`
        - `readable.readableFlowing`
        - `readable.readableHighWaterMark`
        - `readable.readableLength`
        - `readable.readableObjectMode`
        - `readable.resume()`
        - `readable.setEncoding(encoding)`
        - `readable.unpipe([destination])`
        - `readable.unshift(chunk[, encoding])`
        - `readable.wrap(stream)`
        - `readable[Symbol.asyncIterator]()`
        - `readable[Symbol.for('Stream.toAsyncStreamable')]()`
        - `readable[Symbol.asyncDispose]()`
        - `readable.compose(stream[, options])`
        - `readable.iterator([options])`
        - `readable.map(fn[, options])`
        - `readable.filter(fn[, options])`
        - `readable.forEach(fn[, options])`
        - `readable.toArray([options])`
        - `readable.some(fn[, options])`
        - `readable.find(fn[, options])`
        - `readable.every(fn[, options])`
        - `readable.flatMap(fn[, options])`
        - `readable.drop(limit[, options])`
        - `readable.take(limit[, options])`
        - `readable.reduce(fn[, initial[, options]])`
    - Duplex and transform streams
      - Class: `stream.Duplex`
        - `duplex.allowHalfOpen`
      - Class: `stream.Transform`
        - `transform.destroy([error])`
      - `stream.duplexPair([options])`
    - `stream.finished(stream[, options], callback)`
    - `stream.pipeline(source[, ...transforms], destination, callback)`
    - `stream.pipeline(streams, callback)`
    - `stream.compose(...streams)`
    - `stream.isDestroyed(stream)`
    - `stream.isErrored(stream)`
    - `stream.isReadable(stream)`
    - `stream.isWritable(stream)`
    - `stream.Readable.from(iterable[, options])`
    - `stream.Readable.fromWeb(readableStream[, options])`
    - `stream.Readable.isDisturbed(stream)`
    - `stream.Readable.toWeb(streamReadable[, options])`
    - `stream.Writable.fromWeb(writableStream[, options])`
    - `stream.Writable.toWeb(streamWritable)`
    - `stream.Duplex.from(src)`
    - `stream.Duplex.fromWeb(pair[, options])`
    - `stream.Duplex.toWeb(streamDuplex[, options])`
    - `stream.addAbortSignal(signal, stream)`
    - `stream.getDefaultHighWaterMark(objectMode)`
    - `stream.setDefaultHighWaterMark(objectMode, value)`
  - API for stream implementers
    - Simplified construction
    - Implementing a writable stream
      - `new stream.Writable([options])`
      - `writable._construct(callback)`
      - `writable._write(chunk, encoding, callback)`
      - `writable._writev(chunks, callback)`
      - `writable._destroy(err, callback)`
      - `writable._final(callback)`
      - Errors while writing
      - An example writable stream
      - Decoding buffers in a writable stream
    - Implementing a readable stream
      - `new stream.Readable([options])`
      - `readable._construct(callback)`
      - `readable._read(size)`
      - `readable._destroy(err, callback)`
      - `readable.push(chunk[, encoding])`
      - Errors while reading
      - An example counting stream
    - Implementing a duplex stream
      - `new stream.Duplex(options)`
      - An example duplex stream
      - Object mode duplex streams
    - Implementing a transform stream
      - `new stream.Transform([options])`
      - Event: `'end'`
      - Event: `'finish'`
      - `transform._flush(callback)`
      - `transform._transform(chunk, encoding, callback)`
      - Class: `stream.PassThrough`
  - Additional notes
    - Streams compatibility with async generators and async iterators
      - Consuming readable streams with async iterators
      - Creating readable streams with async generators
      - Piping to writable streams from async iterators
    - Compatibility with older Node.js versions
    - `readable.read(0)`
    - `readable.push('')`
    - `highWaterMark` discrepancy after calling `readable.setEncoding()`

- Iterable Streams
  - Concepts
    - Byte streams
    - Batching
    - Transforms
    - Pull vs. push
    - Backpressure
      - The two-buffer model
      - Strict (default)
      - Unbounded
      - Drop-oldest
      - Drop-newest
    - Writer interface
      - `writer.canWrite`
      - `writer.end([options])`
      - `writer.endSync()`
      - `writer.fail(reason)`
      - `writer.write(chunk[, options])`
      - `writer.writeSync(chunk)`
      - `writer.writev(chunks[, options])`
      - `writer.writevSync(chunks)`
  - The `stream/iter` module
  - Sources
    - `from(input)`
    - `fromSync(input)`
  - Pipelines
    - `pipeTo(source[, ...transforms], writer[, options])`
    - `pipeToSync(source[, ...transforms], writer[, options])`
    - `pull(source[, ...transforms][, options])`
    - `pullSync(source[, ...transforms])`
  - Push streams
    - `push([...transforms][, options])`
  - Duplex channels
    - `duplex([options])`
  - Consumers
    - `array(source[, options])`
    - `arrayBuffer(source[, options])`
    - `arrayBufferSync(source[, options])`
    - `arraySync(source[, options])`
    - `bytes(source[, options])`
    - `bytesSync(source[, options])`
    - `text(source[, options])`
    - `textSync(source[, options])`
  - Utilities
    - `ondrain(drainable)`
    - `merge(...sources[, options])`
    - `tap(callback)`
    - `tapSync(callback)`
  - Multi-consumer
    - `broadcast([options])`
      - `broadcast.cancel([reason])`
      - `broadcast.consumerCount`
      - `broadcast.push([...transforms][, options])`
      - `broadcast[Symbol.dispose]()`
    - `Broadcast.from(input[, options])`
    - `share(source[, options])`
    - Class: `Share`
      - Static method: `Share.from(input[, options])`
      - `share.cancel([reason])`
      - `share.consumerCount`
      - `share.pull([...transforms][, options])`
      - `share[Symbol.dispose]()`
    - Interface: `Shareable`
      - `sharable[Symbol.for('Stream.shareProtocol')]`
    - Interface: `SyncShareable`
      - `sharable[Symbol.for('Stream.shareSyncProtocol')]`
    - `shareSync(source[, options])`
    - Class: `SyncShare`
      - Static method: `SyncShare.fromSync(input[, options])`
      - `share.bufferSize`
      - `share.cancel([reason])`
      - `share.consumerCount`
      - `share.pull([...transforms][, options])`
      - `share[Symbol.dispose]()`
  - Compression and decompression transforms
  - Classic stream interop
    - `fromReadable(readable)`
    - `fromWritable(writable[, options])`
    - `toReadable(source[, options])`
    - `toReadableSync(source[, options])`
    - `toWritable(writer)`
  - Protocol symbols
    - `Stream.broadcastProtocol`
    - `Stream.drainableProtocol`
    - `Stream.shareProtocol`
    - `Stream.shareSyncProtocol`
    - `Stream.toAsyncStreamable`
    - `Stream.toStreamable`

- Modules: `node:module` API
  - The `Module` object
    - `module.builtinModules`
    - `module.createRequire(filename)`
    - `module.findPackageJSON(specifier[, base])`
    - `module.isBuiltin(moduleName)`
    - `module.register(specifier[, parentURL][, options])`
    - `module.registerHooks(options)`
    - `module.stripTypeScriptTypes(code[, options])`
    - `module.syncBuiltinESMExports()`
  - Module compile cache
    - Portability of the compile cache
    - Limitations of the compile cache
    - `module.constants.compileCacheStatus`
    - `module.enableCompileCache([options])`
    - `module.flushCompileCache()`
    - `module.getCompileCacheDir()`
  - Customization Hooks
    - Synchronous customization hooks
      - Registration of synchronous customization hooks
        - Registering hooks before application code runs with flags
        - Registering hooks before application code runs programmatically
        - Registering hooks before application code runs with a `data:` URL
      - Convention of hooks and chaining
      - Deregistration of synchronous customization hooks
      - Hook functions accepted by `module.registerHooks()`
      - Synchronous `resolve(specifier, context, nextResolve)`
      - Synchronous `load(url, context, nextLoad)`
        - Accepted final formats returned by `load`
    - Asynchronous customization hooks
      - Caveats of asynchronous customization hooks
      - Registration of asynchronous customization hooks
      - Chaining of asynchronous customization hooks
      - Communication with asynchronous module customization hooks
      - Asynchronous hooks accepted by `module.register()`
      - `initialize()`
      - Asynchronous `resolve(specifier, context, nextResolve)`
      - Asynchronous `load(url, context, nextLoad)`
    - Examples
      - Import from HTTPS
      - Transpilation
        - Asynchronous version
        - Synchronous version
      - Running hooks
      - Import maps
        - Asynchronous version
        - Synchronous version
        - Using the hooks
  - Source Map Support
    - `module.getSourceMapsSupport()`
    - `module.findSourceMap(path)`
    - `module.setSourceMapsSupport(enabled[, options])`
    - Class: `module.SourceMap`
      - `new SourceMap(payload[, { lineLengths }])`
      - `sourceMap.payload`
      - `sourceMap.findEntry(lineOffset, columnOffset)`
      - `sourceMap.findOrigin(lineNumber, columnNumber)`

- Modules: CommonJS modules
  - Enabling
  - Accessing the main module
  - Package manager tips
  - Loading ECMAScript modules using `require()`
  - All together
  - Caching
    - Module caching caveats
  - Built-in modules
    - Built-in modules with mandatory `node:` prefix
  - Cycles
  - File modules
  - Folders as modules
  - Loading from `node_modules` folders
  - Loading from the global folders
  - The module wrapper
  - The module scope
    - `__dirname`
    - `__filename`
    - `exports`
    - `module`
    - `require(id)`
      - `require.cache`
      - `require.extensions`
      - `require.main`
      - `require.resolve(request[, options])`
        - `require.resolve.paths(request)`
  - The `module` object
    - `module.children`
    - `module.exports`
      - `exports` shortcut
    - `module.filename`
    - `module.id`
    - `module.isPreloading`
    - `module.loaded`
    - `module.parent`
    - `module.path`
    - `module.paths`
    - `module.require(id)`
  - The `Module` object
  - Source map v3 support

- Modules: ECMAScript modules
  - Introduction
  - Enabling
  - Packages
  - `import` Specifiers
    - Terminology
    - Mandatory file extensions
    - URLs
      - `file:` URLs
      - `data:` imports
      - `node:` imports
  - Import attributes
  - Built-in modules
  - `import()` expressions
  - `import.meta`
    - `import.meta.dirname`
    - `import.meta.filename`
    - `import.meta.url`
    - `import.meta.main`
    - `import.meta.resolve(specifier)`
  - Interoperability with CommonJS
    - `import` statements
    - `require`
    - CommonJS Namespaces
    - Differences between ES modules and CommonJS
      - No `require`, `exports`, or `module.exports`
      - No `__filename` or `__dirname`
      - No Addon Loading
      - No `require.main`
      - No `require.resolve`
      - No `NODE_PATH`
      - No `require.extensions`
      - No `require.cache`
  - JSON modules
  - Text modules
  - Wasm modules
    - Wasm Source Phase Imports
    - JavaScript String Builtins
    - Wasm Instance Phase Imports
    - Reserved Wasm Namespaces
  - Top-level `await`
  - Loaders
  - Resolution and loading algorithm
    - Features
    - Resolution algorithm
    - Resolution Algorithm Specification
    - Customizing ESM specifier resolution algorithm

- Modules: Packages
  - Introduction
  - Determining module system
    - Introduction
    - Syntax detection
    - Module resolution and loading
    - `package.json` and file extensions
    - `--input-type` flag
  - Package entry points
    - Main entry point export
    - Subpath exports
      - Extensions in subpaths
      - Path Rules and Validation for Export Targets
        - Targets must be relative URLs
        - No path traversal or invalid segments
    - Exports sugar
    - Subpath imports
    - Subpath patterns
    - Conditional exports
    - Nested conditions
    - Resolving user conditions
    - Community Conditions Definitions
    - Self-referencing a package using its name
  - Dual CommonJS/ES module packages
  - Package maps
    - Configuration file format
    - Resolution algorithm
    - Multiple package versions
    - Multiple packages for the same URL
    - Interaction with other resolution
    - Limitations
  - Node.js `package.json` field definitions
    - `"name"`
    - `"main"`
    - `"type"`
    - `"exports"`
    - `"imports"`

- Modules: TypeScript
  - Enabling
  - Full TypeScript support
  - Type stripping
    - Determining module system
    - TypeScript features
    - Importing types without `type` keyword
    - Non-file forms of input
    - Source maps
    - Type stripping in dependencies
    - Paths aliases

- Net
  - IPC support
    - Identifying paths for IPC connections
  - Class: `net.BlockList`
    - `blockList.addAddress(address[, type])`
    - `blockList.addRange(start, end[, type])`
    - `blockList.addSubnet(net, prefix[, type])`
    - `blockList.check(address[, type])`
    - `blockList.rules`
    - `BlockList.isBlockList(value)`
    - `blockList.fromJSON(value)`
    - `blockList.toJSON()`
  - Class: `net.SocketAddress`
    - `new net.SocketAddress([options])`
    - `socketaddress.address`
    - `socketaddress.family`
    - `socketaddress.flowlabel`
    - `socketaddress.port`
    - `SocketAddress.parse(input)`
  - Class: `net.Server`
    - `new net.Server([options][, connectionListener])`
    - Event: `'close'`
    - Event: `'connection'`
    - Event: `'error'`
    - Event: `'listening'`
    - Event: `'drop'`
    - `server.address()`
    - `server.close([callback])`
    - `server[Symbol.asyncDispose]()`
    - `server.getConnections(callback)`
    - `server.listen()`
      - `server.listen(handle[, backlog][, callback])`
      - `server.listen(options[, callback])`
      - `server.listen(path[, backlog][, callback])`
      - `server.listen([port[, host[, backlog]]][, callback])`
    - `server.listening`
    - `server.maxConnections`
    - `server.dropMaxConnection`
    - `server.ref()`
    - `server.unref()`
  - Class: `net.Socket`
    - Transferring TCP handles to other threads
    - `new net.Socket([options])`
    - Event: `'close'`
    - Event: `'connect'`
    - Event: `'connectionAttempt'`
    - Event: `'connectionAttemptFailed'`
    - Event: `'connectionAttemptTimeout'`
    - Event: `'data'`
    - Event: `'drain'`
    - Event: `'end'`
    - Event: `'error'`
    - Event: `'lookup'`
    - Event: `'ready'`
    - Event: `'timeout'`
    - `socket.address()`
    - `socket.autoSelectFamilyAttemptedAddresses`
    - `socket.bufferSize`
    - `socket.bytesRead`
    - `socket.bytesWritten`
    - `socket.connect()`
      - `socket.connect(options[, connectListener])`
      - `socket.connect(path[, connectListener])`
      - `socket.connect(port[, host][, connectListener])`
    - `socket.connecting`
    - `socket.destroy([error])`
    - `socket.destroyed`
    - `socket.destroySoon()`
    - `socket.end([data[, encoding]][, callback])`
    - `socket.localAddress`
    - `socket.localPort`
    - `socket.localFamily`
    - `socket.pause()`
    - `socket.pending`
    - `socket.ref()`
    - `socket.remoteAddress`
    - `socket.remoteFamily`
    - `socket.remotePort`
    - `socket.server`
    - `socket.resetAndDestroy()`
    - `socket.resume()`
    - `socket.setEncoding([encoding])`
    - `socket.setKeepAlive()`
      - `socket.setKeepAlive([options])`
      - `socket.setKeepAlive([enable][, initialDelay][, interval][, count])`
    - `socket.setNoDelay([noDelay])`
    - `socket.setTimeout(timeout[, callback])`
    - `socket.getTypeOfService()`
    - `socket.setTypeOfService(tos)`
    - `socket.timeout`
    - `socket.unref()`
    - `socket.write(data[, encoding][, callback])`
    - `socket.readyState`
  - Class: `net.BoundSocket`
    - `new net.BoundSocket([options])`
    - `boundSocket.address()`
    - `boundSocket.isPipe`
    - `boundSocket.fd()`
    - `boundSocket.close()`
    - `boundSocket[Symbol.dispose]()`
  - `net.connect()`
    - `net.connect(options[, connectListener])`
    - `net.connect(path[, connectListener])`
    - `net.connect(port[, host][, connectListener])`
  - `net.createConnection()`
    - `net.createConnection(options[, connectListener])`
    - `net.createConnection(path[, connectListener])`
    - `net.createConnection(port[, host][, connectListener])`
  - `net.createServer([options][, connectionListener])`
  - `net.getDefaultAutoSelectFamily()`
  - `net.setDefaultAutoSelectFamily(value)`
  - `net.getDefaultAutoSelectFamilyAttemptTimeout()`
  - `net.setDefaultAutoSelectFamilyAttemptTimeout(value)`
  - `net.isIP(input)`
  - `net.isIPv4(input)`
  - `net.isIPv6(input)`

- Node-API
  - Writing addons in various programming languages
  - Implications of ABI stability
    - Enum values in ABI stability
  - Building
    - Build tools
      - node-gyp
      - CMake.js
    - Uploading precompiled binaries
      - node-pre-gyp
      - prebuild
      - prebuildify
  - Usage
  - Node-API version matrix
  - Environment life cycle APIs
    - `napi_set_instance_data`
    - `napi_get_instance_data`
  - Basic Node-API data types
    - `napi_status`
    - `napi_extended_error_info`
    - `napi_env`
    - `node_api_basic_env`
    - `napi_value`
    - `napi_threadsafe_function`
    - `napi_threadsafe_function_release_mode`
    - `napi_threadsafe_function_call_mode`
    - Node-API memory management types
      - `napi_handle_scope`
      - `napi_escapable_handle_scope`
      - `napi_ref`
      - `napi_type_tag`
      - `napi_async_cleanup_hook_handle`
    - Node-API callback types
      - `napi_callback_info`
      - `napi_callback`
      - `node_api_basic_finalize`
      - `napi_finalize`
      - `napi_async_execute_callback`
      - `napi_async_complete_callback`
      - `napi_threadsafe_function_call_js`
      - `napi_cleanup_hook`
      - `napi_async_cleanup_hook`
  - Error handling
    - Return values
      - `napi_get_last_error_info`
    - Exceptions
      - `napi_throw`
      - `napi_throw_error`
      - `napi_throw_type_error`
      - `napi_throw_range_error`
      - `node_api_throw_syntax_error`
      - `napi_is_error`
      - `napi_create_error`
      - `napi_create_type_error`
      - `napi_create_range_error`
      - `node_api_create_syntax_error`
      - `napi_get_and_clear_last_exception`
      - `napi_is_exception_pending`
      - `napi_fatal_exception`
    - Fatal errors
      - `napi_fatal_error`
  - Object lifetime management
    - Making handle lifespan shorter than that of the native method
      - `napi_open_handle_scope`
      - `napi_close_handle_scope`
      - `napi_open_escapable_handle_scope`
      - `napi_close_escapable_handle_scope`
      - `napi_escape_handle`
    - References to values with a lifespan longer than that of the native method
      - `napi_create_reference`
      - `napi_delete_reference`
      - `napi_reference_ref`
      - `napi_reference_unref`
      - `napi_get_reference_value`
    - Cleanup on exit of the current Node.js environment
      - `napi_add_env_cleanup_hook`
      - `napi_remove_env_cleanup_hook`
      - `napi_add_async_cleanup_hook`
      - `napi_remove_async_cleanup_hook`
    - Finalization on the exit of the Node.js environment
  - Module registration
  - Working with JavaScript values
    - Enum types
      - `napi_key_collection_mode`
      - `napi_key_filter`
      - `napi_key_conversion`
      - `napi_valuetype`
      - `napi_typedarray_type`
    - Object creation functions
      - `napi_create_array`
      - `napi_create_array_with_length`
      - `napi_create_arraybuffer`
      - `napi_create_buffer`
      - `napi_create_buffer_copy`
      - `napi_create_date`
      - `napi_create_external`
      - `napi_create_external_arraybuffer`
      - `node_api_create_external_sharedarraybuffer`
      - `napi_create_external_buffer`
      - `napi_create_object`
      - `node_api_create_object_with_properties`
      - `napi_create_symbol`
      - `node_api_symbol_for`
      - `napi_create_typedarray`
      - `node_api_create_buffer_from_arraybuffer`
      - `napi_create_dataview`
    - Functions to convert from C types to Node-API
      - `napi_create_int32`
      - `napi_create_uint32`
      - `napi_create_int64`
      - `napi_create_double`
      - `napi_create_bigint_int64`
      - `napi_create_bigint_uint64`
      - `napi_create_bigint_words`
      - `napi_create_string_latin1`
      - `node_api_create_external_string_latin1`
      - `napi_create_string_utf16`
      - `node_api_create_external_string_utf16`
      - `napi_create_string_utf8`
    - Functions to create optimized property keys
      - `node_api_create_property_key_latin1`
      - `node_api_create_property_key_utf16`
      - `node_api_create_property_key_utf8`
    - Functions to convert from Node-API to C types
      - `napi_get_array_length`
      - `napi_get_arraybuffer_info`
      - `napi_get_buffer_info`
      - `napi_get_prototype`
      - `napi_get_typedarray_info`
      - `napi_get_dataview_info`
      - `napi_get_date_value`
      - `napi_get_value_bool`
      - `napi_get_value_double`
      - `napi_get_value_bigint_int64`
      - `napi_get_value_bigint_uint64`
      - `napi_get_value_bigint_words`
      - `napi_get_value_external`
      - `napi_get_value_int32`
      - `napi_get_value_int64`
      - `napi_get_value_string_latin1`
      - `napi_get_value_string_utf8`
      - `napi_get_value_string_utf16`
      - `napi_get_value_uint32`
    - Functions to get global instances
      - `napi_get_boolean`
      - `napi_get_global`
      - `napi_get_null`
      - `napi_get_undefined`
  - Working with JavaScript values and abstract operations
    - `napi_coerce_to_bool`
    - `napi_coerce_to_number`
    - `napi_coerce_to_object`
    - `napi_coerce_to_string`
    - `napi_typeof`
    - `napi_instanceof`
    - `napi_is_array`
    - `napi_is_arraybuffer`
    - `napi_is_buffer`
    - `napi_is_date`
    - `napi_is_error`
    - `napi_is_typedarray`
    - `napi_is_dataview`
    - `napi_strict_equals`
    - `napi_detach_arraybuffer`
    - `napi_is_detached_arraybuffer`
    - `node_api_is_sharedarraybuffer`
    - `node_api_create_sharedarraybuffer`
  - Working with JavaScript properties
    - Structures
      - `napi_property_attributes`
      - `napi_property_descriptor`
    - Functions
      - `napi_get_property_names`
      - `napi_get_all_property_names`
      - `napi_set_property`
      - `napi_get_property`
      - `napi_has_property`
      - `napi_delete_property`
      - `napi_has_own_property`
      - `napi_set_named_property`
      - `napi_get_named_property`
      - `napi_has_named_property`
      - `napi_set_element`
      - `napi_get_element`
      - `napi_has_element`
      - `napi_delete_element`
      - `napi_define_properties`
      - `napi_object_freeze`
      - `napi_object_seal`
      - `node_api_set_prototype`
  - Working with JavaScript functions
    - `napi_call_function`
    - `napi_create_function`
    - `napi_get_cb_info`
    - `napi_get_new_target`
    - `napi_new_instance`
  - Object wrap
    - `napi_define_class`
    - `napi_wrap`
    - `napi_unwrap`
    - `napi_remove_wrap`
    - `napi_type_tag_object`
    - `napi_check_object_type_tag`
    - `napi_add_finalizer`
      - `node_api_post_finalizer`
  - Simple asynchronous operations
    - `napi_create_async_work`
    - `napi_delete_async_work`
    - `napi_queue_async_work`
    - `napi_cancel_async_work`
  - Custom asynchronous operations
    - `napi_async_init`
    - `napi_async_destroy`
    - `napi_make_callback`
    - `napi_open_callback_scope`
    - `napi_close_callback_scope`
  - Version management
    - `napi_get_node_version`
    - `napi_get_version`
  - Memory management
    - `napi_adjust_external_memory`
  - Promises
    - `napi_create_promise`
    - `napi_resolve_deferred`
    - `napi_reject_deferred`
    - `napi_is_promise`
  - Script execution
    - `napi_run_script`
  - libuv event loop
    - `napi_get_uv_event_loop`
  - Asynchronous thread-safe function calls
    - Calling a thread-safe function
    - Reference counting of thread-safe functions
    - Deciding whether to keep the process running
    - `napi_create_threadsafe_function`
    - `napi_get_threadsafe_function_context`
    - `napi_call_threadsafe_function`
    - `napi_acquire_threadsafe_function`
    - `napi_release_threadsafe_function`
    - `napi_ref_threadsafe_function`
    - `napi_unref_threadsafe_function`
  - Miscellaneous utilities
    - `node_api_get_module_file_name`

- OS
  - `os.EOL`
  - `os.availableParallelism()`
  - `os.arch()`
  - `os.constants`
  - `os.cpus()`
  - `os.devNull`
  - `os.endianness()`
  - `os.freemem()`
  - `os.getPriority([pid])`
  - `os.homedir()`
  - `os.hostname()`
  - `os.loadavg()`
  - `os.machine()`
  - `os.networkInterfaces()`
  - `os.platform()`
  - `os.release()`
  - `os.setPriority([pid, ]priority)`
  - `os.tmpdir()`
  - `os.totalmem()`
  - `os.type()`
  - `os.uptime()`
  - `os.userInfo([options])`
  - `os.version()`
  - OS constants
    - Signal constants
    - Error constants
      - POSIX error constants
      - Windows-specific error constants
    - dlopen constants
    - Priority constants
    - libuv constants

- Path
  - Windows vs. POSIX
  - `path.basename(path[, suffix])`
  - `path.delimiter`
  - `path.dirname(path)`
  - `path.extname(path)`
  - `path.format(pathObject)`
  - `path.matchesGlob(path, pattern)`
  - `path.isAbsolute(path)`
  - `path.join([...paths])`
  - `path.normalize(path)`
  - `path.parse(path)`
  - `path.posix`
  - `path.relative(from, to)`
  - `path.resolve([...paths])`
  - `path.sep`
  - `path.toNamespacedPath(path)`
  - `path.win32`

- String decoder
  - Class: `StringDecoder`
    - `new StringDecoder([encoding])`
    - `stringDecoder.end([buffer])`
    - `stringDecoder.write(buffer)`

- Test runner
  - Subtests
  - Rerunning failed tests
  - `describe()` and `it()` aliases
  - Skipping tests
  - TODO tests
  - Expecting tests to fail
  - `only` tests
  - Filtering tests by name
  - Test tags
    - Authoring tagged tests
    - Filtering by tag
    - Reading tags from inside a test
    - Errors
  - Extraneous asynchronous activity
  - Watch mode
  - Global setup and teardown
  - Running tests from the command line
    - Randomizing tests execution order
    - Test runner execution model
      - Child process option inheritance
  - Collecting code coverage
    - Coverage reporters
  - Mocking
    - Timers
    - Dates
  - Snapshot testing
  - Test reporters
    - Custom reporters
    - Multiple reporters
  - `run([options])`
  - `suite([name][, options][, fn])`
  - `suite.skip([name][, options][, fn])`
  - `suite.todo([name][, options][, fn])`
  - `suite.only([name][, options][, fn])`
  - `test([name][, options][, fn])`
  - `test.skip([name][, options][, fn])`
  - `test.todo([name][, options][, fn])`
  - `test.only([name][, options][, fn])`
  - `describe([name][, options][, fn])`
  - `describe.skip([name][, options][, fn])`
  - `describe.todo([name][, options][, fn])`
  - `describe.only([name][, options][, fn])`
  - `it([name][, options][, fn])`
  - `it.skip([name][, options][, fn])`
  - `it.todo([name][, options][, fn])`
  - `it.only([name][, options][, fn])`
  - `before([fn][, options])`
  - `after([fn][, options])`
  - `beforeEach([fn][, options])`
  - `afterEach([fn][, options])`
  - `assert`
    - `assert.register(name, fn)`
  - `snapshot`
    - `snapshot.setDefaultSnapshotSerializers(serializers)`
    - `snapshot.setResolveSnapshotPath(fn)`
  - Class: `MockFunctionContext`
    - `ctx.calls`
    - `ctx.callCount()`
    - `ctx.mockImplementation(implementation)`
    - `ctx.mockImplementationOnce(implementation[, onCall])`
    - `ctx.resetCalls()`
    - `ctx.restore()`
  - Class: `MockModuleContext`
    - `ctx.restore()`
  - Class: `MockPropertyContext`
    - `ctx.accesses`
    - `ctx.accessCount()`
    - `ctx.mockImplementation(value)`
    - `ctx.mockImplementationOnce(value[, onAccess])`
      - Caveat
    - `ctx.resetAccesses()`
    - `ctx.restore()`
  - Class: `MockTracker`
    - `mock.fn([original[, implementation]][, options])`
    - `mock.getter(object, methodName[, implementation][, options])`
    - `mock.method(object, methodName[, implementation][, options])`
    - `mock.module(specifier[, options])`
    - `mock.property(object, propertyName[, value])`
    - `mock.reset()`
    - `mock.restoreAll()`
    - `mock.setter(object, methodName[, implementation][, options])`
  - Class: `MockTimers`
    - `timers.enable([enableOptions])`
    - `timers.reset()`
    - `timers[Symbol.dispose]()`
    - `timers.tick([milliseconds])`
      - Using clear functions
      - Working with Node.js timers modules
    - `timers.runAll()`
    - `timers.setTime(milliseconds)`
      - Dates and Timers working together
  - Class: `TestsStream`
    - Event: `'test:coverage'`
    - Event: `'test:complete'`
    - Event: `'test:dequeue'`
    - Event: `'test:diagnostic'`
    - Event: `'test:enqueue'`
    - Event: `'test:fail'`
    - Event: `'test:interrupted'`
    - Event: `'test:log'`
    - Event: `'test:pass'`
    - Event: `'test:plan'`
    - Event: `'test:start'`
    - Event: `'test:stderr'`
    - Event: `'test:stdout'`
    - Event: `'test:summary'`
    - Event: `'test:watch:drained'`
    - Event: `'test:watch:restarted'`
  - `getTestContext()`
  - Test instrumentation and OpenTelemetry
    - Tracing events
      - Channel: `'tracing:node.test:start'`
      - Channel: `'tracing:node.test:end'`
      - Channel: `'tracing:node.test:error'`
    - Context propagation with `bindStore()`
  - Class: `TestContext`
    - `context.before([fn][, options])`
    - `context.beforeEach([fn][, options])`
    - `context.after([fn][, options])`
    - `context.afterEach([fn][, options])`
    - `context.assert`
      - `context.assert.fileSnapshot(value, path[, options])`
      - `context.assert.snapshot(value[, options])`
    - `context.diagnostic(message)`
    - `context.log(message[, data])`
    - `context.filePath`
    - `context.fullName`
    - `context.name`
    - `context.passed`
    - `context.error`
    - `context.attempt`
    - `context.tags`
    - `context.workerId`
    - `context.plan(count[,options])`
    - `context.runOnly(shouldRunOnlyTests)`
    - `context.signal`
    - `context.skip([message])`
    - `context.todo([message])`
    - `context.test([name][, options][, fn])`
    - `context.waitFor(condition[, options])`
  - Class: `SuiteContext`
    - `context.filePath`
    - `context.fullName`
    - `context.name`
    - `context.signal`
    - `context.passed`
    - `context.attempt`
    - `context.diagnostic(message)`
    - `context.log(message[, data])`

- Timers
  - Class: `Immediate`
    - `immediate.hasRef()`
    - `immediate.ref()`
    - `immediate.unref()`
    - `immediate[Symbol.dispose]()`
  - Class: `Timeout`
    - `timeout.close()`
    - `timeout.hasRef()`
    - `timeout.ref()`
    - `timeout.refresh()`
    - `timeout.unref()`
    - `timeout[Symbol.toPrimitive]()`
    - `timeout[Symbol.dispose]()`
  - Scheduling timers
    - `setImmediate(callback[, ...args])`
    - `setInterval(callback[, delay[, ...args]])`
    - `setTimeout(callback[, delay[, ...args]])`
  - Cancelling timers
    - `clearImmediate(immediate)`
    - `clearInterval(timeout)`
    - `clearTimeout(timeout)`
  - Timers Promises API
    - `timersPromises.setTimeout([delay[, value[, options]]])`
    - `timersPromises.setImmediate([value[, options]])`
    - `timersPromises.setInterval([delay[, value[, options]]])`
    - `timersPromises.scheduler.wait(delay[, options])`
    - `timersPromises.scheduler.yield()`

- TLS (SSL)
  - Determining if crypto support is unavailable
  - TLS/SSL concepts
    - Perfect forward secrecy
    - ALPN and SNI
    - Pre-shared keys
    - Client-initiated renegotiation attack mitigation
    - Session resumption
      - Session identifiers
      - Session tickets
  - Modifying the default TLS cipher suite
  - OpenSSL security level
    - Setting security levels
    - Using `--tls-cipher-list`
  - X509 certificate error codes
  - Class: `tls.Server`
    - Event: `'connection'`
    - Event: `'keylog'`
    - Event: `'newSession'`
    - Event: `'OCSPRequest'`
    - Event: `'resumeSession'`
    - Event: `'secureConnection'`
    - Event: `'tlsClientError'`
    - `server.addContext(hostname, context)`
    - `server.address()`
    - `server.close([callback])`
    - `server.getTicketKeys()`
    - `server.listen()`
    - `server.setSecureContext(options)`
    - `server.setTicketKeys(keys)`
  - Class: `tls.TLSSocket`
    - `new tls.TLSSocket(socket[, options])`
    - Event: `'keylog'`
    - Event: `'OCSPResponse'`
    - Event: `'secure'`
    - Event: `'secureConnect'`
    - Event: `'session'`
    - `tlsSocket.address()`
    - `tlsSocket.alpnProtocol`
    - `tlsSocket.authorizationError`
    - `tlsSocket.authorized`
    - `tlsSocket.disableRenegotiation()`
    - `tlsSocket.enableTrace()`
    - `tlsSocket.encrypted`
    - `tlsSocket.exportKeyingMaterial(length, label[, context])`
    - `tlsSocket.getCertificate()`
    - `tlsSocket.getCipher()`
    - `tlsSocket.getEphemeralKeyInfo()`
    - `tlsSocket.getFinished()`
    - `tlsSocket.getPeerCertificate([detailed])`
      - Certificate object
    - `tlsSocket.getPeerFinished()`
    - `tlsSocket.getPeerX509Certificate()`
    - `tlsSocket.getProtocol()`
    - `tlsSocket.getSession()`
    - `tlsSocket.getSharedSigalgs()`
    - `tlsSocket.getTLSTicket()`
    - `tlsSocket.getX509Certificate()`
    - `tlsSocket.isSessionReused()`
    - `tlsSocket.localAddress`
    - `tlsSocket.localPort`
    - `tlsSocket.remoteAddress`
    - `tlsSocket.remoteFamily`
    - `tlsSocket.remotePort`
    - `tlsSocket.renegotiate(options, callback)`
    - `tlsSocket.servername`
    - `tlsSocket.setKeyCert(context)`
    - `tlsSocket.setMaxSendFragment(size)`
  - `tls.checkServerIdentity(hostname, cert)`
  - `tls.connect(options[, callback])`
  - `tls.connect(path[, options][, callback])`
  - `tls.connect(port[, host][, options][, callback])`
  - `tls.createSecureContext([options])`
  - `tls.createServer([options][, secureConnectionListener])`
  - `tls.setDefaultCACertificates(certs)`
  - `tls.getCACertificates([type])`
  - `tls.getCiphers()`
  - `tls.getCertificateCompressionAlgorithms()`
  - `tls.rootCertificates`
  - `tls.DEFAULT_ECDH_CURVE`
  - `tls.DEFAULT_MAX_VERSION`
  - `tls.DEFAULT_MIN_VERSION`
  - `tls.DEFAULT_CIPHERS`

- Trace events
  - The `node:trace_events` module
    - `Tracing` object
      - `tracing.categories`
      - `tracing.disable()`
      - `tracing.enable()`
      - `tracing.enabled`
    - `trace_events.createTracing(options)`
    - `trace_events.getEnabledCategories()`
  - Examples
    - Collect trace events data by inspector

- TTY
  - Class: `tty.ReadStream`
    - `readStream.isRaw`
    - `readStream.isTTY`
    - `readStream.setRawMode(mode)`
  - Class: `tty.WriteStream`
    - `new tty.ReadStream(fd[, options])`
    - `new tty.WriteStream(fd)`
    - Event: `'resize'`
    - `writeStream.clearLine(dir[, callback])`
    - `writeStream.clearScreenDown([callback])`
    - `writeStream.columns`
    - `writeStream.cursorTo(x[, y][, callback])`
    - `writeStream.getColorDepth([env])`
    - `writeStream.getWindowSize()`
    - `writeStream.hasColors([count][, env])`
    - `writeStream.isTTY`
    - `writeStream.moveCursor(dx, dy[, callback])`
    - `writeStream.rows`
  - `tty.isatty(fd)`

- UDP/datagram sockets
  - Class: `dgram.Socket`
    - Event: `'close'`
    - Event: `'connect'`
    - Event: `'error'`
    - Event: `'listening'`
    - Event: `'message'`
    - `socket.addMembership(multicastAddress[, multicastInterface])`
    - `socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])`
    - `socket.address()`
    - `socket.bind([port][, address][, callback])`
    - `socket.bind(options[, callback])`
    - `socket.bindSync([options])`
    - `socket.close([callback])`
    - `socket[Symbol.asyncDispose]()`
    - `socket.connect(port[, address][, callback])`
    - `socket.connectSync(port[, address])`
    - `socket.disconnect()`
    - `socket.dropMembership(multicastAddress[, multicastInterface])`
    - `socket.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])`
    - `socket.getRecvBufferSize()`
    - `socket.getSendBufferSize()`
    - `socket.getSendQueueSize()`
    - `socket.getSendQueueCount()`
    - `socket.ref()`
    - `socket.remoteAddress()`
    - `socket.send(msg[, offset, length][, port][, address][, callback])`
      - Note about UDP datagram size
    - `socket.setBroadcast(flag)`
    - `socket.setMulticastInterface(multicastInterface)`
      - Example: IPv6 outgoing multicast interface
      - Example: IPv4 outgoing multicast interface
      - Call results
    - `socket.setMulticastLoopback(flag)`
    - `socket.setMulticastTTL(ttl)`
    - `socket.setRecvBufferSize(size)`
    - `socket.setSendBufferSize(size)`
    - `socket.setTTL(ttl)`
    - `socket.unref()`
  - `node:dgram` module functions
    - `dgram.createSocket(options[, callback])`
    - `dgram.createSocket(type[, callback])`

- URL
  - URL strings and URL objects
    - Constructing a URL from component parts and getting the constructed string
  - The WHATWG URL API
    - Class: `URL`
      - `new URL(input[, base])`
      - `url.hash`
      - `url.host`
      - `url.hostname`
      - `url.href`
      - `url.origin`
      - `url.password`
      - `url.pathname`
      - `url.port`
      - `url.protocol`
        - Special schemes
      - `url.search`
      - `url.searchParams`
      - `url.username`
      - `url.toString()`
      - `url.toJSON()`
      - `URL.createObjectURL(blob)`
      - `URL.revokeObjectURL(id)`
      - `URL.canParse(input[, base])`
      - `URL.parse(input[, base])`
    - Class: `URLPattern`
      - `new URLPattern()`
      - `new URLPattern(string[, baseURL][, options])`
      - `new URLPattern(obj[, baseURL][, options])`
      - `urlPattern.exec(input[, baseURL])`
      - `urlPattern.test(input[, baseURL])`
    - Class: `URLSearchParams`
      - `new URLSearchParams()`
      - `new URLSearchParams(string)`
      - `new URLSearchParams(obj)`
      - `new URLSearchParams(iterable)`
      - `urlSearchParams.append(name, value)`
      - `urlSearchParams.delete(name[, value])`
      - `urlSearchParams.entries()`
      - `urlSearchParams.forEach(fn[, thisArg])`
      - `urlSearchParams.get(name)`
      - `urlSearchParams.getAll(name)`
      - `urlSearchParams.has(name[, value])`
      - `urlSearchParams.keys()`
      - `urlSearchParams.set(name, value)`
      - `urlSearchParams.size`
      - `urlSearchParams.sort()`
      - `urlSearchParams.toString()`
      - `urlSearchParams.values()`
      - `urlSearchParams[Symbol.iterator]()`
    - `url.domainToASCII(domain)`
    - `url.domainToUnicode(domain)`
    - `url.fileURLToPath(url[, options])`
    - `url.fileURLToPathBuffer(url[, options])`
    - `url.format(URL[, options])`
    - `url.pathToFileURL(path[, options])`
    - `url.urlToHttpOptions(url)`
  - Legacy URL API
    - Legacy `urlObject`
      - `urlObject.auth`
      - `urlObject.hash`
      - `urlObject.host`
      - `urlObject.hostname`
      - `urlObject.href`
      - `urlObject.path`
      - `urlObject.pathname`
      - `urlObject.port`
      - `urlObject.protocol`
      - `urlObject.query`
      - `urlObject.search`
      - `urlObject.slashes`
    - `url.format(urlObject)`
    - `url.format(urlString)`
    - `url.parse(urlString[, parseQueryString[, slashesDenoteHost]])`
    - `url.resolve(from, to)`
  - Percent-encoding in URLs
    - Legacy API
    - WHATWG API

- Usage and example
  - Usage
  - Example

- Util
  - `util.callbackify(original)`
  - `util.convertProcessSignalToExitCode(signal)`
  - `util.debuglog(section[, callback])`
    - `debuglog().enabled`
  - `util.debug(section)`
  - `util.deprecate(fn, msg[, code[, options]])`
  - `util.diff(actual, expected)`
  - `util.format(format[, ...args])`
  - `util.formatWithOptions(inspectOptions, format[, ...args])`
  - `util.getCallSites([frameCount][, options])`
  - `util.getSystemErrorName(err)`
  - `util.getSystemErrorMap()`
  - `util.getSystemErrorMessage(err)`
  - `util.setTraceSigInt(enable)`
  - `util.inherits(constructor, superConstructor)`
  - `util.inspect(object[, options])`
  - `util.inspect(object[, showHidden[, depth[, colors]]])`
    - Customizing `util.inspect` colors
      - Complex custom coloring
      - Modifiers
      - Foreground colors
      - Background colors
    - Custom inspection functions on objects
    - `util.inspect.custom`
    - `util.inspect.defaultOptions`
  - `util.isDeepStrictEqual(val1, val2[, options])`
  - Class: `util.MIMEType`
    - `new MIMEType(input)`
    - `mime.type`
    - `mime.subtype`
    - `mime.essence`
    - `mime.params`
    - `mime.toString()`
    - `mime.toJSON()`
  - Class: `util.MIMEParams`
    - `new MIMEParams()`
    - `mimeParams.delete(name)`
    - `mimeParams.entries()`
    - `mimeParams.get(name)`
    - `mimeParams.has(name)`
    - `mimeParams.keys()`
    - `mimeParams.set(name, value)`
    - `mimeParams.values()`
    - `mimeParams[Symbol.iterator]()`
  - `util.parseArgs([config])`
    - `parseArgs` `tokens`
  - `util.parseEnv(content)`
  - `util.promisify(original)`
    - Custom promisified functions
    - `util.promisify.custom`
  - `util.stripVTControlCharacters(str)`
  - `util.styleText(format, text[, options])`
  - Class: `util.TextDecoder`
    - WHATWG supported encodings
      - Encodings supported by default (with full ICU data)
      - Encodings supported when Node.js is built with the `small-icu` option
      - Encodings supported when ICU is disabled
    - `new TextDecoder([encoding[, options]])`
    - `textDecoder.decode([input[, options]])`
    - `textDecoder.encoding`
    - `textDecoder.fatal`
    - `textDecoder.ignoreBOM`
  - Class: `util.TextEncoder`
    - `textEncoder.encode([input])`
    - `textEncoder.encodeInto(src, dest)`
    - `textEncoder.encoding`
  - `util.toUSVString(string)`
  - `util.transferableAbortController()`
  - `util.transferableAbortSignal(signal)`
  - `util.aborted(signal, resource)`
  - `util.types`
    - `util.types.isAnyArrayBuffer(value)`
    - `util.types.isArrayBufferView(value)`
    - `util.types.isArgumentsObject(value)`
    - `util.types.isArrayBuffer(value)`
    - `util.types.isAsyncFunction(value)`
    - `util.types.isBigInt64Array(value)`
    - `util.types.isBigIntObject(value)`
    - `util.types.isBigUint64Array(value)`
    - `util.types.isBooleanObject(value)`
    - `util.types.isBoxedPrimitive(value)`
    - `util.types.isCryptoKey(value)`
    - `util.types.isDataView(value)`
    - `util.types.isDate(value)`
    - `util.types.isExternal(value)`
    - `util.types.isFloat16Array(value)`
    - `util.types.isFloat32Array(value)`
    - `util.types.isFloat64Array(value)`
    - `util.types.isGeneratorFunction(value)`
    - `util.types.isGeneratorObject(value)`
    - `util.types.isInt8Array(value)`
    - `util.types.isInt16Array(value)`
    - `util.types.isInt32Array(value)`
    - `util.types.isKeyObject(value)`
    - `util.types.isMap(value)`
    - `util.types.isMapIterator(value)`
    - `util.types.isModuleNamespaceObject(value)`
    - `util.types.isNativeError(value)`
    - `util.types.isNumberObject(value)`
    - `util.types.isPromise(value)`
    - `util.types.isProxy(value)`
    - `util.types.isRegExp(value)`
    - `util.types.isSet(value)`
    - `util.types.isSetIterator(value)`
    - `util.types.isSharedArrayBuffer(value)`
    - `util.types.isStringObject(value)`
    - `util.types.isSymbolObject(value)`
    - `util.types.isTypedArray(value)`
    - `util.types.isUint8Array(value)`
    - `util.types.isUint8ClampedArray(value)`
    - `util.types.isUint16Array(value)`
    - `util.types.isUint32Array(value)`
    - `util.types.isWeakMap(value)`
    - `util.types.isWeakSet(value)`
  - Deprecated APIs
    - `util._extend(target, source)`
    - `util.isArray(object)`

- Console
  - Class: `Console`
    - `new Console(stdout[, stderr][, ignoreErrors])`
    - `new Console(options)`
    - `console.assert(value[, ...message])`
    - `console.clear()`
    - `console.count([label])`
    - `console.countReset([label])`
    - `console.debug(data[, ...args])`
    - `console.dir(obj[, options])`
    - `console.dirxml(...data)`
    - `console.error([data][, ...args])`
    - `console.group([...label])`
    - `console.groupCollapsed()`
    - `console.groupEnd()`
    - `console.info([data][, ...args])`
    - `console.log([data][, ...args])`
    - `console.table(tabularData[, properties])`
    - `console.time([label])`
    - `console.timeEnd([label])`
    - `console.timeLog([label][, ...data])`
    - `console.trace([message][, ...args])`
    - `console.warn([data][, ...args])`
  - Inspector only methods
    - `console.profile([label])`
    - `console.profileEnd([label])`
    - `console.timeStamp([label])`

- Crypto
  - Determining if crypto support is unavailable
  - Asymmetric key types
    - Key formats
      - KeyObject
      - PEM and DER
      - JSON Web Key (JWK)
      - Raw key formats
    - Choosing a key format
      - Importing keys
      - Inline key material in operations
    - Examples
  - Class: `Certificate`
    - Static method: `Certificate.exportChallenge(spkac[, encoding])`
    - Static method: `Certificate.exportPublicKey(spkac[, encoding])`
    - Static method: `Certificate.verifySpkac(spkac[, encoding])`
    - Legacy API
      - `new crypto.Certificate()`
      - `certificate.exportChallenge(spkac[, encoding])`
      - `certificate.exportPublicKey(spkac[, encoding])`
      - `certificate.verifySpkac(spkac[, encoding])`
  - Class: `Cipheriv`
    - `cipher.final([outputEncoding])`
    - `cipher.getAuthTag()`
    - `cipher.setAAD(buffer[, options])`
    - `cipher.setAutoPadding([autoPadding])`
    - `cipher.update(data[, inputEncoding][, outputEncoding])`
  - Class: `Decipheriv`
    - `decipher.final([outputEncoding])`
    - `decipher.setAAD(buffer[, options])`
    - `decipher.setAuthTag(buffer[, encoding])`
    - `decipher.setAutoPadding([autoPadding])`
    - `decipher.update(data[, inputEncoding][, outputEncoding])`
  - Class: `DiffieHellman`
    - `diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])`
    - `diffieHellman.generateKeys([encoding])`
    - `diffieHellman.getGenerator([encoding])`
    - `diffieHellman.getPrime([encoding])`
    - `diffieHellman.getPrivateKey([encoding])`
    - `diffieHellman.getPublicKey([encoding])`
    - `diffieHellman.setPrivateKey(privateKey[, encoding])`
    - `diffieHellman.setPublicKey(publicKey[, encoding])`
    - `diffieHellman.verifyError`
  - Class: `DiffieHellmanGroup`
  - Class: `ECDH`
    - Static method: `ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])`
    - `ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])`
    - `ecdh.generateKeys([encoding[, format]])`
    - `ecdh.getPrivateKey([encoding])`
    - `ecdh.getPublicKey([encoding][, format])`
    - `ecdh.setPrivateKey(privateKey[, encoding])`
    - `ecdh.setPublicKey(publicKey[, encoding])`
  - Class: `Hash`
    - `hash.copy([options])`
    - `hash.digest([encoding])`
    - `hash.update(data[, inputEncoding])`
  - Class: `Hmac`
    - `hmac.digest([encoding])`
    - `hmac.update(data[, inputEncoding])`
  - Class: `KeyObject`
    - Static method: `KeyObject.from(key)`
    - `keyObject.asymmetricKeyDetails`
    - `keyObject.asymmetricKeyType`
    - `keyObject.equals(otherKeyObject)`
    - `keyObject.export([options])`
    - `keyObject.symmetricKeySize`
    - `keyObject.toCryptoKey(algorithm, extractable, keyUsages)`
    - `keyObject.type`
  - Class: `Sign`
    - `sign.sign(privateKey[, outputEncoding])`
    - `sign.update(data[, inputEncoding])`
  - Class: `Verify`
    - `verify.update(data[, inputEncoding])`
    - `verify.verify(key, signature[, signatureEncoding])`
  - Class: `X509Certificate`
    - `new X509Certificate(buffer)`
    - `x509.ca`
    - `x509.checkEmail(email[, options])`
    - `x509.checkHost(name[, options])`
    - `x509.checkIP(ip)`
    - `x509.checkIssued(otherCert)`
    - `x509.checkPrivateKey(privateKey)`
    - `x509.fingerprint`
    - `x509.fingerprint256`
    - `x509.fingerprint512`
    - `x509.infoAccess`
    - `x509.issuer`
    - `x509.issuerCertificate`
    - `x509.keyUsage`
    - `x509.publicKey`
    - `x509.raw`
    - `x509.serialNumber`
    - `x509.subject`
    - `x509.subjectAltName`
    - `x509.toJSON()`
    - `x509.toLegacyObject()`
    - `x509.toString()`
    - `x509.validFrom`
    - `x509.validFromDate`
    - `x509.validTo`
    - `x509.validToDate`
    - `x509.signatureAlgorithm`
    - `x509.signatureAlgorithmOid`
    - `x509.verify(publicKey)`
  - `node:crypto` module methods and properties
    - `crypto.argon2(algorithm, parameters, callback)`
    - `crypto.argon2Sync(algorithm, parameters)`
    - `crypto.checkPrime(candidate[, options], callback)`
    - `crypto.checkPrimeSync(candidate[, options])`
    - `crypto.constants`
    - `crypto.createCipheriv(algorithm, key, iv[, options])`
    - `crypto.createDecipheriv(algorithm, key, iv[, options])`
    - `crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])`
    - `crypto.createDiffieHellman(primeLength[, generator])`
    - `crypto.createDiffieHellmanGroup(name)`
    - `crypto.createECDH(curveName)`
    - `crypto.createHash(algorithm[, options])`
    - `crypto.createHmac(algorithm, key[, options])`
    - `crypto.createPrivateKey(key)`
      - Private keys from OpenSSL STORE loaders
    - `crypto.createPublicKey(key)`
    - `crypto.createSecretKey(key[, encoding])`
    - `crypto.createSign(algorithm[, options])`
    - `crypto.createVerify(algorithm[, options])`
    - `crypto.decapsulate(key, ciphertext[, callback])`
    - `crypto.diffieHellman(options[, callback])`
    - `crypto.encapsulate(key[, callback])`
    - `crypto.fips`
    - `crypto.generateKey(type, options, callback)`
    - `crypto.generateKeyPair(type, options, callback)`
    - `crypto.generateKeyPairSync(type, options)`
    - `crypto.generateKeySync(type, options)`
    - `crypto.generatePrime(size[, options], callback)`
    - `crypto.generatePrimeSync(size[, options])`
    - `crypto.getCipherInfo(nameOrNid[, options])`
    - `crypto.getCiphers()`
    - `crypto.getCurves()`
    - `crypto.getDiffieHellman(groupName)`
    - `crypto.getFips()`
    - `crypto.getHashes()`
    - `crypto.getRandomValues(typedArray)`
    - `crypto.hash(algorithm, data[, options])`
    - `crypto.hkdf(digest, ikm, salt, info, keylen, callback)`
    - `crypto.hkdfSync(digest, ikm, salt, info, keylen)`
    - `crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)`
    - `crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)`
    - `crypto.privateDecrypt(privateKey, buffer)`
    - `crypto.privateEncrypt(privateKey, buffer)`
    - `crypto.publicDecrypt(key, buffer)`
    - `crypto.publicEncrypt(key, buffer)`
    - `crypto.randomBytes(size[, callback])`
    - `crypto.randomFill(buffer[, offset][, size], callback)`
    - `crypto.randomFillSync(buffer[, offset][, size])`
    - `crypto.randomInt([min, ]max[, callback])`
    - `crypto.randomUUID([options])`
    - `crypto.randomUUIDv7([options])`
    - `crypto.scrypt(password, salt, keylen[, options], callback)`
    - `crypto.scryptSync(password, salt, keylen[, options])`
    - `crypto.secureHeapUsed()`
    - `crypto.setEngine(engine[, flags])`
    - `crypto.setFips(bool)`
    - `crypto.sign(algorithm, data, key[, callback])`
    - `crypto.subtle`
    - `crypto.timingSafeEqual(a, b)`
    - `crypto.verify(algorithm, data, key, signature[, callback])`
    - `crypto.webcrypto`
  - Notes
    - Using strings as inputs to cryptographic APIs
    - Legacy streams API (prior to Node.js 0.10)
    - Support for weak or compromised algorithms
    - CCM mode
    - FIPS mode
  - Crypto constants
    - OpenSSL options
    - OpenSSL engine constants
    - Other OpenSSL constants
    - Node.js crypto constants

- Debugger
  - Interactive mode
    - Watchers
  - Command reference
    - Stepping
      - Breakpoints
      - Information
      - Execution control
      - Various
  - Probe mode
    - Probe output format
    - Output and exit codes
    - Probing multiple expressions at the same execution point
    - Selecting the probe location
    - Probe examples
      - Probing a variable conditionally
  - Advanced usage
    - V8 inspector integration for Node.js

- Deprecated APIs
  - Revoking deprecations
  - List of deprecated APIs
    - DEP0001: `http.OutgoingMessage.prototype.flush`
    - DEP0002: `require('_linklist')`
    - DEP0003: `_writableState.buffer`
    - DEP0004: `CryptoStream.prototype.readyState`
    - DEP0005: `Buffer()` constructor
    - DEP0006: `child_process` `options.customFds`
    - DEP0007: Replace `cluster` `worker.suicide` with `worker.exitedAfterDisconnect`
    - DEP0008: `require('node:constants')`
    - DEP0009: `crypto.pbkdf2` without digest
    - DEP0010: `crypto.createCredentials`
    - DEP0011: `crypto.Credentials`
    - DEP0012: `Domain.dispose`
    - DEP0013: `fs` asynchronous function without callback
    - DEP0014: `fs.read` legacy String interface
    - DEP0015: `fs.readSync` legacy String interface
    - DEP0016: `GLOBAL`/`root`
    - DEP0017: `Intl.v8BreakIterator`
    - DEP0018: Unhandled promise rejections
    - DEP0019: `require('.')` resolved outside directory
    - DEP0020: `Server.connections`
    - DEP0021: `Server.listenFD`
    - DEP0022: `os.tmpDir()`
    - DEP0023: `os.getNetworkInterfaces()`
    - DEP0024: `REPLServer.prototype.convertToContext()`
    - DEP0025: `require('node:sys')`
    - DEP0026: `util.print()`
    - DEP0027: `util.puts()`
    - DEP0028: `util.debug()`
    - DEP0029: `util.error()`
    - DEP0030: `SlowBuffer`
    - DEP0031: `ecdh.setPublicKey()`
    - DEP0032: `node:domain` module
    - DEP0033: `EventEmitter.listenerCount()`
    - DEP0034: `fs.exists(path, callback)`
    - DEP0035: `fs.lchmod(path, mode, callback)`
    - DEP0036: `fs.lchmodSync(path, mode)`
    - DEP0037: `fs.lchown(path, uid, gid, callback)`
    - DEP0038: `fs.lchownSync(path, uid, gid)`
    - DEP0039: `require.extensions`
    - DEP0040: `node:punycode` module
    - DEP0041: `NODE_REPL_HISTORY_FILE` environment variable
    - DEP0042: `tls.CryptoStream`
    - DEP0043: `tls.SecurePair`
    - DEP0044: `util.isArray()`
    - DEP0045: `util.isBoolean()`
    - DEP0046: `util.isBuffer()`
    - DEP0047: `util.isDate()`
    - DEP0048: `util.isError()`
    - DEP0049: `util.isFunction()`
    - DEP0050: `util.isNull()`
    - DEP0051: `util.isNullOrUndefined()`
    - DEP0052: `util.isNumber()`
    - DEP0053: `util.isObject()`
    - DEP0054: `util.isPrimitive()`
    - DEP0055: `util.isRegExp()`
    - DEP0056: `util.isString()`
    - DEP0057: `util.isSymbol()`
    - DEP0058: `util.isUndefined()`
    - DEP0059: `util.log()`
    - DEP0060: `util._extend()`
    - DEP0061: `fs.SyncWriteStream`
    - DEP0062: `node --debug`
    - DEP0063: `ServerResponse.prototype.writeHeader()`
    - DEP0064: `tls.createSecurePair()`
    - DEP0065: `repl.REPL_MODE_MAGIC` and `NODE_REPL_MODE=magic`
    - DEP0066: `OutgoingMessage.prototype._headers, OutgoingMessage.prototype._headerNames`
    - DEP0067: `OutgoingMessage.prototype._renderHeaders`
    - DEP0068: `node debug`
    - DEP0069: `vm.runInDebugContext(string)`
    - DEP0070: `async_hooks.currentId()`
    - DEP0071: `async_hooks.triggerId()`
    - DEP0072: `async_hooks.AsyncResource.triggerId()`
    - DEP0073: Several internal properties of `net.Server`
    - DEP0074: `REPLServer.bufferedCommand`
    - DEP0075: `REPLServer.parseREPLKeyword()`
    - DEP0076: `tls.parseCertString()`
    - DEP0077: `Module._debug()`
    - DEP0078: `REPLServer.turnOffEditorMode()`
    - DEP0079: Custom inspection function on objects via `.inspect()`
    - DEP0080: `path._makeLong()`
    - DEP0081: `fs.truncate()` using a file descriptor
    - DEP0082: `REPLServer.prototype.memory()`
    - DEP0083: Disabling ECDH by setting `ecdhCurve` to `false`
    - DEP0084: requiring bundled internal dependencies
    - DEP0085: AsyncHooks sensitive API
    - DEP0086: Remove `runInAsyncIdScope`
    - DEP0089: `require('node:assert')`
    - DEP0090: Invalid GCM authentication tag lengths
    - DEP0091: `crypto.DEFAULT_ENCODING`
    - DEP0092: Top-level `this` bound to `module.exports`
    - DEP0093: `crypto.fips` is deprecated and replaced
    - DEP0094: Using `assert.fail()` with more than one argument
    - DEP0095: `timers.enroll()`
    - DEP0096: `timers.unenroll()`
    - DEP0097: `MakeCallback` with `domain` property
    - DEP0098: AsyncHooks embedder `AsyncResource.emitBefore` and `AsyncResource.emitAfter` APIs
    - DEP0099: Async context-unaware `node::MakeCallback` C++ APIs
    - DEP0100: `process.assert()`
    - DEP0101: `--with-lttng`
    - DEP0102: Using `noAssert` in `Buffer#(read|write)` operations
    - DEP0103: `process.binding('util').is[...]` typechecks
    - DEP0104: `process.env` string coercion
    - DEP0105: `decipher.finaltol`
    - DEP0106: `crypto.createCipher` and `crypto.createDecipher`
    - DEP0107: `tls.convertNPNProtocols()`
    - DEP0108: `zlib.bytesRead`
    - DEP0109: `http`, `https`, and `tls` support for invalid URLs
    - DEP0110: `vm.Script` cached data
    - DEP0111: `process.binding()`
    - DEP0112: `dgram` private APIs
    - DEP0113: `Cipher.setAuthTag()`, `Decipher.getAuthTag()`
    - DEP0114: `crypto._toBuf()`
    - DEP0115: `crypto.prng()`, `crypto.pseudoRandomBytes()`, `crypto.rng()`
    - DEP0116: Legacy URL API
    - DEP0117: Native crypto handles
    - DEP0118: `dns.lookup()` support for a falsy host name
    - DEP0119: `process.binding('uv').errname()` private API
    - DEP0120: Windows Performance Counter support
    - DEP0121: `net._setSimultaneousAccepts()`
    - DEP0122: `tls` `Server.prototype.setOptions()`
    - DEP0123: setting the TLS ServerName to an IP address
    - DEP0124: using `REPLServer.rli`
    - DEP0125: `require('node:_stream_wrap')`
    - DEP0126: `timers.active()`
    - DEP0127: `timers._unrefActive()`
    - DEP0128: modules with an invalid `main` entry and an `index.js` file
    - DEP0129: `ChildProcess._channel`
    - DEP0130: `Module.createRequireFromPath()`
    - DEP0131: Legacy HTTP parser
    - DEP0132: `worker.terminate()` with callback
    - DEP0133: `http` `connection`
    - DEP0134: `process._tickCallback`
    - DEP0135: `WriteStream.open()` and `ReadStream.open()` are internal
    - DEP0136: `http` `finished`
    - DEP0137: Closing fs.FileHandle on garbage collection
    - DEP0138: `process.mainModule`
    - DEP0139: `process.umask()` with no arguments
    - DEP0140: Use `request.destroy()` instead of `request.abort()`
    - DEP0141: `repl.inputStream` and `repl.outputStream`
    - DEP0142: `repl._builtinLibs`
    - DEP0143: `Transform._transformState`
    - DEP0144: `module.parent`
    - DEP0145: `socket.bufferSize`
    - DEP0146: `new crypto.Certificate()`
    - DEP0147: `fs.rmdir(path, { recursive: true })`
    - DEP0148: Folder mappings in `"exports"` (trailing `"/"`)
    - DEP0149: `http.IncomingMessage#connection`
    - DEP0150: Changing the value of `process.config`
    - DEP0151: Main index lookup and extension searching
    - DEP0152: Extension PerformanceEntry properties
    - DEP0153: `dns.lookup` and `dnsPromises.lookup` options type coercion
    - DEP0154: RSA-PSS generate key pair options
    - DEP0155: Trailing slashes in pattern specifier resolutions
    - DEP0156: `.aborted` property and `'abort'`, `'aborted'` event in `http`
    - DEP0157: Thenable support in streams
    - DEP0158: `buffer.slice(start, end)`
    - DEP0159: `ERR_INVALID_CALLBACK`
    - DEP0160: `process.on('multipleResolves', handler)`
    - DEP0161: `process._getActiveRequests()` and `process._getActiveHandles()`
    - DEP0162: `fs.write()`, `fs.writeFileSync()` coercion to string
    - DEP0163: `channel.subscribe(onMessage)`, `channel.unsubscribe(onMessage)`
    - DEP0164: `process.exit(code)`, `process.exitCode` coercion to integer
    - DEP0165: `--trace-atomics-wait`
    - DEP0166: Double slashes in imports and exports targets
    - DEP0167: Weak `DiffieHellmanGroup` instances (`modp1`, `modp2`, `modp5`)
    - DEP0168: Unhandled exception in Node-API callbacks
    - DEP0169: Insecure url.parse()
    - DEP0170: Invalid port when using `url.parse()`
    - DEP0171: Setters for `http.IncomingMessage` headers and trailers
    - DEP0172: The `asyncResource` property of `AsyncResource` bound functions
    - DEP0173: the `assert.CallTracker` class
    - DEP0174: calling `promisify` on a function that returns a `Promise`
    - DEP0175: `util.toUSVString`
    - DEP0176: `fs.F_OK`, `fs.R_OK`, `fs.W_OK`, `fs.X_OK`
    - DEP0177: `util.types.isWebAssemblyCompiledModule`
    - DEP0178: `dirent.path`
    - DEP0179: `Hash` constructor
    - DEP0180: `fs.Stats` constructor
    - DEP0181: `Hmac` constructor
    - DEP0182: Short GCM authentication tags without explicit `authTagLength`
    - DEP0183: OpenSSL engine-based APIs
    - DEP0184: Instantiating `node:zlib` classes without `new`
    - DEP0185: Instantiating `node:repl` classes without `new`
    - DEP0187: Passing invalid argument types to `fs.existsSync`
    - DEP0188: `process.features.ipv6` and `process.features.uv`
    - DEP0189: `process.features.tls_*`
    - DEP0190: Passing `args` to `node:child_process` `execFile`/`spawn` with `shell` option
    - DEP0191: `repl.builtinModules`
    - DEP0192: `require('node:_tls_common')` and `require('node:_tls_wrap')`
    - DEP0193: `require('node:_stream_*')`
    - DEP0194: HTTP/2 priority signaling
    - DEP0195: Instantiating `node:http` classes without `new`
    - DEP0196: Calling `node:child_process` functions with `options.shell` as an empty string
    - DEP0197: `util.types.isNativeError()`
    - DEP0198: Creating SHAKE-128 and SHAKE-256 digests without an explicit `options.outputLength`
    - DEP0199: `require('node:_http_*')`
    - DEP0200: Closing fs.Dir on garbage collection
    - DEP0201: Passing `options.type` to `Duplex.toWeb()`
    - DEP0202: `Http1IncomingMessage` and `Http1ServerResponse` options of HTTP/2 servers
    - DEP0203: Passing `CryptoKey` to `node:crypto` APIs
    - DEP0204: `KeyObject.from()` with non-extractable `CryptoKey`
    - DEP0205: `module.register()`
    - DEP0206: Calling `digest()` on an already-finalized `Hmac` instance

- Diagnostic report
  - Usage
  - Report Version
    - Version history
      - Version 5
      - Version 4
      - Version 3
      - Version 2
      - Version 1
  - Configuration
  - Interaction with workers

- Diagnostics Channel
  - Public API
    - Overview
      - `diagnostics_channel.hasSubscribers(name)`
      - `diagnostics_channel.channel(name)`
      - `diagnostics_channel.subscribe(name, onMessage)`
      - `diagnostics_channel.unsubscribe(name, onMessage)`
      - `diagnostics_channel.tracingChannel(nameOrChannels)`
      - `diagnostics_channel.boundedChannel(nameOrChannels)`
    - Class: `Channel`
      - `channel.hasSubscribers`
      - `channel.publish(message)`
      - `channel.subscribe(onMessage)`
      - `channel.unsubscribe(onMessage)`
      - `channel.bindStore(store[, transform])`
      - `channel.unbindStore(store)`
      - `channel.runStores(context, fn[, thisArg[, ...args]])`
      - `channel.withStoreScope(data)`
    - Class: `RunStoresScope`
    - Class: `TracingChannel`
      - `tracingChannel.subscribe(subscribers)`
      - `tracingChannel.unsubscribe(subscribers)`
      - `tracingChannel.traceSync(fn[, context[, thisArg[, ...args]]])`
      - `tracingChannel.tracePromise(fn[, context[, thisArg[, ...args]]])`
      - `tracingChannel.traceCallback(fn[, position[, context[, thisArg[, ...args]]]])`
      - `tracingChannel.hasSubscribers`
    - Class: `BoundedChannel`
      - `boundedChannel.hasSubscribers`
      - `boundedChannel.subscribe(handlers)`
      - `boundedChannel.unsubscribe(handlers)`
      - `boundedChannel.run(context, fn[, thisArg[, ...args]])`
      - `boundedChannel.withScope([context])`
    - Class: `BoundedChannelScope`
    - BoundedChannel Channels
    - TracingChannel Channels
      - `start(event)`
      - `end(event)`
      - `asyncStart(event)`
      - `asyncEnd(event)`
      - `error(event)`
    - Built-in Channels
      - Console
        - Event: `'console.log'`
        - Event: `'console.info'`
        - Event: `'console.debug'`
        - Event: `'console.warn'`
        - Event: `'console.error'`
      - HTTP
        - Event: `'http.client.request.created'`
        - Event: `'http.client.request.start'`
        - Event: `'http.client.request.error'`
        - Event: `'http.client.response.finish'`
        - Event: `'http.server.request.start'`
        - Event: `'http.server.response.created'`
        - Event: `'http.server.response.finish'`
      - HTTP/2
        - Event: `'http2.client.stream.created'`
        - Event: `'http2.client.stream.start'`
        - Event: `'http2.client.stream.error'`
        - Event: `'http2.client.stream.finish'`
        - Event: `'http2.client.stream.bodyChunkSent'`
        - Event: `'http2.client.stream.bodySent'`
        - Event: `'http2.client.stream.close'`
        - Event: `'http2.server.stream.created'`
        - Event: `'http2.server.stream.start'`
        - Event: `'http2.server.stream.error'`
        - Event: `'http2.server.stream.finish'`
        - Event: `'http2.server.stream.close'`
      - Modules
        - Event: `'tracing:module.require:start'`
        - Event: `'tracing:module.require:end'`
        - Event: `'tracing:module.require:error'`
        - Event: `'tracing:module.import:asyncStart'`
        - Event: `'tracing:module.import:asyncEnd'`
        - Event: `'tracing:module.import:error'`
      - NET
        - Event: `'net.client.socket'`
        - Event: `'net.server.socket'`
        - Event: `'tracing:net.server.listen:asyncStart'`
        - Event: `'tracing:net.server.listen:asyncEnd'`
        - Event: `'tracing:net.server.listen:error'`
      - UDP
        - Event: `'udp.socket'`
      - Process
        - Event: `'child_process'`
        - Event: `'process.execve'`
      - Web Locks
        - Event: `'locks.request.start'`
        - Event: `'locks.request.grant'`
        - Event: `'locks.request.miss'`
        - Event: `'locks.request.end'`
      - Worker Thread
        - Event: `'worker_threads'`

- DNS
  - Class: `dns.Resolver`
    - `Resolver([options])`
    - `resolver.cancel()`
    - `resolver.setLocalAddress([ipv4][, ipv6])`
  - `dns.getServers()`
  - `dns.lookup(hostname[, options], callback)`
    - Supported getaddrinfo flags
  - `dns.lookupService(address, port, callback)`
  - `dns.resolve(hostname[, rrtype], callback)`
  - `dns.resolve4(hostname[, options], callback)`
  - `dns.resolve6(hostname[, options], callback)`
  - `dns.resolveAny(hostname, callback)`
  - `dns.resolveCname(hostname, callback)`
  - `dns.resolveCaa(hostname, callback)`
  - `dns.resolveMx(hostname, callback)`
  - `dns.resolveNaptr(hostname, callback)`
  - `dns.resolveNs(hostname, callback)`
  - `dns.resolvePtr(hostname, callback)`
  - `dns.resolveSoa(hostname, callback)`
  - `dns.resolveSrv(hostname, callback)`
  - `dns.resolveTlsa(hostname, callback)`
  - `dns.resolveTxt(hostname, callback)`
  - `dns.reverse(ip, callback)`
  - `dns.setDefaultResultOrder(order)`
  - `dns.getDefaultResultOrder()`
  - `dns.setServers(servers)`
  - DNS promises API
    - Class: `dnsPromises.Resolver`
    - `resolver.cancel()`
    - `dnsPromises.getServers()`
    - `dnsPromises.lookup(hostname[, options])`
    - `dnsPromises.lookupService(address, port)`
    - `dnsPromises.resolve(hostname[, rrtype])`
    - `dnsPromises.resolve4(hostname[, options])`
    - `dnsPromises.resolve6(hostname[, options])`
    - `dnsPromises.resolveAny(hostname)`
    - `dnsPromises.resolveCaa(hostname)`
    - `dnsPromises.resolveCname(hostname)`
    - `dnsPromises.resolveMx(hostname)`
    - `dnsPromises.resolveNaptr(hostname)`
    - `dnsPromises.resolveNs(hostname)`
    - `dnsPromises.resolvePtr(hostname)`
    - `dnsPromises.resolveSoa(hostname)`
    - `dnsPromises.resolveSrv(hostname)`
    - `dnsPromises.resolveTlsa(hostname)`
    - `dnsPromises.resolveTxt(hostname)`
    - `dnsPromises.reverse(ip)`
    - `dnsPromises.setDefaultResultOrder(order)`
    - `dnsPromises.getDefaultResultOrder()`
    - `dnsPromises.setServers(servers)`
  - Error codes
  - Implementation considerations
    - `dns.lookup()`
    - `dns.resolve()`, `dns.resolve*()`, and `dns.reverse()`

- Domain
  - Warning: Don't ignore errors!
  - Additions to `Error` objects
  - Implicit binding
  - Explicit binding
  - `domain.create()`
  - Class: `Domain`
    - `domain.members`
    - `domain.add(emitter)`
    - `domain.bind(callback)`
    - `domain.enter()`
    - `domain.exit()`
    - `domain.intercept(callback)`
    - `domain.remove(emitter)`
    - `domain.run(fn[, ...args])`
  - Domains and promises

- Environment Variables
  - CLI Environment Variables
  - `process.env`
  - DotEnv
    - .env files
      - Variable Names
      - Variable Values
      - Spacing
      - Comments
      - `export` prefixes
    - CLI Options
    - Programmatic APIs

- Errors
  - Error propagation and interception
  - Class: `Error`
    - `new Error(message[, options])`
    - `Error.captureStackTrace(targetObject[, constructorOpt])`
    - `Error.stackTraceLimit`
    - `error.cause`
    - `error.code`
    - `error.message`
    - `error.stack`
  - Class: `AssertionError`
  - Class: `RangeError`
  - Class: `ReferenceError`
  - Class: `SyntaxError`
  - Class: `SystemError`
    - `error.address`
    - `error.code`
    - `error.dest`
    - `error.errno`
    - `error.info`
    - `error.message`
    - `error.path`
    - `error.port`
    - `error.syscall`
    - Common system errors
  - Class: `TypeError`
  - Exceptions vs. errors
  - OpenSSL errors
    - `error.opensslErrorStack`
    - `error.function`
    - `error.library`
    - `error.reason`
  - Node.js error codes
    - `ABORT_ERR`
    - `ERR_ACCESS_DENIED`
    - `ERR_AMBIGUOUS_ARGUMENT`
    - `ERR_ARG_NOT_ITERABLE`
    - `ERR_ASSERTION`
    - `ERR_ASYNC_CALLBACK`
    - `ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED`
    - `ERR_ASYNC_TYPE`
    - `ERR_BROTLI_COMPRESSION_FAILED`
    - `ERR_BROTLI_INVALID_PARAM`
    - `ERR_BUFFER_CONTEXT_NOT_AVAILABLE`
    - `ERR_BUFFER_OUT_OF_BOUNDS`
    - `ERR_BUFFER_TOO_LARGE`
    - `ERR_CANNOT_WATCH_SIGINT`
    - `ERR_CHILD_CLOSED_BEFORE_REPLY`
    - `ERR_CHILD_PROCESS_IPC_REQUIRED`
    - `ERR_CHILD_PROCESS_STDIO_MAXBUFFER`
    - `ERR_CLOSED_MESSAGE_PORT`
    - `ERR_CONSOLE_WRITABLE_STREAM`
    - `ERR_CONSTRUCT_CALL_INVALID`
    - `ERR_CONSTRUCT_CALL_REQUIRED`
    - `ERR_CONTEXT_NOT_INITIALIZED`
    - `ERR_CPU_PROFILE_ALREADY_STARTED`
    - `ERR_CPU_PROFILE_NOT_STARTED`
    - `ERR_CPU_PROFILE_TOO_MANY`
    - `ERR_CRYPTO_ARGON2_NOT_SUPPORTED`
    - `ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED`
    - `ERR_CRYPTO_ECDH_INVALID_FORMAT`
    - `ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY`
    - `ERR_CRYPTO_ENGINE_UNKNOWN`
    - `ERR_CRYPTO_FIPS_FORCED`
    - `ERR_CRYPTO_FIPS_UNAVAILABLE`
    - `ERR_CRYPTO_HASH_FINALIZED`
    - `ERR_CRYPTO_HASH_UPDATE_FAILED`
    - `ERR_CRYPTO_INCOMPATIBLE_KEY`
    - `ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS`
    - `ERR_CRYPTO_INITIALIZATION_FAILED`
    - `ERR_CRYPTO_INVALID_AUTH_TAG`
    - `ERR_CRYPTO_INVALID_COUNTER`
    - `ERR_CRYPTO_INVALID_CURVE`
    - `ERR_CRYPTO_INVALID_DIGEST`
    - `ERR_CRYPTO_INVALID_IV`
    - `ERR_CRYPTO_INVALID_JWK`
    - `ERR_CRYPTO_INVALID_KEYLEN`
    - `ERR_CRYPTO_INVALID_KEYPAIR`
    - `ERR_CRYPTO_INVALID_KEYTYPE`
    - `ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE`
    - `ERR_CRYPTO_INVALID_MESSAGELEN`
    - `ERR_CRYPTO_INVALID_SCRYPT_PARAMS`
    - `ERR_CRYPTO_INVALID_STATE`
    - `ERR_CRYPTO_INVALID_TAG_LENGTH`
    - `ERR_CRYPTO_JOB_INIT_FAILED`
    - `ERR_CRYPTO_JWK_UNSUPPORTED_CURVE`
    - `ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE`
    - `ERR_CRYPTO_KEM_NOT_SUPPORTED`
    - `ERR_CRYPTO_OPERATION_FAILED`
    - `ERR_CRYPTO_PBKDF2_ERROR`
    - `ERR_CRYPTO_SCRYPT_NOT_SUPPORTED`
    - `ERR_CRYPTO_SIGN_KEY_REQUIRED`
    - `ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH`
    - `ERR_CRYPTO_UNKNOWN_CIPHER`
    - `ERR_CRYPTO_UNKNOWN_DH_GROUP`
    - `ERR_CRYPTO_UNSUPPORTED_OPERATION`
    - `ERR_DEBUGGER_ERROR`
    - `ERR_DEBUGGER_STARTUP_ERROR`
    - `ERR_DIR_CLOSED`
    - `ERR_DIR_CONCURRENT_OPERATION`
    - `ERR_DLOPEN_DISABLED`
    - `ERR_DLOPEN_FAILED`
    - `ERR_DNS_SET_SERVERS_FAILED`
    - `ERR_DOMAIN_CALLBACK_NOT_AVAILABLE`
    - `ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE`
    - `ERR_DUPLICATE_STARTUP_SNAPSHOT_MAIN_FUNCTION`
    - `ERR_ENCODING_INVALID_ENCODED_DATA`
    - `ERR_ENCODING_NOT_SUPPORTED`
    - `ERR_EVAL_ESM_CANNOT_PRINT`
    - `ERR_EVENT_RECURSION`
    - `ERR_EXECUTION_ENVIRONMENT_NOT_AVAILABLE`
    - `ERR_FALSY_VALUE_REJECTION`
    - `ERR_FEATURE_UNAVAILABLE_ON_PLATFORM`
    - `ERR_FFI_CALL_FAILED`
    - `ERR_FFI_INVALID_POINTER`
    - `ERR_FFI_LIBRARY_CLOSED`
    - `ERR_FS_CP_DIR_TO_NON_DIR`
    - `ERR_FS_CP_EEXIST`
    - `ERR_FS_CP_EINVAL`
    - `ERR_FS_CP_FIFO_PIPE`
    - `ERR_FS_CP_NON_DIR_TO_DIR`
    - `ERR_FS_CP_SOCKET`
    - `ERR_FS_CP_SYMLINK_TO_SUBDIRECTORY`
    - `ERR_FS_CP_UNKNOWN`
    - `ERR_FS_EISDIR`
    - `ERR_FS_FILE_TOO_LARGE`
    - `ERR_FS_WATCH_QUEUE_OVERFLOW`
    - `ERR_HTTP2_ALTSVC_INVALID_ORIGIN`
    - `ERR_HTTP2_ALTSVC_LENGTH`
    - `ERR_HTTP2_CONNECT_AUTHORITY`
    - `ERR_HTTP2_CONNECT_PATH`
    - `ERR_HTTP2_CONNECT_SCHEME`
    - `ERR_HTTP2_ERROR`
    - `ERR_HTTP2_GOAWAY_SESSION`
    - `ERR_HTTP2_HEADERS_AFTER_RESPOND`
    - `ERR_HTTP2_HEADERS_SENT`
    - `ERR_HTTP2_HEADER_SINGLE_VALUE`
    - `ERR_HTTP2_INFO_STATUS_NOT_ALLOWED`
    - `ERR_HTTP2_INVALID_CONNECTION_HEADERS`
    - `ERR_HTTP2_INVALID_HEADER_VALUE`
    - `ERR_HTTP2_INVALID_INFO_STATUS`
    - `ERR_HTTP2_INVALID_ORIGIN`
    - `ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH`
    - `ERR_HTTP2_INVALID_PSEUDOHEADER`
    - `ERR_HTTP2_INVALID_SESSION`
    - `ERR_HTTP2_INVALID_SETTING_VALUE`
    - `ERR_HTTP2_INVALID_STREAM`
    - `ERR_HTTP2_MAX_PENDING_SETTINGS_ACK`
    - `ERR_HTTP2_NESTED_PUSH`
    - `ERR_HTTP2_NO_MEM`
    - `ERR_HTTP2_NO_SOCKET_MANIPULATION`
    - `ERR_HTTP2_ORIGIN_LENGTH`
    - `ERR_HTTP2_OUT_OF_STREAMS`
    - `ERR_HTTP2_PAYLOAD_FORBIDDEN`
    - `ERR_HTTP2_PING_CANCEL`
    - `ERR_HTTP2_PING_LENGTH`
    - `ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED`
    - `ERR_HTTP2_PUSH_DISABLED`
    - `ERR_HTTP2_SEND_FILE`
    - `ERR_HTTP2_SEND_FILE_NOSEEK`
    - `ERR_HTTP2_SESSION_ERROR`
    - `ERR_HTTP2_SETTINGS_CANCEL`
    - `ERR_HTTP2_SOCKET_BOUND`
    - `ERR_HTTP2_SOCKET_UNBOUND`
    - `ERR_HTTP2_STATUS_101`
    - `ERR_HTTP2_STATUS_INVALID`
    - `ERR_HTTP2_STREAM_CANCEL`
    - `ERR_HTTP2_STREAM_ERROR`
    - `ERR_HTTP2_STREAM_SELF_DEPENDENCY`
    - `ERR_HTTP2_TOO_MANY_CUSTOM_SETTINGS`
    - `ERR_HTTP2_TOO_MANY_INVALID_FRAMES`
    - `ERR_HTTP2_TOO_MANY_ORIGINS`
    - `ERR_HTTP2_TRAILERS_ALREADY_SENT`
    - `ERR_HTTP2_TRAILERS_NOT_READY`
    - `ERR_HTTP2_UNSUPPORTED_PROTOCOL`
    - `ERR_HTTP_BODY_NOT_ALLOWED`
    - `ERR_HTTP_CONTENT_LENGTH_MISMATCH`
    - `ERR_HTTP_HEADERS_SENT`
    - `ERR_HTTP_INVALID_HEADER_VALUE`
    - `ERR_HTTP_INVALID_STATUS_CODE`
    - `ERR_HTTP_REQUEST_TIMEOUT`
    - `ERR_HTTP_SOCKET_ASSIGNED`
    - `ERR_HTTP_SOCKET_ENCODING`
    - `ERR_HTTP_TRAILER_INVALID`
    - `ERR_ILLEGAL_CONSTRUCTOR`
    - `ERR_IMPORT_ATTRIBUTE_MISSING`
    - `ERR_IMPORT_ATTRIBUTE_TYPE_INCOMPATIBLE`
    - `ERR_IMPORT_ATTRIBUTE_UNSUPPORTED`
    - `ERR_INCOMPATIBLE_OPTION_PAIR`
    - `ERR_INPUT_TYPE_NOT_ALLOWED`
    - `ERR_INSPECTOR_ALREADY_ACTIVATED`
    - `ERR_INSPECTOR_ALREADY_CONNECTED`
    - `ERR_INSPECTOR_CLOSED`
    - `ERR_INSPECTOR_COMMAND`
    - `ERR_INSPECTOR_NOT_ACTIVE`
    - `ERR_INSPECTOR_NOT_AVAILABLE`
    - `ERR_INSPECTOR_NOT_CONNECTED`
    - `ERR_INSPECTOR_NOT_WORKER`
    - `ERR_INTERNAL_ASSERTION`
    - `ERR_INVALID_ADDRESS`
    - `ERR_INVALID_ADDRESS_FAMILY`
    - `ERR_INVALID_ARG_TYPE`
    - `ERR_INVALID_ARG_VALUE`
    - `ERR_INVALID_ASYNC_ID`
    - `ERR_INVALID_BUFFER_SIZE`
    - `ERR_INVALID_CHAR`
    - `ERR_INVALID_CURSOR_POS`
    - `ERR_INVALID_FD`
    - `ERR_INVALID_FD_TYPE`
    - `ERR_INVALID_FILE_URL_HOST`
    - `ERR_INVALID_FILE_URL_PATH`
    - `ERR_INVALID_HANDLE_TYPE`
    - `ERR_INVALID_HTTP_TOKEN`
    - `ERR_INVALID_IP_ADDRESS`
    - `ERR_INVALID_MIME_SYNTAX`
    - `ERR_INVALID_MODULE`
    - `ERR_INVALID_MODULE_SPECIFIER`
    - `ERR_INVALID_OBJECT_DEFINE_PROPERTY`
    - `ERR_INVALID_PACKAGE_CONFIG`
    - `ERR_INVALID_PACKAGE_TARGET`
    - `ERR_INVALID_PROTOCOL`
    - `ERR_INVALID_REPL_EVAL_CONFIG`
    - `ERR_INVALID_REPL_INPUT`
    - `ERR_INVALID_RETURN_PROPERTY`
    - `ERR_INVALID_RETURN_PROPERTY_VALUE`
    - `ERR_INVALID_RETURN_VALUE`
    - `ERR_INVALID_STATE`
    - `ERR_INVALID_SYNC_FORK_INPUT`
    - `ERR_INVALID_THIS`
    - `ERR_INVALID_TUPLE`
    - `ERR_INVALID_TYPESCRIPT_SYNTAX`
    - `ERR_INVALID_URI`
    - `ERR_INVALID_URL`
    - `ERR_INVALID_URL_PATTERN`
    - `ERR_INVALID_URL_SCHEME`
    - `ERR_IPC_CHANNEL_CLOSED`
    - `ERR_IPC_DISCONNECTED`
    - `ERR_IPC_ONE_PIPE`
    - `ERR_IPC_SYNC_FORK`
    - `ERR_IP_BLOCKED`
    - `ERR_LOADER_CHAIN_INCOMPLETE`
    - `ERR_LOAD_SQLITE_EXTENSION`
    - `ERR_MEMORY_ALLOCATION_FAILED`
    - `ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE`
    - `ERR_METHOD_NOT_IMPLEMENTED`
    - `ERR_MISSING_ARGS`
    - `ERR_MISSING_OPTION`
    - `ERR_MISSING_PASSPHRASE`
    - `ERR_MISSING_PLATFORM_FOR_WORKER`
    - `ERR_MODULE_LINK_MISMATCH`
    - `ERR_MODULE_NOT_FOUND`
    - `ERR_MULTIPLE_CALLBACK`
    - `ERR_NAPI_CONS_FUNCTION`
    - `ERR_NAPI_INVALID_DATAVIEW_ARGS`
    - `ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT`
    - `ERR_NAPI_INVALID_TYPEDARRAY_LENGTH`
    - `ERR_NAPI_TSFN_CALL_JS`
    - `ERR_NAPI_TSFN_GET_UNDEFINED`
    - `ERR_NON_CONTEXT_AWARE_DISABLED`
    - `ERR_NOT_BUILDING_SNAPSHOT`
    - `ERR_NOT_IN_SINGLE_EXECUTABLE_APPLICATION`
    - `ERR_NOT_SUPPORTED_IN_SNAPSHOT`
    - `ERR_NO_CRYPTO`
    - `ERR_NO_ICU`
    - `ERR_NO_TEMPORAL`
    - `ERR_NO_TYPESCRIPT`
    - `ERR_OPERATION_FAILED`
    - `ERR_OPTIONS_BEFORE_BOOTSTRAPPING`
    - `ERR_OUT_OF_RANGE`
    - `ERR_PACKAGE_IMPORT_NOT_DEFINED`
    - `ERR_PACKAGE_MAP_EXTERNAL_FILE`
    - `ERR_PACKAGE_MAP_INVALID`
    - `ERR_PACKAGE_MAP_KEY_NOT_FOUND`
    - `ERR_PACKAGE_PATH_NOT_EXPORTED`
    - `ERR_PARSE_ARGS_INVALID_OPTION_VALUE`
    - `ERR_PARSE_ARGS_UNEXPECTED_POSITIONAL`
    - `ERR_PARSE_ARGS_UNKNOWN_OPTION`
    - `ERR_PERFORMANCE_INVALID_TIMESTAMP`
    - `ERR_PERFORMANCE_MEASURE_INVALID_OPTIONS`
    - `ERR_PROTO_ACCESS`
    - `ERR_PROXY_INVALID_CONFIG`
    - `ERR_PROXY_TUNNEL`
    - `ERR_QUIC_APPLICATION_ERROR`
    - `ERR_QUIC_CONNECTION_FAILED`
    - `ERR_QUIC_ENDPOINT_CLOSED`
    - `ERR_QUIC_OPEN_STREAM_FAILED`
    - `ERR_QUIC_STREAM_ABORTED`
    - `ERR_QUIC_STREAM_RESET`
    - `ERR_QUIC_TRANSPORT_ERROR`
    - `ERR_QUIC_VERSION_NEGOTIATION_ERROR`
    - `ERR_REQUIRE_ASYNC_MODULE`
    - `ERR_REQUIRE_CYCLE_MODULE`
    - `ERR_REQUIRE_ESM`
    - `ERR_REQUIRE_ESM_RACE_CONDITION`
    - `ERR_SCRIPT_EXECUTION_INTERRUPTED`
    - `ERR_SCRIPT_EXECUTION_TIMEOUT`
    - `ERR_SERVER_ALREADY_LISTEN`
    - `ERR_SERVER_NOT_RUNNING`
    - `ERR_SINGLE_EXECUTABLE_APPLICATION_ASSET_NOT_FOUND`
    - `ERR_SOCKET_ALREADY_BOUND`
    - `ERR_SOCKET_BAD_BUFFER_SIZE`
    - `ERR_SOCKET_BAD_PORT`
    - `ERR_SOCKET_BAD_TYPE`
    - `ERR_SOCKET_BUFFER_SIZE`
    - `ERR_SOCKET_CLOSED`
    - `ERR_SOCKET_CLOSED_BEFORE_CONNECTION`
    - `ERR_SOCKET_CONNECTION_TIMEOUT`
    - `ERR_SOCKET_DGRAM_IS_CONNECTED`
    - `ERR_SOCKET_DGRAM_NOT_CONNECTED`
    - `ERR_SOCKET_DGRAM_NOT_RUNNING`
    - `ERR_SOCKET_HANDLE_ADOPTED`
    - `ERR_SOURCE_MAP_CORRUPT`
    - `ERR_SOURCE_MAP_MISSING_SOURCE`
    - `ERR_SOURCE_PHASE_NOT_DEFINED`
    - `ERR_SQLITE_ERROR`
    - `ERR_SRI_PARSE`
    - `ERR_STREAM_ALREADY_FINISHED`
    - `ERR_STREAM_CANNOT_PIPE`
    - `ERR_STREAM_DESTROYED`
    - `ERR_STREAM_ITER_MISSING_FLAG`
    - `ERR_STREAM_NULL_VALUES`
    - `ERR_STREAM_PREMATURE_CLOSE`
    - `ERR_STREAM_PUSH_AFTER_EOF`
    - `ERR_STREAM_UNABLE_TO_PIPE`
    - `ERR_STREAM_UNSHIFT_AFTER_END_EVENT`
    - `ERR_STREAM_WRAP`
    - `ERR_STREAM_WRITE_AFTER_END`
    - `ERR_STRING_TOO_LONG`
    - `ERR_SYNTHETIC`
    - `ERR_SYSTEM_ERROR`
    - `ERR_TEST_FAILURE`
    - `ERR_TLS_ALPN_CALLBACK_INVALID_RESULT`
    - `ERR_TLS_ALPN_CALLBACK_WITH_PROTOCOLS`
    - `ERR_TLS_CERT_ALTNAME_FORMAT`
    - `ERR_TLS_CERT_ALTNAME_INVALID`
    - `ERR_TLS_DH_PARAM_SIZE`
    - `ERR_TLS_HANDSHAKE_TIMEOUT`
    - `ERR_TLS_INVALID_CONTEXT`
    - `ERR_TLS_INVALID_PROTOCOL_METHOD`
    - `ERR_TLS_INVALID_PROTOCOL_VERSION`
    - `ERR_TLS_INVALID_STATE`
    - `ERR_TLS_PROTOCOL_VERSION_CONFLICT`
    - `ERR_TLS_PSK_SET_IDENTITY_HINT_FAILED`
    - `ERR_TLS_RENEGOTIATION_DISABLED`
    - `ERR_TLS_RENEGOTIATION_UNSUPPORTED`
    - `ERR_TLS_REQUIRED_SERVER_NAME`
    - `ERR_TLS_SESSION_ATTACK`
    - `ERR_TLS_SNI_FROM_SERVER`
    - `ERR_TRACE_EVENTS_CATEGORY_REQUIRED`
    - `ERR_TRACE_EVENTS_UNAVAILABLE`
    - `ERR_TRAILING_JUNK_AFTER_STREAM_END`
    - `ERR_TRANSFORM_ALREADY_TRANSFORMING`
    - `ERR_TRANSFORM_WITH_LENGTH_0`
    - `ERR_TTY_INIT_FAILED`
    - `ERR_UNAVAILABLE_DURING_EXIT`
    - `ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET`
    - `ERR_UNESCAPED_CHARACTERS`
    - `ERR_UNHANDLED_ERROR`
    - `ERR_UNKNOWN_BUILTIN_MODULE`
    - `ERR_UNKNOWN_CREDENTIAL`
    - `ERR_UNKNOWN_ENCODING`
    - `ERR_UNKNOWN_FILE_EXTENSION`
    - `ERR_UNKNOWN_MODULE_FORMAT`
    - `ERR_UNKNOWN_SIGNAL`
    - `ERR_UNSUPPORTED_DIR_IMPORT`
    - `ERR_UNSUPPORTED_ESM_URL_SCHEME`
    - `ERR_UNSUPPORTED_NODE_MODULES_TYPE_STRIPPING`
    - `ERR_UNSUPPORTED_RESOLVE_REQUEST`
    - `ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX`
    - `ERR_USE_AFTER_CLOSE`
    - `ERR_VALID_PERFORMANCE_ENTRY_TYPE`
    - `ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING`
    - `ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING_FLAG`
    - `ERR_VM_MODULE_ALREADY_LINKED`
    - `ERR_VM_MODULE_CACHED_DATA_REJECTED`
    - `ERR_VM_MODULE_CANNOT_CREATE_CACHED_DATA`
    - `ERR_VM_MODULE_DIFFERENT_CONTEXT`
    - `ERR_VM_MODULE_LINK_FAILURE`
    - `ERR_VM_MODULE_NOT_MODULE`
    - `ERR_VM_MODULE_STATUS`
    - `ERR_WASI_ALREADY_STARTED`
    - `ERR_WASI_NOT_STARTED`
    - `ERR_WEBASSEMBLY_NOT_SUPPORTED`
    - `ERR_WEBASSEMBLY_RESPONSE`
    - `ERR_WORKER_HANDLE_NOT_TRANSFERABLE`
    - `ERR_WORKER_INIT_FAILED`
    - `ERR_WORKER_INVALID_EXEC_ARGV`
    - `ERR_WORKER_MESSAGING_ERRORED`
    - `ERR_WORKER_MESSAGING_FAILED`
    - `ERR_WORKER_MESSAGING_SAME_THREAD`
    - `ERR_WORKER_MESSAGING_TIMEOUT`
    - `ERR_WORKER_NOT_RUNNING`
    - `ERR_WORKER_OUT_OF_MEMORY`
    - `ERR_WORKER_PATH`
    - `ERR_WORKER_UNSERIALIZABLE_ERROR`
    - `ERR_WORKER_UNSUPPORTED_OPERATION`
    - `ERR_ZLIB_INITIALIZATION_FAILED`
    - `ERR_ZSTD_INVALID_PARAM`
    - `HPE_CHUNK_EXTENSIONS_OVERFLOW`
    - `HPE_HEADER_OVERFLOW`
    - `HPE_UNEXPECTED_CONTENT_LENGTH`
    - `MODULE_NOT_FOUND`
  - Legacy Node.js error codes
    - `ERR_CANNOT_TRANSFER_OBJECT`
    - `ERR_CPU_USAGE`
    - `ERR_CRYPTO_HASH_DIGEST_NO_UTF16`
    - `ERR_CRYPTO_SCRYPT_INVALID_PARAMETER`
    - `ERR_FS_INVALID_SYMLINK_TYPE`
    - `ERR_HTTP2_FRAME_ERROR`
    - `ERR_HTTP2_HEADERS_OBJECT`
    - `ERR_HTTP2_HEADER_REQUIRED`
    - `ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND`
    - `ERR_HTTP2_STREAM_CLOSED`
    - `ERR_HTTP_INVALID_CHAR`
    - `ERR_IMPORT_ASSERTION_TYPE_FAILED`
    - `ERR_IMPORT_ASSERTION_TYPE_MISSING`
    - `ERR_IMPORT_ASSERTION_TYPE_UNSUPPORTED`
    - `ERR_INDEX_OUT_OF_RANGE`
    - `ERR_INVALID_OPT_VALUE`
    - `ERR_INVALID_OPT_VALUE_ENCODING`
    - `ERR_INVALID_PERFORMANCE_MARK`
    - `ERR_INVALID_TRANSFER_OBJECT`
    - `ERR_MANIFEST_ASSERT_INTEGRITY`
    - `ERR_MANIFEST_DEPENDENCY_MISSING`
    - `ERR_MANIFEST_INTEGRITY_MISMATCH`
    - `ERR_MANIFEST_INVALID_RESOURCE_FIELD`
    - `ERR_MANIFEST_INVALID_SPECIFIER`
    - `ERR_MANIFEST_PARSE_POLICY`
    - `ERR_MANIFEST_TDZ`
    - `ERR_MANIFEST_UNKNOWN_ONERROR`
    - `ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST`
    - `ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST`
    - `ERR_NAPI_CONS_PROTOTYPE_OBJECT`
    - `ERR_NAPI_TSFN_START_IDLE_LOOP`
    - `ERR_NAPI_TSFN_STOP_IDLE_LOOP`
    - `ERR_NO_LONGER_SUPPORTED`
    - `ERR_OUTOFMEMORY`
    - `ERR_PARSE_HISTORY_DATA`
    - `ERR_SOCKET_CANNOT_SEND`
    - `ERR_STDERR_CLOSE`
    - `ERR_STDOUT_CLOSE`
    - `ERR_STREAM_READ_NOT_IMPLEMENTED`
    - `ERR_TAP_LEXER_ERROR`
    - `ERR_TAP_PARSER_ERROR`
    - `ERR_TAP_VALIDATION_ERROR`
    - `ERR_TLS_RENEGOTIATION_FAILED`
    - `ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER`
    - `ERR_UNKNOWN_STDIN_TYPE`
    - `ERR_UNKNOWN_STREAM_TYPE`
    - `ERR_V8BREAKITERATOR`
    - `ERR_VALUE_OUT_OF_RANGE`
    - `ERR_VM_MODULE_LINKING_ERRORED`
    - `ERR_VM_MODULE_NOT_LINKED`
    - `ERR_WORKER_UNSUPPORTED_EXTENSION`
    - `ERR_ZLIB_BINDING_CLOSED`
  - OpenSSL Error Codes
    - Time Validity Errors
      - `CERT_NOT_YET_VALID`
      - `CERT_HAS_EXPIRED`
      - `CRL_NOT_YET_VALID`
      - `CRL_HAS_EXPIRED`
      - `CERT_REVOKED`
    - Trust or Chain Related Errors
      - `UNABLE_TO_GET_ISSUER_CERT`
      - `UNABLE_TO_GET_ISSUER_CERT_LOCALLY`
      - `DEPTH_ZERO_SELF_SIGNED_CERT`
      - `SELF_SIGNED_CERT_IN_CHAIN`
      - `CERT_CHAIN_TOO_LONG`
      - `UNABLE_TO_GET_CRL`
      - `UNABLE_TO_VERIFY_LEAF_SIGNATURE`
      - `CERT_UNTRUSTED`
    - Basic Extension Errors
      - `INVALID_CA`
      - `PATH_LENGTH_EXCEEDED`
    - Name Related Errors
      - `HOSTNAME_MISMATCH`
    - Usage and Policy Errors
      - `INVALID_PURPOSE`
      - `CERT_REJECTED`
    - Formatting Errors
      - `CERT_SIGNATURE_FAILURE`
      - `CRL_SIGNATURE_FAILURE`
      - `ERROR_IN_CERT_NOT_BEFORE_FIELD`
      - `ERROR_IN_CERT_NOT_AFTER_FIELD`
      - `ERROR_IN_CRL_LAST_UPDATE_FIELD`
      - `ERROR_IN_CRL_NEXT_UPDATE_FIELD`
      - `UNABLE_TO_DECRYPT_CERT_SIGNATURE`
      - `UNABLE_TO_DECRYPT_CRL_SIGNATURE`
      - `UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY`
    - Other OpenSSL Errors
      - `OUT_OF_MEM`

## Events#

Source Code: lib/events.js

Stability: 2 - Stable

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") emit named events that cause `Function` objects ("listeners") to be called.

For instance: a `net.Server` object emits an event each time a peer connects to it; a `fs.ReadStream` emits an event when the file is opened; a stream emits an event whenever data is available to be read.

All objects that emit events are instances of the `EventEmitter` class. These objects expose an `eventEmitter.on()` function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the `EventEmitter` object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and discarded.

The following example shows a simple `EventEmitter` instance with a single listener. The `eventEmitter.on()` method is used to register listeners, while the `eventEmitter.emit()` method is used to trigger the event.

    import { EventEmitter } from 'node:events';

    class MyEmitter extends EventEmitter {}

    const myEmitter = new MyEmitter();
    myEmitter.on('event', () => {
      console.log('an event occurred!');
    });
    myEmitter.emit('event');
    const EventEmitter = require('node:events');

    class MyEmitter extends EventEmitter {}

    const myEmitter = new MyEmitter();
    myEmitter.on('event', () => {
      console.log('an event occurred!');
    });
    myEmitter.emit('event');
    javascript

### Passing arguments and `this` to listeners#

The `eventEmitter.emit()` method allows an arbitrary set of arguments to be passed to the listener functions. Keep in mind that when an ordinary listener function is called, the standard `this` keyword is intentionally set to reference the `EventEmitter` instance to which the listener is attached.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('event', function(a, b) {
      console.log(a, b, this, this === myEmitter);
      // Prints:
      //   a b MyEmitter {
      //     _events: [Object: null prototype] { event: [Function (anonymous)] },
      //     _eventsCount: 1,
      //     _maxListeners: undefined,
      //     Symbol(shapeMode): false,
      //     Symbol(kCapture): false
      //   } true
    });
    myEmitter.emit('event', 'a', 'b');
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('event', function(a, b) {
      console.log(a, b, this, this === myEmitter);
      // Prints:
      //   a b MyEmitter {
      //     _events: [Object: null prototype] { event: [Function (anonymous)] },
      //     _eventsCount: 1,
      //     _maxListeners: undefined,
      //     Symbol(shapeMode): false,
      //     Symbol(kCapture): false
      //   } true
    });
    myEmitter.emit('event', 'a', 'b');
    javascript

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the `this` keyword will no longer reference the `EventEmitter` instance:

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('event', (a, b) => {
      console.log(a, b, this);
      // Prints: a b undefined
    });
    myEmitter.emit('event', 'a', 'b');
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('event', (a, b) => {
      console.log(a, b, this);
      // Prints: a b {}
    });
    myEmitter.emit('event', 'a', 'b');
    javascript

### Asynchronous vs. synchronous#

The `EventEmitter` calls all listeners synchronously in the order in which they were registered. This ensures the proper sequencing of events and helps avoid race conditions and logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the `setImmediate()` or `process.nextTick()` methods:

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('event', (a, b) => {
      setImmediate(() => {
        console.log('this happens asynchronously');
      });
    });
    myEmitter.emit('event', 'a', 'b');
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('event', (a, b) => {
      setImmediate(() => {
        console.log('this happens asynchronously');
      });
    });
    myEmitter.emit('event', 'a', 'b');
    javascript

### Handling events only once#

When a listener is registered using the `eventEmitter.on()` method, that listener is invoked every time the named event is emitted.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    let m = 0;
    myEmitter.on('event', () => {
      console.log(++m);
    });
    myEmitter.emit('event');
    // Prints: 1
    myEmitter.emit('event');
    // Prints: 2
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    let m = 0;
    myEmitter.on('event', () => {
      console.log(++m);
    });
    myEmitter.emit('event');
    // Prints: 1
    myEmitter.emit('event');
    // Prints: 2
    javascript

Using the `eventEmitter.once()` method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    let m = 0;
    myEmitter.once('event', () => {
      console.log(++m);
    });
    myEmitter.emit('event');
    // Prints: 1
    myEmitter.emit('event');
    // Ignored
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    let m = 0;
    myEmitter.once('event', () => {
      console.log(++m);
    });
    myEmitter.emit('event');
    // Prints: 1
    myEmitter.emit('event');
    // Ignored
    javascript

### Error events#

When an error occurs within an `EventEmitter` instance, the typical action is for an `'error'` event to be emitted. These are treated as special cases within Node.js.

If an `EventEmitter` does not have at least one listener registered for the `'error'` event, and an `'error'` event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.emit('error', new Error('whoops!'));
    // Throws and crashes Node.js
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.emit('error', new Error('whoops!'));
    // Throws and crashes Node.js
    javascript

To guard against crashing the Node.js process the `domain` module can be used. (Note, however, that the `node:domain` module is deprecated.)

As a best practice, listeners should always be added for the `'error'` events.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('error', (err) => {
      console.error('whoops! there was an error');
    });
    myEmitter.emit('error', new Error('whoops!'));
    // Prints: whoops! there was an error
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    myEmitter.on('error', (err) => {
      console.error('whoops! there was an error');
    });
    myEmitter.emit('error', new Error('whoops!'));
    // Prints: whoops! there was an error
    javascript

It is possible to monitor `'error'` events without consuming the emitted error by installing a listener using the symbol `events.errorMonitor`.

    import { EventEmitter, errorMonitor } from 'node:events';

    const myEmitter = new EventEmitter();
    myEmitter.on(errorMonitor, (err) => {
      MyMonitoringTool.log(err);
    });
    myEmitter.emit('error', new Error('whoops!'));
    // Still throws and crashes Node.js
    const { EventEmitter, errorMonitor } = require('node:events');

    const myEmitter = new EventEmitter();
    myEmitter.on(errorMonitor, (err) => {
      MyMonitoringTool.log(err);
    });
    myEmitter.emit('error', new Error('whoops!'));
    // Still throws and crashes Node.js
    javascript

### Capture rejections of promises#

Using `async` functions with event handlers is problematic, because it can lead to an unhandled rejection in case of a thrown exception:

    import { EventEmitter } from 'node:events';
    const ee = new EventEmitter();
    ee.on('something', async (value) => {
      throw new Error('kaboom');
    });
    const EventEmitter = require('node:events');
    const ee = new EventEmitter();
    ee.on('something', async (value) => {
      throw new Error('kaboom');
    });
    javascript

The `captureRejections` option in the `EventEmitter` constructor or the global setting change this behavior, installing a `.then(undefined, handler)` handler on the `Promise`. This handler routes the exception asynchronously to the `Symbol.for('nodejs.rejection')` method if there is one, or to `'error'` event handler if there is none.

    import { EventEmitter } from 'node:events';
    const ee1 = new EventEmitter({ captureRejections: true });
    ee1.on('something', async (value) => {
      throw new Error('kaboom');
    });

    ee1.on('error', console.log);

    const ee2 = new EventEmitter({ captureRejections: true });
    ee2.on('something', async (value) => {
      throw new Error('kaboom');
    });

    ee2[Symbol.for('nodejs.rejection')] = console.log;
    const EventEmitter = require('node:events');
    const ee1 = new EventEmitter({ captureRejections: true });
    ee1.on('something', async (value) => {
      throw new Error('kaboom');
    });

    ee1.on('error', console.log);

    const ee2 = new EventEmitter({ captureRejections: true });
    ee2.on('something', async (value) => {
      throw new Error('kaboom');
    });

    ee2[Symbol.for('nodejs.rejection')] = console.log;
    javascript

Setting `events.captureRejections = true` will change the default for all new instances of `EventEmitter`.

    import { EventEmitter } from 'node:events';

    EventEmitter.captureRejections = true;
    const ee1 = new EventEmitter();
    ee1.on('something', async (value) => {
      throw new Error('kaboom');
    });

    ee1.on('error', console.log);
    const events = require('node:events');
    events.captureRejections = true;
    const ee1 = new events.EventEmitter();
    ee1.on('something', async (value) => {
      throw new Error('kaboom');
    });

    ee1.on('error', console.log);
    javascript

The `'error'` events that are generated by the `captureRejections` behavior do not have a catch handler to avoid infinite error loops: the recommendation is to not use `async` functions as `'error'` event handlers.

### Class: `EventEmitter`#

Added in: v0.1.26
History

Version | Changes
--- | ---
v13.4.0, v12.16.0 | Added captureRejections option.

The `EventEmitter` class is defined and exposed by the `node:events` module:

    import { EventEmitter } from 'node:events';
    const EventEmitter = require('node:events');
    javascript

All `EventEmitter`s emit the event `'newListener'` when new listeners are added and `'removeListener'` when existing listeners are removed.

It supports the following option:

- `captureRejections` `<boolean>` It enables automatic capturing of promise rejection. Default: `false`.

#### Event: `'newListener'`#

Added in: v0.1.26

- `eventName` `<string>` | `<symbol>` The name of the event being listened for
- `listener` `<Function>` The event handler function

The `EventEmitter` instance will emit its own `'newListener'` event before a listener is added to its internal array of listeners.

Listeners registered for the `'newListener'` event are passed the event name and a reference to the listener being added.

The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same `name` within the `'newListener'` callback are inserted before the listener that is in the process of being added.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}

    const myEmitter = new MyEmitter();
    // Only do this once so we don't loop forever
    myEmitter.once('newListener', (event, listener) => {
      if (event === 'event') {
        // Insert a new listener in front
        myEmitter.on('event', () => {
          console.log('B');
        });
      }
    });
    myEmitter.on('event', () => {
      console.log('A');
    });
    myEmitter.emit('event');
    // Prints:
    //   B
    //   A
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}

    const myEmitter = new MyEmitter();
    // Only do this once so we don't loop forever
    myEmitter.once('newListener', (event, listener) => {
      if (event === 'event') {
        // Insert a new listener in front
        myEmitter.on('event', () => {
          console.log('B');
        });
      }
    });
    myEmitter.on('event', () => {
      console.log('A');
    });
    myEmitter.emit('event');
    // Prints:
    //   B
    //   A
    javascript

#### Event: `'removeListener'`#

Added in: v0.9.3
History

Version | Changes
--- | ---
v6.1.0, v4.7.0 | For listeners attached using `.once()`, the `listener` argument now yields the original listener function.

- `eventName` `<string>` | `<symbol>` The event name
- `listener` `<Function>` The event handler function

The `'removeListener'` event is emitted after the `listener` is removed.

#### `emitter.addListener(eventName, listener)`#

Added in: v0.1.26

- `eventName` `<string>` | `<symbol>`
- `listener` `<Function>`

Alias for `emitter.on(eventName, listener)`.

#### `emitter.emit(eventName[, ...args])`#

Added in: v0.1.26

- `eventName` `<string>` | `<symbol>`
- `...args` `<any>`
- Returns: `<boolean>`

Synchronously calls each of the listeners registered for the event named `eventName`, in the order they were registered, passing the supplied arguments to each.

Returns `true` if the event had listeners, `false` otherwise.

    import { EventEmitter } from 'node:events';
    const myEmitter = new EventEmitter();

    // First listener
    myEmitter.on('event', function firstListener() {
      console.log('Helloooo! first listener');
    });
    // Second listener
    myEmitter.on('event', function secondListener(arg1, arg2) {
      console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
    });
    // Third listener
    myEmitter.on('event', function thirdListener(...args) {
      const parameters = args.join(', ');
      console.log(`event with parameters ${parameters} in third listener`);
    });

    console.log(myEmitter.listeners('event'));

    myEmitter.emit('event', 1, 2, 3, 4, 5);

    // Prints:
    // [
    //   [Function: firstListener],
    //   [Function: secondListener],
    //   [Function: thirdListener]
    // ]
    // Helloooo! first listener
    // event with parameters 1, 2 in second listener
    // event with parameters 1, 2, 3, 4, 5 in third listener
    const EventEmitter = require('node:events');
    const myEmitter = new EventEmitter();

    // First listener
    myEmitter.on('event', function firstListener() {
      console.log('Helloooo! first listener');
    });
    // Second listener
    myEmitter.on('event', function secondListener(arg1, arg2) {
      console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
    });
    // Third listener
    myEmitter.on('event', function thirdListener(...args) {
      const parameters = args.join(', ');
      console.log(`event with parameters ${parameters} in third listener`);
    });

    console.log(myEmitter.listeners('event'));

    myEmitter.emit('event', 1, 2, 3, 4, 5);

    // Prints:
    // [
    //   [Function: firstListener],
    //   [Function: secondListener],
    //   [Function: thirdListener]
    // ]
    // Helloooo! first listener
    // event with parameters 1, 2 in second listener
    // event with parameters 1, 2, 3, 4, 5 in third listener
    javascript

#### `emitter.eventNames()`#

Added in: v6.0.0

- Returns: `<string>`[] | `<symbol>`[]

Returns an array listing the events for which the emitter has registered listeners.

    import { EventEmitter } from 'node:events';

    const myEE = new EventEmitter();
    myEE.on('foo', () => {});
    myEE.on('bar', () => {});

    const sym = Symbol('symbol');
    myEE.on(sym, () => {});

    console.log(myEE.eventNames());
    // Prints: [ 'foo', 'bar', Symbol(symbol) ]
    const EventEmitter = require('node:events');

    const myEE = new EventEmitter();
    myEE.on('foo', () => {});
    myEE.on('bar', () => {});

    const sym = Symbol('symbol');
    myEE.on(sym, () => {});

    console.log(myEE.eventNames());
    // Prints: [ 'foo', 'bar', Symbol(symbol) ]
    javascript

#### `emitter.getMaxListeners()`#

Added in: v1.0.0

- Returns: `<integer>`

Returns the current max listener value for the `EventEmitter` which is either set by `emitter.setMaxListeners(n)` or defaults to `events.defaultMaxListeners`.

#### `emitter.listenerCount(eventName[, listener])`#

Added in: v3.2.0
History

Version | Changes
--- | ---
v19.8.0, v18.16.0 | Added the `listener` argument.

- `eventName` `<string>` | `<symbol>` The name of the event being listened for
- `listener` `<Function>` The event handler function
- Returns: `<integer>`

Returns the number of listeners listening for the event named `eventName`. If `listener` is provided, it will return how many times the listener is found in the list of the listeners of the event.

#### `emitter.listeners(eventName)`#

Added in: v0.1.26
History

Version | Changes
--- | ---
v7.0.0 | For listeners attached using `.once()` this returns the original listeners instead of wrapper functions now.

- `eventName` `<string>` | `<symbol>`
- Returns: `<Function>`[]

Returns a copy of the array of listeners for the event named `eventName`.

    server.on('connection', (stream) => {
      console.log('someone connected!');
    });
    console.log(util.inspect(server.listeners('connection')));
    // Prints: [ [Function] ]
    js

#### `emitter.off(eventName, listener)`#

Added in: v10.0.0

- `eventName` `<string>` | `<symbol>`
- `listener` `<Function>`
- Returns: `<EventEmitter>`

Alias for `emitter.removeListener()`.

#### `emitter.on(eventName, listener)`#

Added in: v0.1.101

- `eventName` `<string>` | `<symbol>` The name of the event.
- `listener` `<Function>` The callback function
- Returns: `<EventEmitter>`

Adds the `listener` function to the end of the listeners array for the event named `eventName`. No checks are made to see if the `listener` has already been added. Multiple calls passing the same combination of `eventName` and `listener` will result in the `listener` being added, and called, multiple times.

    server.on('connection', (stream) => {
      console.log('someone connected!');
    });
    js

Returns a reference to the `EventEmitter`, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The `emitter.prependListener()` method can be used as an alternative to add the event listener to the beginning of the listeners array.

    import { EventEmitter } from 'node:events';
    const myEE = new EventEmitter();
    myEE.on('foo', () => console.log('a'));
    myEE.prependListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    //   b
    //   a
    const EventEmitter = require('node:events');
    const myEE = new EventEmitter();
    myEE.on('foo', () => console.log('a'));
    myEE.prependListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    //   b
    //   a
    javascript

#### `emitter.once(eventName, listener)`#

Added in: v0.3.0

- `eventName` `<string>` | `<symbol>` The name of the event.
- `listener` `<Function>` The callback function
- Returns: `<EventEmitter>`

Adds a one-time `listener` function for the event named `eventName`. The next time `eventName` is triggered, this listener is removed and then invoked.

    server.once('connection', (stream) => {
      console.log('Ah, we have our first user!');
    });
    js

Returns a reference to the `EventEmitter`, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The `emitter.prependOnceListener()` method can be used as an alternative to add the event listener to the beginning of the listeners array.

    import { EventEmitter } from 'node:events';
    const myEE = new EventEmitter();
    myEE.once('foo', () => console.log('a'));
    myEE.prependOnceListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    //   b
    //   a
    const EventEmitter = require('node:events');
    const myEE = new EventEmitter();
    myEE.once('foo', () => console.log('a'));
    myEE.prependOnceListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    //   b
    //   a
    javascript

#### `emitter.prependListener(eventName, listener)`#

Added in: v6.0.0

- `eventName` `<string>` | `<symbol>` The name of the event.
- `listener` `<Function>` The callback function
- Returns: `<EventEmitter>`

Adds the `listener` function to the beginning of the listeners array for the event named `eventName`. No checks are made to see if the `listener` has already been added. Multiple calls passing the same combination of `eventName` and `listener` will result in the `listener` being added, and called, multiple times.

    server.prependListener('connection', (stream) => {
      console.log('someone connected!');
    });
    js

Returns a reference to the `EventEmitter`, so that calls can be chained.

#### `emitter.prependOnceListener(eventName, listener)`#

Added in: v6.0.0

- `eventName` `<string>` | `<symbol>` The name of the event.
- `listener` `<Function>` The callback function
- Returns: `<EventEmitter>`

Adds a one-time `listener` function for the event named `eventName` to the beginning of the listeners array. The next time `eventName` is triggered, this listener is removed, and then invoked.

    server.prependOnceListener('connection', (stream) => {
      console.log('Ah, we have our first user!');
    });
    js

Returns a reference to the `EventEmitter`, so that calls can be chained.

#### `emitter.removeAllListeners([eventName])`#

Added in: v0.1.26

- `eventName` `<string>` | `<symbol>`
- Returns: `<EventEmitter>`

Removes all listeners, or those of the specified `eventName`.

It is bad practice to remove listeners added elsewhere in the code, particularly when the `EventEmitter` instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the `EventEmitter`, so that calls can be chained.

#### `emitter.removeListener(eventName, listener)`#

Added in: v0.1.26

- `eventName` `<string>` | `<symbol>`
- `listener` `<Function>`
- Returns: `<EventEmitter>`

Removes the specified `listener` from the listener array for the event named `eventName`.

    const callback = (stream) => {
      console.log('someone connected!');
    };
    server.on('connection', callback);
    // ...
    server.removeListener('connection', callback);
    js

`removeListener()` will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified `eventName`, then `removeListener()` must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any `removeListener()` or `removeAllListeners()` calls after emitting and before the last listener finishes execution will not remove them from `emit()` in progress. Subsequent events behave as expected.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();

    const callbackA = () => {
      console.log('A');
      myEmitter.removeListener('event', callbackB);
    };

    const callbackB = () => {
      console.log('B');
    };

    myEmitter.on('event', callbackA);

    myEmitter.on('event', callbackB);

    // callbackA removes listener callbackB but it will still be called.
    // Internal listener array at time of emit [callbackA, callbackB]
    myEmitter.emit('event');
    // Prints:
    //   A
    //   B

    // callbackB is now removed.
    // Internal listener array [callbackA]
    myEmitter.emit('event');
    // Prints:
    //   A
    const EventEmitter = require('node:events');
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();

    const callbackA = () => {
      console.log('A');
      myEmitter.removeListener('event', callbackB);
    };

    const callbackB = () => {
      console.log('B');
    };

    myEmitter.on('event', callbackA);

    myEmitter.on('event', callbackB);

    // callbackA removes listener callbackB but it will still be called.
    // Internal listener array at time of emit [callbackA, callbackB]
    myEmitter.emit('event');
    // Prints:
    //   A
    //   B

    // callbackB is now removed.
    // Internal listener array [callbackA]
    myEmitter.emit('event');
    // Prints:
    //   A
    javascript

Because listeners are managed using an internal array, calling this will change the position indexes of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the `emitter.listeners()` method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), `removeListener()` will remove the most recently added instance. In the example the `once('ping')` listener is removed:

    import { EventEmitter } from 'node:events';
    const ee = new EventEmitter();

    function pong() {
      console.log('pong');
    }

    ee.on('ping', pong);
    ee.once('ping', pong);
    ee.removeListener('ping', pong);

    ee.emit('ping');
    ee.emit('ping');
    const EventEmitter = require('node:events');
    const ee = new EventEmitter();

    function pong() {
      console.log('pong');
    }

    ee.on('ping', pong);
    ee.once('ping', pong);
    ee.removeListener('ping', pong);

    ee.emit('ping');
    ee.emit('ping');
    javascript

Returns a reference to the `EventEmitter`, so that calls can be chained.

#### `emitter.setMaxListeners(n)`#

Added in: v0.3.5

- `n` `<integer>`
- Returns: `<EventEmitter>`

By default `EventEmitter`s will print a warning if more than `10` listeners are added for a particular event. This is a useful default that helps finding memory leaks. The `emitter.setMaxListeners()` method allows the limit to be modified for this specific `EventEmitter` instance. The value can be set to `Infinity` (or `0`) to indicate an unlimited number of listeners.

Returns a reference to the `EventEmitter`, so that calls can be chained.

#### `emitter.rawListeners(eventName)`#

Added in: v9.4.0

- `eventName` `<string>` | `<symbol>`
- Returns: `<Function>`[]

Returns a copy of the array of listeners for the event named `eventName`, including any wrappers (such as those created by `.once()`).

    import { EventEmitter } from 'node:events';
    const emitter = new EventEmitter();
    emitter.once('log', () => console.log('log once'));

    // Returns a new Array with a function `onceWrapper` which has a property
    // `listener` which contains the original listener bound above
    const listeners = emitter.rawListeners('log');
    const logFnWrapper = listeners[0];

    // Logs "log once" to the console and does not unbind the `once` event
    logFnWrapper.listener();

    // Logs "log once" to the console and removes the listener
    logFnWrapper();

    emitter.on('log', () => console.log('log persistently'));
    // Will return a new Array with a single function bound by `.on()` above
    const newListeners = emitter.rawListeners('log');

    // Logs "log persistently" twice
    newListeners[0]();
    emitter.emit('log');
    const EventEmitter = require('node:events');
    const emitter = new EventEmitter();
    emitter.once('log', () => console.log('log once'));

    // Returns a new Array with a function `onceWrapper` which has a property
    // `listener` which contains the original listener bound above
    const listeners = emitter.rawListeners('log');
    const logFnWrapper = listeners[0];

    // Logs "log once" to the console and does not unbind the `once` event
    logFnWrapper.listener();

    // Logs "log once" to the console and removes the listener
    logFnWrapper();

    emitter.on('log', () => console.log('log persistently'));
    // Will return a new Array with a single function bound by `.on()` above
    const newListeners = emitter.rawListeners('log');

    // Logs "log persistently" twice
    newListeners[0]();
    emitter.emit('log');
    javascript

#### `emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])`#

Added in: v13.4.0, v12.16.0
History

Version | Changes
--- | ---
v17.4.0, v16.14.0 | No longer experimental.

- `err` `<Error>`
- `eventName` `<string>` | `<symbol>`
- `...args` `<any>`

The `Symbol.for('nodejs.rejection')` method is called in case a promise rejection happens when emitting an event and `captureRejections` is enabled on the emitter. It is possible to use `events.captureRejectionSymbol` in place of `Symbol.for('nodejs.rejection')`.

    import { EventEmitter, captureRejectionSymbol } from 'node:events';

    class MyClass extends EventEmitter {
      constructor() {
        super({ captureRejections: true });
      }

      [captureRejectionSymbol](err, event, ...args) {
        console.log('rejection happened for', event, 'with', err, ...args);
        this.destroy(err);
      }

      destroy(err) {
        // Tear the resource down here.
      }
    }
    const { EventEmitter, captureRejectionSymbol } = require('node:events');

    class MyClass extends EventEmitter {
      constructor() {
        super({ captureRejections: true });
      }

      [captureRejectionSymbol](err, event, ...args) {
        console.log('rejection happened for', event, 'with', err, ...args);
        this.destroy(err);
      }

      destroy(err) {
        // Tear the resource down here.
      }
    }
    javascript

### `events.defaultMaxListeners`#

Added in: v0.11.2

By default, a maximum of `10` listeners can be registered for any single event. This limit can be changed for individual `EventEmitter` instances using the `emitter.setMaxListeners(n)` method. To change the default for all `EventEmitter` instances, the `events.defaultMaxListeners` property can be used. If this value is not a positive number, a `RangeError` is thrown.

Take caution when setting the `events.defaultMaxListeners` because the change affects all `EventEmitter` instances, including those created before the change is made. However, calling `emitter.setMaxListeners(n)` still has precedence over `events.defaultMaxListeners`.

This is not a hard limit. The `EventEmitter` instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single `EventEmitter`, the `emitter.getMaxListeners()` and `emitter.setMaxListeners()` methods can be used to temporarily avoid this warning:

`defaultMaxListeners` has no effect on `AbortSignal` instances. While it is still possible to use `emitter.setMaxListeners(n)` to set a warning limit for individual `AbortSignal` instances, per default `AbortSignal` instances will not warn.

    import { EventEmitter } from 'node:events';
    const emitter = new EventEmitter();
    emitter.setMaxListeners(emitter.getMaxListeners() + 1);
    emitter.once('event', () => {
      // do stuff
      emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
    });
    const EventEmitter = require('node:events');
    const emitter = new EventEmitter();
    emitter.setMaxListeners(emitter.getMaxListeners() + 1);
    emitter.once('event', () => {
      // do stuff
      emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
    });
    javascript

The `--trace-warnings` command-line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with `process.on('warning')` and will have the additional `emitter`, `type`, and `count` properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its `name` property is set to `'MaxListenersExceededWarning'`.

### `events.errorMonitor`#

Added in: v13.6.0, v12.17.0

This symbol shall be used to install a listener for only monitoring `'error'` events. Listeners installed using this symbol are called before the regular `'error'` listeners are called.

Installing a listener using this symbol does not change the behavior once an `'error'` event is emitted. Therefore, the process will still crash if no regular `'error'` listener is installed.

### `events.getEventListeners(emitterOrTarget, eventName)`#

Added in: v15.2.0, v14.17.0

- `emitterOrTarget` `<EventEmitter>` | `<EventTarget>`
- `eventName` `<string>` | `<symbol>`
- Returns: `<Function>`[]

Returns a copy of the array of listeners for the event named `eventName`.

For `EventEmitter`s this behaves exactly the same as calling `.listeners` on the emitter.

For `EventTarget`s this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

    import { getEventListeners, EventEmitter } from 'node:events';

    {
      const ee = new EventEmitter();
      const listener = () => console.log('Events are fun');
      ee.on('foo', listener);
      console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
    }
    {
      const et = new EventTarget();
      const listener = () => console.log('Events are fun');
      et.addEventListener('foo', listener);
      console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
    }
    const { getEventListeners, EventEmitter } = require('node:events');

    {
      const ee = new EventEmitter();
      const listener = () => console.log('Events are fun');
      ee.on('foo', listener);
      console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
    }
    {
      const et = new EventTarget();
      const listener = () => console.log('Events are fun');
      et.addEventListener('foo', listener);
      console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
    }
    javascript

### `events.getMaxListeners(emitterOrTarget)`#

Added in: v19.9.0, v18.17.0

- `emitterOrTarget` `<EventEmitter>` | `<EventTarget>`
- Returns: `<number>`

Returns the currently set max amount of listeners.

For `EventEmitter`s this behaves exactly the same as calling `.getMaxListeners` on the emitter.

For `EventTarget`s this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

    import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';

    {
      const ee = new EventEmitter();
      console.log(getMaxListeners(ee)); // 10
      setMaxListeners(11, ee);
      console.log(getMaxListeners(ee)); // 11
    }
    {
      const et = new EventTarget();
      console.log(getMaxListeners(et)); // 10
      setMaxListeners(11, et);
      console.log(getMaxListeners(et)); // 11
    }
    const { getMaxListeners, setMaxListeners, EventEmitter } = require('node:events');

    {
      const ee = new EventEmitter();
      console.log(getMaxListeners(ee)); // 10
      setMaxListeners(11, ee);
      console.log(getMaxListeners(ee)); // 11
    }
    {
      const et = new EventTarget();
      console.log(getMaxListeners(et)); // 10
      setMaxListeners(11, et);
      console.log(getMaxListeners(et)); // 11
    }
    javascript

### `events.once(emitter, name[, options])`#

Added in: v11.13.0, v10.16.0
History

Version | Changes
--- | ---
v15.0.0 | The `signal` option is supported now.

- `emitter` `<EventEmitter>`
- `name` `<string>` | `<symbol>`
- `options` `<Object>`
  - `signal` `<AbortSignal>` Can be used to cancel waiting for the event.
- Returns: `<Promise>`

Creates a `Promise` that is fulfilled when the `EventEmitter` emits the given event or that is rejected if the `EventEmitter` emits `'error'` while waiting. The `Promise` will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special `'error'` event semantics and does not listen to the `'error'` event.

    import { once, EventEmitter } from 'node:events';
    import process from 'node:process';

    const ee = new EventEmitter();

    process.nextTick(() => {
      ee.emit('myevent', 42);
    });

    const [value] = await once(ee, 'myevent');
    console.log(value);

    const err = new Error('kaboom');
    process.nextTick(() => {
      ee.emit('error', err);
    });

    try {
      await once(ee, 'myevent');
    } catch (err) {
      console.error('error happened', err);
    }
    const { once, EventEmitter } = require('node:events');

    async function run() {
      const ee = new EventEmitter();

      process.nextTick(() => {
        ee.emit('myevent', 42);
      });

      const [value] = await once(ee, 'myevent');
      console.log(value);

      const err = new Error('kaboom');
      process.nextTick(() => {
        ee.emit('error', err);
      });

      try {
        await once(ee, 'myevent');
      } catch (err) {
        console.error('error happened', err);
      }
    }

    run();
    javascript

The special handling of the `'error'` event is only used when `events.once()` is used to wait for another event. If `events.once()` is used to wait for the '`error'` event itself, then it is treated as any other kind of event without special handling:

    import { EventEmitter, once } from 'node:events';

    const ee = new EventEmitter();

    once(ee, 'error')
      .then(([err]) => console.log('ok', err.message))
      .catch((err) => console.error('error', err.message));

    ee.emit('error', new Error('boom'));

    // Prints: ok boom
    const { EventEmitter, once } = require('node:events');

    const ee = new EventEmitter();

    once(ee, 'error')
      .then(([err]) => console.log('ok', err.message))
      .catch((err) => console.error('error', err.message));

    ee.emit('error', new Error('boom'));

    // Prints: ok boom
    javascript

An `<AbortSignal>` can be used to cancel waiting for the event:

    import { EventEmitter, once } from 'node:events';

    const ee = new EventEmitter();
    const ac = new AbortController();

    async function foo(emitter, event, signal) {
      try {
        await once(emitter, event, { signal });
        console.log('event emitted!');
      } catch (error) {
        if (error.name === 'AbortError') {
          console.error('Waiting for the event was canceled!');
        } else {
          console.error('There was an error', error.message);
        }
      }
    }

    foo(ee, 'foo', ac.signal);
    ac.abort(); // Prints: Waiting for the event was canceled!
    const { EventEmitter, once } = require('node:events');

    const ee = new EventEmitter();
    const ac = new AbortController();

    async function foo(emitter, event, signal) {
      try {
        await once(emitter, event, { signal });
        console.log('event emitted!');
      } catch (error) {
        if (error.name === 'AbortError') {
          console.error('Waiting for the event was canceled!');
        } else {
          console.error('There was an error', error.message);
        }
      }
    }

    foo(ee, 'foo', ac.signal);
    ac.abort(); // Prints: Waiting for the event was canceled!
    javascript

#### Caveats when awaiting multiple events#

It is important to be aware of execution order when using the `events.once()` method to await multiple events.

Conventional event listeners are called synchronously when the event is emitted. This guarantees that execution will not proceed beyond the emitted event until all listeners have finished executing.

The same is not true when awaiting Promises returned by `events.once()`. Promise tasks are not handled until after the current execution stack runs to completion, which means that multiple events could be emitted before asynchronous execution continues from the relevant `await` statement.

As a result, events can be "missed" if a series of `await events.once()` statements is used to listen to multiple events, since there might be times where more than one event is emitted during the same phase of the event loop. (The same is true when using `process.nextTick()` to emit events, because the tasks queued by `process.nextTick()` are executed before Promise tasks.)

    import { EventEmitter, once } from 'node:events';
    import process from 'node:process';

    const myEE = new EventEmitter();

    async function listen() {
      await once(myEE, 'foo');
      console.log('foo');

      // This Promise will never resolve, because the 'bar' event will
      // have already been emitted before the next line is executed.
      await once(myEE, 'bar');
      console.log('bar');
    }

    process.nextTick(() => {
      myEE.emit('foo');
      myEE.emit('bar');
    });

    listen().then(() => console.log('done'));
    const { EventEmitter, once } = require('node:events');

    const myEE = new EventEmitter();

    async function listen() {
      await once(myEE, 'foo');
      console.log('foo');

      // This Promise will never resolve, because the 'bar' event will
      // have already been emitted before the next line is executed.
      await once(myEE, 'bar');
      console.log('bar');
    }

    process.nextTick(() => {
      myEE.emit('foo');
      myEE.emit('bar');
    });

    listen().then(() => console.log('done'));
    javascript

To catch multiple events, create all of the Promises before awaiting any of them. This is usually made easier by using `Promise.all()`, `Promise.race()`, or `Promise.allSettled()`:

    import { EventEmitter, once } from 'node:events';
    import process from 'node:process';

    const myEE = new EventEmitter();

    async function listen() {
      await Promise.all([
        once(myEE, 'foo'),
        once(myEE, 'bar'),
      ]);
      console.log('foo', 'bar');
    }

    process.nextTick(() => {
      myEE.emit('foo');
      myEE.emit('bar');
    });

    listen().then(() => console.log('done'));
    const { EventEmitter, once } = require('node:events');

    const myEE = new EventEmitter();

    async function listen() {
      await Promise.all([
        once(myEE, 'bar'),
        once(myEE, 'foo'),
      ]);
      console.log('foo', 'bar');
    }

    process.nextTick(() => {
      myEE.emit('foo');
      myEE.emit('bar');
    });

    listen().then(() => console.log('done'));
    javascript

### `events.captureRejections`#

Added in: v13.4.0, v12.16.0
History

Version | Changes
--- | ---
v17.4.0, v16.14.0 | No longer experimental.

- Type: `<boolean>`

Change the default `captureRejections` option on all new `EventEmitter` objects.

### `events.captureRejectionSymbol`#

Added in: v13.4.0, v12.16.0
History

Version | Changes
--- | ---
v17.4.0, v16.14.0 | No longer experimental.

- Type: `<symbol>` `Symbol.for('nodejs.rejection')`

See how to write a custom rejection handler.

### `events.listenerCount(emitterOrTarget, eventName)`#

Added in: v0.9.12
History

Version | Changes
--- | ---
v25.4.0, v24.14.0 | Now accepts EventTarget arguments.
v25.4.0, v24.14.0 | Deprecation revoked.
v3.2.0 | Documentation-only deprecation.

- `emitterOrTarget` `<EventEmitter>` | `<EventTarget>`
- `eventName` `<string>` | `<symbol>`
- Returns: `<integer>`

Returns the number of registered listeners for the event named `eventName`.

For `EventEmitter`s this behaves exactly the same as calling `.listenerCount` on the emitter.

For `EventTarget`s this is the only way to obtain the listener count. This can be useful for debugging and diagnostic purposes.

    import { EventEmitter, listenerCount } from 'node:events';

    {
      const ee = new EventEmitter();
      ee.on('event', () => {});
      ee.on('event', () => {});
      console.log(listenerCount(ee, 'event')); // 2
    }
    {
      const et = new EventTarget();
      et.addEventListener('event', () => {});
      et.addEventListener('event', () => {});
      console.log(listenerCount(et, 'event')); // 2
    }
    const { EventEmitter, listenerCount } = require('node:events');

    {
      const ee = new EventEmitter();
      ee.on('event', () => {});
      ee.on('event', () => {});
      console.log(listenerCount(ee, 'event')); // 2
    }
    {
      const et = new EventTarget();
      et.addEventListener('event', () => {});
      et.addEventListener('event', () => {});
      console.log(listenerCount(et, 'event')); // 2
    }
    javascript

### `events.on(emitter, eventName[, options])`#

Added in: v13.6.0, v12.16.0
History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | Support `highWaterMark` and `lowWaterMark` options, For consistency. Old options are still supported.
v20.0.0 | The `close`, `highWatermark`, and `lowWatermark` options are supported now.

- `emitter` `<EventEmitter>`
- `eventName` `<string>` | `<symbol>` The name of the event being listened for
- `options` `<Object>`
  - `signal` `<AbortSignal>` Can be used to cancel awaiting events.
  - `close` `<string>`[] Names of events that will end the iteration.
  - `highWaterMark` `<integer>` Default: `Number.MAX_SAFE_INTEGER` The high watermark. The emitter is paused every time the size of events being buffered is higher than it. Supported only on emitters implementing `pause()` and `resume()` methods.
  - `lowWaterMark` `<integer>` Default: `1` The low watermark. The emitter is resumed every time the size of events being buffered is lower than it. Supported only on emitters implementing `pause()` and `resume()` methods.
- Returns: `<AsyncIterator>` that iterates `eventName` events emitted by the `emitter`

    import { on, EventEmitter } from 'node:events';
    import process from 'node:process';

    const ee = new EventEmitter();

    // Emit later on
    process.nextTick(() => {
      ee.emit('foo', 'bar');
      ee.emit('foo', 42);
    });

    for await (const event of on(ee, 'foo')) {
      // The execution of this inner block is synchronous and it
      // processes one event at a time (even with await). Do not use
      // if concurrent execution is required.
      console.log(event); // prints ['bar'] [42]
    }
    // Unreachable here
    const { on, EventEmitter } = require('node:events');

    (async () => {
      const ee = new EventEmitter();

      // Emit later on
      process.nextTick(() => {
        ee.emit('foo', 'bar');
        ee.emit('foo', 42);
      });

      for await (const event of on(ee, 'foo')) {
        // The execution of this inner block is synchronous and it
        // processes one event at a time (even with await). Do not use
        // if concurrent execution is required.
        console.log(event); // prints ['bar'] [42]
      }
      // Unreachable here
    })();
    javascript

Returns an `AsyncIterator` that iterates `eventName` events. It will throw if the `EventEmitter` emits `'error'`. It removes all listeners when exiting the loop. The `value` returned by each iteration is an array composed of the emitted event arguments.

An `<AbortSignal>` can be used to cancel waiting on events:

    import { on, EventEmitter } from 'node:events';
    import process from 'node:process';

    const ac = new AbortController();

    (async () => {
      const ee = new EventEmitter();

      // Emit later on
      process.nextTick(() => {
        ee.emit('foo', 'bar');
        ee.emit('foo', 42);
      });

      for await (const event of on(ee, 'foo', { signal: ac.signal })) {
        // The execution of this inner block is synchronous and it
        // processes one event at a time (even with await). Do not use
        // if concurrent execution is required.
        console.log(event); // prints ['bar'] [42]
      }
      // Unreachable here
    })();

    process.nextTick(() => ac.abort());
    const { on, EventEmitter } = require('node:events');

    const ac = new AbortController();

    (async () => {
      const ee = new EventEmitter();

      // Emit later on
      process.nextTick(() => {
        ee.emit('foo', 'bar');
        ee.emit('foo', 42);
      });

      for await (const event of on(ee, 'foo', { signal: ac.signal })) {
        // The execution of this inner block is synchronous and it
        // processes one event at a time (even with await). Do not use
        // if concurrent execution is required.
        console.log(event); // prints ['bar'] [42]
      }
      // Unreachable here
    })();

    process.nextTick(() => ac.abort());
    javascript

### `events.setMaxListeners(n[, ...eventTargets])`#

Added in: v15.4.0

- `n` `<number>` A non-negative number. The maximum number of listeners per `EventTarget` event.
- `...eventsTargets` `<EventTarget>`[] | `<EventEmitter>`[] Zero or more `<EventTarget>` or `<EventEmitter>` instances. If none are specified, `n` is set as the default max for all newly created `<EventTarget>` and `<EventEmitter>` objects.

    import { setMaxListeners, EventEmitter } from 'node:events';

    const target = new EventTarget();
    const emitter = new EventEmitter();

    setMaxListeners(5, target, emitter);
    const {
      setMaxListeners,
      EventEmitter,
    } = require('node:events');

    const target = new EventTarget();
    const emitter = new EventEmitter();

    setMaxListeners(5, target, emitter);
    javascript

### `events.addAbortListener(signal, listener)`#

Added in: v20.5.0, v18.18.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Change stability index for this feature from Experimental to Stable.

- `signal` `<AbortSignal>`
- `listener` `<Function>` | `<EventListener>`
- Returns: `<Disposable>` A Disposable that removes the `abort` listener.

Listens once to the `abort` event on the provided `signal`.

Listening to the `abort` event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call `e.stopImmediatePropagation()`. Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.

This API allows safely using `AbortSignal`s in Node.js APIs by solving these two issues by listening to the event such that `stopImmediatePropagation` does not prevent the listener from running.

Returns a disposable so that it may be unsubscribed from more easily.

    const { addAbortListener } = require('node:events');

    function example(signal) {
      signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
      // addAbortListener() returns a disposable, so the `using` keyword ensures
      // the abort listener is automatically removed when this scope exits.
      using _ = addAbortListener(signal, (e) => {
        // Do something when signal is aborted.
      });
    }
    import { addAbortListener } from 'node:events';

    function example(signal) {
      signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
      // addAbortListener() returns a disposable, so the `using` keyword ensures
      // the abort listener is automatically removed when this scope exits.
      using _ = addAbortListener(signal, (e) => {
        // Do something when signal is aborted.
      });
    }
    javascript

### Class: `events.EventEmitterAsyncResource extends EventEmitter`#

Added in: v17.4.0, v16.14.0

Integrates `EventEmitter` with `<AsyncResource>` for `EventEmitter`s that require manual async tracking. Specifically, all events emitted by instances of `events.EventEmitterAsyncResource` will run within its async context.

    import { EventEmitterAsyncResource, EventEmitter } from 'node:events';
    import { notStrictEqual, strictEqual } from 'node:assert';
    import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';

    // Async tracking tooling will identify this as 'Q'.
    const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

    // 'foo' listeners will run in the EventEmitters async context.
    ee1.on('foo', () => {
      strictEqual(executionAsyncId(), ee1.asyncId);
      strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
    });

    const ee2 = new EventEmitter();

    // 'foo' listeners on ordinary EventEmitters that do not track async
    // context, however, run in the same async context as the emit().
    ee2.on('foo', () => {
      notStrictEqual(executionAsyncId(), ee2.asyncId);
      notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
    });

    Promise.resolve().then(() => {
      ee1.emit('foo');
      ee2.emit('foo');
    });
    const { EventEmitterAsyncResource, EventEmitter } = require('node:events');
    const { notStrictEqual, strictEqual } = require('node:assert');
    const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');

    // Async tracking tooling will identify this as 'Q'.
    const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

    // 'foo' listeners will run in the EventEmitters async context.
    ee1.on('foo', () => {
      strictEqual(executionAsyncId(), ee1.asyncId);
      strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
    });

    const ee2 = new EventEmitter();

    // 'foo' listeners on ordinary EventEmitters that do not track async
    // context, however, run in the same async context as the emit().
    ee2.on('foo', () => {
      notStrictEqual(executionAsyncId(), ee2.asyncId);
      notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
    });

    Promise.resolve().then(() => {
      ee1.emit('foo');
      ee2.emit('foo');
    });
    javascript

The `EventEmitterAsyncResource` class has the same methods and takes the same options as `EventEmitter` and `AsyncResource` themselves.

#### `new events.EventEmitterAsyncResource([options])`#

- `options` `<Object>`
  - `captureRejections` `<boolean>` It enables automatic capturing of promise rejection. Default: `false`.
  - `name` `<string>` The type of async event. Default: `new.target.name`.
  - `triggerAsyncId` `<number>` The ID of the execution context that created this async event. Default: `executionAsyncId()`.
  - `requireManualDestroy` `<boolean>` If set to `true`, disables `emitDestroy` when the object is garbage collected. This usually does not need to be set (even if `emitDestroy` is called manually), unless the resource's `asyncId` is retrieved and the sensitive API's `emitDestroy` is called with it. When set to `false`, the `emitDestroy` call on garbage collection will only take place if there is at least one active `destroy` hook. Default: `false`.

#### `eventemitterasyncresource.asyncId`#

- Type: `<number>` The unique `asyncId` assigned to the resource.

#### `eventemitterasyncresource.asyncResource`#

- Type: `<AsyncResource>` The underlying `<AsyncResource>`.

The returned `AsyncResource` object has an additional `eventEmitter` property that provides a reference to this `EventEmitterAsyncResource`.

#### `eventemitterasyncresource.emitDestroy()`#

Call all `destroy` hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the `destroy` hooks will never be called.

#### `eventemitterasyncresource.triggerAsyncId`#

- Type: `<number>` The same `triggerAsyncId` that is passed to the `AsyncResource` constructor.

### `EventTarget` and `Event` API#

Added in: v14.5.0
History

Version | Changes
--- | ---
v16.0.0 | changed EventTarget error handling.
v15.4.0 | No longer experimental.
v15.0.0 | The `EventTarget` and `Event` classes are now available as globals.

The `EventTarget` and `Event` objects are a Node.js-specific implementation of the `EventTarget` Web API that are exposed by some Node.js core APIs.

    const target = new EventTarget();

    target.addEventListener('foo', (event) => {
      console.log('foo event happened!');
    });
    js

#### Node.js `EventTarget` vs. DOM `EventTarget`#

There are two key differences between the Node.js `EventTarget` and the `EventTarget` Web API:

1. Whereas DOM `EventTarget` instances may be hierarchical, there is no concept of hierarchy and event propagation in Node.js. That is, an event dispatched to an `EventTarget` does not propagate through a hierarchy of nested target objects that may each have their own set of handlers for the event.
2. In the Node.js `EventTarget`, if an event listener is an async function or returns a `Promise`, and the returned `Promise` rejects, the rejection is automatically captured and handled the same way as a listener that throws synchronously (see `EventTarget` error handling for details).

#### `NodeEventTarget` vs. `EventEmitter`#

The `NodeEventTarget` object implements a modified subset of the `EventEmitter` API that allows it to closely emulate an `EventEmitter` in certain situations. A `NodeEventTarget` is not an instance of `EventEmitter` and cannot be used in place of an `EventEmitter` in most cases.

1. Unlike `EventEmitter`, any given `listener` can be registered at most once per event `type`. Attempts to register a `listener` multiple times are ignored.
2. The `NodeEventTarget` does not emulate the full `EventEmitter` API. Specifically the `prependListener()`, `prependOnceListener()`, `rawListeners()`, and `errorMonitor` APIs are not emulated. The `'newListener'` and `'removeListener'` events will also not be emitted.
3. The `NodeEventTarget` does not implement any special default behavior for events with type `'error'`.
4. The `NodeEventTarget` supports `EventListener` objects as well as functions as handlers for all event types.

#### Event listener#

Event listeners registered for an event `type` may either be JavaScript functions or objects with a `handleEvent` property whose value is a function.

In either case, the handler function is invoked with the `event` argument passed to the `eventTarget.dispatchEvent()` function.

Async functions may be used as event listeners. If an async handler function rejects, the rejection is captured and handled as described in `EventTarget` error handling.

An error thrown by one handler function does not prevent the other handlers from being invoked.

The return value of a handler function is ignored.

Handlers are always invoked in the order they were added.

Handler functions may mutate the `event` object.

    function handler1(event) {
      console.log(event.type);  // Prints 'foo'
      event.a = 1;
    }

    async function handler2(event) {
      console.log(event.type);  // Prints 'foo'
      console.log(event.a);  // Prints 1
    }

    const handler3 = {
      handleEvent(event) {
        console.log(event.type);  // Prints 'foo'
      },
    };

    const handler4 = {
      async handleEvent(event) {
        console.log(event.type);  // Prints 'foo'
      },
    };

    const target = new EventTarget();

    target.addEventListener('foo', handler1);
    target.addEventListener('foo', handler2);
    target.addEventListener('foo', handler3);
    target.addEventListener('foo', handler4, { once: true });
    js

#### `EventTarget` error handling#

When a registered event listener throws (or returns a Promise that rejects), by default the error is treated as an uncaught exception on `process.nextTick()`. This means uncaught exceptions in `EventTarget`s will terminate the Node.js process by default.

Throwing within an event listener will not stop the other registered handlers from being invoked.

The `EventTarget` does not implement any special default handling for `'error'` type events like `EventEmitter`.

Currently errors are first forwarded to the `process.on('error')` event before reaching `process.on('uncaughtException')`. This behavior is deprecated and will change in a future release to align `EventTarget` with other Node.js APIs. Any code relying on the `process.on('error')` event should be aligned with the new behavior.

#### Class: `Event`#

Added in: v14.5.0
History

Version | Changes
--- | ---
v15.0.0 | The `Event` class is now available through the global object.

The `Event` object is an adaptation of the `Event` Web API. Instances are created internally by Node.js.

##### `event.bubbles`#

Added in: v14.5.0

- Type: `<boolean>` Always returns `false`.

This is not used in Node.js and is provided purely for completeness.

##### `event.cancelBubble`#

Added in: v14.5.0

Stability: 3 - Legacy: Use `event.stopPropagation()` instead.

- Type: `<boolean>`

Alias for `event.stopPropagation()` if set to `true`. This is not used in Node.js and is provided purely for completeness.

##### `event.cancelable`#

Added in: v14.5.0

- Type: `<boolean>` True if the event was created with the `cancelable` option.

##### `event.composed`#

Added in: v14.5.0

- Type: `<boolean>` Always returns `false`.

This is not used in Node.js and is provided purely for completeness.

##### `event.composedPath()`#

Added in: v14.5.0

Returns an array containing the current `EventTarget` as the only entry or empty if the event is not being dispatched. This is not used in Node.js and is provided purely for completeness.

##### `event.currentTarget`#

Added in: v14.5.0

- Type: `<EventTarget>` The `EventTarget` dispatching the event.

Alias for `event.target`.

##### `event.defaultPrevented`#

Added in: v14.5.0

- Type: `<boolean>`

Is `true` if `cancelable` is `true` and `event.preventDefault()` has been called.

##### `event.eventPhase`#

Added in: v14.5.0

- Type: `<number>` Returns `0` while an event is not being dispatched, `2` while it is being dispatched.

This is not used in Node.js and is provided purely for completeness.

##### `event.initEvent(type[, bubbles[, cancelable]])`#

Added in: v19.5.0

Stability: 3 - Legacy: The WHATWG spec considers it deprecated and users shouldn't use it at all.

- `type` `<string>`
- `bubbles` `<boolean>`
- `cancelable` `<boolean>`

Redundant with event constructors and incapable of setting `composed`. This is not used in Node.js and is provided purely for completeness.

##### `event.isTrusted`#

Added in: v14.5.0

- Type: `<boolean>`

The `<AbortSignal>` `"abort"` event is emitted with `isTrusted` set to `true`. The value is `false` in all other cases.

##### `event.preventDefault()`#

Added in: v14.5.0

Sets the `defaultPrevented` property to `true` if `cancelable` is `true`.

##### `event.returnValue`#

Added in: v14.5.0

Stability: 3 - Legacy: Use `event.defaultPrevented` instead.

- Type: `<boolean>` True if the event has not been canceled.

The value of `event.returnValue` is always the opposite of `event.defaultPrevented`. This is not used in Node.js and is provided purely for completeness.

##### `event.srcElement`#

Added in: v14.5.0

Stability: 3 - Legacy: Use `event.target` instead.

- Type: `<EventTarget>` The `EventTarget` dispatching the event.

Alias for `event.target`.

##### `event.stopImmediatePropagation()`#

Added in: v14.5.0

Stops the invocation of event listeners after the current one completes.

##### `event.stopPropagation()`#

Added in: v14.5.0

This is not used in Node.js and is provided purely for completeness.

##### `event.target`#

Added in: v14.5.0

- Type: `<EventTarget>` The `EventTarget` dispatching the event.

##### `event.timeStamp`#

Added in: v14.5.0

- Type: `<number>`

The millisecond timestamp when the `Event` object was created.

##### `event.type`#

Added in: v14.5.0

- Type: `<string>`

The event type identifier.

#### Class: `EventTarget`#

Added in: v14.5.0
History

Version | Changes
--- | ---
v15.0.0 | The `EventTarget` class is now available through the global object.

##### `eventTarget.addEventListener(type, listener[, options])`#

Added in: v14.5.0
History

Version | Changes
--- | ---
v15.4.0 | add support for `signal` option.

- `type` `<string>`
- `listener` `<Function>` | `<EventListener>`
- `options` `<Object>`
  - `once` `<boolean>` When `true`, the listener is automatically removed when it is first invoked. Default: `false`.
  - `passive` `<boolean>` When `true`, serves as a hint that the listener will not call the `Event` object's `preventDefault()` method. Default: `false`.
  - `capture` `<boolean>` Not directly used by Node.js. Added for API completeness. Default: `false`.
  - `signal` `<AbortSignal>` The listener will be removed when the given AbortSignal object's `abort()` method is called.

Adds a new handler for the `type` event. Any given `listener` is added only once per `type` and per `capture` option value.

If the `once` option is `true`, the `listener` is removed after the next time a `type` event is dispatched.

The `capture` option is not used by Node.js in any functional way other than tracking registered event listeners per the `EventTarget` specification. Specifically, the `capture` option is used as part of the key when registering a `listener`. Any individual `listener` may be added once with `capture = false`, and once with `capture = true`.

    function handler(event) {}

    const target = new EventTarget();
    target.addEventListener('foo', handler, { capture: true });  // first
    target.addEventListener('foo', handler, { capture: false }); // second

    // Removes the second instance of handler
    target.removeEventListener('foo', handler);

    // Removes the first instance of handler
    target.removeEventListener('foo', handler, { capture: true });
    js

##### `eventTarget.dispatchEvent(event)`#

Added in: v14.5.0

- `event` `<Event>`
- Returns: `<boolean>` `true` if either event's `cancelable` attribute value is false or its `preventDefault()` method was not invoked, otherwise `false`.

Dispatches the `event` to the list of handlers for `event.type`.

The registered event listeners is synchronously invoked in the order they were registered.

##### `eventTarget.removeEventListener(type, listener[, options])`#

Added in: v14.5.0

- `type` `<string>`
- `listener` `<Function>` | `<EventListener>`
- `options` `<Object>`
  - `capture` `<boolean>`

Removes the `listener` from the list of handlers for event `type`.

#### Class: `CustomEvent`#

Added in: v18.7.0, v16.17.0
History

Version | Changes
--- | ---
v23.0.0 | No longer experimental.
v22.1.0, v20.13.0 | CustomEvent is now stable.
v19.0.0 | No longer behind `--experimental-global-customevent` CLI flag.

- Extends: `<Event>`

The `CustomEvent` object is an adaptation of the `CustomEvent` Web API. Instances are created internally by Node.js.

##### `event.detail`#

Added in: v18.7.0, v16.17.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | CustomEvent is now stable.

- Type: `<any>` Returns custom data passed when initializing.

Read-only.

#### Class: `NodeEventTarget`#

Added in: v14.5.0

- Extends: `<EventTarget>`

The `NodeEventTarget` is a Node.js-specific extension to `EventTarget` that emulates a subset of the `EventEmitter` API.

##### `nodeEventTarget.addListener(type, listener)`#

Added in: v14.5.0

- `type` `<string>`

- `listener` `<Function>` | `<EventListener>`

- Returns: `<EventTarget>` this

Node.js-specific extension to the `EventTarget` class that emulates the equivalent `EventEmitter` API. The only difference between `addListener()` and `addEventListener()` is that `addListener()` will return a reference to the `EventTarget`.

##### `nodeEventTarget.emit(type, arg)`#

Added in: v15.2.0

- `type` `<string>`
- `arg` `<any>`
- Returns: `<boolean>` `true` if event listeners registered for the `type` exist, otherwise `false`.

Node.js-specific extension to the `EventTarget` class that dispatches the `arg` to the list of handlers for `type`.

##### `nodeEventTarget.eventNames()`#

Added in: v14.5.0

- Returns: `<string>`[]

Node.js-specific extension to the `EventTarget` class that returns an array of event `type` names for which event listeners are registered.

##### `nodeEventTarget.listenerCount(type)`#

Added in: v14.5.0

- `type` `<string>`

- Returns: `<number>`

Node.js-specific extension to the `EventTarget` class that returns the number of event listeners registered for the `type`.

##### `nodeEventTarget.setMaxListeners(n)`#

Added in: v14.5.0

- `n` `<number>`

Node.js-specific extension to the `EventTarget` class that sets the number of max event listeners as `n`.

##### `nodeEventTarget.getMaxListeners()`#

Added in: v14.5.0

- Returns: `<number>`

Node.js-specific extension to the `EventTarget` class that returns the number of max event listeners.

##### `nodeEventTarget.off(type, listener[, options])`#

Added in: v14.5.0

- `type` `<string>`

- `listener` `<Function>` | `<EventListener>`

- `options` `<Object>`

  - `capture` `<boolean>`

- Returns: `<EventTarget>` this

Node.js-specific alias for `eventTarget.removeEventListener()`.

##### `nodeEventTarget.on(type, listener)`#

Added in: v14.5.0

- `type` `<string>`

- `listener` `<Function>` | `<EventListener>`

- Returns: `<EventTarget>` this

Node.js-specific alias for `eventTarget.addEventListener()`.

##### `nodeEventTarget.once(type, listener)`#

Added in: v14.5.0

- `type` `<string>`

- `listener` `<Function>` | `<EventListener>`

- Returns: `<EventTarget>` this

Node.js-specific extension to the `EventTarget` class that adds a `once` listener for the given event `type`. This is equivalent to calling `on` with the `once` option set to `true`.

##### `nodeEventTarget.removeAllListeners([type])`#

Added in: v14.5.0

- `type` `<string>`

- Returns: `<EventTarget>` this

Node.js-specific extension to the `EventTarget` class. If `type` is specified, removes all registered listeners for `type`, otherwise removes all registered listeners.

##### `nodeEventTarget.removeListener(type, listener[, options])`#

Added in: v14.5.0

- `type` `<string>`

- `listener` `<Function>` | `<EventListener>`

- `options` `<Object>`

  - `capture` `<boolean>`

- Returns: `<EventTarget>` this

Node.js-specific extension to the `EventTarget` class that removes the `listener` for the given `type`. The only difference between `removeListener()` and `removeEventListener()` is that `removeListener()` will return a reference to the `EventTarget`.

## FFI#

Source Code: lib/ffi.js Added in: v26.1.0

Stability: 1 - Experimental

The `node:ffi` module provides an experimental foreign function interface for loading dynamic libraries and calling native symbols from JavaScript.

This API is unsafe. Passing invalid pointers, using an incorrect symbol signature, or accessing memory after it has been freed can crash the process or corrupt memory.

To access it:

    import ffi from 'node:ffi';
    const ffi = require('node:ffi');
    javascript

This module is only available under the `node:` scheme in builds with FFI support and is gated by the `--experimental-ffi` flag.

Building Node.js with `node:ffi` support is available via the bundled `libffi` on platforms where `libffi` provides a compatible static backend, or via a shared `libffi` using the `--shared-ffi` configure flag. The unofficial GN build does not support `node:ffi`.

The following targets are not supported by bundled libffi:

- `s390x`.
- `mips`, `mipsel`, and `mips64el` on targets other than FreeBSD, Linux, and OpenBSD.
- `ppc64` on Android, CloudABI, iOS, OpenHarmony, OS/400, Solaris, and Windows.

When using the Permission Model, FFI APIs are restricted unless the `--allow-ffi` flag is provided.

### Overview#

The `node:ffi` module exposes two groups of APIs:

- Dynamic library APIs for loading libraries, resolving symbols, and creating callable JavaScript wrappers.
- Raw memory helpers for reading and writing primitive values through pointers, converting pointers to JavaScript strings, `Buffer` instances, and `ArrayBuffer` instances, and for copying data back into native memory.

### Type names#

FFI signatures use string type names.

Supported type names:

- `void`
- `char`
- `i8`, `int8`
- `u8`, `uint8`, `bool`
- `i16`, `int16`
- `u16`, `uint16`
- `i32`, `int32`
- `u32`, `uint32`
- `i64`, `int64`
- `u64`, `uint64`
- `f32`, `float`, `float32`
- `f64`, `double`, `float64`
- `pointer`, `ptr`
- `string`, `str`
- `buffer`
- `arraybuffer`
- `function`

These type names are also exposed as constants on `ffi.types`:

- `ffi.types.VOID` = `'void'`
- `ffi.types.POINTER` = `'pointer'`
- `ffi.types.BUFFER` = `'buffer'`
- `ffi.types.ARRAY_BUFFER` = `'arraybuffer'`
- `ffi.types.FUNCTION` = `'function'`
- `ffi.types.BOOL` = `'bool'`
- `ffi.types.CHAR` = `'char'`
- `ffi.types.STRING` = `'string'`
- `ffi.types.FLOAT` = `'float'`
- `ffi.types.DOUBLE` = `'double'`
- `ffi.types.INT_8` = `'int8'`
- `ffi.types.UINT_8` = `'uint8'`
- `ffi.types.INT_16` = `'int16'`
- `ffi.types.UINT_16` = `'uint16'`
- `ffi.types.INT_32` = `'int32'`
- `ffi.types.UINT_32` = `'uint32'`
- `ffi.types.INT_64` = `'int64'`
- `ffi.types.UINT_64` = `'uint64'`
- `ffi.types.FLOAT_32` = `'float32'`
- `ffi.types.FLOAT_64` = `'float64'`

Pointer-like types (`pointer`, `string`, `buffer`, `arraybuffer`, and `function`) are all passed through the native layer as pointers.

When `Buffer`, `ArrayBuffer`, or typed array values are passed as pointer-like arguments, Node.js borrows a raw pointer to their backing memory for the duration of the native call. The caller must ensure that backing store remains valid and stable for the entire call.

It is unsupported and dangerous to resize, transfer, detach, or otherwise invalidate that backing store while the native call is active, including through reentrant JavaScript such as FFI callbacks. Doing so may crash the process, produce incorrect output, or corrupt memory.

The `char` type follows the platform C ABI. On platforms where plain C `char` is signed it behaves like `i8`; otherwise it behaves like `u8`.

The `bool` type is marshaled as an 8-bit unsigned integer. Pass numeric values such as `0` and `1`; JavaScript `true` and `false` are not accepted.

On optimized Fast FFI calls, `pointer`, `ptr`, and `function` parameters accept raw pointer `bigint` values. For pointer-like parameters, `null`, `undefined`, strings, `Buffer`, typed array, `DataView`, and `ArrayBuffer` values are converted on the JavaScript side before calling the optimized native wrapper.

Optimized Fast FFI calls support at most 8 function arguments, but the exact limit depends on the architecture and on the argument types, because each argument must fit in the registers used by the platform trampoline. Integer and pointer arguments are limited to 7 on AArch64 and to 6 on x86-64, while floating-point arguments can use up to 8 on both. Functions that exceed these limits, including any function with more than 8 arguments, use the generic FFI call path instead.

### Signature objects#

Functions and callbacks are described with signature objects.

Signature objects may contain the following properties, both of which are optional:

- `return` `<string>` A type name specifying the return type of the function or callback. Default: `'void'`.
- `arguments` `<string>`[] An array of type names specifying the argument type list of the function or callback. Default: `[]`.

    const signature = {
      return: 'i32',
      arguments: ['i32', 'i32'],
    };
    js

### `ffi.suffix`#

Added in: v26.1.0

- `<string>`

The native shared library suffix for the current platform:

- `'dylib'` on macOS
- `'so'` on Unix-like platforms
- `'dll'` on Windows

This can be used to build portable library paths:

    const { suffix } = require('node:ffi');

    const path = `libsqlite3.${suffix}`;
    cjs

### `ffi.dlopen(path[, definitions])`#

Added in: v26.1.0

- `path` `<string>` | `<null>` Path to a dynamic library, or `null` to resolve symbols from the current process image.
- `definitions` `<Object>` Symbol definitions to resolve immediately.
- Returns: `<Object>`

Loads a dynamic library and resolves the requested function definitions.

On Windows passing `null` is not supported.

When `definitions` is omitted, `functions` is returned as an empty object until symbols are resolved explicitly.

The returned object contains:

- `lib` `<DynamicLibrary>` The loaded library handle.
- `functions` `<Object>` Callable wrappers for the requested symbols.

The returned object also implements the explicit resource management protocol, so it can be used with the `using` declaration. Disposing the returned object closes the library handle.

    import { dlopen, suffix } from 'node:ffi';

    {
      using handle = dlopen(`./mylib.${suffix}`, {
        add_i32: { arguments: ['i32', 'i32'], return: 'i32' },
      });
      console.log(handle.functions.add_i32(20, 22));
    } // handle.lib.close() is invoked automatically here.
    mjs

    import { dlopen, suffix } from 'node:ffi';

    const { lib, functions } = dlopen(`./mylib.${suffix}`, {
      add_i32: { arguments: ['i32', 'i32'], return: 'i32' },
      string_length: { arguments: ['pointer'], return: 'u64' },
    });

    console.log(functions.add_i32(20, 22));
    const { dlopen, suffix } = require('node:ffi');

    const { lib, functions } = dlopen(`./mylib.${suffix}`, {
      add_i32: { arguments: ['i32', 'i32'], return: 'i32' },
      string_length: { arguments: ['pointer'], return: 'u64' },
    });

    console.log(functions.add_i32(20, 22));
    javascript

### `ffi.dlclose(handle)`#

Added in: v26.1.0

- `handle` `<DynamicLibrary>`

Closes a dynamic library.

This is equivalent to calling `handle.close()`.

### `ffi.dlsym(handle, symbol)`#

Added in: v26.1.0

- `handle` `<DynamicLibrary>`
- `symbol` `<string>`
- Returns: `<bigint>`

Resolves a symbol address from a loaded library.

This is equivalent to calling `handle.getSymbol(symbol)`.

### Class: `DynamicLibrary`#

Added in: v26.1.0

Represents a loaded dynamic library.

#### `new DynamicLibrary(path)`#

- `path` `<string>` | `<null>` Path to a dynamic library, or `null` to resolve symbols from the current process image.

Loads the dynamic library without resolving any functions eagerly.

On Windows passing `null` is not supported.

    const { DynamicLibrary, suffix } = require('node:ffi');

    const lib = new DynamicLibrary(`./mylib.${suffix}`);
    cjs

#### `library.path`#

- `<string>`

The path used to load the library.

#### `library.functions`#

- `<Object>`

An object containing previously resolved function wrappers.

#### `library.symbols`#

- `<Object>`

An object containing previously resolved symbol addresses as `bigint` values.

#### `library.close()`#

Closes the library handle.

`DynamicLibrary` implements the explicit resource management protocol, so a library instance can be managed with the `using` declaration. Leaving the enclosing scope invokes `library.close()` automatically.

    import { DynamicLibrary, suffix } from 'node:ffi';

    {
      using lib = new DynamicLibrary(`./mylib.${suffix}`);
      // Use `lib` here; `lib.close()` is called when the block exits.
    }
    mjs

Calling `library.close()` (or disposing the library) more than once is a no-op.

After a library has been closed:

- Resolved function wrappers become invalid.
- Further symbol and function resolution throws.
- Registered callbacks are invalidated.

Closing a library does not make previously exported callback pointers safe to reuse. Node.js does not track or revoke callback pointers that have already been handed to native code.

If native code still holds a callback pointer after `library.close()` or after `library.unregisterCallback(pointer)`, invoking that pointer has undefined behavior, is not allowed, and is dangerous: it can crash the process, produce incorrect output, or corrupt memory. Native code must stop using callback addresses before the library is closed or before the callback is unregistered.

Calling `library.close()` from one of the library's active callbacks is unsupported and dangerous. The callback must return before the library is closed.

#### `library[Symbol.dispose]()`#

Added in: v26.1.0

Calls `library.close()`. This allows `DynamicLibrary` instances to be used with the `using` declaration for automatic cleanup when the enclosing scope exits. It is a no-op on a library that has already been closed.

#### `library.getFunction(name, signature)`#

- `name` `<string>`
- `signature` `<Object>`
- Returns: `<Function>`

Resolves a symbol and returns a callable JavaScript wrapper.

The returned function has a `.pointer` property containing the native function address as a `bigint`.

If the same symbol has already been resolved, requesting it again with a different signature throws.

    const { DynamicLibrary, suffix } = require('node:ffi');

    const lib = new DynamicLibrary(`./mylib.${suffix}`);
    const add = lib.getFunction('add_i32', {
      arguments: ['i32', 'i32'],
      return: 'i32',
    });

    console.log(add(20, 22));
    console.log(add.pointer);
    cjs

#### `library.getFunctions([definitions])`#

- `definitions` `<Object>`
- Returns: `<Object>`

When `definitions` is provided, resolves each named symbol and returns an object containing callable wrappers.

When `definitions` is omitted, returns wrappers for all functions that have already been resolved on the library.

#### `library.getSymbol(name)`#

- `name` `<string>`
- Returns: `<bigint>`

Resolves a symbol and returns its native address as a `bigint`.

#### `library.getSymbols()`#

- Returns: `<Object>`

Returns an object containing all previously resolved symbol addresses.

#### `library.registerCallback([signature,] callback)`#

- `signature` `<Object>`
- `callback` `<Function>`
- Returns: `<bigint>`

Creates a native callback pointer backed by a JavaScript function.

When `signature` is omitted, the callback uses a default `void ()` signature.

The return value is the callback pointer address as a `bigint`. It can be passed to native functions expecting a callback pointer.

    const { DynamicLibrary, suffix } = require('node:ffi');

    const lib = new DynamicLibrary(`./mylib.${suffix}`);

    const callback = lib.registerCallback(
      { arguments: ['i32'], return: 'i32' },
      (value) => value * 2,
    );
    cjs

Callbacks are subject to the following restrictions:

- They must be invoked on the same system thread where they were created.
- They must not throw exceptions.
- They must not return promises.
- They must return a value compatible with the declared return type.
- They must not call `library.close()` on their owning library while running.
- They must not unregister themselves while running.

Closing the owning library or unregistering the currently executing callback from inside the callback is unsupported and dangerous. Doing so may crash the process, produce incorrect output, or corrupt memory.

#### `library.unregisterCallback(pointer)`#

- `pointer` `<bigint>`

Releases a callback previously created with `library.registerCallback()`.

Calling `library.unregisterCallback(pointer)` for a callback that is currently executing is unsupported and dangerous. The callback must return before it is unregistered.

After `library.unregisterCallback(pointer)` returns, invoking that callback pointer from native code has undefined behavior, is not allowed, and is dangerous: it can crash the process, produce incorrect output, or corrupt memory.

#### `library.refCallback(pointer)`#

- `pointer` `<bigint>`

Keeps the callback strongly referenced by JavaScript.

Throws `ERR_INVALID_ARG_VALUE` if the callback function has already been garbage collected after a previous `library.unrefCallback(pointer)` call, since a collected function cannot be referenced again.

#### `library.unrefCallback(pointer)`#

- `pointer` `<bigint>`

Allows the callback to become weakly referenced by JavaScript.

If the callback function is later garbage collected, subsequent native invocations become a no-op. Non-void return values are zero-initialized before returning to native code.

Throws `ERR_INVALID_ARG_VALUE` if the callback function has already been garbage collected.

### Calling native functions#

Argument conversion depends on the declared FFI type.

For 8-, 16-, and 32-bit integer types and for floating-point types, pass JavaScript `number` values that match the declared type.

For 64-bit integer types (`i64` and `u64`), pass JavaScript `bigint` values.

For pointer-like arguments:

- `null` and `undefined` are passed as null pointers.
- `string` values are copied to temporary NUL-terminated UTF-8 strings for the duration of the call.
- `Buffer`, typed arrays, and `DataView` instances pass a pointer to their backing memory.
- `ArrayBuffer` passes a pointer to its backing memory.
- `bigint` values are passed as raw pointer addresses.

Pointer return values are exposed as `bigint` addresses.

### Primitive memory access helpers#

The following helpers read and write primitive values at a native pointer, optionally with a byte offset:

- `ffi.getInt8(pointer[, offset])`
- `ffi.getUint8(pointer[, offset])`
- `ffi.getInt16(pointer[, offset])`
- `ffi.getUint16(pointer[, offset])`
- `ffi.getInt32(pointer[, offset])`
- `ffi.getUint32(pointer[, offset])`
- `ffi.getInt64(pointer[, offset])`
- `ffi.getUint64(pointer[, offset])`
- `ffi.getFloat32(pointer[, offset])`
- `ffi.getFloat64(pointer[, offset])`
- `ffi.setInt8(pointer, offset, value)`
- `ffi.setUint8(pointer, offset, value)`
- `ffi.setInt16(pointer, offset, value)`
- `ffi.setUint16(pointer, offset, value)`
- `ffi.setInt32(pointer, offset, value)`
- `ffi.setUint32(pointer, offset, value)`
- `ffi.setInt64(pointer, offset, value)`
- `ffi.setUint64(pointer, offset, value)`
- `ffi.setFloat32(pointer, offset, value)`
- `ffi.setFloat64(pointer, offset, value)`

These helpers perform direct memory reads and writes. `pointer` must be a `bigint` referring to valid readable or writable native memory. `offset`, when provided, is interpreted as a byte offset from `pointer`.

The getter helpers return JavaScript `number` values for 8-, 16-, and 32-bit integer types and for floating-point types. They return `bigint` values for 64-bit integer types.

The setter helpers require an explicit byte offset and validate the supplied JavaScript value against the target native type before writing it into memory. For `setInt64()` and `setUint64()`, `bigint` values are accepted directly; numeric inputs must be integers within JavaScript's safe integer range.

    const {
      getInt32,
      setInt32,
    } = require('node:ffi');

    setInt32(ptr, 0, 42);
    console.log(getInt32(ptr, 0));
    cjs

Like the other raw memory helpers in this module, these APIs do not track ownership, bounds, or lifetime. Passing an invalid pointer, using the wrong offset, or writing through a stale pointer can corrupt memory or crash the process.

### `ffi.toString(pointer)`#

Added in: v26.1.0

- `pointer` `<bigint>`
- Returns: `<string>` | `<null>`

Reads a NUL-terminated UTF-8 string from native memory.

If `pointer` is `0n`, `null` is returned.

This function does not validate that `pointer` refers to readable memory or that the pointed-to data is terminated with `\0`. Passing an invalid pointer, a pointer to freed memory, or a pointer to bytes without a terminating NUL can read unrelated memory, crash the process, or produce truncated or garbled output.

    const { toString } = require('node:ffi');

    const value = toString(ptr);
    cjs

### `ffi.toBuffer(pointer, length[, copy])`#

Added in: v26.1.0

- `pointer` `<bigint>`
- `length` `<number>`
- `copy` `<boolean>` When `false`, creates a zero-copy view. Default: `true`.
- Returns: `<Buffer>`

Creates a `Buffer` from native memory.

When `copy` is `true`, the returned `Buffer` owns its own copied memory. When `copy` is `false`, the returned `Buffer` references the original native memory directly.

Using `copy: false` is a zero-copy escape hatch. The returned `Buffer` is a writable view onto foreign memory, so writes in JavaScript update the original native memory directly. The caller must guarantee that:

- `pointer` remains valid for the entire lifetime of the returned `Buffer`.
- `length` stays within the allocated native region.
- no native code frees or repurposes that memory while JavaScript still uses the `Buffer`.
- Memory protection is observed. For example, read-only memory pages must not be written to.

If these guarantees are not met, reading or writing the `Buffer` can corrupt memory or crash the process.

### `ffi.toArrayBuffer(pointer, length[, copy])`#

Added in: v26.1.0

- `pointer` `<bigint>`
- `length` `<number>`
- `copy` `<boolean>` When `false`, creates a zero-copy view. Default: `true`.
- Returns: `<ArrayBuffer>`

Creates an `ArrayBuffer` from native memory.

When `copy` is `true`, the returned `ArrayBuffer` contains copied bytes. When `copy` is `false`, the returned `ArrayBuffer` references the original native memory directly.

The same lifetime and bounds requirements described for `ffi.toBuffer(pointer, length, copy)` apply here. With `copy: false`, the returned `ArrayBuffer` is a zero-copy view of foreign memory and is only safe while that memory remains allocated, unchanged in layout, and valid for the entire exposed range.

### `ffi.exportString(string, pointer, length[, encoding])`#

Added in: v26.1.0

- `string` `<string>`
- `pointer` `<bigint>`
- `length` `<number>`
- `encoding` `<string>` Default: `'utf8'`.

Copies a JavaScript string into native memory and appends a trailing NUL terminator.

`length` must be large enough to hold the full encoded string plus the trailing NUL terminator. For UTF-16 and UCS-2 encodings, the trailing terminator uses two zero bytes.

`pointer` must refer to writable native memory with at least `length` bytes of available storage. This function does not allocate memory on its own.

`string` must be a JavaScript string. `encoding` must be a string.

### `ffi.exportBuffer(buffer, pointer, length)`#

Added in: v26.1.0

- `buffer` `<Buffer>`
- `pointer` `<bigint>`
- `length` `<number>`

Copies bytes from a `Buffer` into native memory.

`length` must be at least `buffer.length`.

`pointer` must refer to writable native memory with at least `length` bytes of available storage. This function does not allocate memory on its own.

`buffer` must be a Node.js `Buffer`.

### `ffi.exportArrayBuffer(arrayBuffer, pointer, length)`#

Added in: v26.1.0

- `arrayBuffer` `<ArrayBuffer>`
- `pointer` `<bigint>`
- `length` `<number>`

Copies bytes from an `ArrayBuffer` into native memory.

`length` must be at least `arrayBuffer.byteLength`.

`pointer` must refer to writable native memory with at least `length` bytes of available storage. This function does not allocate memory on its own.

### `ffi.exportArrayBufferView(arrayBufferView, pointer, length)`#

Added in: v26.1.0

- `arrayBufferView` `<ArrayBufferView>`
- `pointer` `<bigint>`
- `length` `<number>`

Copies bytes from an `ArrayBufferView` into native memory.

`length` must be at least `arrayBufferView.byteLength`.

`pointer` must refer to writable native memory with at least `length` bytes of available storage. This function does not allocate memory on its own.

### `ffi.getRawPointer(source)`#

Added in: v26.1.0

- `source` `<Buffer>` | `<ArrayBuffer>` | `<ArrayBufferView>`
- Returns: `<bigint>`

Returns the raw memory address of JavaScript-managed byte storage.

This is unsafe and dangerous. The returned pointer can become invalid if the underlying memory is detached, resized, transferred, or otherwise invalidated. Using stale pointers can cause memory corruption or process crashes.

### `ffi.getCurrentEventLoop()`#

Added in: v26.6.0

- Returns: `<bigint>`

Returns the address of the current thread's `uv_loop_t` as a `bigint`.

The returned address is for the current Node.js environment. In the main thread, this is the main thread event loop. In a worker thread, this is that worker's event loop.

This is unsafe and dangerous. The returned pointer is only valid for the lifetime of the current environment. Using it after the environment exits, or from native code that assumes a different thread or lifetime, can crash the process or corrupt memory.

### Safety notes#

The `node:ffi` module does not track pointer validity, memory ownership, or native object lifetimes.

In particular:

- Do not read from or write to freed memory.
- Do not use zero-copy views after the native memory has been released.
- Do not declare incorrect signatures for native symbols.
- Do not unregister callbacks while native code may still call them.
- Do not call callback pointers after `library.close()` or `library.unregisterCallback(pointer)`.
- Assume undefined callback behavior can crash the process, produce incorrect output, or corrupt memory.
- Do not assume pointer return values imply ownership; whether the caller must free the returned address depends entirely on the native API.

As a general rule, prefer copied values unless zero-copy access is required, and keep callback and pointer lifetimes explicit on the native side.

## File system#

Source Code: lib/fs.js

Stability: 2 - Stable

The `node:fs` module enables interacting with the file system in a way modeled on standard POSIX functions.

To use the promise-based APIs:

    import * as fs from 'node:fs/promises';
    const fs = require('node:fs/promises');
    javascript

To use the callback and sync APIs:

    import * as fs from 'node:fs';
    const fs = require('node:fs');
    javascript

All file system operations have synchronous, callback, and promise-based forms, and are accessible using both CommonJS syntax and ES6 Modules (ESM).

### Promise example#

Promise-based operations return a promise that is fulfilled when the asynchronous operation is complete.

    import { unlink } from 'node:fs/promises';

    try {
      await unlink('/tmp/hello');
      console.log('successfully deleted /tmp/hello');
    } catch (error) {
      console.error('there was an error:', error.message);
    }
    const { unlink } = require('node:fs/promises');

    (async function(path) {
      try {
        await unlink(path);
        console.log(`successfully deleted ${path}`);
      } catch (error) {
        console.error('there was an error:', error.message);
      }
    })('/tmp/hello');
    javascript

### Callback example#

The callback form takes a completion callback function as its last argument and invokes the operation asynchronously. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation is completed successfully, then the first argument is `null` or `undefined`.

    import { unlink } from 'node:fs';

    unlink('/tmp/hello', (err) => {
      if (err) throw err;
      console.log('successfully deleted /tmp/hello');
    });
    const { unlink } = require('node:fs');

    unlink('/tmp/hello', (err) => {
      if (err) throw err;
      console.log('successfully deleted /tmp/hello');
    });
    javascript

The callback-based versions of the `node:fs` module APIs are preferable over the use of the promise APIs when maximal performance (both in terms of execution time and memory allocation) is required.

### Synchronous example#

The synchronous APIs block the Node.js event loop and further JavaScript execution until the operation is complete. Exceptions are thrown immediately and can be handled using `try…catch`, or can be allowed to bubble up.

    import { unlinkSync } from 'node:fs';

    try {
      unlinkSync('/tmp/hello');
      console.log('successfully deleted /tmp/hello');
    } catch (err) {
      // handle the error
    }
    const { unlinkSync } = require('node:fs');

    try {
      unlinkSync('/tmp/hello');
      console.log('successfully deleted /tmp/hello');
    } catch (err) {
      // handle the error
    }
    javascript

### Promises API#

Added in: v10.0.0
History

Version | Changes
--- | ---
v14.0.0 | Exposed as `require('fs/promises')`.
v11.14.0, v10.17.0 | This API is no longer experimental.
v10.1.0 | The API is accessible via `require('fs').promises` only.

The `fs/promises` API provides asynchronous file system methods that return promises.

The promise APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.

#### Class: `FileHandle`#

Added in: v10.0.0

A `<FileHandle>` object is an object wrapper for a numeric file descriptor.

Instances of the `<FileHandle>` object are created by the `fsPromises.open()` method.

All `<FileHandle>` objects are `<EventEmitter>`s.

If a `<FileHandle>` is not closed using the `filehandle.close()` method, it will try to automatically close the file descriptor and emit a process warning, helping to prevent memory leaks. Please do not rely on this behavior because it can be unreliable and the file may not be closed. Instead, always explicitly close `<FileHandle>`s. Node.js may change this behavior in the future.

##### Event: `'close'`#

Added in: v15.4.0

The `'close'` event is emitted when the `<FileHandle>` has been closed and can no longer be used.

##### `filehandle.appendFile(data[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v21.1.0, v20.10.0 | The `flush` option is now supported.
v15.14.0, v14.18.0 | The `data` argument supports `AsyncIterable`, `Iterable`, and `Stream`.
v14.0.0 | The `data` parameter won't coerce unsupported input to strings anymore.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<AsyncIterable>` | `<Iterable>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `signal` `<AbortSignal>` | `<undefined>` allows aborting an in-progress writeFile. Default: `undefined`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Alias of `filehandle.writeFile()`.

When operating on file handles, the mode cannot be changed from what it was set to with `fsPromises.open()`. Therefore, this is equivalent to `filehandle.writeFile()`.

##### `filehandle.chmod(mode)`#

Added in: v10.0.0

- `mode` `<integer>` the file mode bit mask.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Modifies the permissions on the file. See `chmod(2)`.

##### `filehandle.chown(uid, gid)`#

Added in: v10.0.0

- `uid` `<integer>` The file's new owner's user id.
- `gid` `<integer>` The file's new group's group id.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Changes the ownership of the file. A wrapper for `chown(2)`.

##### `filehandle.close()`#

Added in: v10.0.0

- Returns: `<Promise>` Fulfills with `undefined` upon success.

Closes the file handle after waiting for any pending operation on the handle to complete.

    import { open } from 'node:fs/promises';

    let filehandle;
    try {
      filehandle = await open('thefile.txt', 'r');
    } finally {
      await filehandle?.close();
    }
    mjs

##### `filehandle.createReadStream([options])`#

Added in: v16.11.0

- `options` `<Object>`
  - `encoding` `<string>` Default: `null`
  - `autoClose` `<boolean>` Default: `true`
  - `emitClose` `<boolean>` Default: `true`
  - `start` `<integer>`
  - `end` `<integer>` Default: `Infinity`
  - `highWaterMark` `<integer>` Default: `64 * 1024`
  - `signal` `<AbortSignal>` | `<undefined>` Default: `undefined`
- Returns: `<fs.ReadStream>`

`options` can include `start` and `end` values to read a range of bytes from the file instead of the entire file. Both `start` and `end` are inclusive and start counting at 0, allowed values are in the [0, `Number.MAX_SAFE_INTEGER`] range. If `start` is omitted or `undefined`, `filehandle.createReadStream()` reads sequentially from the current file position. The `encoding` can be any one of those accepted by `<Buffer>`.

If the `FileHandle` points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally.

By default, the stream will emit a `'close'` event after it has been destroyed. Set the `emitClose` option to `false` to change this behavior.

    import { open } from 'node:fs/promises';

    const fd = await open('/dev/input/event0');
    // Create a stream from some character device.
    const stream = fd.createReadStream();
    setTimeout(() => {
      stream.close(); // This may not close the stream.
      // Artificially marking end-of-stream, as if the underlying resource had
      // indicated end-of-file by itself, allows the stream to close.
      // This does not cancel pending read operations, and if there is such an
      // operation, the process may still not be able to exit successfully
      // until it finishes.
      stream.push(null);
      stream.read(0);
    }, 100);
    mjs

If `autoClose` is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If `autoClose` is set to true (default behavior), on `'error'` or `'end'` the file descriptor will be closed automatically.

An example to read the last 10 bytes of a file which is 100 bytes long:

    import { open } from 'node:fs/promises';

    const fd = await open('sample.txt');
    fd.createReadStream({ start: 90, end: 99 });
    mjs

##### `filehandle.createWriteStream([options])`#

Added in: v16.11.0
History

Version | Changes
--- | ---
v21.0.0, v20.10.0 | The `flush` option is now supported.

- `options` `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
  - `autoClose` `<boolean>` Default: `true`
  - `emitClose` `<boolean>` Default: `true`
  - `start` `<integer>`
  - `highWaterMark` `<number>` Default: `16384`
  - `flush` `<boolean>` If `true`, the underlying file descriptor is flushed prior to closing it. Default: `false`.
- Returns: `<fs.WriteStream>`

`options` may also include a `start` option to allow writing data at some position past the beginning of the file, allowed values are in the [0, `Number.MAX_SAFE_INTEGER`] range. Modifying a file rather than replacing it may require the `flags` `open` option to be set to `r+` rather than the default `r`. The `encoding` can be any one of those accepted by `<Buffer>`.

If `autoClose` is set to true (default behavior) on `'error'` or `'finish'` the file descriptor will be closed automatically. If `autoClose` is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak.

By default, the stream will emit a `'close'` event after it has been destroyed. Set the `emitClose` option to `false` to change this behavior.

##### `filehandle.datasync()`#

Added in: v10.0.0

- Returns: `<Promise>` Fulfills with `undefined` upon success.

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX `fdatasync(2)` documentation for details.

Unlike `filehandle.sync` this method does not flush modified metadata.

##### `filehandle.fd`#

Added in: v10.0.0

- Type: `<number>` The numeric file descriptor managed by the `<FileHandle>` object.

##### `filehandle.pull([...transforms][, options])`#

Added in: v25.9.0

Stability: 1 - Experimental

- `...transforms` `<Function>` | `<Object>` Optional transforms to apply via `stream/iter pull()`.
- `options` `<Object>`
  - `signal` `<AbortSignal>`
  - `autoClose` `<boolean>` Close the file handle when the stream ends. Default: `false`.
  - `start` `<number>` Byte offset to begin reading from. When specified, reads use explicit positioning (`pread` semantics). Default: current file position.
  - `limit` `<number>` Maximum number of bytes to read before ending the iterator. Reads stop when `limit` bytes have been delivered or EOF is reached, whichever comes first. Default: read until EOF.
  - `chunkSize` `<number>` Size in bytes of the buffer allocated for each read operation. Default: `131072` (128 KB).
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Return the file contents as an async iterable using the `node:stream/iter` pull model. Reads are performed in `chunkSize`-byte chunks (default 128 KB). If transforms are provided, they are applied via `stream/iter pull()`.

The file handle is locked while the iterable is being consumed and unlocked when iteration completes, an error occurs, or the consumer breaks.

This function is only available when the `--experimental-stream-iter` flag is enabled.

    import { open } from 'node:fs/promises';
    import { text } from 'node:stream/iter';
    import { compressGzip } from 'node:zlib/iter';

    const fh = await open('input.txt', 'r');

    // Read as text
    console.log(await text(fh.pull({ autoClose: true })));

    // Read 1 KB starting at byte 100
    const fh2 = await open('input.txt', 'r');
    console.log(await text(fh2.pull({ start: 100, limit: 1024, autoClose: true })));

    // Read with compression
    const fh3 = await open('input.txt', 'r');
    const compressed = fh3.pull(compressGzip(), { autoClose: true });
    const { open } = require('node:fs/promises');
    const { text } = require('node:stream/iter');
    const { compressGzip } = require('node:zlib/iter');

    async function run() {
      const fh = await open('input.txt', 'r');

      // Read as text
      console.log(await text(fh.pull({ autoClose: true })));

      // Read 1 KB starting at byte 100
      const fh2 = await open('input.txt', 'r');
      console.log(await text(fh2.pull({ start: 100, limit: 1024, autoClose: true })));

      // Read with compression
      const fh3 = await open('input.txt', 'r');
      const compressed = fh3.pull(compressGzip(), { autoClose: true });
    }

    run().catch(console.error);
    javascript

##### `filehandle.pullSync([...transforms][, options])`#

Added in: v25.9.0

Stability: 1 - Experimental

- `...transforms` `<Function>` | `<Object>` Optional transforms to apply via `stream/iter pullSync()`.
- `options` `<Object>`
  - `autoClose` `<boolean>` Close the file handle when the stream ends. Default: `false`.
  - `start` `<number>` Byte offset to begin reading from. When specified, reads use explicit positioning. Default: current file position.
  - `limit` `<number>` Maximum number of bytes to read before ending the iterator. Default: read until EOF.
  - `chunkSize` `<number>` Size in bytes of the buffer allocated for each read operation. Default: `131072` (128 KB).
- Returns: `<Iterable>` whose chunks return `<Uint8Array>`[]

Synchronous counterpart of `filehandle.pull()`. Returns a sync iterable that reads the file using synchronous I/O on the main thread. Reads are performed in `chunkSize`-byte chunks (default 128 KB).

The file handle is locked while the iterable is being consumed. Unlike the async `pull()`, this method does not support `AbortSignal` since all operations are synchronous.

This function is only available when the `--experimental-stream-iter` flag is enabled.

    import { open } from 'node:fs/promises';
    import { textSync, pipeToSync } from 'node:stream/iter';
    import { compressGzipSync, decompressGzipSync } from 'node:zlib/iter';

    const fh = await open('input.txt', 'r');

    // Read as text (sync)
    console.log(textSync(fh.pullSync({ autoClose: true })));

    // Sync compress pipeline: file -> gzip -> file
    const src = await open('input.txt', 'r');
    const dst = await open('output.gz', 'w');
    pipeToSync(src.pullSync(compressGzipSync(), { autoClose: true }), dst.writer({ autoClose: true }));
    const { open } = require('node:fs/promises');
    const { textSync, pipeToSync } = require('node:stream/iter');
    const { compressGzipSync, decompressGzipSync } = require('node:zlib/iter');

    async function run() {
      const fh = await open('input.txt', 'r');

      // Read as text (sync)
      console.log(textSync(fh.pullSync({ autoClose: true })));

      // Sync compress pipeline: file -> gzip -> file
      const src = await open('input.txt', 'r');
      const dst = await open('output.gz', 'w');
      pipeToSync(
        src.pullSync(compressGzipSync(), { autoClose: true }),
        dst.writer({ autoClose: true }),
      );
    }

    run().catch(console.error);
    javascript

##### `filehandle.read(buffer, offset, length, position)`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v21.0.0 | Accepts bigint values as `position`.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A buffer that will be filled with the file data read.
- `offset` `<integer>` The location in the buffer at which to start filling. Default: `0`
- `length` `<integer>` The number of bytes to read. Default: `buffer.byteLength - offset`
- `position` `<integer>` | `<bigint>` | `<null>` The location where to begin reading data from the file. If `null` or `-1`, data will be read from the current file position, and the position will be updated. If `position` is a non-negative integer, the current file position will remain unchanged. Default: `null`
- Returns: `<Promise>` Fulfills upon success with an object with two properties:
  - `bytesRead` `<integer>` The number of bytes read
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A reference to the passed in `buffer` argument.

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

##### `filehandle.read([options])`#

Added in: v13.11.0, v12.17.0
History

Version | Changes
--- | ---
v21.0.0 | Accepts bigint values as `position`.

- `options` `<Object>`
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A buffer that will be filled with the file data read. Default: `Buffer.alloc(16384)`
  - `offset` `<integer>` The location in the buffer at which to start filling. Default: `0`
  - `length` `<integer>` The number of bytes to read. Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<bigint>` | `<null>` The location where to begin reading data from the file. If `null` or `-1`, data will be read from the current file position, and the position will be updated. If `position` is a non-negative integer, the current file position will remain unchanged. Default:: `null`
- Returns: `<Promise>` Fulfills upon success with an object with two properties:
  - `bytesRead` `<integer>` The number of bytes read
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A reference to the passed in `buffer` argument.

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

##### `filehandle.read(buffer[, options])`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v21.0.0 | Accepts bigint values as `position`.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A buffer that will be filled with the file data read.
- `options` `<Object>`
  - `offset` `<integer>` The location in the buffer at which to start filling. Default: `0`
  - `length` `<integer>` The number of bytes to read. Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<bigint>` | `<null>` The location where to begin reading data from the file. If `null` or `-1`, data will be read from the current file position, and the position will be updated. If `position` is a non-negative integer, the current file position will remain unchanged. Default:: `null`
- Returns: `<Promise>` Fulfills upon success with an object with two properties:
  - `bytesRead` `<integer>` The number of bytes read
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A reference to the passed in `buffer` argument.

Reads data from the file and stores that in the given buffer.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

##### `filehandle.readableWebStream([options])`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.
v23.8.0, v22.15.0 | Removed option to create a 'bytes' stream. Streams are now always 'bytes' streams.
v20.0.0, v18.17.0 | Added option to create a 'bytes' stream.

- `options` `<Object>`
  - `autoClose` `<boolean>` When true, causes the `<FileHandle>` to be closed when the stream is closed. Default: `false`
- Returns: `<ReadableStream>`

Returns a byte-oriented `ReadableStream` that may be used to read the file's contents.

An error will be thrown if this method is called more than once or is called after the `FileHandle` is closed or closing.

    import {
      open,
    } from 'node:fs/promises';

    const file = await open('./some/file/to/read');

    for await (const chunk of file.readableWebStream())
      console.log(chunk);

    await file.close();
    const {
      open,
    } = require('node:fs/promises');

    (async () => {
      const file = await open('./some/file/to/read');

      for await (const chunk of file.readableWebStream())
        console.log(chunk);

      await file.close();
    })();
    javascript

While the `ReadableStream` will read the file to completion, it will not close the `FileHandle` automatically. User code must still call the `fileHandle.close()` method unless the `autoClose` option is set to `true`.

##### `filehandle.readFile(options)`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v26.4.0 | Added support for the `buffer` option.

- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `null`
  - `signal` `<AbortSignal>` allows aborting an in-progress readFile
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<Function>` A buffer to read into, or a function called with the file size that returns the buffer.
- Returns: `<Promise>` Fulfills upon a successful read with the contents of the file. If no encoding is specified (using `options.encoding`), the data is returned as a `<Buffer>` object. Otherwise, the data will be a string.

Asynchronously reads the entire contents of a file.

If `options` is a string, then it specifies the `encoding`.

If `buffer` is provided and no encoding is specified, the returned `<Buffer>` is a view over the supplied buffer containing only the bytes read. If the supplied buffer is too small to contain the entire file, the operation will fail.

The `<FileHandle>` has to support reading.

If one or more `filehandle.read()` calls are made on a file handle and then a `filehandle.readFile()` call is made, the data will be read from the current position till the end of the file. It doesn't always read from the beginning of the file.

An example using the `buffer` option with a pre-allocated buffer:

    import { Buffer } from 'node:buffer';
    import { open } from 'node:fs/promises';

    const file = await open('./some/file/to/read');
    try {
      const buf = Buffer.alloc(16384);
      const contents = await file.readFile({ buffer: buf });
      console.log(contents); // A view over `buf` containing only the bytes read
    } finally {
      await file.close();
    }
    mjs

An example using the `buffer` option with a function returning a buffer:

    import { Buffer } from 'node:buffer';
    import { open } from 'node:fs/promises';

    const file = await open('./some/file/to/read');
    try {
      const contents = await file.readFile({
        buffer: (size) => Buffer.alloc(size),
      });
      console.log(contents);
    } finally {
      await file.close();
    }
    mjs

##### `filehandle.readLines([options])`#

Added in: v18.11.0

- `options` `<Object>`
  - `encoding` `<string>` Default: `null`
  - `autoClose` `<boolean>` Default: `true`
  - `emitClose` `<boolean>` Default: `true`
  - `start` `<integer>`
  - `end` `<integer>` Default: `Infinity`
  - `highWaterMark` `<integer>` Default: `64 * 1024`
- Returns: `<readline.InterfaceConstructor>`

Convenience method to create a `readline` interface and stream over the file. See `filehandle.createReadStream()` for the options.

    import { open } from 'node:fs/promises';

    const file = await open('./some/file/to/read');

    for await (const line of file.readLines()) {
      console.log(line);
    }
    const { open } = require('node:fs/promises');

    (async () => {
      const file = await open('./some/file/to/read');

      for await (const line of file.readLines()) {
        console.log(line);
      }
    })();
    javascript

##### `filehandle.readv(buffers[, position])`#

Added in: v13.13.0, v12.17.0

- `buffers` `<Buffer>`[] | `<TypedArray>`[] | `<DataView>`[]
- `position` `<integer>` | `<null>` The offset from the beginning of the file where the data should be read from. If `position` is not a `number`, the data will be read from the current position. Default: `null`
- Returns: `<Promise>` Fulfills upon success an object containing two properties:
  - `bytesRead` `<integer>` the number of bytes read
  - `buffers` `<Buffer>`[] | `<TypedArray>`[] | `<DataView>`[] property containing a reference to the `buffers` input.

Read from a file and write to an array of `<ArrayBufferView>`s

##### `filehandle.stat([options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v26.1.0 | Now accepts an additional `signal` property to allow aborting the operation.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.

- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
  - `signal` `<AbortSignal>` An AbortSignal to cancel the operation. Default: `undefined`.
- Returns: `<Promise>` Fulfills with an `<fs.Stats>` for the file.

##### `filehandle.sync()`#

Added in: v10.0.0

- Returns: `<Promise>` Fulfills with `undefined` upon success.

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX `fsync(2)` documentation for more detail.

##### `filehandle.truncate(len)`#

Added in: v10.0.0

- `len` `<integer>` Default: `0`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Truncates the file.

If the file was larger than `len` bytes, only the first `len` bytes will be retained in the file.

The following example retains only the first four bytes of the file:

    import { open } from 'node:fs/promises';

    let filehandle = null;
    try {
      filehandle = await open('temp.txt', 'r+');
      await filehandle.truncate(4);
    } finally {
      await filehandle?.close();
    }
    mjs

If the file previously was shorter than `len` bytes, it is extended, and the extended part is filled with null bytes (`'\0'`):

If `len` is negative then `0` will be used.

##### `filehandle.utimes(atime, mtime)`#

Added in: v10.0.0

- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- Returns: `<Promise>`

Change the file system timestamps of the object referenced by the `<FileHandle>` then fulfills the promise with no arguments upon success.

##### `filehandle.write(buffer, offset[, length[, position]])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v14.0.0 | The `buffer` parameter won't coerce unsupported input to buffers anymore.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `offset` `<integer>` The start position from within `buffer` where the data to write begins.
- `length` `<integer>` The number of bytes from `buffer` to write. Default: `buffer.byteLength - offset`
- `position` `<integer>` | `<null>` The offset from the beginning of the file where the data from `buffer` should be written. If `position` is not a `number`, the data will be written at the current position. See the POSIX `pwrite(2)` documentation for more detail. Default: `null`
- Returns: `<Promise>`

Write `buffer` to the file.

The promise is fulfilled with an object containing two properties:

- `bytesWritten` `<integer>` the number of bytes written
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` a reference to the `buffer` written.

It is unsafe to use `filehandle.write()` multiple times on the same file without waiting for the promise to be fulfilled (or rejected). For this scenario, use `filehandle.createWriteStream()`.

On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

##### `filehandle.write(buffer[, options])`#

Added in: v18.3.0, v16.17.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>`
  - `offset` `<integer>` Default: `0`
  - `length` `<integer>` Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<null>` Default: `null`
- Returns: `<Promise>`

Write `buffer` to the file.

Similar to the above `filehandle.write` function, this version takes an optional `options` object. If no `options` object is specified, it will default with the above values.

##### `filehandle.write(string[, position[, encoding]])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v14.0.0 | The `string` parameter won't coerce unsupported input to strings anymore.

- `string` `<string>`
- `position` `<integer>` | `<null>` The offset from the beginning of the file where the data from `string` should be written. If `position` is not a `number` the data will be written at the current position. See the POSIX `pwrite(2)` documentation for more detail. Default: `null`
- `encoding` `<string>` The expected string encoding. Default: `'utf8'`
- Returns: `<Promise>`

Write `string` to the file. If `string` is not a string, the promise is rejected with an error.

The promise is fulfilled with an object containing two properties:

- `bytesWritten` `<integer>` the number of bytes written
- `buffer` `<string>` a reference to the `string` written.

It is unsafe to use `filehandle.write()` multiple times on the same file without waiting for the promise to be fulfilled (or rejected). For this scenario, use `filehandle.createWriteStream()`.

On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

##### `filehandle.writeFile(data, options)`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v15.14.0, v14.18.0 | The `data` argument supports `AsyncIterable`, `Iterable`, and `Stream`.
v14.0.0 | The `data` parameter won't coerce unsupported input to strings anymore.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<AsyncIterable>` | `<Iterable>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` The expected character encoding when `data` is a string. Default: `'utf8'`
  - `signal` `<AbortSignal>` | `<undefined>` allows aborting an in-progress writeFile. Default: `undefined`
- Returns: `<Promise>`

Asynchronously writes data to a file, replacing the file if it already exists. `data` can be a string, a buffer, an `<AsyncIterable>`, or an `<Iterable>` object. The promise is fulfilled with no arguments upon success.

If `options` is a string, then it specifies the `encoding`.

The `<FileHandle>` has to support writing.

It is unsafe to use `filehandle.writeFile()` multiple times on the same file without waiting for the promise to be fulfilled (or rejected).

If one or more `filehandle.write()` calls are made on a file handle and then a `filehandle.writeFile()` call is made, the data will be written from the current position till the end of the file. It doesn't always write from the beginning of the file.

##### `filehandle.writev(buffers[, position])`#

Added in: v12.9.0

- `buffers` `<Buffer>`[] | `<TypedArray>`[] | `<DataView>`[]
- `position` `<integer>` | `<null>` The offset from the beginning of the file where the data from `buffers` should be written. If `position` is not a `number`, the data will be written at the current position. Default: `null`
- Returns: `<Promise>`

Write an array of `<ArrayBufferView>`s to the file.

The promise is fulfilled with an object containing a two properties:

- `bytesWritten` `<integer>` the number of bytes written
- `buffers` `<Buffer>`[] | `<TypedArray>`[] | `<DataView>`[] a reference to the `buffers` input.

It is unsafe to call `writev()` multiple times on the same file without waiting for the promise to be fulfilled (or rejected).

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

##### `filehandle.writer([options])`#

Added in: v25.9.0

Stability: 1 - Experimental

- `options` `<Object>`
  - `autoClose` `<boolean>` Close the file handle when the writer ends or fails. Default: `false`.
  - `start` `<number>` Byte offset to start writing at. When specified, writes use explicit positioning. Default: current file position.
  - `limit` `<number>` Maximum number of bytes the writer will accept. Async writes (`write()`, `writev()`) that would exceed the limit reject with `ERR_OUT_OF_RANGE`. Sync writes (`writeSync()`, `writevSync()`) return `false`. Default: no limit.
  - `chunkSize` `<number>` Maximum chunk size in bytes for synchronous write operations. Writes larger than this threshold fall back to async I/O. Set this to match the reader's `chunkSize` for optimal `pipeTo()` performance. Default: `131072` (128 KB).
- Returns: `<Object>`
  - `write(chunk[, options])` `<Function>` Returns `<Promise>`. Accepts `Uint8Array`, `Buffer`, or string (UTF-8 encoded).
    - `chunk` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<string>`
    - `options` `<Object>`
      - `signal` `<AbortSignal>` If the signal is already aborted, the write rejects with `AbortError` without performing I/O.
  - `writev(chunks[, options])` `<Function>` Returns `<Promise>`. Uses scatter/gather I/O via a single `writev()` syscall. Accepts mixed `Uint8Array`/string arrays.
    - `chunks` `<Buffer>`[] | `<TypedArray>`[] | `<DataView>`[] | `<string>`[]
    - `options` `<Object>`
      - `signal` `<AbortSignal>` If the signal is already aborted, the write rejects with `AbortError` without performing I/O.
  - `writeSync(chunk)` `<Function>` Returns `<boolean>`. Attempts a synchronous write. Returns `true` if the write succeeded, `false` if the caller should fall back to async `write()`. Returns `false` when: the writer is closed/errored, an async operation is in flight, the chunk exceeds `chunkSize`, or the write would exceed `limit`.
    - `chunk` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<string>`
  - `writevSync(chunks)` `<Function>` Returns `<boolean>`. Synchronous batch write. Same fallback semantics as `writeSync()`.
    - `chunks` `<Buffer>`[] | `<TypedArray>`[] | `<DataView>`[] | `<string>`[]
  - `end([options])` `<Function>` Returns `<Promise>`, fulfills with the total number of bytes written. Idempotent: returns `totalBytesWritten` if already closed, returns the pending promise if already closing. Rejects if the writer is in an errored state.
    - `options` `<Object>`
      - `signal` `<AbortSignal>` If the signal is already aborted, `end()` rejects with `AbortError` and the writer remains open.
  - `endSync()` `<Function>` Returns `<number>` | `<number>` total bytes written on success, `-1` if the writer is errored or an async operation is in flight. Idempotent when already closed.
  - `fail(reason)` `<Function>` Puts the writer into a terminal error state. Synchronous. If the writer is already closed or errored, this is a no-op. If `autoClose` is true, closes the file handle synchronously.

Return a `node:stream/iter` writer backed by this file handle.

The writer supports both `Symbol.asyncDispose` and `Symbol.dispose`:

- `await using w = fh.writer()` — if the writer is still open (no `end()` called), `asyncDispose` calls `fail()`. If `end()` is pending, it waits for it to complete.
- `using w = fh.writer()` — calls `fail()` unconditionally.

The `writeSync()` and `writevSync()` methods enable the try-sync fast path used by `stream/iter pipeTo()`. When the reader's chunk size matches the writer's `chunkSize`, all writes in a `pipeTo()` pipeline complete synchronously with zero promise overhead.

This function is only available when the `--experimental-stream-iter` flag is enabled.

    import { open } from 'node:fs/promises';
    import { from, pipeTo } from 'node:stream/iter';
    import { compressGzip } from 'node:zlib/iter';

    // Async pipeline
    const fh = await open('output.gz', 'w');
    await pipeTo(from('Hello!'), compressGzip(), fh.writer({ autoClose: true }));

    // Sync pipeline with limit
    const src = await open('input.txt', 'r');
    const dst = await open('output.txt', 'w');
    const w = dst.writer({ limit: 1024 * 1024 }); // Max 1 MB
    await pipeTo(src.pull({ autoClose: true }), w);
    await w.end();
    await dst.close();
    const { open } = require('node:fs/promises');
    const { from, pipeTo } = require('node:stream/iter');
    const { compressGzip } = require('node:zlib/iter');

    async function run() {
      // Async pipeline
      const fh = await open('output.gz', 'w');
      await pipeTo(from('Hello!'), compressGzip(), fh.writer({ autoClose: true }));

      // Sync pipeline with limit
      const src = await open('input.txt', 'r');
      const dst = await open('output.txt', 'w');
      const w = dst.writer({ limit: 1024 * 1024 }); // Max 1 MB
      await pipeTo(src.pull({ autoClose: true }), w);
      await w.end();
      await dst.close();
    }

    run().catch(console.error);
    javascript

##### `filehandle[Symbol.asyncDispose]()`#

Added in: v20.4.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `filehandle.close()` and returns a promise that fulfills when the filehandle is closed.

This method enables the filehandle to be used with `await using`, which will automatically close the file when the scope exits. For more information, see the MDN documentation on `using` statements.

#### `fsPromises.access(path[, mode])`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<integer>` Default: `fs.constants.F_OK`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Tests a user's permissions for the file or directory specified by `path`. The `mode` argument is an optional integer that specifies the accessibility checks to be performed. `mode` should be either the value `fs.constants.F_OK` or a mask consisting of the bitwise OR of any of `fs.constants.R_OK`, `fs.constants.W_OK`, and `fs.constants.X_OK` (e.g. `fs.constants.W_OK | fs.constants.R_OK`). Check File access constants for possible values of `mode`.

If the accessibility check is successful, the promise is fulfilled with no value. If any of the accessibility checks fail, the promise is rejected with an `<Error>` object. The following example checks if the file `/etc/passwd` can be read and written by the current process.

    import { access, constants } from 'node:fs/promises';

    try {
      await access('/etc/passwd', constants.R_OK | constants.W_OK);
      console.log('can access');
    } catch {
      console.error('cannot access');
    }
    mjs

Using `fsPromises.access()` to check for the accessibility of a file before calling `fsPromises.open()` is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

#### `fsPromises.appendFile(path, data[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v21.1.0, v20.10.0 | The `flush` option is now supported.
v15.14.0, v14.18.0 | The `data` argument supports `AsyncIterable`, `Iterable`, and `Stream`.

- `path` `<string>` | `<Buffer>` | `<URL>` | `<FileHandle>` filename or `<FileHandle>`
- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<AsyncIterable>` | `<Iterable>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `mode` `<integer>` Default: `0o666`
  - `flag` `<string>` See support of file system `flags`. Default: `'a'`.
  - `flush` `<boolean>` If `true`, the underlying file descriptor is flushed prior to closing it. Default: `false`.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Asynchronously append data to a file, creating the file if it does not yet `data` can be a string, a buffer, an `<AsyncIterable>`, or an `<Iterable>` object.

If `options` is a string, then it specifies the `encoding`.

The `mode` option only affects the newly created file. See `fs.open()` for more details.

The `path` may be specified as a `<FileHandle>` that has been opened for appending (using `fsPromises.open()`).

#### `fsPromises.chmod(path, mode)`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<string>` | `<integer>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Changes the permissions of a file.

#### `fsPromises.chown(path, uid, gid)`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `uid` `<integer>`
- `gid` `<integer>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Changes the ownership of a file.

#### `fsPromises.copyFile(src, dest[, mode])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v14.0.0 | Changed `flags` argument to `mode` and imposed stricter type validation.

- `src` `<string>` | `<Buffer>` | `<URL>` source filename to copy
- `dest` `<string>` | `<Buffer>` | `<URL>` destination filename of the copy operation
- `mode` `<integer>` Optional modifiers that specify the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. `fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE`) Default: `0`.
  - `fs.constants.COPYFILE_EXCL`: The copy operation will fail if `dest` already exists.
  - `fs.constants.COPYFILE_FICLONE`: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  - `fs.constants.COPYFILE_FICLONE_FORCE`: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Asynchronously copies `src` to `dest`. By default, `dest` is overwritten if it already exists.

No guarantees are made about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, an attempt will be made to remove the destination.

    import { copyFile, constants } from 'node:fs/promises';

    try {
      await copyFile('source.txt', 'destination.txt');
      console.log('source.txt was copied to destination.txt');
    } catch {
      console.error('The file could not be copied');
    }

    // By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
    try {
      await copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
      console.log('source.txt was copied to destination.txt');
    } catch {
      console.error('The file could not be copied');
    }
    mjs

#### `fsPromises.cp(src, dest[, options])`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v22.3.0 | This API is no longer experimental.
v20.1.0, v18.17.0 | Accept an additional `mode` option to specify the copy behavior as the `mode` argument of `fs.copyFile()`.
v17.6.0, v16.15.0 | Accepts an additional `verbatimSymlinks` option to specify whether to perform path resolution for symlinks.

- `src` `<string>` | `<URL>` source path to copy.
- `dest` `<string>` | `<URL>` destination path to copy to.
- `options` `<Object>`
  - `dereference` `<boolean>` dereference symlinks. Default: `false`.
  - `errorOnExist` `<boolean>` when `force` is `false`, and the destination exists, throw an error. Default: `false`.
  - `filter` `<Function>` Function to filter copied files/directories. Return `true` to copy the item, `false` to ignore it. When ignoring a directory, all of its contents will be skipped as well. Can also return a `Promise` that resolves to `true` or `false` Default: `undefined`.
    - `src` `<string>` source path to copy.
    - `dest` `<string>` destination path to copy to.
    - Returns: `<boolean>` | `<Promise>` A value that is coercible to `boolean` or a `Promise` that fulfils with such value.
  - `force` `<boolean>` overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the `errorOnExist` option to change this behavior. Default: `true`.
  - `mode` `<integer>` modifiers for copy operation. Default: `0`. See `mode` flag of `fsPromises.copyFile()`.
  - `preserveTimestamps` `<boolean>` When `true` timestamps from `src` will be preserved. Default: `false`.
  - `recursive` `<boolean>` copy directories recursively Default: `false`
  - `verbatimSymlinks` `<boolean>` When `true`, path resolution for symlinks will be skipped. Default: `false`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Asynchronously copies the entire directory structure from `src` to `dest`, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to `cp dir1/ dir2/`.

#### `fsPromises.glob(pattern[, options])`#

Added in: v22.0.0
History

Version | Changes
--- | ---
v26.1.0 | Add support for the `followSymlinks` option.
v24.1.0, v22.17.0 | Add support for `URL` instances for `cwd` option.
v24.0.0, v22.17.0 | Marking the API stable.
v23.7.0, v22.14.0 | Add support for `exclude` option to accept glob patterns.
v22.2.0 | Add support for `withFileTypes` as an option.

- `pattern` `<string>` | `<string>`[]
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` current working directory. Default: `process.cwd()`
  - `exclude` `<Function>` | `<string>`[] Function to filter out files/directories or a list of glob patterns to be excluded. If a function is provided, return `true` to exclude the item, `false` to include it. Default: `undefined`. If a string array is provided, each string should be a glob pattern that specifies paths to exclude. Note: Negation patterns (e.g., '!foo.js') are not supported.
  - `followSymlinks` `<boolean>` When `true`, symbolic links to directories are followed while expanding `**` patterns. Default: `false`.
  - `withFileTypes` `<boolean>` `true` if the glob should return paths as Dirents, `false` otherwise. Default: `false`.
- Returns: `<AsyncIterator>` An AsyncIterator that yields the paths of files that match the pattern.

When `followSymlinks` is enabled, detected symbolic link cycles are not traversed recursively.

    import { glob } from 'node:fs/promises';

    for await (const entry of glob('**/*.js'))
      console.log(entry);
    const { glob } = require('node:fs/promises');

    (async () => {
      for await (const entry of glob('**/*.js'))
        console.log(entry);
    })();
    javascript

#### `fsPromises.lchmod(path, mode)`#

Deprecated in: v10.0.0

Stability: 0 - Deprecated

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<integer>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Changes the permissions on a symbolic link.

This method is only implemented on macOS.

#### `fsPromises.lchown(path, uid, gid)`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v10.6.0 | This API is no longer deprecated.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `uid` `<integer>`
- `gid` `<integer>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Changes the ownership on a symbolic link.

#### `fsPromises.lutimes(path, atime, mtime)`#

Added in: v14.5.0, v12.19.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Changes the access and modification times of a file in the same way as `fsPromises.utimes()`, with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.

#### `fsPromises.link(existingPath, newPath)`#

Added in: v10.0.0

- `existingPath` `<string>` | `<Buffer>` | `<URL>`
- `newPath` `<string>` | `<Buffer>` | `<URL>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Creates a new link from the `existingPath` to the `newPath`. See the POSIX `link(2)` documentation for more detail.

#### `fsPromises.lstat(path[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
- Returns: `<Promise>` Fulfills with the `<fs.Stats>` object for the given symbolic link `path`.

Equivalent to `fsPromises.stat()` unless `path` refers to a symbolic link, in which case the link itself is stat-ed, not the file that it refers to. Refer to the POSIX `lstat(2)` document for more detail.

#### `fsPromises.mkdir(path[, options])`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>` | `<integer>`
  - `recursive` `<boolean>` Default: `false`
  - `mode` `<string>` | `<integer>` Not supported on Windows. See File modes for more details. Default: `0o777`.
- Returns: `<Promise>` Upon success, fulfills with `undefined` if `recursive` is `false`, or the first directory path created if `recursive` is `true`.

Asynchronously creates a directory.

The optional `options` argument can be an integer specifying `mode` (permission and sticky bits), or an object with a `mode` property and a `recursive` property indicating whether parent directories should be created. Calling `fsPromises.mkdir()` when `path` is a directory that exists results in a rejection only when `recursive` is false.

    import { mkdir } from 'node:fs/promises';

    try {
      const projectFolder = new URL('./test/project/', import.meta.url);
      const createDir = await mkdir(projectFolder, { recursive: true });

      console.log(`created ${createDir}`);
    } catch (err) {
      console.error(err.message);
    }
    const { mkdir } = require('node:fs/promises');
    const { join } = require('node:path');

    async function makeDirectory() {
      const projectFolder = join(__dirname, 'test', 'project');
      const dirCreation = await mkdir(projectFolder, { recursive: true });

      console.log(dirCreation);
      return dirCreation;
    }

    makeDirectory().catch(console.error);
    javascript

#### `fsPromises.mkdtemp(prefix[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | The `prefix` parameter now accepts buffers and URL.
v16.5.0, v14.18.0 | The `prefix` parameter now accepts an empty string.

- `prefix` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<Promise>` Fulfills with a string containing the file system path of the newly created temporary directory.

Creates a unique temporary directory. A unique directory name is generated by appending six random characters to the end of the provided `prefix`. Due to platform inconsistencies, avoid trailing `X` characters in `prefix`. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing `X` characters in `prefix` with random characters.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use.

    import { mkdtemp } from 'node:fs/promises';
    import { join } from 'node:path';
    import { tmpdir } from 'node:os';

    try {
      await mkdtemp(join(tmpdir(), 'foo-'));
    } catch (err) {
      console.error(err);
    }
    mjs

The `fsPromises.mkdtemp()` method will append the six randomly selected characters directly to the `prefix` string. For instance, given a directory `/tmp`, if the intention is to create a temporary directory within `/tmp`, the `prefix` must end with a trailing platform-specific path separator (`require('node:path').sep`).

#### `fsPromises.mkdtempDisposable(prefix[, options])`#

Added in: v24.4.0

- `prefix` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<Promise>` Fulfills with a Promise for an async-disposable Object:
  - `path` `<string>` The path of the created directory.
  - `remove` `<AsyncFunction>` A function which removes the created directory.
  - `[Symbol.asyncDispose]` `<AsyncFunction>` The same as `remove`.

The resulting Promise holds an async-disposable object whose `path` property holds the created directory path. When the object is disposed, the directory and its contents will be removed asynchronously if it still exists. If the directory cannot be deleted, disposal will throw an error. The object has an async `remove()` method which will perform the same task.

Both this function and the disposal function on the resulting object are async, so it should be used with `await` + `await using` as in `await using dir = await fsPromises.mkdtempDisposable('prefix')`.

See the MDN documentation on `using` statements for more information about explicit resource management.

For detailed information, see the documentation of `fsPromises.mkdtemp()`.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use.

#### `fsPromises.open(path, flags[, mode])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v11.1.0 | The `flags` argument is now optional and defaults to `'r'`.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `flags` `<string>` | `<number>` See support of file system `flags`. Default: `'r'`.
- `mode` `<string>` | `<integer>` Sets the file mode (permission and sticky bits) if the file is created. See File modes for more details. Default: `0o666` (readable and writable)
- Returns: `<Promise>` Fulfills with a `<FileHandle>` object.

Opens a `<FileHandle>`.

Refer to the POSIX `open(2)` documentation for more detail.

Some characters (`< > : " / \ | ? *`) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

#### `fsPromises.opendir(path[, options])`#

Added in: v12.12.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | Added `recursive` option.
v13.1.0, v12.16.0 | The `bufferSize` option was introduced.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `bufferSize` `<number>` Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: `32`
  - `recursive` `<boolean>` Resolved `Dir` will be an `<AsyncIterable>` containing all sub files and directories. Default: `false`
- Returns: `<Promise>` Fulfills with an `<fs.Dir>`.

Asynchronously open a directory for iterative scanning. See the POSIX `opendir(3)` documentation for more detail.

Creates an `<fs.Dir>`, which contains all further functions for reading from and cleaning up the directory.

The `encoding` option sets the encoding for the `path` while opening the directory and subsequent read operations.

Example using async iteration:

    import { opendir } from 'node:fs/promises';

    try {
      const dir = await opendir('./');
      for await (const dirent of dir)
        console.log(dirent.name);
    } catch (err) {
      console.error(err);
    }
    mjs

When using the async iterator, the `<fs.Dir>` object will be automatically closed after the iterator exits.

#### `fsPromises.readdir(path[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | Added `recursive` option.
v10.11.0 | New option `withFileTypes` was added.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
  - `withFileTypes` `<boolean>` Default: `false`
  - `recursive` `<boolean>` If `true`, reads the contents of a directory recursively. In recursive mode, it will list all files, sub files, and directories. Default: `false`.
- Returns: `<Promise>` Fulfills with an array of the names of the files in the directory excluding `'.'` and `'..'`.

Reads the contents of a directory.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the filenames. If the `encoding` is set to `'buffer'`, the filenames returned will be passed as `<Buffer>` objects.

If `options.withFileTypes` is set to `true`, the returned array will contain `<fs.Dirent>` objects.

    import { readdir } from 'node:fs/promises';

    try {
      const files = await readdir(path);
      for (const file of files)
        console.log(file);
    } catch (err) {
      console.error(err);
    }
    mjs

#### `fsPromises.readFile(path[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v26.4.0 | Added support for the `buffer` option.
v15.2.0, v14.17.0 | The options argument may include an AbortSignal to abort an ongoing readFile request.

- `path` `<string>` | `<Buffer>` | `<URL>` | `<FileHandle>` filename or `FileHandle`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `null`
  - `flag` `<string>` See support of file system `flags`. Default: `'r'`.
  - `signal` `<AbortSignal>` allows aborting an in-progress readFile
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<Function>` A buffer to read into, or a function called with the file size that returns the buffer.
- Returns: `<Promise>` Fulfills with the contents of the file.

Asynchronously reads the entire contents of a file.

If no encoding is specified (using `options.encoding`), the data is returned as a `<Buffer>` object. Otherwise, the data will be a string.

If `options` is a string, then it specifies the encoding.

If `buffer` is provided and no encoding is specified, the returned `<Buffer>` is a view over the supplied buffer containing only the bytes read. If the supplied buffer is too small to contain the entire file, the promise will be rejected.

When the `path` is a directory, the behavior of `fsPromises.readFile()` is platform-specific. On macOS, Linux, and Windows, the promise will be rejected with an error. On FreeBSD, a representation of the directory's contents will be returned.

An example of reading a `package.json` file located in the same directory of the running code:

    import { readFile } from 'node:fs/promises';
    try {
      const filePath = new URL('./package.json', import.meta.url);
      const contents = await readFile(filePath, { encoding: 'utf8' });
      console.log(contents);
    } catch (err) {
      console.error(err.message);
    }
    const { readFile } = require('node:fs/promises');
    const { resolve } = require('node:path');
    async function logFile() {
      try {
        const filePath = resolve('./package.json');
        const contents = await readFile(filePath, { encoding: 'utf8' });
        console.log(contents);
      } catch (err) {
        console.error(err.message);
      }
    }
    logFile();
    javascript

It is possible to abort an ongoing `readFile` using an `<AbortSignal>`. If a request is aborted the promise returned is rejected with an `AbortError`:

    import { readFile } from 'node:fs/promises';

    try {
      const controller = new AbortController();
      const { signal } = controller;
      const promise = readFile(fileName, { signal });

      // Abort the request before the promise settles.
      controller.abort();

      await promise;
    } catch (err) {
      // When a request is aborted - err is an AbortError
      console.error(err);
    }
    mjs

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering `fs.readFile` performs.

Any specified `<FileHandle>` has to support reading.

An example using the `buffer` option with a pre-allocated buffer:

    import { Buffer } from 'node:buffer';
    import { readFile } from 'node:fs/promises';

    const buf = Buffer.alloc(16384);
    const contents = await readFile('/path/to/file', { buffer: buf });
    console.log(contents); // A view over `buf` containing only the bytes read
    mjs

An example using the `buffer` option with a function returning a buffer:

    import { Buffer } from 'node:buffer';
    import { readFile } from 'node:fs/promises';

    const contents = await readFile('/path/to/file', {
      buffer: (size) => Buffer.alloc(size),
    });
    console.log(contents);
    mjs

#### `fsPromises.readlink(path[, options])`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<Promise>` Fulfills with the `linkString` upon success.

Reads the contents of the symbolic link referred to by `path`. See the POSIX `readlink(2)` documentation for more detail. The promise is fulfilled with the `linkString` upon success.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the link path returned. If the `encoding` is set to `'buffer'`, the link path returned will be passed as a `<Buffer>` object.

#### `fsPromises.realpath(path[, options])`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<Promise>` Fulfills with the resolved path upon success.

Determines the actual location of `path` using the same semantics as the `fs.realpath.native()` function.

Only paths that can be converted to UTF8 strings are supported.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the path. If the `encoding` is set to `'buffer'`, the path returned will be passed as a `<Buffer>` object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on `/proc` in order for this function to work. Glibc does not have this restriction.

#### `fsPromises.rename(oldPath, newPath)`#

Added in: v10.0.0

- `oldPath` `<string>` | `<Buffer>` | `<URL>`
- `newPath` `<string>` | `<Buffer>` | `<URL>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Renames `oldPath` to `newPath`.

#### `fsPromises.rmdir(path[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v25.0.0 | Remove `recursive` option.
v16.0.0 | Using `fsPromises.rmdir(path, { recursive: true })` on a `path` that is a file is no longer permitted and results in an `ENOENT` error on Windows and an `ENOTDIR` error on POSIX.
v16.0.0 | Using `fsPromises.rmdir(path, { recursive: true })` on a `path` that does not exist is no longer permitted and results in a `ENOENT` error.
v16.0.0 | The `recursive` option is deprecated, using it triggers a deprecation warning.
v14.14.0 | The `recursive` option is deprecated, use `fsPromises.rm` instead.
v13.3.0, v12.16.0 | The `maxBusyTries` option is renamed to `maxRetries`, and its default is 0. The `emfileWait` option has been removed, and `EMFILE` errors use the same retry logic as other errors. The `retryDelay` option is now supported. `ENFILE` errors are now retried.
v12.10.0 | The `recursive`, `maxBusyTries`, and `emfileWait` options are now supported.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>` There are currently no options exposed. There used to be options for `recursive`, `maxBusyTries`, and `emfileWait` but they were deprecated and removed. The `options` argument is still accepted for backwards compatibility but it is not used.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Removes the directory identified by `path`.

Using `fsPromises.rmdir()` on a file (not a directory) results in the promise being rejected with an `ENOENT` error on Windows and an `ENOTDIR` error on POSIX.

To get a behavior similar to the `rm -rf` Unix command, use `fsPromises.rm()` with options `{ recursive: true, force: true }`.

#### `fsPromises.rm(path[, options])`#

Added in: v14.14.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `force` `<boolean>` When `true`, exceptions will be ignored if `path` does not exist. Default: `false`.
  - `maxRetries` `<integer>` If an `EBUSY`, `EMFILE`, `ENFILE`, `ENOTEMPTY`, or `EPERM` error is encountered, Node.js will retry the operation with a linear backoff wait of `retryDelay` milliseconds longer on each try. This option represents the number of retries. This option is ignored if the `recursive` option is not `true`. Default: `0`.
  - `recursive` `<boolean>` If `true`, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: `false`.
  - `retryDelay` `<integer>` The amount of time in milliseconds to wait between retries. This option is ignored if the `recursive` option is not `true`. Default: `100`.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Removes files and directories (modeled on the standard POSIX `rm` utility).

#### `fsPromises.stat(path[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v25.7.0 | Accepts a `throwIfNoEntry` option to specify whether an exception should be thrown if the entry does not exist.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
  - `throwIfNoEntry` `<boolean>` Whether an exception will be thrown if no file system entry exists, rather than returning `undefined`. Default: `true`.
- Returns: `<Promise>` Fulfills with the `<fs.Stats>` object for the given `path`.

#### `fsPromises.statfs(path[, options])`#

Added in: v19.6.0, v18.15.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.StatFs>` object should be `bigint`. Default: `false`.
- Returns: `<Promise>` Fulfills with the `<fs.StatFs>` object for the given `path`.

#### `fsPromises.symlink(target, path[, type])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v19.0.0 | If the `type` argument is `null` or omitted, Node.js will autodetect `target` type and automatically select `dir` or `file`.

- `target` `<string>` | `<Buffer>` | `<URL>`
- `path` `<string>` | `<Buffer>` | `<URL>`
- `type` `<string>` | `<null>` Default: `null`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Creates a symbolic link.

The `type` argument is only used on Windows platforms and can be one of `'dir'`, `'file'`, or `'junction'`. If the `type` argument is `null`, Node.js will autodetect `target` type and use `'file'` or `'dir'`. If the `target` does not exist, `'file'` will be used. Windows junction points require the destination path to be absolute. When using `'junction'`, the `target` argument will automatically be normalized to absolute path. Junction points on NTFS volumes can only point to directories.

#### `fsPromises.truncate(path[, len])`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `len` `<integer>` Default: `0`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Truncates (shortens or extends the length) of the content at `path` to `len` bytes.

#### `fsPromises.unlink(path)`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

If `path` refers to a symbolic link, then the link is removed without affecting the file or directory to which that link refers. If the `path` refers to a file path that is not a symbolic link, the file is deleted. See the POSIX `unlink(2)` documentation for more detail.

#### `fsPromises.utimes(path, atime, mtime)`#

Added in: v10.0.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Change the file system timestamps of the object referenced by `path`.

The `atime` and `mtime` arguments follow these rules:

- Values can be either numbers representing Unix epoch time, `Date`s, or a numeric string like `'123456789.0'`.
- If the value can not be converted to a number, or is `NaN`, `Infinity`, or `-Infinity`, an `Error` will be thrown.

#### `fsPromises.watch(filename[, options])`#

Added in: v15.9.0, v14.18.0

- `filename` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `persistent` `<boolean>` Indicates whether the process should continue to run as long as files are being watched. Default: `true`.
  - `recursive` `<boolean>` Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default: `false`.
  - `encoding` `<string>` Specifies the character encoding to be used for the filename passed to the listener. Default: `'utf8'`.
  - `signal` `<AbortSignal>` An `<AbortSignal>` used to signal when the watcher should stop.
  - `maxQueue` `<number>` Specifies the number of events to queue between iterations of the `<AsyncIterator>` returned. Default: `2048`.
  - `overflow` `<string>` Either `'ignore'` or `'throw'` when there are more events to be queued than `maxQueue` allows. `'ignore'` means overflow events are dropped and a warning is emitted, while `'throw'` means to throw an exception. Default: `'ignore'`.
  - `ignore` `<string>` | `<RegExp>` | `<Function>` | `<Array>` Pattern(s) to ignore. Strings are glob patterns (using `minimatch`), RegExp patterns are tested against the filename, and functions receive the filename and return `true` to ignore. Default: `undefined`.
- Returns: `<AsyncIterator>` of objects with the properties:
  - `eventType` `<string>` The type of change
  - `filename` `<string>` | `<Buffer>` | `<null>` The name of the file changed.

Returns an async iterator that watches for changes on `filename`, where `filename` is either a file or a directory.

    const { watch } = require('node:fs/promises');

    const ac = new AbortController();
    const { signal } = ac;
    setTimeout(() => ac.abort(), 10000);

    (async () => {
      try {
        const watcher = watch(__filename, { signal });
        for await (const event of watcher)
          console.log(event);
      } catch (err) {
        if (err.name === 'AbortError')
          return;
        throw err;
      }
    })();
    js

On most platforms, `'rename'` is emitted whenever a filename appears or disappears in the directory.

All the caveats for `fs.watch()` also apply to `fsPromises.watch()`.

#### `fsPromises.writeFile(file, data[, options])`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v21.0.0, v20.10.0 | The `flush` option is now supported.
v15.14.0, v14.18.0 | The `data` argument supports `AsyncIterable`, `Iterable`, and `Stream`.
v15.2.0, v14.17.0 | The options argument may include an AbortSignal to abort an ongoing writeFile request.
v14.0.0 | The `data` parameter won't coerce unsupported input to strings anymore.

- `file` `<string>` | `<Buffer>` | `<URL>` | `<FileHandle>` filename or `FileHandle`
- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<AsyncIterable>` | `<Iterable>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `mode` `<integer>` Default: `0o666`
  - `flag` `<string>` See support of file system `flags`. Default: `'w'`.
  - `flush` `<boolean>` If all data is successfully written to the file, and `flush` is `true`, `filehandle.sync()` is used to flush the data. Default: `false`.
  - `signal` `<AbortSignal>` allows aborting an in-progress writeFile
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Asynchronously writes data to a file, replacing the file if it already exists. `data` can be a string, a buffer, an `<AsyncIterable>`, or an `<Iterable>` object.

The `encoding` option is ignored if `data` is a buffer.

If `options` is a string, then it specifies the encoding.

The `mode` option only affects the newly created file. See `fs.open()` for more details.

Any specified `<FileHandle>` has to support writing.

It is unsafe to use `fsPromises.writeFile()` multiple times on the same file without waiting for the promise to be settled.

Similarly to `fsPromises.readFile` - `fsPromises.writeFile` is a convenience method that performs multiple `write` calls internally to write the buffer passed to it. For performance sensitive code consider using `fs.createWriteStream()` or `filehandle.createWriteStream()`.

It is possible to use an `<AbortSignal>` to cancel an `fsPromises.writeFile()`. Cancelation is "best effort", and some amount of data is likely still to be written.

    import { writeFile } from 'node:fs/promises';
    import { Buffer } from 'node:buffer';

    try {
      const controller = new AbortController();
      const { signal } = controller;
      const data = new Uint8Array(Buffer.from('Hello Node.js'));
      const promise = writeFile('message.txt', data, { signal });

      // Abort the request before the promise settles.
      controller.abort();

      await promise;
    } catch (err) {
      // When a request is aborted - err is an AbortError
      console.error(err);
    }
    mjs

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering `fs.writeFile` performs.

#### `fsPromises.constants`#

Added in: v18.4.0, v16.17.0

- Type: `<Object>`

Returns an object containing commonly used constants for file system operations. The object is the same as `fs.constants`. See FS constants for more details.

### Callback API#

The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.

The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.

#### `fs.access(path[, mode], callback)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v25.0.0 | The constants `fs.F_OK`, `fs.R_OK`, `fs.W_OK` and `fs.X_OK` which were present directly on `fs` are removed.
v20.8.0 | The constants `fs.F_OK`, `fs.R_OK`, `fs.W_OK` and `fs.X_OK` which were present directly on `fs` are deprecated.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v6.3.0 | The constants like `fs.R_OK`, etc which were present directly on `fs` were moved into `fs.constants` as a soft deprecation. Thus for Node.js `< v6.3.0` use `fs` to access those constants, or do something like `(fs.constants || fs).R_OK` to work with all versions.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<integer>` Default: `fs.constants.F_OK`
- `callback` `<Function>`
  - `err` `<Error>`

Tests a user's permissions for the file or directory specified by `path`. The `mode` argument is an optional integer that specifies the accessibility checks to be performed. `mode` should be either the value `fs.constants.F_OK` or a mask consisting of the bitwise OR of any of `fs.constants.R_OK`, `fs.constants.W_OK`, and `fs.constants.X_OK` (e.g. `fs.constants.W_OK | fs.constants.R_OK`). Check File access constants for possible values of `mode`.

The final argument, `callback`, is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be an `Error` object. The following examples check if `package.json` exists, and if it is readable or writable.

    import { access, constants } from 'node:fs';

    const file = 'package.json';

    // Check if the file exists in the current directory.
    access(file, constants.F_OK, (err) => {
      console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
    });

    // Check if the file is readable.
    access(file, constants.R_OK, (err) => {
      console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
    });

    // Check if the file is writable.
    access(file, constants.W_OK, (err) => {
      console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
    });

    // Check if the file is readable and writable.
    access(file, constants.R_OK | constants.W_OK, (err) => {
      console.log(`${file} ${err ? 'is not' : 'is'} readable and writable`);
    });
    mjs

Do not use `fs.access()` to check for the accessibility of a file before calling `fs.open()`, `fs.readFile()`, or `fs.writeFile()`. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

write (NOT RECOMMENDED)

    import { access, open, close } from 'node:fs';

    access('myfile', (err) => {
      if (!err) {
        console.error('myfile already exists');
        return;
      }

      open('myfile', 'wx', (err, fd) => {
        if (err) throw err;

        try {
          writeMyData(fd);
        } finally {
          close(fd, (err) => {
            if (err) throw err;
          });
        }
      });
    });
    mjs

write (RECOMMENDED)

    import { open, close } from 'node:fs';

    open('myfile', 'wx', (err, fd) => {
      if (err) {
        if (err.code === 'EEXIST') {
          console.error('myfile already exists');
          return;
        }

        throw err;
      }

      try {
        writeMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
    mjs

read (NOT RECOMMENDED)

    import { access, open, close } from 'node:fs';
    access('myfile', (err) => {
      if (err) {
        if (err.code === 'ENOENT') {
          console.error('myfile does not exist');
          return;
        }

        throw err;
      }

      open('myfile', 'r', (err, fd) => {
        if (err) throw err;

        try {
          readMyData(fd);
        } finally {
          close(fd, (err) => {
            if (err) throw err;
          });
        }
      });
    });
    mjs

read (RECOMMENDED)

    import { open, close } from 'node:fs';

    open('myfile', 'r', (err, fd) => {
      if (err) {
        if (err.code === 'ENOENT') {
          console.error('myfile does not exist');
          return;
        }

        throw err;
      }

      try {
        readMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
    mjs

The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the accessibility of a file only if the file will not be used directly, for example when its accessibility is a signal from another process.

On Windows, access-control policies (ACLs) on a directory may limit access to a file or directory. The `fs.access()` function, however, does not check the ACL and therefore may report that a path is accessible even if the ACL restricts the user from reading or writing to it.

#### `fs.appendFile(path, data[, options], callback)`#

Added in: v0.6.7
History

Version | Changes
--- | ---
v21.1.0, v20.10.0 | The `flush` option is now supported.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v7.0.0 | The passed `options` object will never be modified.
v5.0.0 | The `file` parameter can be a file descriptor now.

- `path` `<string>` | `<Buffer>` | `<URL>` | `<number>` filename or file descriptor
- `data` `<string>` | `<Buffer>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `mode` `<integer>` Default: `0o666`
  - `flag` `<string>` See support of file system `flags`. Default: `'a'`.
  - `flush` `<boolean>` If `true`, the underlying file descriptor is flushed prior to closing it. Default: `false`.
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously append data to a file, creating the file if it does not yet exist. `data` can be a string or a `<Buffer>`.

The `mode` option only affects the newly created file. See `fs.open()` for more details.

    import { appendFile } from 'node:fs';

    appendFile('message.txt', 'data to append', (err) => {
      if (err) throw err;
      console.log('The "data to append" was appended to file!');
    });
    mjs

If `options` is a string, then it specifies the encoding:

    import { appendFile } from 'node:fs';

    appendFile('message.txt', 'data to append', 'utf8', callback);
    mjs

The `path` may be specified as a numeric file descriptor that has been opened for appending (using `fs.open()` or `fs.openSync()`). The file descriptor will not be closed automatically.

    import { open, close, appendFile } from 'node:fs';

    function closeFd(fd) {
      close(fd, (err) => {
        if (err) throw err;
      });
    }

    open('message.txt', 'a', (err, fd) => {
      if (err) throw err;

      try {
        appendFile(fd, 'data to append', 'utf8', (err) => {
          closeFd(fd);
          if (err) throw err;
        });
      } catch (err) {
        closeFd(fd);
        throw err;
      }
    });
    mjs

#### `fs.chmod(path, mode, callback)`#

Added in: v0.1.30
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<string>` | `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.

See the POSIX `chmod(2)` documentation for more detail.

    import { chmod } from 'node:fs';

    chmod('my_file.txt', 0o775, (err) => {
      if (err) throw err;
      console.log('The permissions for file "my_file.txt" have been changed!');
    });
    mjs

##### File modes#

The `mode` argument used in both the `fs.chmod()` and `fs.chmodSync()` methods is a numeric bitmask created using a logical OR of the following constants:

Constant | Octal | Description
--- | --- | ---
`fs.constants.S_IRUSR` | `0o400` | read by owner
`fs.constants.S_IWUSR` | `0o200` | write by owner
`fs.constants.S_IXUSR` | `0o100` | execute/search by owner
`fs.constants.S_IRGRP` | `0o40` | read by group
`fs.constants.S_IWGRP` | `0o20` | write by group
`fs.constants.S_IXGRP` | `0o10` | execute/search by group
`fs.constants.S_IROTH` | `0o4` | read by others
`fs.constants.S_IWOTH` | `0o2` | write by others
`fs.constants.S_IXOTH` | `0o1` | execute/search by others

An easier method of constructing the `mode` is to use a sequence of three octal digits (e.g. `765`). The left-most digit (`7` in the example), specifies the permissions for the file owner. The middle digit (`6` in the example), specifies permissions for the group. The right-most digit (`5` in the example), specifies the permissions for others.

Number | Description
--- | ---
`7` | read, write, and execute
`6` | read and write
`5` | read and execute
`4` | read only
`3` | write and execute
`2` | write only
`1` | execute only
`0` | no permission

For example, the octal value `0o765` means:

- The owner may read, write, and execute the file.
- The group may read and write the file.
- Others may read and execute the file.

When using raw numbers where file modes are expected, any value larger than `0o777` may result in platform-specific behaviors that are not supported to work consistently. Therefore constants like `S_ISVTX`, `S_ISGID`, or `S_ISUID` are not exposed in `fs.constants`.

Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner, or others is not implemented.

#### `fs.chown(path, uid, gid, callback)`#

Added in: v0.1.97
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `uid` `<integer>`
- `gid` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.

See the POSIX `chown(2)` documentation for more detail.

#### `fs.close(fd[, callback])`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v15.9.0, v14.17.0 | A default callback is now used if one is not provided.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Closes the file descriptor. No arguments other than a possible exception are given to the completion callback.

Calling `fs.close()` on any file descriptor (`fd`) that is currently in use through any other `fs` operation may lead to undefined behavior.

See the POSIX `close(2)` documentation for more detail.

#### `fs.copyFile(src, dest[, mode], callback)`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v14.0.0 | Changed `flags` argument to `mode` and imposed stricter type validation.

- `src` `<string>` | `<Buffer>` | `<URL>` source filename to copy
- `dest` `<string>` | `<Buffer>` | `<URL>` destination filename of the copy operation
- `mode` `<integer>` modifiers for copy operation. Default: `0`.
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously copies `src` to `dest`. By default, `dest` is overwritten if it already exists. No arguments other than a possible exception are given to the callback function. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

`mode` is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. `fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE`).

- `fs.constants.COPYFILE_EXCL`: The copy operation will fail if `dest` already exists.
- `fs.constants.COPYFILE_FICLONE`: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
- `fs.constants.COPYFILE_FICLONE_FORCE`: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.

    import { copyFile, constants } from 'node:fs';

    function callback(err) {
      if (err) throw err;
      console.log('source.txt was copied to destination.txt');
    }

    // destination.txt will be created or overwritten by default.
    copyFile('source.txt', 'destination.txt', callback);

    // By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
    copyFile('source.txt', 'destination.txt', constants.COPYFILE_EXCL, callback);
    mjs

#### `fs.cp(src, dest[, options], callback)`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v22.3.0 | This API is no longer experimental.
v20.1.0, v18.17.0 | Accept an additional `mode` option to specify the copy behavior as the `mode` argument of `fs.copyFile()`.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v17.6.0, v16.15.0 | Accepts an additional `verbatimSymlinks` option to specify whether to perform path resolution for symlinks.

- `src` `<string>` | `<URL>` source path to copy.
- `dest` `<string>` | `<URL>` destination path to copy to.
- `options` `<Object>`
  - `dereference` `<boolean>` dereference symlinks. Default: `false`.
  - `errorOnExist` `<boolean>` when `force` is `false`, and the destination exists, throw an error. Default: `false`.
  - `filter` `<Function>` Function to filter copied files/directories. Return `true` to copy the item, `false` to ignore it. When ignoring a directory, all of its contents will be skipped as well. Can also return a `Promise` that fulfills with `true` or `false`. Default: `undefined`.
    - `src` `<string>` source path to copy.
    - `dest` `<string>` destination path to copy to.
    - Returns: `<boolean>` | `<Promise>` A value that is coercible to `boolean` or a `Promise` that fulfils with such value.
  - `force` `<boolean>` overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the `errorOnExist` option to change this behavior. Default: `true`.
  - `mode` `<integer>` modifiers for copy operation. Default: `0`. See `mode` flag of `fs.copyFile()`.
  - `preserveTimestamps` `<boolean>` When `true` timestamps from `src` will be preserved. Default: `false`.
  - `recursive` `<boolean>` copy directories recursively Default: `false`
  - `verbatimSymlinks` `<boolean>` When `true`, path resolution for symlinks will be skipped. Default: `false`
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously copies the entire directory structure from `src` to `dest`, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to `cp dir1/ dir2/`.

#### `fs.createReadStream(path[, options])`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v16.10.0 | The `fs` option does not need `open` method if an `fd` was provided.
v16.10.0 | The `fs` option does not need `close` method if `autoClose` is `false`.
v15.5.0 | Add support for `AbortSignal`.
v15.4.0 | The `fd` option accepts FileHandle arguments.
v14.0.0 | Change `emitClose` default to `true`.
v13.6.0, v12.17.0 | The `fs` options allow overriding the used `fs` implementation.
v12.10.0 | Enable `emitClose` option.
v11.0.0 | Impose new restrictions on `start` and `end`, throwing more appropriate errors in cases when we cannot reasonably handle the input values.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The passed `options` object will never be modified.
v2.3.0 | The passed `options` object can be a string now.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `flags` `<string>` See support of file system `flags`. Default: `'r'`.
  - `encoding` `<string>` Default: `null`
  - `fd` `<integer>` | `<FileHandle>` Default: `null`
  - `mode` `<integer>` Default: `0o666`
  - `autoClose` `<boolean>` Default: `true`
  - `emitClose` `<boolean>` Default: `true`
  - `start` `<integer>`
  - `end` `<integer>` Default: `Infinity`
  - `highWaterMark` `<integer>` Default: `64 * 1024`
  - `fs` `<Object>` | `<null>` Default: `null`
  - `signal` `<AbortSignal>` | `<null>` Default: `null`
- Returns: `<fs.ReadStream>`

`options` can include `start` and `end` values to read a range of bytes from the file instead of the entire file. Both `start` and `end` are inclusive and start counting at 0, allowed values are in the [0, `Number.MAX_SAFE_INTEGER`] range. If `fd` is specified and `start` is omitted or `undefined`, `fs.createReadStream()` reads sequentially from the current file position. The `encoding` can be any one of those accepted by `<Buffer>`.

If `fd` is specified, `ReadStream` will ignore the `path` argument and will use the specified file descriptor. This means that no `'open'` event will be emitted. `fd` should be blocking; non-blocking `fd`s should be passed to `<net.Socket>`.

If `fd` points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally.

By default, the stream will emit a `'close'` event after it has been destroyed. Set the `emitClose` option to `false` to change this behavior.

By providing the `fs` option, it is possible to override the corresponding `fs` implementations for `open`, `read`, and `close`. When providing the `fs` option, an override for `read` is required. If no `fd` is provided, an override for `open` is also required. If `autoClose` is `true`, an override for `close` is also required.

    import { createReadStream } from 'node:fs';

    // Create a stream from some character device.
    const stream = createReadStream('/dev/input/event0');
    setTimeout(() => {
      stream.close(); // This may not close the stream.
      // Artificially marking end-of-stream, as if the underlying resource had
      // indicated end-of-file by itself, allows the stream to close.
      // This does not cancel pending read operations, and if there is such an
      // operation, the process may still not be able to exit successfully
      // until it finishes.
      stream.push(null);
      stream.read(0);
    }, 100);
    mjs

If `autoClose` is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If `autoClose` is set to true (default behavior), on `'error'` or `'end'` the file descriptor will be closed automatically.

`mode` sets the file mode (permission and sticky bits), but only if the file was created.

An example to read the last 10 bytes of a file which is 100 bytes long:

    import { createReadStream } from 'node:fs';

    createReadStream('sample.txt', { start: 90, end: 99 });
    mjs

If `options` is a string, then it specifies the encoding.

#### `fs.createWriteStream(path[, options])`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v21.0.0, v20.10.0 | The `flush` option is now supported.
v16.10.0 | The `fs` option does not need `open` method if an `fd` was provided.
v16.10.0 | The `fs` option does not need `close` method if `autoClose` is `false`.
v15.5.0 | Add support for `AbortSignal`.
v15.4.0 | The `fd` option accepts FileHandle arguments.
v14.0.0 | Change `emitClose` default to `true`.
v13.6.0, v12.17.0 | The `fs` options allow overriding the used `fs` implementation.
v12.10.0 | Enable `emitClose` option.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The passed `options` object will never be modified.
v5.5.0 | The `autoClose` option is supported now.
v2.3.0 | The passed `options` object can be a string now.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `flags` `<string>` See support of file system `flags`. Default: `'w'`.
  - `encoding` `<string>` Default: `'utf8'`
  - `fd` `<integer>` | `<FileHandle>` Default: `null`
  - `mode` `<integer>` Default: `0o666`
  - `autoClose` `<boolean>` Default: `true`
  - `emitClose` `<boolean>` Default: `true`
  - `start` `<integer>`
  - `fs` `<Object>` | `<null>` Default: `null`
  - `signal` `<AbortSignal>` | `<null>` Default: `null`
  - `highWaterMark` `<number>` Default: `16384`
  - `flush` `<boolean>` If `true`, the underlying file descriptor is flushed prior to closing it. Default: `false`.
- Returns: `<fs.WriteStream>`

`options` may also include a `start` option to allow writing data at some position past the beginning of the file, allowed values are in the [0, `Number.MAX_SAFE_INTEGER`] range. Modifying a file rather than replacing it may require the `flags` option to be set to `r+` rather than the default `w`. The `encoding` can be any one of those accepted by `<Buffer>`.

If `autoClose` is set to true (default behavior) on `'error'` or `'finish'` the file descriptor will be closed automatically. If `autoClose` is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak.

By default, the stream will emit a `'close'` event after it has been destroyed. Set the `emitClose` option to `false` to change this behavior.

By providing the `fs` option it is possible to override the corresponding `fs` implementations for `open`, `write`, `writev`, and `close`. Overriding `write()` without `writev()` can reduce performance as some optimizations (`_writev()`) will be disabled. When providing the `fs` option, overrides for at least one of `write` and `writev` are required. If no `fd` option is supplied, an override for `open` is also required. If `autoClose` is `true`, an override for `close` is also required.

Like `<fs.ReadStream>`, if `fd` is specified, `<fs.WriteStream>` will ignore the `path` argument and will use the specified file descriptor. This means that no `'open'` event will be emitted. `fd` should be blocking; non-blocking `fd`s should be passed to `<net.Socket>`.

If `options` is a string, then it specifies the encoding.

#### `fs.exists(path, callback)`#

Added in: v0.0.2Deprecated in: v1.0.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

Stability: 0 - Deprecated: Use `fs.stat()` or `fs.access()` instead.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `callback` `<Function>`
  - `exists` `<boolean>`

Test whether or not the element at the given `path` exists by checking with the file system. Then call the `callback` argument with either true or false:

    import { exists } from 'node:fs';

    exists('/etc/passwd', (e) => {
      console.log(e ? 'it exists' : 'no passwd!');
    });
    mjs

The parameters for this callback are not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an `err` parameter, optionally followed by other parameters. The `fs.exists()` callback has only one boolean parameter. This is one reason `fs.access()` is recommended instead of `fs.exists()`.

If `path` is a symbolic link, it is followed. Thus, if `path` exists but points to a non-existent element, the callback will receive the value `false`.

Using `fs.exists()` to check for the existence of a file before calling `fs.open()`, `fs.readFile()`, or `fs.writeFile()` is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

write (NOT RECOMMENDED)

    import { exists, open, close } from 'node:fs';

    exists('myfile', (e) => {
      if (e) {
        console.error('myfile already exists');
      } else {
        open('myfile', 'wx', (err, fd) => {
          if (err) throw err;

          try {
            writeMyData(fd);
          } finally {
            close(fd, (err) => {
              if (err) throw err;
            });
          }
        });
      }
    });
    mjs

write (RECOMMENDED)

    import { open, close } from 'node:fs';
    open('myfile', 'wx', (err, fd) => {
      if (err) {
        if (err.code === 'EEXIST') {
          console.error('myfile already exists');
          return;
        }

        throw err;
      }

      try {
        writeMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
    mjs

read (NOT RECOMMENDED)

    import { open, close, exists } from 'node:fs';

    exists('myfile', (e) => {
      if (e) {
        open('myfile', 'r', (err, fd) => {
          if (err) throw err;

          try {
            readMyData(fd);
          } finally {
            close(fd, (err) => {
              if (err) throw err;
            });
          }
        });
      } else {
        console.error('myfile does not exist');
      }
    });
    mjs

read (RECOMMENDED)

    import { open, close } from 'node:fs';

    open('myfile', 'r', (err, fd) => {
      if (err) {
        if (err.code === 'ENOENT') {
          console.error('myfile does not exist');
          return;
        }

        throw err;
      }

      try {
        readMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
    mjs

The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the existence of a file only if the file won't be used directly, for example when its existence is a signal from another process.

#### `fs.fchmod(fd, mode, callback)`#

Added in: v0.4.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `mode` `<string>` | `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Sets the permissions on the file. No arguments other than a possible exception are given to the completion callback.

See the POSIX `fchmod(2)` documentation for more detail.

#### `fs.fchown(fd, uid, gid, callback)`#

Added in: v0.4.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `uid` `<integer>`
- `gid` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Sets the owner of the file. No arguments other than a possible exception are given to the completion callback.

See the POSIX `fchown(2)` documentation for more detail.

#### `fs.fdatasync(fd, callback)`#

Added in: v0.1.96
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX `fdatasync(2)` documentation for details. No arguments other than a possible exception are given to the completion callback.

#### `fs.fstat(fd[, options], callback)`#

Added in: v0.1.95
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `stats` `<fs.Stats>`

Invokes the callback with the `<fs.Stats>` for the file descriptor.

See the POSIX `fstat(2)` documentation for more detail.

#### `fs.fsync(fd, callback)`#

Added in: v0.1.96
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX `fsync(2)` documentation for more detail. No arguments other than a possible exception are given to the completion callback.

#### `fs.ftruncate(fd[, len], callback)`#

Added in: v0.8.6
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `len` `<integer>` Default: `0`
- `callback` `<Function>`
  - `err` `<Error>`

Truncates the file descriptor. No arguments other than a possible exception are given to the completion callback.

See the POSIX `ftruncate(2)` documentation for more detail.

If the file referred to by the file descriptor was larger than `len` bytes, only the first `len` bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file:

    import { open, close, ftruncate } from 'node:fs';

    function closeFd(fd) {
      close(fd, (err) => {
        if (err) throw err;
      });
    }

    open('temp.txt', 'r+', (err, fd) => {
      if (err) throw err;

      try {
        ftruncate(fd, 4, (err) => {
          closeFd(fd);
          if (err) throw err;
        });
      } catch (err) {
        closeFd(fd);
        if (err) throw err;
      }
    });
    mjs

If the file previously was shorter than `len` bytes, it is extended, and the extended part is filled with null bytes (`'\0'`):

If `len` is negative then `0` will be used.

#### `fs.futimes(fd, atime, mtime, callback)`#

Added in: v0.4.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v4.1.0 | Numeric strings, `NaN`, and `Infinity` are now allowed time specifiers.

- `fd` `<integer>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- `callback` `<Function>`
  - `err` `<Error>`

Change the file system timestamps of the object referenced by the supplied file descriptor. See `fs.utimes()`.

#### `fs.glob(pattern[, options], callback)`#

Added in: v22.0.0
History

Version | Changes
--- | ---
v26.1.0 | Add support for the `followSymlinks` option.
v24.1.0, v22.17.0 | Add support for `URL` instances for `cwd` option.
v24.0.0, v22.17.0 | Marking the API stable.
v23.7.0, v22.14.0 | Add support for `exclude` option to accept glob patterns.
v22.2.0 | Add support for `withFileTypes` as an option.

- `pattern` `<string>` | `<string>`[]

- `options` `<Object>`

  - `cwd` `<string>` | `<URL>` current working directory. Default: `process.cwd()`
  - `exclude` `<Function>` | `<string>`[] Function to filter out files/directories or a list of glob patterns to be excluded. If a function is provided, return `true` to exclude the item, `false` to include it. Default: `undefined`.
  - `followSymlinks` `<boolean>` When `true`, symbolic links to directories are followed while expanding `**` patterns. Default: `false`.
  - `withFileTypes` `<boolean>` `true` if the glob should return paths as Dirents, `false` otherwise. Default: `false`.

- `callback` `<Function>`

  - `err` `<Error>`

- Retrieves the files matching the specified pattern.

When `followSymlinks` is enabled, detected symbolic link cycles are not traversed recursively.

    import { glob } from 'node:fs';

    glob('**/*.js', (err, matches) => {
      if (err) throw err;
      console.log(matches);
    });
    const { glob } = require('node:fs');

    glob('**/*.js', (err, matches) => {
      if (err) throw err;
      console.log(matches);
    });
    javascript

#### `fs.lchmod(path, mode, callback)`#

Deprecated in: v0.4.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v16.0.0 | The error returned may be an `AggregateError` if more than one error is returned.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

Stability: 0 - Deprecated

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>` | `<AggregateError>`

Changes the permissions on a symbolic link. No arguments other than a possible exception are given to the completion callback.

This method is only implemented on macOS.

See the POSIX `lchmod(2)` documentation for more detail.

#### `fs.lchown(path, uid, gid, callback)`#

History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.6.0 | This API is no longer deprecated.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v0.4.7 | Documentation-only deprecation.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `uid` `<integer>`
- `gid` `<integer>`
- `callback` `<Function>`
  - `err` `<Error>`

Set the owner of the symbolic link. No arguments other than a possible exception are given to the completion callback.

See the POSIX `lchown(2)` documentation for more detail.

#### `fs.lutimes(path, atime, mtime, callback)`#

Added in: v14.5.0, v12.19.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- `callback` `<Function>`
  - `err` `<Error>`

Changes the access and modification times of a file in the same way as `fs.utimes()`, with the difference that if the path refers to a symbolic link, then the link is not dereferenced: instead, the timestamps of the symbolic link itself are changed.

No arguments other than a possible exception are given to the completion callback.

#### `fs.link(existingPath, newPath, callback)`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `existingPath` and `newPath` parameters can be WHATWG `URL` objects using `file:` protocol. Support is currently still experimental.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `existingPath` `<string>` | `<Buffer>` | `<URL>`
- `newPath` `<string>` | `<Buffer>` | `<URL>`
- `callback` `<Function>`
  - `err` `<Error>`

Creates a new link from the `existingPath` to the `newPath`. See the POSIX `link(2)` documentation for more detail. No arguments other than a possible exception are given to the completion callback.

#### `fs.lstat(path[, options], callback)`#

Added in: v0.1.30
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `stats` `<fs.Stats>`

Retrieves the `<fs.Stats>` for the symbolic link referred to by the path. The callback gets two arguments `(err, stats)` where `stats` is a `<fs.Stats>` object. `lstat()` is identical to `stat()`, except that if `path` is a symbolic link, then the link itself is stat-ed, not the file that it refers to.

See the POSIX `lstat(2)` documentation for more details.

#### `fs.mkdir(path[, options], callback)`#

Added in: v0.1.8
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v13.11.0, v12.17.0 | In `recursive` mode, the callback now receives the first created path as an argument.
v10.12.0 | The second argument can now be an `options` object with `recursive` and `mode` properties.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>` | `<integer>`
  - `recursive` `<boolean>` Default: `false`
  - `mode` `<string>` | `<integer>` Not supported on Windows. See File modes for more details. Default: `0o777`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `path` `<string>` | `<undefined>` Present only if a directory is created with `recursive` set to `true`.

Asynchronously creates a directory.

The callback is given a possible exception and, if `recursive` is `true`, the first directory path created, `(err[, path])`. `path` can still be `undefined` when `recursive` is `true`, if no directory was created (for instance, if it was previously created).

The optional `options` argument can be an integer specifying `mode` (permission and sticky bits), or an object with a `mode` property and a `recursive` property indicating whether parent directories should be created. Calling `fs.mkdir()` when `path` is a directory that exists results in an error only when `recursive` is false. If `recursive` is false and the directory exists, an `EEXIST` error occurs.

    import { mkdir } from 'node:fs';

    // Create ./tmp/a/apple, regardless of whether ./tmp and ./tmp/a exist.
    mkdir('./tmp/a/apple', { recursive: true }, (err) => {
      if (err) throw err;
    });
    mjs

On Windows, using `fs.mkdir()` on the root directory even with recursion will result in an error:

    import { mkdir } from 'node:fs';

    mkdir('/', { recursive: true }, (err) => {
      // => [Error: EPERM: operation not permitted, mkdir 'C:\']
    });
    mjs

See the POSIX `mkdir(2)` documentation for more details.

#### `fs.mkdtemp(prefix[, options], callback)`#

Added in: v5.10.0
History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | The `prefix` parameter now accepts buffers and URL.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v16.5.0, v14.18.0 | The `prefix` parameter now accepts an empty string.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v6.2.1 | The `callback` parameter is optional now.

- `prefix` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- `callback` `<Function>`
  - `err` `<Error>`
  - `directory` `<string>`

Creates a unique temporary directory.

Generates six random characters to be appended behind a required `prefix` to create a unique temporary directory. Due to platform inconsistencies, avoid trailing `X` characters in `prefix`. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing `X` characters in `prefix` with random characters.

The created directory path is passed as a string to the callback's second parameter.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use.

    import { mkdtemp } from 'node:fs';
    import { join } from 'node:path';
    import { tmpdir } from 'node:os';

    mkdtemp(join(tmpdir(), 'foo-'), (err, directory) => {
      if (err) throw err;
      console.log(directory);
      // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
    });
    mjs

The `fs.mkdtemp()` method will append the six randomly selected characters directly to the `prefix` string. For instance, given a directory `/tmp`, if the intention is to create a temporary directory within `/tmp`, the `prefix` must end with a trailing platform-specific path separator (`require('node:path').sep`).

    import { tmpdir } from 'node:os';
    import { mkdtemp } from 'node:fs';

    // The parent directory for the new temporary directory
    const tmpDir = tmpdir();

    // This method is *INCORRECT*:
    mkdtemp(tmpDir, (err, directory) => {
      if (err) throw err;
      console.log(directory);
      // Will print something similar to `/tmpabc123`.
      // A new temporary directory is created at the file system root
      // rather than *within* the /tmp directory.
    });

    // This method is *CORRECT*:
    import { sep } from 'node:path';
    mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
      if (err) throw err;
      console.log(directory);
      // Will print something similar to `/tmp/abc123`.
      // A new temporary directory is created within
      // the /tmp directory.
    });
    mjs

#### `fs.open(path[, flags[, mode]], callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v11.1.0 | The `flags` argument is now optional and defaults to `'r'`.
v9.9.0 | The `as` and `as+` flags are supported now.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `flags` `<string>` | `<number>` See support of file system `flags`. Default: `'r'`.
- `mode` `<string>` | `<integer>` Default: `0o666` (readable and writable)
- `callback` `<Function>`
  - `err` `<Error>`
  - `fd` `<integer>`

Asynchronous file open. See the POSIX `open(2)` documentation for more details.

`mode` sets the file mode (permission and sticky bits), but only if the file was created. On Windows, only the write permission can be manipulated; see `fs.chmod()`.

The callback gets two arguments `(err, fd)`.

Some characters (`< > : " / \ | ? *`) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

Functions based on `fs.open()` exhibit this behavior as well: `fs.writeFile()`, `fs.readFile()`, etc.

#### `fs.openAsBlob(path[, options])`#

Added in: v19.8.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `type` `<string>` An optional mime type for the blob.
- Returns: `<Promise>` Fulfills with a `<Blob>` upon success.

Returns a `<Blob>` whose data is backed by the given file.

The file must not be modified after the `<Blob>` is created. Any modifications will cause reading the `<Blob>` data to fail with a `DOMException` error. Synchronous stat operations on the file when the `Blob` is created, and before each read in order to detect whether the file data has been modified on disk.

    import { openAsBlob } from 'node:fs';

    const blob = await openAsBlob('the.file.txt');
    const ab = await blob.arrayBuffer();
    blob.stream();
    const { openAsBlob } = require('node:fs');

    (async () => {
      const blob = await openAsBlob('the.file.txt');
      const ab = await blob.arrayBuffer();
      blob.stream();
    })();
    javascript

#### `fs.opendir(path[, options], callback)`#

Added in: v12.12.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | Added `recursive` option.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v13.1.0, v12.16.0 | The `bufferSize` option was introduced.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `bufferSize` `<number>` Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: `32`
  - `recursive` `<boolean>` Default: `false`
- `callback` `<Function>`
  - `err` `<Error>`
  - `dir` `<fs.Dir>`

Asynchronously open a directory. See the POSIX `opendir(3)` documentation for more details.

Creates an `<fs.Dir>`, which contains all further functions for reading from and cleaning up the directory.

The `encoding` option sets the encoding for the `path` while opening the directory and subsequent read operations.

#### `fs.read(fd, buffer, offset, length, position, callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.10.0 | The `buffer` parameter can now be any `TypedArray`, or a `DataView`.
v7.4.0 | The `buffer` parameter can now be a `Uint8Array`.
v6.0.0 | The `length` parameter can now be `0`.

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` The buffer that the data will be written to.
- `offset` `<integer>` The position in `buffer` to write the data to.
- `length` `<integer>` The number of bytes to read.
- `position` `<integer>` | `<bigint>` | `<null>` Specifies where to begin reading from in the file. If `position` is `null` or `-1` , data will be read from the current file position, and the file position will be updated. If `position` is a non-negative integer, the file position will be unchanged.
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesRead` `<integer>`
  - `buffer` `<Buffer>`

Read data from the file specified by `fd`.

The callback is given the three arguments, `(err, bytesRead, buffer)`.

If the file is not modified concurrently, the end-of-file is reached when the number of bytes read is zero.

If this method is invoked as its `util.promisify()`ed version, it returns a promise for an `Object` with `bytesRead` and `buffer` properties.

The `fs.read()` method reads data from the file specified by the file descriptor (`fd`). The `length` argument indicates the maximum number of bytes that Node.js will attempt to read from the kernel. However, the actual number of bytes read (`bytesRead`) can be lower than the specified `length` for various reasons.

For example:

- If the file is shorter than the specified `length`, `bytesRead` will be set to the actual number of bytes read.
- If the file encounters EOF (End of File) before the buffer could be filled, Node.js will read all available bytes until EOF is encountered, and the `bytesRead` parameter in the callback will indicate the actual number of bytes read, which may be less than the specified `length`.
- If the file is on a slow network `filesystem` or encounters any other issue during reading, `bytesRead` can be lower than the specified `length`.

Therefore, when using `fs.read()`, it's important to check the `bytesRead` value to determine how many bytes were actually read from the file. Depending on your application logic, you may need to handle cases where `bytesRead` is lower than the specified `length`, such as by wrapping the read call in a loop if you require a minimum amount of bytes.

This behavior is similar to the POSIX `preadv2` function.

#### `fs.read(fd[, options], callback)`#

Added in: v13.11.0, v12.17.0
History

Version | Changes
--- | ---
v13.11.0, v12.17.0 | Options object can be passed in to make buffer, offset, length, and position optional.

- `fd` `<integer>`
- `options` `<Object>`
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` Default: `Buffer.alloc(16384)`
  - `offset` `<integer>` Default: `0`
  - `length` `<integer>` Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<bigint>` | `<null>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesRead` `<integer>`
  - `buffer` `<Buffer>`

Similar to the `fs.read()` function, this version takes an optional `options` object. If no `options` object is specified, it will default with the above values.

#### `fs.read(fd, buffer[, options], callback)`#

Added in: v18.2.0, v16.17.0

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` The buffer that the data will be written to.
- `options` `<Object>`
  - `offset` `<integer>` Default: `0`
  - `length` `<integer>` Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<bigint>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesRead` `<integer>`
  - `buffer` `<Buffer>`

Similar to the `fs.read()` function, this version takes an optional `options` object. If no `options` object is specified, it will default with the above values.

#### `fs.readdir(path[, options], callback)`#

Added in: v0.1.8
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | Added `recursive` option.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.10.0 | New option `withFileTypes` was added.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v6.0.0 | The `options` parameter was added.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
  - `withFileTypes` `<boolean>` Default: `false`
  - `recursive` `<boolean>` If `true`, reads the contents of a directory recursively. In recursive mode, it will list all files, sub files and directories. Default: `false`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `files` `<string>`[] | `<Buffer>`[] | `<fs.Dirent>`[]

Reads the contents of a directory. The callback gets two arguments `(err, files)` where `files` is an array of the names of the files in the directory excluding `'.'` and `'..'`.

See the POSIX `readdir(3)` documentation for more details.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the filenames passed to the callback. If the `encoding` is set to `'buffer'`, the filenames returned will be passed as `<Buffer>` objects.

If `options.withFileTypes` is set to `true`, the `files` array will contain `<fs.Dirent>` objects.

#### `fs.readFile(path[, options], callback)`#

Added in: v0.1.29
History

Version | Changes
--- | ---
v26.4.0 | Added support for the `buffer` option.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v16.0.0 | The error returned may be an `AggregateError` if more than one error is returned.
v15.2.0, v14.17.0 | The options argument may include an AbortSignal to abort an ongoing readFile request.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v5.1.0 | The `callback` will always be called with `null` as the `error` parameter in case of success.
v5.0.0 | The `path` parameter can be a file descriptor now.

- `path` `<string>` | `<Buffer>` | `<URL>` | `<integer>` filename or file descriptor
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `null`
  - `flag` `<string>` See support of file system `flags`. Default: `'r'`.
  - `signal` `<AbortSignal>` allows aborting an in-progress readFile
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<Function>` A buffer to read into, or a function called with the file size that returns the buffer.
- `callback` `<Function>`
  - `err` `<Error>` | `<AggregateError>`
  - `data` `<string>` | `<Buffer>`

Asynchronously reads the entire contents of a file.

    import { readFile } from 'node:fs';

    readFile('/etc/passwd', (err, data) => {
      if (err) throw err;
      console.log(data);
    });
    mjs

The callback is passed two arguments `(err, data)`, where `data` is the contents of the file.

If no encoding is specified, then the raw buffer is returned.

If `buffer` is provided and no encoding is specified, the returned `<Buffer>` is a view over the supplied buffer containing only the bytes read. If the supplied buffer is too small to contain the entire file, the callback is called with an error.

If `options` is a string, then it specifies the encoding:

    import { readFile } from 'node:fs';

    readFile('/etc/passwd', 'utf8', callback);
    mjs

When the path is a directory, the behavior of `fs.readFile()` and `fs.readFileSync()` is platform-specific. On macOS, Linux, and Windows, an error will be returned. On FreeBSD, a representation of the directory's contents will be returned.

    import { readFile } from 'node:fs';

    // macOS, Linux, and Windows
    readFile('<directory>', (err, data) => {
      // => [Error: EISDIR: illegal operation on a directory, read <directory>]
    });

    //  FreeBSD
    readFile('<directory>', (err, data) => {
      // => null, <data>
    });
    mjs

It is possible to abort an ongoing request using an `AbortSignal`. If a request is aborted the callback is called with an `AbortError`:

    import { readFile } from 'node:fs';

    const controller = new AbortController();
    const signal = controller.signal;
    readFile(fileInfo[0].name, { signal }, (err, buf) => {
      // ...
    });
    // When you want to abort the request
    controller.abort();
    mjs

The `fs.readFile()` function buffers the entire file. To minimize memory costs, when possible prefer streaming via `fs.createReadStream()`.

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering `fs.readFile` performs.

An example using the `buffer` option with a pre-allocated buffer:

    import { Buffer } from 'node:buffer';
    import { readFile } from 'node:fs';

    const buf = Buffer.alloc(16384);
    readFile('/path/to/file', { buffer: buf }, (err, data) => {
      if (err) throw err;
      console.log(data); // A view over `buf` containing only the bytes read
    });
    mjs

An example using the `buffer` option with a function returning a buffer:

    import { Buffer } from 'node:buffer';
    import { readFile } from 'node:fs';

    readFile('/path/to/file', {
      buffer: (size) => Buffer.alloc(size),
    }, (err, data) => {
      if (err) throw err;
      console.log(data);
    });
    mjs

##### File descriptors#

1. Any specified file descriptor has to support reading.
2. If a file descriptor is specified as the `path`, it will not be closed automatically.
3. The reading will begin at the current position. For example, if the file already had `'Hello World'` and six bytes are read with the file descriptor, the call to `fs.readFile()` with the same file descriptor, would give `'World'`, rather than `'Hello World'`.

##### Performance Considerations#

The `fs.readFile()` method asynchronously reads the contents of a file into memory one chunk at a time, allowing the event loop to turn between each chunk. This allows the read operation to have less impact on other activity that may be using the underlying libuv thread pool but means that it will take longer to read a complete file into memory.

The additional read overhead can vary broadly on different systems and depends on the type of file being read. If the file type is not a regular file (a pipe for instance) and Node.js is unable to determine an actual file size, each read operation will load on 64 KiB of data. For regular files, each read will process 512 KiB of data.

For applications that require as-fast-as-possible reading of file contents, it is better to use `fs.read()` directly and for application code to manage reading the full contents of the file itself.

The Node.js GitHub issue #25741 provides more information and a detailed analysis on the performance of `fs.readFile()` for multiple file sizes in different Node.js versions.

#### `fs.readlink(path[, options], callback)`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- `callback` `<Function>`
  - `err` `<Error>`
  - `linkString` `<string>` | `<Buffer>`

Reads the contents of the symbolic link referred to by `path`. The callback gets two arguments `(err, linkString)`.

See the POSIX `readlink(2)` documentation for more details.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the link path passed to the callback. If the `encoding` is set to `'buffer'`, the link path returned will be passed as a `<Buffer>` object.

#### `fs.readv(fd, buffers[, position], callback)`#

Added in: v13.13.0, v12.17.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `fd` `<integer>`
- `buffers` `<ArrayBufferView>`[]
- `position` `<integer>` | `<null>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesRead` `<integer>`
  - `buffers` `<ArrayBufferView>`[]

Read from a file specified by `fd` and write to an array of `ArrayBufferView`s using `readv()`.

`position` is the offset from the beginning of the file from where data should be read. If `typeof position !== 'number'`, the data will be read from the current position.

The callback will be given three arguments: `err`, `bytesRead`, and `buffers`. `bytesRead` is how many bytes were read from the file.

If this method is invoked as its `util.promisify()`ed version, it returns a promise for an `Object` with `bytesRead` and `buffers` properties.

#### `fs.realpath(path[, options], callback)`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v8.0.0 | Pipe/Socket resolve support was added.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v6.4.0 | Calling `realpath` now works again for various edge cases on Windows.
v6.0.0 | The `cache` parameter was removed.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- `callback` `<Function>`
  - `err` `<Error>`
  - `resolvedPath` `<string>` | `<Buffer>`

Asynchronously computes the canonical pathname by resolving `.`, `..`, and symbolic links.

A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.

This function behaves like `realpath(3)`, with some exceptions:

1. No case conversion is performed on case-insensitive file systems.

2. The maximum number of symbolic links is platform-independent and generally (much) higher than what the native `realpath(3)` implementation supports.

The `callback` gets two arguments `(err, resolvedPath)`. May use `process.cwd` to resolve relative paths.

Only paths that can be converted to UTF8 strings are supported.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the path passed to the callback. If the `encoding` is set to `'buffer'`, the path returned will be passed as a `<Buffer>` object.

If `path` resolves to a socket or a pipe, the function will return a system dependent name for that object.

A path that does not exist results in an ENOENT error. `error.path` is the absolute file path.

#### `fs.realpath.native(path[, options], callback)`#

Added in: v9.2.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- `callback` `<Function>`
  - `err` `<Error>`
  - `resolvedPath` `<string>` | `<Buffer>`

Asynchronous `realpath(3)`.

The `callback` gets two arguments `(err, resolvedPath)`.

Only paths that can be converted to UTF8 strings are supported.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the path passed to the callback. If the `encoding` is set to `'buffer'`, the path returned will be passed as a `<Buffer>` object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on `/proc` in order for this function to work. Glibc does not have this restriction.

#### `fs.rename(oldPath, newPath, callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `oldPath` and `newPath` parameters can be WHATWG `URL` objects using `file:` protocol. Support is currently still experimental.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `oldPath` `<string>` | `<Buffer>` | `<URL>`
- `newPath` `<string>` | `<Buffer>` | `<URL>`
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously rename file at `oldPath` to the pathname provided as `newPath`. In the case that `newPath` already exists, it will be overwritten. If there is a directory at `newPath`, an error will be raised instead. No arguments other than a possible exception are given to the completion callback.

See also: `rename(2)`.

    import { rename } from 'node:fs';

    rename('oldFile.txt', 'newFile.txt', (err) => {
      if (err) throw err;
      console.log('Rename complete!');
    });
    mjs

#### `fs.rmdir(path[, options], callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v25.0.0 | Remove `recursive` option.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v16.0.0 | Using `fs.rmdir(path, { recursive: true })` on a `path` that is a file is no longer permitted and results in an `ENOENT` error on Windows and an `ENOTDIR` error on POSIX.
v16.0.0 | Using `fs.rmdir(path, { recursive: true })` on a `path` that does not exist is no longer permitted and results in a `ENOENT` error.
v16.0.0 | The `recursive` option is deprecated, using it triggers a deprecation warning.
v14.14.0 | The `recursive` option is deprecated, use `fs.rm` instead.
v13.3.0, v12.16.0 | The `maxBusyTries` option is renamed to `maxRetries`, and its default is 0. The `emfileWait` option has been removed, and `EMFILE` errors use the same retry logic as other errors. The `retryDelay` option is now supported. `ENFILE` errors are now retried.
v12.10.0 | The `recursive`, `maxBusyTries`, and `emfileWait` options are now supported.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameters can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>` There are currently no options exposed. There used to be options for `recursive`, `maxBusyTries`, and `emfileWait` but they were deprecated and removed. The `options` argument is still accepted for backwards compatibility but it is not used.
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronous `rmdir(2)`. No arguments other than a possible exception are given to the completion callback.

Using `fs.rmdir()` on a file (not a directory) results in an `ENOENT` error on Windows and an `ENOTDIR` error on POSIX.

To get a behavior similar to the `rm -rf` Unix command, use `fs.rm()` with options `{ recursive: true, force: true }`.

#### `fs.rm(path[, options], callback)`#

Added in: v14.14.0
History

Version | Changes
--- | ---
v17.3.0, v16.14.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `force` `<boolean>` When `true`, exceptions will be ignored if `path` does not exist. Default: `false`.
  - `maxRetries` `<integer>` If an `EBUSY`, `EMFILE`, `ENFILE`, `ENOTEMPTY`, or `EPERM` error is encountered, Node.js will retry the operation with a linear backoff wait of `retryDelay` milliseconds longer on each try. This option represents the number of retries. This option is ignored if the `recursive` option is not `true`. Default: `0`.
  - `recursive` `<boolean>` If `true`, perform a recursive removal. In recursive mode operations are retried on failure. Default: `false`.
  - `retryDelay` `<integer>` The amount of time in milliseconds to wait between retries. This option is ignored if the `recursive` option is not `true`. Default: `100`.
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously removes files and directories (modeled on the standard POSIX `rm` utility). No arguments other than a possible exception are given to the completion callback.

#### `fs.stat(path[, options], callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v25.7.0 | Accepts a `throwIfNoEntry` option to specify whether an exception should be thrown if the entry does not exist.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
  - `throwIfNoEntry` `<boolean>` Whether an exception will be thrown if no file system entry exists, rather than returning `undefined`. Default: `true`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `stats` `<fs.Stats>`

Asynchronous `stat(2)`. The callback gets two arguments `(err, stats)` where `stats` is an `<fs.Stats>` object.

In case of an error, the `err.code` will be one of Common System Errors.

`fs.stat()` follows symbolic links. Use `fs.lstat()` to look at the links themselves.

Using `fs.stat()` to check for the existence of a file before calling `fs.open()`, `fs.readFile()`, or `fs.writeFile()` is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available.

To check if a file exists without manipulating it afterwards, `fs.access()` is recommended.

For example, given the following directory structure:

    - txtDir
    -- file.txt
    - app.js
    text

The next program will check for the stats of the given paths:

    import { stat } from 'node:fs';

    const pathsToCheck = ['./txtDir', './txtDir/file.txt'];

    for (let i = 0; i < pathsToCheck.length; i++) {
      stat(pathsToCheck[i], (err, stats) => {
        console.log(stats.isDirectory());
        console.log(stats);
      });
    }
    mjs

The resulting output will resemble:

    true
    Stats {
      dev: 16777220,
      mode: 16877,
      nlink: 3,
      uid: 501,
      gid: 20,
      rdev: 0,
      blksize: 4096,
      ino: 14214262,
      size: 96,
      blocks: 0,
      atimeMs: 1561174653071.963,
      mtimeMs: 1561174614583.3518,
      ctimeMs: 1561174626623.5366,
      birthtimeMs: 1561174126937.2893,
      atime: 2019-06-22T03:37:33.072Z,
      mtime: 2019-06-22T03:36:54.583Z,
      ctime: 2019-06-22T03:37:06.624Z,
      birthtime: 2019-06-22T03:28:46.937Z,
      atimeInstant: 2019-06-22T03:37:33.071963Z,
      mtimeInstant: 2019-06-22T03:36:54.5833518Z,
      ctimeInstant: 2019-06-22T03:37:06.6235366Z,
      birthtimeInstant: 2019-06-22T03:28:46.9372893Z
    }
    false
    Stats {
      dev: 16777220,
      mode: 33188,
      nlink: 1,
      uid: 501,
      gid: 20,
      rdev: 0,
      blksize: 4096,
      ino: 14214074,
      size: 8,
      blocks: 8,
      atimeMs: 1561174616618.8555,
      mtimeMs: 1561174614584,
      ctimeMs: 1561174614583.8145,
      birthtimeMs: 1561174007710.7478,
      atime: 2019-06-22T03:36:56.619Z,
      mtime: 2019-06-22T03:36:54.584Z,
      ctime: 2019-06-22T03:36:54.584Z,
      birthtime: 2019-06-22T03:26:47.711Z,
      atimeInstant: 2019-06-22T03:36:56.6188555Z,
      mtimeInstant: 2019-06-22T03:36:54.584Z,
      ctimeInstant: 2019-06-22T03:36:54.5838145Z,
      birthtimeInstant: 2019-06-22T03:26:47.7107478Z
    }
    console

#### `fs.statfs(path[, options], callback)`#

Added in: v19.6.0, v18.15.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.StatFs>` object should be `bigint`. Default: `false`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `stats` `<fs.StatFs>`

Asynchronous `statfs(2)`. Returns information about the mounted file system which contains `path`. The callback gets two arguments `(err, stats)` where `stats` is an `<fs.StatFs>` object.

In case of an error, the `err.code` will be one of Common System Errors.

#### `fs.symlink(target, path[, type], callback)`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v12.0.0 | If the `type` argument is left undefined, Node will autodetect `target` type and automatically select `dir` or `file`.
v7.6.0 | The `target` and `path` parameters can be WHATWG `URL` objects using `file:` protocol. Support is currently still experimental.

- `target` `<string>` | `<Buffer>` | `<URL>`
- `path` `<string>` | `<Buffer>` | `<URL>`
- `type` `<string>` | `<null>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`

Creates the link called `path` pointing to `target`. No arguments other than a possible exception are given to the completion callback.

See the POSIX `symlink(2)` documentation for more details.

The `type` argument is only available on Windows and ignored on other platforms. It can be set to `'dir'`, `'file'`, or `'junction'`. If the `type` argument is `null`, Node.js will autodetect `target` type and use `'file'` or `'dir'`. If the `target` does not exist, `'file'` will be used. Windows junction points require the destination path to be absolute. When using `'junction'`, the `target` argument will automatically be normalized to absolute path. Junction points on NTFS volumes can only point to directories.

Relative targets are relative to the link's parent directory.

    import { symlink } from 'node:fs';

    symlink('./mew', './mewtwo', callback);
    mjs

The above example creates a symbolic link `mewtwo` which points to `mew` in the same directory:

    $ tree .
    .
    ├── mew
    └── mewtwo -> ./mew
    bash

#### `fs.truncate(path[, len], callback)`#

Added in: v0.8.6
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v16.0.0 | The error returned may be an `AggregateError` if more than one error is returned.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `len` `<integer>` Default: `0`
- `callback` `<Function>`
  - `err` `<Error>` | `<AggregateError>`

Truncates the file. No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, `fs.ftruncate()` is called.

    import { truncate } from 'node:fs';
    // Assuming that 'path/file.txt' is a regular file.
    truncate('path/file.txt', (err) => {
      if (err) throw err;
      console.log('path/file.txt was truncated');
    });
    const { truncate } = require('node:fs');
    // Assuming that 'path/file.txt' is a regular file.
    truncate('path/file.txt', (err) => {
      if (err) throw err;
      console.log('path/file.txt was truncated');
    });
    javascript

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

See the POSIX `truncate(2)` documentation for more details.

#### `fs.unlink(path, callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.

    import { unlink } from 'node:fs';
    // Assuming that 'path/file.txt' is a regular file.
    unlink('path/file.txt', (err) => {
      if (err) throw err;
      console.log('path/file.txt was deleted');
    });
    mjs

`fs.unlink()` will not work on a directory, empty or otherwise. To remove a directory, use `fs.rmdir()`.

See the POSIX `unlink(2)` documentation for more details.

#### `fs.unwatchFile(filename[, listener])`#

Added in: v0.1.31

- `filename` `<string>` | `<Buffer>` | `<URL>`
- `listener` `<Function>` Optional, a listener previously attached using `fs.watchFile()`

Stop watching for changes on `filename`. If `listener` is specified, only that particular listener is removed. Otherwise, all listeners are removed, effectively stopping watching of `filename`.

Calling `fs.unwatchFile()` with a filename that is not being watched is a no-op, not an error.

Using `fs.watch()` is more efficient than `fs.watchFile()` and `fs.unwatchFile()`. `fs.watch()` should be used instead of `fs.watchFile()` and `fs.unwatchFile()` when possible.

#### `fs.utimes(path, atime, mtime, callback)`#

Added in: v0.4.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v8.0.0 | `NaN`, `Infinity`, and `-Infinity` are no longer valid time specifiers.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v4.1.0 | Numeric strings, `NaN`, and `Infinity` are now allowed time specifiers.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- `callback` `<Function>`
  - `err` `<Error>`

Change the file system timestamps of the object referenced by `path`.

The `atime` and `mtime` arguments follow these rules:

- Values can be either numbers representing Unix epoch time in seconds, `Date`s, or a numeric string like `'123456789.0'`.
- If the value can not be converted to a number, or is `NaN`, `Infinity`, or `-Infinity`, an `Error` will be thrown.

#### `fs.watch(filename[, options][, listener])`#

Added in: v0.5.10
History

Version | Changes
--- | ---
v26.1.0 | Added `throwIfNoEntry` option.
v19.1.0 | Added recursive support for Linux, AIX and IBMi.
v15.9.0, v14.17.0 | Added support for closing the watcher with an AbortSignal.
v7.6.0 | The `filename` parameter can be a WHATWG `URL` object using `file:` protocol.
v7.0.0 | The passed `options` object will never be modified.

- `filename` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `persistent` `<boolean>` Indicates whether the process should continue to run as long as files are being watched. Default: `true`.
  - `recursive` `<boolean>` Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See caveats). Default: `false`.
  - `encoding` `<string>` Specifies the character encoding to be used for the filename passed to the listener. Default: `'utf8'`.
  - `signal` `<AbortSignal>` allows closing the watcher with an AbortSignal.
  - `throwIfNoEntry` `<boolean>` Indicates whether an exception should be thrown when the path does not exist. Default: `true`.
  - `ignore` `<string>` | `<RegExp>` | `<Function>` | `<Array>` Pattern(s) to ignore. Strings are glob patterns (using `minimatch`), RegExp patterns are tested against the filename, and functions receive the filename and return `true` to ignore. Default: `undefined`.
- `listener` `<Function>` | `<undefined>` Default: `undefined`
  - `eventType` `<string>`
  - `filename` `<string>` | `<Buffer>` | `<null>`
- Returns: `<fs.FSWatcher>`

Watch for changes on `filename`, where `filename` is either a file or a directory.

The second argument is optional. If `options` is provided as a string, it specifies the `encoding`. Otherwise `options` should be passed as an object.

The listener callback gets two arguments `(eventType, filename)`. `eventType` is either `'rename'` or `'change'`, and `filename` is the name of the file which triggered the event.

On most platforms, `'rename'` is emitted whenever a filename appears or disappears in the directory.

The listener callback is attached to the `'change'` event fired by `<fs.FSWatcher>`, but it is not the same thing as the `'change'` value of `eventType`.

If a `signal` is passed, aborting the corresponding AbortController will close the returned `<fs.FSWatcher>`.

##### Caveats#

The `fs.watch` API is not 100% consistent across platforms, and is unavailable in some situations.

On Windows, no events will be emitted if the watched directory is moved or renamed. An `EPERM` error is reported when the watched directory is deleted.

The `fs.watch` API does not provide any protection with respect to malicious actions on the file system. For example, on Windows it is implemented by monitoring changes in a directory versus specific files. This allows substitution of a file and fs reporting changes on the new file with the same filename.

###### Availability#

This feature depends on the underlying operating system providing a way to be notified of file system changes.

- On Linux systems, this uses `inotify(7)`.
- On BSD systems, this uses `kqueue(2)`.
- On macOS, this uses `kqueue(2)` for files and `FSEvents` for directories.
- On SunOS systems (including Solaris and SmartOS), this uses `event ports`.
- On Windows systems, this feature depends on `ReadDirectoryChangesW`.
- On AIX systems, this feature depends on `AHAFS`, which must be enabled.
- On IBM i systems, this feature is not supported.

If the underlying functionality is not available for some reason, then `fs.watch()` will not be able to function and may throw an exception. For example, watching files or directories can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc) or host file systems when using virtualization software such as Vagrant or Docker.

It is still possible to use `fs.watchFile()`, which uses stat polling, but this method is slower and less reliable.

###### Inodes#

On Linux and macOS systems, `fs.watch()` resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior.

AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).

###### Filename argument#

Providing `filename` argument in the callback is only supported on Linux, macOS, Windows, and AIX. Even on supported platforms, `filename` is not always guaranteed to be provided. Therefore, don't assume that `filename` argument is always provided in the callback, and have some fallback logic if it is `null`.

    import { watch } from 'node:fs';
    watch('somedir', (eventType, filename) => {
      console.log(`event type is: ${eventType}`);
      if (filename) {
        console.log(`filename provided: ${filename}`);
      } else {
        console.log('filename not provided');
      }
    });
    mjs

#### `fs.watchFile(filename[, options], listener)`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v10.5.0 | The `bigint` option is now supported.
v7.6.0 | The `filename` parameter can be a WHATWG `URL` object using `file:` protocol.

- `filename` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Default: `false`
  - `persistent` `<boolean>` Default: `true`
  - `interval` `<integer>` Default: `5007`
- `listener` `<Function>`
  - `current` `<fs.Stats>`
  - `previous` `<fs.Stats>`
- Returns: `<fs.StatWatcher>`

Watch for changes on `filename`. The callback `listener` will be called each time the file is accessed.

The `options` argument may be omitted. If provided, it should be an object. The `options` object may contain a boolean named `persistent` that indicates whether the process should continue to run as long as files are being watched. The `options` object may specify an `interval` property indicating how often the target should be polled in milliseconds.

The `listener` gets two arguments the current stat object and the previous stat object:

    import { watchFile } from 'node:fs';

    watchFile('message.text', (curr, prev) => {
      console.log(`the current mtime is: ${curr.mtime}`);
      console.log(`the previous mtime was: ${prev.mtime}`);
    });
    mjs

These stat objects are instances of `fs.Stat`. If the `bigint` option is `true`, the numeric values in these objects are specified as `BigInt`s.

To be notified when the file was modified, not just accessed, it is necessary to compare `curr.mtimeMs` and `prev.mtimeMs`.

When an `fs.watchFile` operation results in an `ENOENT` error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.

Using `fs.watch()` is more efficient than `fs.watchFile` and `fs.unwatchFile`. `fs.watch` should be used instead of `fs.watchFile` and `fs.unwatchFile` when possible.

When a file being watched by `fs.watchFile()` disappears and reappears, then the contents of `previous` in the second callback event (the file's reappearance) will be the same as the contents of `previous` in the first callback event (its disappearance).

This happens when:

- the file is deleted, followed by a restore
- the file is renamed and then renamed a second time back to its original name

#### `fs.write(fd, buffer, offset[, length[, position]], callback)`#

Added in: v0.0.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v14.0.0 | The `buffer` parameter won't coerce unsupported input to strings anymore.
v10.10.0 | The `buffer` parameter can now be any `TypedArray` or a `DataView`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.4.0 | The `buffer` parameter can now be a `Uint8Array`.
v7.2.0 | The `offset` and `length` parameters are optional now.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `offset` `<integer>` Default: `0`
- `length` `<integer>` Default: `buffer.byteLength - offset`
- `position` `<integer>` | `<null>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesWritten` `<integer>`
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`

Write `buffer` to the file specified by `fd`.

`offset` determines the part of the buffer to be written, and `length` is an integer specifying the number of bytes to write.

`position` refers to the offset from the beginning of the file where this data should be written. If `typeof position !== 'number'`, the data will be written at the current position. See `pwrite(2)`.

The callback will be given three arguments `(err, bytesWritten, buffer)` where `bytesWritten` specifies how many bytes were written from `buffer`.

If this method is invoked as its `util.promisify()`ed version, it returns a promise for an `Object` with `bytesWritten` and `buffer` properties.

It is unsafe to use `fs.write()` multiple times on the same file without waiting for the callback. For this scenario, `fs.createWriteStream()` is recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

#### `fs.write(fd, buffer[, options], callback)`#

Added in: v18.3.0, v16.17.0

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>`
  - `offset` `<integer>` Default: `0`
  - `length` `<integer>` Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<null>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesWritten` `<integer>`
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`

Write `buffer` to the file specified by `fd`.

Similar to the above `fs.write` function, this version takes an optional `options` object. If no `options` object is specified, it will default with the above values.

#### `fs.write(fd, string[, position[, encoding]], callback)`#

Added in: v0.11.5
History

Version | Changes
--- | ---
v19.0.0 | Passing to the `string` parameter an object with an own `toString` function is no longer supported.
v17.8.0 | Passing to the `string` parameter an object with an own `toString` function is deprecated.
v14.12.0 | The `string` parameter will stringify an object with an explicit `toString` function.
v14.0.0 | The `string` parameter won't coerce unsupported input to strings anymore.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.2.0 | The `position` parameter is optional now.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

- `fd` `<integer>`
- `string` `<string>`
- `position` `<integer>` | `<null>` Default: `null`
- `encoding` `<string>` Default: `'utf8'`
- `callback` `<Function>`
  - `err` `<Error>`
  - `written` `<integer>`
  - `string` `<string>`

Write `string` to the file specified by `fd`. If `string` is not a string, an exception is thrown.

`position` refers to the offset from the beginning of the file where this data should be written. If `typeof position !== 'number'` the data will be written at the current position. See `pwrite(2)`.

`encoding` is the expected string encoding.

The callback will receive the arguments `(err, written, string)` where `written` specifies how many bytes the passed string required to be written. Bytes written is not necessarily the same as string characters written. See `Buffer.byteLength`.

It is unsafe to use `fs.write()` multiple times on the same file without waiting for the callback. For this scenario, `fs.createWriteStream()` is recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

On Windows, if the file descriptor is connected to the console (e.g. `fd == 1` or `stdout`) a string containing non-ASCII characters will not be rendered properly by default, regardless of the encoding used. It is possible to configure the console to render UTF-8 properly by changing the active codepage with the `chcp 65001` command. See the chcp docs for more details.

#### `fs.writeFile(file, data[, options], callback)`#

Added in: v0.1.29
History

Version | Changes
--- | ---
v21.0.0, v20.10.0 | The `flush` option is now supported.
v19.0.0 | Passing to the `string` parameter an object with an own `toString` function is no longer supported.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v17.8.0 | Passing to the `string` parameter an object with an own `toString` function is deprecated.
v16.0.0 | The error returned may be an `AggregateError` if more than one error is returned.
v15.2.0, v14.17.0 | The options argument may include an AbortSignal to abort an ongoing writeFile request.
v14.12.0 | The `data` parameter will stringify an object with an explicit `toString` function.
v14.0.0 | The `data` parameter won't coerce unsupported input to strings anymore.
v10.10.0 | The `data` parameter can now be any `TypedArray` or a `DataView`.
v10.0.0 | The `callback` parameter is no longer optional. Not passing it will throw a `TypeError` at runtime.
v7.4.0 | The `data` parameter can now be a `Uint8Array`.
v7.0.0 | The `callback` parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.
v5.0.0 | The `file` parameter can be a file descriptor now.

- `file` `<string>` | `<Buffer>` | `<URL>` | `<integer>` filename or file descriptor
- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `mode` `<integer>` Default: `0o666`
  - `flag` `<string>` See support of file system `flags`. Default: `'w'`.
  - `flush` `<boolean>` If all data is successfully written to the file, and `flush` is `true`, `fs.fsync()` is used to flush the data. Default: `false`.
  - `signal` `<AbortSignal>` allows aborting an in-progress writeFile
- `callback` `<Function>`
  - `err` `<Error>` | `<AggregateError>`

When `file` is a filename, asynchronously writes data to the file, replacing the file if it already exists. `data` can be a string or a buffer.

When `file` is a file descriptor, the behavior is similar to calling `fs.write()` directly (which is recommended). See the notes below on using a file descriptor.

The `encoding` option is ignored if `data` is a buffer.

The `mode` option only affects the newly created file. See `fs.open()` for more details.

    import { writeFile } from 'node:fs';
    import { Buffer } from 'node:buffer';

    const data = new Uint8Array(Buffer.from('Hello Node.js'));
    writeFile('message.txt', data, (err) => {
      if (err) throw err;
      console.log('The file has been saved!');
    });
    mjs

If `options` is a string, then it specifies the encoding:

    import { writeFile } from 'node:fs';

    writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
    mjs

It is unsafe to use `fs.writeFile()` multiple times on the same file without waiting for the callback. For this scenario, `fs.createWriteStream()` is recommended.

Similarly to `fs.readFile` - `fs.writeFile` is a convenience method that performs multiple `write` calls internally to write the buffer passed to it. For performance sensitive code consider using `fs.createWriteStream()`.

It is possible to use an `<AbortSignal>` to cancel an `fs.writeFile()`. Cancelation is "best effort", and some amount of data is likely still to be written.

    import { writeFile } from 'node:fs';
    import { Buffer } from 'node:buffer';

    const controller = new AbortController();
    const { signal } = controller;
    const data = new Uint8Array(Buffer.from('Hello Node.js'));
    writeFile('message.txt', data, { signal }, (err) => {
      // When a request is aborted - the callback is called with an AbortError
    });
    // When the request should be aborted
    controller.abort();
    mjs

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering `fs.writeFile` performs.

##### Using `fs.writeFile()` with file descriptors#

When `file` is a file descriptor, the behavior is almost identical to directly calling `fs.write()` like:

    import { write } from 'node:fs';
    import { Buffer } from 'node:buffer';

    write(fd, Buffer.from(data, options.encoding), callback);
    mjs

The difference from directly calling `fs.write()` is that under some unusual conditions, `fs.write()` might write only part of the buffer and need to be retried to write the remaining data, whereas `fs.writeFile()` retries until the data is entirely written (or an error occurs).

The implications of this are a common source of confusion. In the file descriptor case, the file is not replaced! The data is not necessarily written to the beginning of the file, and the file's original data may remain before and/or after the newly written data.

For example, if `fs.writeFile()` is called twice in a row, first to write the string `'Hello'`, then to write the string `', World'`, the file would contain `'Hello, World'`, and might contain some of the file's original data (depending on the size of the original file, and the position of the file descriptor). If a file name had been used instead of a descriptor, the file would be guaranteed to contain only `', World'`.

#### `fs.writev(fd, buffers[, position], callback)`#

Added in: v12.9.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `fd` `<integer>`
- `buffers` `<ArrayBufferView>`[]
- `position` `<integer>` | `<null>` Default: `null`
- `callback` `<Function>`
  - `err` `<Error>`
  - `bytesWritten` `<integer>`
  - `buffers` `<ArrayBufferView>`[]

Write an array of `ArrayBufferView`s to the file specified by `fd` using `writev()`.

`position` is the offset from the beginning of the file where this data should be written. If `typeof position !== 'number'`, the data will be written at the current position.

The callback will be given three arguments: `err`, `bytesWritten`, and `buffers`. `bytesWritten` is how many bytes were written from `buffers`.

If this method is `util.promisify()`ed, it returns a promise for an `Object` with `bytesWritten` and `buffers` properties.

It is unsafe to use `fs.writev()` multiple times on the same file without waiting for the callback. For this scenario, use `fs.createWriteStream()`.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

### Synchronous API#

The synchronous APIs perform all operations synchronously, blocking the event loop until the operation completes or fails.

#### `fs.accessSync(path[, mode])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<integer>` Default: `fs.constants.F_OK`

Synchronously tests a user's permissions for the file or directory specified by `path`. The `mode` argument is an optional integer that specifies the accessibility checks to be performed. `mode` should be either the value `fs.constants.F_OK` or a mask consisting of the bitwise OR of any of `fs.constants.R_OK`, `fs.constants.W_OK`, and `fs.constants.X_OK` (e.g. `fs.constants.W_OK | fs.constants.R_OK`). Check File access constants for possible values of `mode`.

If any of the accessibility checks fail, an `Error` will be thrown. Otherwise, the method will return `undefined`.

    import { accessSync, constants } from 'node:fs';

    try {
      accessSync('etc/passwd', constants.R_OK | constants.W_OK);
      console.log('can read/write');
    } catch (err) {
      console.error('no access!');
    }
    mjs

#### `fs.appendFileSync(path, data[, options])`#

Added in: v0.6.7
History

Version | Changes
--- | ---
v21.1.0, v20.10.0 | The `flush` option is now supported.
v7.0.0 | The passed `options` object will never be modified.
v5.0.0 | The `file` parameter can be a file descriptor now.

- `path` `<string>` | `<Buffer>` | `<URL>` | `<number>` filename or file descriptor
- `data` `<string>` | `<Buffer>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `mode` `<integer>` Default: `0o666`
  - `flag` `<string>` See support of file system `flags`. Default: `'a'`.
  - `flush` `<boolean>` If `true`, the underlying file descriptor is flushed prior to closing it. Default: `false`.

Synchronously append data to a file, creating the file if it does not yet exist. `data` can be a string or a `<Buffer>`.

The `mode` option only affects the newly created file. See `fs.open()` for more details.

    import { appendFileSync } from 'node:fs';

    try {
      appendFileSync('message.txt', 'data to append');
      console.log('The "data to append" was appended to file!');
    } catch (err) {
      /* Handle the error */
    }
    mjs

If `options` is a string, then it specifies the encoding:

    import { appendFileSync } from 'node:fs';

    appendFileSync('message.txt', 'data to append', 'utf8');
    mjs

The `path` may be specified as a numeric file descriptor that has been opened for appending (using `fs.open()` or `fs.openSync()`). The file descriptor will not be closed automatically.

    import { openSync, closeSync, appendFileSync } from 'node:fs';

    let fd;

    try {
      fd = openSync('message.txt', 'a');
      appendFileSync(fd, 'data to append', 'utf8');
    } catch (err) {
      /* Handle the error */
    } finally {
      if (fd !== undefined)
        closeSync(fd);
    }
    mjs

#### `fs.chmodSync(path, mode)`#

Added in: v0.6.7
History

Version | Changes
--- | ---
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<string>` | `<integer>`

For detailed information, see the documentation of the asynchronous version of this API: `fs.chmod()`.

See the POSIX `chmod(2)` documentation for more detail.

#### `fs.chownSync(path, uid, gid)`#

Added in: v0.1.97
History

Version | Changes
--- | ---
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `uid` `<integer>`
- `gid` `<integer>`

Synchronously changes owner and group of a file. Returns `undefined`. This is the synchronous version of `fs.chown()`.

See the POSIX `chown(2)` documentation for more detail.

#### `fs.closeSync(fd)`#

Added in: v0.1.21

- `fd` `<integer>`

Closes the file descriptor. Returns `undefined`.

Calling `fs.closeSync()` on any file descriptor (`fd`) that is currently in use through any other `fs` operation may lead to undefined behavior.

See the POSIX `close(2)` documentation for more detail.

#### `fs.copyFileSync(src, dest[, mode])`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v14.0.0 | Changed `flags` argument to `mode` and imposed stricter type validation.

- `src` `<string>` | `<Buffer>` | `<URL>` source filename to copy
- `dest` `<string>` | `<Buffer>` | `<URL>` destination filename of the copy operation
- `mode` `<integer>` modifiers for copy operation. Default: `0`.

Synchronously copies `src` to `dest`. By default, `dest` is overwritten if it already exists. Returns `undefined`. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

`mode` is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. `fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE`).

- `fs.constants.COPYFILE_EXCL`: The copy operation will fail if `dest` already exists.
- `fs.constants.COPYFILE_FICLONE`: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
- `fs.constants.COPYFILE_FICLONE_FORCE`: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.

    import { copyFileSync, constants } from 'node:fs';

    // destination.txt will be created or overwritten by default.
    copyFileSync('source.txt', 'destination.txt');
    console.log('source.txt was copied to destination.txt');

    // By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
    copyFileSync('source.txt', 'destination.txt', constants.COPYFILE_EXCL);
    mjs

#### `fs.cpSync(src, dest[, options])`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v22.3.0 | This API is no longer experimental.
v20.1.0, v18.17.0 | Accept an additional `mode` option to specify the copy behavior as the `mode` argument of `fs.copyFile()`.
v17.6.0, v16.15.0 | Accepts an additional `verbatimSymlinks` option to specify whether to perform path resolution for symlinks.

- `src` `<string>` | `<URL>` source path to copy.
- `dest` `<string>` | `<URL>` destination path to copy to.
- `options` `<Object>`
  - `dereference` `<boolean>` dereference symlinks. Default: `false`.
  - `errorOnExist` `<boolean>` when `force` is `false`, and the destination exists, throw an error. Default: `false`.
  - `filter` `<Function>` Function to filter copied files/directories. Return `true` to copy the item, `false` to ignore it. When ignoring a directory, all of its contents will be skipped as well. Default: `undefined`
    - `src` `<string>` source path to copy.
    - `dest` `<string>` destination path to copy to.
    - Returns: `<boolean>` Any non-`Promise` value that is coercible to `boolean`.
  - `force` `<boolean>` overwrite existing file or directory. The copy operation will ignore errors if you set this to false and the destination exists. Use the `errorOnExist` option to change this behavior. Default: `true`.
  - `mode` `<integer>` modifiers for copy operation. Default: `0`. See `mode` flag of `fs.copyFileSync()`.
  - `preserveTimestamps` `<boolean>` When `true` timestamps from `src` will be preserved. Default: `false`.
  - `recursive` `<boolean>` copy directories recursively Default: `false`
  - `verbatimSymlinks` `<boolean>` When `true`, path resolution for symlinks will be skipped. Default: `false`

Synchronously copies the entire directory structure from `src` to `dest`, including subdirectories and files.

When copying a directory to another directory, globs are not supported and behavior is similar to `cp dir1/ dir2/`.

#### `fs.existsSync(path)`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- Returns: `<boolean>`

Returns `true` if the path exists, `false` otherwise.

For detailed information, see the documentation of the asynchronous version of this API: `fs.exists()`.

`fs.exists()` is deprecated, but `fs.existsSync()` is not. The `callback` parameter to `fs.exists()` accepts parameters that are inconsistent with other Node.js callbacks. `fs.existsSync()` does not use a callback.

    import { existsSync } from 'node:fs';

    if (existsSync('/etc/passwd'))
      console.log('The path exists.');
    mjs

#### `fs.fchmodSync(fd, mode)`#

Added in: v0.4.7

- `fd` `<integer>`
- `mode` `<string>` | `<integer>`

Sets the permissions on the file. Returns `undefined`.

See the POSIX `fchmod(2)` documentation for more detail.

#### `fs.fchownSync(fd, uid, gid)`#

Added in: v0.4.7

- `fd` `<integer>`
- `uid` `<integer>` The file's new owner's user id.
- `gid` `<integer>` The file's new group's group id.

Sets the owner of the file. Returns `undefined`.

See the POSIX `fchown(2)` documentation for more detail.

#### `fs.fdatasyncSync(fd)`#

Added in: v0.1.96

- `fd` `<integer>`

Forces all currently queued I/O operations associated with the file to the operating system's synchronized I/O completion state. Refer to the POSIX `fdatasync(2)` documentation for details. Returns `undefined`.

#### `fs.fstatSync(fd[, options])`#

Added in: v0.1.95
History

Version | Changes
--- | ---
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.

- `fd` `<integer>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
- Returns: `<fs.Stats>`

Retrieves the `<fs.Stats>` for the file descriptor.

See the POSIX `fstat(2)` documentation for more detail.

#### `fs.fsyncSync(fd)`#

Added in: v0.1.96

- `fd` `<integer>`

Request that all data for the open file descriptor is flushed to the storage device. The specific implementation is operating system and device specific. Refer to the POSIX `fsync(2)` documentation for more detail. Returns `undefined`.

#### `fs.ftruncateSync(fd[, len])`#

Added in: v0.8.6

- `fd` `<integer>`
- `len` `<integer>` Default: `0`

Truncates the file descriptor. Returns `undefined`.

For detailed information, see the documentation of the asynchronous version of this API: `fs.ftruncate()`.

#### `fs.futimesSync(fd, atime, mtime)`#

Added in: v0.4.2
History

Version | Changes
--- | ---
v4.1.0 | Numeric strings, `NaN`, and `Infinity` are now allowed time specifiers.

- `fd` `<integer>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`

Synchronous version of `fs.futimes()`. Returns `undefined`.

#### `fs.globSync(pattern[, options])`#

Added in: v22.0.0
History

Version | Changes
--- | ---
v26.1.0 | Add support for the `followSymlinks` option.
v24.1.0, v22.17.0 | Add support for `URL` instances for `cwd` option.
v24.0.0, v22.17.0 | Marking the API stable.
v23.7.0, v22.14.0 | Add support for `exclude` option to accept glob patterns.
v22.2.0 | Add support for `withFileTypes` as an option.

- `pattern` `<string>` | `<string>`[]
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` current working directory. Default: `process.cwd()`
  - `exclude` `<Function>` | `<string>`[] Function to filter out files/directories or a list of glob patterns to be excluded. If a function is provided, return `true` to exclude the item, `false` to include it. Default: `undefined`.
  - `followSymlinks` `<boolean>` When `true`, symbolic links to directories are followed while expanding `**` patterns. Default: `false`.
  - `withFileTypes` `<boolean>` `true` if the glob should return paths as Dirents, `false` otherwise. Default: `false`.
- Returns: `<string>`[] paths of files that match the pattern.

When `followSymlinks` is enabled, detected symbolic link cycles are not traversed recursively.

    import { globSync } from 'node:fs';

    console.log(globSync('**/*.js'));
    const { globSync } = require('node:fs');

    console.log(globSync('**/*.js'));
    javascript

#### `fs.lchmodSync(path, mode)`#

Deprecated in: v0.4.7

Stability: 0 - Deprecated

- `path` `<string>` | `<Buffer>` | `<URL>`
- `mode` `<integer>`

Changes the permissions on a symbolic link. Returns `undefined`.

This method is only implemented on macOS.

See the POSIX `lchmod(2)` documentation for more detail.

#### `fs.lchownSync(path, uid, gid)`#

History

Version | Changes
--- | ---
v10.6.0 | This API is no longer deprecated.
v0.4.7 | Documentation-only deprecation.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `uid` `<integer>` The file's new owner's user id.
- `gid` `<integer>` The file's new group's group id.

Set the owner for the path. Returns `undefined`.

See the POSIX `lchown(2)` documentation for more details.

#### `fs.lutimesSync(path, atime, mtime)`#

Added in: v14.5.0, v12.19.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`

Change the file system timestamps of the symbolic link referenced by `path`. Returns `undefined`, or throws an exception when parameters are incorrect or the operation fails. This is the synchronous version of `fs.lutimes()`.

#### `fs.linkSync(existingPath, newPath)`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v7.6.0 | The `existingPath` and `newPath` parameters can be WHATWG `URL` objects using `file:` protocol. Support is currently still experimental.

- `existingPath` `<string>` | `<Buffer>` | `<URL>`
- `newPath` `<string>` | `<Buffer>` | `<URL>`

Creates a new link from the `existingPath` to the `newPath`. See the POSIX `link(2)` documentation for more detail. Returns `undefined`.

#### `fs.lstatSync(path[, options])`#

Added in: v0.1.30
History

Version | Changes
--- | ---
v15.3.0, v14.17.0 | Accepts a `throwIfNoEntry` option to specify whether an exception should be thrown if the entry does not exist.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
  - `throwIfNoEntry` `<boolean>` Whether an exception will be thrown if no file system entry exists, rather than returning `undefined`. Default: `true`.
- Returns: `<fs.Stats>`

Retrieves the `<fs.Stats>` for the symbolic link referred to by `path`.

See the POSIX `lstat(2)` documentation for more details.

#### `fs.mkdirSync(path[, options])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v13.11.0, v12.17.0 | In `recursive` mode, the first created path is returned now.
v10.12.0 | The second argument can now be an `options` object with `recursive` and `mode` properties.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>` | `<integer>`
  - `recursive` `<boolean>` Default: `false`
  - `mode` `<string>` | `<integer>` Not supported on Windows. Default: `0o777`.
- Returns: `<string>` | `<undefined>`

Synchronously creates a directory. Returns `undefined`, or if `recursive` is `true`, the first directory path created. This is the synchronous version of `fs.mkdir()`.

See the POSIX `mkdir(2)` documentation for more details.

#### `fs.mkdtempSync(prefix[, options])`#

Added in: v5.10.0
History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | The `prefix` parameter now accepts buffers and URL.
v16.5.0, v14.18.0 | The `prefix` parameter now accepts an empty string.

- `prefix` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<string>`

Returns the created directory path.

For detailed information, see the documentation of the asynchronous version of this API: `fs.mkdtemp()`.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use.

#### `fs.mkdtempDisposableSync(prefix[, options])`#

Added in: v24.4.0

- `prefix` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<Object>` A disposable object:
  - `path` `<string>` The path of the created directory.
  - `remove` `<Function>` A function which removes the created directory.
  - `[Symbol.dispose]` `<Function>` The same as `remove`.

Returns a disposable object whose `path` property holds the created directory path. When the object is disposed, the directory and its contents will be removed if it still exists. If the directory cannot be deleted, disposal will throw an error. The object has a `remove()` method which will perform the same task.

See the MDN documentation on `using` statements for more information about explicit resource management.

For detailed information, see the documentation of `fs.mkdtemp()`.

There is no callback-based version of this API because it is designed for use with the `using` syntax.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use.

#### `fs.opendirSync(path[, options])`#

Added in: v12.12.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | Added `recursive` option.
v13.1.0, v12.16.0 | The `bufferSize` option was introduced.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `bufferSize` `<number>` Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: `32`
  - `recursive` `<boolean>` Default: `false`
- Returns: `<fs.Dir>`

Synchronously open a directory. See `opendir(3)`.

Creates an `<fs.Dir>`, which contains all further functions for reading from and cleaning up the directory.

The `encoding` option sets the encoding for the `path` while opening the directory and subsequent read operations.

#### `fs.openSync(path[, flags[, mode]])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v11.1.0 | The `flags` argument is now optional and defaults to `'r'`.
v9.9.0 | The `as` and `as+` flags are supported now.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `flags` `<string>` | `<number>` Default: `'r'`. See support of file system `flags`.
- `mode` `<string>` | `<integer>` Default: `0o666`
- Returns: `<number>`

Returns an integer representing the file descriptor.

For detailed information, see the documentation of the asynchronous version of this API: `fs.open()`.

#### `fs.readdirSync(path[, options])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | Added `recursive` option.
v10.10.0 | New option `withFileTypes` was added.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
  - `withFileTypes` `<boolean>` Default: `false`
  - `recursive` `<boolean>` If `true`, reads the contents of a directory recursively. In recursive mode, it will list all files, sub files, and directories. Default: `false`.
- Returns: `<string>`[] | `<Buffer>`[] | `<fs.Dirent>`[]

Reads the contents of the directory.

See the POSIX `readdir(3)` documentation for more details.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the filenames returned. If the `encoding` is set to `'buffer'`, the filenames returned will be passed as `<Buffer>` objects.

If `options.withFileTypes` is set to `true`, the result will contain `<fs.Dirent>` objects.

#### `fs.readFileSync(path[, options])`#

Added in: v0.1.8
History

Version | Changes
--- | ---
v26.4.0 | Added support for the `buffer` option.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v5.0.0 | The `path` parameter can be a file descriptor now.

- `path` `<string>` | `<Buffer>` | `<URL>` | `<integer>` filename or file descriptor
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `null`
  - `flag` `<string>` See support of file system `flags`. Default: `'r'`.
  - `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<Function>` A buffer to read into, or a function called with the file size that returns the buffer.
- Returns: `<string>` | `<Buffer>`

Returns the contents of the `path`.

For detailed information, see the documentation of the asynchronous version of this API: `fs.readFile()`.

If the `encoding` option is specified then this function returns a string. Otherwise it returns a buffer.

If `buffer` is provided and no encoding is specified, the returned `<Buffer>` is a view over the supplied buffer containing only the bytes read. If the supplied buffer is too small to contain the entire file, an error will be thrown.

Similar to `fs.readFile()`, when the path is a directory, the behavior of `fs.readFileSync()` is platform-specific.

    import { readFileSync } from 'node:fs';

    // macOS, Linux, and Windows
    readFileSync('<directory>');
    // => [Error: EISDIR: illegal operation on a directory, read <directory>]

    //  FreeBSD
    readFileSync('<directory>'); // => <data>
    mjs

#### `fs.readlinkSync(path[, options])`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<string>` | `<Buffer>`

Returns the symbolic link's string value.

See the POSIX `readlink(2)` documentation for more details.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the link path returned. If the `encoding` is set to `'buffer'`, the link path returned will be passed as a `<Buffer>` object.

#### `fs.readSync(fd, buffer, offset, length[, position])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v10.10.0 | The `buffer` parameter can now be any `TypedArray` or a `DataView`.
v6.0.0 | The `length` parameter can now be `0`.

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `offset` `<integer>`
- `length` `<integer>`
- `position` `<integer>` | `<bigint>` | `<null>` Default: `null`
- Returns: `<number>`

Returns the number of `bytesRead`.

For detailed information, see the documentation of the asynchronous version of this API: `fs.read()`.

#### `fs.readSync(fd, buffer[, options])`#

Added in: v13.13.0, v12.17.0
History

Version | Changes
--- | ---
v13.13.0, v12.17.0 | Options object can be passed in to make offset, length, and position optional.

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>`
  - `offset` `<integer>` Default: `0`
  - `length` `<integer>` Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<bigint>` | `<null>` Default: `null`
- Returns: `<number>`

Returns the number of `bytesRead`.

Similar to the above `fs.readSync` function, this version takes an optional `options` object. If no `options` object is specified, it will default with the above values.

For detailed information, see the documentation of the asynchronous version of this API: `fs.read()`.

#### `fs.readvSync(fd, buffers[, position])`#

Added in: v13.13.0, v12.17.0

- `fd` `<integer>`
- `buffers` `<ArrayBufferView>`[]
- `position` `<integer>` | `<null>` Default: `null`
- Returns: `<number>` The number of bytes read.

For detailed information, see the documentation of the asynchronous version of this API: `fs.readv()`.

#### `fs.realpathSync(path[, options])`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v8.0.0 | Pipe/Socket resolve support was added.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v6.4.0 | Calling `realpathSync` now works again for various edge cases on Windows.
v6.0.0 | The `cache` parameter was removed.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<string>` | `<Buffer>`

Returns the resolved pathname.

For detailed information, see the documentation of the asynchronous version of this API: `fs.realpath()`.

#### `fs.realpathSync.native(path[, options])`#

Added in: v9.2.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<string>` | `<Object>`
  - `encoding` `<string>` Default: `'utf8'`
- Returns: `<string>` | `<Buffer>`

Synchronous `realpath(3)`.

Only paths that can be converted to UTF8 strings are supported.

The optional `options` argument can be a string specifying an encoding, or an object with an `encoding` property specifying the character encoding to use for the path returned. If the `encoding` is set to `'buffer'`, the path returned will be passed as a `<Buffer>` object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on `/proc` in order for this function to work. Glibc does not have this restriction.

#### `fs.renameSync(oldPath, newPath)`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v7.6.0 | The `oldPath` and `newPath` parameters can be WHATWG `URL` objects using `file:` protocol. Support is currently still experimental.

- `oldPath` `<string>` | `<Buffer>` | `<URL>`
- `newPath` `<string>` | `<Buffer>` | `<URL>`

Renames the file from `oldPath` to `newPath`. Returns `undefined`.

See the POSIX `rename(2)` documentation for more details.

#### `fs.rmdirSync(path[, options])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v25.0.0 | Remove `recursive` option.
v16.0.0 | Using `fs.rmdirSync(path, { recursive: true })` on a `path` that is a file is no longer permitted and results in an `ENOENT` error on Windows and an `ENOTDIR` error on POSIX.
v16.0.0 | Using `fs.rmdirSync(path, { recursive: true })` on a `path` that does not exist is no longer permitted and results in a `ENOENT` error.
v16.0.0 | The `recursive` option is deprecated, using it triggers a deprecation warning.
v14.14.0 | The `recursive` option is deprecated, use `fs.rmSync` instead.
v13.3.0, v12.16.0 | The `maxBusyTries` option is renamed to `maxRetries`, and its default is 0. The `emfileWait` option has been removed, and `EMFILE` errors use the same retry logic as other errors. The `retryDelay` option is now supported. `ENFILE` errors are now retried.
v12.10.0 | The `recursive`, `maxBusyTries`, and `emfileWait` options are now supported.
v7.6.0 | The `path` parameters can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>` There are currently no options exposed. There used to be options for `recursive`, `maxBusyTries`, and `emfileWait` but they were deprecated and removed. The `options` argument is still accepted for backwards compatibility but it is not used.

Synchronous `rmdir(2)`. Returns `undefined`.

Using `fs.rmdirSync()` on a file (not a directory) results in an `ENOENT` error on Windows and an `ENOTDIR` error on POSIX.

To get a behavior similar to the `rm -rf` Unix command, use `fs.rmSync()` with options `{ recursive: true, force: true }`.

#### `fs.rmSync(path[, options])`#

Added in: v14.14.0
History

Version | Changes
--- | ---
v17.3.0, v16.14.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `force` `<boolean>` When `true`, exceptions will be ignored if `path` does not exist. Default: `false`.
  - `maxRetries` `<integer>` If an `EBUSY`, `EMFILE`, `ENFILE`, `ENOTEMPTY`, or `EPERM` error is encountered, Node.js will retry the operation with a linear backoff wait of `retryDelay` milliseconds longer on each try. This option represents the number of retries. This option is ignored if the `recursive` option is not `true`. Default: `0`.
  - `recursive` `<boolean>` If `true`, perform a recursive directory removal. In recursive mode operations are retried on failure. Default: `false`.
  - `retryDelay` `<integer>` The amount of time in milliseconds to wait between retries. This option is ignored if the `recursive` option is not `true`. Default: `100`.

Synchronously removes files and directories (modeled on the standard POSIX `rm` utility). Returns `undefined`.

#### `fs.statSync(path[, options])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v15.3.0, v14.17.0 | Accepts a `throwIfNoEntry` option to specify whether an exception should be thrown if the entry does not exist.
v10.5.0 | Accepts an additional `options` object to specify whether the numeric values returned should be bigint.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.Stats>` object should be `bigint`. Default: `false`.
  - `throwIfNoEntry` `<boolean>` Whether an exception will be thrown if no file system entry exists, rather than returning `undefined`. Default: `true`.
- Returns: `<fs.Stats>`

Retrieves the `<fs.Stats>` for the path.

#### `fs.statfsSync(path[, options])`#

Added in: v19.6.0, v18.15.0

- `path` `<string>` | `<Buffer>` | `<URL>`
- `options` `<Object>`
  - `bigint` `<boolean>` Whether the numeric values in the returned `<fs.StatFs>` object should be `bigint`. Default: `false`.
- Returns: `<fs.StatFs>`

Synchronous `statfs(2)`. Returns information about the mounted file system which contains `path`.

In case of an error, the `err.code` will be one of Common System Errors.

#### `fs.symlinkSync(target, path[, type])`#

Added in: v0.1.31
History

Version | Changes
--- | ---
v12.0.0 | If the `type` argument is left undefined, Node will autodetect `target` type and automatically select `dir` or `file`.
v7.6.0 | The `target` and `path` parameters can be WHATWG `URL` objects using `file:` protocol. Support is currently still experimental.

- `target` `<string>` | `<Buffer>` | `<URL>`
- `path` `<string>` | `<Buffer>` | `<URL>`
- `type` `<string>` | `<null>` Default: `null`
- Returns: `undefined`.

For detailed information, see the documentation of the asynchronous version of this API: `fs.symlink()`.

#### `fs.truncateSync(path[, len])`#

Added in: v0.8.6

- `path` `<string>` | `<Buffer>` | `<URL>`
- `len` `<integer>` Default: `0`

Truncates the file. Returns `undefined`. A file descriptor can also be passed as the first argument. In this case, `fs.ftruncateSync()` is called.

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

#### `fs.unlinkSync(path)`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.

- `path` `<string>` | `<Buffer>` | `<URL>`

Synchronous `unlink(2)`. Returns `undefined`.

#### `fs.utimesSync(path, atime, mtime)`#

Added in: v0.4.2
History

Version | Changes
--- | ---
v8.0.0 | `NaN`, `Infinity`, and `-Infinity` are no longer valid time specifiers.
v7.6.0 | The `path` parameter can be a WHATWG `URL` object using `file:` protocol.
v4.1.0 | Numeric strings, `NaN`, and `Infinity` are now allowed time specifiers.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `atime` `<number>` | `<string>` | `<Date>`
- `mtime` `<number>` | `<string>` | `<Date>`
- Returns: `undefined`.

For detailed information, see the documentation of the asynchronous version of this API: `fs.utimes()`.

#### `fs.writeFileSync(file, data[, options])`#

Added in: v0.1.29
History

Version | Changes
--- | ---
v21.0.0, v20.10.0 | The `flush` option is now supported.
v19.0.0 | Passing to the `data` parameter an object with an own `toString` function is no longer supported.
v17.8.0 | Passing to the `data` parameter an object with an own `toString` function is deprecated.
v14.12.0 | The `data` parameter will stringify an object with an explicit `toString` function.
v14.0.0 | The `data` parameter won't coerce unsupported input to strings anymore.
v10.10.0 | The `data` parameter can now be any `TypedArray` or a `DataView`.
v7.4.0 | The `data` parameter can now be a `Uint8Array`.
v5.0.0 | The `file` parameter can be a file descriptor now.

- `file` `<string>` | `<Buffer>` | `<URL>` | `<integer>` filename or file descriptor
- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>` | `<string>`
  - `encoding` `<string>` | `<null>` Default: `'utf8'`
  - `mode` `<integer>` Default: `0o666`
  - `flag` `<string>` See support of file system `flags`. Default: `'w'`.
  - `flush` `<boolean>` If all data is successfully written to the file, and `flush` is `true`, `fs.fsyncSync()` is used to flush the data.
- Returns: `undefined`.

The `mode` option only affects the newly created file. See `fs.open()` for more details.

For detailed information, see the documentation of the asynchronous version of this API: `fs.writeFile()`.

#### `fs.writeSync(fd, buffer, offset[, length[, position]])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v14.0.0 | The `buffer` parameter won't coerce unsupported input to strings anymore.
v10.10.0 | The `buffer` parameter can now be any `TypedArray` or a `DataView`.
v7.4.0 | The `buffer` parameter can now be a `Uint8Array`.
v7.2.0 | The `offset` and `length` parameters are optional now.

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `offset` `<integer>` Default: `0`
- `length` `<integer>` Default: `buffer.byteLength - offset`
- `position` `<integer>` | `<null>` Default: `null`
- Returns: `<number>` The number of bytes written.

For detailed information, see the documentation of the asynchronous version of this API: `fs.write(fd, buffer...)`.

#### `fs.writeSync(fd, buffer[, options])`#

Added in: v18.3.0, v16.17.0

- `fd` `<integer>`
- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>`
  - `offset` `<integer>` Default: `0`
  - `length` `<integer>` Default: `buffer.byteLength - offset`
  - `position` `<integer>` | `<null>` Default: `null`
- Returns: `<number>` The number of bytes written.

For detailed information, see the documentation of the asynchronous version of this API: `fs.write(fd, buffer...)`.

#### `fs.writeSync(fd, string[, position[, encoding]])`#

Added in: v0.11.5
History

Version | Changes
--- | ---
v14.0.0 | The `string` parameter won't coerce unsupported input to strings anymore.
v7.2.0 | The `position` parameter is optional now.

- `fd` `<integer>`
- `string` `<string>`
- `position` `<integer>` | `<null>` Default: `null`
- `encoding` `<string>` Default: `'utf8'`
- Returns: `<number>` The number of bytes written.

For detailed information, see the documentation of the asynchronous version of this API: `fs.write(fd, string...)`.

#### `fs.writevSync(fd, buffers[, position])`#

Added in: v12.9.0

- `fd` `<integer>`
- `buffers` `<ArrayBufferView>`[]
- `position` `<integer>` | `<null>` Default: `null`
- Returns: `<number>` The number of bytes written.

For detailed information, see the documentation of the asynchronous version of this API: `fs.writev()`.

### Common Objects#

The common objects are shared by all of the file system API variants (promise, callback, and synchronous).

#### Class: `fs.Dir`#

Added in: v12.12.0

A class representing a directory stream.

Created by `fs.opendir()`, `fs.opendirSync()`, or `fsPromises.opendir()`.

    import { opendir } from 'node:fs/promises';

    try {
      const dir = await opendir('./');
      for await (const dirent of dir)
        console.log(dirent.name);
    } catch (err) {
      console.error(err);
    }
    mjs

When using the async iterator, the `<fs.Dir>` object will be automatically closed after the iterator exits.

##### `dir.close()`#

Added in: v12.12.0

- Returns: `<Promise>`

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

A promise is returned that will be fulfilled after the resource has been closed.

##### `dir.close(callback)`#

Added in: v12.12.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `callback` `<Function>`
  - `err` `<Error>`

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

The `callback` will be called after the resource handle has been closed.

##### `dir.closeSync()`#

Added in: v12.12.0

Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

##### `dir.path`#

Added in: v12.12.0

- Type: `<string>`

The read-only path of this directory as was provided to `fs.opendir()`, `fs.opendirSync()`, or `fsPromises.opendir()`.

##### `dir.read()`#

Added in: v12.12.0

- Returns: `<Promise>` Fulfills with a `<fs.Dirent>` | `<null>`

Asynchronously read the next directory entry via `readdir(3)` as an `<fs.Dirent>`.

A promise is returned that will be fulfilled with an `<fs.Dirent>`, or `null` if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

##### `dir.read(callback)`#

Added in: v12.12.0

- `callback` `<Function>`
  - `err` `<Error>`
  - `dirent` `<fs.Dirent>` | `<null>`

Asynchronously read the next directory entry via `readdir(3)` as an `<fs.Dirent>`.

After the read is completed, the `callback` will be called with an `<fs.Dirent>`, or `null` if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

##### `dir.readSync()`#

Added in: v12.12.0

- Returns: `<fs.Dirent>` | `<null>`

Synchronously read the next directory entry as an `<fs.Dirent>`. See the POSIX `readdir(3)` documentation for more detail.

If there are no more directory entries to read, `null` will be returned.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

##### `dir[Symbol.asyncIterator]()`#

Added in: v12.12.0

- Returns: `<AsyncIterator>` An AsyncIterator of `<fs.Dirent>`

Asynchronously iterates over the directory until all entries have been read. Refer to the POSIX `readdir(3)` documentation for more detail.

Entries returned by the async iterator are always an `<fs.Dirent>`. The `null` case from `dir.read()` is handled internally.

See `<fs.Dir>` for an example.

Directory entries returned by this iterator are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory might not be included in the iteration results.

##### `dir[Symbol.asyncDispose]()`#

Added in: v24.1.0, v22.1.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `dir.close()` if the directory handle is open, and returns a promise that fulfills when disposal is complete.

This method enables the directory to be used with `await using`, which will automatically close the directory when the scope exits. For more information, see the MDN documentation on `using` statements.

##### `dir[Symbol.dispose]()`#

Added in: v24.1.0, v22.1.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `dir.closeSync()` if the directory handle is open, and returns `undefined`.

This method enables the directory to be used with `using`, which will automatically close the directory when the scope exits. For more information, see the MDN documentation on `using` statements.

#### Class: `fs.Dirent`#

Added in: v10.10.0

A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by reading from an `<fs.Dir>`. The directory entry is a combination of the file name and file type pairs.

Additionally, when `fs.readdir()` or `fs.readdirSync()` is called with the `withFileTypes` option set to `true`, the resulting array is filled with `<fs.Dirent>` objects, rather than strings or `<Buffer>`s.

##### `dirent.isBlockDevice()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a block device.

##### `dirent.isCharacterDevice()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a character device.

##### `dirent.isDirectory()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a file system directory.

##### `dirent.isFIFO()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a first-in-first-out (FIFO) pipe.

##### `dirent.isFile()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a regular file.

##### `dirent.isSocket()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a socket.

##### `dirent.isSymbolicLink()`#

Added in: v10.10.0

- Returns: `<boolean>`

Returns `true` if the `<fs.Dirent>` object describes a symbolic link.

##### `dirent.name`#

Added in: v10.10.0

- Type: `<string>` | `<Buffer>`

The file name that this `<fs.Dirent>` object refers to. The type of this value is determined by the `options.encoding` passed to `fs.readdir()` or `fs.readdirSync()`.

##### `dirent.parentPath`#

Added in: v21.4.0, v20.12.0, v18.20.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- Type: `<string>`

The path to the parent directory of the file this `<fs.Dirent>` object refers to.

#### Class: `fs.FSWatcher`#

Added in: v0.5.8

- Extends `<EventEmitter>`

A successful call to `fs.watch()` method will return a new `<fs.FSWatcher>` object.

All `<fs.FSWatcher>` objects emit a `'change'` event whenever a specific watched file is modified.

##### Event: `'change'`#

Added in: v0.5.8

- `eventType` `<string>` The type of change event that has occurred
- `filename` `<string>` | `<Buffer>` The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file. See more details in `fs.watch()`.

The `filename` argument may not be provided depending on operating system support. If `filename` is provided, it will be provided as a `<Buffer>` if `fs.watch()` is called with its `encoding` option set to `'buffer'`, otherwise `filename` will be a UTF-8 string.

    import { watch } from 'node:fs';
    // Example when handled through fs.watch() listener
    watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
      if (filename) {
        console.log(filename);
        // Prints: <Buffer ...>
      }
    });
    mjs

##### Event: `'close'`#

Added in: v10.0.0

Emitted when the watcher stops watching for changes. The closed `<fs.FSWatcher>` object is no longer usable in the event handler.

##### Event: `'error'`#

Added in: v0.5.8

- `error` `<Error>`

Emitted when an error occurs while watching the file. The errored `<fs.FSWatcher>` object is no longer usable in the event handler.

##### `watcher.close()`#

Added in: v0.5.8

Stop watching for changes on the given `<fs.FSWatcher>`. Once stopped, the `<fs.FSWatcher>` object is no longer usable.

##### `watcher.ref()`#

Added in: v14.3.0, v12.20.0

- Returns: `<fs.FSWatcher>`

When called, requests that the Node.js event loop not exit so long as the `<fs.FSWatcher>` is active. Calling `watcher.ref()` multiple times will have no effect.

By default, all `<fs.FSWatcher>` objects are "ref'ed", making it normally unnecessary to call `watcher.ref()` unless `watcher.unref()` had been called previously.

##### `watcher.unref()`#

Added in: v14.3.0, v12.20.0

- Returns: `<fs.FSWatcher>`

When called, the active `<fs.FSWatcher>` object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the `<fs.FSWatcher>` object's callback is invoked. Calling `watcher.unref()` multiple times will have no effect.

#### Class: `fs.StatWatcher`#

Added in: v14.3.0, v12.20.0

- Extends `<EventEmitter>`

A successful call to `fs.watchFile()` method will return a new `<fs.StatWatcher>` object.

##### `watcher.ref()`#

Added in: v14.3.0, v12.20.0

- Returns: `<fs.StatWatcher>`

When called, requests that the Node.js event loop not exit so long as the `<fs.StatWatcher>` is active. Calling `watcher.ref()` multiple times will have no effect.

By default, all `<fs.StatWatcher>` objects are "ref'ed", making it normally unnecessary to call `watcher.ref()` unless `watcher.unref()` had been called previously.

##### `watcher.unref()`#

Added in: v14.3.0, v12.20.0

- Returns: `<fs.StatWatcher>`

When called, the active `<fs.StatWatcher>` object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the `<fs.StatWatcher>` object's callback is invoked. Calling `watcher.unref()` multiple times will have no effect.

#### Class: `fs.ReadStream`#

Added in: v0.1.93

- Extends: `<stream.Readable>`

Instances of `<fs.ReadStream>` cannot be constructed directly. They are created and returned using the `fs.createReadStream()` function.

##### Event: `'close'`#

Added in: v0.1.93

Emitted when the `<fs.ReadStream>`'s underlying file descriptor has been closed.

##### Event: `'open'`#

Added in: v0.1.93

- `fd` `<integer>` Integer file descriptor used by the `<fs.ReadStream>`.

Emitted when the `<fs.ReadStream>`'s file descriptor has been opened.

##### Event: `'ready'`#

Added in: v9.11.0

Emitted when the `<fs.ReadStream>` is ready to be used.

Fires immediately after `'open'`.

##### `readStream.bytesRead`#

Added in: v6.4.0

- Type: `<number>`

The number of bytes that have been read so far.

##### `readStream.path`#

Added in: v0.1.93

- Type: `<string>` | `<Buffer>`

The path to the file the stream is reading from as specified in the first argument to `fs.createReadStream()`. If `path` is passed as a string, then `readStream.path` will be a string. If `path` is passed as a `<Buffer>`, then `readStream.path` will be a `<Buffer>`. If `fd` is specified, then `readStream.path` will be `undefined`.

##### `readStream.pending`#

Added in: v11.2.0, v10.16.0

- Type: `<boolean>`

This property is `true` if the underlying file has not been opened yet, i.e. before the `'ready'` event is emitted.

#### Class: `fs.Stats`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.2.0 | Added `Temporal.Instant` support.
v22.0.0, v20.13.0 | Public constructor is deprecated.
v8.1.0 | Added times as numbers.

A `<fs.Stats>` object provides information about a file.

Objects returned from `fs.stat()`, `fs.lstat()`, `fs.fstat()`, and their synchronous counterparts are of this type. If `bigint` in the `options` passed to those methods is true, the numeric values will be `bigint` instead of `number`, and the object will contain additional nanosecond-precision properties suffixed with `Ns`. `Stat` objects are not to be created directly using the `new` keyword.

    Stats {
      dev: 2114,
      ino: 48064969,
      mode: 33188,
      nlink: 1,
      uid: 85,
      gid: 100,
      rdev: 0,
      size: 527,
      blksize: 4096,
      blocks: 8,
      atimeMs: 1318289051000.1,
      mtimeMs: 1318289051000.1,
      ctimeMs: 1318289051000.1,
      birthtimeMs: 1318289051000.1,

      // Instances of Date
      atime: Mon, 10 Oct 2011 23:24:11 GMT,
      mtime: Mon, 10 Oct 2011 23:24:11 GMT,
      ctime: Mon, 10 Oct 2011 23:24:11 GMT,
      birthtime: Mon, 10 Oct 2011 23:24:11 GMT,

      // Instances of Temporal.Instant
      atimeInstant: 2011-10-10T23:24:11.0001Z,
      mtimeInstant: 2011-10-10T23:24:11.0001Z,
      ctimeInstant: 2011-10-10T23:24:11.0001Z,
      birthtimeInstant: 2011-10-10T23:24:11.0001Z
    }
    console

`bigint` version:

    BigIntStats {
      dev: 2114n,
      ino: 48064969n,
      mode: 33188n,
      nlink: 1n,
      uid: 85n,
      gid: 100n,
      rdev: 0n,
      size: 527n,
      blksize: 4096n,
      blocks: 8n,
      atimeMs: 1318289051000n,
      mtimeMs: 1318289051000n,
      ctimeMs: 1318289051000n,
      birthtimeMs: 1318289051000n,
      atimeNs: 1318289051000000000n,
      mtimeNs: 1318289051000000000n,
      ctimeNs: 1318289051000000000n,
      birthtimeNs: 1318289051000000000n,

      // Instances of Date
      atime: Mon, 10 Oct 2011 23:24:11 GMT,
      mtime: Mon, 10 Oct 2011 23:24:11 GMT,
      ctime: Mon, 10 Oct 2011 23:24:11 GMT,
      birthtime: Mon, 10 Oct 2011 23:24:11 GMT,

      // Instances of Temporal.Instant
      atimeInstant: 2011-10-10T23:24:11Z,
      mtimeInstant: 2011-10-10T23:24:11Z,
      ctimeInstant: 2011-10-10T23:24:11Z,
      birthtimeInstant: 2011-10-10T23:24:11Z
    }
    console

##### `stats.isBlockDevice()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a block device.

##### `stats.isCharacterDevice()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a character device.

##### `stats.isDirectory()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a file system directory.

If the `<fs.Stats>` object was obtained from calling `fs.lstat()` on a symbolic link which resolves to a directory, this method will return `false`. This is because `fs.lstat()` returns information about a symbolic link itself and not the path it resolves to.

##### `stats.isFIFO()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a first-in-first-out (FIFO) pipe.

##### `stats.isFile()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a regular file.

##### `stats.isSocket()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a socket.

##### `stats.isSymbolicLink()`#

Added in: v0.1.10

- Returns: `<boolean>`

Returns `true` if the `<fs.Stats>` object describes a symbolic link.

This method is only valid when using `fs.lstat()`.

##### `stats.dev`#

- Type: `<number>` | `<bigint>`

The numeric identifier of the device containing the file.

##### `stats.ino`#

- Type: `<number>` | `<bigint>`

The file system specific "Inode" number for the file.

##### `stats.mode`#

- Type: `<number>` | `<bigint>`

A bit-field describing the file type and mode.

##### `stats.nlink`#

- Type: `<number>` | `<bigint>`

The number of hard-links that exist for the file.

##### `stats.uid`#

- Type: `<number>` | `<bigint>`

The numeric user identifier of the user that owns the file (POSIX).

##### `stats.gid`#

- Type: `<number>` | `<bigint>`

The numeric group identifier of the group that owns the file (POSIX).

##### `stats.rdev`#

- Type: `<number>` | `<bigint>`

A numeric device identifier if the file represents a device.

##### `stats.size`#

- Type: `<number>` | `<bigint>`

The size of the file in bytes.

If the underlying file system does not support getting the size of the file, this will be `0`.

##### `stats.blksize`#

- Type: `<number>` | `<bigint>`

The file system block size for i/o operations.

##### `stats.blocks`#

- Type: `<number>` | `<bigint>`

The number of blocks allocated for this file.

##### `stats.atimeMs`#

Added in: v8.1.0

- Type: `<number>` | `<bigint>`

The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.

##### `stats.mtimeMs`#

Added in: v8.1.0

- Type: `<number>` | `<bigint>`

The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.

##### `stats.ctimeMs`#

Added in: v8.1.0

- Type: `<number>` | `<bigint>`

The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.

##### `stats.birthtimeMs`#

Added in: v8.1.0

- Type: `<number>` | `<bigint>`

The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.

##### `stats.atimeNs`#

Added in: v12.10.0

- Type: `<bigint>`

Only present when `bigint: true` is passed into the method that generates the object. The timestamp indicating the last time this file was accessed expressed in nanoseconds since the POSIX Epoch.

##### `stats.mtimeNs`#

Added in: v12.10.0

- Type: `<bigint>`

Only present when `bigint: true` is passed into the method that generates the object. The timestamp indicating the last time this file was modified expressed in nanoseconds since the POSIX Epoch.

##### `stats.ctimeNs`#

Added in: v12.10.0

- Type: `<bigint>`

Only present when `bigint: true` is passed into the method that generates the object. The timestamp indicating the last time the file status was changed expressed in nanoseconds since the POSIX Epoch.

##### `stats.birthtimeNs`#

Added in: v12.10.0

- Type: `<bigint>`

Only present when `bigint: true` is passed into the method that generates the object. The timestamp indicating the creation time of this file expressed in nanoseconds since the POSIX Epoch.

##### `stats.atime`#

Added in: v0.11.13

- Type: `<Date>`

The timestamp indicating the last time this file was accessed.

##### `stats.mtime`#

Added in: v0.11.13

- Type: `<Date>`

The timestamp indicating the last time this file was modified.

##### `stats.ctime`#

Added in: v0.11.13

- Type: `<Date>`

The timestamp indicating the last time the file status was changed.

##### `stats.birthtime`#

Added in: v0.11.13

- Type: `<Date>`

The timestamp indicating the creation time of this file.

##### Stat time values#

The `atimeMs`, `mtimeMs`, `ctimeMs`, `birthtimeMs` properties are numeric values that hold the corresponding times in milliseconds. Their precision is platform specific. When `bigint: true` is passed into the method that generates the object, the properties will be bigints, otherwise they will be numbers.

The `atimeNs`, `mtimeNs`, `ctimeNs`, `birthtimeNs` properties are bigints that hold the corresponding times in nanoseconds. They are only present when `bigint: true` is passed into the method that generates the object. Their precision is platform specific.

`atime`, `mtime`, `ctime`, and `birthtime` are `Date` object alternate representations of the various times. The `Date` and number values are not connected. Assigning a new number value, or mutating the `Date` value, will not be reflected in the corresponding alternate representation.

The times in the stat object have the following semantics:

- `atime` "Access Time": Time when file data last accessed. Changed by the `mknod(2)`, `utimes(2)`, and `read(2)` system calls.
- `mtime` "Modified Time": Time when file data last modified. Changed by the `mknod(2)`, `utimes(2)`, and `write(2)` system calls.
- `ctime` "Change Time": Time when file status was last changed (inode data modification). Changed by the `chmod(2)`, `chown(2)`, `link(2)`, `mknod(2)`, `rename(2)`, `unlink(2)`, `utimes(2)`, `read(2)`, and `write(2)` system calls.
- `birthtime` "Birth Time": Time of file creation. Set once when the file is created. On file systems where birthtime is not available, this field may instead hold either the `ctime` or `1970-01-01T00:00Z` (ie, Unix epoch timestamp `0`). This value may be greater than `atime` or `mtime` in this case. On Darwin and other FreeBSD variants, also set if the `atime` is explicitly set to an earlier value than the current `birthtime` using the `utimes(2)` system call.

Prior to Node.js 0.12, the `ctime` held the `birthtime` on Windows systems. As of 0.12, `ctime` is not "creation time", and on Unix systems, it never was.

#### Class: `fs.StatFs`#

Added in: v19.6.0, v18.15.0

Provides information about a mounted file system.

Objects returned from `fs.statfs()` and its synchronous counterpart are of this type. If `bigint` in the `options` passed to those methods is `true`, the numeric values will be `bigint` instead of `number`.

    StatFs {
      type: 1397114950,
      bsize: 4096,
      frsize: 4096,
      blocks: 121938943,
      bfree: 61058895,
      bavail: 61058895,
      files: 999,
      ffree: 1000000
    }
    console

`bigint` version:

    StatFs {
      type: 1397114950n,
      bsize: 4096n,
      frsize: 4096n,
      blocks: 121938943n,
      bfree: 61058895n,
      bavail: 61058895n,
      files: 999n,
      ffree: 1000000n
    }
    console

##### `statfs.bavail`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Free blocks available to unprivileged users. Multiply by `statfs.bsize` to get the number of available bytes.

    import { statfs } from 'node:fs/promises';

    const stats = await statfs('/');
    const availableBytes = stats.bsize * stats.bavail;
    console.log(`Available space: ${availableBytes} bytes`);
    const { statfs } = require('node:fs/promises');

    (async () => {
      const stats = await statfs('/');
      const availableBytes = stats.bsize * stats.bavail;
      console.log(`Available space: ${availableBytes} bytes`);
    })();
    javascript

##### `statfs.bfree`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Free blocks in file system. Multiply by `statfs.bsize` to get the number of free bytes.

    import { statfs } from 'node:fs/promises';

    const stats = await statfs('/');
    const freeBytes = stats.bsize * stats.bfree;
    console.log(`Free space: ${freeBytes} bytes`);
    const { statfs } = require('node:fs/promises');

    (async () => {
      const stats = await statfs('/');
      const freeBytes = stats.bsize * stats.bfree;
      console.log(`Free space: ${freeBytes} bytes`);
    })();
    javascript

##### `statfs.blocks`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Total data blocks in file system. Multiply by `statfs.bsize` to get the total size in bytes.

    import { statfs } from 'node:fs/promises';

    const stats = await statfs('/');
    const totalBytes = stats.bsize * stats.blocks;
    console.log(`Total space: ${totalBytes} bytes`);
    const { statfs } = require('node:fs/promises');

    (async () => {
      const stats = await statfs('/');
      const totalBytes = stats.bsize * stats.blocks;
      console.log(`Total space: ${totalBytes} bytes`);
    })();
    javascript

##### `statfs.bsize`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Optimal transfer block size in bytes.

##### `statfs.frsize`#

Added in: v26.1.0

- Type: `<number>` | `<bigint>`

Fundamental file system block size.

##### `statfs.ffree`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Free file nodes in file system.

##### `statfs.files`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Total file nodes in file system.

##### `statfs.type`#

Added in: v19.6.0, v18.15.0

- Type: `<number>` | `<bigint>`

Type of file system. A platform-specific numeric identifier for the type of file system. This value corresponds to the `f_type` field returned by `statfs(2)` on POSIX systems (for example, `0xEF53` for ext4 on Linux). Its meaning is OS-dependent and is not guaranteed to be consistent across platforms.

#### Class: `fs.Utf8Stream`#

Added in: v24.6.0

Stability: 1 - Experimental

An optimized UTF-8 stream writer that allows for flushing all the internal buffering on demand. It handles `EAGAIN` errors correctly, allowing for customization, for example, by dropping content if the disk is busy.

##### Event: `'close'`#

The `'close'` event is emitted when the stream is fully closed.

##### Event: `'drain'`#

The `'drain'` event is emitted when the internal buffer has drained sufficiently to allow continued writing.

##### Event: `'drop'`#

The `'drop'` event is emitted when the maximal length is reached and that data will not be written. The data that was dropped is passed as the first argument to the event handler.

##### Event: `'error'`#

The `'error'` event is emitted when an error occurs.

##### Event: `'finish'`#

The `'finish'` event is emitted when the stream has been ended and all data has been flushed to the underlying file.

##### Event: `'ready'`#

The `'ready'` event is emitted when the stream is ready to accept writes.

##### Event: `'write'`#

The `'write'` event is emitted when a write operation has completed. The number of bytes written is passed as the first argument to the event handler.

##### `new fs.Utf8Stream([options])`#

- `options` `<Object>`
  - `append`: `<boolean>` Appends writes to dest file instead of truncating it. Default: `true`.
  - `contentMode`: `<string>` Which type of data you can send to the write function, supported values are `'utf8'` or `'buffer'`. Default: `'utf8'`.
  - `dest`: `<string>` A path to a file to be written to (mode controlled by the append option).
  - `fd`: `<number>` A file descriptor, something that is returned by `fs.open()` or `fs.openSync()`.
  - `fs`: `<Object>` An object that has the same API as the `fs` module, useful for mocking, testing, or customizing the behavior of the stream.
  - `fsync`: `<boolean>` Perform a `fs.fsyncSync()` every time a write is completed.
  - `maxLength`: `<number>` The maximum length of the internal buffer. If a write operation would cause the buffer to exceed `maxLength`, the data written is dropped and a drop event is emitted with the dropped data
  - `maxWrite`: `<number>` The maximum number of bytes that can be written; Default: `16384`
  - `minLength`: `<number>` The minimum length of the internal buffer that is required to be full before flushing.
  - `mkdir`: `<boolean>` Ensure directory for `dest` file exists when true. Default: `false`.
  - `mode`: `<number>` | `<string>` Specify the creating file mode (see `fs.open()`).
  - `periodicFlush`: `<number>` Calls flush every `periodicFlush` milliseconds.
  - `retryEAGAIN` `<Function>` A function that will be called when `write()`, `writeSync()`, or `flushSync()` encounters an `EAGAIN` or `EBUSY` error. If the return value is `true` the operation will be retried, otherwise it will bubble the error. The `err` is the error that caused this function to be called, `writeBufferLen` is the length of the buffer that was written, and `remainingBufferLen` is the length of the remaining buffer that the stream did not try to write.
    - `err` `<any>` An error or `null`.
    - `writeBufferLen` `<number>`
    - `remainingBufferLen`: `<number>`
  - `sync`: `<boolean>` Perform writes synchronously.

##### `utf8Stream.append`#

- `<boolean>` Whether the stream is appending to the file or truncating it.

##### `utf8Stream.contentMode`#

- `<string>` The type of data that can be written to the stream. Supported values are `'utf8'` or `'buffer'`. Default: `'utf8'`.

##### `utf8Stream.destroy()`#

Close the stream immediately, without flushing the internal buffer.

##### `utf8Stream.end()`#

Close the stream gracefully, flushing the internal buffer before closing.

##### `utf8Stream.fd`#

- `<number>` The file descriptor that is being written to.

##### `utf8Stream.file`#

- `<string>` The file that is being written to.

##### `utf8Stream.flush(callback)`#

- `callback` `<Function>`
  - `err` `<Error>` | `<null>` An error if the flush failed, otherwise `null`.

Writes the current buffer to the file if a write was not in progress. Do nothing if `minLength` is zero or if it is already writing.

##### `utf8Stream.flushSync()`#

Flushes the buffered data synchronously. This is a costly operation.

##### `utf8Stream.fsync`#

- `<boolean>` Whether the stream is performing a `fs.fsyncSync()` after every write operation.

##### `utf8Stream.maxLength`#

- `<number>` The maximum length of the internal buffer. If a write operation would cause the buffer to exceed `maxLength`, the data written is dropped and a drop event is emitted with the dropped data.

##### `utf8Stream.minLength`#

- `<number>` The minimum length of the internal buffer that is required to be full before flushing.

##### `utf8Stream.mkdir`#

- `<boolean>` Whether the stream should ensure that the directory for the `dest` file exists. If `true`, it will create the directory if it does not exist. Default: `false`.

##### `utf8Stream.mode`#

- `<number>` | `<string>` The mode of the file that is being written to.

##### `utf8Stream.periodicFlush`#

- `<number>` The number of milliseconds between flushes. If set to `0`, no periodic flushes will be performed.

##### `utf8Stream.reopen(file)`#

- `file`: `<string>` | `<Buffer>` | `<URL>` A path to a file to be written to (mode controlled by the append option).

Reopen the file in place, useful for log rotation.

##### `utf8Stream.sync`#

- `<boolean>` Whether the stream is writing synchronously or asynchronously.

##### `utf8Stream.write(data)`#

- `data` `<string>` | `<Buffer>` The data to write.
- Returns `<boolean>`

When the `options.contentMode` is set to `'utf8'` when the stream is created, the `data` argument must be a string. If the `contentMode` is set to `'buffer'`, the `data` argument must be a `<Buffer>`.

##### `utf8Stream.writing`#

- `<boolean>` Whether the stream is currently writing data to the file.

##### `utf8Stream[Symbol.dispose]()`#

Calls `utf8Stream.destroy()`.

This method enables the stream to be used with `using`, which will automatically destroy the stream when the scope exits. For more information, see the MDN documentation on `using` statements.

#### Class: `fs.WriteStream`#

Added in: v0.1.93

- Extends `<stream.Writable>`

Instances of `<fs.WriteStream>` cannot be constructed directly. They are created and returned using the `fs.createWriteStream()` function.

##### Event: `'close'`#

Added in: v0.1.93

Emitted when the `<fs.WriteStream>`'s underlying file descriptor has been closed.

##### Event: `'open'`#

Added in: v0.1.93

- `fd` `<integer>` Integer file descriptor used by the `<fs.WriteStream>`.

Emitted when the `<fs.WriteStream>`'s file is opened.

##### Event: `'ready'`#

Added in: v9.11.0

Emitted when the `<fs.WriteStream>` is ready to be used.

Fires immediately after `'open'`.

##### `writeStream.bytesWritten`#

Added in: v0.4.7

The number of bytes written so far. Does not include data that is still queued for writing.

##### `writeStream.close([callback])`#

Added in: v0.9.4

- `callback` `<Function>`
  - `err` `<Error>`

Closes `writeStream`. Optionally accepts a callback that will be executed once the `writeStream` is closed.

##### `writeStream.path`#

Added in: v0.1.93

The path to the file the stream is writing to as specified in the first argument to `fs.createWriteStream()`. If `path` is passed as a string, then `writeStream.path` will be a string. If `path` is passed as a `<Buffer>`, then `writeStream.path` will be a `<Buffer>`.

##### `writeStream.pending`#

Added in: v11.2.0

- Type: `<boolean>`

This property is `true` if the underlying file has not been opened yet, i.e. before the `'ready'` event is emitted.

#### `fs.constants`#

- Type: `<Object>`

Returns an object containing commonly used constants for file system operations.

##### FS constants#

The following constants are exported by `fs.constants` and `fsPromises.constants`.

Not every constant will be available on every operating system; this is especially important for Windows, where many of the POSIX specific definitions are not available. For portable applications it is recommended to check for their presence before use.

To use more than one constant, use the bitwise OR `|` operator.

Example:

    import { open, constants } from 'node:fs';

    const {
      O_RDWR,
      O_CREAT,
      O_EXCL,
    } = constants;

    open('/path/to/my/file', O_RDWR | O_CREAT | O_EXCL, (err, fd) => {
      // ...
    });
    mjs

###### File access constants#

The following constants are meant for use as the `mode` parameter passed to `fsPromises.access()`, `fs.access()`, and `fs.accessSync()`.

Constant | Description
--- | ---
`F_OK` | Flag indicating that the file is visible to the calling process. This is useful for determining if a file exists, but says nothing about `rwx` permissions. Default if no mode is specified.
`R_OK` | Flag indicating that the file can be read by the calling process.
`W_OK` | Flag indicating that the file can be written by the calling process.
`X_OK` | Flag indicating that the file can be executed by the calling process. This has no effect on Windows (will behave like `fs.constants.F_OK`).

The definitions are also available on Windows.

###### File copy constants#

The following constants are meant for use with `fs.copyFile()`.

Constant | Description
--- | ---
`COPYFILE_EXCL` | If present, the copy operation will fail with an error if the destination path already exists.
`COPYFILE_FICLONE` | If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used.
`COPYFILE_FICLONE_FORCE` | If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error.

The definitions are also available on Windows.

###### File open constants#

The following constants are meant for use with `fs.open()`.

Constant | Description
--- | ---
`O_RDONLY` | Flag indicating to open a file for read-only access.
`O_WRONLY` | Flag indicating to open a file for write-only access.
`O_RDWR` | Flag indicating to open a file for read-write access.
`O_CREAT` | Flag indicating to create the file if it does not already exist.
`O_EXCL` | Flag indicating that opening a file should fail if the `O_CREAT` flag is set and the file already exists.
`O_NOCTTY` | Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one).
`O_TRUNC` | Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero.
`O_APPEND` | Flag indicating that data will be appended to the end of the file.
`O_DIRECTORY` | Flag indicating that the open should fail if the path is not a directory.
`O_NOATIME` | Flag indicating reading accesses to the file system will no longer result in an update to the `atime` information associated with the file. This flag is available on Linux operating systems only.
`O_NOFOLLOW` | Flag indicating that the open should fail if the path is a symbolic link.
`O_SYNC` | Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity.
`O_DSYNC` | Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity.
`O_SYMLINK` | Flag indicating to open the symbolic link itself rather than the resource it is pointing to.
`O_DIRECT` | When set, an attempt will be made to minimize caching effects of file I/O.
`O_NONBLOCK` | Flag indicating to open the file in nonblocking mode when possible.
`UV_FS_O_FILEMAP` | When set, a memory file mapping is used to access the file. This flag is available on Windows operating systems only. On other operating systems, this flag is ignored.

On Windows, only `O_APPEND`, `O_CREAT`, `O_EXCL`, `O_RDONLY`, `O_RDWR`, `O_TRUNC`, `O_WRONLY`, and `UV_FS_O_FILEMAP` are available.

###### File type constants#

The following constants are meant for use with the `<fs.Stats>` object's `mode` property for determining a file's type.

Constant | Description
--- | ---
`S_IFMT` | Bit mask used to extract the file type code.
`S_IFREG` | File type constant for a regular file.
`S_IFDIR` | File type constant for a directory.
`S_IFCHR` | File type constant for a character-oriented device file.
`S_IFBLK` | File type constant for a block-oriented device file.
`S_IFIFO` | File type constant for a FIFO/pipe.
`S_IFLNK` | File type constant for a symbolic link.
`S_IFSOCK` | File type constant for a socket.

On Windows, only `S_IFCHR`, `S_IFDIR`, `S_IFLNK`, `S_IFMT`, and `S_IFREG`, are available.

###### File mode constants#

The following constants are meant for use with the `<fs.Stats>` object's `mode` property for determining the access permissions for a file.

Constant | Description
--- | ---
`S_IRWXU` | File mode indicating readable, writable, and executable by owner.
`S_IRUSR` | File mode indicating readable by owner.
`S_IWUSR` | File mode indicating writable by owner.
`S_IXUSR` | File mode indicating executable by owner.
`S_IRWXG` | File mode indicating readable, writable, and executable by group.
`S_IRGRP` | File mode indicating readable by group.
`S_IWGRP` | File mode indicating writable by group.
`S_IXGRP` | File mode indicating executable by group.
`S_IRWXO` | File mode indicating readable, writable, and executable by others.
`S_IROTH` | File mode indicating readable by others.
`S_IWOTH` | File mode indicating writable by others.
`S_IXOTH` | File mode indicating executable by others.

On Windows, only `S_IRUSR` and `S_IWUSR` are available.

### Notes#

#### Ordering of callback and promise-based operations#

Because they are executed asynchronously by the underlying thread pool, there is no guaranteed ordering when using either the callback or promise-based methods.

For example, the following is prone to error because the `fs.stat()` operation might complete before the `fs.rename()` operation:

    const fs = require('node:fs');

    fs.rename('/tmp/hello', '/tmp/world', (err) => {
      if (err) throw err;
      console.log('renamed complete');
    });
    fs.stat('/tmp/world', (err, stats) => {
      if (err) throw err;
      console.log(`stats: ${JSON.stringify(stats)}`);
    });
    js

It is important to correctly order the operations by awaiting the results of one before invoking the other:

    import { rename, stat } from 'node:fs/promises';

    const oldPath = '/tmp/hello';
    const newPath = '/tmp/world';

    try {
      await rename(oldPath, newPath);
      const stats = await stat(newPath);
      console.log(`stats: ${JSON.stringify(stats)}`);
    } catch (error) {
      console.error('there was an error:', error.message);
    }
    const { rename, stat } = require('node:fs/promises');

    (async function(oldPath, newPath) {
      try {
        await rename(oldPath, newPath);
        const stats = await stat(newPath);
        console.log(`stats: ${JSON.stringify(stats)}`);
      } catch (error) {
        console.error('there was an error:', error.message);
      }
    })('/tmp/hello', '/tmp/world');
    javascript

Or, when using the callback APIs, move the `fs.stat()` call into the callback of the `fs.rename()` operation:

    import { rename, stat } from 'node:fs';

    rename('/tmp/hello', '/tmp/world', (err) => {
      if (err) throw err;
      stat('/tmp/world', (err, stats) => {
        if (err) throw err;
        console.log(`stats: ${JSON.stringify(stats)}`);
      });
    });
    const { rename, stat } = require('node:fs');

    rename('/tmp/hello', '/tmp/world', (err) => {
      if (err) throw err;
      stat('/tmp/world', (err, stats) => {
        if (err) throw err;
        console.log(`stats: ${JSON.stringify(stats)}`);
      });
    });
    javascript

#### File paths#

Most `fs` operations accept file paths that may be specified in the form of a string, a `<Buffer>`, or a `<URL>` object using the `file:` protocol.

##### String paths#

String paths are interpreted as UTF-8 character sequences identifying the absolute or relative filename. Relative paths will be resolved relative to the current working directory as determined by calling `process.cwd()`.

Example using an absolute path on POSIX:

    import { open } from 'node:fs/promises';

    let fd;
    try {
      fd = await open('/open/some/file.txt', 'r');
      // Do something with the file
    } finally {
      await fd?.close();
    }
    mjs

Example using a relative path on POSIX (relative to `process.cwd()`):

    import { open } from 'node:fs/promises';

    let fd;
    try {
      fd = await open('file.txt', 'r');
      // Do something with the file
    } finally {
      await fd?.close();
    }
    mjs

##### File URL paths#

Added in: v7.6.0

For most `node:fs` module functions, the `path` or `filename` argument may be passed as a `<URL>` object using the `file:` protocol.

    import { readFileSync } from 'node:fs';

    readFileSync(new URL('file:///tmp/hello'));
    mjs

`file:` URLs are always absolute paths.

###### Platform-specific considerations#

On Windows, `file:` `<URL>`s with a host name convert to UNC paths, while `file:` `<URL>`s with drive letters convert to local absolute paths. `file:` `<URL>`s with no host name and no drive letter will result in an error:

    import { readFileSync } from 'node:fs';
    // On Windows :

    // - WHATWG file URLs with hostname convert to UNC path
    // file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
    readFileSync(new URL('file://hostname/p/a/t/h/file'));

    // - WHATWG file URLs with drive letters convert to absolute path
    // file:///C:/tmp/hello => C:\tmp\hello
    readFileSync(new URL('file:///C:/tmp/hello'));

    // - WHATWG file URLs without hostname must have a drive letters
    readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
    readFileSync(new URL('file:///c/p/a/t/h/file'));
    // TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute
    mjs

`file:` `<URL>`s with drive letters must use `:` as a separator just after the drive letter. Using another separator will result in an error.

On all other platforms, `file:` `<URL>`s with a host name are unsupported and will result in an error:

    import { readFileSync } from 'node:fs';
    // On other platforms:

    // - WHATWG file URLs with hostname are unsupported
    // file://hostname/p/a/t/h/file => throw!
    readFileSync(new URL('file://hostname/p/a/t/h/file'));
    // TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute

    // - WHATWG file URLs convert to absolute path
    // file:///tmp/hello => /tmp/hello
    readFileSync(new URL('file:///tmp/hello'));
    mjs

A `file:` `<URL>` having encoded slash characters will result in an error on all platforms:

    import { readFileSync } from 'node:fs';

    // On Windows
    readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
    readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
    /* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
    \ or / characters */

    // On POSIX
    readFileSync(new URL('file:///p/a/t/h/%2F'));
    readFileSync(new URL('file:///p/a/t/h/%2f'));
    /* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
    / characters */
    mjs

On Windows, `file:` `<URL>`s having encoded backslash will result in an error:

    import { readFileSync } from 'node:fs';

    // On Windows
    readFileSync(new URL('file:///C:/path/%5C'));
    readFileSync(new URL('file:///C:/path/%5c'));
    /* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
    \ or / characters */
    mjs

##### Buffer paths#

Paths specified using a `<Buffer>` are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, `<Buffer>` paths may be relative or absolute:

Example using an absolute path on POSIX:

    import { open } from 'node:fs/promises';
    import { Buffer } from 'node:buffer';

    let fd;
    try {
      fd = await open(Buffer.from('/open/some/file.txt'), 'r');
      // Do something with the file
    } finally {
      await fd?.close();
    }
    mjs

##### Per-drive working directories on Windows#

On Windows, Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example `fs.readdirSync('C:\\')` can potentially return a different result than `fs.readdirSync('C:')`. For more information, see this MSDN page.

#### File descriptors#

On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the differences between operating systems and assigns all open files a numeric file descriptor.

The callback-based `fs.open()`, and synchronous `fs.openSync()` methods open a file and allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file.

Operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.

    import { open, close, fstat } from 'node:fs';

    function closeFd(fd) {
      close(fd, (err) => {
        if (err) throw err;
      });
    }

    open('/open/some/file.txt', 'r', (err, fd) => {
      if (err) throw err;
      try {
        fstat(fd, (err, stat) => {
          if (err) {
            closeFd(fd);
            throw err;
          }

          // use stat

          closeFd(fd);
        });
      } catch (err) {
        closeFd(fd);
        throw err;
      }
    });
    mjs

The promise-based APIs use a `<FileHandle>` object in place of the numeric file descriptor. These objects are better managed by the system to ensure that resources are not leaked. However, it is still required that they are closed when operations are completed:

    import { open } from 'node:fs/promises';

    let file;
    try {
      file = await open('/open/some/file.txt', 'r');
      const stat = await file.stat();
      // use stat
    } finally {
      await file.close();
    }
    mjs

#### Threadpool usage#

All callback and promise-based file system APIs (with the exception of `fs.FSWatcher()`) use libuv's threadpool. This can have surprising and negative performance implications for some applications. See the `UV_THREADPOOL_SIZE` documentation for more information.

#### File system flags#

The following flags are available wherever the `flag` option takes a string.

- `'a'`: Open file for appending. The file is created if it does not exist.

- `'ax'`: Like `'a'` but fails if the path exists.

- `'a+'`: Open file for reading and appending. The file is created if it does not exist.

- `'ax+'`: Like `'a+'` but fails if the path exists.

- `'as'`: Open file for appending in synchronous mode. The file is created if it does not exist.

- `'as+'`: Open file for reading and appending in synchronous mode. The file is created if it does not exist.

- `'r'`: Open file for reading. An exception occurs if the file does not exist.

- `'rs'`: Open file for reading in synchronous mode. An exception occurs if the file does not exist.

- `'r+'`: Open file for reading and writing. An exception occurs if the file does not exist.

- `'rs+'`: Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.

  This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.

  This doesn't turn `fs.open()` or `fsPromises.open()` into a synchronous blocking call. If synchronous operation is desired, something like `fs.openSync()` should be used.

- `'w'`: Open file for writing. The file is created (if it does not exist) or truncated (if it exists).

- `'wx'`: Like `'w'` but fails if the path exists.

- `'w+'`: Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).

- `'wx+'`: Like `'w+'` but fails if the path exists.

`flag` can also be a number as documented by `open(2)`; commonly used constants are available from `fs.constants`. On Windows, flags are translated to their equivalent ones where applicable, e.g. `O_WRONLY` to `FILE_GENERIC_WRITE`, or `O_EXCL|O_CREAT` to `CREATE_NEW`, as accepted by `CreateFileW`.

The exclusive flag `'x'` (`O_EXCL` flag in `open(2)`) causes the operation to return an error if the path already exists. On POSIX, if the path is a symbolic link, using `O_EXCL` returns an error even if the link is to a path that does not exist. The exclusive flag might not work with network file systems.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

Modifying a file rather than replacing it may require the `flag` option to be set to `'r+'` rather than the default `'w'`.

The behavior of some flags are platform-specific. As such, opening a directory on macOS and Linux with the `'a+'` flag, as in the example below, will return an error. In contrast, on Windows and FreeBSD, a file descriptor or a `FileHandle` will be returned.

    // macOS and Linux
    fs.open('<directory>', 'a+', (err, fd) => {
      // => [Error: EISDIR: illegal operation on a directory, open <directory>]
    });

    // Windows and FreeBSD
    fs.open('<directory>', 'a+', (err, fd) => {
      // => null, <fd>
    });
    js

On Windows, opening an existing hidden file using the `'w'` flag (either through `fs.open()`, `fs.writeFile()`, or `fsPromises.open()`) will fail with `EPERM`. Existing hidden files can be opened for writing with the `'r+'` flag.

A call to `fs.ftruncate()` or `filehandle.truncate()` can be used to reset the file contents.

## Global objects#

Stability: 2 - Stable

These objects are available in all modules.

The following variables may appear to be global but are not. They exist only in the scope of CommonJS modules:

- `__dirname`
- `__filename`
- `exports`
- `module`
- `require()`

The objects listed here are specific to Node.js. There are built-in objects that are part of the JavaScript language itself, which are also globally accessible.

### `__dirname`#

This variable may appear to be global but is not. See `__dirname`.

### `__filename`#

This variable may appear to be global but is not. See `__filename`.

### Class: `AbortController`#

Added in: v15.0.0, v14.17.0
History

Version | Changes
--- | ---
v15.4.0 | No longer experimental.

A utility class used to signal cancelation in selected `Promise`-based APIs. The API is based on the Web API `<AbortController>`.

    const ac = new AbortController();

    ac.signal.addEventListener('abort', () => console.log('Aborted!'),
                               { once: true });

    ac.abort();

    console.log(ac.signal.aborted);  // Prints true
    js

#### `abortController.abort([reason])`#

Added in: v15.0.0, v14.17.0
History

Version | Changes
--- | ---
v17.2.0, v16.14.0 | Added the new optional reason argument.

- `reason` `<any>` An optional reason, retrievable on the `AbortSignal`'s `reason` property.

Triggers the abort signal, causing the `abortController.signal` to emit the `'abort'` event.

#### `abortController.signal`#

Added in: v15.0.0, v14.17.0

- Type: `<AbortSignal>`

### Class: `AbortSignal`#

Added in: v15.0.0, v14.17.0

- Extends: `<EventTarget>`

The `AbortSignal` is used to notify observers when the `abortController.abort()` method is called.

#### Static method: `AbortSignal.abort([reason])`#

Added in: v15.12.0, v14.17.0
History

Version | Changes
--- | ---
v17.2.0, v16.14.0 | Added the new optional reason argument.

- `reason` `<any>`
- Returns: `<AbortSignal>`

Returns a new already aborted `AbortSignal`.

#### Static method: `AbortSignal.timeout(delay)`#

Added in: v17.3.0, v16.14.0

- `delay` `<number>` The number of milliseconds to wait before triggering the AbortSignal.

Returns a new `AbortSignal` which will be aborted in `delay` milliseconds.

#### Static method: `AbortSignal.any(signals)`#

Added in: v20.3.0, v18.17.0

- `signals` `<AbortSignal>`[] The `AbortSignal`s of which to compose a new `AbortSignal`.

Returns a new `AbortSignal` which will be aborted if any of the provided signals are aborted. Its `abortSignal.reason` will be set to whichever one of the `signals` caused it to be aborted.

#### Event: `'abort'`#

Added in: v15.0.0, v14.17.0

The `'abort'` event is emitted when the `abortController.abort()` method is called. The callback is invoked with a single object argument with a single `type` property set to `'abort'`:

    const ac = new AbortController();

    // Use either the onabort property...
    ac.signal.onabort = () => console.log('aborted!');

    // Or the EventTarget API...
    ac.signal.addEventListener('abort', (event) => {
      console.log(event.type);  // Prints 'abort'
    }, { once: true });

    ac.abort();
    js

The `AbortController` with which the `AbortSignal` is associated will only ever trigger the `'abort'` event once. We recommended that code check that the `abortSignal.aborted` attribute is `false` before adding an `'abort'` event listener.

Any event listeners attached to the `AbortSignal` should use the `{ once: true }` option (or, if using the `EventEmitter` APIs to attach a listener, use the `once()` method) to ensure that the event listener is removed as soon as the `'abort'` event is handled. Failure to do so may result in memory leaks.

#### `abortSignal.aborted`#

Added in: v15.0.0, v14.17.0

- Type: `<boolean>`

True after the `AbortController` has been aborted.

#### `abortSignal.onabort`#

Added in: v15.0.0, v14.17.0

- Type: `<Function>`

An optional callback function that may be set by user code to be notified when the `abortController.abort()` function has been called.

#### `abortSignal.reason`#

Added in: v17.2.0, v16.14.0

- Type: `<any>`

An optional reason specified when the `AbortSignal` was triggered.

    const ac = new AbortController();
    ac.abort(new Error('boom!'));
    console.log(ac.signal.reason);  // Error: boom!
    js

#### `abortSignal.throwIfAborted()`#

Added in: v17.3.0, v16.17.0

If `abortSignal.aborted` is `true`, throws `abortSignal.reason`.

### `atob(data)`#

Added in: v16.0.0

Stability: 3 - Legacy. Use `Buffer.from(data, 'base64')` instead.

Global alias for `buffer.atob()`.

An automated migration is available (source):

    npx codemod@latest @nodejs/buffer-atob-btoa
    bash

### Class: `Blob`#

Added in: v18.0.0

See `<Blob>`.

### Class: `BroadcastChannel`#

Added in: v18.0.0

See `<BroadcastChannel>`.

### `btoa(data)`#

Added in: v16.0.0

Stability: 3 - Legacy. Use `buf.toString('base64')` instead.

Global alias for `buffer.btoa()`.

An automated migration is available (source):

    npx codemod@latest @nodejs/buffer-atob-btoa
    bash

### Class: `Buffer`#

Added in: v0.1.103

- Type: `<Function>`

Used to handle binary data. See the buffer section.

### Class: `ByteLengthQueuingStrategy`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ByteLengthQueuingStrategy`.

### `clearImmediate(immediateObject)`#

Added in: v0.9.1

`clearImmediate` is described in the timers section.

### `clearInterval(intervalObject)`#

Added in: v0.0.1

`clearInterval` is described in the timers section.

### `clearTimeout(timeoutObject)`#

Added in: v0.0.1

`clearTimeout` is described in the timers section.

### Class: `CloseEvent`#

Added in: v23.0.0

A browser-compatible implementation of `<CloseEvent>`. Disable this API with the `--no-experimental-websocket` CLI flag.

### Class: `CompressionStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v24.7.0, v22.20.0 | format now accepts `brotli` value.
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `CompressionStream`.

### `console`#

Added in: v0.1.100

- Type: `<Object>`

Used to print to stdout and stderr. See the `console` section.

### Class: `CountQueuingStrategy`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `CountQueuingStrategy`.

### Class: `Crypto`#

Added in: v17.6.0, v16.15.0
History

Version | Changes
--- | ---
v23.0.0 | No longer experimental.
v19.0.0 | No longer behind `--experimental-global-webcrypto` CLI flag.

A browser-compatible implementation of `<Crypto>`. This global is available only if the Node.js binary was compiled with including support for the `node:crypto` module.

### `crypto`#

Added in: v17.6.0, v16.15.0
History

Version | Changes
--- | ---
v23.0.0 | No longer experimental.
v19.0.0 | No longer behind `--experimental-global-webcrypto` CLI flag.

A browser-compatible implementation of the Web Crypto API.

### Class: `CryptoKey`#

Added in: v17.6.0, v16.15.0
History

Version | Changes
--- | ---
v23.0.0 | No longer experimental.
v19.0.0 | No longer behind `--experimental-global-webcrypto` CLI flag.

A browser-compatible implementation of `<CryptoKey>`. This global is available only if the Node.js binary was compiled with including support for the `node:crypto` module.

### Class: `CustomEvent`#

Added in: v18.7.0, v16.17.0
History

Version | Changes
--- | ---
v23.0.0 | No longer experimental.
v22.1.0, v20.13.0 | CustomEvent is now stable.
v19.0.0 | No longer behind `--experimental-global-customevent` CLI flag.

A browser-compatible implementation of `<CustomEvent>`.

### Class: `DecompressionStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v24.7.0, v22.20.0 | format now accepts `brotli` value.
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `DecompressionStream`.

### Class: `DOMException`#

Added in: v17.0.0

The WHATWG `<DOMException>` class.

### `ErrorEvent`#

Added in: v25.0.0

A browser-compatible implementation of `<ErrorEvent>`.

### Class: `Event`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v15.4.0 | No longer experimental.

A browser-compatible implementation of the `Event` class. See `EventTarget` and `Event` API for more details.

### Class: `EventSource`#

Added in: v22.3.0, v20.18.0

Stability: 1 - Experimental. Enable this API with the `--experimental-eventsource` CLI flag.

A browser-compatible implementation of `<EventSource>`.

### Class: `EventTarget`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v15.4.0 | No longer experimental.

A browser-compatible implementation of the `EventTarget` class. See `EventTarget` and `Event` API for more details.

### `exports`#

This variable may appear to be global but is not. See `exports`.

### `fetch`#

Added in: v17.5.0, v16.15.0
History

Version | Changes
--- | ---
v21.0.0 | No longer experimental.
v18.0.0 | No longer behind `--experimental-fetch` CLI flag.

A browser-compatible implementation of the `fetch()` function.

    const res = await fetch('https://nodejs.org/api/documentation.json');
    if (res.ok) {
      const data = await res.json();
      console.log(data);
    }
    mjs

The implementation is based upon undici, an HTTP/1.1 client written from scratch for Node.js. You can figure out which version of `undici` is bundled in your Node.js process reading the `process.versions.undici` property.

#### Custom dispatcher#

You can use a custom dispatcher to dispatch requests passing it in fetch's options object. The dispatcher must be compatible with `undici`'s `Dispatcher` class.

    fetch(url, { dispatcher: new MyAgent() });
    js

It is possible to change the global dispatcher in Node.js by installing `undici` and using the `setGlobalDispatcher()` method. Calling this method will affect both `undici` and Node.js.

    import { setGlobalDispatcher } from 'undici';
    setGlobalDispatcher(new MyAgent());
    mjs

#### Related classes#

The following globals are available to use with `fetch`:

- `FormData`
- `Headers`
- `Request`
- `Response`

### Class: `File`#

Added in: v20.0.0

See `<File>`.

### Class: `FormData`#

Added in: v17.6.0, v16.15.0
History

Version | Changes
--- | ---
v21.0.0 | No longer experimental.
v18.0.0 | No longer behind `--experimental-fetch` CLI flag.

A browser-compatible implementation of `<FormData>`.

### `global`#

Added in: v0.1.27

Stability: 3 - Legacy. Use `globalThis` instead.

- Type: `<Object>` The global namespace object.

In browsers, the top-level scope has traditionally been the global scope. This means that `var something` will define a new global variable, except within ECMAScript modules. In Node.js, this is different. The top-level scope is not the global scope; `var something` inside a Node.js module will be local to that module, regardless of whether it is a CommonJS module or an ECMAScript module.

### Class: `Headers`#

Added in: v17.5.0, v16.15.0
History

Version | Changes
--- | ---
v21.0.0 | No longer experimental.
v18.0.0 | No longer behind `--experimental-fetch` CLI flag.

A browser-compatible implementation of `<Headers>`.

### `localStorage`#

Added in: v22.4.0
History

Version | Changes
--- | ---
v26.0.0 | Accessing the `localStorage` global without providing `--localstorage-file` now throws a `DOMException`, for compliance with the Web Storage specification.
v25.0.0 | When webstorage is enabled and `--localstorage-file` is not provided, accessing the `localStorage` global now returns an empty object.
v25.0.0 | This API is no longer behind `--experimental-webstorage` runtime flag.

Stability: 1.2 - Release candidate. Disable this API with `--no-experimental-webstorage`.

A browser-compatible implementation of `localStorage`. Data is stored unencrypted in the file specified by the `--localstorage-file` CLI flag. The maximum amount of data that can be stored is 10 MB. Any modification of this data outside of the Web Storage API is not supported. `localStorage` data is not stored per user or per request when used in the context of a server, it is shared across all users and requests.

### Class: `MessageChannel`#

Added in: v15.0.0

The `MessageChannel` class. See `MessageChannel` for more details.

### Class: `MessageEvent`#

Added in: v15.0.0

A browser-compatible implementation of `<MessageEvent>`.

### Class: `MessagePort`#

Added in: v15.0.0

The `MessagePort` class. See `MessagePort` for more details.

### `module`#

This variable may appear to be global but is not. See `module`.

### Class: `Navigator`#

Added in: v21.0.0

Stability: 1.1 - Active development. Disable this API with the `--no-experimental-global-navigator` CLI flag.

A partial implementation of the Navigator API.

### `navigator`#

Added in: v21.0.0

Stability: 1.1 - Active development. Disable this API with the `--no-experimental-global-navigator` CLI flag.

A partial implementation of `window.navigator`.

#### `navigator.hardwareConcurrency`#

Added in: v21.0.0

- Type: `<number>`

The `navigator.hardwareConcurrency` read-only property returns the number of logical processors available to the current Node.js instance.

    console.log(`This process is running on ${navigator.hardwareConcurrency} logical processors`);
    js

#### `navigator.language`#

Added in: v21.2.0

- Type: `<string>`

The `navigator.language` read-only property returns a string representing the preferred language of the Node.js instance. The language will be determined by the ICU library used by Node.js at runtime based on the default language of the operating system.

The value is representing the language version as defined in RFC 5646.

The fallback value on builds without ICU is `'en-US'`.

    console.log(`The preferred language of the Node.js instance has the tag '${navigator.language}'`);
    js

#### `navigator.languages`#

Added in: v21.2.0

- Type: `<string>`[]

The `navigator.languages` read-only property returns an array of strings representing the preferred languages of the Node.js instance. By default `navigator.languages` contains only the value of `navigator.language`, which will be determined by the ICU library used by Node.js at runtime based on the default language of the operating system.

The fallback value on builds without ICU is `['en-US']`.

    console.log(`The preferred languages are '${navigator.languages}'`);
    js

#### `navigator.locks`#

Added in: v24.5.0

Stability: 1 - Experimental

The `navigator.locks` read-only property returns a `LockManager` instance that can be used to coordinate access to resources that may be shared across multiple threads within the same process. This global implementation matches the semantics of the browser `LockManager` API.

    // Request an exclusive lock
    await navigator.locks.request('my_resource', async (lock) => {
      // The lock has been acquired.
      console.log(`Lock acquired: ${lock.name}`);
      // Lock is automatically released when the function returns
    });

    // Request a shared lock
    await navigator.locks.request('shared_resource', { mode: 'shared' }, async (lock) => {
      // Multiple shared locks can be held simultaneously
      console.log(`Shared lock acquired: ${lock.name}`);
    });
    // Request an exclusive lock
    navigator.locks.request('my_resource', async (lock) => {
      // The lock has been acquired.
      console.log(`Lock acquired: ${lock.name}`);
      // Lock is automatically released when the function returns
    }).then(() => {
      console.log('Lock released');
    });

    // Request a shared lock
    navigator.locks.request('shared_resource', { mode: 'shared' }, async (lock) => {
      // Multiple shared locks can be held simultaneously
      console.log(`Shared lock acquired: ${lock.name}`);
    }).then(() => {
      console.log('Shared lock released');
    });
    javascript

See `worker_threads.locks` for detailed API documentation.

#### `navigator.platform`#

Added in: v21.2.0

- Type: `<string>`

The `navigator.platform` read-only property returns a string identifying the platform on which the Node.js instance is running.

    console.log(`This process is running on ${navigator.platform}`);
    js

#### `navigator.userAgent`#

Added in: v21.1.0

- Type: `<string>`

The `navigator.userAgent` read-only property returns user agent consisting of the runtime name and major version number.

    console.log(`The user-agent is ${navigator.userAgent}`); // Prints "Node.js/21"
    js

### `performance`#

Added in: v16.0.0

The `perf_hooks.performance` object.

### Class: `PerformanceEntry`#

Added in: v19.0.0

The `PerformanceEntry` class. See `PerformanceEntry` for more details.

### Class: `PerformanceMark`#

Added in: v19.0.0

The `PerformanceMark` class. See `PerformanceMark` for more details.

### Class: `PerformanceMeasure`#

Added in: v19.0.0

The `PerformanceMeasure` class. See `PerformanceMeasure` for more details.

### Class: `PerformanceObserver`#

Added in: v19.0.0

The `PerformanceObserver` class. See `PerformanceObserver` for more details.

### Class: `PerformanceObserverEntryList`#

Added in: v19.0.0

The `PerformanceObserverEntryList` class. See `PerformanceObserverEntryList` for more details.

### Class: `PerformanceResourceTiming`#

Added in: v19.0.0

The `PerformanceResourceTiming` class. See `PerformanceResourceTiming` for more details.

### `process`#

Added in: v0.1.7

- Type: `<Object>`

The process object. See the `process` object section.

### `queueMicrotask(callback)`#

Added in: v11.0.0

- `callback` `<Function>` Function to be queued.

The `queueMicrotask()` method queues a microtask to invoke `callback`. If `callback` throws an exception, the `process` object `'uncaughtException'` event will be emitted.

The microtask queue is managed by V8 and may be used in a similar manner to the `process.nextTick()` queue, which is managed by Node.js. The `process.nextTick()` queue is always processed before the microtask queue within each turn of the Node.js event loop.

    // Here, `queueMicrotask()` is used to ensure the 'load' event is always
    // emitted asynchronously, and therefore consistently. Using
    // `process.nextTick()` here would result in the 'load' event always emitting
    // before any other promise jobs.

    DataHandler.prototype.load = async function load(key) {
      const hit = this._cache.get(key);
      if (hit !== undefined) {
        queueMicrotask(() => {
          this.emit('load', hit);
        });
        return;
      }

      const data = await fetchData(key);
      this._cache.set(key, data);
      this.emit('load', data);
    };
    js

### Class: `QuotaExceededError`#

Added in: v26.0.0

The WHATWG `<QuotaExceededError>` class. Extends `<DOMException>`.

### Class: `ReadableByteStreamController`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ReadableByteStreamController`.

### Class: `ReadableStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ReadableStream`.

### Class: `ReadableStreamBYOBReader`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ReadableStreamBYOBReader`.

### Class: `ReadableStreamBYOBRequest`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ReadableStreamBYOBRequest`.

### Class: `ReadableStreamDefaultController`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ReadableStreamDefaultController`.

### Class: `ReadableStreamDefaultReader`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `ReadableStreamDefaultReader`.

### Class: `Request`#

Added in: v17.5.0, v16.15.0
History

Version | Changes
--- | ---
v21.0.0 | No longer experimental.
v18.0.0 | No longer behind `--experimental-fetch` CLI flag.

A browser-compatible implementation of `<Request>`.

### `require()`#

This variable may appear to be global but is not. See `require()`.

### Class: `Response`#

Added in: v17.5.0, v16.15.0
History

Version | Changes
--- | ---
v21.0.0 | No longer experimental.
v18.0.0 | No longer behind `--experimental-fetch` CLI flag.

A browser-compatible implementation of `<Response>`.

### `sessionStorage`#

Added in: v22.4.0
History

Version | Changes
--- | ---
v25.0.0 | This API is no longer behind `--experimental-webstorage` runtime flag.

Stability: 1.2 - Release candidate. Disable this API with `--no-experimental-webstorage`.

A browser-compatible implementation of `sessionStorage`. Data is stored in memory, with a storage quota of 10 MB. `sessionStorage` data persists only within the currently running process, and is not shared between workers.

### `setImmediate(callback[, ...args])`#

Added in: v0.9.1

`setImmediate` is described in the timers section.

### `setInterval(callback, delay[, ...args])`#

Added in: v0.0.1

`setInterval` is described in the timers section.

### `setTimeout(callback, delay[, ...args])`#

Added in: v0.0.1

`setTimeout` is described in the timers section.

### Class: `Storage`#

Added in: v22.4.0

Stability: 1.2 - Release candidate. Disable this API with `--no-experimental-webstorage`.

A browser-compatible implementation of `<Storage>`.

### `structuredClone(value[, options])`#

Added in: v17.0.0

The WHATWG `structuredClone` method.

### Class: `SubtleCrypto`#

Added in: v17.6.0, v16.15.0
History

Version | Changes
--- | ---
v19.0.0 | No longer behind `--experimental-global-webcrypto` CLI flag.

A browser-compatible implementation of `<SubtleCrypto>`. This global is available only if the Node.js binary was compiled with including support for the `node:crypto` module.

### Class: `TextDecoder`#

Added in: v11.0.0

The WHATWG `TextDecoder` class. See the `TextDecoder` section.

### Class: `TextDecoderStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `TextDecoderStream`.

### Class: `TextEncoder`#

Added in: v11.0.0

The WHATWG `TextEncoder` class. See the `TextEncoder` section.

### Class: `TextEncoderStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `TextEncoderStream`.

### Class: `TransformStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `TransformStream`.

### Class: `TransformStreamDefaultController`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `TransformStreamDefaultController`.

### Class: `URL`#

Added in: v10.0.0

The WHATWG `URL` class. See the `URL` section.

### Class: `URLPattern`#

Added in: v24.0.0

Stability: 1 - Experimental

The WHATWG `URLPattern` class. See the `URLPattern` section.

### Class: `URLSearchParams`#

Added in: v10.0.0

The WHATWG `URLSearchParams` class. See the `URLSearchParams` section.

### Class: `WebAssembly`#

Added in: v8.0.0

- Type: `<Object>`

The object that acts as the namespace for all W3C WebAssembly related functionality. See the Mozilla Developer Network for usage and compatibility.

### Class: `WebSocket`#

Added in: v21.0.0, v20.10.0
History

Version | Changes
--- | ---
v22.4.0 | No longer experimental.
v22.0.0 | No longer behind `--experimental-websocket` CLI flag.

A browser-compatible implementation of `<WebSocket>`. Disable this API with the `--no-experimental-websocket` CLI flag.

### Class: `WritableStream`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `WritableStream`.

### Class: `WritableStreamDefaultController`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `WritableStreamDefaultController`.

### Class: `WritableStreamDefaultWriter`#

Added in: v18.0.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

A browser-compatible implementation of `WritableStreamDefaultWriter`.

## HTTP#

Source Code: lib/http.js

Stability: 2 - Stable

This module, containing both a client and server, can be imported via `require('node:http')` (CommonJS) or `import * as http from 'node:http'` (ES module).

The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses, so the user is able to stream data.

HTTP message headers are represented by an object like this:

    { "content-length": "123",
      "content-type": "text/plain",
      "connection": "keep-alive",
      "host": "example.com",
      "accept": "*/*" }
    json

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, the Node.js HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.

See `message.headers` for details on how duplicate headers are handled.

The raw headers as they were received are retained in the `rawHeaders` property, which is an array of `[key, value, key2, value2, ...]`. For example, the previous message header object might have a `rawHeaders` list like the following:

    [ "ConTent-Length", "123456",
      "content-LENGTH", "123",
      "content-type", "text/plain",
      "CONNECTION", "keep-alive",
      "Host", "example.com",
      "accepT", "*/*" ]
    json

### Class: `http.Agent`#

Added in: v0.3.4

An `Agent` is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the `keepAlive` option.

Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The `Agent` will still make the requests to that server, but each one will occur over a new connection.

#### Response ordering with connection reuse#

On a reused HTTP/1.1 keep-alive connection, responses are associated with requests by their order on that connection. HTTP/1.1 keep-alive does not provide per-request response attribution beyond that ordering. Applications that require per-request connection isolation can use a separate `Agent`, disable keep-alive, or pass `agent: false`.

When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see `socket.unref()`).

It is good practice, to `destroy()` an `Agent` instance when it is no longer in use, because unused sockets consume OS resources.

Sockets are removed from an agent when the socket emits either a `'close'` event or an `'agentRemove'` event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done:

    http.get(options, (res) => {
      // Do stuff
    }).on('socket', (socket) => {
      socket.emit('agentRemove');
    });
    js

An agent may also be used for an individual request. By providing `{agent: false}` as an option to the `http.get()` or `http.request()` functions, a one-time use `Agent` with default options will be used for the client connection.

`agent:false`:

    http.get({
      hostname: 'localhost',
      port: 80,
      path: '/',
      agent: false,  // Create a new agent just for this one request
    }, (res) => {
      // Do stuff with response
    });
    js

Use `agent: false` to avoid connection reuse for a request.

#### `new Agent([options])`#

Added in: v0.3.4
History

Version | Changes
--- | ---
v24.7.0, v22.20.0 | Add support for `agentKeepAliveTimeoutBuffer`.
v24.5.0, v22.21.0 | Add support for `proxyEnv`.
v24.5.0, v22.21.0 | Add support for `defaultPort` and `protocol`.
v15.6.0, v14.17.0 | Change the default scheduling from 'fifo' to 'lifo'.
v14.5.0, v12.19.0 | Add `maxTotalSockets` option to agent constructor.
v14.5.0, v12.20.0 | Add `scheduling` option to specify the free socket scheduling strategy.

- `options` `<Object>` Set of configurable options to set on the agent. Can have the following fields:
  - `keepAlive` `<boolean>` Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Not to be confused with the `keep-alive` value of the `Connection` header. The `Connection: keep-alive` header is always sent when using an agent except when the `Connection` header is explicitly specified or when the `keepAlive` and `maxSockets` options are respectively set to `false` and `Infinity`, in which case `Connection: close` will be used. Default: `false`.
  - `keepAliveMsecs` `<number>` When using the `keepAlive` option, specifies the initial delay for TCP Keep-Alive packets. Ignored when the `keepAlive` option is `false` or `undefined`. Default: `1000`.
  - `agentKeepAliveTimeoutBuffer` `<number>` Milliseconds to subtract from the server-provided `keep-alive: timeout=...` hint when determining socket expiration time. This buffer helps ensure the agent closes the socket slightly before the server does, reducing the chance of sending a request on a socket that’s about to be closed by the server. Default: `1000`.
  - `maxSockets` `<number>` Maximum number of sockets to allow per host. If the same host opens multiple concurrent connections, each request will use new socket until the `maxSockets` value is reached. If the host attempts to open more connections than `maxSockets`, the additional requests will enter into a pending request queue, and will enter active connection state when an existing connection terminates. This makes sure there are at most `maxSockets` active connections at any point in time, from a given host. Default: `Infinity`.
  - `maxTotalSockets` `<number>` Maximum number of sockets allowed for all hosts in total. Each request will use a new socket until the maximum is reached. Default: `Infinity`.
  - `maxFreeSockets` `<number>` Maximum number of sockets per host to leave open in a free state. Only relevant if `keepAlive` is set to `true`. Default: `256`.
  - `scheduling` `<string>` Scheduling strategy to apply when picking the next free socket to use. It can be `'fifo'` or `'lifo'`. The main difference between the two scheduling strategies is that `'lifo'` selects the most recently used socket, while `'fifo'` selects the least recently used socket. In case of a low rate of request per second, the `'lifo'` scheduling will lower the risk of picking a socket that might have been closed by the server due to inactivity. In case of a high rate of request per second, the `'fifo'` scheduling will maximize the number of open sockets, while the `'lifo'` scheduling will keep it as low as possible. Default: `'lifo'`.
  - `timeout` `<number>` Socket timeout in milliseconds. This will set the timeout when the socket is created.
  - `proxyEnv` `<Object>` | `<undefined>` Environment variables for proxy configuration. See Built-in Proxy Support for details. Default: `undefined`
    - `HTTP_PROXY` `<string>` | `<undefined>` URL for the proxy server that HTTP requests should use. If undefined, no proxy is used for HTTP requests.
    - `HTTPS_PROXY` `<string>` | `<undefined>` URL for the proxy server that HTTPS requests should use. If undefined, no proxy is used for HTTPS requests.
    - `NO_PROXY` `<string>` | `<undefined>` Patterns specifying the endpoints that should not be routed through a proxy.
    - `http_proxy` `<string>` | `<undefined>` Same as `HTTP_PROXY`. If both are set, `http_proxy` takes precedence.
    - `https_proxy` `<string>` | `<undefined>` Same as `HTTPS_PROXY`. If both are set, `https_proxy` takes precedence.
    - `no_proxy` `<string>` | `<undefined>` Same as `NO_PROXY`. If both are set, `no_proxy` takes precedence.
  - `defaultPort` `<number>` Default port to use when the port is not specified in requests. Default: `80`.
  - `protocol` `<string>` The protocol to use for the agent. Default: `'http:'`.

`options` in `socket.connect()` are also supported.

To configure any of them, a custom `http.Agent` instance must be created.

    import { Agent, request } from 'node:http';
    const keepAliveAgent = new Agent({ keepAlive: true });
    options.agent = keepAliveAgent;
    request(options, onResponseCallback);
    const http = require('node:http');
    const keepAliveAgent = new http.Agent({ keepAlive: true });
    options.agent = keepAliveAgent;
    http.request(options, onResponseCallback);
    javascript

#### `agent.createConnection(options[, callback])`#

Added in: v0.11.4

- `options` `<Object>` Options containing connection details. Check `net.createConnection()` for the format of the options. For custom agents, this object is passed to the custom `createConnection` function.
- `callback` `<Function>` (Optional, primarily for custom agents) A function to be called by a custom `createConnection` implementation when the socket is created, especially for asynchronous operations.
  - `err` `<Error>` | `<null>` An error object if socket creation failed.
  - `socket` `<stream.Duplex>` The created socket.
- Returns: `<stream.Duplex>` The created socket. This is returned by the default implementation or by a custom synchronous `createConnection` implementation. If a custom `createConnection` uses the `callback` for asynchronous operation, this return value might not be the primary way to obtain the socket.

Produces a socket/stream to be used for HTTP requests.

By default, this function behaves identically to `net.createConnection()`, synchronously returning the created socket. The optional `callback` parameter in the signature is not used by this default implementation.

However, custom agents may override this method to provide greater flexibility, for example, to create sockets asynchronously. When overriding `createConnection`:

1. Synchronous socket creation: The overriding method can return the socket/stream directly.
2. Asynchronous socket creation: The overriding method can accept the `callback` and pass the created socket/stream to it (e.g., `callback(null, newSocket)`). If an error occurs during socket creation, it should be passed as the first argument to the `callback` (e.g., `callback(err)`).

The agent will call the provided `createConnection` function with `options` and this internal `callback`. The `callback` provided by the agent has a signature of `(err, stream)`.

#### `agent.keepSocketAlive(socket)`#

Added in: v8.1.0

- `socket` `<stream.Duplex>`

Called when `socket` is detached from a request and could be persisted by the `Agent`. Default behavior is to:

    socket.setKeepAlive(true, this.keepAliveMsecs);
    socket.unref();
    return true;
    js

This method can be overridden by a particular `Agent` subclass. If this method returns a falsy value, the socket will be destroyed instead of persisting it for use with the next request.

The `socket` argument can be an instance of `<net.Socket>`, a subclass of `<stream.Duplex>`.

#### `agent.reuseSocket(socket, request)`#

Added in: v8.1.0

- `socket` `<stream.Duplex>`
- `request` `<http.ClientRequest>`

Called when `socket` is attached to `request` after being persisted because of the keep-alive options. Default behavior is to:

    socket.ref();
    js

This method can be overridden by a particular `Agent` subclass.

The `socket` argument can be an instance of `<net.Socket>`, a subclass of `<stream.Duplex>`.

#### `agent.destroy()`#

Added in: v0.11.4

Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if using an agent with `keepAlive` enabled, then it is best to explicitly shut down the agent when it is no longer needed. Otherwise, sockets might stay open for quite a long time before the server terminates them.

#### `agent.freeSockets`#

Added in: v0.11.4
History

Version | Changes
--- | ---
v16.0.0 | The property now has a `null` prototype.

- Type: `<Object>`

An object which contains arrays of sockets currently awaiting use by the agent when `keepAlive` is enabled. Do not modify.

Sockets in the `freeSockets` list will be automatically destroyed and removed from the array on `'timeout'`.

#### `agent.getName([options])`#

Added in: v0.11.4
History

Version | Changes
--- | ---
v17.7.0, v16.15.0 | The `options` parameter is now optional.

- `options` `<Object>` A set of options providing information for name generation
  - `host` `<string>` A domain name or IP address of the server to issue the request to
  - `port` `<number>` Port of remote server
  - `localAddress` `<string>` Local interface to bind for network connections when issuing the request
  - `family` `<integer>` Must be 4 or 6 if this doesn't equal `undefined`.
- Returns: `<string>`

Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns `host:port:localAddress` or `host:port:localAddress:family`. For an HTTPS agent, the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options that determine socket reusability.

#### `agent.maxFreeSockets`#

Added in: v0.11.7

- Type: `<number>`

By default set to 256. For agents with `keepAlive` enabled, this sets the maximum number of sockets that will be left open in the free state.

#### `agent.maxSockets`#

Added in: v0.3.6

- Type: `<number>`

By default set to `Infinity`. Determines how many concurrent sockets the agent can have open per origin. Origin is the returned value of `agent.getName()`.

#### `agent.maxTotalSockets`#

Added in: v14.5.0, v12.19.0

- Type: `<number>`

By default set to `Infinity`. Determines how many concurrent sockets the agent can have open. Unlike `maxSockets`, this parameter applies across all origins.

#### `agent.requests`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v16.0.0 | The property now has a `null` prototype.

- Type: `<Object>`

An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.

#### `agent.sockets`#

Added in: v0.3.6
History

Version | Changes
--- | ---
v16.0.0 | The property now has a `null` prototype.

- Type: `<Object>`

An object which contains arrays of sockets currently in use by the agent. Do not modify.

### Class: `http.ClientRequest`#

Added in: v0.1.17

- Extends: `<http.OutgoingMessage>`

This object is created internally and returned from `http.request()`. It represents an in-progress request whose header has already been queued. The header is still mutable using the `setHeader(name, value)`, `getHeader(name)`, `removeHeader(name)` API. The actual header will be sent along with the first data chunk or when calling `request.end()`.

To get the response, add a listener for `'response'` to the request object. `'response'` will be emitted from the request object when the response headers have been received. The `'response'` event is executed with one argument which is an instance of `http.IncomingMessage`.

During the `'response'` event, one can add listeners to the response object; particularly to listen for the `'data'` event.

If no `'response'` handler is added, then the response will be entirely discarded. However, if a `'response'` event handler is added, then the data from the response object must be consumed, either by calling `response.read()` whenever there is a `'readable'` event, or by adding a `'data'` handler, or by calling the `.resume()` method. Until the data is consumed, the `'end'` event will not fire. Also, until the data is read it will consume memory that can eventually lead to a 'process out of memory' error.

For backward compatibility, `res` will only emit `'error'` if there is an `'error'` listener registered.

Set `Content-Length` header to limit the response body size. If `response.strictContentLength` is set to `true`, mismatching the `Content-Length` header value will result in an `Error` being thrown, identified by `code:` `'ERR_HTTP_CONTENT_LENGTH_MISMATCH'`.

`Content-Length` value should be in bytes, not characters. Use `Buffer.byteLength()` to determine the length of the body in bytes.

#### Event: `'abort'`#

Added in: v1.4.1Deprecated in: v17.0.0, v16.12.0

Stability: 0 - Deprecated. Listen for the `'close'` event instead.

Emitted when the request has been aborted by the client. This event is only emitted on the first call to `abort()`.

#### Event: `'close'`#

Added in: v0.5.4

Indicates that the request is completed, or its underlying connection was terminated prematurely (before the response completion).

#### Event: `'connect'`#

Added in: v0.7.0

- `response` `<http.IncomingMessage>`
- `socket` `<stream.Duplex>`
- `head` `<Buffer>`

Emitted each time a server responds to a request with a `CONNECT` method. If this event is not being listened for, clients receiving a `CONNECT` method will have their connections closed.

This event is guaranteed to be passed an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specifies a socket type other than `<net.Socket>`.

A client and server pair demonstrating how to listen for the `'connect'` event:

    import { createServer, request } from 'node:http';
    import { connect } from 'node:net';
    import { URL } from 'node:url';

    // Create an HTTP tunneling proxy
    const proxy = createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('okay');
    });
    proxy.on('connect', (req, clientSocket, head) => {
      // Connect to an origin server
      const { port, hostname } = new URL(`http://${req.url}`);
      const serverSocket = connect(port || 80, hostname, () => {
        clientSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                        'Proxy-agent: Node.js-Proxy\r\n' +
                        '\r\n');
        serverSocket.write(head);
        serverSocket.pipe(clientSocket);
        clientSocket.pipe(serverSocket);
      });
    });

    // Now that proxy is running
    proxy.listen(1337, '127.0.0.1', () => {

      // Make a request to a tunneling proxy
      const options = {
        port: 1337,
        host: '127.0.0.1',
        method: 'CONNECT',
        path: 'www.google.com:80',
      };

      const req = request(options);
      req.end();

      req.on('connect', (res, socket, head) => {
        console.log('got connected!');

        // Make a request over an HTTP tunnel
        socket.write('GET / HTTP/1.1\r\n' +
                     'Host: www.google.com:80\r\n' +
                     'Connection: close\r\n' +
                     '\r\n');
        socket.on('data', (chunk) => {
          console.log(chunk.toString());
        });
        socket.on('end', () => {
          proxy.close();
        });
      });
    });
    const http = require('node:http');
    const net = require('node:net');
    const { URL } = require('node:url');

    // Create an HTTP tunneling proxy
    const proxy = http.createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('okay');
    });
    proxy.on('connect', (req, clientSocket, head) => {
      // Connect to an origin server
      const { port, hostname } = new URL(`http://${req.url}`);
      const serverSocket = net.connect(port || 80, hostname, () => {
        clientSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                        'Proxy-agent: Node.js-Proxy\r\n' +
                        '\r\n');
        serverSocket.write(head);
        serverSocket.pipe(clientSocket);
        clientSocket.pipe(serverSocket);
      });
    });

    // Now that proxy is running
    proxy.listen(1337, '127.0.0.1', () => {

      // Make a request to a tunneling proxy
      const options = {
        port: 1337,
        host: '127.0.0.1',
        method: 'CONNECT',
        path: 'www.google.com:80',
      };

      const req = http.request(options);
      req.end();

      req.on('connect', (res, socket, head) => {
        console.log('got connected!');

        // Make a request over an HTTP tunnel
        socket.write('GET / HTTP/1.1\r\n' +
                     'Host: www.google.com:80\r\n' +
                     'Connection: close\r\n' +
                     '\r\n');
        socket.on('data', (chunk) => {
          console.log(chunk.toString());
        });
        socket.on('end', () => {
          proxy.close();
        });
      });
    });
    javascript

#### Event: `'continue'`#

Added in: v0.3.2

Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.

#### Event: `'finish'`#

Added in: v0.3.6

Emitted when the request has been sent. More specifically, this event is emitted when the last segment of the request headers and body have been handed off to the operating system for transmission over the network. It does not imply that the server has received anything yet.

#### Event: `'information'`#

Added in: v10.0.0

- `info` `<Object>`
  - `httpVersion` `<string>`
  - `httpVersionMajor` `<integer>`
  - `httpVersionMinor` `<integer>`
  - `statusCode` `<integer>`
  - `statusMessage` `<string>`
  - `headers` `<Object>`
  - `rawHeaders` `<string>`[]

Emitted when the server sends a 1xx intermediate response (excluding 101 Upgrade). The listeners of this event will receive an object containing the HTTP version, status code, status message, key-value headers object, and array with the raw header names followed by their respective values.

    import { request } from 'node:http';

    const options = {
      host: '127.0.0.1',
      port: 8080,
      path: '/length_request',
    };

    // Make a request
    const req = request(options);
    req.end();

    req.on('information', (info) => {
      console.log(`Got information prior to main response: ${info.statusCode}`);
    });
    const http = require('node:http');

    const options = {
      host: '127.0.0.1',
      port: 8080,
      path: '/length_request',
    };

    // Make a request
    const req = http.request(options);
    req.end();

    req.on('information', (info) => {
      console.log(`Got information prior to main response: ${info.statusCode}`);
    });
    javascript

101 Upgrade statuses do not fire this event due to their break from the traditional HTTP request/response chain, such as web sockets, in-place TLS upgrades, or HTTP 2.0. To be notified of 101 Upgrade notices, listen for the `'upgrade'` event instead.

#### Event: `'response'`#

Added in: v0.1.0

- `response` `<http.IncomingMessage>`

Emitted when a response is received to this request. This event is emitted only once.

#### Event: `'socket'`#

Added in: v0.5.3

- `socket` `<stream.Duplex>`

This event is guaranteed to be passed an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specifies a socket type other than `<net.Socket>`.

#### Event: `'timeout'`#

Added in: v0.7.8

Emitted when the underlying socket times out from inactivity. This only notifies that the socket has been idle. The request must be destroyed manually.

See also: `request.setTimeout()`.

#### Event: `'upgrade'`#

Added in: v0.1.94

- `response` `<http.IncomingMessage>`
- `stream` `<stream.Duplex>`
- `head` `<Buffer>`

Emitted each time a server responds to a request with an upgrade. If this event is not being listened for and the response status code is 101 Switching Protocols, clients receiving an upgrade header will have their connections closed.

This event is guaranteed to be passed an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specifies a socket type other than `<net.Socket>`.

A client server pair demonstrating how to listen for the `'upgrade'` event.

    import http from 'node:http';
    import process from 'node:process';

    // Create an HTTP server
    const server = http.createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('okay');
    });
    server.on('upgrade', (req, stream, head) => {
      stream.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
                   'Upgrade: WebSocket\r\n' +
                   'Connection: Upgrade\r\n' +
                   '\r\n');

      stream.pipe(stream); // echo back
    });

    // Now that server is running
    server.listen(1337, '127.0.0.1', () => {

      // make a request
      const options = {
        port: 1337,
        host: '127.0.0.1',
        headers: {
          'Connection': 'Upgrade',
          'Upgrade': 'websocket',
        },
      };

      const req = http.request(options);
      req.end();

      req.on('upgrade', (res, stream, upgradeHead) => {
        console.log('got upgraded!');
        stream.end();
        process.exit(0);
      });
    });
    const http = require('node:http');

    // Create an HTTP server
    const server = http.createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('okay');
    });
    server.on('upgrade', (req, stream, head) => {
      stream.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
                   'Upgrade: WebSocket\r\n' +
                   'Connection: Upgrade\r\n' +
                   '\r\n');

      stream.pipe(stream); // echo back
    });

    // Now that server is running
    server.listen(1337, '127.0.0.1', () => {

      // make a request
      const options = {
        port: 1337,
        host: '127.0.0.1',
        headers: {
          'Connection': 'Upgrade',
          'Upgrade': 'websocket',
        },
      };

      const req = http.request(options);
      req.end();

      req.on('upgrade', (res, stream, upgradeHead) => {
        console.log('got upgraded!');
        stream.end();
        process.exit(0);
      });
    });
    javascript

#### `request.abort()`#

Added in: v0.3.8Deprecated in: v14.1.0, v13.14.0

Stability: 0 - Deprecated: Use `request.destroy()` instead.

Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

#### `request.aborted`#

Added in: v0.11.14Deprecated in: v17.0.0, v16.12.0
History

Version | Changes
--- | ---
v11.0.0 | The `aborted` property is no longer a timestamp number.

Stability: 0 - Deprecated. Check `request.destroyed` instead.

- Type: `<boolean>`

The `request.aborted` property will be `true` if the request has been aborted.

#### `request.connection`#

Added in: v0.3.0Deprecated in: v13.0.0

Stability: 0 - Deprecated. Use `request.socket`.

- Type: `<stream.Duplex>`

See `request.socket`.

#### `request.cork()`#

Added in: v13.2.0, v12.16.0

See `writable.cork()`.

#### `request.end([data[, encoding]][, callback])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v15.0.0 | The `data` parameter can now be a `Uint8Array`.
v10.0.0 | This method now returns a reference to `ClientRequest`.

- `data` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>`
- `callback` `<Function>`
- Returns: `<this>`

Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating `'0\r\n\r\n'`.

If `data` is specified, it is equivalent to calling `request.write(data, encoding)` followed by `request.end(callback)`.

If `callback` is specified, it will be called when the request stream is finished.

#### `request.destroy([error])`#

Added in: v0.3.0
History

Version | Changes
--- | ---
v14.5.0 | The function returns `this` for consistency with other Readable streams.

- `error` `<Error>` Optional, an error to emit with `'error'` event.
- Returns: `<this>`

Destroy the request. Optionally emit an `'error'` event, and emit a `'close'` event. Calling this will cause remaining data in the response to be dropped, and the socket to be destroyed if used, or returned to the corresponding Agent pool otherwise if possible.

See `writable.destroy()` for further details.

##### `request.destroyed`#

Added in: v14.1.0, v13.14.0

- Type: `<boolean>`

Is `true` after `request.destroy()` has been called.

See `writable.destroyed` for further details.

#### `request.finished`#

Added in: v0.0.1Deprecated in: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use `request.writableEnded`.

- Type: `<boolean>`

The `request.finished` property will be `true` if `request.end()` has been called. `request.end()` will automatically be called if the request was initiated via `http.get()`.

#### `request.flushHeaders()`#

Added in: v1.6.0

Flushes the request headers.

For efficiency reasons, Node.js normally buffers the request headers until `request.end()` is called or the first chunk of request data is written. It then tries to pack the request headers and data into a single TCP packet.

That's usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later. `request.flushHeaders()` bypasses the optimization and kickstarts the request.

#### `request.getHeader(name)`#

Added in: v1.6.0

- `name` `<string>`
- Returns: `<any>`

Reads out a header on the request. The name is case-insensitive. The type of the return value depends on the arguments provided to `request.setHeader()`.

    request.setHeader('content-type', 'text/html');
    request.setHeader('Content-Length', Buffer.byteLength(body));
    request.setHeader('Cookie', ['type=ninja', 'language=javascript']);
    const contentType = request.getHeader('Content-Type');
    // 'contentType' is 'text/html'
    const contentLength = request.getHeader('Content-Length');
    // 'contentLength' is of type number
    const cookie = request.getHeader('Cookie');
    // 'cookie' is of type string[]
    js

#### `request.getHeaderNames()`#

Added in: v7.7.0

- Returns: `<string>`[]

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

    request.setHeader('Foo', 'bar');
    request.setHeader('Cookie', ['foo=bar', 'bar=baz']);

    const headerNames = request.getHeaderNames();
    // headerNames === ['foo', 'cookie']
    js

#### `request.getHeaders()`#

Added in: v7.7.0

- Returns: `<Object>`

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the `request.getHeaders()` method does not prototypically inherit from the JavaScript `Object`. This means that typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others are not defined and will not work.

    request.setHeader('Foo', 'bar');
    request.setHeader('Cookie', ['foo=bar', 'bar=baz']);

    const headers = request.getHeaders();
    // headers === { foo: 'bar', 'cookie': ['foo=bar', 'bar=baz'] }
    js

#### `request.getRawHeaderNames()`#

Added in: v15.13.0, v14.17.0

- Returns: `<string>`[]

Returns an array containing the unique names of the current outgoing raw headers. Header names are returned with their exact casing being set.

    request.setHeader('Foo', 'bar');
    request.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

    const headerNames = request.getRawHeaderNames();
    // headerNames === ['Foo', 'Set-Cookie']
    js

#### `request.hasHeader(name)`#

Added in: v7.7.0

- `name` `<string>`
- Returns: `<boolean>`

Returns `true` if the header identified by `name` is currently set in the outgoing headers. The header name matching is case-insensitive.

    const hasContentType = request.hasHeader('content-type');
    js

#### `request.maxHeadersCount`#

- Type: `<number>` Default: `2000`

Limits maximum response headers count. If set to 0, no limit will be applied.

#### `request.path`#

Added in: v0.4.0

- Type: `<string>` The request path.

#### `request.method`#

Added in: v0.1.97

- Type: `<string>` The request method.

#### `request.host`#

Added in: v14.5.0, v12.19.0

- Type: `<string>` The request host.

#### `request.protocol`#

Added in: v14.5.0, v12.19.0

- Type: `<string>` The request protocol.

#### `request.removeHeader(name)`#

Added in: v1.6.0

- `name` `<string>`

Removes a header that's already defined into headers object.

    request.removeHeader('Content-Type');
    js

#### `request.reusedSocket`#

Added in: v13.0.0, v12.16.0

- Type: `<boolean>` Whether the request is sent through a reused socket.

When sending request through a keep-alive enabled agent, the underlying socket might be reused. But if server closes connection at unfortunate time, client may run into a 'ECONNRESET' error.

    import http from 'node:http';
    const agent = new http.Agent({ keepAlive: true });

    // Server has a 5 seconds keep-alive timeout by default
    http
      .createServer((req, res) => {
        res.write('hello\n');
        res.end();
      })
      .listen(3000);

    setInterval(() => {
      // Adapting a keep-alive agent
      http.get('http://localhost:3000', { agent }, (res) => {
        res.on('data', (data) => {
          // Do nothing
        });
      });
    }, 5000); // Sending request on 5s interval so it's easy to hit idle timeout
    const http = require('node:http');
    const agent = new http.Agent({ keepAlive: true });

    // Server has a 5 seconds keep-alive timeout by default
    http
      .createServer((req, res) => {
        res.write('hello\n');
        res.end();
      })
      .listen(3000);

    setInterval(() => {
      // Adapting a keep-alive agent
      http.get('http://localhost:3000', { agent }, (res) => {
        res.on('data', (data) => {
          // Do nothing
        });
      });
    }, 5000); // Sending request on 5s interval so it's easy to hit idle timeout
    javascript

By marking a request whether it reused socket or not, we can do automatic error retry base on it.

    import http from 'node:http';
    const agent = new http.Agent({ keepAlive: true });

    function retriableRequest() {
      const req = http
        .get('http://localhost:3000', { agent }, (res) => {
          // ...
        })
        .on('error', (err) => {
          // Check if retry is needed
          if (req.reusedSocket && err.code === 'ECONNRESET') {
            retriableRequest();
          }
        });
    }

    retriableRequest();
    const http = require('node:http');
    const agent = new http.Agent({ keepAlive: true });

    function retriableRequest() {
      const req = http
        .get('http://localhost:3000', { agent }, (res) => {
          // ...
        })
        .on('error', (err) => {
          // Check if retry is needed
          if (req.reusedSocket && err.code === 'ECONNRESET') {
            retriableRequest();
          }
        });
    }

    retriableRequest();
    javascript

#### `request.setHeader(name, value)`#

Added in: v1.6.0

- `name` `<string>`
- `value` `<any>`

Sets a single header value for headers object. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, `request.getHeader()` may return non-string values. However, the non-string values will be converted to strings for network transmission.

    request.setHeader('Content-Type', 'application/json');
    js

or

    request.setHeader('Cookie', ['type=ninja', 'language=javascript']);
    js

When the value is a string an exception will be thrown if it contains characters outside the `latin1` encoding.

If you need to pass UTF-8 characters in the value please encode the value using the RFC 8187 standard.

    const filename = 'Rock 🎵.txt';
    request.setHeader('Content-Disposition', `attachment; filename*=utf-8''${encodeURIComponent(filename)}`);
    js

#### `request.setNoDelay([noDelay])`#

Added in: v0.5.9

- `noDelay` `<boolean>`

Once a socket is assigned to this request and is connected `socket.setNoDelay()` will be called.

#### `request.setSocketKeepAlive([enable][, initialDelay])`#

Added in: v0.5.9

- `enable` `<boolean>`
- `initialDelay` `<number>`

Once a socket is assigned to this request and is connected `socket.setKeepAlive()` will be called.

#### `request.setTimeout(timeout[, callback])`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v9.0.0 | Consistently set socket timeout only when the socket connects.

- `timeout` `<number>` Milliseconds before a request times out.
- `callback` `<Function>` Optional function to be called when a timeout occurs. Same as binding to the `'timeout'` event.
- Returns: `<http.ClientRequest>`

Once a socket is assigned to this request and is connected `socket.setTimeout()` will be called.

#### `request.socket`#

Added in: v0.3.0

- Type: `<stream.Duplex>`

Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit `'readable'` events because of how the protocol parser attaches to the socket.

    import http from 'node:http';
    const options = {
      host: 'www.google.com',
    };
    const req = http.get(options);
    req.end();
    req.once('response', (res) => {
      const ip = req.socket.localAddress;
      const port = req.socket.localPort;
      console.log(`Your IP address is ${ip} and your source port is ${port}.`);
      // Consume response object
    });
    const http = require('node:http');
    const options = {
      host: 'www.google.com',
    };
    const req = http.get(options);
    req.end();
    req.once('response', (res) => {
      const ip = req.socket.localAddress;
      const port = req.socket.localPort;
      console.log(`Your IP address is ${ip} and your source port is ${port}.`);
      // Consume response object
    });
    javascript

This property is guaranteed to be an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specified a socket type other than `<net.Socket>`.

#### `request.uncork()`#

Added in: v13.2.0, v12.16.0

See `writable.uncork()`.

#### `request.writableEnded`#

Added in: v12.9.0

- Type: `<boolean>`

Is `true` after `request.end()` has been called. This property does not indicate whether the data has been flushed, for this use `request.writableFinished` instead.

#### `request.writableFinished`#

Added in: v12.7.0

- Type: `<boolean>`

Is `true` if all data has been flushed to the underlying system, immediately before the `'finish'` event is emitted.

#### `request.write(chunk[, encoding][, callback])`#

Added in: v0.1.29
History

Version | Changes
--- | ---
v15.0.0 | The `chunk` parameter can now be a `Uint8Array`.

- `chunk` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>`
- `callback` `<Function>`
- Returns: `<boolean>`

Sends a chunk of the body. This method can be called multiple times. If no `Content-Length` is set, data will automatically be encoded in HTTP Chunked transfer encoding, so that server knows when the data ends. The `Transfer-Encoding: chunked` header is added. Calling `request.end()` is necessary to finish sending the request.

The `encoding` argument is optional and only applies when `chunk` is a string. Defaults to `'utf8'`.

The `callback` argument is optional and will be called when this chunk of data is flushed, but only if the chunk is non-empty.

Returns `true` if the entire data was flushed successfully to the kernel buffer. Returns `false` if all or part of the data was queued in user memory. `'drain'` will be emitted when the buffer is free again.

When `write` function is called with empty string or buffer, it does nothing and waits for more input.

### Class: `http.Server`#

Added in: v0.1.17

- Extends: `<net.Server>`

#### Event: `'checkContinue'`#

Added in: v0.3.0

- `request` `<http.IncomingMessage>`
- `response` `<http.ServerResponse>`

Emitted each time a request with an HTTP `Expect: 100-continue` is received. If this event is not listened for, the server will automatically respond with a `100 Continue` as appropriate.

Handling this event involves calling `response.writeContinue()` if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the `'request'` event will not be emitted.

#### Event: `'checkExpectation'`#

Added in: v5.5.0

- `request` `<http.IncomingMessage>`
- `response` `<http.ServerResponse>`

Emitted each time a request with an HTTP `Expect` header is received, where the value is not `100-continue`. If this event is not listened for, the server will automatically respond with a `417 Expectation Failed` as appropriate.

When this event is emitted and handled, the `'request'` event will not be emitted.

#### Event: `'clientError'`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v12.0.0 | The default behavior will return a 431 Request Header Fields Too Large if an HPE_HEADER_OVERFLOW error occurs.
v9.4.0 | The `rawPacket` is the current buffer that just parsed. Adding this buffer to the error object of `'clientError'` event is to make it possible that developers can log the broken packet.
v6.0.0 | The default action of calling `.destroy()` on the `socket` will no longer take place if there are listeners attached for `'clientError'`.

- `exception` `<Error>`
- `socket` `<stream.Duplex>`

If a client connection emits an `'error'` event, it will be forwarded here. Listener of this event is responsible for closing/destroying the underlying socket. For example, one may wish to more gracefully close the socket with a custom HTTP response instead of abruptly severing the connection. The socket must be closed or destroyed before the listener ends.

This event is guaranteed to be passed an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specifies a socket type other than `<net.Socket>`.

Default behavior is to try close the socket with an HTTP '400 Bad Request', or an HTTP '431 Request Header Fields Too Large' in the case of an `HPE_HEADER_OVERFLOW` error. If the socket is not writable or headers of the current attached `http.ServerResponse` has been sent, it is immediately destroyed.

`socket` is the `net.Socket` object that the error originated from.

    import http from 'node:http';

    const server = http.createServer((req, res) => {
      res.end();
    });
    server.on('clientError', (err, socket) => {
      socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
    });
    server.listen(8000);
    const http = require('node:http');

    const server = http.createServer((req, res) => {
      res.end();
    });
    server.on('clientError', (err, socket) => {
      socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
    });
    server.listen(8000);
    javascript

When the `'clientError'` event occurs, there is no `request` or `response` object, so any HTTP response sent, including response headers and payload, must be written directly to the `socket` object. Care must be taken to ensure the response is a properly formatted HTTP response message.

`err` is an instance of `Error` with two extra columns:

- `bytesParsed`: the bytes count of request packet that Node.js may have parsed correctly;
- `rawPacket`: the raw packet of current request.

In some cases, the client has already received the response and/or the socket has already been destroyed, like in case of `ECONNRESET` errors. Before trying to send data to the socket, it is better to check that it is still writable.

    server.on('clientError', (err, socket) => {
      if (err.code === 'ECONNRESET' || !socket.writable) {
        return;
      }

      socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
    });
    js

#### Event: `'close'`#

Added in: v0.1.4

Emitted when the server closes.

#### Event: `'connect'`#

Added in: v0.7.0

- `request` `<http.IncomingMessage>` Arguments for the HTTP request, as it is in the `'request'` event
- `socket` `<stream.Duplex>` Network socket between the server and client
- `head` `<Buffer>` The first packet of the tunneling stream (may be empty)

Emitted each time a client requests an HTTP `CONNECT` method. If this event is not listened for, then clients requesting a `CONNECT` method will have their connections closed.

This event is guaranteed to be passed an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specifies a socket type other than `<net.Socket>`.

After this event is emitted, the request's socket will not have a `'data'` event listener, meaning it will need to be bound in order to handle data sent to the server on that socket.

#### Event: `'connection'`#

Added in: v0.1.0

- `socket` `<stream.Duplex>`

This event is emitted when a new TCP stream is established. `socket` is typically an object of type `net.Socket`. Usually users will not want to access this event. In particular, the socket will not emit `'readable'` events because of how the protocol parser attaches to the socket. The `socket` can also be accessed at `request.socket`.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any `Duplex` stream can be passed.

If `socket.setTimeout()` is called here, the timeout will be replaced with `server.keepAliveTimeout` when the socket has served a request (if `server.keepAliveTimeout` is non-zero).

This event is guaranteed to be passed an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specifies a socket type other than `<net.Socket>`.

#### Event: `'dropRequest'`#

Added in: v18.7.0, v16.17.0

- `request` `<http.IncomingMessage>` Arguments for the HTTP request, as it is in the `'request'` event
- `socket` `<stream.Duplex>` Network socket between the server and client

When the number of requests on a socket reaches the threshold of `server.maxRequestsPerSocket`, the server will drop new requests and emit `'dropRequest'` event instead, then send `503` to client.

#### Event: `'request'`#

Added in: v0.1.0

- `request` `<http.IncomingMessage>`
- `response` `<http.ServerResponse>`

Emitted each time there is a request. There may be multiple requests per connection (in the case of HTTP Keep-Alive connections).

#### Event: `'upgrade'`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v26.0.0 | Request bodies are no longer exposed raw (unparsed) on the socket argument. Instead, if a body is received, the stream argument will be a duplex that emits socket content only after the request body, while the parsed request body data will be emitted from the request, just as in normal server `'request'` events.
v24.9.0, v22.21.0 | Whether this event is fired can now be controlled by the `shouldUpgradeCallback` and sockets will be destroyed if upgraded while no event handler is listening.
v10.0.0 | Not listening to this event no longer causes the socket to be destroyed if a client sends an Upgrade header.

- `request` `<http.IncomingMessage>` Arguments for the HTTP request, as it is in the `'request'` event
- `stream` `<stream.Duplex>` The upgraded stream between the server and client
- `head` `<Buffer>` The first packet of the upgraded stream (may be empty)

Emitted each time a client's HTTP upgrade request is accepted. By default all HTTP upgrade requests are ignored (i.e. only regular `'request'` events are emitted, sticking with the normal HTTP request/response flow) unless you listen to this event, in which case they are all accepted (i.e. the `'upgrade'` event is emitted instead, and future communication must handled directly through the raw stream). You can control this more precisely by using the server `shouldUpgradeCallback` option.

Listening to this event is optional and clients cannot insist on a protocol change.

If an upgrade is accepted by `shouldUpgradeCallback` but no event handler is registered then the socket will be destroyed, resulting in an immediate connection closure for the client.

In the uncommon case that the incoming request has a body, this body will be parsed as normal, separate to the upgrade stream, and the raw stream data will only begin after it has completed. To ensure that reading from the stream isn't blocked by waiting for the request body to be read, any reads on the stream will start the request body flowing automatically. If you want to read the request body, ensure that you do so (i.e. you attach `'data'` listeners) before starting to read from the upgraded stream.

The stream argument will typically be the `<net.Socket>` instance used by the request, but in some cases (such as with a request body) it may be a duplex stream. If required, you can access the raw connection underlying the request via `request.socket`, which is guaranteed to be an instance of `<net.Socket>` unless the user specified another socket type.

#### `server.close([callback])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v19.0.0 | The method closes idle connections before returning.

- `callback` `<Function>`

Stops the server from accepting new connections and closes all connections connected to this server which are not sending a request or waiting for a response. See `net.Server.close()`.

    const http = require('node:http');

    const server = http.createServer({ keepAliveTimeout: 60000 }, (req, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    // Close the server after 10 seconds
    setTimeout(() => {
      server.close(() => {
        console.log('server on port 8000 closed successfully');
      });
    }, 10000);
    js

#### `server.closeAllConnections()`#

Added in: v18.2.0

Closes all established HTTP(S) connections connected to this server, including active connections connected to this server which are sending a request or waiting for a response. This does not destroy sockets upgraded to a different protocol, such as WebSocket or HTTP/2.

> This is a forceful way of closing all connections and should be used with caution. Whenever using this in conjunction with `server.close`, calling this after `server.close` is recommended as to avoid race conditions where new connections are created between a call to this and a call to `server.close`.

    const http = require('node:http');

    const server = http.createServer({ keepAliveTimeout: 60000 }, (req, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    // Close the server after 10 seconds
    setTimeout(() => {
      server.close(() => {
        console.log('server on port 8000 closed successfully');
      });
      // Closes all connections, ensuring the server closes successfully
      server.closeAllConnections();
    }, 10000);
    js

#### `server.closeIdleConnections()`#

Added in: v18.2.0

Closes all connections connected to this server which are not sending a request or waiting for a response.

> Starting with Node.js 19.0.0, there's no need for calling this method in conjunction with `server.close` to reap `keep-alive` connections. Using it won't cause any harm though, and it can be useful to ensure backwards compatibility for libraries and applications that need to support versions older than 19.0.0. Whenever using this in conjunction with `server.close`, calling this after `server.close` is recommended as to avoid race conditions where new connections are created between a call to this and a call to `server.close`.

    const http = require('node:http');

    const server = http.createServer({ keepAliveTimeout: 60000 }, (req, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    // Close the server after 10 seconds
    setTimeout(() => {
      server.close(() => {
        console.log('server on port 8000 closed successfully');
      });
      // Closes idle connections, such as keep-alive connections. Server will close
      // once remaining active connections are terminated
      server.closeIdleConnections();
    }, 10000);
    js

#### `server.headersTimeout`#

Added in: v11.3.0, v10.14.0
History

Version | Changes
--- | ---
v19.4.0, v18.14.0 | The default is now set to the minimum between 60000 (60 seconds) or `requestTimeout`.

- Type: `<number>` Default: The minimum between `server.requestTimeout` or `60000`.

Limit the amount of time the parser will wait to receive the complete HTTP headers.

If the timeout expires, the server responds with status 408 without forwarding the request to the request listener and then closes the connection.

It must be set to a non-zero value (e.g. 120 seconds) to protect against potential Denial-of-Service attacks in case the server is deployed without a reverse proxy in front.

#### `server.listen()`#

Starts the HTTP server listening for connections. This method is identical to `server.listen()` from `net.Server`.

#### `server.listening`#

Added in: v5.7.0

- Type: `<boolean>` Indicates whether or not the server is listening for connections.

#### `server.maxHeadersCount`#

Added in: v0.7.0

- Type: `<number>` Default: `2000`

Limits maximum incoming headers count. If set to 0, no limit will be applied.

#### `server.requestTimeout`#

Added in: v14.11.0
History

Version | Changes
--- | ---
v18.0.0 | The default request timeout changed from no timeout to 300s (5 minutes).

- Type: `<number>` Default: `300000`

Sets the timeout value in milliseconds for receiving the entire request from the client.

If the timeout expires, the server responds with status 408 without forwarding the request to the request listener and then closes the connection.

It must be set to a non-zero value (e.g. 120 seconds) to protect against potential Denial-of-Service attacks in case the server is deployed without a reverse proxy in front.

#### `server.setTimeout([msecs][, callback])`#

Added in: v0.9.12
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

- `msecs` `<number>` Default: 0 (no timeout)
- `callback` `<Function>`
- Returns: `<http.Server>`

Sets the timeout value for sockets, and emits a `'timeout'` event on the Server object, passing the socket as an argument, if a timeout occurs.

If there is a `'timeout'` event listener on the Server object, then it will be called with the timed-out socket as an argument.

By default, the Server does not timeout sockets. However, if a callback is assigned to the Server's `'timeout'` event, timeouts must be handled explicitly.

#### `server.maxRequestsPerSocket`#

Added in: v16.10.0

- Type: `<number>` Requests per socket. Default: 0 (no limit)

The maximum number of requests socket can handle before closing keep alive connection.

A value of `0` will disable the limit.

When the limit is reached it will set the `Connection` header value to `close`, but will not actually close the connection, subsequent requests sent after the limit is reached will get `503 Service Unavailable` as a response.

#### `server.timeout`#

Added in: v0.9.12
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

- Type: `<number>` Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of `0` will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

#### `server.keepAliveTimeout`#

Added in: v8.0.0

- Type: `<number>` Timeout in milliseconds. Default: `5000` (5 seconds).

The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed.

This timeout value is combined with the `server.keepAliveTimeoutBuffer` option to determine the actual socket timeout, calculated as: socketTimeout = keepAliveTimeout + keepAliveTimeoutBuffer If the server receives new data before the keep-alive timeout has fired, it will reset the regular inactivity timeout, i.e., `server.timeout`.

A value of `0` will disable the keep-alive timeout behavior on incoming connections. A value of `0` makes the HTTP server behave similarly to Node.js versions prior to 8.0.0, which did not have a keep-alive timeout.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

#### `server.keepAliveTimeoutBuffer`#

Added in: v24.6.0, v22.19.0

- Type: `<number>` Timeout in milliseconds. Default: `1000` (1 second).

An additional buffer time added to the `server.keepAliveTimeout` to extend the internal socket timeout.

This buffer helps reduce connection reset (`ECONNRESET`) errors by increasing the socket timeout slightly beyond the advertised keep-alive timeout.

This option applies only to new incoming connections.

#### `server[Symbol.asyncDispose]()`#

Added in: v20.4.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `server.close()` and returns a promise that fulfills when the server has closed.

### Class: `http.ServerResponse`#

Added in: v0.1.17

- Extends: `<http.OutgoingMessage>`

This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the `'request'` event.

#### Event: `'close'`#

Added in: v0.6.7

Indicates that the response is completed, or its underlying connection was terminated prematurely (before the response completion).

#### Event: `'finish'`#

Added in: v0.3.6

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.

#### `response.addTrailers(headers)`#

Added in: v0.3.0

- `headers` `<Object>`

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.

HTTP requires the `Trailer` header to be sent in order to emit trailers, with a list of the header fields in its value. E.g.,

    response.writeHead(200, { 'Content-Type': 'text/plain',
                              'Trailer': 'Content-MD5' });
    response.write(fileData);
    response.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
    response.end();
    js

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

#### `response.connection`#

Added in: v0.3.0Deprecated in: v13.0.0

Stability: 0 - Deprecated. Use `response.socket`.

- Type: `<stream.Duplex>`

See `response.socket`.

#### `response.cork()`#

Added in: v13.2.0, v12.16.0

See `writable.cork()`.

#### `response.end([data[, encoding]][, callback])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v15.0.0 | The `data` parameter can now be a `Uint8Array`.
v10.0.0 | This method now returns a reference to `ServerResponse`.

- `data` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>`
- `callback` `<Function>`
- Returns: `<this>`

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, `response.end()`, MUST be called on each response.

If `data` is specified, it is similar in effect to calling `response.write(data, encoding)` followed by `response.end(callback)`.

If `callback` is specified, it will be called when the response stream is finished.

#### `response.finished`#

Added in: v0.0.2Deprecated in: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use `response.writableEnded`.

- Type: `<boolean>`

The `response.finished` property will be `true` if `response.end()` has been called.

#### `response.flushHeaders()`#

Added in: v1.6.0

Flushes the response headers. See also: `request.flushHeaders()`.

#### `response.getHeader(name)`#

Added in: v0.4.0

- `name` `<string>`
- Returns: `<number>` | `<string>` | `<string>`[] | `<undefined>`

Reads out a header that's already been queued but not sent to the client. The name is case-insensitive. The type of the return value depends on the arguments provided to `response.setHeader()`.

    response.setHeader('Content-Type', 'text/html');
    response.setHeader('Content-Length', Buffer.byteLength(body));
    response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
    const contentType = response.getHeader('content-type');
    // contentType is 'text/html'
    const contentLength = response.getHeader('Content-Length');
    // contentLength is of type number
    const setCookie = response.getHeader('set-cookie');
    // setCookie is of type string[]
    js

#### `response.getHeaderNames()`#

Added in: v7.7.0

- Returns: `<string>`[]

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

    response.setHeader('Foo', 'bar');
    response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

    const headerNames = response.getHeaderNames();
    // headerNames === ['foo', 'set-cookie']
    js

#### `response.getHeaders()`#

Added in: v7.7.0

- Returns: `<Object>`

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the `response.getHeaders()` method does not prototypically inherit from the JavaScript `Object`. This means that typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others are not defined and will not work.

    response.setHeader('Foo', 'bar');
    response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

    const headers = response.getHeaders();
    // headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
    js

#### `response.hasHeader(name)`#

Added in: v7.7.0

- `name` `<string>`
- Returns: `<boolean>`

Returns `true` if the header identified by `name` is currently set in the outgoing headers. The header name matching is case-insensitive.

    const hasContentType = response.hasHeader('content-type');
    js

#### `response.headersSent`#

Added in: v0.9.3

- Type: `<boolean>`

Boolean (read-only). True if headers were sent, false otherwise.

#### `response.removeHeader(name)`#

Added in: v0.4.0

- `name` `<string>`

Removes a header that's queued for implicit sending.

    response.removeHeader('Content-Encoding');
    js

#### `response.req`#

Added in: v15.7.0

- Type: `<http.IncomingMessage>`

A reference to the original HTTP `request` object.

#### `response.sendDate`#

Added in: v0.7.5

- Type: `<boolean>`

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; the Date header is required in most HTTP responses (see RFC 9110 Section 6.6.1 for details).

#### `response.setHeader(name, value)`#

Added in: v0.4.0

- `name` `<string>`
- `value` `<number>` | `<string>` | `<string>`[]
- Returns: `<http.ServerResponse>`

Returns the response object.

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, `response.getHeader()` may return non-string values. However, the non-string values will be converted to strings for network transmission. The same response object is returned to the caller, to enable call chaining.

    response.setHeader('Content-Type', 'text/html');
    js

or

    response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
    js

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

When headers have been set with `response.setHeader()`, they will be merged with any headers passed to `response.writeHead()`, with the headers passed to `response.writeHead()` given precedence.

    // Returns content-type = text/plain
    const server = http.createServer((req, res) => {
      res.setHeader('Content-Type', 'text/html');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('ok');
    });
    js

If `response.writeHead()` method is called and this method has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the `response.getHeader()` on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use `response.setHeader()` instead of `response.writeHead()`.

#### `response.setTimeout(msecs[, callback])`#

Added in: v0.9.12

- `msecs` `<number>`
- `callback` `<Function>`
- Returns: `<http.ServerResponse>`

Sets the Socket's timeout value to `msecs`. If a callback is provided, then it is added as a listener on the `'timeout'` event on the response object.

If no `'timeout'` listener is added to the request, the response, or the server, then sockets are destroyed when they time out. If a handler is assigned to the request, the response, or the server's `'timeout'` events, timed out sockets must be handled explicitly.

#### `response.socket`#

Added in: v0.3.0

- Type: `<stream.Duplex>`

Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit `'readable'` events because of how the protocol parser attaches to the socket. After `response.end()`, the property is nulled.

    import http from 'node:http';
    const server = http.createServer((req, res) => {
      const ip = res.socket.remoteAddress;
      const port = res.socket.remotePort;
      res.end(`Your IP address is ${ip} and your source port is ${port}.`);
    }).listen(3000);
    const http = require('node:http');
    const server = http.createServer((req, res) => {
      const ip = res.socket.remoteAddress;
      const port = res.socket.remotePort;
      res.end(`Your IP address is ${ip} and your source port is ${port}.`);
    }).listen(3000);
    javascript

This property is guaranteed to be an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specified a socket type other than `<net.Socket>`.

#### `response.statusCode`#

Added in: v0.4.0

- Type: `<number>` Default: `200`

When using implicit headers (not calling `response.writeHead()` explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

    response.statusCode = 404;
    js

After response header was sent to the client, this property indicates the status code which was sent out.

#### `response.statusMessage`#

Added in: v0.11.8

- Type: `<string>`

When using implicit headers (not calling `response.writeHead()` explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left as `undefined` then the standard message for the status code will be used.

    response.statusMessage = 'Not found';
    js

After response header was sent to the client, this property indicates the status message which was sent out.

#### `response.strictContentLength`#

Added in: v18.10.0, v16.18.0

- Type: `<boolean>` Default: `false`

If set to `true`, Node.js will check whether the `Content-Length` header value and the size of the body, in bytes, are equal. Mismatching the `Content-Length` header value will result in an `Error` being thrown, identified by `code:` `'ERR_HTTP_CONTENT_LENGTH_MISMATCH'`.

#### `response.uncork()`#

Added in: v13.2.0, v12.16.0

See `writable.uncork()`.

#### `response.writableEnded`#

Added in: v12.9.0

- Type: `<boolean>`

Is `true` after `response.end()` has been called. This property does not indicate whether the data has been flushed, for this use `response.writableFinished` instead.

#### `response.writableFinished`#

Added in: v12.7.0

- Type: `<boolean>`

Is `true` if all data has been flushed to the underlying system, immediately before the `'finish'` event is emitted.

#### `response.write(chunk[, encoding][, callback])`#

Added in: v0.1.29
History

Version | Changes
--- | ---
v15.0.0 | The `chunk` parameter can now be a `Uint8Array`.

- `chunk` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>` Default: `'utf8'`
- `callback` `<Function>`
- Returns: `<boolean>`

If this method is called and `response.writeHead()` has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

If `rejectNonStandardBodyWrites` is set to true in `createServer` then writing to the body is not allowed when the request method or response status do not support content. If an attempt is made to write to the body for a HEAD request or as part of a `204` or `304`response, a synchronous `Error` with the code `ERR_HTTP_BODY_NOT_ALLOWED` is thrown.

`chunk` can be a string or a buffer. If `chunk` is a string, the second parameter specifies how to encode it into a byte stream. `callback` will be called when this chunk of data is flushed.

This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time `response.write()` is called, it will send the buffered header information and the first chunk of the body to the client. The second time `response.write()` is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body.

Returns `true` if the entire data was flushed successfully to the kernel buffer. Returns `false` if all or part of the data was queued in user memory. `'drain'` will be emitted when the buffer is free again.

#### `response.writeContinue()`#

Added in: v0.3.0

Sends an HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the `'checkContinue'` event on `Server`.

#### `response.writeEarlyHints(hints[, callback])`#

Added in: v18.11.0
History

Version | Changes
--- | ---
v18.11.0 | Allow passing hints as an object.

- `hints` `<Object>`
- `callback` `<Function>`

Sends an HTTP/1.1 103 Early Hints message to the client with a Link header, indicating that the user agent can preload/preconnect the linked resources. The `hints` is an object containing the values of headers to be sent with early hints message. The optional `callback` argument will be called when the response message has been written.

Example

    const earlyHintsLink = '</styles.css>; rel=preload; as=style';
    response.writeEarlyHints({
      'link': earlyHintsLink,
    });

    const earlyHintsLinks = [
      '</styles.css>; rel=preload; as=style',
      '</scripts.js>; rel=preload; as=script',
    ];
    response.writeEarlyHints({
      'link': earlyHintsLinks,
      'x-trace-id': 'id for diagnostics',
    });

    const earlyHintsCallback = () => console.log('early hints message sent');
    response.writeEarlyHints({
      'link': earlyHintsLinks,
    }, earlyHintsCallback);
    js

#### `response.writeHead(statusCode[, statusMessage][, headers])`#

Added in: v0.1.30
History

Version | Changes
--- | ---
v14.14.0 | Allow passing headers as an array.
v11.10.0, v10.17.0 | Return `this` from `writeHead()` to allow chaining with `end()`.
v5.11.0, v4.4.5 | A `RangeError` is thrown if `statusCode` is not a number in the range `[100, 999]`.

- `statusCode` `<number>`
- `statusMessage` `<string>`
- `headers` `<Object>` | `<Array>`
- Returns: `<http.ServerResponse>`

Sends a response header to the request. The status code is a 3-digit HTTP status code, like `404`. The last argument, `headers`, are the response headers. Optionally one can give a human-readable `statusMessage` as the second argument.

`headers` may be an `Array` where the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. The array is in the same format as `request.rawHeaders`.

Returns a reference to the `ServerResponse`, so that calls can be chained.

    const body = 'hello world';
    response
      .writeHead(200, {
        'Content-Length': Buffer.byteLength(body),
        'Content-Type': 'text/plain',
      })
      .end(body);
    js

This method must only be called once on a message and it must be called before `response.end()` is called.

If `response.write()` or `response.end()` are called before calling this, the implicit/mutable headers will be calculated and call this function.

When headers have been set with `response.setHeader()`, they will be merged with any headers passed to `response.writeHead()`, with the headers passed to `response.writeHead()` given precedence.

If this method is called and `response.setHeader()` has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the `response.getHeader()` on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use `response.setHeader()` instead.

    // Returns content-type = text/plain
    const server = http.createServer((req, res) => {
      res.setHeader('Content-Type', 'text/html');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('ok');
    });
    js

`Content-Length` is read in bytes, not characters. Use `Buffer.byteLength()` to determine the length of the body in bytes. Node.js will check whether `Content-Length` and the length of the body which has been transmitted are equal or not.

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

#### `response.writeInformation(statusCode[, headers][, callback])`#

Added in: v26.2.0

- `statusCode` `<number>` An HTTP 1xx informational status code, between `100` and `199` inclusive, excluding `101` (Switching Protocols) which is only available through the `'upgrade'` event.
- `headers` `<Object>` | `<Array>` An optional set of headers to send with the informational response. Accepts the same shapes as `response.writeHead()`.
- `callback` `<Function>` Optional, called once the message has been written to the socket.

Sends an arbitrary HTTP/1.1 1xx informational response to the client. This is a generic equivalent of `response.writeContinue()`, `response.writeProcessing()` and `response.writeEarlyHints()`, and can be called multiple times before the final response. After the final response headers have been sent (via `response.writeHead()` or an implicit header), calling this method throws `ERR_HTTP_HEADERS_SENT`.

Clients receive these responses via the `'information'` event on `http.ClientRequest`.

    response.writeInformation(110, { 'X-Progress': '50%' });
    js

#### `response.writeProcessing()`#

Added in: v10.0.0

Sends an HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent.

### Class: `http.IncomingMessage`#

Added in: v0.1.17
History

Version | Changes
--- | ---
v15.5.0 | The `destroyed` value returns `true` after the incoming data is consumed.
v13.1.0, v12.16.0 | The `readableHighWaterMark` value mirrors that of the socket.

- Extends: `<stream.Readable>`

An `IncomingMessage` object is created by `http.Server` or `http.ClientRequest` and passed as the first argument to the `'request'` and `'response'` event respectively. It may be used to access response status, headers, and data.

Different from its `socket` value which is a subclass of `<stream.Duplex>`, the `IncomingMessage` itself extends `<stream.Readable>` and is created separately to parse and emit the incoming HTTP headers and payload, as the underlying socket may be reused multiple times in case of keep-alive.

#### Event: `'aborted'`#

Added in: v0.3.8Deprecated in: v17.0.0, v16.12.0

Stability: 0 - Deprecated. Listen for `'close'` event instead.

Emitted when the request has been aborted.

#### Event: `'close'`#

Added in: v0.4.2
History

Version | Changes
--- | ---
v16.0.0 | The close event is now emitted when the request has been completed and not when the underlying socket is closed.

Emitted when the request has been completed.

#### `message.aborted`#

Added in: v10.1.0Deprecated in: v17.0.0, v16.12.0

Stability: 0 - Deprecated. Check `message.destroyed` from `<stream.Readable>`.

- Type: `<boolean>`

The `message.aborted` property will be `true` if the request has been aborted.

#### `message.complete`#

Added in: v0.3.0

- Type: `<boolean>`

The `message.complete` property will be `true` if a complete HTTP message has been received and successfully parsed.

This property is particularly useful as a means of determining if a client or server fully transmitted a message before a connection was terminated:

    const req = http.request({
      host: '127.0.0.1',
      port: 8080,
      method: 'POST',
    }, (res) => {
      res.resume();
      res.on('end', () => {
        if (!res.complete)
          console.error(
            'The connection was terminated while the message was still being sent');
      });
    });
    js

#### `message.connection`#

Added in: v0.1.90Deprecated in: v16.0.0

Stability: 0 - Deprecated. Use `message.socket`.

Alias for `message.socket`.

#### `message.destroy([error])`#

Added in: v0.3.0
History

Version | Changes
--- | ---
v14.5.0, v12.19.0 | The function returns `this` for consistency with other Readable streams.

- `error` `<Error>`
- Returns: `<this>`

Calls `destroy()` on the socket that received the `IncomingMessage`. If `error` is provided, an `'error'` event is emitted on the socket and `error` is passed as an argument to any listeners on the event.

#### `message.headers`#

Added in: v0.1.5
History

Version | Changes
--- | ---
v19.5.0, v18.14.0 | The `joinDuplicateHeaders` option in the `http.request()` and `http.createServer()` functions ensures that duplicate headers are not discarded, but rather combined using a comma separator, in accordance with RFC 9110 Section 5.3.
v15.1.0 | `message.headers` is now lazily computed using an accessor property on the prototype and is no longer enumerable.

- Type: `<Object>`

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.

    // Prints something like:
    //
    // { 'user-agent': 'curl/7.22.0',
    //   host: '127.0.0.1:8000',
    //   accept: '*/*' }
    console.log(request.headers);
    js

Duplicates in raw headers are handled in the following ways, depending on the header name:

- Duplicates of `age`, `authorization`, `content-length`, `content-type`, `etag`, `expires`, `from`, `host`, `if-modified-since`, `if-unmodified-since`, `last-modified`, `location`, `max-forwards`, `proxy-authorization`, `referer`, `retry-after`, `server`, or `user-agent` are discarded. To allow duplicate values of the headers listed above to be joined, use the option `joinDuplicateHeaders` in `http.request()` and `http.createServer()`. See RFC 9110 Section 5.3 for more information.
- `set-cookie` is always an array. Duplicates are added to the array.
- For duplicate `cookie` headers, the values are joined together with `;` .
- For all other headers, the values are joined together with `,` .

#### `message.headersDistinct`#

Added in: v18.3.0, v16.17.0

- Type: `<Object>`

Similar to `message.headers`, but there is no join logic and the values are always arrays of strings, even for headers received just once.

    // Prints something like:
    //
    // { 'user-agent': ['curl/7.22.0'],
    //   host: ['127.0.0.1:8000'],
    //   accept: ['*/*'] }
    console.log(request.headersDistinct);
    js

#### `message.httpVersion`#

Added in: v0.1.1

- Type: `<string>`

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Probably either `'1.1'` or `'1.0'`.

Also `message.httpVersionMajor` is the first integer and `message.httpVersionMinor` is the second.

#### `message.method`#

Added in: v0.1.1

- Type: `<string>`

Only valid for request obtained from `http.Server`.

The request method as a string. Read only. Examples: `'GET'`, `'DELETE'`.

#### `message.rawHeaders`#

Added in: v0.11.6

- Type: `<string>`[]

The raw request/response headers list exactly as they were received.

The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

    // Prints something like:
    //
    // [ 'user-agent',
    //   'this is invalid because there can be only one',
    //   'User-Agent',
    //   'curl/7.22.0',
    //   'Host',
    //   '127.0.0.1:8000',
    //   'ACCEPT',
    //   '*/*' ]
    console.log(request.rawHeaders);
    js

#### `message.rawTrailers`#

Added in: v0.11.6

- Type: `<string>`[]

The raw request/response trailer keys and values exactly as they were received. Only populated at the `'end'` event.

#### `message.setTimeout(msecs[, callback])`#

Added in: v0.5.9

- `msecs` `<number>`
- `callback` `<Function>`
- Returns: `<http.IncomingMessage>`

Calls `message.socket.setTimeout(msecs, callback)`.

#### `message.signal`#

Added in: v26.1.0
History

Version | Changes
--- | ---
v26.7.0 | The signal is no longer aborted after the message completes normally.

- Type: `<AbortSignal>`

An `<AbortSignal>` that is aborted when the message is destroyed before completion or when its underlying socket closes before request handling or response reading completes. The signal is created lazily on first access — no `<AbortController>` is allocated for requests that never use this property.

This is useful for cancelling downstream asynchronous work such as database queries or `fetch` calls when a client disconnects mid-request.

    import http from 'node:http';

    http.createServer(async (req, res) => {
      try {
        const data = await fetch('https://example.com/api', { signal: req.signal });
        res.end(JSON.stringify(await data.json()));
      } catch (err) {
        if (err.name === 'AbortError') return;
        res.statusCode = 500;
        res.end('Internal Server Error');
      }
    }).listen(3000);
    const http = require('node:http');

    http.createServer(async (req, res) => {
      try {
        const data = await fetch('https://example.com/api', { signal: req.signal });
        res.end(JSON.stringify(await data.json()));
      } catch (err) {
        if (err.name === 'AbortError') return;
        res.statusCode = 500;
        res.end('Internal Server Error');
      }
    }).listen(3000);
    javascript

#### `message.socket`#

Added in: v0.3.0

- Type: `<stream.Duplex>`

The `net.Socket` object associated with the connection.

With HTTPS support, use `request.socket.getPeerCertificate()` to obtain the client's authentication details.

This property is guaranteed to be an instance of the `<net.Socket>` class, a subclass of `<stream.Duplex>`, unless the user specified a socket type other than `<net.Socket>` or internally nulled.

#### `message.statusCode`#

Added in: v0.1.1

- Type: `<number>`

Only valid for response obtained from `http.ClientRequest`.

The 3-digit HTTP response status code. E.G. `404`.

#### `message.statusMessage`#

Added in: v0.11.10

- Type: `<string>`

Only valid for response obtained from `http.ClientRequest`.

The HTTP response status message (reason phrase). E.G. `OK` or `Internal Server Error`.

#### `message.trailers`#

Added in: v0.3.0

- Type: `<Object>`

The request/response trailers object. Only populated at the `'end'` event.

#### `message.trailersDistinct`#

Added in: v18.3.0, v16.17.0

- Type: `<Object>`

Similar to `message.trailers`, but there is no join logic and the values are always arrays of strings, even for headers received just once. Only populated at the `'end'` event.

#### `message.url`#

Added in: v0.1.90

- Type: `<string>`

Only valid for request obtained from `http.Server`.

Request URL string. This contains only the URL that is present in the actual HTTP request. Take the following request:

    GET /status?name=ryan HTTP/1.1
    Accept: text/plain
    http

To parse the URL into its parts:

    new URL(`http://${process.env.HOST ?? 'localhost'}${request.url}`);
    js

When `request.url` is `'/status?name=ryan'` and `process.env.HOST` is undefined:

    $ node
    > new URL(`http://${process.env.HOST ?? 'localhost'}${request.url}`);
    URL {
      href: 'http://localhost/status?name=ryan',
      origin: 'http://localhost',
      protocol: 'http:',
      username: '',
      password: '',
      host: 'localhost',
      hostname: 'localhost',
      port: '',
      pathname: '/status',
      search: '?name=ryan',
      searchParams: URLSearchParams { 'name' => 'ryan' },
      hash: ''
    }
    console

Ensure that you set `process.env.HOST` to the server's host name, or consider replacing this part entirely. If using `req.headers.host`, ensure proper validation is used, as clients may specify a custom `Host` header.

### Class: `http.OutgoingMessage`#

Added in: v0.1.17

- Extends: `<Stream>`

This class serves as the parent class of `http.ClientRequest` and `http.ServerResponse`. It is an abstract outgoing message from the perspective of the participants of an HTTP transaction.

#### Event: `'drain'`#

Added in: v0.3.6

Emitted when the buffer of the message is free again.

#### Event: `'finish'`#

Added in: v0.1.17

Emitted when the transmission is finished successfully.

#### Event: `'prefinish'`#

Added in: v0.11.6

Emitted after `outgoingMessage.end()` is called. When the event is emitted, all data has been processed but not necessarily completely flushed.

#### `outgoingMessage.addTrailers(headers)`#

Added in: v0.3.0

- `headers` `<Object>`

Adds HTTP trailers (headers but at the end of the message) to the message.

Trailers will only be emitted if the message is chunked encoded. If not, the trailers will be silently discarded.

HTTP requires the `Trailer` header to be sent to emit trailers, with a list of header field names in its value, e.g.

    message.writeHead(200, { 'Content-Type': 'text/plain',
                             'Trailer': 'Content-MD5' });
    message.write(fileData);
    message.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
    message.end();
    js

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

#### `outgoingMessage.appendHeader(name, value)`#

Added in: v18.3.0, v16.17.0

- `name` `<string>` Header name
- `value` `<string>` | `<string>`[] Header value
- Returns: `<this>`

Append a single header value to the header object.

If the value is an array, this is equivalent to calling this method multiple times.

If there were no previous values for the header, this is equivalent to calling `outgoingMessage.setHeader(name, value)`.

Depending of the value of `options.uniqueHeaders` when the client request or the server were created, this will end up in the header being sent multiple times or a single time with values joined using `;` .

#### `outgoingMessage.connection`#

Added in: v0.3.0Deprecated in: v15.12.0, v14.17.1

Stability: 0 - Deprecated: Use `outgoingMessage.socket` instead.

Alias of `outgoingMessage.socket`.

#### `outgoingMessage.cork()`#

Added in: v13.2.0, v12.16.0

See `writable.cork()`.

#### `outgoingMessage.destroy([error])`#

Added in: v0.3.0

- `error` `<Error>` Optional, an error to emit with `error` event
- Returns: `<this>`

Destroys the message. Once a socket is associated with the message and is connected, that socket will be destroyed as well.

#### `outgoingMessage.end(chunk[, encoding][, callback])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v15.0.0 | The `chunk` parameter can now be a `Uint8Array`.
v0.11.6 | add `callback` argument.

- `chunk` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>` Optional, Default: `utf8`
- `callback` `<Function>` Optional
- Returns: `<this>`

Finishes the outgoing message. If any parts of the body are unsent, it will flush them to the underlying system. If the message is chunked, it will send the terminating chunk `0\r\n\r\n`, and send the trailers (if any).

If `chunk` is specified, it is equivalent to calling `outgoingMessage.write(chunk, encoding)`, followed by `outgoingMessage.end(callback)`.

If `callback` is provided, it will be called when the message is finished (equivalent to a listener of the `'finish'` event).

#### `outgoingMessage.flushHeaders()`#

Added in: v1.6.0

Flushes the message headers.

For efficiency reason, Node.js normally buffers the message headers until `outgoingMessage.end()` is called or the first chunk of message data is written. It then tries to pack the headers and data into a single TCP packet.

It is usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later. `outgoingMessage.flushHeaders()` bypasses the optimization and kickstarts the message.

#### `outgoingMessage.getHeader(name)`#

Added in: v0.4.0

- `name` `<string>` Name of header
- Returns: `<number>` | `<string>` | `<string>`[] | `<undefined>`

Gets the value of the HTTP header with the given name. If that header is not set, the returned value will be `undefined`.

#### `outgoingMessage.getHeaderNames()`#

Added in: v7.7.0

- Returns: `<string>`[]

Returns an array containing the unique names of the current outgoing headers. All names are lowercase.

#### `outgoingMessage.getHeaders()`#

Added in: v7.7.0

- Returns: `<Object>`

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related HTTP module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the `outgoingMessage.getHeaders()` method does not prototypically inherit from the JavaScript `Object`. This means that typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others are not defined and will not work.

    outgoingMessage.setHeader('Foo', 'bar');
    outgoingMessage.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

    const headers = outgoingMessage.getHeaders();
    // headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
    js

#### `outgoingMessage.hasHeader(name)`#

Added in: v7.7.0

- `name` `<string>`
- Returns: `<boolean>`

Returns `true` if the header identified by `name` is currently set in the outgoing headers. The header name is case-insensitive.

    const hasContentType = outgoingMessage.hasHeader('content-type');
    js

#### `outgoingMessage.headersSent`#

Added in: v0.9.3

- Type: `<boolean>`

Read-only. `true` if the headers were sent, otherwise `false`.

#### `outgoingMessage.pipe()`#

Added in: v9.0.0

Overrides the `stream.pipe()` method inherited from the legacy `Stream` class which is the parent class of `http.OutgoingMessage`.

Calling this method will throw an `Error` because `outgoingMessage` is a write-only stream.

#### `outgoingMessage.removeHeader(name)`#

Added in: v0.4.0

- `name` `<string>` Header name

Removes a header that is queued for implicit sending.

    outgoingMessage.removeHeader('Content-Encoding');
    js

#### `outgoingMessage.setHeader(name, value)`#

Added in: v0.4.0

- `name` `<string>` Header name
- `value` `<number>` | `<string>` | `<string>`[] Header value
- Returns: `<this>`

Sets a single header value. If the header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings to send multiple headers with the same name.

#### `outgoingMessage.setHeaders(headers)`#

Added in: v19.6.0, v18.15.0

- `headers` `<Headers>` | `<Map>`
- Returns: `<this>`

Sets multiple header values for implicit headers. `headers` must be an instance of `Headers` or `Map`, if a header already exists in the to-be-sent headers, its value will be replaced.

    const headers = new Headers({ foo: 'bar' });
    outgoingMessage.setHeaders(headers);
    js

or

    const headers = new Map([['foo', 'bar']]);
    outgoingMessage.setHeaders(headers);
    js

When headers have been set with `outgoingMessage.setHeaders()`, they will be merged with any headers passed to `response.writeHead()`, with the headers passed to `response.writeHead()` given precedence.

    // Returns content-type = text/plain
    const server = http.createServer((req, res) => {
      const headers = new Headers({ 'Content-Type': 'text/html' });
      res.setHeaders(headers);
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('ok');
    });
    js

#### `outgoingMessage.setTimeout(msecs[, callback])`#

Added in: v0.9.12

- `msecs` `<number>`
- `callback` `<Function>` Optional function to be called when a timeout occurs. Same as binding to the `timeout` event.
- Returns: `<this>`

Once a socket is associated with the message and is connected, `socket.setTimeout()` will be called with `msecs` as the first parameter.

#### `outgoingMessage.socket`#

Added in: v0.3.0

- Type: `<stream.Duplex>`

Reference to the underlying socket. Usually, users will not want to access this property.

After calling `outgoingMessage.end()`, this property will be nulled.

#### `outgoingMessage.uncork()`#

Added in: v13.2.0, v12.16.0

See `writable.uncork()`

#### `outgoingMessage.writableCorked`#

Added in: v13.2.0, v12.16.0

- Type: `<number>`

The number of times `outgoingMessage.cork()` has been called.

#### `outgoingMessage.writableEnded`#

Added in: v12.9.0

- Type: `<boolean>`

Is `true` if `outgoingMessage.end()` has been called. This property does not indicate whether the data has been flushed. For that purpose, use `message.writableFinished` instead.

#### `outgoingMessage.writableFinished`#

Added in: v12.7.0

- Type: `<boolean>`

Is `true` if all data has been flushed to the underlying system.

#### `outgoingMessage.writableHighWaterMark`#

Added in: v12.9.0

- Type: `<number>`

The `highWaterMark` of the underlying socket if assigned. Otherwise, the default buffer level when `writable.write()` starts returning false (`16384`).

#### `outgoingMessage.writableLength`#

Added in: v12.9.0

- Type: `<number>`

The number of buffered bytes.

#### `outgoingMessage.writableObjectMode`#

Added in: v12.9.0

- Type: `<boolean>`

Always `false`.

#### `outgoingMessage.write(chunk[, encoding][, callback])`#

Added in: v0.1.29
History

Version | Changes
--- | ---
v15.0.0 | The `chunk` parameter can now be a `Uint8Array`.
v0.11.6 | The `callback` argument was added.

- `chunk` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>` Default: `utf8`
- `callback` `<Function>`
- Returns: `<boolean>`

Sends a chunk of the body. This method can be called multiple times.

The `encoding` argument is only relevant when `chunk` is a string. Defaults to `'utf8'`.

The `callback` argument is optional and will be called when this chunk of data is flushed.

Returns `true` if the entire data was flushed successfully to the kernel buffer. Returns `false` if all or part of the data was queued in the user memory. The `'drain'` event will be emitted when the buffer is free again.

### `http.METHODS`#

Added in: v0.11.8

- Type: `<string>`[]

A list of the HTTP methods that are supported by the parser.

### `http.STATUS_CODES`#

Added in: v0.1.22

- Type: `<Object>`

A collection of all the standard HTTP response status codes, and the short description of each. For example, `http.STATUS_CODES[404] === 'Not Found'`.

### `http.createServer([options][, requestListener])`#

Added in: v0.1.13
History

Version | Changes
--- | ---
v26.3.0 | The `httpValidation` option is supported now.
v25.1.0, v24.12.0 | Add optimizeEmptyRequests option.
v24.9.0, v22.21.0 | The `shouldUpgradeCallback` option is now supported.
v20.1.0, v18.17.0 | The `highWaterMark` option is supported now.
v18.0.0 | The `requestTimeout`, `headersTimeout`, `keepAliveTimeout`, and `connectionsCheckingInterval` options are supported now.
v18.0.0 | The `noDelay` option now defaults to `true`.
v17.7.0, v16.15.0 | The `noDelay`, `keepAlive` and `keepAliveInitialDelay` options are supported now.
v13.8.0, v12.15.0, v10.19.0 | The `insecureHTTPParser` option is supported now.
v13.3.0 | The `maxHeaderSize` option is supported now.
v9.6.0, v8.12.0 | The `options` argument is supported now.

- `options` `<Object>`

  - `connectionsCheckingInterval`: Sets the interval value in milliseconds to check for request and headers timeout in incomplete requests. Default: `30000`.
  - `headersTimeout`: Sets the timeout value in milliseconds for receiving the complete HTTP headers from the client. See `server.headersTimeout` for more information. Default: `60000`.
  - `highWaterMark` `<number>` Optionally overrides all `socket`s' `readableHighWaterMark` and `writableHighWaterMark`. This affects `highWaterMark` property of both `IncomingMessage` and `ServerResponse`. Default: See `stream.getDefaultHighWaterMark()`.
  - `httpValidation` `<string>` Controls HTTP header value validation strictness for incoming requests. Accepted values are:
    - `'strict'`: Strictest validation; rejects any non-ASCII or control characters in header values.
    - `'relaxed'`: Allows a limited set of non-ASCII characters in header values, aligning with the Fetch specification.
    - `'insecure'`: Disables all header value validation (equivalent to `insecureHTTPParser: true`). Cannot be used together with `insecureHTTPParser`. Default: `'strict'`.
  - `insecureHTTPParser` `<boolean>` If set to `true`, it will use an HTTP parser with leniency flags enabled. Using the insecure parser should be avoided. See `--insecure-http-parser` for more information. Default: `false`.
  - `IncomingMessage` `<http.IncomingMessage>` Specifies the `IncomingMessage` class to be used. Useful for extending the original `IncomingMessage`. Default: `IncomingMessage`.
  - `joinDuplicateHeaders` `<boolean>` If set to `true`, this option allows joining the field line values of multiple headers in a request with a comma (`,` ) instead of discarding the duplicates. For more information, refer to `message.headers`. Default: `false`.
  - `keepAlive` `<boolean>` If set to `true`, it enables keep-alive functionality on the socket immediately after a new incoming connection is received, similarly on what is done in `socket.setKeepAlive()`. Default: `false`.
  - `keepAliveInitialDelay` `<number>` If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket. Default: `0`.
  - `keepAliveTimeout`: The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. See `server.keepAliveTimeout` for more information. Default: `65000`.
  - `maxHeaderSize` `<number>` Optionally overrides the value of `--max-http-header-size` for requests received by this server, i.e. the maximum length of request headers in bytes. Default: 16384 (16 KiB).
  - `noDelay` `<boolean>` If set to `true`, it disables the use of Nagle's algorithm immediately after a new incoming connection is received. Default: `true`.
  - `requestTimeout`: Sets the timeout value in milliseconds for receiving the entire request from the client. See `server.requestTimeout` for more information. Default: `300000`.
  - `requireHostHeader` `<boolean>` If set to `true`, it forces the server to respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header (as mandated by the specification). Default: `true`.
  - `ServerResponse` `<http.ServerResponse>` Specifies the `ServerResponse` class to be used. Useful for extending the original `ServerResponse`. Default: `ServerResponse`.
  - `shouldUpgradeCallback(request)` `<Function>` A callback which receives an incoming request and returns a boolean, to control which upgrade attempts should be accepted. Accepted upgrades will fire an `'upgrade'` event (or their sockets will be destroyed, if no listener is registered) while rejected upgrades will fire a `'request'` event like any non-upgrade request. This options defaults to `() => server.listenerCount('upgrade') > 0`.
  - `uniqueHeaders` `<Array>` A list of response headers that should be sent only once. If the header's value is an array, the items will be joined using `;` .
  - `rejectNonStandardBodyWrites` `<boolean>` If set to `true`, an error is thrown when writing to an HTTP response which does not have a body. Default: `false`.
  - `optimizeEmptyRequests` `<boolean>` If set to `true`, requests without `Content-Length` or `Transfer-Encoding` headers (indicating no body) will be initialized with an already-ended body stream, so they will never emit any stream events (like `'data'` or `'end'`). You can use `req.readableEnded` to detect this case. Default: `false`.

- `requestListener` `<Function>`

- Returns: `<http.Server>`

Returns a new instance of `http.Server`.

The `requestListener` is a function which is automatically added to the `'request'` event.

    import http from 'node:http';

    // Create a local server to receive data from
    const server = http.createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    const http = require('node:http');

    // Create a local server to receive data from
    const server = http.createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    javascript

    import http from 'node:http';

    // Create a local server to receive data from
    const server = http.createServer();

    // Listen to the request event
    server.on('request', (request, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    const http = require('node:http');

    // Create a local server to receive data from
    const server = http.createServer();

    // Listen to the request event
    server.on('request', (request, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    javascript

### `http.get(options[, callback])`#

### `http.get(url[, options][, callback])`#

Added in: v0.3.6
History

Version | Changes
--- | ---
v10.9.0 | The `url` parameter can now be passed along with a separate `options` object.
v7.5.0 | The `options` parameter can be a WHATWG `URL` object.

- `url` `<string>` | `<URL>`
- `options` `<Object>` Accepts the same `options` as `http.request()`, with the method set to GET by default.
- `callback` `<Function>`
- Returns: `<http.ClientRequest>`

Since most requests are GET requests without bodies, Node.js provides this convenience method. The only difference between this method and `http.request()` is that it sets the method to GET by default and calls `req.end()` automatically. The callback must take care to consume the response data for reasons stated in `http.ClientRequest` section.

The `callback` is invoked with a single argument that is an instance of `http.IncomingMessage`.

JSON fetching example:

    http.get('http://localhost:8000/', (res) => {
      const { statusCode } = res;
      const contentType = res.headers['content-type'];

      let error;
      // Any 2xx status code signals a successful response but
      // here we're only checking for 200.
      if (statusCode !== 200) {
        error = new Error('Request Failed.\n' +
                          `Status Code: ${statusCode}`);
      } else if (!/^application\/json/.test(contentType)) {
        error = new Error('Invalid content-type.\n' +
                          `Expected application/json but received ${contentType}`);
      }
      if (error) {
        console.error(error.message);
        // Consume response data to free up memory
        res.resume();
        return;
      }

      res.setEncoding('utf8');
      let rawData = '';
      res.on('data', (chunk) => { rawData += chunk; });
      res.on('end', () => {
        try {
          const parsedData = JSON.parse(rawData);
          console.log(parsedData);
        } catch (e) {
          console.error(e.message);
        }
      });
    }).on('error', (e) => {
      console.error(`Got error: ${e.message}`);
    });

    // Create a local server to receive data from
    const server = http.createServer((req, res) => {
      res.writeHead(200, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify({
        data: 'Hello World!',
      }));
    });

    server.listen(8000);
    js

### `http.globalAgent`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v19.0.0 | The agent now uses HTTP Keep-Alive and a 5 second timeout by default.

- Type: `<http.Agent>`

Global instance of `Agent` which is used as the default for all HTTP client requests. Diverges from a default `Agent` configuration by having `keepAlive` enabled and a `timeout` of 5 seconds.

### `http.maxHeaderSize`#

Added in: v11.6.0, v10.15.0

- Type: `<number>`

Read-only property specifying the maximum allowed size of HTTP headers in bytes. Defaults to 16 KiB. Configurable using the `--max-http-header-size` CLI option.

This can be overridden for servers and client requests by passing the `maxHeaderSize` option.

### `http.request(options[, callback])`#

### `http.request(url[, options][, callback])`#

Added in: v0.3.6
History

Version | Changes
--- | ---
v26.3.0 | The `httpValidation` option is supported now.
v16.7.0, v14.18.0 | When using a `URL` object parsed username and password will now be properly URI decoded.
v15.3.0, v14.17.0 | It is possible to abort a request with an AbortSignal.
v13.8.0, v12.15.0, v10.19.0 | The `insecureHTTPParser` option is supported now.
v13.3.0 | The `maxHeaderSize` option is supported now.
v10.9.0 | The `url` parameter can now be passed along with a separate `options` object.
v7.5.0 | The `options` parameter can be a WHATWG `URL` object.

- `url` `<string>` | `<URL>`
- `options` `<Object>`
  - `agent` `<http.Agent>` | `<boolean>` Controls `Agent` behavior. Possible values:
    - `undefined` (default): use `http.globalAgent` for this host and port.
    - `Agent` object: explicitly use the passed in `Agent`.
    - `false`: causes a new `Agent` with default values to be used.
  - `auth` `<string>` Basic authentication (`'user:password'`) to compute an Authorization header.
  - `createConnection` `<Function>` A function that produces a socket/stream to use for the request when the `agent` option is not used. This can be used to avoid creating a custom `Agent` class just to override the default `createConnection` function. See `agent.createConnection()` for more details. Any `Duplex` stream is a valid return value.
  - `defaultPort` `<number>` Default port for the protocol. Default: `agent.defaultPort` if an `Agent` is used, else `undefined`.
  - `family` `<number>` IP address family to use when resolving `host` or `hostname`. Valid values are `4` or `6`. When unspecified, both IP v4 and v6 will be used.
  - `headers` `<Object>` | `<Array>` An object or an array of strings containing request headers. The array is in the same format as `message.rawHeaders`.
  - `hints` `<number>` Optional `dns.lookup()` hints.
  - `host` `<string>` A domain name or IP address of the server to issue the request to. Default: `'localhost'`.
  - `hostname` `<string>` Alias for `host`. To support `url.parse()`, `hostname` will be used if both `host` and `hostname` are specified.
  - `httpValidation` `<string>` Controls HTTP header value validation strictness for outgoing requests. Accepted values are:
    - `'strict'`: Strictest validation; rejects any non-ASCII or control characters in header values.
    - `'relaxed'`: Allows a limited set of non-ASCII characters in header values, aligning with the Fetch specification.
    - `'insecure'`: Disables all header value validation (equivalent to `insecureHTTPParser: true`). Cannot be used together with `insecureHTTPParser`. Default: `'strict'`.
  - `insecureHTTPParser` `<boolean>` If set to `true`, it will use an HTTP parser with leniency flags enabled. Using the insecure parser should be avoided. See `--insecure-http-parser` for more information. Default: `false`
  - `joinDuplicateHeaders` `<boolean>` It joins the field line values of multiple headers in a request with `,` instead of discarding the duplicates. See `message.headers` for more information. Default: `false`.
  - `localAddress` `<string>` Local interface to bind for network connections.
  - `localPort` `<number>` Local port to connect from.
  - `lookup` `<Function>` Custom lookup function. Default: `dns.lookup()`.
  - `maxHeaderSize` `<number>` Optionally overrides the value of `--max-http-header-size` (the maximum length of response headers in bytes) for responses received from the server. Default: 16384 (16 KiB).
  - `method` `<string>` A string specifying the HTTP request method. Default: `'GET'`.
  - `path` `<string>` Request path. Should include query string if any. E.G. `'/index.html?page=12'`. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future. Default: `'/'`. The content in `path` is sent as the request target in the HTTP 1.1 message. When `path` is an absolute URL, this means the request target in the message in absolute form. If the receiving server is a proxy, the server typically forwards the request to the destination specified in the request target, and ignores the `Host` header. The user needs to make sure that `path`, `host` and the Host headers conform to the requirement of the request target in the HTTP specification. When the receiving server is known to be a proxy because the request is routed through Built-in Proxy Support, `http.request` will additionally perform a best-effort check to see that the `host` option or `Host` in `headers` agrees with the authority in `path` during the initial construction of the request. It gives up rewriting the request target for proxying and throws an error if they don't match at request construction time, though there won't be checks for later header mutations done by the user.
  - `port` `<number>` Port of remote server. Default: `defaultPort` if set, else `80`.
  - `protocol` `<string>` Protocol to use. Default: `'http:'`.
  - `setDefaultHeaders` `<boolean>`: Specifies whether or not to automatically add default headers such as `Connection`, `Content-Length`, `Transfer-Encoding`, and `Host`. If set to `false` then all necessary headers must be added manually. Defaults to `true`.
  - `setHost` `<boolean>`: Specifies whether or not to automatically add the `Host` header. If provided, this overrides `setDefaultHeaders`. Defaults to `true`.
  - `signal` `<AbortSignal>`: An AbortSignal that may be used to abort an ongoing request.
  - `socketPath` `<string>` Unix domain socket. Cannot be used if one of `host` or `port` is specified, as those specify a TCP Socket.
  - `timeout` `<number>`: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
  - `uniqueHeaders` `<Array>` A list of request headers that should be sent only once. If the header's value is an array, the items will be joined using `;` .
- `callback` `<Function>`
- Returns: `<http.ClientRequest>`

`options` in `socket.connect()` are also supported.

Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.

`url` can be a string or a `URL` object. If `url` is a string, it is automatically parsed with `new URL()`. If it is a `URL` object, it will be automatically converted to an ordinary `options` object.

If both `url` and `options` are specified, the objects are merged, with the `options` properties taking precedence.

The optional `callback` parameter will be added as a one-time listener for the `'response'` event.

`http.request()` returns an instance of the `http.ClientRequest` class. The `ClientRequest` instance is a writable stream. If one needs to upload a file with a POST request, then write to the `ClientRequest` object.

    import http from 'node:http';
    import { Buffer } from 'node:buffer';

    const postData = JSON.stringify({
      'msg': 'Hello World!',
    });

    const options = {
      hostname: 'www.google.com',
      port: 80,
      path: '/upload',
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Content-Length': Buffer.byteLength(postData),
      },
    };

    const req = http.request(options, (res) => {
      console.log(`STATUS: ${res.statusCode}`);
      console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
      res.setEncoding('utf8');
      res.on('data', (chunk) => {
        console.log(`BODY: ${chunk}`);
      });
      res.on('end', () => {
        console.log('No more data in response.');
      });
    });

    req.on('error', (e) => {
      console.error(`problem with request: ${e.message}`);
    });

    // Write data to request body
    req.write(postData);
    req.end();
    const http = require('node:http');

    const postData = JSON.stringify({
      'msg': 'Hello World!',
    });

    const options = {
      hostname: 'www.google.com',
      port: 80,
      path: '/upload',
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Content-Length': Buffer.byteLength(postData),
      },
    };

    const req = http.request(options, (res) => {
      console.log(`STATUS: ${res.statusCode}`);
      console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
      res.setEncoding('utf8');
      res.on('data', (chunk) => {
        console.log(`BODY: ${chunk}`);
      });
      res.on('end', () => {
        console.log('No more data in response.');
      });
    });

    req.on('error', (e) => {
      console.error(`problem with request: ${e.message}`);
    });

    // Write data to request body
    req.write(postData);
    req.end();
    javascript

In the example `req.end()` was called. With `http.request()` one must always call `req.end()` to signify the end of the request - even if there is no data being written to the request body.

If any error is encountered during the request (be that with DNS resolution, TCP level errors, or actual HTTP parse errors) an `'error'` event is emitted on the returned request object. As with all `'error'` events, if no listeners are registered the error will be thrown.

There are a few special headers that should be noted.

- Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.

- Sending a 'Content-Length' header will disable the default chunked encoding.

- Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', both a timeout and a listener for the `'continue'` event should be set. See RFC 2616 Section 8.2.3 for more information.

- Sending an Authorization header will override using the `auth` option to compute basic authentication.

Example using a `URL` as `options`:

    const options = new URL('http://abc:xyz@example.com');

    const req = http.request(options, (res) => {
      // ...
    });
    js

In a successful request, the following events will be emitted in the following order:

- `'socket'`
- `'response'`
  - `'data'` any number of times, on the `res` object (`'data'` will not be emitted at all if the response body is empty, for instance, in most redirects)
  - `'end'` on the `res` object
- `'close'`

In the case of a connection error, the following events will be emitted:

- `'socket'`
- `'error'`
- `'close'`

In the case of a premature connection close before the response is received, the following events will be emitted in the following order:

- `'socket'`
- `'error'` with an error with message `'Error: socket hang up'` and code `'ECONNRESET'`
- `'close'`

In the case of a premature connection close after the response is received, the following events will be emitted in the following order:

- `'socket'`
- `'response'`
  - `'data'` any number of times, on the `res` object
- (connection closed here)
- `'aborted'` on the `res` object
- `'close'`
- `'error'` on the `res` object with an error with message `'Error: aborted'` and code `'ECONNRESET'`
- `'close'` on the `res` object

If `req.destroy()` is called before a socket is assigned, the following events will be emitted in the following order:

- (`req.destroy()` called here)
- `'error'` with an error with message `'Error: socket hang up'` and code `'ECONNRESET'`, or the error with which `req.destroy()` was called
- `'close'`

If `req.destroy()` is called before the connection succeeds, the following events will be emitted in the following order:

- `'socket'`
- (`req.destroy()` called here)
- `'error'` with an error with message `'Error: socket hang up'` and code `'ECONNRESET'`, or the error with which `req.destroy()` was called
- `'close'`

If `req.destroy()` is called after the response is received, the following events will be emitted in the following order:

- `'socket'`
- `'response'`
  - `'data'` any number of times, on the `res` object
- (`req.destroy()` called here)
- `'aborted'` on the `res` object
- `'close'`
- `'error'` on the `res` object with an error with message `'Error: aborted'` and code `'ECONNRESET'`, or the error with which `req.destroy()` was called
- `'close'` on the `res` object

If `req.abort()` is called before a socket is assigned, the following events will be emitted in the following order:

- (`req.abort()` called here)
- `'abort'`
- `'close'`

If `req.abort()` is called before the connection succeeds, the following events will be emitted in the following order:

- `'socket'`
- (`req.abort()` called here)
- `'abort'`
- `'error'` with an error with message `'Error: socket hang up'` and code `'ECONNRESET'`
- `'close'`

If `req.abort()` is called after the response is received, the following events will be emitted in the following order:

- `'socket'`
- `'response'`
  - `'data'` any number of times, on the `res` object
- (`req.abort()` called here)
- `'abort'`
- `'aborted'` on the `res` object
- `'error'` on the `res` object with an error with message `'Error: aborted'` and code `'ECONNRESET'`.
- `'close'`
- `'close'` on the `res` object

Setting the `timeout` option or using the `setTimeout()` function will not abort the request or do anything besides add a `'timeout'` event.

Passing an `AbortSignal` and then calling `abort()` on the corresponding `AbortController` will behave the same way as calling `.destroy()` on the request. Specifically, the `'error'` event will be emitted with an error with the message `'AbortError: The operation was aborted'`, the code `'ABORT_ERR'` and the `cause`, if one was provided.

### `http.validateHeaderName(name[, label])`#

Added in: v14.3.0
History

Version | Changes
--- | ---
v19.5.0, v18.14.0 | The `label` parameter is added.

- `name` `<string>`
- `label` `<string>` Label for error message. Default: `'Header name'`.

Performs the low-level validations on the provided `name` that are done when `res.setHeader(name, value)` is called.

Passing illegal value as `name` will result in a `TypeError` being thrown, identified by `code: 'ERR_INVALID_HTTP_TOKEN'`.

It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers.

Example:

    import { validateHeaderName } from 'node:http';

    try {
      validateHeaderName('');
    } catch (err) {
      console.error(err instanceof TypeError); // --> true
      console.error(err.code); // --> 'ERR_INVALID_HTTP_TOKEN'
      console.error(err.message); // --> 'Header name must be a valid HTTP token [""]'
    }
    const { validateHeaderName } = require('node:http');

    try {
      validateHeaderName('');
    } catch (err) {
      console.error(err instanceof TypeError); // --> true
      console.error(err.code); // --> 'ERR_INVALID_HTTP_TOKEN'
      console.error(err.message); // --> 'Header name must be a valid HTTP token [""]'
    }
    javascript

### `http.validateHeaderValue(name, value)`#

Added in: v14.3.0

- `name` `<string>`
- `value` `<any>`

Performs the low-level validations on the provided `value` that are done when `res.setHeader(name, value)` is called.

Passing illegal value as `value` will result in a `TypeError` being thrown.

- Undefined value error is identified by `code: 'ERR_HTTP_INVALID_HEADER_VALUE'`.
- Invalid value character error is identified by `code: 'ERR_INVALID_CHAR'`.

It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers.

Examples:

    import { validateHeaderValue } from 'node:http';

    try {
      validateHeaderValue('x-my-header', undefined);
    } catch (err) {
      console.error(err instanceof TypeError); // --> true
      console.error(err.code === 'ERR_HTTP_INVALID_HEADER_VALUE'); // --> true
      console.error(err.message); // --> 'Invalid value "undefined" for header "x-my-header"'
    }

    try {
      validateHeaderValue('x-my-header', 'oʊmɪɡə');
    } catch (err) {
      console.error(err instanceof TypeError); // --> true
      console.error(err.code === 'ERR_INVALID_CHAR'); // --> true
      console.error(err.message); // --> 'Invalid character in header content ["x-my-header"]'
    }
    const { validateHeaderValue } = require('node:http');

    try {
      validateHeaderValue('x-my-header', undefined);
    } catch (err) {
      console.error(err instanceof TypeError); // --> true
      console.error(err.code === 'ERR_HTTP_INVALID_HEADER_VALUE'); // --> true
      console.error(err.message); // --> 'Invalid value "undefined" for header "x-my-header"'
    }

    try {
      validateHeaderValue('x-my-header', 'oʊmɪɡə');
    } catch (err) {
      console.error(err instanceof TypeError); // --> true
      console.error(err.code === 'ERR_INVALID_CHAR'); // --> true
      console.error(err.message); // --> 'Invalid character in header content ["x-my-header"]'
    }
    javascript

### `http.setMaxIdleHTTPParsers(max)`#

Added in: v18.8.0, v16.18.0

- `max` `<number>` Default: `1000`.

Set the maximum number of idle HTTP parsers.

### `http.setGlobalProxyFromEnv([proxyEnv])`#

Added in: v25.4.0, v24.14.0

- `proxyEnv` `<Object>` An object containing proxy configuration. This accepts the same options as the `proxyEnv` option accepted by `Agent`. Default: `process.env`.
- Returns: `<Function>` A function that restores the original agent and dispatcher settings to the state before this `http.setGlobalProxyFromEnv()` is invoked.

Dynamically resets the global configurations to enable built-in proxy support for `fetch()` and `http.request()`/`https.request()` at runtime, as an alternative to using the `--use-env-proxy` flag or `NODE_USE_ENV_PROXY` environment variable. It can also be used to override settings configured from the environment variables.

As this function resets the global configurations, any previously configured `http.globalAgent`, `https.globalAgent` or undici global dispatcher would be overridden after this function is invoked. It's recommended to invoke it before any requests are made and avoid invoking it in the middle of any requests.

See Built-in Proxy Support for details on proxy URL formats and `NO_PROXY` syntax.

### Class: `WebSocket`#

Added in: v22.5.0

A browser-compatible implementation of `<WebSocket>`.

### Built-in Proxy Support#

Added in: v24.5.0, v22.21.0

Stability: 1.1 - Active development

When Node.js creates the global agent, if the `NODE_USE_ENV_PROXY` environment variable is set to `1` or `--use-env-proxy` is enabled, the global agent will be constructed with `proxyEnv: process.env`, enabling proxy support based on the environment variables.

To enable proxy support dynamically and globally, use `http.setGlobalProxyFromEnv()`.

Custom agents can also be created with proxy support by passing a `proxyEnv` option when constructing the agent. The value can be `process.env` if they just want to inherit the configuration from the environment variables, or an object with specific setting overriding the environment.

The following properties of the `proxyEnv` are checked to configure proxy support.

- `HTTP_PROXY` or `http_proxy`: Proxy server URL for HTTP requests. If both are set, `http_proxy` takes precedence.
- `HTTPS_PROXY` or `https_proxy`: Proxy server URL for HTTPS requests. If both are set, `https_proxy` takes precedence.
- `NO_PROXY` or `no_proxy`: Comma-separated list of hosts to bypass the proxy. If both are set, `no_proxy` takes precedence.

If the request is made to a Unix domain socket, the proxy settings will be ignored.

#### Proxy security considerations#

Built-in proxy support routes outbound requests through an HTTP(S) proxy, often because a firewall requires one to access external networks. It is not an anonymity or traffic-hiding feature and does not attempt to hide traffic from the proxy, the local network, network operators, or authorities that govern the deployment.

Configure only proxies that are trusted and authorized for the deployment. A proxy can observe connection metadata; for plain HTTP requests, or when TLS is terminated or intercepted by the proxy, it can also observe request and response contents. Node.js does not support treating an untrusted proxy as a privacy boundary. Deployment operators are responsible for controlling proxy configuration and for meeting deployment-specific network policy and legal requirements.

#### Proxy URL Format#

Proxy URLs can use either HTTP or HTTPS protocols:

- HTTP proxy: `http://proxy.example.com:8080`
- HTTPS proxy: `https://proxy.example.com:8080`
- Proxy with authentication: `http://username:password@proxy.example.com:8080`

#### `NO_PROXY` Format#

The `NO_PROXY` environment variable supports several formats:

- `*` - Bypass proxy for all hosts
- `example.com` - Exact host name match
- `.example.com` - Domain suffix match (matches `sub.example.com`)
- `*.example.com` - Wildcard domain match
- `192.168.1.100` - Exact IP address match
- `192.168.1.1-192.168.1.100` - IP address range
- `example.com:8080` - Hostname with specific port

Multiple entries should be separated by commas.

#### Example#

To start a Node.js process with proxy support enabled for all requests sent through the default global agent, either use the `NODE_USE_ENV_PROXY` environment variable:

    NODE_USE_ENV_PROXY=1 HTTP_PROXY=http://proxy.example.com:8080 NO_PROXY=localhost,127.0.0.1 node client.js
    console

Or the `--use-env-proxy` flag.

    HTTP_PROXY=http://proxy.example.com:8080 NO_PROXY=localhost,127.0.0.1 node --use-env-proxy client.js
    console

To enable proxy support dynamically and globally with `process.env` (the default option of `http.setGlobalProxyFromEnv()`):

    const http = require('node:http');

    // Reads proxy-related environment variables from process.env
    const restore = http.setGlobalProxyFromEnv();

    // Subsequent requests will use the configured proxies from environment variables
    http.get('http://www.example.com', (res) => {
      // This request will be proxied if HTTP_PROXY or http_proxy is set
    });

    fetch('https://www.example.com', (res) => {
      // This request will be proxied if HTTPS_PROXY or https_proxy is set
    });

    // To restore the original global agent and dispatcher settings, call the returned function.
    // restore();
    import http from 'node:http';

    // Reads proxy-related environment variables from process.env
    http.setGlobalProxyFromEnv();

    // Subsequent requests will use the configured proxies from environment variables
    http.get('http://www.example.com', (res) => {
      // This request will be proxied if HTTP_PROXY or http_proxy is set
    });

    fetch('https://www.example.com', (res) => {
      // This request will be proxied if HTTPS_PROXY or https_proxy is set
    });

    // To restore the original global agent and dispatcher settings, call the returned function.
    // restore();
    javascript

To enable proxy support dynamically and globally with custom settings:

    const http = require('node:http');

    const restore = http.setGlobalProxyFromEnv({
      http_proxy: 'http://proxy.example.com:8080',
      https_proxy: 'https://proxy.example.com:8443',
      no_proxy: 'localhost,127.0.0.1,.internal.example.com',
    });

    // Subsequent requests will use the configured proxies
    http.get('http://www.example.com', (res) => {
      // This request will be proxied through proxy.example.com:8080
    });

    fetch('https://www.example.com', (res) => {
      // This request will be proxied through proxy.example.com:8443
    });
    import http from 'node:http';

    http.setGlobalProxyFromEnv({
      http_proxy: 'http://proxy.example.com:8080',
      https_proxy: 'https://proxy.example.com:8443',
      no_proxy: 'localhost,127.0.0.1,.internal.example.com',
    });

    // Subsequent requests will use the configured proxies
    http.get('http://www.example.com', (res) => {
      // This request will be proxied through proxy.example.com:8080
    });

    fetch('https://www.example.com', (res) => {
      // This request will be proxied through proxy.example.com:8443
    });
    javascript

To create a custom agent with built-in proxy support:

    const http = require('node:http');

    // Creating a custom agent with custom proxy support.
    const agent = new http.Agent({ proxyEnv: { HTTP_PROXY: 'http://proxy.example.com:8080' } });

    http.request({
      hostname: 'www.example.com',
      port: 80,
      path: '/',
      agent,
    }, (res) => {
      // This request will be proxied through proxy.example.com:8080 using the HTTP protocol.
      console.log(`STATUS: ${res.statusCode}`);
    });
    cjs

Alternatively, the following also works:

    const http = require('node:http');
    // Use lower-cased option name.
    const agent1 = new http.Agent({ proxyEnv: { http_proxy: 'http://proxy.example.com:8080' } });
    // Use values inherited from the environment variables, if the process is started with
    // HTTP_PROXY=http://proxy.example.com:8080 this will use the proxy server specified
    // in process.env.HTTP_PROXY.
    const agent2 = new http.Agent({ proxyEnv: process.env });
    cjs

## HTTP/2#

Source Code: lib/http2.js Added in: v8.4.0
History

Version | Changes
--- | ---
v15.3.0, v14.17.0 | It is possible to abort a request with an AbortSignal.
v15.0.0 | Requests with the `host` header (with or without `:authority`) can now be sent/received.
v10.10.0 | HTTP/2 is now Stable. Previously, it had been Experimental.

Stability: 2 - Stable

The `node:http2` module provides an implementation of the HTTP/2 protocol. It can be accessed using:

    const http2 = require('node:http2');
    js

### Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the `node:crypto` module. In such cases, attempting to `import` from `node:http2` or calling `require('node:http2')` will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

    let http2;
    try {
      http2 = require('node:http2');
    } catch (err) {
      console.error('http2 support is disabled!');
    }
    cjs

When using the lexical ESM `import` keyword, the error can only be caught if a handler for `process.on('uncaughtException')` is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the `import()` function instead of the lexical `import` keyword:

    let http2;
    try {
      http2 = await import('node:http2');
    } catch (err) {
      console.error('http2 support is disabled!');
    }
    mjs

### Core API#

The Core API provides a low-level interface designed specifically around support for HTTP/2 protocol features. It is specifically not designed for compatibility with the existing HTTP/1 module API. However, the Compatibility API is.

The `http2` Core API is much more symmetric between client and server than the `http` API. For instance, most events, like `'error'`, `'connect'` and `'stream'`, can be emitted either by client-side code or server-side code.

#### Server-side example#

The following illustrates a simple HTTP/2 server using the Core API. Since there are no browsers known that support unencrypted HTTP/2, the use of `http2.createSecureServer()` is necessary when communicating with browser clients.

    import { createSecureServer } from 'node:http2';
    import { readFileSync } from 'node:fs';

    const server = createSecureServer({
      key: readFileSync('localhost-privkey.pem'),
      cert: readFileSync('localhost-cert.pem'),
    });

    server.on('error', (err) => console.error(err));

    server.on('stream', (stream, headers) => {
      // stream is a Duplex
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8443);
    const http2 = require('node:http2');
    const fs = require('node:fs');

    const server = http2.createSecureServer({
      key: fs.readFileSync('localhost-privkey.pem'),
      cert: fs.readFileSync('localhost-cert.pem'),
    });
    server.on('error', (err) => console.error(err));

    server.on('stream', (stream, headers) => {
      // stream is a Duplex
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8443);
    javascript

To generate the certificate and key for this example, run:

    openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
      -keyout localhost-privkey.pem -out localhost-cert.pem
    bash

#### Client-side example#

The following illustrates an HTTP/2 client:

    import { connect } from 'node:http2';
    import { readFileSync } from 'node:fs';

    const client = connect('https://localhost:8443', {
      ca: readFileSync('localhost-cert.pem'),
    });
    client.on('error', (err) => console.error(err));

    const req = client.request({ ':path': '/' });

    req.on('response', (headers, flags) => {
      for (const name in headers) {
        console.log(`${name}: ${headers[name]}`);
      }
    });

    req.setEncoding('utf8');
    let data = '';
    req.on('data', (chunk) => { data += chunk; });
    req.on('end', () => {
      console.log(`\n${data}`);
      client.close();
    });
    req.end();
    const http2 = require('node:http2');
    const fs = require('node:fs');

    const client = http2.connect('https://localhost:8443', {
      ca: fs.readFileSync('localhost-cert.pem'),
    });
    client.on('error', (err) => console.error(err));

    const req = client.request({ ':path': '/' });

    req.on('response', (headers, flags) => {
      for (const name in headers) {
        console.log(`${name}: ${headers[name]}`);
      }
    });

    req.setEncoding('utf8');
    let data = '';
    req.on('data', (chunk) => { data += chunk; });
    req.on('end', () => {
      console.log(`\n${data}`);
      client.close();
    });
    req.end();
    javascript

#### Class: `Http2Session`#

Added in: v8.4.0

- Extends: `<EventEmitter>`

Instances of the `http2.Http2Session` class represent an active communications session between an HTTP/2 client and server. Instances of this class are not intended to be constructed directly by user code.

Each `Http2Session` instance will exhibit slightly different behaviors depending on whether it is operating as a server or a client. The `http2session.type` property can be used to determine the mode in which an `Http2Session` is operating. On the server side, user code should rarely have occasion to work with the `Http2Session` object directly, with most actions typically taken through interactions with either the `Http2Server` or `Http2Stream` objects.

User code will not create `Http2Session` instances directly. Server-side `Http2Session` instances are created by the `Http2Server` instance when a new HTTP/2 connection is received. Client-side `Http2Session` instances are created using the `http2.connect()` method.

##### `Http2Session` and sockets#

Every `Http2Session` instance is associated with exactly one `net.Socket` or `tls.TLSSocket` when it is created. When either the `Socket` or the `Http2Session` are destroyed, both will be destroyed.

Because of the specific serialization and processing requirements imposed by the HTTP/2 protocol, it is not recommended for user code to read data from or write data to a `Socket` instance bound to a `Http2Session`. Doing so can put the HTTP/2 session into an indeterminate state causing the session and the socket to become unusable.

Once a `Socket` has been bound to an `Http2Session`, user code should rely solely on the API of the `Http2Session`.

##### Event: `'close'`#

Added in: v8.4.0

The `'close'` event is emitted once the `Http2Session` has been destroyed. Its listener does not expect any arguments.

##### Event: `'connect'`#

Added in: v8.4.0

- `session` `<Http2Session>`
- `socket` `<net.Socket>`

The `'connect'` event is emitted once the `Http2Session` has been successfully connected to the remote peer and communication may begin.

User code will typically not listen for this event directly.

##### Event: `'error'`#

Added in: v8.4.0

- `error` `<Error>`

The `'error'` event is emitted when an error occurs during the processing of an `Http2Session`.

##### Event: `'frameError'`#

Added in: v8.4.0

- `type` `<integer>` The frame type.
- `code` `<integer>` The error code.
- `id` `<integer>` The stream id (or `0` if the frame isn't associated with a stream).

The `'frameError'` event is emitted when an error occurs while attempting to send a frame on the session. If the frame that could not be sent is associated with a specific `Http2Stream`, an attempt to emit a `'frameError'` event on the `Http2Stream` is made.

If the `'frameError'` event is associated with a stream, the stream will be closed and destroyed immediately following the `'frameError'` event. If the event is not associated with a stream, the `Http2Session` will be shut down immediately following the `'frameError'` event.

##### Event: `'goaway'`#

Added in: v8.4.0

- `errorCode` `<number>` The HTTP/2 error code specified in the `GOAWAY` frame.
- `lastStreamID` `<number>` The ID of the last stream the remote peer successfully processed (or `0` if no ID is specified).
- `opaqueData` `<Buffer>` If additional opaque data was included in the `GOAWAY` frame, a `Buffer` instance will be passed containing that data.

The `'goaway'` event is emitted when a `GOAWAY` frame is received.

The `Http2Session` instance will be shut down automatically when the `'goaway'` event is emitted.

##### Event: `'localSettings'`#

Added in: v8.4.0

- `settings` `<HTTP/2 Settings Object>` A copy of the `SETTINGS` frame received.

The `'localSettings'` event is emitted when an acknowledgment `SETTINGS` frame has been received.

When using `http2session.settings()` to submit new settings, the modified settings do not take effect until the `'localSettings'` event is emitted.

    session.settings({ enablePush: false });

    session.on('localSettings', (settings) => {
      /* Use the new settings */
    });
    js

##### Event: `'ping'`#

Added in: v10.12.0

- `payload` `<Buffer>` The `PING` frame 8-byte payload

The `'ping'` event is emitted whenever a `PING` frame is received from the connected peer.

##### Event: `'remoteSettings'`#

Added in: v8.4.0

- `settings` `<HTTP/2 Settings Object>` A copy of the `SETTINGS` frame received.

The `'remoteSettings'` event is emitted when a new `SETTINGS` frame is received from the connected peer.

    session.on('remoteSettings', (settings) => {
      /* Use the new settings */
    });
    js

##### Event: `'stream'`#

Added in: v8.4.0

- `stream` `<Http2Stream>` A reference to the stream
- `headers` `<HTTP/2 Headers Object>` An object describing the headers
- `flags` `<number>` The associated numeric flags
- `rawHeaders` `<HTTP/2 Raw Headers>` An array containing the raw headers

The `'stream'` event is emitted when a new `Http2Stream` is created.

    session.on('stream', (stream, headers, flags) => {
      const method = headers[':method'];
      const path = headers[':path'];
      // ...
      stream.respond({
        ':status': 200,
        'content-type': 'text/plain; charset=utf-8',
      });
      stream.write('hello ');
      stream.end('world');
    });
    js

On the server side, user code will typically not listen for this event directly, and would instead register a handler for the `'stream'` event emitted by the `net.Server` or `tls.Server` instances returned by `http2.createServer()` and `http2.createSecureServer()`, respectively, as in the example below:

    import { createServer } from 'node:http2';

    // Create an unencrypted HTTP/2 server
    const server = createServer();

    server.on('stream', (stream, headers) => {
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.on('error', (error) => console.error(error));
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8000);
    const http2 = require('node:http2');

    // Create an unencrypted HTTP/2 server
    const server = http2.createServer();

    server.on('stream', (stream, headers) => {
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.on('error', (error) => console.error(error));
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8000);
    javascript

Even though HTTP/2 streams and network sockets are not in a 1:1 correspondence, a network error will destroy each individual stream and must be handled on the stream level, as shown above.

##### Event: `'timeout'`#

Added in: v8.4.0

After the `http2session.setTimeout()` method is used to set the timeout period for this `Http2Session`, the `'timeout'` event is emitted if there is no activity on the `Http2Session` after the configured number of milliseconds. Its listener does not expect any arguments.

    session.setTimeout(2000);
    session.on('timeout', () => { /* .. */ });
    js

##### `http2session.alpnProtocol`#

Added in: v9.4.0

- Type: `<string>` | `<undefined>`

Value will be `undefined` if the `Http2Session` is not yet connected to a socket, `h2c` if the `Http2Session` is not connected to a `TLSSocket`, or will return the value of the connected `TLSSocket`'s own `alpnProtocol` property.

##### `http2session.close([callback])`#

Added in: v9.4.0

- `callback` `<Function>`

Gracefully closes the `Http2Session`, allowing any existing streams to complete on their own and preventing new `Http2Stream` instances from being created. Once closed, `http2session.destroy()` might be called if there are no open `Http2Stream` instances.

If specified, the `callback` function is registered as a handler for the `'close'` event.

##### `http2session.closed`#

Added in: v9.4.0

- Type: `<boolean>`

Will be `true` if this `Http2Session` instance has been closed, otherwise `false`.

##### `http2session.connecting`#

Added in: v10.0.0

- Type: `<boolean>`

Will be `true` if this `Http2Session` instance is still connecting, will be set to `false` before emitting `connect` event and/or calling the `http2.connect` callback.

##### `http2session.destroy([error][, code])`#

Added in: v8.4.0

- `error` `<Error>` An `Error` object if the `Http2Session` is being destroyed due to an error.
- `code` `<number>` The HTTP/2 error code to send in the final `GOAWAY` frame. If unspecified, and `error` is not undefined, the default is `INTERNAL_ERROR`, otherwise defaults to `NO_ERROR`.

Immediately terminates the `Http2Session` and the associated `net.Socket` or `tls.TLSSocket`.

Once destroyed, the `Http2Session` will emit the `'close'` event. If `error` is not undefined, an `'error'` event will be emitted immediately before the `'close'` event.

If there are any remaining open `Http2Streams` associated with the `Http2Session`, those will also be destroyed.

##### `http2session.destroyed`#

Added in: v8.4.0

- Type: `<boolean>`

Will be `true` if this `Http2Session` instance has been destroyed and must no longer be used, otherwise `false`.

##### `http2session.encrypted`#

Added in: v9.4.0

- Type: `<boolean>` | `<undefined>`

Value is `undefined` if the `Http2Session` session socket has not yet been connected, `true` if the `Http2Session` is connected with a `TLSSocket`, and `false` if the `Http2Session` is connected to any other kind of socket or stream.

##### `http2session.goaway([code[, lastStreamID[, opaqueData]]])`#

Added in: v9.4.0

- `code` `<number>` An HTTP/2 error code
- `lastStreamID` `<number>` The numeric ID of the last processed `Http2Stream`
- `opaqueData` `<Buffer>` | `<TypedArray>` | `<DataView>` A `TypedArray` or `DataView` instance containing additional data to be carried within the `GOAWAY` frame.

Transmits a `GOAWAY` frame to the connected peer without shutting down the `Http2Session`.

##### `http2session.localSettings`#

Added in: v8.4.0

- Type: `<HTTP/2 Settings Object>`

A prototype-less object describing the current local settings of this `Http2Session`. The local settings are local to this `Http2Session` instance.

##### `http2session.originSet`#

Added in: v9.4.0

- Type: `<string>`[] | `<undefined>`

If the `Http2Session` is connected to a `TLSSocket`, the `originSet` property will return an `Array` of origins for which the `Http2Session` may be considered authoritative.

The `originSet` property is only available when using a secure TLS connection.

##### `http2session.pendingSettingsAck`#

Added in: v8.4.0

- Type: `<boolean>`

Indicates whether the `Http2Session` is currently waiting for acknowledgment of a sent `SETTINGS` frame. Will be `true` after calling the `http2session.settings()` method. Will be `false` once all sent `SETTINGS` frames have been acknowledged.

##### `http2session.ping([payload, ]callback)`#

Added in: v8.9.3
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `payload` `<Buffer>` | `<TypedArray>` | `<DataView>` Optional ping payload.
- `callback` `<Function>`
- Returns: `<boolean>`

Sends a `PING` frame to the connected HTTP/2 peer. A `callback` function must be provided. The method will return `true` if the `PING` was sent, `false` otherwise.

The maximum number of outstanding (unacknowledged) pings is determined by the `maxOutstandingPings` configuration option. The default maximum is 10.

If provided, the `payload` must be a `Buffer`, `TypedArray`, or `DataView` containing 8 bytes of data that will be transmitted with the `PING` and returned with the ping acknowledgment.

The callback will be invoked with three arguments: an error argument that will be `null` if the `PING` was successfully acknowledged, a `duration` argument that reports the number of milliseconds elapsed since the ping was sent and the acknowledgment was received, and a `Buffer` containing the 8-byte `PING` payload.

    session.ping(Buffer.from('abcdefgh'), (err, duration, payload) => {
      if (!err) {
        console.log(`Ping acknowledged in ${duration} milliseconds`);
        console.log(`With payload '${payload.toString()}'`);
      }
    });
    js

If the `payload` argument is not specified, the default payload will be the 64-bit timestamp (little endian) marking the start of the `PING` duration.

##### `http2session.ref()`#

Added in: v9.4.0

Calls `ref()` on this `Http2Session` instance's underlying `net.Socket`.

##### `http2session.remoteSettings`#

Added in: v8.4.0

- Type: `<HTTP/2 Settings Object>`

A prototype-less object describing the current remote settings of this `Http2Session`. The remote settings are set by the connected HTTP/2 peer.

##### `http2session.setLocalWindowSize(windowSize)`#

Added in: v15.3.0, v14.18.0

- `windowSize` `<number>`

Sets the local endpoint's window size. The `windowSize` is the total window size to set, not the delta.

    import { createServer } from 'node:http2';

    const server = createServer();
    const expectedWindowSize = 2 ** 20;
    server.on('session', (session) => {

      // Set local window size to be 2 ** 20
      session.setLocalWindowSize(expectedWindowSize);
    });
    const http2 = require('node:http2');

    const server = http2.createServer();
    const expectedWindowSize = 2 ** 20;
    server.on('session', (session) => {

      // Set local window size to be 2 ** 20
      session.setLocalWindowSize(expectedWindowSize);
    });
    javascript

For http2 clients the proper event is either `'connect'` or `'remoteSettings'`.

##### `http2session.setTimeout(msecs, callback)`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `msecs` `<number>`
- `callback` `<Function>`

Used to set a callback function that is called when there is no activity on the `Http2Session` after `msecs` milliseconds. The given `callback` is registered as a listener on the `'timeout'` event.

##### `http2session.socket`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v26.6.0 | Calling `destroy` no longer throws and instead destroys the `Http2Session`.

- Type: `<net.Socket>` | `<tls.TLSSocket>`

Returns a `Proxy` object that acts as a `net.Socket` (or `tls.TLSSocket`) but limits available methods to ones safe to use with HTTP/2.

`emit`, `end`, `pause`, `read`, `resume`, and `write` will throw an error with code `ERR_HTTP2_NO_SOCKET_MANIPULATION`. See `Http2Session` and Sockets for more information.

`destroy`, `setTimeout`, `ref`, and `unref` methods will be called on this `Http2Session`.

All other interactions will be routed directly to the socket.

##### `http2session.state`#

Added in: v8.4.0

Provides miscellaneous information about the current state of the `Http2Session`.

- Type: `<Object>`
  - `effectiveLocalWindowSize` `<number>` The current local (receive) flow control window size for the `Http2Session`.
  - `effectiveRecvDataLength` `<number>` The current number of bytes that have been received since the last flow control `WINDOW_UPDATE`.
  - `nextStreamID` `<number>` The numeric identifier to be used the next time a new `Http2Stream` is created by this `Http2Session`.
  - `localWindowSize` `<number>` The number of bytes that the remote peer can send without receiving a `WINDOW_UPDATE`.
  - `lastProcStreamID` `<number>` The numeric id of the `Http2Stream` for which a `HEADERS` or `DATA` frame was most recently received.
  - `remoteWindowSize` `<number>` The number of bytes that this `Http2Session` may send without receiving a `WINDOW_UPDATE`.
  - `outboundQueueSize` `<number>` The number of frames currently within the outbound queue for this `Http2Session`.
  - `deflateDynamicTableSize` `<number>` The current size in bytes of the outbound header compression state table.
  - `inflateDynamicTableSize` `<number>` The current size in bytes of the inbound header compression state table.

An object describing the current status of this `Http2Session`.

##### `http2session.settings([settings][, callback])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `settings` `<HTTP/2 Settings Object>`
- `callback` `<Function>` Callback that is called once the session is connected or right away if the session is already connected.
  - `err` `<Error>` | `<null>`
  - `settings` `<HTTP/2 Settings Object>` The updated `settings` object.
  - `duration` `<integer>`

Updates the current local settings for this `Http2Session` and sends a new `SETTINGS` frame to the connected HTTP/2 peer.

Once called, the `http2session.pendingSettingsAck` property will be `true` while the session is waiting for the remote peer to acknowledge the new settings.

The new settings will not become effective until the `SETTINGS` acknowledgment is received and the `'localSettings'` event is emitted. It is possible to send multiple `SETTINGS` frames while acknowledgment is still pending.

##### `http2session.type`#

Added in: v8.4.0

- Type: `<number>`

The `http2session.type` will be equal to `http2.constants.NGHTTP2_SESSION_SERVER` if this `Http2Session` instance is a server, and `http2.constants.NGHTTP2_SESSION_CLIENT` if the instance is a client.

##### `http2session.unref()`#

Added in: v9.4.0

Calls `unref()` on this `Http2Session` instance's underlying `net.Socket`.

#### Class: `ServerHttp2Session`#

Added in: v8.4.0

- Extends: `<Http2Session>`

##### `serverhttp2session.altsvc(alt, originOrStream)`#

Added in: v9.4.0

- `alt` `<string>` A description of the alternative service configuration as defined by RFC 7838.
- `originOrStream` `<number>` | `<string>` | `<URL>` | `<Object>` Either a URL string specifying the origin (or an `Object` with an `origin` property) or the numeric identifier of an active `Http2Stream` as given by the `http2stream.id` property.

Submits an `ALTSVC` frame (as defined by RFC 7838) to the connected client.

    import { createServer } from 'node:http2';

    const server = createServer();
    server.on('session', (session) => {
      // Set altsvc for origin https://example.org:80
      session.altsvc('h2=":8000"', 'https://example.org:80');
    });

    server.on('stream', (stream) => {
      // Set altsvc for a specific stream
      stream.session.altsvc('h2=":8000"', stream.id);
    });
    const http2 = require('node:http2');

    const server = http2.createServer();
    server.on('session', (session) => {
      // Set altsvc for origin https://example.org:80
      session.altsvc('h2=":8000"', 'https://example.org:80');
    });

    server.on('stream', (stream) => {
      // Set altsvc for a specific stream
      stream.session.altsvc('h2=":8000"', stream.id);
    });
    javascript

Sending an `ALTSVC` frame with a specific stream ID indicates that the alternate service is associated with the origin of the given `Http2Stream`.

The `alt` and origin string must contain only ASCII bytes and are strictly interpreted as a sequence of ASCII bytes. The special value `'clear'` may be passed to clear any previously set alternative service for a given domain.

When a string is passed for the `originOrStream` argument, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL `'https://example.org/foo/bar'` is the ASCII string `'https://example.org'`. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A `URL` object, or any object with an `origin` property, may be passed as `originOrStream`, in which case the value of the `origin` property will be used. The value of the `origin` property must be a properly serialized ASCII origin.

##### Specifying alternative services#

The format of the `alt` parameter is strictly defined by RFC 7838 as an ASCII string containing a comma-delimited list of "alternative" protocols associated with a specific host and port.

For example, the value `'h2="example.org:81"'` indicates that the HTTP/2 protocol is available on the host `'example.org'` on TCP/IP port 81. The host and port must be contained within the quote (`"`) characters.

Multiple alternatives may be specified, for instance: `'h2="example.org:81", h2=":82"'`.

The protocol identifier (`'h2'` in the examples) may be any valid ALPN Protocol ID.

The syntax of these values is not validated by the Node.js implementation and are passed through as provided by the user or received from the peer.

##### `serverhttp2session.origin(...origins)`#

Added in: v10.12.0

- `origins` { string | URL | Object } One or more URL Strings passed as separate arguments.

Submits an `ORIGIN` frame (as defined by RFC 8336) to the connected client to advertise the set of origins for which the server is capable of providing authoritative responses.

    import { createSecureServer } from 'node:http2';
    const options = getSecureOptionsSomehow();
    const server = createSecureServer(options);
    server.on('stream', (stream) => {
      stream.respond();
      stream.end('ok');
    });
    server.on('session', (session) => {
      session.origin('https://example.com', 'https://example.org');
    });
    const http2 = require('node:http2');
    const options = getSecureOptionsSomehow();
    const server = http2.createSecureServer(options);
    server.on('stream', (stream) => {
      stream.respond();
      stream.end('ok');
    });
    server.on('session', (session) => {
      session.origin('https://example.com', 'https://example.org');
    });
    javascript

When a string is passed as an `origin`, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL `'https://example.org/foo/bar'` is the ASCII string `'https://example.org'`. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A `URL` object, or any object with an `origin` property, may be passed as an `origin`, in which case the value of the `origin` property will be used. The value of the `origin` property must be a properly serialized ASCII origin.

Alternatively, the `origins` option may be used when creating a new HTTP/2 server using the `http2.createSecureServer()` method:

    import { createSecureServer } from 'node:http2';
    const options = getSecureOptionsSomehow();
    options.origins = ['https://example.com', 'https://example.org'];
    const server = createSecureServer(options);
    server.on('stream', (stream) => {
      stream.respond();
      stream.end('ok');
    });
    const http2 = require('node:http2');
    const options = getSecureOptionsSomehow();
    options.origins = ['https://example.com', 'https://example.org'];
    const server = http2.createSecureServer(options);
    server.on('stream', (stream) => {
      stream.respond();
      stream.end('ok');
    });
    javascript

#### Class: `ClientHttp2Session`#

Added in: v8.4.0

- Extends: `<Http2Session>`

##### Event: `'altsvc'`#

Added in: v9.4.0

- `alt` `<string>`
- `origin` `<string>`
- `streamId` `<number>`

The `'altsvc'` event is emitted whenever an `ALTSVC` frame is received by the client. The event is emitted with the `ALTSVC` value, origin, and stream ID. If no `origin` is provided in the `ALTSVC` frame, `origin` will be an empty string.

    import { connect } from 'node:http2';
    const client = connect('https://example.org');

    client.on('altsvc', (alt, origin, streamId) => {
      console.log(alt);
      console.log(origin);
      console.log(streamId);
    });
    const http2 = require('node:http2');
    const client = http2.connect('https://example.org');

    client.on('altsvc', (alt, origin, streamId) => {
      console.log(alt);
      console.log(origin);
      console.log(streamId);
    });
    javascript

##### Event: `'origin'`#

Added in: v10.12.0

- `origins` `<string>`[]

The `'origin'` event is emitted whenever an `ORIGIN` frame is received by the client. The event is emitted with an array of `origin` strings. The `http2session.originSet` will be updated to include the received origins.

    import { connect } from 'node:http2';
    const client = connect('https://example.org');

    client.on('origin', (origins) => {
      for (let n = 0; n < origins.length; n++)
        console.log(origins[n]);
    });
    const http2 = require('node:http2');
    const client = http2.connect('https://example.org');

    client.on('origin', (origins) => {
      for (let n = 0; n < origins.length; n++)
        console.log(origins[n]);
    });
    javascript

The `'origin'` event is only emitted when using a secure TLS connection.

##### `clienthttp2session.request(headers[, options])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v24.2.0 | The `weight` option is now ignored, setting it will trigger a runtime warning.
v24.2.0, v22.17.0, v20.19.6 | Following the deprecation of priority signaling as of RFC 9113, `weight` option is deprecated.
v24.0.0, v22.17.0 | Allow passing headers in raw array format.

- `headers` `<HTTP/2 Headers Object>` | `<HTTP/2 Raw Headers>`

- `options` `<Object>`

  - `endStream` `<boolean>` `true` if the `Http2Stream` writable side should be closed initially, such as when sending a `GET` request that should not expect a payload body.
  - `exclusive` `<boolean>` When `true` and `parent` identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: `false`.
  - `parent` `<number>` Specifies the numeric identifier of a stream the newly created stream is dependent on.
  - `waitForTrailers` `<boolean>` When `true`, the `Http2Stream` will emit the `'wantTrailers'` event after the final `DATA` frame has been sent.
  - `signal` `<AbortSignal>` An AbortSignal that may be used to abort an ongoing request.

- Returns: `<ClientHttp2Stream>`

For HTTP/2 Client `Http2Session` instances only, the `http2session.request()` creates and returns an `Http2Stream` instance that can be used to send an HTTP/2 request to the connected server.

When a `ClientHttp2Session` is first created, the socket may not yet be connected. If `clienthttp2session.request()` is called during this time, the actual request will be deferred until the socket is ready to go.

If the session becomes unavailable before the request can be created, the returned stream will emit `ERR_HTTP2_GOAWAY_SESSION` or `ERR_HTTP2_INVALID_SESSION` asynchronously.

This method is only available if `http2session.type` is equal to `http2.constants.NGHTTP2_SESSION_CLIENT`.

    import { connect, constants } from 'node:http2';
    const clientSession = connect('https://localhost:1234');
    const {
      HTTP2_HEADER_PATH,
      HTTP2_HEADER_STATUS,
    } = constants;

    const req = clientSession.request({ [HTTP2_HEADER_PATH]: '/' });
    req.on('response', (headers) => {
      console.log(headers[HTTP2_HEADER_STATUS]);
      req.on('data', (chunk) => { /* .. */ });
      req.on('end', () => { /* .. */ });
    });
    const http2 = require('node:http2');
    const clientSession = http2.connect('https://localhost:1234');
    const {
      HTTP2_HEADER_PATH,
      HTTP2_HEADER_STATUS,
    } = http2.constants;

    const req = clientSession.request({ [HTTP2_HEADER_PATH]: '/' });
    req.on('response', (headers) => {
      console.log(headers[HTTP2_HEADER_STATUS]);
      req.on('data', (chunk) => { /* .. */ });
      req.on('end', () => { /* .. */ });
    });
    javascript

When the `options.waitForTrailers` option is set, the `'wantTrailers'` event is emitted immediately after queuing the last chunk of payload data to be sent. The `http2stream.sendTrailers()` method can then be called to send trailing headers to the peer.

When `options.waitForTrailers` is set, the `Http2Stream` will not automatically close when the final `DATA` frame is transmitted. User code must call either `http2stream.sendTrailers()` or `http2stream.close()` to close the `Http2Stream`.

When `options.signal` is set with an `AbortSignal` and then `abort` on the corresponding `AbortController` is called, the request will emit an `'error'` event with an `AbortError` error.

The `:method` and `:path` pseudo-headers are not specified within `headers`, they respectively default to:

- `:method` = `'GET'`
- `:path` = `/`

#### Class: `Http2Stream`#

Added in: v8.4.0

- Extends: `<stream.Duplex>`

Each instance of the `Http2Stream` class represents a bidirectional HTTP/2 communications stream over an `Http2Session` instance. Any single `Http2Session` may have up to 231-1 `Http2Stream` instances over its lifetime.

User code will not construct `Http2Stream` instances directly. Rather, these are created, managed, and provided to user code through the `Http2Session` instance. On the server, `Http2Stream` instances are created either in response to an incoming HTTP request (and handed off to user code via the `'stream'` event), or in response to a call to the `http2stream.pushStream()` method. On the client, `Http2Stream` instances are created and returned when either the `http2session.request()` method is called, or in response to an incoming `'push'` event.

The `Http2Stream` class is a base for the `ServerHttp2Stream` and `ClientHttp2Stream` classes, each of which is used specifically by either the Server or Client side, respectively.

All `Http2Stream` instances are `Duplex` streams. The `Writable` side of the `Duplex` is used to send data to the connected peer, while the `Readable` side is used to receive data sent by the connected peer.

The default text character encoding for an `Http2Stream` is UTF-8. When using an `Http2Stream` to send text, use the `'content-type'` header to set the character encoding.

    stream.respond({
      'content-type': 'text/html; charset=utf-8',
      ':status': 200,
    });
    js

##### `Http2Stream` Lifecycle#

###### Creation#

On the server side, instances of `ServerHttp2Stream` are created either when:

- A new HTTP/2 `HEADERS` frame with a previously unused stream ID is received;
- The `http2stream.pushStream()` method is called.

On the client side, instances of `ClientHttp2Stream` are created when the `http2session.request()` method is called.

On the client, the `Http2Stream` instance returned by `http2session.request()` may not be immediately ready for use if the parent `Http2Session` has not yet been fully established. In such cases, operations called on the `Http2Stream` will be buffered until the `'ready'` event is emitted. User code should rarely, if ever, need to handle the `'ready'` event directly. The ready status of an `Http2Stream` can be determined by checking the value of `http2stream.id`. If the value is `undefined`, the stream is not yet ready for use.

###### Destruction#

All `Http2Stream` instances are destroyed either when:

- An `RST_STREAM` frame for the stream is received by the connected peer, and (for client streams only) pending data has been read.
- The `http2stream.close()` method is called, and (for client streams only) pending data has been read.
- The `http2stream.destroy()` or `http2session.destroy()` methods are called.

When an `Http2Stream` instance is destroyed, an attempt will be made to send an `RST_STREAM` frame to the connected peer.

When the `Http2Stream` instance is destroyed, the `'close'` event will be emitted. Because `Http2Stream` is an instance of `stream.Duplex`, the `'end'` event will also be emitted if the stream data is currently flowing. The `'error'` event may also be emitted if `http2stream.destroy()` was called with an `Error` passed as the first argument.

After the `Http2Stream` has been destroyed, the `http2stream.destroyed` property will be `true` and the `http2stream.rstCode` property will specify the `RST_STREAM` error code. The `Http2Stream` instance is no longer usable once destroyed.

##### Event: `'aborted'`#

Added in: v8.4.0

The `'aborted'` event is emitted whenever a `Http2Stream` instance is abnormally aborted in mid-communication. Its listener does not expect any arguments.

The `'aborted'` event will only be emitted if the `Http2Stream` writable side has not been ended.

##### Event: `'close'`#

Added in: v8.4.0

The `'close'` event is emitted when the `Http2Stream` is destroyed. Once this event is emitted, the `Http2Stream` instance is no longer usable.

The HTTP/2 error code used when closing the stream can be retrieved using the `http2stream.rstCode` property. If the code is any value other than `NGHTTP2_NO_ERROR` (`0`), an `'error'` event will have also been emitted.

##### Event: `'error'`#

Added in: v8.4.0

- `error` `<Error>`

The `'error'` event is emitted when an error occurs during the processing of an `Http2Stream`.

##### Event: `'frameError'`#

Added in: v8.4.0

- `type` `<integer>` The frame type.
- `code` `<integer>` The error code.
- `id` `<integer>` The stream id (or `0` if the frame isn't associated with a stream).

The `'frameError'` event is emitted when an error occurs while attempting to send a frame. When invoked, the handler function will receive an integer argument identifying the frame type, and an integer argument identifying the error code. The `Http2Stream` instance will be destroyed immediately after the `'frameError'` event is emitted.

##### Event: `'ready'`#

Added in: v8.4.0

The `'ready'` event is emitted when the `Http2Stream` has been opened, has been assigned an `id`, and can be used. The listener does not expect any arguments.

##### Event: `'timeout'`#

Added in: v8.4.0

The `'timeout'` event is emitted after no activity is received for this `Http2Stream` within the number of milliseconds set using `http2stream.setTimeout()`. Its listener does not expect any arguments.

##### Event: `'trailers'`#

Added in: v8.4.0

- `headers` `<HTTP/2 Headers Object>` An object describing the headers
- `flags` `<number>` The associated numeric flags
- `rawHeaders` `<HTTP/2 Raw Headers>`

The `'trailers'` event is emitted when a block of headers associated with trailing header fields is received. The listener callback is passed the HTTP/2 Headers Object, flags associated with the headers, and the headers in raw format (see HTTP/2 Raw Headers).

This event might not be emitted if `http2stream.end()` is called before trailers are received and the incoming data is not being read or listened for.

    stream.on('trailers', (headers, flags) => {
      console.log(headers);
    });
    js

##### Event: `'wantTrailers'`#

Added in: v10.0.0

The `'wantTrailers'` event is emitted when the `Http2Stream` has queued the final `DATA` frame to be sent on a frame and the `Http2Stream` is ready to send trailing headers. When initiating a request or response, the `waitForTrailers` option must be set for this event to be emitted.

##### `http2stream.aborted`#

Added in: v8.4.0

- Type: `<boolean>`

Set to `true` if the `Http2Stream` instance was aborted abnormally. When set, the `'aborted'` event will have been emitted.

##### `http2stream.bufferSize`#

Added in: v11.2.0, v10.16.0

- Type: `<number>`

This property shows the number of characters currently buffered to be written. See `net.Socket.bufferSize` for details.

##### `http2stream.close(code[, callback])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `code` `<number>` Unsigned 32-bit integer identifying the error code. Default: `http2.constants.NGHTTP2_NO_ERROR` (`0x00`).
- `callback` `<Function>` An optional function registered to listen for the `'close'` event.

Closes the `Http2Stream` instance by sending an `RST_STREAM` frame to the connected HTTP/2 peer.

##### `http2stream.closed`#

Added in: v9.4.0

- Type: `<boolean>`

Set to `true` if the `Http2Stream` instance has been closed.

##### `http2stream.destroyed`#

Added in: v8.4.0

- Type: `<boolean>`

Set to `true` if the `Http2Stream` instance has been destroyed and is no longer usable.

##### `http2stream.endAfterHeaders`#

Added in: v10.11.0

- Type: `<boolean>`

Set to `true` if the `END_STREAM` flag was set in the request or response HEADERS frame received, indicating that no additional data should be received and the readable side of the `Http2Stream` will be closed.

##### `http2stream.id`#

Added in: v8.4.0

- Type: `<number>` | `<undefined>`

The numeric stream identifier of this `Http2Stream` instance. Set to `undefined` if the stream identifier has not yet been assigned.

##### `http2stream.pending`#

Added in: v9.4.0

- Type: `<boolean>`

Set to `true` if the `Http2Stream` instance has not yet been assigned a numeric stream identifier.

##### `http2stream.priority(options)`#

Added in: v8.4.0Deprecated in: v24.2.0, v22.17.0, v20.19.6
History

Version | Changes
--- | ---
v24.2.0 | This method no longer sets the priority of the stream. Using it now triggers a runtime warning.

Stability: 0 - Deprecated: support for priority signaling has been deprecated in the RFC 9113 and is no longer supported in Node.js.

Empty method, only there to maintain some backward compatibility.

##### `http2stream.rstCode`#

Added in: v8.4.0

- Type: `<number>`

Set to the `RST_STREAM` error code reported when the `Http2Stream` is destroyed after either receiving an `RST_STREAM` frame from the connected peer, calling `http2stream.close()`, or `http2stream.destroy()`. Will be `undefined` if the `Http2Stream` has not been closed.

##### `http2stream.sentHeaders`#

Added in: v9.5.0

- Type: `<HTTP/2 Headers Object>`

An object containing the outbound headers sent for this `Http2Stream`.

##### `http2stream.sentInfoHeaders`#

Added in: v9.5.0

- Type: `<HTTP/2 Headers Object>`[]

An array of objects containing the outbound informational (additional) headers sent for this `Http2Stream`.

##### `http2stream.sentTrailers`#

Added in: v9.5.0

- Type: `<HTTP/2 Headers Object>`

An object containing the outbound trailers sent for this `HttpStream`.

##### `http2stream.session`#

Added in: v8.4.0

- Type: `<Http2Session>`

A reference to the `Http2Session` instance that owns this `Http2Stream`. The value will be `undefined` after the `Http2Stream` instance is destroyed.

##### `http2stream.setTimeout(msecs, callback)`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `msecs` `<number>`
- `callback` `<Function>`

    import { connect, constants } from 'node:http2';
    const client = connect('http://example.org:8000');
    const { NGHTTP2_CANCEL } = constants;
    const req = client.request({ ':path': '/' });

    // Cancel the stream if there's no activity after 5 seconds
    req.setTimeout(5000, () => req.close(NGHTTP2_CANCEL));
    const http2 = require('node:http2');
    const client = http2.connect('http://example.org:8000');
    const { NGHTTP2_CANCEL } = http2.constants;
    const req = client.request({ ':path': '/' });

    // Cancel the stream if there's no activity after 5 seconds
    req.setTimeout(5000, () => req.close(NGHTTP2_CANCEL));
    javascript

##### `http2stream.state`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v24.2.0 | The `state.weight` property is now always set to 16 and `sumDependencyWeight` is always set to 0.
v24.2.0, v22.17.0, v20.19.6 | Following the deprecation of priority signaling as of RFC 9113, `weight` and `sumDependencyWeight` options are deprecated.

Provides miscellaneous information about the current state of the `Http2Stream`.

- Type: `<Object>`
  - `localWindowSize` `<number>` The number of bytes the connected peer may send for this `Http2Stream` without receiving a `WINDOW_UPDATE`.
  - `state` `<number>` A flag indicating the low-level current state of the `Http2Stream` as determined by `nghttp2`.
  - `localClose` `<number>` `1` if this `Http2Stream` has been closed locally.
  - `remoteClose` `<number>` `1` if this `Http2Stream` has been closed remotely.
  - `sumDependencyWeight` `<number>` Legacy property, always set to `0`.
  - `weight` `<number>` Legacy property, always set to `16`.

A current state of this `Http2Stream`.

##### `http2stream.sendTrailers(headers)`#

Added in: v10.0.0

- `headers` `<HTTP/2 Headers Object>`

Sends a trailing `HEADERS` frame to the connected HTTP/2 peer. This method will cause the `Http2Stream` to be immediately closed and must only be called after the `'wantTrailers'` event has been emitted. When sending a request or sending a response, the `options.waitForTrailers` option must be set in order to keep the `Http2Stream` open after the final `DATA` frame so that trailers can be sent.

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      stream.respond(undefined, { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ xyz: 'abc' });
      });
      stream.end('Hello World');
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      stream.respond(undefined, { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ xyz: 'abc' });
      });
      stream.end('Hello World');
    });
    javascript

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. `':method'`, `':path'`, etc).

#### Class: `ClientHttp2Stream`#

Added in: v8.4.0

- Extends `<Http2Stream>`

The `ClientHttp2Stream` class is an extension of `Http2Stream` that is used exclusively on HTTP/2 Clients. `Http2Stream` instances on the client provide events such as `'response'` and `'push'` that are only relevant on the client.

##### Event: `'continue'`#

Added in: v8.5.0

Emitted when the server sends a `100 Continue` status, usually because the request contained `Expect: 100-continue`. This is an instruction that the client should send the request body.

##### Event: `'headers'`#

Added in: v8.4.0

- `headers` `<HTTP/2 Headers Object>`
- `flags` `<number>`
- `rawHeaders` `<HTTP/2 Raw Headers>`

The `'headers'` event is emitted when an additional block of headers is received for a stream, such as when a block of `1xx` informational headers is received. The listener callback is passed the HTTP/2 Headers Object, flags associated with the headers, and the headers in raw format (see HTTP/2 Raw Headers).

    stream.on('headers', (headers, flags) => {
      console.log(headers);
    });
    js

##### Event: `'push'`#

Added in: v8.4.0

- `headers` `<HTTP/2 Headers Object>`
- `flags` `<number>`
- `rawHeaders` `<HTTP/2 Raw Headers>`

The `'push'` event is emitted when response headers for a Server Push stream are received. The listener callback is passed the HTTP/2 Headers Object, flags associated with the headers, and the headers in raw format (see HTTP/2 Raw Headers).

    stream.on('push', (headers, flags) => {
      console.log(headers);
    });
    js

##### Event: `'response'`#

Added in: v8.4.0

- `headers` `<HTTP/2 Headers Object>`
- `flags` `<number>`
- `rawHeaders` `<HTTP/2 Raw Headers>`

The `'response'` event is emitted when a response `HEADERS` frame has been received for this stream from the connected HTTP/2 server. The listener is invoked with three arguments: an `Object` containing the received HTTP/2 Headers Object, flags associated with the headers, and the headers in raw format (see HTTP/2 Raw Headers).

    import { connect } from 'node:http2';
    const client = connect('https://localhost');
    const req = client.request({ ':path': '/' });
    req.on('response', (headers, flags) => {
      console.log(headers[':status']);
    });
    const http2 = require('node:http2');
    const client = http2.connect('https://localhost');
    const req = client.request({ ':path': '/' });
    req.on('response', (headers, flags) => {
      console.log(headers[':status']);
    });
    javascript

#### Class: `ServerHttp2Stream`#

Added in: v8.4.0

- Extends: `<Http2Stream>`

The `ServerHttp2Stream` class is an extension of `Http2Stream` that is used exclusively on HTTP/2 Servers. `Http2Stream` instances on the server provide additional methods such as `http2stream.pushStream()` and `http2stream.respond()` that are only relevant on the server.

##### `http2stream.additionalHeaders(headers)`#

Added in: v8.4.0

- `headers` `<HTTP/2 Headers Object>`

Sends an additional informational `HEADERS` frame to the connected HTTP/2 peer.

##### `http2stream.headersSent`#

Added in: v8.4.0

- Type: `<boolean>`

True if headers were sent, false otherwise (read-only).

##### `http2stream.pushAllowed`#

Added in: v8.4.0

- Type: `<boolean>`

Read-only property mapped to the `SETTINGS_ENABLE_PUSH` flag of the remote client's most recent `SETTINGS` frame. Will be `true` if the remote peer accepts push streams, `false` otherwise. Settings are the same for every `Http2Stream` in the same `Http2Session`.

##### `http2stream.pushStream(headers[, options], callback)`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `headers` `<HTTP/2 Headers Object>`
- `options` `<Object>`
  - `exclusive` `<boolean>` When `true` and `parent` identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: `false`.
  - `parent` `<number>` Specifies the numeric identifier of a stream the newly created stream is dependent on.
- `callback` `<Function>` Callback that is called once the push stream has been initiated.
  - `err` `<Error>`
  - `pushStream` `<ServerHttp2Stream>` The returned `pushStream` object.
  - `headers` `<HTTP/2 Headers Object>` Headers object the `pushStream` was initiated with.

Initiates a push stream. The callback is invoked with the new `Http2Stream` instance created for the push stream passed as the second argument, or an `Error` passed as the first argument.

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      stream.respond({ ':status': 200 });
      stream.pushStream({ ':path': '/' }, (err, pushStream, headers) => {
        if (err) throw err;
        pushStream.respond({ ':status': 200 });
        pushStream.end('some pushed data');
      });
      stream.end('some data');
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      stream.respond({ ':status': 200 });
      stream.pushStream({ ':path': '/' }, (err, pushStream, headers) => {
        if (err) throw err;
        pushStream.respond({ ':status': 200 });
        pushStream.end('some pushed data');
      });
      stream.end('some data');
    });
    javascript

Setting the weight of a push stream is not allowed in the `HEADERS` frame. Pass a `weight` value to `http2stream.priority` with the `silent` option set to `true` to enable server-side bandwidth balancing between concurrent streams.

Calling `http2stream.pushStream()` from within a pushed stream is not permitted and will throw an error.

##### `http2stream.respond([headers[, options]])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v24.7.0, v22.20.0 | Allow passing headers in raw array format.
v14.5.0, v12.19.0 | Allow explicitly setting date headers.

- `headers` `<HTTP/2 Headers Object>` | `<HTTP/2 Raw Headers>`
- `options` `<Object>`
  - `endStream` `<boolean>` Set to `true` to indicate that the response will not include payload data.
  - `waitForTrailers` `<boolean>` When `true`, the `Http2Stream` will emit the `'wantTrailers'` event after the final `DATA` frame has been sent.

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      stream.respond({ ':status': 200 });
      stream.end('some data');
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      stream.respond({ ':status': 200 });
      stream.end('some data');
    });
    javascript

Initiates a response. When the `options.waitForTrailers` option is set, the `'wantTrailers'` event will be emitted immediately after queuing the last chunk of payload data to be sent. The `http2stream.sendTrailers()` method can then be used to send trailing header fields to the peer.

When `options.waitForTrailers` is set, the `Http2Stream` will not automatically close when the final `DATA` frame is transmitted. User code must call either `http2stream.sendTrailers()` or `http2stream.close()` to close the `Http2Stream`.

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      stream.respond({ ':status': 200 }, { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ ABC: 'some value to send' });
      });
      stream.end('some data');
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      stream.respond({ ':status': 200 }, { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ ABC: 'some value to send' });
      });
      stream.end('some data');
    });
    javascript

##### `http2stream.respondWithFD(fd[, headers[, options]])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v14.5.0, v12.19.0 | Allow explicitly setting date headers.
v12.12.0 | The `fd` option may now be a `FileHandle`.
v10.0.0 | Any readable file descriptor, not necessarily for a regular file, is supported now.

- `fd` `<number>` | `<FileHandle>` A readable file descriptor.
- `headers` `<HTTP/2 Headers Object>`
- `options` `<Object>`
  - `statCheck` `<Function>`
  - `waitForTrailers` `<boolean>` When `true`, the `Http2Stream` will emit the `'wantTrailers'` event after the final `DATA` frame has been sent.
  - `offset` `<number>` The offset position at which to begin reading.
  - `length` `<number>` The amount of data from the fd to send.

Initiates a response whose data is read from the given file descriptor. No validation is performed on the given file descriptor. If an error occurs while attempting to read data using the file descriptor, the `Http2Stream` will be closed using an `RST_STREAM` frame using the standard `INTERNAL_ERROR` code.

When used, the `Http2Stream` object's `Duplex` interface will be closed automatically.

    import { createServer } from 'node:http2';
    import { openSync, fstatSync, closeSync } from 'node:fs';

    const server = createServer();
    server.on('stream', (stream) => {
      const fd = openSync('/some/file', 'r');

      const stat = fstatSync(fd);
      const headers = {
        'content-length': stat.size,
        'last-modified': stat.mtime.toUTCString(),
        'content-type': 'text/plain; charset=utf-8',
      };
      stream.respondWithFD(fd, headers);
      stream.on('close', () => closeSync(fd));
    });
    const http2 = require('node:http2');
    const fs = require('node:fs');

    const server = http2.createServer();
    server.on('stream', (stream) => {
      const fd = fs.openSync('/some/file', 'r');

      const stat = fs.fstatSync(fd);
      const headers = {
        'content-length': stat.size,
        'last-modified': stat.mtime.toUTCString(),
        'content-type': 'text/plain; charset=utf-8',
      };
      stream.respondWithFD(fd, headers);
      stream.on('close', () => fs.closeSync(fd));
    });
    javascript

The optional `options.statCheck` function may be specified to give user code an opportunity to set additional content headers based on the `fs.Stat` details of the given fd. If the `statCheck` function is provided, the `http2stream.respondWithFD()` method will perform an `fs.fstat()` call to collect details on the provided file descriptor.

The `offset` and `length` options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests.

The file descriptor or `FileHandle` is not closed when the stream is closed, so it will need to be closed manually once it is no longer needed. Using the same file descriptor concurrently for multiple streams is not supported and may result in data loss. Re-using a file descriptor after a stream has finished is supported.

When the `options.waitForTrailers` option is set, the `'wantTrailers'` event will be emitted immediately after queuing the last chunk of payload data to be sent. The `http2stream.sendTrailers()` method can then be used to send trailing header fields to the peer.

When `options.waitForTrailers` is set, the `Http2Stream` will not automatically close when the final `DATA` frame is transmitted. User code must call either `http2stream.sendTrailers()` or `http2stream.close()` to close the `Http2Stream`.

    import { createServer } from 'node:http2';
    import { openSync, fstatSync, closeSync } from 'node:fs';

    const server = createServer();
    server.on('stream', (stream) => {
      const fd = openSync('/some/file', 'r');

      const stat = fstatSync(fd);
      const headers = {
        'content-length': stat.size,
        'last-modified': stat.mtime.toUTCString(),
        'content-type': 'text/plain; charset=utf-8',
      };
      stream.respondWithFD(fd, headers, { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ ABC: 'some value to send' });
      });

      stream.on('close', () => closeSync(fd));
    });
    const http2 = require('node:http2');
    const fs = require('node:fs');

    const server = http2.createServer();
    server.on('stream', (stream) => {
      const fd = fs.openSync('/some/file', 'r');

      const stat = fs.fstatSync(fd);
      const headers = {
        'content-length': stat.size,
        'last-modified': stat.mtime.toUTCString(),
        'content-type': 'text/plain; charset=utf-8',
      };
      stream.respondWithFD(fd, headers, { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ ABC: 'some value to send' });
      });

      stream.on('close', () => fs.closeSync(fd));
    });
    javascript

##### `http2stream.respondWithFile(path[, headers[, options]])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v14.5.0, v12.19.0 | Allow explicitly setting date headers.
v10.0.0 | Any readable file, not necessarily a regular file, is supported now.

- `path` `<string>` | `<Buffer>` | `<URL>`
- `headers` `<HTTP/2 Headers Object>`
- `options` `<Object>`
  - `statCheck` `<Function>`
  - `onError` `<Function>` Callback function invoked in the case of an error before send.
  - `waitForTrailers` `<boolean>` When `true`, the `Http2Stream` will emit the `'wantTrailers'` event after the final `DATA` frame has been sent.
  - `offset` `<number>` The offset position at which to begin reading.
  - `length` `<number>` The amount of data from the fd to send.

Sends a regular file as the response. The `path` must specify a regular file or an `'error'` event will be emitted on the `Http2Stream` object.

When used, the `Http2Stream` object's `Duplex` interface will be closed automatically.

The optional `options.statCheck` function may be specified to give user code an opportunity to set additional content headers based on the `fs.Stat` details of the given file:

If an error occurs while attempting to read the file data, the `Http2Stream` will be closed using an `RST_STREAM` frame using the standard `INTERNAL_ERROR` code. If the `onError` callback is defined, then it will be called. Otherwise the stream will be destroyed.

Example using a file path:

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      function statCheck(stat, headers) {
        headers['last-modified'] = stat.mtime.toUTCString();
      }

      function onError(err) {
        // stream.respond() can throw if the stream has been destroyed by
        // the other side.
        try {
          if (err.code === 'ENOENT') {
            stream.respond({ ':status': 404 });
          } else {
            stream.respond({ ':status': 500 });
          }
        } catch (err) {
          // Perform actual error handling.
          console.error(err);
        }
        stream.end();
      }

      stream.respondWithFile('/some/file',
                             { 'content-type': 'text/plain; charset=utf-8' },
                             { statCheck, onError });
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      function statCheck(stat, headers) {
        headers['last-modified'] = stat.mtime.toUTCString();
      }

      function onError(err) {
        // stream.respond() can throw if the stream has been destroyed by
        // the other side.
        try {
          if (err.code === 'ENOENT') {
            stream.respond({ ':status': 404 });
          } else {
            stream.respond({ ':status': 500 });
          }
        } catch (err) {
          // Perform actual error handling.
          console.error(err);
        }
        stream.end();
      }

      stream.respondWithFile('/some/file',
                             { 'content-type': 'text/plain; charset=utf-8' },
                             { statCheck, onError });
    });
    javascript

The `options.statCheck` function may also be used to cancel the send operation by returning `false`. For instance, a conditional request may check the stat results to determine if the file has been modified to return an appropriate `304` response:

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      function statCheck(stat, headers) {
        // Check the stat here...
        stream.respond({ ':status': 304 });
        return false; // Cancel the send operation
      }
      stream.respondWithFile('/some/file',
                             { 'content-type': 'text/plain; charset=utf-8' },
                             { statCheck });
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      function statCheck(stat, headers) {
        // Check the stat here...
        stream.respond({ ':status': 304 });
        return false; // Cancel the send operation
      }
      stream.respondWithFile('/some/file',
                             { 'content-type': 'text/plain; charset=utf-8' },
                             { statCheck });
    });
    javascript

The `content-length` header field will be automatically set.

The `offset` and `length` options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests.

The `options.onError` function may also be used to handle all the errors that could happen before the delivery of the file is initiated. The default behavior is to destroy the stream.

When the `options.waitForTrailers` option is set, the `'wantTrailers'` event will be emitted immediately after queuing the last chunk of payload data to be sent. The `http2stream.sendTrailers()` method can then be used to send trailing header fields to the peer.

When `options.waitForTrailers` is set, the `Http2Stream` will not automatically close when the final `DATA` frame is transmitted. User code must call either `http2stream.sendTrailers()` or `http2stream.close()` to close the `Http2Stream`.

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream) => {
      stream.respondWithFile('/some/file',
                             { 'content-type': 'text/plain; charset=utf-8' },
                             { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ ABC: 'some value to send' });
      });
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream) => {
      stream.respondWithFile('/some/file',
                             { 'content-type': 'text/plain; charset=utf-8' },
                             { waitForTrailers: true });
      stream.on('wantTrailers', () => {
        stream.sendTrailers({ ABC: 'some value to send' });
      });
    });
    javascript

#### Class: `Http2Server`#

Added in: v8.4.0

- Extends: `<net.Server>`

Instances of `Http2Server` are created using the `http2.createServer()` function. The `Http2Server` class is not exported directly by the `node:http2` module.

##### Event: `'checkContinue'`#

Added in: v8.5.0

- `request` `<http2.Http2ServerRequest>`
- `response` `<http2.Http2ServerResponse>`

If a `'request'` listener is registered or `http2.createServer()` is supplied a callback function, the `'checkContinue'` event is emitted each time a request with an HTTP `Expect: 100-continue` is received. If this event is not listened for, the server will automatically respond with a status `100 Continue` as appropriate.

Handling this event involves calling `response.writeContinue()` if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the `'request'` event will not be emitted.

##### Event: `'connection'`#

Added in: v8.4.0

- `socket` `<stream.Duplex>`

This event is emitted when a new TCP stream is established. `socket` is typically an object of type `net.Socket`. Usually users will not want to access this event.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any `Duplex` stream can be passed.

##### Event: `'request'`#

Added in: v8.4.0

- `request` `<http2.Http2ServerRequest>`
- `response` `<http2.Http2ServerResponse>`

Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.

##### Event: `'session'`#

Added in: v8.4.0

- `session` `<ServerHttp2Session>`

The `'session'` event is emitted when a new `Http2Session` is created by the `Http2Server`.

##### Event: `'sessionError'`#

Added in: v8.4.0

- `error` `<Error>`
- `session` `<ServerHttp2Session>`

The `'sessionError'` event is emitted when an `'error'` event is emitted by an `Http2Session` object associated with the `Http2Server`.

##### Event: `'stream'`#

Added in: v8.4.0

- `stream` `<Http2Stream>` A reference to the stream
- `headers` `<HTTP/2 Headers Object>` An object describing the headers
- `flags` `<number>` The associated numeric flags
- `rawHeaders` `<HTTP/2 Raw Headers>` An array containing the raw headers

The `'stream'` event is emitted when a `'stream'` event has been emitted by an `Http2Session` associated with the server.

See also `Http2Session`'s `'stream'` event.

    import { createServer, constants } from 'node:http2';
    const {
      HTTP2_HEADER_METHOD,
      HTTP2_HEADER_PATH,
      HTTP2_HEADER_STATUS,
      HTTP2_HEADER_CONTENT_TYPE,
    } = constants;

    const server = createServer();
    server.on('stream', (stream, headers, flags) => {
      const method = headers[HTTP2_HEADER_METHOD];
      const path = headers[HTTP2_HEADER_PATH];
      // ...
      stream.respond({
        [HTTP2_HEADER_STATUS]: 200,
        [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
      });
      stream.write('hello ');
      stream.end('world');
    });
    const http2 = require('node:http2');
    const {
      HTTP2_HEADER_METHOD,
      HTTP2_HEADER_PATH,
      HTTP2_HEADER_STATUS,
      HTTP2_HEADER_CONTENT_TYPE,
    } = http2.constants;

    const server = http2.createServer();
    server.on('stream', (stream, headers, flags) => {
      const method = headers[HTTP2_HEADER_METHOD];
      const path = headers[HTTP2_HEADER_PATH];
      // ...
      stream.respond({
        [HTTP2_HEADER_STATUS]: 200,
        [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
      });
      stream.write('hello ');
      stream.end('world');
    });
    javascript

##### Event: `'timeout'`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

The `'timeout'` event is emitted when there is no activity on the Server for a given number of milliseconds set using `http2server.setTimeout()`. Default: 0 (no timeout)

##### `server.close([callback])`#

Added in: v8.4.0

- `callback` `<Function>`

Stops the server from establishing new sessions and streams.

If `callback` is provided, it is not invoked until all active sessions have been closed, although the server has already stopped allowing new sessions. See `net.Server.close()` for more details.

##### `server[Symbol.asyncDispose]()`#

Added in: v20.4.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `server.close()` and returns a promise that fulfills when the server has closed.

##### `server.setTimeout([msecs][, callback])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

- `msecs` `<number>` Default: 0 (no timeout)
- `callback` `<Function>`
- Returns: `<Http2Server>`

Used to set the timeout value for http2 server requests, and sets a callback function that is called when there is no activity on the `Http2Server` after `msecs` milliseconds.

The given callback is registered as a listener on the `'timeout'` event.

In case if `callback` is not a function, a new `ERR_INVALID_ARG_TYPE` error will be thrown.

##### `server.timeout`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

- Type: `<number>` Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of `0` will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

##### `server.updateSettings([settings])`#

Added in: v15.1.0, v14.17.0

- `settings` `<HTTP/2 Settings Object>`

Used to update the server with the provided settings.

Throws `ERR_HTTP2_INVALID_SETTING_VALUE` for invalid `settings` values.

Throws `ERR_INVALID_ARG_TYPE` for invalid `settings` argument.

#### Class: `Http2SecureServer`#

Added in: v8.4.0

- Extends: `<tls.Server>`

Instances of `Http2SecureServer` are created using the `http2.createSecureServer()` function. The `Http2SecureServer` class is not exported directly by the `node:http2` module.

##### Event: `'checkContinue'`#

Added in: v8.5.0

- `request` `<http2.Http2ServerRequest>`
- `response` `<http2.Http2ServerResponse>`

If a `'request'` listener is registered or `http2.createSecureServer()` is supplied a callback function, the `'checkContinue'` event is emitted each time a request with an HTTP `Expect: 100-continue` is received. If this event is not listened for, the server will automatically respond with a status `100 Continue` as appropriate.

Handling this event involves calling `response.writeContinue()` if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the `'request'` event will not be emitted.

##### Event: `'connection'`#

Added in: v8.4.0

- `socket` `<stream.Duplex>`

This event is emitted when a new TCP stream is established, before the TLS handshake begins. `socket` is typically an object of type `net.Socket`. Usually users will not want to access this event.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any `Duplex` stream can be passed.

##### Event: `'request'`#

Added in: v8.4.0

- `request` `<http2.Http2ServerRequest>`
- `response` `<http2.Http2ServerResponse>`

Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.

##### Event: `'session'`#

Added in: v8.4.0

- `session` `<ServerHttp2Session>`

The `'session'` event is emitted when a new `Http2Session` is created by the `Http2SecureServer`.

##### Event: `'sessionError'`#

Added in: v8.4.0

- `error` `<Error>`
- `session` `<ServerHttp2Session>`

The `'sessionError'` event is emitted when an `'error'` event is emitted by an `Http2Session` object associated with the `Http2SecureServer`.

##### Event: `'stream'`#

Added in: v8.4.0

- `stream` `<Http2Stream>` A reference to the stream
- `headers` `<HTTP/2 Headers Object>` An object describing the headers
- `flags` `<number>` The associated numeric flags
- `rawHeaders` `<HTTP/2 Raw Headers>` An array containing the raw headers

The `'stream'` event is emitted when a `'stream'` event has been emitted by an `Http2Session` associated with the server.

See also `Http2Session`'s `'stream'` event.

    import { createSecureServer, constants } from 'node:http2';
    const {
      HTTP2_HEADER_METHOD,
      HTTP2_HEADER_PATH,
      HTTP2_HEADER_STATUS,
      HTTP2_HEADER_CONTENT_TYPE,
    } = constants;

    const options = getOptionsSomehow();

    const server = createSecureServer(options);
    server.on('stream', (stream, headers, flags) => {
      const method = headers[HTTP2_HEADER_METHOD];
      const path = headers[HTTP2_HEADER_PATH];
      // ...
      stream.respond({
        [HTTP2_HEADER_STATUS]: 200,
        [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
      });
      stream.write('hello ');
      stream.end('world');
    });
    const http2 = require('node:http2');
    const {
      HTTP2_HEADER_METHOD,
      HTTP2_HEADER_PATH,
      HTTP2_HEADER_STATUS,
      HTTP2_HEADER_CONTENT_TYPE,
    } = http2.constants;

    const options = getOptionsSomehow();

    const server = http2.createSecureServer(options);
    server.on('stream', (stream, headers, flags) => {
      const method = headers[HTTP2_HEADER_METHOD];
      const path = headers[HTTP2_HEADER_PATH];
      // ...
      stream.respond({
        [HTTP2_HEADER_STATUS]: 200,
        [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain; charset=utf-8',
      });
      stream.write('hello ');
      stream.end('world');
    });
    javascript

##### Event: `'timeout'`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

The `'timeout'` event is emitted when there is no activity on the Server for a given number of milliseconds set using `http2secureServer.setTimeout()`. Default: 0 (no timeout)

##### Event: `'unknownProtocol'`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v19.0.0 | This event will only be emitted if the client did not transmit an ALPN extension during the TLS handshake.

- `socket` `<stream.Duplex>`

The `'unknownProtocol'` event is emitted when a connecting client fails to negotiate an allowed protocol (i.e. HTTP/2 or HTTP/1.1). The event handler receives the socket for handling. If no listener is registered for this event, the connection is terminated. A timeout may be specified using the `'unknownProtocolTimeout'` option passed to `http2.createSecureServer()`.

In earlier versions of Node.js, this event would be emitted if `allowHTTP1` is `false` and, during the TLS handshake, the client either does not send an ALPN extension or sends an ALPN extension that does not include HTTP/2 (`h2`). Newer versions of Node.js only emit this event if `allowHTTP1` is `false` and the client does not send an ALPN extension. If the client sends an ALPN extension that does not include HTTP/2 (or HTTP/1.1 if `allowHTTP1` is `true`), the TLS handshake will fail and no secure connection will be established.

See the Compatibility API.

##### `server.close([callback])`#

Added in: v8.4.0

- `callback` `<Function>`

Stops the server from establishing new sessions and streams.

If `callback` is provided, it is not invoked until all active sessions have been closed, although the server has already stopped allowing new sessions. See `tls.Server.close()` for more details.

##### `server.setTimeout([msecs][, callback])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `msecs` `<number>` Default: `120000` (2 minutes)
- `callback` `<Function>`
- Returns: `<Http2SecureServer>`

Used to set the timeout value for http2 secure server requests, and sets a callback function that is called when there is no activity on the `Http2SecureServer` after `msecs` milliseconds.

The given callback is registered as a listener on the `'timeout'` event.

In case if `callback` is not a function, a new `ERR_INVALID_ARG_TYPE` error will be thrown.

##### `server.timeout`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

- Type: `<number>` Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of `0` will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

##### `server.updateSettings([settings])`#

Added in: v15.1.0, v14.17.0

- `settings` `<HTTP/2 Settings Object>`

Used to update the server with the provided settings.

Throws `ERR_HTTP2_INVALID_SETTING_VALUE` for invalid `settings` values.

Throws `ERR_INVALID_ARG_TYPE` for invalid `settings` argument.

#### `http2.createServer([options][, onRequestHandler])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v25.7.0 | Added the `strictSingleValueFields` option.
v25.7.0 | Added `http1Options` option. The `Http1IncomingMessage` and `Http1ServerResponse` options are now deprecated.
v23.0.0, v22.10.0 | Added `streamResetBurst` and `streamResetRate`.
v15.10.0, v14.16.0, v12.21.0, v10.24.0 | Added `unknownProtocolTimeout` option with a default of 10000.
v14.4.0, v12.18.0, v10.21.0 | Added `maxSettings` option with a default of 32.
v13.3.0, v12.16.0 | Added `maxSessionRejectedStreams` option with a default of 100.
v13.3.0, v12.16.0 | Added `maxSessionInvalidFrames` option with a default of 1000.
v13.0.0 | The `PADDING_STRATEGY_CALLBACK` has been made equivalent to providing `PADDING_STRATEGY_ALIGNED` and `selectPadding` has been removed.
v12.4.0 | The `options` parameter now supports `net.createServer()` options.
v9.6.0 | Added the `Http1IncomingMessage` and `Http1ServerResponse` option.
v8.9.3 | Added the `maxOutstandingPings` option with a default limit of 10.
v8.9.3 | Added the `maxHeaderListPairs` option with a default limit of 128 header pairs.

- `options` `<Object>`
  - `maxDeflateDynamicTableSize` `<number>` Sets the maximum dynamic table size for deflating header fields. Default: `4Kib`.
  - `maxSettings` `<number>` Sets the maximum number of settings entries per `SETTINGS` frame. The minimum value allowed is `1`. Default: `32`.
  - `maxSessionMemory``<number>` Sets the maximum memory that the `Http2Session` is permitted to use. The value is expressed in terms of number of megabytes, e.g. `1` equal 1 megabyte. The minimum value allowed is `1`. This is a credit based limit, existing `Http2Stream`s may cause this limit to be exceeded, but new `Http2Stream` instances will be rejected while this limit is exceeded. The current number of `Http2Stream` sessions, the current memory use of the header compression tables, header blocks retained by open streams, current data queued to be sent, and unacknowledged `PING` and `SETTINGS` frames are all counted towards the current limit. Default: `10`.
  - `maxHeaderListPairs` `<number>` Sets the maximum number of header entries. This is similar to `server.maxHeadersCount` or `request.maxHeadersCount` in the `node:http` module. The minimum value is `4`. Default: `128`.
  - `maxOutstandingPings` `<number>` Sets the maximum number of outstanding, unacknowledged pings. Default: `10`.
  - `maxSendHeaderBlockLength` `<number>` Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a `'frameError'` event being emitted and the stream being closed and destroyed. While this sets the maximum allowed size to the entire block of headers, `nghttp2` (the internal http2 library) has a limit of `65536` for each decompressed key/value pair.
  - `paddingStrategy` `<number>` The strategy used for determining the amount of padding to use for `HEADERS` and `DATA` frames. Default: `http2.constants.PADDING_STRATEGY_NONE`. Value may be one of:
    - `http2.constants.PADDING_STRATEGY_NONE`: No padding is applied.
    - `http2.constants.PADDING_STRATEGY_MAX`: The maximum amount of padding, determined by the internal implementation, is applied.
    - `http2.constants.PADDING_STRATEGY_ALIGNED`: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
  - `peerMaxConcurrentStreams` `<number>` Sets the maximum number of concurrent streams for the remote peer as if a `SETTINGS` frame had been received. Will be overridden if the remote peer sets its own value for `maxConcurrentStreams`. Default: `100`.
  - `maxSessionInvalidFrames` `<integer>` Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default: `1000`.
  - `maxSessionRejectedStreams` `<integer>` Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with an `NGHTTP2_ENHANCE_YOUR_CALM` error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default: `100`.
  - `settings` `<HTTP/2 Settings Object>` The initial settings to send to the remote peer upon connection.
  - `streamResetBurst` `<number>` and `streamResetRate` `<number>` Sets the rate limit for the incoming stream reset (RST_STREAM frame). Both settings must be set to have any effect, and default to 1000 and 33 respectively.
  - `remoteCustomSettings` `<Array>` The array of integer values determines the settings types, which are included in the `CustomSettings`-property of the received remoteSettings. Please see the `CustomSettings`-property of the `Http2Settings` object for more information, on the allowed setting types.
  - `Http1IncomingMessage` `<http.IncomingMessage>` Specifies the `IncomingMessage` class to used for HTTP/1 fallback. Useful for extending the original `http.IncomingMessage`. Default: `http.IncomingMessage`. Deprecated. Use `http1Options.IncomingMessage` instead. See DEP0202.
  - `Http1ServerResponse` `<http.ServerResponse>` Specifies the `ServerResponse` class to used for HTTP/1 fallback. Useful for extending the original `http.ServerResponse`. Default: `http.ServerResponse`. Deprecated. Use `http1Options.ServerResponse` instead. See DEP0202.
  - `http1Options` `<Object>` An options object for configuring the HTTP/1 fallback when `allowHTTP1` is `true`. These options are passed to the underlying HTTP/1 server. See `http.createServer()` for available options. Among others, the following are supported:
    - `IncomingMessage` `<http.IncomingMessage>` Specifies the `IncomingMessage` class to use for HTTP/1 fallback. Default: `http.IncomingMessage`.
    - `ServerResponse` `<http.ServerResponse>` Specifies the `ServerResponse` class to use for HTTP/1 fallback. Default: `http.ServerResponse`.
    - `keepAliveTimeout` `<number>` The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. Default: `5000`.
  - `Http2ServerRequest` `<http2.Http2ServerRequest>` Specifies the `Http2ServerRequest` class to use. Useful for extending the original `Http2ServerRequest`. Default: `Http2ServerRequest`.
  - `Http2ServerResponse` `<http2.Http2ServerResponse>` Specifies the `Http2ServerResponse` class to use. Useful for extending the original `Http2ServerResponse`. Default: `Http2ServerResponse`.
  - `unknownProtocolTimeout` `<number>` Specifies a timeout in milliseconds that a server should wait when an `'unknownProtocol'` is emitted. If the socket has not been destroyed by that time the server will destroy it. Default: `10000`.
  - `strictFieldWhitespaceValidation` `<boolean>` If `true`, it turns on strict leading and trailing whitespace validation for HTTP/2 header field names and values as per RFC-9113. Default: `true`.
  - `strictSingleValueFields` `<boolean>` If `true`, strict validation is used for headers and trailers defined as having only a single value, such that an error is thrown if multiple values are provided. Default: `true`.
  - `...options` `<Object>` Any `net.createServer()` option can be provided.
- `onRequestHandler` `<Function>` See Compatibility API
- Returns: `<Http2Server>`

Returns a `net.Server` instance that creates and manages `Http2Session` instances.

Since there are no browsers known that support unencrypted HTTP/2, the use of `http2.createSecureServer()` is necessary when communicating with browser clients.

    import { createServer } from 'node:http2';

    // Create an unencrypted HTTP/2 server.
    // Since there are no browsers known that support
    // unencrypted HTTP/2, the use of `createSecureServer()`
    // is necessary when communicating with browser clients.
    const server = createServer();

    server.on('stream', (stream, headers) => {
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8000);
    const http2 = require('node:http2');

    // Create an unencrypted HTTP/2 server.
    // Since there are no browsers known that support
    // unencrypted HTTP/2, the use of `http2.createSecureServer()`
    // is necessary when communicating with browser clients.
    const server = http2.createServer();

    server.on('stream', (stream, headers) => {
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8000);
    javascript

#### `http2.createSecureServer(options[, onRequestHandler])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v25.7.0 | Added the `strictSingleValueFields` option.
v25.7.0 | Added `http1Options` option.
v15.10.0, v14.16.0, v12.21.0, v10.24.0 | Added `unknownProtocolTimeout` option with a default of 10000.
v14.4.0, v12.18.0, v10.21.0 | Added `maxSettings` option with a default of 32.
v13.3.0, v12.16.0 | Added `maxSessionRejectedStreams` option with a default of 100.
v13.3.0, v12.16.0 | Added `maxSessionInvalidFrames` option with a default of 1000.
v13.0.0 | The `PADDING_STRATEGY_CALLBACK` has been made equivalent to providing `PADDING_STRATEGY_ALIGNED` and `selectPadding` has been removed.
v10.12.0 | Added the `origins` option to automatically send an `ORIGIN` frame on `Http2Session` startup.
v8.9.3 | Added the `maxOutstandingPings` option with a default limit of 10.
v8.9.3 | Added the `maxHeaderListPairs` option with a default limit of 128 header pairs.

- `options` `<Object>`
  - `allowHTTP1` `<boolean>` Incoming client connections that do not support HTTP/2 will be downgraded to HTTP/1.x when set to `true`. See the `'unknownProtocol'` event. See ALPN negotiation. Default: `false`.
  - `maxDeflateDynamicTableSize` `<number>` Sets the maximum dynamic table size for deflating header fields. Default: `4Kib`.
  - `maxSettings` `<number>` Sets the maximum number of settings entries per `SETTINGS` frame. The minimum value allowed is `1`. Default: `32`.
  - `maxSessionMemory``<number>` Sets the maximum memory that the `Http2Session` is permitted to use. The value is expressed in terms of number of megabytes, e.g. `1` equal 1 megabyte. The minimum value allowed is `1`. This is a credit based limit, existing `Http2Stream`s may cause this limit to be exceeded, but new `Http2Stream` instances will be rejected while this limit is exceeded. The current number of `Http2Stream` sessions, the current memory use of the header compression tables, header blocks retained by open streams, current data queued to be sent, and unacknowledged `PING` and `SETTINGS` frames are all counted towards the current limit. Default: `10`.
  - `maxHeaderListPairs` `<number>` Sets the maximum number of header entries. This is similar to `server.maxHeadersCount` or `request.maxHeadersCount` in the `node:http` module. The minimum value is `4`. Default: `128`.
  - `maxOutstandingPings` `<number>` Sets the maximum number of outstanding, unacknowledged pings. Default: `10`.
  - `maxSendHeaderBlockLength` `<number>` Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a `'frameError'` event being emitted and the stream being closed and destroyed.
  - `paddingStrategy` `<number>` Strategy used for determining the amount of padding to use for `HEADERS` and `DATA` frames. Default: `http2.constants.PADDING_STRATEGY_NONE`. Value may be one of:
    - `http2.constants.PADDING_STRATEGY_NONE`: No padding is applied.
    - `http2.constants.PADDING_STRATEGY_MAX`: The maximum amount of padding, determined by the internal implementation, is applied.
    - `http2.constants.PADDING_STRATEGY_ALIGNED`: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
  - `peerMaxConcurrentStreams` `<number>` Sets the maximum number of concurrent streams for the remote peer as if a `SETTINGS` frame had been received. Will be overridden if the remote peer sets its own value for `maxConcurrentStreams`. Default: `100`.
  - `maxSessionInvalidFrames` `<integer>` Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default: `1000`.
  - `maxSessionRejectedStreams` `<integer>` Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with an `NGHTTP2_ENHANCE_YOUR_CALM` error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default: `100`.
  - `settings` `<HTTP/2 Settings Object>` The initial settings to send to the remote peer upon connection.
  - `streamResetBurst` `<number>` and `streamResetRate` `<number>` Sets the rate limit for the incoming stream reset (RST_STREAM frame). Both settings must be set to have any effect, and default to 1000 and 33 respectively.
  - `remoteCustomSettings` `<Array>` The array of integer values determines the settings types, which are included in the `customSettings`-property of the received remoteSettings. Please see the `customSettings`-property of the `Http2Settings` object for more information, on the allowed setting types.
  - `...options` `<Object>` Any `tls.createServer()` options can be provided. For servers, the identity options (`pfx` or `key`/`cert`) are usually required.
  - `origins` `<string>`[] An array of origin strings to send within an `ORIGIN` frame immediately following creation of a new server `Http2Session`.
  - `unknownProtocolTimeout` `<number>` Specifies a timeout in milliseconds that a server should wait when an `'unknownProtocol'` event is emitted. If the socket has not been destroyed by that time the server will destroy it. Default: `10000`.
  - `strictFieldWhitespaceValidation` `<boolean>` If `true`, it turns on strict leading and trailing whitespace validation for HTTP/2 header field names and values as per RFC-9113. Default: `true`.
  - `strictSingleValueFields` `<boolean>` If `true`, strict validation is used for headers and trailers defined as having only a single value, such that an error is thrown if multiple values are provided. Default: `true`.
  - `http1Options` `<Object>` An options object for configuring the HTTP/1 fallback when `allowHTTP1` is `true`. These options are passed to the underlying HTTP/1 server. See `http.createServer()` for available options. Among others, the following are supported:
    - `IncomingMessage` `<http.IncomingMessage>` Specifies the `IncomingMessage` class to use for HTTP/1 fallback. Default: `http.IncomingMessage`.
    - `ServerResponse` `<http.ServerResponse>` Specifies the `ServerResponse` class to use for HTTP/1 fallback. Default: `http.ServerResponse`.
    - `keepAliveTimeout` `<number>` The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. Default: `5000`.
- `onRequestHandler` `<Function>` See Compatibility API
- Returns: `<Http2SecureServer>`

Returns a `tls.Server` instance that creates and manages `Http2Session` instances.

    import { createSecureServer } from 'node:http2';
    import { readFileSync } from 'node:fs';

    const options = {
      key: readFileSync('server-key.pem'),
      cert: readFileSync('server-cert.pem'),
    };

    // Create a secure HTTP/2 server
    const server = createSecureServer(options);

    server.on('stream', (stream, headers) => {
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8443);
    const http2 = require('node:http2');
    const fs = require('node:fs');

    const options = {
      key: fs.readFileSync('server-key.pem'),
      cert: fs.readFileSync('server-cert.pem'),
    };

    // Create a secure HTTP/2 server
    const server = http2.createSecureServer(options);

    server.on('stream', (stream, headers) => {
      stream.respond({
        'content-type': 'text/html; charset=utf-8',
        ':status': 200,
      });
      stream.end('<h1>Hello World</h1>');
    });

    server.listen(8443);
    javascript

#### `http2.connect(authority[, options][, listener])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v15.10.0, v14.16.0, v12.21.0, v10.24.0 | Added `unknownProtocolTimeout` option with a default of 10000.
v14.4.0, v12.18.0, v10.21.0 | Added `maxSettings` option with a default of 32.
v13.0.0 | The `PADDING_STRATEGY_CALLBACK` has been made equivalent to providing `PADDING_STRATEGY_ALIGNED` and `selectPadding` has been removed.
v8.9.3 | Added the `maxOutstandingPings` option with a default limit of 10.
v8.9.3 | Added the `maxHeaderListPairs` option with a default limit of 128 header pairs.

- `authority` `<string>` | `<URL>` The remote HTTP/2 server to connect to. This must be in the form of a minimal, valid URL with the `http://` or `https://` prefix, host name, and IP port (if a non-default port is used). Userinfo (user ID and password), path, querystring, and fragment details in the URL will be ignored.
- `options` `<Object>`
  - `maxDeflateDynamicTableSize` `<number>` Sets the maximum dynamic table size for deflating header fields. Default: `4Kib`.
  - `maxSettings` `<number>` Sets the maximum number of settings entries per `SETTINGS` frame. The minimum value allowed is `1`. Default: `32`.
  - `maxSessionMemory``<number>` Sets the maximum memory that the `Http2Session` is permitted to use. The value is expressed in terms of number of megabytes, e.g. `1` equal 1 megabyte. The minimum value allowed is `1`. This is a credit based limit, existing `Http2Stream`s may cause this limit to be exceeded, but new `Http2Stream` instances will be rejected while this limit is exceeded. The current number of `Http2Stream` sessions, the current memory use of the header compression tables, header blocks retained by open streams, current data queued to be sent, and unacknowledged `PING` and `SETTINGS` frames are all counted towards the current limit. Default: `10`.
  - `maxHeaderListPairs` `<number>` Sets the maximum number of header entries. This is similar to `server.maxHeadersCount` or `request.maxHeadersCount` in the `node:http` module. The minimum value is `1`. Default: `128`.
  - `maxOriginSetSize` `<number>` Sets the maximum number of uniq origin the sever can send via ORIGIN frames. Default: `128`.
  - `maxOutstandingPings` `<number>` Sets the maximum number of outstanding, unacknowledged pings. Default: `10`.
  - `maxReservedRemoteStreams` `<number>` Sets the maximum number of reserved push streams the client will accept at any given time. Once the current number of currently reserved push streams exceeds reaches this limit, new push streams sent by the server will be automatically rejected. The minimum allowed value is 0. The maximum allowed value is 232-1. A negative value sets this option to the maximum allowed value. Default: `200`.
  - `maxSendHeaderBlockLength` `<number>` Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a `'frameError'` event being emitted and the stream being closed and destroyed.
  - `paddingStrategy` `<number>` Strategy used for determining the amount of padding to use for `HEADERS` and `DATA` frames. Default: `http2.constants.PADDING_STRATEGY_NONE`. Value may be one of:
    - `http2.constants.PADDING_STRATEGY_NONE`: No padding is applied.
    - `http2.constants.PADDING_STRATEGY_MAX`: The maximum amount of padding, determined by the internal implementation, is applied.
    - `http2.constants.PADDING_STRATEGY_ALIGNED`: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
  - `peerMaxConcurrentStreams` `<number>` Sets the maximum number of concurrent streams for the remote peer as if a `SETTINGS` frame had been received. Will be overridden if the remote peer sets its own value for `maxConcurrentStreams`. Default: `100`.
  - `protocol` `<string>` The protocol to connect with, if not set in the `authority`. Value may be either `'http:'` or `'https:'`. Default: `'https:'`
  - `settings` `<HTTP/2 Settings Object>` The initial settings to send to the remote peer upon connection.
  - `remoteCustomSettings` `<Array>` The array of integer values determines the settings types, which are included in the `CustomSettings`-property of the received remoteSettings. Please see the `CustomSettings`-property of the `Http2Settings` object for more information, on the allowed setting types.
  - `createConnection` `<Function>` An optional callback that receives the `URL` instance passed to `connect` and the `options` object, and returns any `Duplex` stream that is to be used as the connection for this session.
  - `...options` `<Object>` Any `net.connect()` or `tls.connect()` options can be provided.
  - `unknownProtocolTimeout` `<number>` Specifies a timeout in milliseconds that a server should wait when an `'unknownProtocol'` event is emitted. If the socket has not been destroyed by that time the server will destroy it. Default: `10000`.
  - `strictFieldWhitespaceValidation` `<boolean>` If `true`, it turns on strict leading and trailing whitespace validation for HTTP/2 header field names and values as per RFC-9113. Default: `true`.
- `listener` `<Function>` Will be registered as a one-time listener of the `'connect'` event.
- Returns: `<ClientHttp2Session>`

Returns a `ClientHttp2Session` instance.

    import { connect } from 'node:http2';
    const client = connect('https://localhost:1234');

    /* Use the client */

    client.close();
    const http2 = require('node:http2');
    const client = http2.connect('https://localhost:1234');

    /* Use the client */

    client.close();
    javascript

#### `http2.constants`#

Added in: v8.4.0

##### Error codes for `RST_STREAM` and `GOAWAY`#

Value | Name | Constant
--- | --- | ---
`0x00` | No Error | `http2.constants.NGHTTP2_NO_ERROR`
`0x01` | Protocol Error | `http2.constants.NGHTTP2_PROTOCOL_ERROR`
`0x02` | Internal Error | `http2.constants.NGHTTP2_INTERNAL_ERROR`
`0x03` | Flow Control Error | `http2.constants.NGHTTP2_FLOW_CONTROL_ERROR`
`0x04` | Settings Timeout | `http2.constants.NGHTTP2_SETTINGS_TIMEOUT`
`0x05` | Stream Closed | `http2.constants.NGHTTP2_STREAM_CLOSED`
`0x06` | Frame Size Error | `http2.constants.NGHTTP2_FRAME_SIZE_ERROR`
`0x07` | Refused Stream | `http2.constants.NGHTTP2_REFUSED_STREAM`
`0x08` | Cancel | `http2.constants.NGHTTP2_CANCEL`
`0x09` | Compression Error | `http2.constants.NGHTTP2_COMPRESSION_ERROR`
`0x0a` | Connect Error | `http2.constants.NGHTTP2_CONNECT_ERROR`
`0x0b` | Enhance Your Calm | `http2.constants.NGHTTP2_ENHANCE_YOUR_CALM`
`0x0c` | Inadequate Security | `http2.constants.NGHTTP2_INADEQUATE_SECURITY`
`0x0d` | HTTP/1.1 Required | `http2.constants.NGHTTP2_HTTP_1_1_REQUIRED`

The `'timeout'` event is emitted when there is no activity on the Server for a given number of milliseconds set using `http2server.setTimeout()`.

#### `http2.getDefaultSettings()`#

Added in: v8.4.0

- Returns: `<HTTP/2 Settings Object>`

Returns an object containing the default settings for an `Http2Session` instance. This method returns a new object instance every time it is called so instances returned may be safely modified for use.

#### `http2.getPackedSettings([settings])`#

Added in: v8.4.0

- `settings` `<HTTP/2 Settings Object>`
- Returns: `<Buffer>`

Returns a `Buffer` instance containing serialized representation of the given HTTP/2 settings as specified in the HTTP/2 specification. This is intended for use with the `HTTP2-Settings` header field.

    import { getPackedSettings } from 'node:http2';

    const packed = getPackedSettings({ enablePush: false });

    console.log(packed.toString('base64'));
    // Prints: AAIAAAAA
    const http2 = require('node:http2');

    const packed = http2.getPackedSettings({ enablePush: false });

    console.log(packed.toString('base64'));
    // Prints: AAIAAAAA
    javascript

#### `http2.getUnpackedSettings(buf)`#

Added in: v8.4.0

- `buf` `<Buffer>` | `<TypedArray>` The packed settings.
- Returns: `<HTTP/2 Settings Object>`

Returns a HTTP/2 Settings Object containing the deserialized settings from the given `Buffer` as generated by `http2.getPackedSettings()`.

#### `http2.performServerHandshake(socket[, options])`#

Added in: v21.7.0, v20.12.0

- `socket` `<stream.Duplex>`
- `options` `<Object>` Any `http2.createServer()` option can be provided.
- Returns: `<ServerHttp2Session>`

Create an HTTP/2 server session from an existing socket.

#### `http2.sensitiveHeaders`#

Added in: v15.0.0, v14.18.0

- Type: `<symbol>`

This symbol can be set as a property on the HTTP/2 headers object with an array value in order to provide a list of headers considered sensitive. See Sensitive headers for more details.

#### Headers object#

Headers are represented as own-properties on JavaScript objects. The property keys will be serialized to lower-case. Property values should be strings (if they are not they will be coerced to strings) or an `Array` of strings (in order to send more than one value per header field).

    const headers = {
      ':status': '200',
      'content-type': 'text-plain',
      'ABC': ['has', 'more', 'than', 'one', 'value'],
    };

    stream.respond(headers);
    js

Header objects passed to callback functions will have a `null` prototype. This means that normal JavaScript object methods such as `Object.prototype.toString()` and `Object.prototype.hasOwnProperty()` will not work.

For incoming headers:

- The `:status` header is converted to `number`.
- Duplicates of `:status`, `:method`, `:authority`, `:scheme`, `:path`, `:protocol`, `age`, `authorization`, `access-control-allow-credentials`, `access-control-max-age`, `access-control-request-method`, `content-encoding`, `content-language`, `content-length`, `content-location`, `content-md5`, `content-range`, `content-type`, `date`, `dnt`, `etag`, `expires`, `from`, `host`, `if-match`, `if-modified-since`, `if-none-match`, `if-range`, `if-unmodified-since`, `last-modified`, `location`, `max-forwards`, `proxy-authorization`, `range`, `referer`,`retry-after`, `tk`, `upgrade-insecure-requests`, `user-agent` or `x-content-type-options` are discarded.
- `set-cookie` is always an array. Duplicates are added to the array.
- For duplicate `cookie` headers, the values are joined together with '; '.
- For all other headers, the values are joined together with ', '.

    import { createServer } from 'node:http2';
    const server = createServer();
    server.on('stream', (stream, headers) => {
      console.log(headers[':path']);
      console.log(headers.ABC);
    });
    const http2 = require('node:http2');
    const server = http2.createServer();
    server.on('stream', (stream, headers) => {
      console.log(headers[':path']);
      console.log(headers.ABC);
    });
    javascript

##### Raw headers#

In some APIs, in addition to object format, headers can also be passed or accessed as a raw flat array, preserving details of ordering and duplicate keys to match the raw transmission format.

In this format the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. Duplicate headers are not merged and so each key-value pair will appear separately.

This can be useful for cases such as proxies, where existing headers should be exactly forwarded as received, or as a performance optimization when the headers are already available in raw format.

    const rawHeaders = [
      ':status',
      '404',
      'content-type',
      'text/plain',
    ];

    stream.respond(rawHeaders);
    js

##### Sensitive headers#

HTTP2 headers can be marked as sensitive, which means that the HTTP/2 header compression algorithm will never index them. This can make sense for header values with low entropy and that may be considered valuable to an attacker, for example `Cookie` or `Authorization`. To achieve this, add the header name to the `[http2.sensitiveHeaders]` property as an array:

    const headers = {
      ':status': '200',
      'content-type': 'text-plain',
      'cookie': 'some-cookie',
      'other-sensitive-header': 'very secret data',
      [http2.sensitiveHeaders]: ['cookie', 'other-sensitive-header'],
    };

    stream.respond(headers);
    js

For some headers, such as `Authorization` and short `Cookie` headers, this flag is set automatically.

This property is also set for received headers. It will contain the names of all headers marked as sensitive, including ones marked that way automatically.

For raw headers, this should still be set as a property on the array, like `rawHeadersArray[http2.sensitiveHeaders] = ['cookie']`, not as a separate key and value pair within the array itself.

#### Settings object#

Added in: v8.4.0
History

Version | Changes
--- | ---
v12.12.0 | The `maxConcurrentStreams` setting is stricter.
v8.9.3 | The `maxHeaderListSize` setting is now strictly enforced.

The `http2.getDefaultSettings()`, `http2.getPackedSettings()`, `http2.createServer()`, `http2.createSecureServer()`, `http2session.settings()`, `http2session.localSettings`, and `http2session.remoteSettings` APIs either return or receive as input an object that defines configuration settings for an `Http2Session` object. These objects are ordinary JavaScript objects containing the following properties.

- `headerTableSize` `<number>` Specifies the maximum number of bytes used for header compression. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: `4096`.
- `enablePush` `<boolean>` Specifies `true` if HTTP/2 Push Streams are to be permitted on the `Http2Session` instances. Default: `true`.
- `initialWindowSize` `<number>` Specifies the sender's initial window size in bytes for stream-level flow control. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: `65535`.
- `maxFrameSize` `<number>` Specifies the size in bytes of the largest frame payload. The minimum allowed value is 16,384. The maximum allowed value is 224-1. Default: `16384`.
- `maxConcurrentStreams` `<number>` Specifies the maximum number of concurrent streams permitted on an `Http2Session`. There is no default value which implies, at least theoretically, 232-1 streams may be open concurrently at any given time in an `Http2Session`. The minimum value is 0. The maximum allowed value is 232-1. Default: `4294967295`.
- `maxHeaderListSize` `<number>` Specifies the maximum size (uncompressed octets) of header list that will be accepted. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: `65535`.
- `maxHeaderSize` `<number>` Alias for `maxHeaderListSize`.
- `enableConnectProtocol``<boolean>` Specifies `true` if the "Extended Connect Protocol" defined by RFC 8441 is to be enabled. This setting is only meaningful if sent by the server. Once the `enableConnectProtocol` setting has been enabled for a given `Http2Session`, it cannot be disabled. Default: `false`.
- `customSettings` `<Object>` Specifies additional settings, yet not implemented in node and the underlying libraries. The key of the object defines the numeric value of the settings type (as defined in the "HTTP/2 SETTINGS" registry established by [RFC 7540]) and the values the actual numeric value of the settings. The settings type has to be an integer in the range from 1 to 2^16-1. It should not be a settings type already handled by node, i.e. currently it should be greater than 6, although it is not an error. The values need to be unsigned integers in the range from 0 to 2^32-1. Currently, a maximum of up 10 custom settings is supported. It is only supported for sending SETTINGS, or for receiving settings values specified in the `remoteCustomSettings` options of the server or client object. Do not mix the `customSettings`-mechanism for a settings id with interfaces for the natively handled settings, in case a setting becomes natively supported in a future node version.

All additional properties on the settings object are ignored.

#### Error handling#

There are several types of error conditions that may arise when using the `node:http2` module:

Validation errors occur when an incorrect argument, option, or setting value is passed in. These will always be reported by a synchronous `throw`.

State errors occur when an action is attempted at an incorrect time (for instance, attempting to send data on a stream after it has closed). These will be reported using either a synchronous `throw` or via an `'error'` event on the `Http2Stream`, `Http2Session` or HTTP/2 Server objects, depending on where and when the error occurs.

Internal errors occur when an HTTP/2 session fails unexpectedly. These will be reported via an `'error'` event on the `Http2Session` or HTTP/2 Server objects.

Protocol errors occur when various HTTP/2 protocol constraints are violated. These will be reported using either a synchronous `throw` or via an `'error'` event on the `Http2Stream`, `Http2Session` or HTTP/2 Server objects, depending on where and when the error occurs.

#### Invalid character handling in header names and values#

The HTTP/2 implementation applies stricter handling of invalid characters in HTTP header names and values than the HTTP/1 implementation.

Header field names are case-insensitive and are transmitted over the wire strictly as lower-case strings. The API provided by Node.js allows header names to be set as mixed-case strings (e.g. `Content-Type`) but will convert those to lower-case (e.g. `content-type`) upon transmission.

Header field-names must only contain one or more of the following ASCII characters: `a`-`z`, `A`-`Z`, `0`-`9`, `!`, `#`, `$`, `%`, `&`, `'`, `*`, `+`, `-`, `.`, `^`, `_`, ``` (backtick), `|`, and `~`.

Using invalid characters within an HTTP header field name will cause the stream to be closed with a protocol error being reported.

Header field values are handled with more leniency but should not contain new-line or carriage return characters and should be limited to US-ASCII characters, per the requirements of the HTTP specification.

#### Push streams on the client#

To receive pushed streams on the client, set a listener for the `'stream'` event on the `ClientHttp2Session`:

    import { connect } from 'node:http2';

    const client = connect('http://localhost');

    client.on('stream', (pushedStream, requestHeaders) => {
      pushedStream.on('push', (responseHeaders) => {
        // Process response headers
      });
      pushedStream.on('data', (chunk) => { /* handle pushed data */ });
    });

    const req = client.request({ ':path': '/' });
    const http2 = require('node:http2');

    const client = http2.connect('http://localhost');

    client.on('stream', (pushedStream, requestHeaders) => {
      pushedStream.on('push', (responseHeaders) => {
        // Process response headers
      });
      pushedStream.on('data', (chunk) => { /* handle pushed data */ });
    });

    const req = client.request({ ':path': '/' });
    javascript

#### Supporting the `CONNECT` method#

The `CONNECT` method is used to allow an HTTP/2 server to be used as a proxy for TCP/IP connections.

A simple TCP Server:

    import { createServer } from 'node:net';

    const server = createServer((socket) => {
      let name = '';
      socket.setEncoding('utf8');
      socket.on('data', (chunk) => name += chunk);
      socket.on('end', () => socket.end(`hello ${name}`));
    });

    server.listen(8000);
    const net = require('node:net');

    const server = net.createServer((socket) => {
      let name = '';
      socket.setEncoding('utf8');
      socket.on('data', (chunk) => name += chunk);
      socket.on('end', () => socket.end(`hello ${name}`));
    });

    server.listen(8000);
    javascript

An HTTP/2 CONNECT proxy:

    import { createServer, constants } from 'node:http2';
    const { NGHTTP2_REFUSED_STREAM, NGHTTP2_CONNECT_ERROR } = constants;
    import { connect } from 'node:net';

    const proxy = createServer();
    proxy.on('stream', (stream, headers) => {
      if (headers[':method'] !== 'CONNECT') {
        // Only accept CONNECT requests
        stream.close(NGHTTP2_REFUSED_STREAM);
        return;
      }
      const auth = new URL(`tcp://${headers[':authority']}`);
      // It's a very good idea to verify that hostname and port are
      // things this proxy should be connecting to.
      const socket = connect(auth.port, auth.hostname, () => {
        stream.respond();
        socket.pipe(stream);
        stream.pipe(socket);
      });
      socket.on('error', (error) => {
        stream.close(NGHTTP2_CONNECT_ERROR);
      });
    });

    proxy.listen(8001);
    const http2 = require('node:http2');
    const { NGHTTP2_REFUSED_STREAM } = http2.constants;
    const net = require('node:net');

    const proxy = http2.createServer();
    proxy.on('stream', (stream, headers) => {
      if (headers[':method'] !== 'CONNECT') {
        // Only accept CONNECT requests
        stream.close(NGHTTP2_REFUSED_STREAM);
        return;
      }
      const auth = new URL(`tcp://${headers[':authority']}`);
      // It's a very good idea to verify that hostname and port are
      // things this proxy should be connecting to.
      const socket = net.connect(auth.port, auth.hostname, () => {
        stream.respond();
        socket.pipe(stream);
        stream.pipe(socket);
      });
      socket.on('error', (error) => {
        stream.close(http2.constants.NGHTTP2_CONNECT_ERROR);
      });
    });

    proxy.listen(8001);
    javascript

An HTTP/2 CONNECT client:

    import { connect, constants } from 'node:http2';

    const client = connect('http://localhost:8001');

    // Must not specify the ':path' and ':scheme' headers
    // for CONNECT requests or an error will be thrown.
    const req = client.request({
      ':method': 'CONNECT',
      ':authority': 'localhost:8000',
    });

    req.on('response', (headers) => {
      console.log(headers[constants.HTTP2_HEADER_STATUS]);
    });
    let data = '';
    req.setEncoding('utf8');
    req.on('data', (chunk) => data += chunk);
    req.on('end', () => {
      console.log(`The server says: ${data}`);
      client.close();
    });
    req.end('Jane');
    const http2 = require('node:http2');

    const client = http2.connect('http://localhost:8001');

    // Must not specify the ':path' and ':scheme' headers
    // for CONNECT requests or an error will be thrown.
    const req = client.request({
      ':method': 'CONNECT',
      ':authority': 'localhost:8000',
    });

    req.on('response', (headers) => {
      console.log(headers[http2.constants.HTTP2_HEADER_STATUS]);
    });
    let data = '';
    req.setEncoding('utf8');
    req.on('data', (chunk) => data += chunk);
    req.on('end', () => {
      console.log(`The server says: ${data}`);
      client.close();
    });
    req.end('Jane');
    javascript

#### The extended `CONNECT` protocol#

RFC 8441 defines an "Extended CONNECT Protocol" extension to HTTP/2 that may be used to bootstrap the use of an `Http2Stream` using the `CONNECT` method as a tunnel for other communication protocols (such as WebSockets).

The use of the Extended CONNECT Protocol is enabled by HTTP/2 servers by using the `enableConnectProtocol` setting:

    import { createServer } from 'node:http2';
    const settings = { enableConnectProtocol: true };
    const server = createServer({ settings });
    const http2 = require('node:http2');
    const settings = { enableConnectProtocol: true };
    const server = http2.createServer({ settings });
    javascript

Once the client receives the `SETTINGS` frame from the server indicating that the extended CONNECT may be used, it may send `CONNECT` requests that use the `':protocol'` HTTP/2 pseudo-header:

    import { connect } from 'node:http2';
    const client = connect('http://localhost:8080');
    client.on('remoteSettings', (settings) => {
      if (settings.enableConnectProtocol) {
        const req = client.request({ ':method': 'CONNECT', ':protocol': 'foo' });
        // ...
      }
    });
    const http2 = require('node:http2');
    const client = http2.connect('http://localhost:8080');
    client.on('remoteSettings', (settings) => {
      if (settings.enableConnectProtocol) {
        const req = client.request({ ':method': 'CONNECT', ':protocol': 'foo' });
        // ...
      }
    });
    javascript

### Compatibility API#

The Compatibility API has the goal of providing a similar developer experience of HTTP/1 when using HTTP/2, making it possible to develop applications that support both HTTP/1 and HTTP/2. This API targets only the public API of the HTTP/1. However many modules use internal methods or state, and those are not supported as it is a completely different implementation.

The following example creates an HTTP/2 server using the compatibility API:

    import { createServer } from 'node:http2';
    const server = createServer((req, res) => {
      res.setHeader('Content-Type', 'text/html');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
      res.end('ok');
    });
    const http2 = require('node:http2');
    const server = http2.createServer((req, res) => {
      res.setHeader('Content-Type', 'text/html');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
      res.end('ok');
    });
    javascript

In order to create a mixed HTTPS and HTTP/2 server, refer to the ALPN negotiation section. Upgrading from non-tls HTTP/1 servers is not supported.

The HTTP/2 compatibility API is composed of `Http2ServerRequest` and `Http2ServerResponse`. They aim at API compatibility with HTTP/1, but they do not hide the differences between the protocols. As an example, the status message for HTTP codes is ignored.

#### ALPN negotiation#

ALPN negotiation allows supporting both HTTPS and HTTP/2 over the same socket. The `req` and `res` objects can be either HTTP/1 or HTTP/2, and an application must restrict itself to the public API of HTTP/1, and detect if it is possible to use the more advanced features of HTTP/2.

The following example creates a server that supports both protocols:

    import { createSecureServer } from 'node:http2';
    import { readFileSync } from 'node:fs';

    const cert = readFileSync('./cert.pem');
    const key = readFileSync('./key.pem');

    const server = createSecureServer(
      { cert, key, allowHTTP1: true },
      onRequest,
    ).listen(8000);

    function onRequest(req, res) {
      // Detects if it is an HTTPS request or HTTP/2
      const { socket: { alpnProtocol } } = req.httpVersion === '2.0' ?
        req.stream.session : req;
      res.writeHead(200, { 'content-type': 'application/json' });
      res.end(JSON.stringify({
        alpnProtocol,
        httpVersion: req.httpVersion,
      }));
    }
    const { createSecureServer } = require('node:http2');
    const { readFileSync } = require('node:fs');

    const cert = readFileSync('./cert.pem');
    const key = readFileSync('./key.pem');

    const server = createSecureServer(
      { cert, key, allowHTTP1: true },
      onRequest,
    ).listen(4443);

    function onRequest(req, res) {
      // Detects if it is an HTTPS request or HTTP/2
      const { socket: { alpnProtocol } } = req.httpVersion === '2.0' ?
        req.stream.session : req;
      res.writeHead(200, { 'content-type': 'application/json' });
      res.end(JSON.stringify({
        alpnProtocol,
        httpVersion: req.httpVersion,
      }));
    }
    javascript

The `'request'` event works identically on both HTTPS and HTTP/2.

#### Class: `http2.Http2ServerRequest`#

Added in: v8.4.0

- Extends: `<stream.Readable>`

A `Http2ServerRequest` object is created by `http2.Server` or `http2.SecureServer` and passed as the first argument to the `'request'` event. It may be used to access a request status, headers, and data.

##### Event: `'aborted'`#

Added in: v8.4.0

The `'aborted'` event is emitted whenever a `Http2ServerRequest` instance is abnormally aborted in mid-communication.

The `'aborted'` event will only be emitted if the `Http2ServerRequest` writable side has not been ended.

##### Event: `'close'`#

Added in: v8.4.0

Indicates that the underlying `Http2Stream` was closed. Just like `'end'`, this event occurs only once per response.

##### `request.aborted`#

Added in: v10.1.0

- Type: `<boolean>`

The `request.aborted` property will be `true` if the request has been aborted.

##### `request.authority`#

Added in: v8.4.0

- Type: `<string>`

The request authority pseudo header field. Because HTTP/2 allows requests to set either `:authority` or `host`, this value is derived from `req.headers[':authority']` if present. Otherwise, it is derived from `req.headers['host']`.

##### `request.complete`#

Added in: v12.10.0

- Type: `<boolean>`

The `request.complete` property will be `true` if the request has been completed, aborted, or destroyed.

##### `request.connection`#

Added in: v8.4.0Deprecated in: v13.0.0

Stability: 0 - Deprecated. Use `request.socket`.

- Type: `<net.Socket>` | `<tls.TLSSocket>`

See `request.socket`.

##### `request.destroy([error])`#

Added in: v8.4.0

- `error` `<Error>`

Calls `destroy()` on the `Http2Stream` that received the `Http2ServerRequest`. If `error` is provided, an `'error'` event is emitted and `error` is passed as an argument to any listeners on the event.

It does nothing if the stream was already destroyed.

##### `request.headers`#

Added in: v8.4.0

- Type: `<Object>`

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.

    // Prints something like:
    //
    // { 'user-agent': 'curl/7.22.0',
    //   host: '127.0.0.1:8000',
    //   accept: '*/*' }
    console.log(request.headers);
    js

See HTTP/2 Headers Object.

In HTTP/2, the request path, host name, protocol, and method are represented as special headers prefixed with the `:` character (e.g. `':path'`). These special headers will be included in the `request.headers` object. Care must be taken not to inadvertently modify these special headers or errors may occur. For instance, removing all headers from the request will cause errors to occur:

    removeAllHeaders(request.headers);
    assert(request.url);   // Fails because the :path header has been removed
    js

##### `request.httpVersion`#

Added in: v8.4.0

- Type: `<string>`

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Returns `'2.0'`.

Also `message.httpVersionMajor` is the first integer and `message.httpVersionMinor` is the second.

##### `request.method`#

Added in: v8.4.0

- Type: `<string>`

The request method as a string. Read-only. Examples: `'GET'`, `'DELETE'`.

##### `request.rawHeaders`#

Added in: v8.4.0

- Type: `<HTTP/2 Raw Headers>`

The raw request/response headers list exactly as they were received.

    // Prints something like:
    //
    // [ 'user-agent',
    //   'this is invalid because there can be only one',
    //   'User-Agent',
    //   'curl/7.22.0',
    //   'Host',
    //   '127.0.0.1:8000',
    //   'ACCEPT',
    //   '*/*' ]
    console.log(request.rawHeaders);
    js

##### `request.rawTrailers`#

Added in: v8.4.0

- Type: `<string>`[]

The raw request/response trailer keys and values exactly as they were received. Only populated at the `'end'` event.

##### `request.scheme`#

Added in: v8.4.0

- Type: `<string>`

The request scheme pseudo header field indicating the scheme portion of the target URL.

##### `request.setTimeout(msecs, callback)`#

Added in: v8.4.0

- `msecs` `<number>`
- `callback` `<Function>`
- Returns: `<http2.Http2ServerRequest>`

Sets the `Http2Stream`'s timeout value to `msecs`. If a callback is provided, then it is added as a listener on the `'timeout'` event on the response object.

If no `'timeout'` listener is added to the request, the response, or the server, then `Http2Stream`s are destroyed when they time out. If a handler is assigned to the request, the response, or the server's `'timeout'` events, timed out sockets must be handled explicitly.

##### `request.socket`#

Added in: v8.4.0

- Type: `<net.Socket>` | `<tls.TLSSocket>`

Returns a `Proxy` object that acts as a `net.Socket` (or `tls.TLSSocket`) but applies getters, setters, and methods based on HTTP/2 logic.

`destroyed`, `readable`, and `writable` properties will be retrieved from and set on `request.stream`.

`destroy`, `emit`, `end`, `on` and `once` methods will be called on `request.stream`.

`setTimeout` method will be called on `request.stream.session`.

`pause`, `read`, `resume`, and `write` will throw an error with code `ERR_HTTP2_NO_SOCKET_MANIPULATION`. See `Http2Session` and Sockets for more information.

All other interactions will be routed directly to the socket. With TLS support, use `request.socket.getPeerCertificate()` to obtain the client's authentication details.

##### `request.stream`#

Added in: v8.4.0

- Type: `<Http2Stream>`

The `Http2Stream` object backing the request.

##### `request.trailers`#

Added in: v8.4.0

- Type: `<Object>`

The request/response trailers object. Only populated at the `'end'` event.

##### `request.url`#

Added in: v8.4.0

- Type: `<string>`

Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:

    GET /status?name=ryan HTTP/1.1
    Accept: text/plain
    http

Then `request.url` will be:

    "/status?name=ryan"
    json

To parse the url into its parts, `new URL()` can be used:

    $ node
    > new URL('/status?name=ryan', 'http://example.com')
    URL {
      href: 'http://example.com/status?name=ryan',
      origin: 'http://example.com',
      protocol: 'http:',
      username: '',
      password: '',
      host: 'example.com',
      hostname: 'example.com',
      port: '',
      pathname: '/status',
      search: '?name=ryan',
      searchParams: URLSearchParams { 'name' => 'ryan' },
      hash: ''
    }
    console

#### Class: `http2.Http2ServerResponse`#

Added in: v8.4.0

- Extends: `<Stream>`

This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the `'request'` event.

##### Event: `'close'`#

Added in: v8.4.0

Indicates that the underlying `Http2Stream` was terminated before `response.end()` was called or able to flush.

##### Event: `'finish'`#

Added in: v8.4.0

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the HTTP/2 multiplexing for transmission over the network. It does not imply that the client has received anything yet.

After this event, no more events will be emitted on the response object.

##### `response.addTrailers(headers)`#

Added in: v8.4.0

- `headers` `<Object>`

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

##### `response.appendHeader(name, value)`#

Added in: v21.7.0, v20.12.0

- `name` `<string>`
- `value` `<string>` | `<string>`[]

Append a single header value to the header object.

If the value is an array, this is equivalent to calling this method multiple times.

If there were no previous values for the header, this is equivalent to calling `response.setHeader()`.

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

    // Returns headers including "set-cookie: a" and "set-cookie: b"
    const server = http2.createServer((req, res) => {
      res.setHeader('set-cookie', 'a');
      res.appendHeader('set-cookie', 'b');
      res.writeHead(200);
      res.end('ok');
    });
    js

##### `response.connection`#

Added in: v8.4.0Deprecated in: v13.0.0

Stability: 0 - Deprecated. Use `response.socket`.

- Type: `<net.Socket>` | `<tls.TLSSocket>`

See `response.socket`.

##### `response.createPushResponse(headers, callback)`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `headers` `<HTTP/2 Headers Object>` An object describing the headers
- `callback` `<Function>` Called once `http2stream.pushStream()` is finished, or either when the attempt to create the pushed `Http2Stream` has failed or has been rejected, or the state of `Http2ServerRequest` is closed prior to calling the `http2stream.pushStream()` method
  - `err` `<Error>`
  - `res` `<http2.Http2ServerResponse>` The newly-created `Http2ServerResponse` object

Call `http2stream.pushStream()` with the given headers, and wrap the given `Http2Stream` on a newly created `Http2ServerResponse` as the callback parameter if successful. When `Http2ServerRequest` is closed, the callback is called with an error `ERR_HTTP2_INVALID_STREAM`.

##### `response.end([data[, encoding]][, callback])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v10.0.0 | This method now returns a reference to `ServerResponse`.

- `data` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>`
- `callback` `<Function>`
- Returns: `<this>`

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, `response.end()`, MUST be called on each response.

If `data` is specified, it is equivalent to calling `response.write(data, encoding)` followed by `response.end(callback)`.

If `callback` is specified, it will be called when the response stream is finished.

##### `response.finished`#

Added in: v8.4.0Deprecated in: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use `response.writableEnded`.

- Type: `<boolean>`

Boolean value that indicates whether the response has completed. Starts as `false`. After `response.end()` executes, the value will be `true`.

##### `response.getHeader(name)`#

Added in: v8.4.0

- `name` `<string>`
- Returns: `<string>`

Reads out a header that has already been queued but not sent to the client. The name is case-insensitive.

    const contentType = response.getHeader('content-type');
    js

##### `response.getHeaderNames()`#

Added in: v8.4.0

- Returns: `<string>`[]

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

    response.setHeader('Foo', 'bar');
    response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

    const headerNames = response.getHeaderNames();
    // headerNames === ['foo', 'set-cookie']
    js

##### `response.getHeaders()`#

Added in: v8.4.0

- Returns: `<Object>`

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the `response.getHeaders()` method does not prototypically inherit from the JavaScript `Object`. This means that typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others are not defined and will not work.

    response.setHeader('Foo', 'bar');
    response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

    const headers = response.getHeaders();
    // headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
    js

##### `response.hasHeader(name)`#

Added in: v8.4.0

- `name` `<string>`
- Returns: `<boolean>`

Returns `true` if the header identified by `name` is currently set in the outgoing headers. The header name matching is case-insensitive.

    const hasContentType = response.hasHeader('content-type');
    js

##### `response.headersSent`#

Added in: v8.4.0

- Type: `<boolean>`

True if headers were sent, false otherwise (read-only).

##### `response.removeHeader(name)`#

Added in: v8.4.0

- `name` `<string>`

Removes a header that has been queued for implicit sending.

    response.removeHeader('Content-Encoding');
    js

##### `response.req`#

Added in: v15.7.0

- Type: `<http2.Http2ServerRequest>`

A reference to the original HTTP2 `request` object.

##### `response.sendDate`#

Added in: v8.4.0

- Type: `<boolean>`

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

##### `response.setHeader(name, value)`#

Added in: v8.4.0

- `name` `<string>`
- `value` `<string>` | `<string>`[]

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name.

    response.setHeader('Content-Type', 'text/html; charset=utf-8');
    js

or

    response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
    js

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

When headers have been set with `response.setHeader()`, they will be merged with any headers passed to `response.writeHead()`, with the headers passed to `response.writeHead()` given precedence.

    // Returns content-type = text/plain
    const server = http2.createServer((req, res) => {
      res.setHeader('Content-Type', 'text/html; charset=utf-8');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
      res.end('ok');
    });
    js

##### `response.setTimeout(msecs[, callback])`#

Added in: v8.4.0

- `msecs` `<number>`
- `callback` `<Function>`
- Returns: `<http2.Http2ServerResponse>`

Sets the `Http2Stream`'s timeout value to `msecs`. If a callback is provided, then it is added as a listener on the `'timeout'` event on the response object.

If no `'timeout'` listener is added to the request, the response, or the server, then `Http2Stream`s are destroyed when they time out. If a handler is assigned to the request, the response, or the server's `'timeout'` events, timed out sockets must be handled explicitly.

##### `response.socket`#

Added in: v8.4.0

- Type: `<net.Socket>` | `<tls.TLSSocket>`

Returns a `Proxy` object that acts as a `net.Socket` (or `tls.TLSSocket`) but applies getters, setters, and methods based on HTTP/2 logic.

`destroyed`, `readable`, and `writable` properties will be retrieved from and set on `response.stream`.

`destroy`, `emit`, `end`, `on` and `once` methods will be called on `response.stream`.

`setTimeout` method will be called on `response.stream.session`.

`pause`, `read`, `resume`, and `write` will throw an error with code `ERR_HTTP2_NO_SOCKET_MANIPULATION`. See `Http2Session` and Sockets for more information.

All other interactions will be routed directly to the socket.

    import { createServer } from 'node:http2';
    const server = createServer((req, res) => {
      const ip = req.socket.remoteAddress;
      const port = req.socket.remotePort;
      res.end(`Your IP address is ${ip} and your source port is ${port}.`);
    }).listen(3000);
    const http2 = require('node:http2');
    const server = http2.createServer((req, res) => {
      const ip = req.socket.remoteAddress;
      const port = req.socket.remotePort;
      res.end(`Your IP address is ${ip} and your source port is ${port}.`);
    }).listen(3000);
    javascript

##### `response.statusCode`#

Added in: v8.4.0

- Type: `<number>`

When using implicit headers (not calling `response.writeHead()` explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

    response.statusCode = 404;
    js

After response header was sent to the client, this property indicates the status code which was sent out.

##### `response.statusMessage`#

Added in: v8.4.0

- Type: `<string>`

Status message is not supported by HTTP/2 (RFC 7540 8.1.2.4). It returns an empty string.

##### `response.stream`#

Added in: v8.4.0

- Type: `<Http2Stream>`

The `Http2Stream` object backing the response.

##### `response.writableEnded`#

Added in: v12.9.0

- Type: `<boolean>`

Is `true` after `response.end()` has been called. This property does not indicate whether the data has been flushed, for this use `writable.writableFinished` instead.

##### `response.write(chunk[, encoding][, callback])`#

Added in: v8.4.0

- `chunk` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>`
- `callback` `<Function>`
- Returns: `<boolean>`

If this method is called and `response.writeHead()` has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

In the `node:http` module, the response body is omitted when the request is a HEAD request. Similarly, the `204` and `304` responses must not include a message body.

`chunk` can be a string or a buffer. If `chunk` is a string, the second parameter specifies how to encode it into a byte stream. By default the `encoding` is `'utf8'`. `callback` will be called when this chunk of data is flushed.

This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time `response.write()` is called, it will send the buffered header information and the first chunk of the body to the client. The second time `response.write()` is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body.

Returns `true` if the entire data was flushed successfully to the kernel buffer. Returns `false` if all or part of the data was queued in user memory. `'drain'` will be emitted when the buffer is free again.

##### `response.writeContinue()`#

Added in: v8.4.0

Sends a status `100 Continue` to the client, indicating that the request body should be sent. See the `'checkContinue'` event on `Http2Server` and `Http2SecureServer`.

##### `response.writeEarlyHints(hints)`#

Added in: v18.11.0

- `hints` `<Object>`

Sends a status `103 Early Hints` to the client with a Link header, indicating that the user agent can preload/preconnect the linked resources. The `hints` is an object containing the values of headers to be sent with early hints message.

Example

    const earlyHintsLink = '</styles.css>; rel=preload; as=style';
    response.writeEarlyHints({
      'link': earlyHintsLink,
    });

    const earlyHintsLinks = [
      '</styles.css>; rel=preload; as=style',
      '</scripts.js>; rel=preload; as=script',
    ];
    response.writeEarlyHints({
      'link': earlyHintsLinks,
    });
    js

##### `response.writeInformation(statusCode[, headers])`#

Added in: v26.2.0

- `statusCode` `<number>` An HTTP 1xx informational status code, between `100` and `199` inclusive, excluding `101` (Switching Protocols) which is not allowed in HTTP/2.
- `headers` `<Object>` An optional object of headers to send with the informational response.

Sends an arbitrary HTTP 1xx informational response, equivalent in HTTP/2 to a `HEADERS` frame whose `:status` pseudo-header is a 1xx code. May be called multiple times before the final response. After the final response headers have been sent, this method is a no-op and returns `false`.

This is the generic equivalent of `response.writeContinue()` and `response.writeEarlyHints()`.

    response.writeInformation(110, { 'X-Progress': '50%' });
    js

##### `response.writeHead(statusCode[, statusMessage][, headers])`#

Added in: v8.4.0
History

Version | Changes
--- | ---
v11.10.0, v10.17.0 | Return `this` from `writeHead()` to allow chaining with `end()`.

- `statusCode` `<number>`
- `statusMessage` `<string>`
- `headers` `<HTTP/2 Headers Object>` | `<HTTP/2 Raw Headers>`
- Returns: `<http2.Http2ServerResponse>`

Sends a response header to the request. The status code is a 3-digit HTTP status code, like `404`. The last argument, `headers`, are the response headers.

Returns a reference to the `Http2ServerResponse`, so that calls can be chained.

For compatibility with HTTP/1, a human-readable `statusMessage` may be passed as the second argument. However, because the `statusMessage` has no meaning within HTTP/2, the argument will have no effect and a process warning will be emitted.

    const body = 'hello world';
    response.writeHead(200, {
      'Content-Length': Buffer.byteLength(body),
      'Content-Type': 'text/plain; charset=utf-8',
    });
    js

`Content-Length` is given in bytes not characters. The `Buffer.byteLength()` API may be used to determine the number of bytes in a given encoding. On outbound messages, Node.js does not check if Content-Length and the length of the body being transmitted are equal or not. However, when receiving messages, Node.js will automatically reject messages when the `Content-Length` does not match the actual payload size.

This method may be called at most one time on a message before `response.end()` is called.

If `response.write()` or `response.end()` are called before calling this, the implicit/mutable headers will be calculated and call this function.

When headers have been set with `response.setHeader()`, they will be merged with any headers passed to `response.writeHead()`, with the headers passed to `response.writeHead()` given precedence.

    // Returns content-type = text/plain
    const server = http2.createServer((req, res) => {
      res.setHeader('Content-Type', 'text/html; charset=utf-8');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' });
      res.end('ok');
    });
    js

Attempting to set a header field name or value that contains invalid characters will result in a `TypeError` being thrown.

### Collecting HTTP/2 performance metrics#

The Performance Observer API can be used to collect basic performance metrics for each `Http2Session` and `Http2Stream` instance.

    import { PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((items) => {
      const entry = items.getEntries()[0];
      console.log(entry.entryType);  // prints 'http2'
      if (entry.name === 'Http2Session') {
        // Entry contains statistics about the Http2Session
      } else if (entry.name === 'Http2Stream') {
        // Entry contains statistics about the Http2Stream
      }
    });
    obs.observe({ entryTypes: ['http2'] });
    const { PerformanceObserver } = require('node:perf_hooks');

    const obs = new PerformanceObserver((items) => {
      const entry = items.getEntries()[0];
      console.log(entry.entryType);  // prints 'http2'
      if (entry.name === 'Http2Session') {
        // Entry contains statistics about the Http2Session
      } else if (entry.name === 'Http2Stream') {
        // Entry contains statistics about the Http2Stream
      }
    });
    obs.observe({ entryTypes: ['http2'] });
    javascript

The `entryType` property of the `PerformanceEntry` will be equal to `'http2'`.

The `name` property of the `PerformanceEntry` will be equal to either `'Http2Stream'` or `'Http2Session'`.

If `name` is equal to `Http2Stream`, the `PerformanceEntry` will contain the following additional properties:

- `bytesRead` `<number>` The number of `DATA` frame bytes received for this `Http2Stream`.
- `bytesWritten` `<number>` The number of `DATA` frame bytes sent for this `Http2Stream`.
- `id` `<number>` The identifier of the associated `Http2Stream`
- `timeToFirstByte` `<number>` The number of milliseconds elapsed between the `PerformanceEntry` `startTime` and the reception of the first `DATA` frame.
- `timeToFirstByteSent` `<number>` The number of milliseconds elapsed between the `PerformanceEntry` `startTime` and sending of the first `DATA` frame.
- `timeToFirstHeader` `<number>` The number of milliseconds elapsed between the `PerformanceEntry` `startTime` and the reception of the first header.

If `name` is equal to `Http2Session`, the `PerformanceEntry` will contain the following additional properties:

- `bytesRead` `<number>` The number of bytes received for this `Http2Session`.
- `bytesWritten` `<number>` The number of bytes sent for this `Http2Session`.
- `framesReceived` `<number>` The number of HTTP/2 frames received by the `Http2Session`.
- `framesSent` `<number>` The number of HTTP/2 frames sent by the `Http2Session`.
- `maxConcurrentStreams` `<number>` The maximum number of streams concurrently open during the lifetime of the `Http2Session`.
- `pingRTT` `<number>` The number of milliseconds elapsed since the transmission of a `PING` frame and the reception of its acknowledgment. Only present if a `PING` frame has been sent on the `Http2Session`.
- `streamAverageDuration` `<number>` The average duration (in milliseconds) for all `Http2Stream` instances.
- `streamCount` `<number>` The number of `Http2Stream` instances processed by the `Http2Session`.
- `type` `<string>` Either `'server'` or `'client'` to identify the type of `Http2Session`.

### Note on `:authority` and `host`#

HTTP/2 requires requests to have either the `:authority` pseudo-header or the `host` header. Prefer `:authority` when constructing an HTTP/2 request directly, and `host` when converting from HTTP/1 (in proxies, for instance).

The compatibility API falls back to `host` if `:authority` is not present. See `request.authority` for more information. However, if you don't use the compatibility API (or use `req.headers` directly), you need to implement any fall-back behavior yourself.

## HTTPS#

Source Code: lib/https.js

Stability: 2 - Stable

HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.

### Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the `node:crypto` module. In such cases, attempting to `import` from `https` or calling `require('node:https')` will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

    let https;
    try {
      https = require('node:https');
    } catch (err) {
      console.error('https support is disabled!');
    }
    cjs

When using the lexical ESM `import` keyword, the error can only be caught if a handler for `process.on('uncaughtException')` is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the `import()` function instead of the lexical `import` keyword:

    let https;
    try {
      https = await import('node:https');
    } catch (err) {
      console.error('https support is disabled!');
    }
    mjs

### Class: `https.Agent`#

Added in: v0.4.5
History

Version | Changes
--- | ---
v5.3.0 | support `0` `maxCachedSessions` to disable TLS session caching.
v2.5.0 | parameter `maxCachedSessions` added to `options` for TLS sessions reuse.

An `Agent` object for HTTPS similar to `http.Agent`. See `https.request()` for more information.

Like `http.Agent`, the `createConnection(options[, callback])` method can be overridden to customize how TLS connections are established.

> See `agent.createConnection()` for details on overriding this method, including asynchronous socket creation with a callback.

#### `new Agent([options])`#

History

Version | Changes
--- | ---
v24.5.0, v22.21.0 | Add support for `proxyEnv`.
v24.5.0, v22.21.0 | Add support for `defaultPort` and `protocol`.
v12.5.0 | do not automatically set servername if the target host was specified using an IP address.

- `options` `<Object>` Set of configurable options to set on the agent. Can have the same fields as for `http.Agent(options)`, and

  - `maxCachedSessions` `<number>` maximum number of TLS cached sessions. Use `0` to disable TLS session caching. Default: `100`.

  - `servername` `<string>` the value of Server Name Indication extension to be sent to the server. Use empty string `''` to disable sending the extension. Default: host name of the target server, unless the target server is specified using an IP address, in which case the default is `''` (no extension).

    See `Session Resumption` for information about TLS session reuse.

Requests that specify a custom `checkServerIdentity` option are not eligible for connection reuse or TLS session reuse by an `https.Agent`, unless the `checkServerIdentity` option was specified when constructing the Agent.

##### Event: `'keylog'`#

Added in: v13.2.0, v12.16.0

- `line` `<Buffer>` Line of ASCII text, in NSS `SSLKEYLOGFILE` format.
- `tlsSocket` `<tls.TLSSocket>` The `tls.TLSSocket` instance on which it was generated.

The `keylog` event is emitted when key material is generated or received by a connection managed by this agent (typically before handshake has completed, but not necessarily). This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times for each socket.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

    // ...
    https.globalAgent.on('keylog', (line, tlsSocket) => {
      fs.appendFileSync('/tmp/ssl-keys.log', line, { mode: 0o600 });
    });
    js

### Class: `https.Server`#

Added in: v0.3.4

- Extends: `<tls.Server>`

See `http.Server` for more information.

#### `server.close([callback])`#

Added in: v0.1.90

- `callback` `<Function>`
- Returns: `<https.Server>`

See `server.close()` in the `node:http` module.

#### `server[Symbol.asyncDispose]()`#

Added in: v20.4.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `server.close()` and returns a promise that fulfills when the server has closed.

#### `server.closeAllConnections()`#

Added in: v18.2.0

See `server.closeAllConnections()` in the `node:http` module.

#### `server.closeIdleConnections()`#

Added in: v18.2.0

See `server.closeIdleConnections()` in the `node:http` module.

#### `server.headersTimeout`#

Added in: v11.3.0

- Type: `<number>` Default: `60000`

See `server.headersTimeout` in the `node:http` module.

#### `server.listen()`#

Starts the HTTPS server listening for encrypted connections. This method is identical to `server.listen()` from `net.Server`.

#### `server.maxHeadersCount`#

- Type: `<number>` Default: `2000`

See `server.maxHeadersCount` in the `node:http` module.

#### `server.requestTimeout`#

Added in: v14.11.0
History

Version | Changes
--- | ---
v18.0.0 | The default request timeout changed from no timeout to 300s (5 minutes).

- Type: `<number>` Default: `300000`

See `server.requestTimeout` in the `node:http` module.

#### `server.setTimeout([msecs][, callback])`#

Added in: v0.11.2

- `msecs` `<number>` Default: `120000` (2 minutes)
- `callback` `<Function>`
- Returns: `<https.Server>`

See `server.setTimeout()` in the `node:http` module.

#### `server.timeout`#

Added in: v0.11.2
History

Version | Changes
--- | ---
v13.0.0 | The default timeout changed from 120s to 0 (no timeout).

- Type: `<number>` Default: 0 (no timeout)

See `server.timeout` in the `node:http` module.

#### `server.keepAliveTimeout`#

Added in: v8.0.0

- Type: `<number>` Default: `5000` (5 seconds)

See `server.keepAliveTimeout` in the `node:http` module.

### `https.createServer([options][, requestListener])`#

Added in: v0.3.4

- `options` `<Object>` Accepts `options` from `tls.createServer()`, `tls.createSecureContext()` and `http.createServer()`.
- `requestListener` `<Function>` A listener to be added to the `'request'` event.
- Returns: `<https.Server>`

    // curl -k https://localhost:8000/
    import { createServer } from 'node:https';
    import { readFileSync } from 'node:fs';

    const options = {
      key: readFileSync('private-key.pem'),
      cert: readFileSync('certificate.pem'),
    };

    createServer(options, (req, res) => {
      res.writeHead(200);
      res.end('hello world\n');
    }).listen(8000);
    // curl -k https://localhost:8000/
    const https = require('node:https');
    const fs = require('node:fs');

    const options = {
      key: fs.readFileSync('private-key.pem'),
      cert: fs.readFileSync('certificate.pem'),
    };

    https.createServer(options, (req, res) => {
      res.writeHead(200);
      res.end('hello world\n');
    }).listen(8000);
    javascript

Or

    import { createServer } from 'node:https';
    import { readFileSync } from 'node:fs';

    const options = {
      pfx: readFileSync('test_cert.pfx'),
      passphrase: 'sample',
    };

    createServer(options, (req, res) => {
      res.writeHead(200);
      res.end('hello world\n');
    }).listen(8000);
    const https = require('node:https');
    const fs = require('node:fs');

    const options = {
      pfx: fs.readFileSync('test_cert.pfx'),
      passphrase: 'sample',
    };

    https.createServer(options, (req, res) => {
      res.writeHead(200);
      res.end('hello world\n');
    }).listen(8000);
    javascript

To generate the certificate and key for this example, run:

    openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
      -keyout private-key.pem -out certificate.pem
    bash

Then, to generate the `pfx` certificate for this example, run:

    openssl pkcs12 -certpbe AES-256-CBC -export -out test_cert.pfx \
      -inkey private-key.pem -in certificate.pem -passout pass:sample
    bash

### `https.get(options[, callback])`#

### `https.get(url[, options][, callback])`#

Added in: v0.3.6
History

Version | Changes
--- | ---
v10.9.0 | The `url` parameter can now be passed along with a separate `options` object.
v7.5.0 | The `options` parameter can be a WHATWG `URL` object.

- `url` `<string>` | `<URL>`
- `options` `<Object>` | `<string>` | `<URL>` Accepts the same `options` as `https.request()`, with the method set to GET by default.
- `callback` `<Function>`
- Returns: `<http.ClientRequest>`

Like `http.get()` but for HTTPS.

`options` can be an object, a string, or a `URL` object. If `options` is a string, it is automatically parsed with `new URL()`. If it is a `URL` object, it will be automatically converted to an ordinary `options` object.

    import { get } from 'node:https';
    import process from 'node:process';

    get('https://encrypted.google.com/', (res) => {
      console.log('statusCode:', res.statusCode);
      console.log('headers:', res.headers);

      res.on('data', (d) => {
        process.stdout.write(d);
      });

    }).on('error', (e) => {
      console.error(e);
    });
    const https = require('node:https');

    https.get('https://encrypted.google.com/', (res) => {
      console.log('statusCode:', res.statusCode);
      console.log('headers:', res.headers);

      res.on('data', (d) => {
        process.stdout.write(d);
      });

    }).on('error', (e) => {
      console.error(e);
    });
    javascript

### `https.globalAgent`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v19.0.0 | The agent now uses HTTP Keep-Alive and a 5 second timeout by default.

Global instance of `https.Agent` for all HTTPS client requests. Diverges from a default `https.Agent` configuration by having `keepAlive` enabled and a `timeout` of 5 seconds.

### `https.request(options[, callback])`#

### `https.request(url[, options][, callback])`#

Added in: v0.3.6
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `clientCertEngine` option depends on custom engine support in OpenSSL which is deprecated in OpenSSL 3.
v16.7.0, v14.18.0 | When using a `URL` object parsed username and password will now be properly URI decoded.
v14.1.0, v13.14.0 | The `highWaterMark` option is accepted now.
v10.9.0 | The `url` parameter can now be passed along with a separate `options` object.
v9.3.0 | The `options` parameter can now include `clientCertEngine`.
v7.5.0 | The `options` parameter can be a WHATWG `URL` object.

- `url` `<string>` | `<URL>`
- `options` `<Object>` | `<string>` | `<URL>` Accepts all `options` from `http.request()`, with some differences in default values:
  - `protocol` Default: `'https:'`
  - `port` Default: `443`
  - `agent` Default: `https.globalAgent`
- `callback` `<Function>`
- Returns: `<http.ClientRequest>`

Makes a request to a secure web server.

The following additional `options` from `tls.connect()` are also accepted: `ca`, `cert`, `ciphers`, `clientCertEngine` (deprecated), `crl`, `dhparam`, `ecdhCurve`, `honorCipherOrder`, `key`, `passphrase`, `pfx`, `rejectUnauthorized`, `secureOptions`, `secureProtocol`, `servername`, `sessionIdContext`, `highWaterMark`.

`options` can be an object, a string, or a `URL` object. If `options` is a string, it is automatically parsed with `new URL()`. If it is a `URL` object, it will be automatically converted to an ordinary `options` object.

`https.request()` returns an instance of the `http.ClientRequest` class. The `ClientRequest` instance is a writable stream. If one needs to upload a file with a POST request, then write to the `ClientRequest` object.

    import { request } from 'node:https';
    import process from 'node:process';

    const options = {
      hostname: 'encrypted.google.com',
      port: 443,
      path: '/',
      method: 'GET',
    };

    const req = request(options, (res) => {
      console.log('statusCode:', res.statusCode);
      console.log('headers:', res.headers);

      res.on('data', (d) => {
        process.stdout.write(d);
      });
    });

    req.on('error', (e) => {
      console.error(e);
    });
    req.end();
    const https = require('node:https');

    const options = {
      hostname: 'encrypted.google.com',
      port: 443,
      path: '/',
      method: 'GET',
    };

    const req = https.request(options, (res) => {
      console.log('statusCode:', res.statusCode);
      console.log('headers:', res.headers);

      res.on('data', (d) => {
        process.stdout.write(d);
      });
    });

    req.on('error', (e) => {
      console.error(e);
    });
    req.end();
    javascript

Example using options from `tls.connect()`:

    const options = {
      hostname: 'encrypted.google.com',
      port: 443,
      path: '/',
      method: 'GET',
      key: fs.readFileSync('private-key.pem'),
      cert: fs.readFileSync('certificate.pem'),
    };
    options.agent = new https.Agent(options);

    const req = https.request(options, (res) => {
      // ...
    });
    js

Alternatively, opt out of connection pooling by not using an `Agent`.

    const options = {
      hostname: 'encrypted.google.com',
      port: 443,
      path: '/',
      method: 'GET',
      key: fs.readFileSync('private-key.pem'),
      cert: fs.readFileSync('certificate.pem'),
      agent: false,
    };

    const req = https.request(options, (res) => {
      // ...
    });
    js

Example using a `URL` as `options`:

    const options = new URL('https://abc:xyz@example.com');

    const req = https.request(options, (res) => {
      // ...
    });
    js

Example pinning on certificate fingerprint, or the public key (similar to `pin-sha256`):

    import { checkServerIdentity } from 'node:tls';
    import { Agent, request } from 'node:https';
    import { createHash } from 'node:crypto';

    function sha256(s) {
      return createHash('sha256').update(s).digest('base64');
    }
    const options = {
      hostname: 'github.com',
      port: 443,
      path: '/',
      method: 'GET',
      checkServerIdentity: function(host, cert) {
        // Make sure the certificate is issued to the host we are connected to
        const err = checkServerIdentity(host, cert);
        if (err) {
          return err;
        }

        // Pin the public key, similar to HPKP pin-sha256 pinning
        const pubkey256 = 'SIXvRyDmBJSgatgTQRGbInBaAK+hZOQ18UmrSwnDlK8=';
        if (sha256(cert.pubkey) !== pubkey256) {
          const msg = 'Certificate verification error: ' +
            `The public key of '${cert.subject.CN}' ` +
            'does not match our pinned fingerprint';
          return new Error(msg);
        }

        // Pin the exact certificate, rather than the pub key
        const cert256 = 'FD:6E:9B:0E:F3:98:BC:D9:04:C3:B2:EC:16:7A:7B:' +
          '0F:DA:72:01:C9:03:C5:3A:6A:6A:E5:D0:41:43:63:EF:65';
        if (cert.fingerprint256 !== cert256) {
          const msg = 'Certificate verification error: ' +
            `The certificate of '${cert.subject.CN}' ` +
            'does not match our pinned fingerprint';
          return new Error(msg);
        }

        // This loop is informational only.
        // Print the certificate and public key fingerprints of all certs in the
        // chain. Its common to pin the public key of the issuer on the public
        // internet, while pinning the public key of the service in sensitive
        // environments.
        let lastprint256;
        do {
          console.log('Subject Common Name:', cert.subject.CN);
          console.log('  Certificate SHA256 fingerprint:', cert.fingerprint256);

          const hash = createHash('sha256');
          console.log('  Public key ping-sha256:', sha256(cert.pubkey));

          lastprint256 = cert.fingerprint256;
          cert = cert.issuerCertificate;
        } while (cert.fingerprint256 !== lastprint256);

      },
    };

    options.agent = new Agent(options);
    const req = request(options, (res) => {
      console.log('All OK. Server matched our pinned cert or public key');
      console.log('statusCode:', res.statusCode);

      res.on('data', (d) => {});
    });

    req.on('error', (e) => {
      console.error(e.message);
    });
    req.end();
    const tls = require('node:tls');
    const https = require('node:https');
    const crypto = require('node:crypto');

    function sha256(s) {
      return crypto.createHash('sha256').update(s).digest('base64');
    }
    const options = {
      hostname: 'github.com',
      port: 443,
      path: '/',
      method: 'GET',
      checkServerIdentity: function(host, cert) {
        // Make sure the certificate is issued to the host we are connected to
        const err = tls.checkServerIdentity(host, cert);
        if (err) {
          return err;
        }

        // Pin the public key, similar to HPKP pin-sha256 pinning
        const pubkey256 = 'SIXvRyDmBJSgatgTQRGbInBaAK+hZOQ18UmrSwnDlK8=';
        if (sha256(cert.pubkey) !== pubkey256) {
          const msg = 'Certificate verification error: ' +
            `The public key of '${cert.subject.CN}' ` +
            'does not match our pinned fingerprint';
          return new Error(msg);
        }

        // Pin the exact certificate, rather than the pub key
        const cert256 = 'FD:6E:9B:0E:F3:98:BC:D9:04:C3:B2:EC:16:7A:7B:' +
          '0F:DA:72:01:C9:03:C5:3A:6A:6A:E5:D0:41:43:63:EF:65';
        if (cert.fingerprint256 !== cert256) {
          const msg = 'Certificate verification error: ' +
            `The certificate of '${cert.subject.CN}' ` +
            'does not match our pinned fingerprint';
          return new Error(msg);
        }

        // This loop is informational only.
        // Print the certificate and public key fingerprints of all certs in the
        // chain. Its common to pin the public key of the issuer on the public
        // internet, while pinning the public key of the service in sensitive
        // environments.
        do {
          console.log('Subject Common Name:', cert.subject.CN);
          console.log('  Certificate SHA256 fingerprint:', cert.fingerprint256);

          hash = crypto.createHash('sha256');
          console.log('  Public key ping-sha256:', sha256(cert.pubkey));

          lastprint256 = cert.fingerprint256;
          cert = cert.issuerCertificate;
        } while (cert.fingerprint256 !== lastprint256);

      },
    };

    options.agent = new https.Agent(options);
    const req = https.request(options, (res) => {
      console.log('All OK. Server matched our pinned cert or public key');
      console.log('statusCode:', res.statusCode);

      res.on('data', (d) => {});
    });

    req.on('error', (e) => {
      console.error(e.message);
    });
    req.end();
    javascript

Outputs for example:

    Subject Common Name: github.com
      Certificate SHA256 fingerprint: FD:6E:9B:0E:F3:98:BC:D9:04:C3:B2:EC:16:7A:7B:0F:DA:72:01:C9:03:C5:3A:6A:6A:E5:D0:41:43:63:EF:65
      Public key ping-sha256: SIXvRyDmBJSgatgTQRGbInBaAK+hZOQ18UmrSwnDlK8=
    Subject Common Name: Sectigo ECC Domain Validation Secure Server CA
      Certificate SHA256 fingerprint: 61:E9:73:75:E9:F6:DA:98:2F:F5:C1:9E:2F:94:E6:6C:4E:35:B6:83:7C:E3:B9:14:D2:24:5C:7F:5F:65:82:5F
      Public key ping-sha256: Eep0p/AsSa9lFUH6KT2UY+9s1Z8v7voAPkQ4fGknZ2g=
    Subject Common Name: USERTrust ECC Certification Authority
      Certificate SHA256 fingerprint: A6:CF:64:DB:B4:C8:D5:FD:19:CE:48:89:60:68:DB:03:B5:33:A8:D1:33:6C:62:56:A8:7D:00:CB:B3:DE:F3:EA
      Public key ping-sha256: UJM2FOhG9aTNY0Pg4hgqjNzZ/lQBiMGRxPD5Y2/e0bw=
    Subject Common Name: AAA Certificate Services
      Certificate SHA256 fingerprint: D7:A7:A0:FB:5D:7E:27:31:D7:71:E9:48:4E:BC:DE:F7:1D:5F:0C:3E:0A:29:48:78:2B:C8:3E:E0:EA:69:9E:F4
      Public key ping-sha256: vRU+17BDT2iGsXvOi76E7TQMcTLXAqj0+jGPdW7L1vM=
    All OK. Server matched our pinned cert or public key
    statusCode: 200
    text

## Inspector#

Source Code: lib/inspector.js

Stability: 2 - Stable

The `node:inspector` module provides an API for interacting with the V8 inspector.

It can be accessed using:

    import * as inspector from 'node:inspector/promises';
    const inspector = require('node:inspector/promises');
    javascript

or

    import * as inspector from 'node:inspector';
    const inspector = require('node:inspector');
    javascript

### Promises API#

Added in: v19.0.0

Stability: 1 - Experimental

#### Class: `inspector.Session`#

- Extends: `<EventEmitter>`

The `inspector.Session` is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

##### `new inspector.Session()`#

Added in: v8.0.0

Create a new instance of the `inspector.Session` class. The inspector session needs to be connected through `session.connect()` before the messages can be dispatched to the inspector backend.

When using `Session`, the object outputted by the console API will not be released, unless we performed manually `Runtime.DiscardConsoleEntries` command.

##### Event: `'inspectorNotification'`#

Added in: v8.0.0

- Type: `<Object>` The notification message object

Emitted when any notification from the V8 Inspector is received.

    session.on('inspectorNotification', (message) => console.log(message.method));
    // Debugger.paused
    // Debugger.resumed
    js

> Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

It is also possible to subscribe only to notifications with specific method:

##### Event: `<inspector-protocol-method>`#

Added in: v8.0.0

- Type: `<Object>` The notification message object

Emitted when an inspector notification is received that has its method field set to the `<inspector-protocol-method>` value.

The following snippet installs a listener on the `'Debugger.paused'` event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

    session.on('Debugger.paused', ({ params }) => {
      console.log(params.hitBreakpoints);
    });
    // [ '/the/file/that/has/the/breakpoint.js:11:0' ]
    js

> Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

##### `session.connect()`#

Added in: v8.0.0

Connects a session to the inspector back-end.

##### `session.connectToMainThread()`#

Added in: v12.11.0

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

##### `session.disconnect()`#

Added in: v8.0.0

Immediately close the session. All pending message callbacks will be called with an error. `session.connect()` will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

##### `session.post(method[, params])`#

Added in: v19.0.0

- `method` `<string>`
- `params` `<Object>`
- Returns: `<Promise>`

Posts a message to the inspector back-end.

    import { Session } from 'node:inspector/promises';
    try {
      const session = new Session();
      session.connect();
      const result = await session.post('Runtime.evaluate', { expression: '2 + 2' });
      console.log(result);
    } catch (error) {
      console.error(error);
    }
    // Output: { result: { type: 'number', value: 4, description: '4' } }
    mjs

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

##### Example usage#

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

###### CPU profiler#

Here's an example showing how to use the CPU Profiler:

    import { Session } from 'node:inspector/promises';
    import fs from 'node:fs';
    const session = new Session();
    session.connect();

    await session.post('Profiler.enable');
    await session.post('Profiler.start');
    // Invoke business logic under measurement here...

    // some time later...
    const { profile } = await session.post('Profiler.stop');

    // Write profile to disk, upload, etc.
    fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
    mjs

###### Heap profiler#

Here's an example showing how to use the Heap Profiler:

    import { Session } from 'node:inspector/promises';
    import fs from 'node:fs';
    const session = new Session();

    const fd = fs.openSync('profile.heapsnapshot', 'w');

    session.connect();

    session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
      fs.writeSync(fd, m.params.chunk);
    });

    const result = await session.post('HeapProfiler.takeHeapSnapshot', null);
    console.log('HeapProfiler.takeHeapSnapshot done:', result);
    session.disconnect();
    fs.closeSync(fd);
    mjs

### Callback API#

#### Class: `inspector.Session`#

- Extends: `<EventEmitter>`

The `inspector.Session` is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

##### `new inspector.Session()`#

Added in: v8.0.0

Create a new instance of the `inspector.Session` class. The inspector session needs to be connected through `session.connect()` before the messages can be dispatched to the inspector backend.

When using `Session`, the object outputted by the console API will not be released, unless we performed manually `Runtime.DiscardConsoleEntries` command.

##### Event: `'inspectorNotification'`#

Added in: v8.0.0

- Type: `<Object>` The notification message object

Emitted when any notification from the V8 Inspector is received.

    session.on('inspectorNotification', (message) => console.log(message.method));
    // Debugger.paused
    // Debugger.resumed
    js

> Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

It is also possible to subscribe only to notifications with specific method:

##### Event: `<inspector-protocol-method>`;#

Added in: v8.0.0

- Type: `<Object>` The notification message object

Emitted when an inspector notification is received that has its method field set to the `<inspector-protocol-method>` value.

The following snippet installs a listener on the `'Debugger.paused'` event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

    session.on('Debugger.paused', ({ params }) => {
      console.log(params.hitBreakpoints);
    });
    // [ '/the/file/that/has/the/breakpoint.js:11:0' ]
    js

> Caveat Breakpoints with same-thread session is not recommended, see support of breakpoints.

##### `session.connect()`#

Added in: v8.0.0

Connects a session to the inspector back-end.

##### `session.connectToMainThread()`#

Added in: v12.11.0

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

##### `session.disconnect()`#

Added in: v8.0.0

Immediately close the session. All pending message callbacks will be called with an error. `session.connect()` will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

##### `session.post(method[, params][, callback])`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `method` `<string>`
- `params` `<Object>`
- `callback` `<Function>`

Posts a message to the inspector back-end. `callback` will be notified when a response is received. `callback` is a function that accepts two optional arguments: error and message-specific result.

    session.post('Runtime.evaluate', { expression: '2 + 2' },
                 (error, { result }) => console.log(result));
    // Output: { type: 'number', value: 4, description: '4' }
    js

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

You can not set `reportProgress` to `true` when sending a `HeapProfiler.takeHeapSnapshot` or `HeapProfiler.stopTrackingHeapObjects` command to V8.

##### Example usage#

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

###### CPU profiler#

Here's an example showing how to use the CPU Profiler:

    const inspector = require('node:inspector');
    const fs = require('node:fs');
    const session = new inspector.Session();
    session.connect();

    session.post('Profiler.enable', () => {
      session.post('Profiler.start', () => {
        // Invoke business logic under measurement here...

        // some time later...
        session.post('Profiler.stop', (err, { profile }) => {
          // Write profile to disk, upload, etc.
          if (!err) {
            fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
          }
        });
      });
    });
    js

###### Heap profiler#

Here's an example showing how to use the Heap Profiler:

    const inspector = require('node:inspector');
    const fs = require('node:fs');
    const session = new inspector.Session();

    const fd = fs.openSync('profile.heapsnapshot', 'w');

    session.connect();

    session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
      fs.writeSync(fd, m.params.chunk);
    });

    session.post('HeapProfiler.takeHeapSnapshot', null, (err, r) => {
      console.log('HeapProfiler.takeHeapSnapshot done:', err, r);
      session.disconnect();
      fs.closeSync(fd);
    });
    js

### Common Objects#

#### `inspector.close()`#

Added in: v9.0.0
History

Version | Changes
--- | ---
v18.10.0 | The API is exposed in the worker threads.

Deactivates the inspector. If there are active connections, they are forcibly terminated. Blocks until the inspector server has fully stopped.

#### `inspector.console`#

- Type: `<Object>` An object to send messages to the remote inspector console.

    require('node:inspector').console.log('a message');
    js

The inspector console does not have API parity with Node.js console.

#### `inspector.open([port[, host[, wait]]])`#

History

Version | Changes
--- | ---
v20.6.0 | inspector.open() now returns a `Disposable` object.

- `port` `<number>` Port to listen on for inspector connections. Optional. Default: what was specified on the CLI.
- `host` `<string>` Host to listen on for inspector connections. Optional. Default: what was specified on the CLI.
- `wait` `<boolean>` Block until a client has connected. Optional. Default: `false`.
- Returns: `<Disposable>` A Disposable that calls `inspector.close()`.

Activate inspector on host and port. Equivalent to `node --inspect=[[host:]port]`, but can be done programmatically after node has started.

If wait is `true`, will block until a client has connected to the inspect port and flow control has been passed to the debugger client.

See the security warning regarding the `host` parameter usage.

#### `inspector.url()`#

- Returns: `<string>` | `<undefined>`

Return the URL of the active inspector, or `undefined` if there is none.

    $ node --inspect -p 'inspector.url()'
    Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
    For help, see: https://nodejs.org/learn/getting-started/debugging
    ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34

    $ node --inspect=localhost:3000 -p 'inspector.url()'
    Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
    For help, see: https://nodejs.org/learn/getting-started/debugging
    ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a

    $ node -p 'inspector.url()'
    undefined
    console

#### `inspector.waitForDebugger()`#

Added in: v12.7.0

Blocks until a client (existing or connected later) has sent `Runtime.runIfWaitingForDebugger` command.

An exception will be thrown if there is no active inspector.

### Integration with DevTools#

Stability: 1.1 - Active development

The `node:inspector` module provides an API for integrating with devtools that support Chrome DevTools Protocol. DevTools frontends connected to a running Node.js instance can capture protocol events emitted from the instance and display them accordingly to facilitate debugging. The following methods broadcast a protocol event to all connected frontends. The `params` passed to the methods can be optional, depending on the protocol.

    // The `Network.requestWillBeSent` event will be fired.
    inspector.Network.requestWillBeSent({
      requestId: 'request-id-1',
      timestamp: Date.now() / 1000,
      wallTime: Date.now(),
      request: {
        url: 'https://nodejs.org/en',
        method: 'GET',
      },
    });
    js

#### `inspector.Network.dataReceived([params])`#

Added in: v24.2.0, v22.17.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.dataReceived` event to connected frontends, or buffers the data if `Network.streamResourceContent` command was not invoked for the given request yet.

Also enables `Network.getResponseBody` command to retrieve the response data.

#### `inspector.Network.dataSent([params])`#

Added in: v24.3.0, v22.18.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Enables `Network.getRequestPostData` command to retrieve the request data.

#### `inspector.Network.requestWillBeSent([params])`#

Added in: v22.6.0, v20.18.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.requestWillBeSent` event to connected frontends. This event indicates that the application is about to send an HTTP request.

#### `inspector.Network.responseReceived([params])`#

Added in: v22.6.0, v20.18.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.responseReceived` event to connected frontends. This event indicates that HTTP response is available.

#### `inspector.Network.loadingFinished([params])`#

Added in: v22.6.0, v20.18.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.loadingFinished` event to connected frontends. This event indicates that HTTP request has finished loading.

#### `inspector.Network.loadingFailed([params])`#

Added in: v22.7.0, v20.18.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.loadingFailed` event to connected frontends. This event indicates that HTTP request has failed to load.

#### `inspector.Network.webSocketCreated([params])`#

Added in: v24.7.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.webSocketCreated` event to connected frontends. This event indicates that a WebSocket connection has been initiated.

#### `inspector.Network.webSocketHandshakeResponseReceived([params])`#

Added in: v24.7.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.webSocketHandshakeResponseReceived` event to connected frontends. This event indicates that the WebSocket handshake response has been received.

#### `inspector.Network.webSocketClosed([params])`#

Added in: v24.7.0

- `params` `<Object>`

This feature is only available with the `--experimental-network-inspection` flag enabled.

Broadcasts the `Network.webSocketClosed` event to connected frontends. This event indicates that a WebSocket connection has been closed.

#### `inspector.NetworkResources.put`#

Added in: v24.5.0, v22.19.0

Stability: 1.1 - Active Development

This feature is only available with the `--experimental-inspector-network-resource` flag enabled.

The inspector.NetworkResources.put method is used to provide a response for a loadNetworkResource request issued via the Chrome DevTools Protocol (CDP). This is typically triggered when a source map is specified by URL, and a DevTools frontend—such as Chrome—requests the resource to retrieve the source map.

This method allows developers to predefine the resource content to be served in response to such CDP requests.

    const inspector = require('node:inspector');
    // By preemptively calling put to register the resource, a source map can be resolved when
    // a loadNetworkResource request is made from the frontend.
    async function setNetworkResources() {
      const mapUrl = 'http://localhost:3000/dist/app.js.map';
      const tsUrl = 'http://localhost:3000/src/app.ts';
      const distAppJsMap = await fetch(mapUrl).then((res) => res.text());
      const srcAppTs = await fetch(tsUrl).then((res) => res.text());
      inspector.NetworkResources.put(mapUrl, distAppJsMap);
      inspector.NetworkResources.put(tsUrl, srcAppTs);
    };
    setNetworkResources().then(() => {
      require('./dist/app');
    });
    js

For more details, see the official CDP documentation: Network.loadNetworkResource

#### `inspector.DOMStorage.domStorageItemAdded`#

Added in: v25.5.0

- `params` `<Object>`
  - `storageId` `<Object>`
    - `securityOrigin` `<string>`
    - `storageKey` `<string>`
    - `isLocalStorage` `<boolean>`
  - `key` `<string>`
  - `newValue` `<string>`

This feature is only available with the `--experimental-storage-inspection` flag enabled.

Broadcasts the `DOMStorage.domStorageItemAdded` event to connected frontends. This event indicates that a new item has been added to the storage.

#### `inspector.DOMStorage.domStorageItemRemoved`#

Added in: v25.5.0

- `params` `<Object>`
  - `storageId` `<Object>`
    - `securityOrigin` `<string>`
    - `storageKey` `<string>`
    - `isLocalStorage` `<boolean>`
  - `key` `<string>`

This feature is only available with the `--experimental-storage-inspection` flag enabled.

Broadcasts the `DOMStorage.domStorageItemRemoved` event to connected frontends. This event indicates that an item has been removed from the storage.

#### `inspector.DOMStorage.domStorageItemUpdated`#

Added in: v25.5.0

- `params` `<Object>`
  - `storageId` `<Object>`
    - `securityOrigin` `<string>`
    - `storageKey` `<string>`
    - `isLocalStorage` `<boolean>`
  - `key` `<string>`
  - `oldValue` `<string>`
  - `newValue` `<string>`

This feature is only available with the `--experimental-storage-inspection` flag enabled.

Broadcasts the `DOMStorage.domStorageItemUpdated` event to connected frontends. This event indicates that a storage item has been updated.

#### `inspector.DOMStorage.domStorageItemsCleared`#

Added in: v25.5.0

- `params` `<Object>`
  - `storageId` `<Object>`
    - `securityOrigin` `<string>`
    - `storageKey` `<string>`
    - `isLocalStorage` `<boolean>`

This feature is only available with the `--experimental-storage-inspection` flag enabled.

Broadcasts the `DOMStorage.domStorageItemsCleared` event to connected frontends. This event indicates that all items have been cleared from the storage.

#### `inspector.DOMStorage.registerStorage`#

Added in: v25.5.0

- `params` `<Object>`
  - `isLocalStorage` `<boolean>`
  - `storageMap` `<Object>`

This feature is only available with the `--experimental-storage-inspection` flag enabled.

### Support of breakpoints#

The Chrome DevTools Protocol `Debugger` domain allows an `inspector.Session` to attach to a program and set breakpoints to step through the codes.

However, setting breakpoints with a same-thread `inspector.Session`, which is connected by `session.connect()`, should be avoided as the program being attached and paused is exactly the debugger itself. Instead, try connect to the main thread by `session.connectToMainThread()` and set breakpoints in a worker thread, or connect with a Debugger program over WebSocket connection.

## Internationalization support#

Node.js has many features that make it easier to write internationalized programs. Some of them are:

- Locale-sensitive or Unicode-aware functions in the ECMAScript Language Specification:
  - `String.prototype.normalize()`
  - `String.prototype.toLowerCase()`
  - `String.prototype.toUpperCase()`
- All functionality described in the ECMAScript Internationalization API Specification (aka ECMA-402):
  - `Intl` object
  - Locale-sensitive methods like `String.prototype.localeCompare()` and `Date.prototype.toLocaleString()`
- The WHATWG URL parser's internationalized domain names (IDNs) support
- `require('node:buffer').transcode()`
- More accurate REPL line editing
- `require('node:util').TextDecoder`
- `RegExp` Unicode Property Escapes

Node.js and the underlying V8 engine use International Components for Unicode (ICU) to implement these features in native C/C++ code. The full ICU data set is provided by Node.js by default. However, due to the size of the ICU data file, several options are provided for customizing the ICU data set either when building or running Node.js.

### Options for building Node.js#

To control how ICU is used in Node.js, four `configure` options are available during compilation. Additional details on how to compile Node.js are documented in BUILDING.md.

- `--with-intl=none`/`--without-intl`
- `--with-intl=system-icu`
- `--with-intl=small-icu`
- `--with-intl=full-icu` (default)

An overview of available Node.js and JavaScript features for each `configure` option:

Feature | `none` | `system-icu` | `small-icu` | `full-icu`
--- | --- | --- | --- | ---
`String.prototype.normalize()` | none (function is no-op) | full | full | full
`String.prototype.to*Case()` | full | full | full | full
`Intl` | none (object does not exist) | partial/full (depends on OS) | partial (English-only) | full
`String.prototype.localeCompare()` | partial (not locale-aware) | full | full | full
`String.prototype.toLocale*Case()` | partial (not locale-aware) | full | full | full
`Number.prototype.toLocaleString()` | partial (not locale-aware) | partial/full (depends on OS) | partial (English-only) | full
`Date.prototype.toLocale*String()` | partial (not locale-aware) | partial/full (depends on OS) | partial (English-only) | full
Legacy URL Parser | partial (no IDN support) | full | full | full
WHATWG URL Parser | partial (no IDN support) | full | full | full
`require('node:buffer').transcode()` | none (function does not exist) | full | full | full
REPL | partial (inaccurate line editing) | full | full | full
`require('node:util').TextDecoder` | partial (basic encodings support) | partial/full (depends on OS) | partial (Unicode-only) | full
`RegExp` Unicode Property Escapes | none (invalid `RegExp` error) | full | full | full

The "(not locale-aware)" designation denotes that the function carries out its operation just like the non-`Locale` version of the function, if one exists. For example, under `none` mode, `Date.prototype.toLocaleString()`'s operation is identical to that of `Date.prototype.toString()`.

#### Disable all internationalization features (`none`)#

If this option is chosen, ICU is disabled and most internationalization features mentioned above will be unavailable in the resulting `node` binary.

#### Build with a pre-installed ICU (`system-icu`)#

Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS.

Functionalities that only require the ICU library itself, such as `String.prototype.normalize()` and the WHATWG URL parser, are fully supported under `system-icu`. Features that require ICU locale data in addition, such as `Intl.DateTimeFormat` may be fully or partially supported, depending on the completeness of the ICU data installed on the system.

#### Embed a limited set of ICU data (`small-icu`)#

This option makes the resulting binary link against the ICU library statically, and includes a subset of ICU data (typically only the English locale) within the `node` executable.

Functionalities that only require the ICU library itself, such as `String.prototype.normalize()` and the WHATWG URL parser, are fully supported under `small-icu`. Features that require ICU locale data in addition, such as `Intl.DateTimeFormat`, generally only work with the English locale:

    const january = new Date(9e8);
    const english = new Intl.DateTimeFormat('en', { month: 'long' });
    const spanish = new Intl.DateTimeFormat('es', { month: 'long' });

    console.log(english.format(january));
    // Prints "January"
    console.log(spanish.format(january));
    // Prints either "M01" or "January" on small-icu, depending on the user’s default locale
    // Should print "enero"
    js

This mode provides a balance between features and binary size.

##### Providing ICU data at runtime#

If the `small-icu` option is used, one can still provide additional locale data at runtime so that the JS methods would work for all ICU locales. Assuming the data file is stored at `/runtime/directory/with/dat/file`, it can be made available to ICU through either:

- The `--with-icu-default-data-dir` configure option:

      ./configure --with-icu-default-data-dir=/runtime/directory/with/dat/file --with-intl=small-icu
      bash

  This only embeds the default data directory path into the binary. The actual data file is going to be loaded at runtime from this directory path.

- The `NODE_ICU_DATA` environment variable:

      env NODE_ICU_DATA=/runtime/directory/with/dat/file node
      bash

- The `--icu-data-dir` CLI parameter:

      node --icu-data-dir=/runtime/directory/with/dat/file
      bash

When more than one of them is specified, the `--icu-data-dir` CLI parameter has the highest precedence, then the `NODE_ICU_DATA` environment variable, then the `--with-icu-default-data-dir` configure option.

ICU is able to automatically find and load a variety of data formats, but the data must be appropriate for the ICU version, and the file correctly named. The most common name for the data file is `icudtX[bl].dat`, where `X` denotes the intended ICU version, and `b` or `l` indicates the system's endianness. Node.js would fail to load if the expected data file cannot be read from the specified directory. The name of the data file corresponding to the current Node.js version can be computed with:

    `icudt${process.versions.icu.split('.')[0]}${os.endianness()[0].toLowerCase()}.dat`;
    js

Check "ICU Data" article in the ICU User Guide for other supported formats and more details on ICU data in general.

The full-icu npm module can greatly simplify ICU data installation by detecting the ICU version of the running `node` executable and downloading the appropriate data file. After installing the module through `npm i full-icu`, the data file will be available at `./node_modules/full-icu`. This path can be then passed either to `NODE_ICU_DATA` or `--icu-data-dir` as shown above to enable full `Intl` support.

#### Embed the entire ICU (`full-icu`)#

This option makes the resulting binary link against ICU statically and include a full set of ICU data. A binary created this way has no further external dependencies and supports all locales, but might be rather large. This is the default behavior if no `--with-intl` flag is passed. The official binaries are also built in this mode.

### Detecting internationalization support#

To verify that ICU is enabled at all (`system-icu`, `small-icu`, or `full-icu`), simply checking the existence of `Intl` should suffice:

    const hasICU = typeof Intl === 'object';
    js

Alternatively, checking for `process.versions.icu`, a property defined only when ICU is enabled, works too:

    const hasICU = typeof process.versions.icu === 'string';
    js

To check for support for a non-English locale (i.e. `full-icu` or `system-icu`), `Intl.DateTimeFormat` can be a good distinguishing factor:

    const hasFullICU = (() => {
      try {
        const january = new Date(9e8);
        const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
        return spanish.format(january) === 'enero';
      } catch (err) {
        return false;
      }
    })();
    js

For more verbose tests for `Intl` support, the following resources may be found to be helpful:

- btest402: Generally used to check whether Node.js with `Intl` support is built correctly.
- Test262: ECMAScript's official conformance test suite includes a section dedicated to ECMA-402.

## About this documentation#

Welcome to the official API reference documentation for Node.js!

Node.js is a JavaScript runtime built on the V8 JavaScript engine.

### Contributing#

Report errors in this documentation in the issue tracker. See the contributing guide for directions on how to submit pull requests.

### Stability index#

Throughout the documentation are indications of a section's stability. Some APIs are so proven and so relied upon that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous.

The stability indexes are as follows:

Stability: 0 - Deprecated. The feature may emit warnings. Backward compatibility is not guaranteed.

Stability: 1 - Experimental. The feature is not subject to semantic versioning rules. Non-backward compatible changes or removal may occur in any future release. Use of the feature is not recommended in production environments.

Experimental features are subdivided into stages:

- 1.0 - Early development. Experimental features at this stage are unfinished and subject to substantial change.
- 1.1 - Active development. Experimental features at this stage are nearing minimum viability.
- 1.2 - Release candidate. Experimental features at this stage are hopefully ready to become stable. No further breaking changes are anticipated but may still occur in response to user feedback or the features' underlying specification development. We encourage user testing and feedback so that we can know that this feature is ready to be marked as stable.

Experimental features leave the experimental status typically either by graduating to stable, or are removed without a deprecation cycle.

Stability: 2 - Stable. Compatibility with the npm ecosystem is a high priority.

Stability: 3 - Legacy. Although this feature is unlikely to be removed and is still covered by semantic versioning guarantees, it is no longer actively maintained, and other alternatives are available.

Features are marked as legacy rather than being deprecated if their use does no harm, and they are widely relied upon within the npm ecosystem. Bugs found in legacy features are unlikely to be fixed.

Use caution when making use of Experimental features, particularly when authoring libraries. Users may not be aware that experimental features are being used. Bugs or behavior changes may surprise users when Experimental API modifications occur. To avoid surprises, use of an Experimental feature may need a command-line flag. Experimental features may also emit a warning.

### Stability overview#

API | Stability
--- | ---
Assert | (2) Stable
Async hooks | (1) Experimental
Asynchronous context tracking | (2) Stable
Buffer | (2) Stable
Child process | (2) Stable
Cluster | (2) Stable
Console | (2) Stable
Crypto | (2) Stable
Debugger | (2) Stable
Diagnostic report | (2) Stable
Diagnostics Channel | (2) Stable
DNS | (2) Stable
Domain | (0) Deprecated
Events | (2) Stable
FFI | (1) Experimental
File system | (2) Stable
Global objects | (2) Stable
HTTP | (2) Stable
HTTP/2 | (2) Stable
HTTPS | (2) Stable
Inspector | (2) Stable
Iterable Streams | (1) Experimental – Enable this API with the `--experimental-stream-iter` CLI flag.
Modules: CommonJS modules | (2) Stable
Modules: ECMAScript modules | (2) Stable
Modules: TypeScript | (2) Stable
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Performance measurement APIs | (2) Stable
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Query string | (2) Stable
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REPL | (2) Stable
Single executable applications | (1.1) Active development
SQLite | (1.2) Release candidate.
Stream | (2) Stable
String decoder | (2) Stable
Test runner | (2) Stable
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Trace events | (1) Experimental
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Zlib | (2) Stable

### JSON output#

Added in: v0.6.12

Every `.html` document has a corresponding `.json` document. This is for IDEs and other utilities that consume the documentation.

### System calls and man pages#

Node.js functions which wrap a system call will document that. The docs link to the corresponding man pages which describe how the system call works.

Most Unix system calls have Windows analogues. Still, behavior differences may be unavoidable.

## Assert#

Source Code: lib/assert.js

Stability: 2 - Stable

The `node:assert` module provides a set of assertion functions for verifying invariants.

### Strict assertion mode#

Added in: v9.9.0
History

Version | Changes
--- | ---
v15.0.0 | Exposed as `require('node:assert/strict')`.
v13.9.0, v12.16.2 | Changed "strict mode" to "strict assertion mode" and "legacy mode" to "legacy assertion mode" to avoid confusion with the more usual meaning of "strict mode".
v9.9.0 | Added error diffs to the strict assertion mode.
v9.9.0 | Added strict assertion mode to the assert module.

In strict assertion mode, non-strict methods behave like their corresponding strict methods. For example, `assert.deepEqual()` will behave like `assert.deepStrictEqual()`.

In strict assertion mode, error messages for objects display a diff. In legacy assertion mode, error messages for objects display the objects, often truncated.

#### Message parameter semantics#

For assertion methods that accept an optional `message` parameter, the message may be provided in one of the following forms:

- string: Used as-is. If additional arguments are supplied after the `message` string, they are treated as printf-like substitutions (see `util.format()`).
- Error: If an `Error` instance is provided as `message`, that error is thrown directly instead of an `AssertionError`.
- function: A function of the form `(actual, expected) => string`. It is called only when the assertion fails and should return a string to be used as the error message. Non-string return values are ignored and the default message is used instead.

If additional arguments are passed along with an `Error` or a function as `message`, the call is rejected with `ERR_AMBIGUOUS_ARGUMENT`.

If the first item is neither a string, `Error`, nor function, `ERR_INVALID_ARG_TYPE` is thrown.

To use strict assertion mode:

    import { strict as assert } from 'node:assert';
    const assert = require('node:assert').strict;
    javascript

    import assert from 'node:assert/strict';
    const assert = require('node:assert/strict');
    javascript

Example error diff:

    import { strict as assert } from 'node:assert';

    assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected ... Lines skipped
    //
    //   [
    //     [
    // ...
    //       2,
    // +     3
    // -     '3'
    //     ],
    // ...
    //     5
    //   ]
    const assert = require('node:assert/strict');

    assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected ... Lines skipped
    //
    //   [
    //     [
    // ...
    //       2,
    // +     3
    // -     '3'
    //     ],
    // ...
    //     5
    //   ]
    javascript

To deactivate the colors, use the `NO_COLOR` or `NODE_DISABLE_COLORS` environment variables. This will also deactivate the colors in the REPL. For more on color support in terminal environments, read the tty `getColorDepth()` documentation.

### Legacy assertion mode#

Legacy assertion mode uses the `==` operator in:

- `assert.deepEqual()`
- `assert.equal()`
- `assert.notDeepEqual()`
- `assert.notEqual()`

To use legacy assertion mode:

    import assert from 'node:assert';
    const assert = require('node:assert');
    javascript

Legacy assertion mode may have surprising results, especially when using `assert.deepEqual()`:

    // WARNING: This does not throw an AssertionError in legacy assertion mode!
    assert.deepEqual(/a/gi, new Date());
    cjs

### Class: `assert.AssertionError`#

- Extends: `<errors.Error>`

Indicates the failure of an assertion. All errors thrown by the `node:assert` module will be instances of the `AssertionError` class.

#### `new assert.AssertionError(options)`#

Added in: v0.1.21

- `options` `<Object>`
  - `message` `<string>` If provided, the error message is set to this value.
  - `actual` `<any>` The `actual` property on the error instance.
  - `expected` `<any>` The `expected` property on the error instance.
  - `operator` `<string>` The `operator` property on the error instance.
  - `stackStartFn` `<Function>` If provided, the generated stack trace omits frames before this function.
  - `diff` `<string>` If set to `'full'`, shows the full diff in assertion errors. Defaults to `'simple'`. Accepted values: `'simple'`, `'full'`.

A subclass of `<Error>` that indicates the failure of an assertion.

All instances contain the built-in `Error` properties (`message` and `name`) and:

- `actual` `<any>` Set to the `actual` argument for methods such as `assert.strictEqual()`.
- `expected` `<any>` Set to the `expected` value for methods such as `assert.strictEqual()`.
- `generatedMessage` `<boolean>` Indicates if the message was auto-generated (`true`) or not.
- `code` `<string>` Value is always `ERR_ASSERTION` to show that the error is an assertion error.
- `operator` `<string>` Set to the passed in operator value.

    import assert from 'node:assert';

    // Generate an AssertionError to compare the error message later:
    const { message } = new assert.AssertionError({
      actual: 1,
      expected: 2,
      operator: 'strictEqual',
    });

    // Verify error output:
    try {
      assert.strictEqual(1, 2);
    } catch (err) {
      assert(err instanceof assert.AssertionError);
      assert.strictEqual(err.message, message);
      assert.strictEqual(err.name, 'AssertionError');
      assert.strictEqual(err.actual, 1);
      assert.strictEqual(err.expected, 2);
      assert.strictEqual(err.code, 'ERR_ASSERTION');
      assert.strictEqual(err.operator, 'strictEqual');
      assert.strictEqual(err.generatedMessage, true);
    }
    const assert = require('node:assert');

    // Generate an AssertionError to compare the error message later:
    const { message } = new assert.AssertionError({
      actual: 1,
      expected: 2,
      operator: 'strictEqual',
    });

    // Verify error output:
    try {
      assert.strictEqual(1, 2);
    } catch (err) {
      assert(err instanceof assert.AssertionError);
      assert.strictEqual(err.message, message);
      assert.strictEqual(err.name, 'AssertionError');
      assert.strictEqual(err.actual, 1);
      assert.strictEqual(err.expected, 2);
      assert.strictEqual(err.code, 'ERR_ASSERTION');
      assert.strictEqual(err.operator, 'strictEqual');
      assert.strictEqual(err.generatedMessage, true);
    }
    javascript

### Class: `assert.Assert`#

Added in: v24.6.0, v22.19.0

The `Assert` class allows creating independent assertion instances with custom options.

#### `new assert.Assert([options])`#

History

Version | Changes
--- | ---
v24.9.0 | Added `skipPrototype` option.

- `options` `<Object>`
  - `diff` `<string>` If set to `'full'`, shows the full diff in assertion errors. Defaults to `'simple'`. Accepted values: `'simple'`, `'full'`.
  - `strict` `<boolean>` If set to `true`, non-strict methods behave like their corresponding strict methods. Defaults to `true`.
  - `skipPrototype` `<boolean>` If set to `true`, skips prototype and constructor comparison in deep equality checks. Defaults to `false`.

Creates a new assertion instance. The `diff` option controls the verbosity of diffs in assertion error messages.

    const { Assert } = require('node:assert');
    const assertInstance = new Assert({ diff: 'full' });
    assertInstance.deepStrictEqual({ a: 1 }, { a: 2 });
    // Shows a full diff in the error message.
    js

Important: When destructuring assertion methods from an `Assert` instance, the methods lose their connection to the instance's configuration options (such as `diff`, `strict`, and `skipPrototype` settings). The destructured methods will fall back to default behavior instead.

    const myAssert = new Assert({ diff: 'full' });

    // This works as expected - uses 'full' diff
    myAssert.strictEqual({ a: 1 }, { b: { c: 1 } });

    // This loses the 'full' diff setting - falls back to default 'simple' diff
    const { strictEqual } = myAssert;
    strictEqual({ a: 1 }, { b: { c: 1 } });
    js

The `skipPrototype` option affects all deep equality methods:

    class Foo {
      constructor(a) {
        this.a = a;
      }
    }

    class Bar {
      constructor(a) {
        this.a = a;
      }
    }

    const foo = new Foo(1);
    const bar = new Bar(1);

    // Default behavior - fails due to different constructors
    const assert1 = new Assert();
    assert1.deepStrictEqual(foo, bar); // AssertionError

    // Skip prototype comparison - passes if properties are equal
    const assert2 = new Assert({ skipPrototype: true });
    assert2.deepStrictEqual(foo, bar); // OK
    js

When destructured, methods lose access to the instance's `this` context and revert to default assertion behavior (diff: 'simple', non-strict mode). To maintain custom options when using destructured methods, avoid destructuring and call methods directly on the instance.

### `assert(value[, message])`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.

- `value` `<any>` The input that is checked for being truthy.
- `message` `<string>` | `<Error>` | `<Function>`

An alias of `assert.ok()`.

### `assert.deepEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v25.0.0 | Promises are not considered equal anymore if they are not of the same instance.
v25.0.0 | Invalid dates are now considered equal.
v24.0.0 | Recursion now stops when either side encounters a circular reference.
v22.2.0, v20.15.0 | Error cause and errors properties are now compared as well.
v18.0.0 | Regular expressions lastIndex property is now compared as well.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.
v12.0.0 | The type tags are now properly compared and there are a couple minor comparison adjustments to make the check less surprising.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0, v4.5.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.deepStrictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.deepStrictEqual()` instead.

Tests for deep equality between the `actual` and `expected` parameters. Consider using `assert.deepStrictEqual()` instead. `assert.deepEqual()` can have surprising results.

Deep equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules.

#### Comparison details#

- Primitive values are compared with the `==` operator, with the exception of `<NaN>`. It is treated as being identical in case both sides are `<NaN>`.
- Type tags of objects should be the same.
- Only enumerable "own" properties are considered.
- Object constructors are compared when available.
- `<Error>` names, messages, causes, and errors are always compared, even if these are not enumerable properties.
- Object wrappers are compared both as objects and unwrapped values.
- `Object` properties are compared unordered.
- `<Map>` keys and `<Set>` items are compared unordered.
- Recursion stops when both sides differ or either side encounters a circular reference.
- Implementation does not test the `[[Prototype]]` of objects.
- `<Symbol>` properties are not compared.
- `<WeakMap>`, `<WeakSet>` and `<Promise>` instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different `WeakMap`, `WeakSet`, or `Promise` instances will result in inequality, even if they contain the same content.
- `<RegExp>` lastIndex, flags, and source are always compared, even if these are not enumerable properties.

The following example does not throw an `AssertionError` because the primitives are compared using the `==` operator.

    import assert from 'node:assert';
    // WARNING: This does not throw an AssertionError!

    assert.deepEqual('+00000000', false);
    const assert = require('node:assert');
    // WARNING: This does not throw an AssertionError!

    assert.deepEqual('+00000000', false);
    javascript

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

    import assert from 'node:assert';

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.deepEqual(obj1, obj1);
    // OK

    // Values of b are different:
    assert.deepEqual(obj1, obj2);
    // AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

    assert.deepEqual(obj1, obj3);
    // OK

    // Prototypes are ignored:
    assert.deepEqual(obj1, obj4);
    // AssertionError: { a: { b: 1 } } deepEqual {}
    const assert = require('node:assert');

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.deepEqual(obj1, obj1);
    // OK

    // Values of b are different:
    assert.deepEqual(obj1, obj2);
    // AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

    assert.deepEqual(obj1, obj3);
    // OK

    // Prototypes are ignored:
    assert.deepEqual(obj1, obj4);
    // AssertionError: { a: { b: 1 } } deepEqual {}
    javascript

If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.deepStrictEqual(actual, expected[, message])`#

Added in: v1.2.0
History

Version | Changes
--- | ---
v25.1.0 | Message may now be a `printf`-like format string or function.
v25.0.0 | Promises are not considered equal anymore if they are not of the same instance.
v25.0.0 | Invalid dates are now considered equal.
v24.0.0 | Recursion now stops when either side encounters a circular reference.
v22.2.0, v20.15.0 | Error cause and errors properties are now compared as well.
v18.0.0 | Regular expressions lastIndex property is now compared as well.
v9.0.0 | Enumerable symbol properties are now compared.
v9.0.0 | The `NaN` is now compared using the SameValueZero comparison.
v8.5.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests for deep equality between the `actual` and `expected` parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules.

#### Comparison details#

- Primitive values are compared using `Object.is()`.
- Type tags of objects should be the same.
- `[[Prototype]]` of objects are compared using the `===` operator.
- Only enumerable "own" properties are considered.
- Object constructors are compared when available.
- `<Error>` names, messages, causes, and errors are always compared, even if these are not enumerable properties. `errors` is also compared.
- Enumerable own `<Symbol>` properties are compared as well.
- Object wrappers are compared both as objects and unwrapped values.
- `Object` properties are compared unordered.
- `<Map>` keys and `<Set>` items are compared unordered.
- Recursion stops when both sides differ or either side encounters a circular reference.
- `<WeakMap>`, `<WeakSet>` and `<Promise>` instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different `WeakMap`, `WeakSet`, or `Promise` instances will result in inequality, even if they contain the same content.
- `<RegExp>` lastIndex, flags, and source are always compared, even if these are not enumerable properties.

    import assert from 'node:assert/strict';

    // This fails because 1 !== '1'.
    assert.deepStrictEqual({ a: 1 }, { a: '1' });
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    //   {
    // +   a: 1
    // -   a: '1'
    //   }

    // The following objects don't have own properties
    const date = new Date();
    const object = {};
    const fakeDate = {};
    Object.setPrototypeOf(fakeDate, Date.prototype);

    // Different [[Prototype]]:
    assert.deepStrictEqual(object, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + {}
    // - Date {}

    // Different type tags:
    assert.deepStrictEqual(date, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 2018-04-26T00:49:08.604Z
    // - Date {}

    assert.deepStrictEqual(NaN, NaN);
    // OK because Object.is(NaN, NaN) is true.

    // Different unwrapped numbers:
    assert.deepStrictEqual(new Number(1), new Number(2));
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + [Number: 1]
    // - [Number: 2]

    assert.deepStrictEqual(new String('foo'), Object('foo'));
    // OK because the object and the string are identical when unwrapped.

    assert.deepStrictEqual(-0, -0);
    // OK

    // Different zeros:
    assert.deepStrictEqual(0, -0);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 0
    // - -0

    const symbol1 = Symbol();
    const symbol2 = Symbol();
    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
    // OK, because it is the same symbol on both objects.

    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
    // AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
    //
    // {
    //   Symbol(): 1
    // }

    const weakMap1 = new WeakMap();
    const weakMap2 = new WeakMap();
    const obj = {};

    weakMap1.set(obj, 'value');
    weakMap2.set(obj, 'value');

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakMap1, weakMap2);
    // AssertionError: Values have same structure but are not reference-equal:
    //
    // WeakMap {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakMap1, weakMap1);
    // OK

    const weakSet1 = new WeakSet();
    const weakSet2 = new WeakSet();
    weakSet1.add(obj);
    weakSet2.add(obj);

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakSet1, weakSet2);
    // AssertionError: Values have same structure but are not reference-equal:
    // + actual - expected
    //
    // WeakSet {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakSet1, weakSet1);
    // OK
    const assert = require('node:assert/strict');

    // This fails because 1 !== '1'.
    assert.deepStrictEqual({ a: 1 }, { a: '1' });
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    //   {
    // +   a: 1
    // -   a: '1'
    //   }

    // The following objects don't have own properties
    const date = new Date();
    const object = {};
    const fakeDate = {};
    Object.setPrototypeOf(fakeDate, Date.prototype);

    // Different [[Prototype]]:
    assert.deepStrictEqual(object, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + {}
    // - Date {}

    // Different type tags:
    assert.deepStrictEqual(date, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 2018-04-26T00:49:08.604Z
    // - Date {}

    assert.deepStrictEqual(NaN, NaN);
    // OK because Object.is(NaN, NaN) is true.

    // Different unwrapped numbers:
    assert.deepStrictEqual(new Number(1), new Number(2));
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + [Number: 1]
    // - [Number: 2]

    assert.deepStrictEqual(new String('foo'), Object('foo'));
    // OK because the object and the string are identical when unwrapped.

    assert.deepStrictEqual(-0, -0);
    // OK

    // Different zeros:
    assert.deepStrictEqual(0, -0);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 0
    // - -0

    const symbol1 = Symbol();
    const symbol2 = Symbol();
    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
    // OK, because it is the same symbol on both objects.

    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
    // AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
    //
    // {
    //   Symbol(): 1
    // }

    const weakMap1 = new WeakMap();
    const weakMap2 = new WeakMap();
    const obj = {};

    weakMap1.set(obj, 'value');
    weakMap2.set(obj, 'value');

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakMap1, weakMap2);
    // AssertionError: Values have same structure but are not reference-equal:
    //
    // WeakMap {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakMap1, weakMap1);
    // OK

    const weakSet1 = new WeakSet();
    const weakSet2 = new WeakSet();
    weakSet1.add(obj);
    weakSet2.add(obj);

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakSet1, weakSet2);
    // AssertionError: Values have same structure but are not reference-equal:
    // + actual - expected
    //
    // WeakSet {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakSet1, weakSet1);
    // OK
    javascript

If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.doesNotMatch(string, regexp[, message])`#

Added in: v13.6.0, v12.16.0
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0 | This API is no longer experimental.

- `string` `<string>`
- `regexp` `<RegExp>`
- `message` `<string>` | `<Error>` | `<Function>`

Expects the `string` input not to match the regular expression.

    import assert from 'node:assert/strict';

    assert.doesNotMatch('I will fail', /fail/);
    // AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

    assert.doesNotMatch(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.doesNotMatch('I will pass', /different/);
    // OK
    const assert = require('node:assert/strict');

    assert.doesNotMatch('I will fail', /fail/);
    // AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

    assert.doesNotMatch(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.doesNotMatch('I will pass', /different/);
    // OK
    javascript

If the values do match, or if the `string` argument is of another type than `string`, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.doesNotReject(asyncFn[, error][, message])`#

Added in: v10.0.0

- `asyncFn` `<Function>` | `<Promise>`
- `error` `<RegExp>` | `<Function>`
- `message` `<string>`
- Returns: `<Promise>`

Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is not rejected.

If `asyncFn` is a function and it throws an error synchronously, `assert.doesNotReject()` will return a rejected `Promise` with that error. If the function does not return a promise, `assert.doesNotReject()` will return a rejected `Promise` with an `ERR_INVALID_RETURN_VALUE` error. In both cases the error handler is skipped.

Using `assert.doesNotReject()` is actually not useful because there is little benefit in catching a rejection and then rejecting it again. Instead, consider adding a comment next to the specific code path that should not reject and keep error messages as expressive as possible.

If specified, `error` can be a `Class`, `<RegExp>` or a validation function. See `assert.throws()` for more details.

Besides the async nature to await the completion behaves identically to `assert.doesNotThrow()`.

    import assert from 'node:assert/strict';

    await assert.doesNotReject(
      async () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError,
    );
    const assert = require('node:assert/strict');

    (async () => {
      await assert.doesNotReject(
        async () => {
          throw new TypeError('Wrong value');
        },
        SyntaxError,
      );
    })();
    javascript

    import assert from 'node:assert/strict';

    assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
      .then(() => {
        // ...
      });
    const assert = require('node:assert/strict');

    assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
      .then(() => {
        // ...
      });
    javascript

### `assert.doesNotThrow(fn[, error][, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v5.11.0, v4.4.5 | The `message` parameter is respected now.
v4.2.0 | The `error` parameter can now be an arrow function.

- `fn` `<Function>`
- `error` `<RegExp>` | `<Function>`
- `message` `<string>`

Asserts that the function `fn` does not throw an error.

Using `assert.doesNotThrow()` is actually not useful because there is no benefit in catching an error and then rethrowing it. Instead, consider adding a comment next to the specific code path that should not throw and keep error messages as expressive as possible.

When `assert.doesNotThrow()` is called, it will immediately call the `fn` function.

If an error is thrown and it is the same type as that specified by the `error` parameter, then an `AssertionError` is thrown. If the error is of a different type, or if the `error` parameter is undefined, the error is propagated back to the caller.

If specified, `error` can be a `Class`, `<RegExp>`, or a validation function. See `assert.throws()` for more details.

The following, for instance, will throw the `<TypeError>` because there is no matching error type in the assertion:

    import assert from 'node:assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError,
    );
    const assert = require('node:assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError,
    );
    javascript

However, the following will result in an `AssertionError` with the message 'Got unwanted exception...':

    import assert from 'node:assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      TypeError,
    );
    const assert = require('node:assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      TypeError,
    );
    javascript

If an `AssertionError` is thrown and a value is provided for the `message` parameter, the value of `message` will be appended to the `AssertionError` message:

    import assert from 'node:assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      /Wrong value/,
      'Whoops',
    );
    // Throws: AssertionError: Got unwanted exception: Whoops
    const assert = require('node:assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      /Wrong value/,
      'Whoops',
    );
    // Throws: AssertionError: Got unwanted exception: Whoops
    javascript

### `assert.equal(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.strictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.strictEqual()` instead.

Tests shallow, coercive equality between the `actual` and `expected` parameters using the `==` operator. `NaN` is specially handled and treated as being identical if both sides are `NaN`.

    import assert from 'node:assert';

    assert.equal(1, 1);
    // OK, 1 == 1
    assert.equal(1, '1');
    // OK, 1 == '1'
    assert.equal(NaN, NaN);
    // OK

    assert.equal(1, 2);
    // AssertionError: 1 == 2
    assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
    // AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
    const assert = require('node:assert');

    assert.equal(1, 1);
    // OK, 1 == 1
    assert.equal(1, '1');
    // OK, 1 == '1'
    assert.equal(NaN, NaN);
    // OK

    assert.equal(1, 2);
    // AssertionError: 1 == 2
    assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
    // AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
    javascript

If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.fail([message])`#

Added in: v0.1.21

- `message` `<string>` | `<Error>` Default: `'Failed'`

Throws an `AssertionError` with the provided error message or a default error message. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

    import assert from 'node:assert/strict';

    assert.fail();
    // AssertionError [ERR_ASSERTION]: Failed

    assert.fail('boom');
    // AssertionError [ERR_ASSERTION]: boom

    assert.fail(new TypeError('need array'));
    // TypeError: need array
    const assert = require('node:assert/strict');

    assert.fail();
    // AssertionError [ERR_ASSERTION]: Failed

    assert.fail('boom');
    // AssertionError [ERR_ASSERTION]: boom

    assert.fail(new TypeError('need array'));
    // TypeError: need array
    javascript

### `assert.ifError(value)`#

Added in: v0.1.97
History

Version | Changes
--- | ---
v10.0.0 | Instead of throwing the original error it is now wrapped into an [`AssertionError`][] that contains the full stack trace.
v10.0.0 | Value may now only be `undefined` or `null`. Before all falsy values were handled the same as `null` and did not throw.

- `value` `<any>`

Throws `value` if `value` is not `undefined` or `null`. This is useful when testing the `error` argument in callbacks. The stack trace contains all frames from the error passed to `ifError()` including the potential new frames for `ifError()` itself.

    import assert from 'node:assert/strict';

    assert.ifError(null);
    // OK
    assert.ifError(0);
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
    assert.ifError('error');
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
    assert.ifError(new Error());
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

    // Create some random error frames.
    let err;
    (function errorFrame() {
      err = new Error('test error');
    })();

    (function ifErrorFrame() {
      assert.ifError(err);
    })();
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
    //     at ifErrorFrame
    //     at errorFrame
    const assert = require('node:assert/strict');

    assert.ifError(null);
    // OK
    assert.ifError(0);
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
    assert.ifError('error');
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
    assert.ifError(new Error());
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

    // Create some random error frames.
    let err;
    (function errorFrame() {
      err = new Error('test error');
    })();

    (function ifErrorFrame() {
      assert.ifError(err);
    })();
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
    //     at ifErrorFrame
    //     at errorFrame
    javascript

### `assert.match(string, regexp[, message])`#

Added in: v13.6.0, v12.16.0
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0 | This API is no longer experimental.

- `string` `<string>`
- `regexp` `<RegExp>`
- `message` `<string>` | `<Error>` | `<Function>`

Expects the `string` input to match the regular expression.

    import assert from 'node:assert/strict';

    assert.match('I will fail', /pass/);
    // AssertionError [ERR_ASSERTION]: The input did not match the regular ...

    assert.match(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.match('I will pass', /pass/);
    // OK
    const assert = require('node:assert/strict');

    assert.match('I will fail', /pass/);
    // AssertionError [ERR_ASSERTION]: The input did not match the regular ...

    assert.match(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.match('I will pass', /pass/);
    // OK
    javascript

If the values do not match, or if the `string` argument is of another type than `string`, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notDeepEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0, v4.5.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.notDeepStrictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.notDeepStrictEqual()` instead.

Tests for any deep inequality. Opposite of `assert.deepEqual()`.

    import assert from 'node:assert';

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.notDeepEqual(obj1, obj1);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj2);
    // OK

    assert.notDeepEqual(obj1, obj3);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj4);
    // OK
    const assert = require('node:assert');

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.notDeepEqual(obj1, obj1);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj2);
    // OK

    assert.notDeepEqual(obj1, obj3);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj4);
    // OK
    javascript

If the values are deeply equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notDeepStrictEqual(actual, expected[, message])`#

Added in: v1.2.0
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v9.0.0 | The `-0` and `+0` are not considered equal anymore.
v9.0.0 | The `NaN` is now compared using the SameValueZero comparison.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests for deep strict inequality. Opposite of `assert.deepStrictEqual()`.

    import assert from 'node:assert/strict';

    assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
    // OK
    const assert = require('node:assert/strict');

    assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
    // OK
    javascript

If the values are deeply and strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.notStrictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.notStrictEqual()` instead.

Tests shallow, coercive inequality with the `!=` operator. `NaN` is specially handled and treated as being identical if both sides are `NaN`.

    import assert from 'node:assert';

    assert.notEqual(1, 2);
    // OK

    assert.notEqual(1, 1);
    // AssertionError: 1 != 1

    assert.notEqual(1, '1');
    // AssertionError: 1 != '1'
    const assert = require('node:assert');

    assert.notEqual(1, 2);
    // OK

    assert.notEqual(1, 1);
    // AssertionError: 1 != 1

    assert.notEqual(1, '1');
    // AssertionError: 1 != '1'
    javascript

If the values are equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notStrictEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v10.0.0 | Used comparison changed from Strict Equality to `Object.is()`.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests strict inequality between the `actual` and `expected` parameters as determined by `Object.is()`.

    import assert from 'node:assert/strict';

    assert.notStrictEqual(1, 2);
    // OK

    assert.notStrictEqual(1, 1);
    // AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
    //
    // 1

    assert.notStrictEqual(1, '1');
    // OK
    const assert = require('node:assert/strict');

    assert.notStrictEqual(1, 2);
    // OK

    assert.notStrictEqual(1, 1);
    // AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
    //
    // 1

    assert.notStrictEqual(1, '1');
    // OK
    javascript

If the values are strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.ok(value[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v10.0.0 | The `assert.ok()` (no arguments) will now use a predefined error message.

- `value` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests if `value` is truthy. It is equivalent to `assert.equal(!!value, true, message)`.

If `value` is not truthy, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is `undefined`, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`. If no arguments are passed in at all `message` will be set to the string: `'No value argument passed to `assert.ok()`'`.

Be aware that in the `repl` the error message will be different to the one thrown in a file! See below for further details.

    import assert from 'node:assert/strict';

    assert.ok(true);
    // OK
    assert.ok(1);
    // OK

    assert.ok();
    // AssertionError: No value argument passed to `assert.ok()`

    assert.ok(false, 'it\'s false');
    // AssertionError: it's false

    // In the repl:
    assert.ok(typeof 123 === 'string');
    // AssertionError: false == true

    // In a file (e.g. test.js):
    assert.ok(typeof 123 === 'string');
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(typeof 123 === 'string')

    assert.ok(false);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(false)

    assert.ok(0);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(0)
    const assert = require('node:assert/strict');

    assert.ok(true);
    // OK
    assert.ok(1);
    // OK

    assert.ok();
    // AssertionError: No value argument passed to `assert.ok()`

    assert.ok(false, 'it\'s false');
    // AssertionError: it's false

    // In the repl:
    assert.ok(typeof 123 === 'string');
    // AssertionError: false == true

    // In a file (e.g. test.js):
    assert.ok(typeof 123 === 'string');
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(typeof 123 === 'string')

    assert.ok(false);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(false)

    assert.ok(0);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(0)
    javascript

    import assert from 'node:assert/strict';

    // Using `assert()` works the same:
    assert(2 + 2 > 5);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert(2 + 2 > 5)
    const assert = require('node:assert');

    // Using `assert()` works the same:
    assert(2 + 2 > 5);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert(2 + 2 > 5)
    javascript

### `assert.rejects(asyncFn[, error][, message])`#

Added in: v10.0.0

- `asyncFn` `<Function>` | `<Promise>`
- `error` `<RegExp>` | `<Function>` | `<Object>` | `<Error>`
- `message` `<string>`
- Returns: `<Promise>`

Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is rejected.

If `asyncFn` is a function and it throws an error synchronously, `assert.rejects()` will return a rejected `Promise` with that error. If the function does not return a promise, `assert.rejects()` will return a rejected `Promise` with an `ERR_INVALID_RETURN_VALUE` error. In both cases the error handler is skipped.

Besides the async nature to await the completion behaves identically to `assert.throws()`.

If specified, `error` can be a `Class`, `<RegExp>`, a validation function, an object where each property will be tested for, or an instance of error where each property will be tested for including the non-enumerable `message` and `name` properties.

If specified, `message` will be the message provided by the `AssertionError` if the `asyncFn` fails to reject.

    import assert from 'node:assert/strict';

    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      {
        name: 'TypeError',
        message: 'Wrong value',
      },
    );
    const assert = require('node:assert/strict');

    (async () => {
      await assert.rejects(
        async () => {
          throw new TypeError('Wrong value');
        },
        {
          name: 'TypeError',
          message: 'Wrong value',
        },
      );
    })();
    javascript

    import assert from 'node:assert/strict';

    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      (err) => {
        assert.strictEqual(err.name, 'TypeError');
        assert.strictEqual(err.message, 'Wrong value');
        return true;
      },
    );
    const assert = require('node:assert/strict');

    (async () => {
      await assert.rejects(
        async () => {
          throw new TypeError('Wrong value');
        },
        (err) => {
          assert.strictEqual(err.name, 'TypeError');
          assert.strictEqual(err.message, 'Wrong value');
          return true;
        },
      );
    })();
    javascript

    import assert from 'node:assert/strict';

    assert.rejects(
      Promise.reject(new Error('Wrong value')),
      Error,
    ).then(() => {
      // ...
    });
    const assert = require('node:assert/strict');

    assert.rejects(
      Promise.reject(new Error('Wrong value')),
      Error,
    ).then(() => {
      // ...
    });
    javascript

`error` cannot be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Please read the example in `assert.throws()` carefully if using a string as the second argument gets considered.

### `assert.strictEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v10.0.0 | Used comparison changed from Strict Equality to `Object.is()`.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>` Postfix `printf`-like arguments in case it's used as format string. If message is a function, it is called in case of a comparison failure. The function receives the `actual` and `expected` arguments and has to return a string that is going to be used as error message. `printf`-like format strings and functions are beneficial for performance reasons in case arguments are passed through. In addition, it allows nice formatting with ease.

Tests strict equality between the `actual` and `expected` parameters as determined by `Object.is()`.

    import assert from 'node:assert/strict';

    assert.strictEqual(1, 2);
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    //
    // 1 !== 2

    assert.strictEqual(1, 1);
    // OK

    assert.strictEqual('Hello foobar', 'Hello World!');
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    // + actual - expected
    //
    // + 'Hello foobar'
    // - 'Hello World!'
    //          ^

    const apples = 1;
    const oranges = 2;
    assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(apples, oranges, 'apples %s !== oranges %s', apples, oranges);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
    // TypeError: Inputs are not identical

    assert.strictEqual(apples, oranges, (actual, expected) => {
      // Do 'heavy' computations
      return `I expected ${expected} but I got ${actual}`;
    });
    // AssertionError [ERR_ASSERTION]: I expected oranges but I got apples
    const assert = require('node:assert/strict');

    assert.strictEqual(1, 2);
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    //
    // 1 !== 2

    assert.strictEqual(1, 1);
    // OK

    assert.strictEqual('Hello foobar', 'Hello World!');
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    // + actual - expected
    //
    // + 'Hello foobar'
    // - 'Hello World!'
    //          ^

    const apples = 1;
    const oranges = 2;
    assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(apples, oranges, 'apples %s !== oranges %s', apples, oranges);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
    // TypeError: Inputs are not identical

    assert.strictEqual(apples, oranges, (actual, expected) => {
      // Do 'heavy' computations
      return `I expected ${expected} but I got ${actual}`;
    });
    // AssertionError [ERR_ASSERTION]: I expected oranges but I got apples
    javascript

If the values are not strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.throws(fn[, error][, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v10.2.0 | The `error` parameter can be an object containing regular expressions now.
v9.9.0 | The `error` parameter can now be an object as well.
v4.2.0 | The `error` parameter can now be an arrow function.

- `fn` `<Function>`
- `error` `<RegExp>` | `<Function>` | `<Object>` | `<Error>`
- `message` `<string>`

Expects the function `fn` to throw an error.

If specified, `error` can be a `Class`, `<RegExp>`, a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable `message` and `name` properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples.

If specified, `message` will be appended to the message provided by the `AssertionError` if the `fn` call fails to throw or in case the error validation fails.

Custom validation object/error instance:

    import assert from 'node:assert/strict';

    const err = new TypeError('Wrong value');
    err.code = 404;
    err.foo = 'bar';
    err.info = {
      nested: true,
      baz: 'text',
    };
    err.reg = /abc/i;

    assert.throws(
      () => {
        throw err;
      },
      {
        name: 'TypeError',
        message: 'Wrong value',
        info: {
          nested: true,
          baz: 'text',
        },
        // Only properties on the validation object will be tested for.
        // Using nested objects requires all properties to be present. Otherwise
        // the validation is going to fail.
      },
    );

    // Using regular expressions to validate error properties:
    assert.throws(
      () => {
        throw err;
      },
      {
        // The `name` and `message` properties are strings and using regular
        // expressions on those will match against the string. If they fail, an
        // error is thrown.
        name: /^TypeError$/,
        message: /Wrong/,
        foo: 'bar',
        info: {
          nested: true,
          // It is not possible to use regular expressions for nested properties!
          baz: 'text',
        },
        // The `reg` property contains a regular expression and only if the
        // validation object contains an identical regular expression, it is going
        // to pass.
        reg: /abc/i,
      },
    );

    // Fails due to the different `message` and `name` properties:
    assert.throws(
      () => {
        const otherErr = new Error('Not found');
        // Copy all enumerable properties from `err` to `otherErr`.
        for (const [key, value] of Object.entries(err)) {
          otherErr[key] = value;
        }
        throw otherErr;
      },
      // The error's `message` and `name` properties will also be checked when using
      // an error as validation object.
      err,
    );
    const assert = require('node:assert/strict');

    const err = new TypeError('Wrong value');
    err.code = 404;
    err.foo = 'bar';
    err.info = {
      nested: true,
      baz: 'text',
    };
    err.reg = /abc/i;

    assert.throws(
      () => {
        throw err;
      },
      {
        name: 'TypeError',
        message: 'Wrong value',
        info: {
          nested: true,
          baz: 'text',
        },
        // Only properties on the validation object will be tested for.
        // Using nested objects requires all properties to be present. Otherwise
        // the validation is going to fail.
      },
    );

    // Using regular expressions to validate error properties:
    assert.throws(
      () => {
        throw err;
      },
      {
        // The `name` and `message` properties are strings and using regular
        // expressions on those will match against the string. If they fail, an
        // error is thrown.
        name: /^TypeError$/,
        message: /Wrong/,
        foo: 'bar',
        info: {
          nested: true,
          // It is not possible to use regular expressions for nested properties!
          baz: 'text',
        },
        // The `reg` property contains a regular expression and only if the
        // validation object contains an identical regular expression, it is going
        // to pass.
        reg: /abc/i,
      },
    );

    // Fails due to the different `message` and `name` properties:
    assert.throws(
      () => {
        const otherErr = new Error('Not found');
        // Copy all enumerable properties from `err` to `otherErr`.
        for (const [key, value] of Object.entries(err)) {
          otherErr[key] = value;
        }
        throw otherErr;
      },
      // The error's `message` and `name` properties will also be checked when using
      // an error as validation object.
      err,
    );
    javascript

Validate instanceof using constructor:

    import assert from 'node:assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      Error,
    );
    const assert = require('node:assert/strict');

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      Error,
    );
    javascript

Validate error message using `<RegExp>`:

Using a regular expression runs `.toString` on the error object, and will therefore also include the error name.

    import assert from 'node:assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      /^Error: Wrong value$/,
    );
    const assert = require('node:assert/strict');

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      /^Error: Wrong value$/,
    );
    javascript

Custom error validation:

The function must return `true` to indicate all internal validations passed. It will otherwise fail with an `AssertionError`.

    import assert from 'node:assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      (err) => {
        assert(err instanceof Error);
        assert(/value/.test(err));
        // Avoid returning anything from validation functions besides `true`.
        // Otherwise, it's not clear what part of the validation failed. Instead,
        // throw an error about the specific validation that failed (as done in this
        // example) and add as much helpful debugging information to that error as
        // possible.
        return true;
      },
      'unexpected error',
    );
    const assert = require('node:assert/strict');

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      (err) => {
        assert(err instanceof Error);
        assert(/value/.test(err));
        // Avoid returning anything from validation functions besides `true`.
        // Otherwise, it's not clear what part of the validation failed. Instead,
        // throw an error about the specific validation that failed (as done in this
        // example) and add as much helpful debugging information to that error as
        // possible.
        return true;
      },
      'unexpected error',
    );
    javascript

`error` cannot be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an `ERR_AMBIGUOUS_ARGUMENT` error. Please read the example below carefully if using a string as the second argument gets considered:

    import assert from 'node:assert/strict';

    function throwingFirst() {
      throw new Error('First');
    }

    function throwingSecond() {
      throw new Error('Second');
    }

    function notThrowing() {}

    // The second argument is a string and the input function threw an Error.
    // The first case will not throw as it does not match for the error message
    // thrown by the input function!
    assert.throws(throwingFirst, 'Second');
    // In the next example the message has no benefit over the message from the
    // error and since it is not clear if the user intended to actually match
    // against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
    assert.throws(throwingSecond, 'Second');
    // TypeError [ERR_AMBIGUOUS_ARGUMENT]

    // The string is only used (as message) in case the function does not throw:
    assert.throws(notThrowing, 'Second');
    // AssertionError [ERR_ASSERTION]: Missing expected exception: Second

    // If it was intended to match for the error message do this instead:
    // It does not throw because the error messages match.
    assert.throws(throwingSecond, /Second$/);

    // If the error message does not match, an AssertionError is thrown.
    assert.throws(throwingFirst, /Second$/);
    // AssertionError [ERR_ASSERTION]
    const assert = require('node:assert/strict');

    function throwingFirst() {
      throw new Error('First');
    }

    function throwingSecond() {
      throw new Error('Second');
    }

    function notThrowing() {}

    // The second argument is a string and the input function threw an Error.
    // The first case will not throw as it does not match for the error message
    // thrown by the input function!
    assert.throws(throwingFirst, 'Second');
    // In the next example the message has no benefit over the message from the
    // error and since it is not clear if the user intended to actually match
    // against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
    assert.throws(throwingSecond, 'Second');
    // TypeError [ERR_AMBIGUOUS_ARGUMENT]

    // The string is only used (as message) in case the function does not throw:
    assert.throws(notThrowing, 'Second');
    // AssertionError [ERR_ASSERTION]: Missing expected exception: Second

    // If it was intended to match for the error message do this instead:
    // It does not throw because the error messages match.
    assert.throws(throwingSecond, /Second$/);

    // If the error message does not match, an AssertionError is thrown.
    assert.throws(throwingFirst, /Second$/);
    // AssertionError [ERR_ASSERTION]
    javascript

Due to the confusing error-prone notation, avoid a string as the second argument.

### `assert.partialDeepStrictEqual(actual, expected[, message])`#

Added in: v23.4.0, v22.13.0
History

Version | Changes
--- | ---
v25.0.0 | Promises are not considered equal anymore if they are not of the same instance.
v25.0.0 | Invalid dates are now considered equal.
v24.0.0, v22.17.0 | partialDeepStrictEqual is now Stable. Previously, it had been Experimental.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests for partial deep equality between the `actual` and `expected` parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules. "Partial" equality means that only properties that exist on the `expected` parameter are going to be compared.

This method always passes the same test cases as `assert.deepStrictEqual()`, behaving as a super set of it.

#### Comparison details#

- Primitive values are compared using `Object.is()`.
- Type tags of objects should be the same.
- `[[Prototype]]` of objects are not compared.
- Only enumerable "own" properties are considered.
- `<Error>` names, messages, causes, and errors are always compared, even if these are not enumerable properties. `errors` is also compared.
- Enumerable own `<Symbol>` properties are compared as well.
- Object wrappers are compared both as objects and unwrapped values.
- `Object` properties are compared unordered.
- `<Map>` keys and `<Set>` items are compared unordered.
- Recursion stops when both sides differ or both sides encounter a circular reference.
- `<WeakMap>`, `<WeakSet>` and `<Promise>` instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different `WeakMap`, `WeakSet`, or `Promise` instances will result in inequality, even if they contain the same content.
- `<RegExp>` lastIndex, flags, and source are always compared, even if these are not enumerable properties.
- Holes in sparse arrays are ignored.

    import assert from 'node:assert';

    assert.partialDeepStrictEqual(
      { a: { b: { c: 1 } } },
      { a: { b: { c: 1 } } },
    );
    // OK

    assert.partialDeepStrictEqual(
      { a: 1, b: 2, c: 3 },
      { b: 2 },
    );
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [4, 5, 8],
    );
    // OK

    assert.partialDeepStrictEqual(
      new Set([{ a: 1 }, { b: 1 }]),
      new Set([{ a: 1 }]),
    );
    // OK

    assert.partialDeepStrictEqual(
      new Map([['key1', 'value1'], ['key2', 'value2']]),
      new Map([['key2', 'value2']]),
    );
    // OK

    assert.partialDeepStrictEqual(123n, 123n);
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [5, 4, 8],
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: 1 },
      { a: 1, b: 2 },
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: { b: 2 } },
      { a: { b: '2' } },
    );
    // AssertionError
    const assert = require('node:assert');

    assert.partialDeepStrictEqual(
      { a: { b: { c: 1 } } },
      { a: { b: { c: 1 } } },
    );
    // OK

    assert.partialDeepStrictEqual(
      { a: 1, b: 2, c: 3 },
      { b: 2 },
    );
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [4, 5, 8],
    );
    // OK

    assert.partialDeepStrictEqual(
      new Set([{ a: 1 }, { b: 1 }]),
      new Set([{ a: 1 }]),
    );
    // OK

    assert.partialDeepStrictEqual(
      new Map([['key1', 'value1'], ['key2', 'value2']]),
      new Map([['key2', 'value2']]),
    );
    // OK

    assert.partialDeepStrictEqual(123n, 123n);
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [5, 4, 8],
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: 1 },
      { a: 1, b: 2 },
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: { b: 2 } },
      { a: { b: '2' } },
    );
    // AssertionError
    javascript

## Async hooks#

Source Code: lib/async_hooks.js

Stability: 1 - Experimental. Please migrate away from this API, if you can. We do not recommend using the `createHook`, `AsyncHook`, and `executionAsyncResource` APIs as they have usability issues, safety risks, and performance implications. Async context tracking use cases are better served by the stable `AsyncLocalStorage` API. If you have a use case for `createHook`, `AsyncHook`, or `executionAsyncResource` beyond the context tracking need solved by `AsyncLocalStorage` or diagnostics data currently provided by Diagnostics Channel, please open an issue at https://github.com/nodejs/node/issues describing your use case so we can create a more purpose-focused API.

We strongly discourage the use of the `async_hooks` API. Other APIs that can cover most of its use cases include:

- `AsyncLocalStorage` tracks async context
- `process.getActiveResourcesInfo()` tracks active resources

The `node:async_hooks` module provides an API to track asynchronous resources. It can be accessed using:

    import async_hooks from 'node:async_hooks';
    const async_hooks = require('node:async_hooks');
    javascript

### Terminology#

An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, such as the `'connection'` event in `net.createServer()`, or just a single time like in `fs.open()`. A resource can also be closed before the callback is called. `AsyncHook` does not explicitly distinguish between these different cases but will represent them as the abstract concept that is a resource.

If `Worker`s are used, each thread has an independent `async_hooks` interface, and each thread will use a new set of async IDs.

### Overview#

Following is a simple overview of the public API.

    import async_hooks from 'node:async_hooks';

    // Return the ID of the current execution context.
    const eid = async_hooks.executionAsyncId();

    // Return the ID of the handle responsible for triggering the callback of the
    // current execution scope to call.
    const tid = async_hooks.triggerAsyncId();

    // Create a new AsyncHook instance. All of these callbacks are optional.
    const asyncHook =
        async_hooks.createHook({ init, before, after, destroy, promiseResolve });

    // Allow callbacks of this AsyncHook instance to call. This is not an implicit
    // action after running the constructor, and must be explicitly run to begin
    // executing callbacks.
    asyncHook.enable();

    // Disable listening for new asynchronous events.
    asyncHook.disable();

    //
    // The following are the callbacks that can be passed to createHook().
    //

    // init() is called during object construction. The resource may not have
    // completed construction when this callback runs. Therefore, all fields of the
    // resource referenced by "asyncId" may not have been populated.
    function init(asyncId, type, triggerAsyncId, resource) { }

    // before() is called just before the resource's callback is called. It can be
    // called 0-N times for handles (such as TCPWrap), and will be called exactly 1
    // time for requests (such as FSReqCallback).
    function before(asyncId) { }

    // after() is called just after the resource's callback has finished.
    function after(asyncId) { }

    // destroy() is called when the resource is destroyed.
    function destroy(asyncId) { }

    // promiseResolve() is called only for promise resources, when the
    // resolve() function passed to the Promise constructor is invoked
    // (either directly or through other means of resolving a promise).
    function promiseResolve(asyncId) { }
    const async_hooks = require('node:async_hooks');

    // Return the ID of the current execution context.
    const eid = async_hooks.executionAsyncId();

    // Return the ID of the handle responsible for triggering the callback of the
    // current execution scope to call.
    const tid = async_hooks.triggerAsyncId();

    // Create a new AsyncHook instance. All of these callbacks are optional.
    const asyncHook =
        async_hooks.createHook({ init, before, after, destroy, promiseResolve });

    // Allow callbacks of this AsyncHook instance to call. This is not an implicit
    // action after running the constructor, and must be explicitly run to begin
    // executing callbacks.
    asyncHook.enable();

    // Disable listening for new asynchronous events.
    asyncHook.disable();

    //
    // The following are the callbacks that can be passed to createHook().
    //

    // init() is called during object construction. The resource may not have
    // completed construction when this callback runs. Therefore, all fields of the
    // resource referenced by "asyncId" may not have been populated.
    function init(asyncId, type, triggerAsyncId, resource) { }

    // before() is called just before the resource's callback is called. It can be
    // called 0-N times for handles (such as TCPWrap), and will be called exactly 1
    // time for requests (such as FSReqCallback).
    function before(asyncId) { }

    // after() is called just after the resource's callback has finished.
    function after(asyncId) { }

    // destroy() is called when the resource is destroyed.
    function destroy(asyncId) { }

    // promiseResolve() is called only for promise resources, when the
    // resolve() function passed to the Promise constructor is invoked
    // (either directly or through other means of resolving a promise).
    function promiseResolve(asyncId) { }
    javascript

### `async_hooks.createHook(options)`#

Added in: v8.1.0

- `options` `<Object>` The Hook Callbacks to register
  - `init` `<Function>` The `init` callback.
  - `before` `<Function>` The `before` callback.
  - `after` `<Function>` The `after` callback.
  - `destroy` `<Function>` The `destroy` callback.
  - `promiseResolve` `<Function>` The `promiseResolve` callback.
  - `trackPromises` `<boolean>` Whether the hook should track `Promise`s. Cannot be `false` if `promiseResolve` is set. Default: `true`.
- Returns: `<AsyncHook>` Instance used for disabling and enabling hooks

Registers functions to be called for different lifetime events of each async operation.

The callbacks `init()`/`before()`/`after()`/`destroy()` are called for the respective asynchronous event during a resource's lifetime.

All callbacks are optional. For example, if only resource cleanup needs to be tracked, then only the `destroy` callback needs to be passed. The specifics of all functions that can be passed to `callbacks` is in the Hook Callbacks section.

    import { createHook } from 'node:async_hooks';

    const asyncHook = createHook({
      init(asyncId, type, triggerAsyncId, resource) { },
      destroy(asyncId) { },
    });
    const async_hooks = require('node:async_hooks');

    const asyncHook = async_hooks.createHook({
      init(asyncId, type, triggerAsyncId, resource) { },
      destroy(asyncId) { },
    });
    javascript

The callbacks will be inherited via the prototype chain:

    class MyAsyncCallbacks {
      init(asyncId, type, triggerAsyncId, resource) { }
      destroy(asyncId) {}
    }

    class MyAddedCallbacks extends MyAsyncCallbacks {
      before(asyncId) { }
      after(asyncId) { }
    }

    const asyncHook = async_hooks.createHook(new MyAddedCallbacks());
    js

Because promises are asynchronous resources whose lifecycle is tracked via the async hooks mechanism, the `init()`, `before()`, `after()`, and `destroy()` callbacks must not be async functions that return promises.

#### Error handling#

If any `AsyncHook` callbacks throw, the application will print the stack trace and exit. The exit path does follow that of an uncaught exception, but all `'uncaughtException'` listeners are removed, thus forcing the process to exit. The `'exit'` callbacks will still be called unless the application is run with `--abort-on-uncaught-exception`, in which case a stack trace will be printed and the application exits, leaving a core file.

The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects.

#### Printing in `AsyncHook` callbacks#

Because printing to the console is an asynchronous operation, `console.log()` will cause `AsyncHook` callbacks to be called. Using `console.log()` or similar asynchronous operations inside an `AsyncHook` callback function will cause an infinite recursion. An easy solution to this when debugging is to use a synchronous logging operation such as `fs.writeFileSync(file, msg, flag)`. This will print to the file and will not invoke `AsyncHook` recursively because it is synchronous.

    import { writeFileSync } from 'node:fs';
    import { format } from 'node:util';

    function debug(...args) {
      // Use a function like this one when debugging inside an AsyncHook callback
      writeFileSync('log.out', `${format(...args)}\n`, { flag: 'a' });
    }
    const fs = require('node:fs');
    const util = require('node:util');

    function debug(...args) {
      // Use a function like this one when debugging inside an AsyncHook callback
      fs.writeFileSync('log.out', `${util.format(...args)}\n`, { flag: 'a' });
    }
    javascript

If an asynchronous operation is needed for logging, it is possible to keep track of what caused the asynchronous operation using the information provided by `AsyncHook` itself. The logging should then be skipped when it was the logging itself that caused the `AsyncHook` callback to be called. By doing this, the otherwise infinite recursion is broken.

### Class: `AsyncHook`#

The class `AsyncHook` exposes an interface for tracking lifetime events of asynchronous operations.

#### `asyncHook.enable()`#

- Returns: `<AsyncHook>` A reference to `asyncHook`.

Enable the callbacks for a given `AsyncHook` instance. If no callbacks are provided, enabling is a no-op.

The `AsyncHook` instance is disabled by default. If the `AsyncHook` instance should be enabled immediately after creation, the following pattern can be used.

    import { createHook } from 'node:async_hooks';

    const hook = createHook(callbacks).enable();
    const async_hooks = require('node:async_hooks');

    const hook = async_hooks.createHook(callbacks).enable();
    javascript

#### `asyncHook.disable()`#

- Returns: `<AsyncHook>` A reference to `asyncHook`.

Disable the callbacks for a given `AsyncHook` instance from the global pool of `AsyncHook` callbacks to be executed. Once a hook has been disabled it will not be called again until enabled.

For API consistency `disable()` also returns the `AsyncHook` instance.

#### Hook callbacks#

Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destroyed.

##### `init(asyncId, type, triggerAsyncId, resource)`#

- `asyncId` `<number>` A unique ID for the async resource.
- `type` `<string>` The type of the async resource.
- `triggerAsyncId` `<number>` The unique ID of the async resource in whose execution context this async resource was created.
- `resource` `<Object>` Reference to the resource representing the async operation, needs to be released during destroy.

Called when a class is constructed that has the possibility to emit an asynchronous event. This does not mean the instance must call `before`/`after` before `destroy` is called, only that the possibility exists.

This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this.

    import { createServer } from 'node:net';

    createServer().listen(function() { this.close(); });
    // OR
    clearTimeout(setTimeout(() => {}, 10));
    require('node:net').createServer().listen(function() { this.close(); });
    // OR
    clearTimeout(setTimeout(() => {}, 10));
    javascript

Every new resource is assigned an ID that is unique within the scope of the current Node.js instance.

###### `type`#

The `type` is a string identifying the type of resource that caused `init` to be called. Generally, it will correspond to the name of the resource's constructor.

The `type` of resources created by Node.js itself can change in any Node.js release. Valid values include `TLSWRAP`, `TCPWRAP`, `TCPSERVERWRAP`, `GETADDRINFOREQWRAP`, `FSREQCALLBACK`, `Microtask`, and `Timeout`. Inspect the source code of the Node.js version used to get the full list.

Furthermore users of `AsyncResource` create async resources independent of Node.js itself.

There is also the `PROMISE` resource type, which is used to track `Promise` instances and asynchronous work scheduled by them. The `Promise`s are only tracked when `trackPromises` option is set to `true`.

Users are able to define their own `type` when using the public embedder API.

It is possible to have type name collisions. Embedders are encouraged to use unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.

###### `triggerAsyncId`#

`triggerAsyncId` is the `asyncId` of the resource that caused (or "triggered") the new resource to initialize and that caused `init` to call. This is different from `async_hooks.executionAsyncId()` that only shows when a resource was created, while `triggerAsyncId` shows why a resource was created.

The following is a simple demonstration of `triggerAsyncId`:

    import { createHook, executionAsyncId } from 'node:async_hooks';
    import { stdout } from 'node:process';
    import net from 'node:net';
    import fs from 'node:fs';

    createHook({
      init(asyncId, type, triggerAsyncId) {
        const eid = executionAsyncId();
        fs.writeSync(
          stdout.fd,
          `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
      },
    }).enable();

    net.createServer((conn) => {}).listen(8080);
    const { createHook, executionAsyncId } = require('node:async_hooks');
    const { stdout } = require('node:process');
    const net = require('node:net');
    const fs = require('node:fs');

    createHook({
      init(asyncId, type, triggerAsyncId) {
        const eid = executionAsyncId();
        fs.writeSync(
          stdout.fd,
          `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
      },
    }).enable();

    net.createServer((conn) => {}).listen(8080);
    javascript

Output when hitting the server with `nc localhost 8080`:

    TCPSERVERWRAP(5): trigger: 1 execution: 1
    TCPWRAP(7): trigger: 5 execution: 0
    console

The `TCPSERVERWRAP` is the server which receives the connections.

The `TCPWRAP` is the new connection from the client. When a new connection is made, the `TCPWrap` instance is immediately constructed. This happens outside of any JavaScript stack. (An `executionAsyncId()` of `0` means that it is being executed from C++ with no JavaScript stack above it.) With only that information, it would be impossible to link resources together in terms of what caused them to be created, so `triggerAsyncId` is given the task of propagating what resource is responsible for the new resource's existence.

###### `resource`#

`resource` is an object that represents the actual async resource that has been initialized. The API to access the object may be specified by the creator of the resource. Resources created by Node.js itself are internal and may change at any time. Therefore no API is specified for these.

In some cases the resource object is reused for performance reasons, it is thus not safe to use it as a key in a `WeakMap` or add properties to it.

###### Asynchronous context example#

The context tracking use case is covered by the stable API `AsyncLocalStorage`. This example only illustrates async hooks operation but `AsyncLocalStorage` fits better to this use case.

The following is an example with additional information about the calls to `init` between the `before` and `after` calls, specifically what the callback to `listen()` will look like. The output formatting is slightly more elaborate to make calling context easier to see.

    import async_hooks from 'node:async_hooks';
    import fs from 'node:fs';
    import net from 'node:net';
    import { stdout } from 'node:process';
    const { fd } = stdout;

    let indent = 0;
    async_hooks.createHook({
      init(asyncId, type, triggerAsyncId) {
        const eid = async_hooks.executionAsyncId();
        const indentStr = ' '.repeat(indent);
        fs.writeSync(
          fd,
          `${indentStr}${type}(${asyncId}):` +
          ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
      },
      before(asyncId) {
        const indentStr = ' '.repeat(indent);
        fs.writeSync(fd, `${indentStr}before:  ${asyncId}\n`);
        indent += 2;
      },
      after(asyncId) {
        indent -= 2;
        const indentStr = ' '.repeat(indent);
        fs.writeSync(fd, `${indentStr}after:  ${asyncId}\n`);
      },
      destroy(asyncId) {
        const indentStr = ' '.repeat(indent);
        fs.writeSync(fd, `${indentStr}destroy:  ${asyncId}\n`);
      },
    }).enable();

    net.createServer(() => {}).listen(8080, () => {
      // Let's wait 10ms before logging the server started.
      setTimeout(() => {
        console.log('>>>', async_hooks.executionAsyncId());
      }, 10);
    });
    const async_hooks = require('node:async_hooks');
    const fs = require('node:fs');
    const net = require('node:net');
    const { fd } = process.stdout;

    let indent = 0;
    async_hooks.createHook({
      init(asyncId, type, triggerAsyncId) {
        const eid = async_hooks.executionAsyncId();
        const indentStr = ' '.repeat(indent);
        fs.writeSync(
          fd,
          `${indentStr}${type}(${asyncId}):` +
          ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
      },
      before(asyncId) {
        const indentStr = ' '.repeat(indent);
        fs.writeSync(fd, `${indentStr}before:  ${asyncId}\n`);
        indent += 2;
      },
      after(asyncId) {
        indent -= 2;
        const indentStr = ' '.repeat(indent);
        fs.writeSync(fd, `${indentStr}after:  ${asyncId}\n`);
      },
      destroy(asyncId) {
        const indentStr = ' '.repeat(indent);
        fs.writeSync(fd, `${indentStr}destroy:  ${asyncId}\n`);
      },
    }).enable();

    net.createServer(() => {}).listen(8080, () => {
      // Let's wait 10ms before logging the server started.
      setTimeout(() => {
        console.log('>>>', async_hooks.executionAsyncId());
      }, 10);
    });
    javascript

Output from only starting the server:

    TCPSERVERWRAP(5): trigger: 1 execution: 1
    TickObject(6): trigger: 5 execution: 1
    before:  6
      Timeout(7): trigger: 6 execution: 6
    after:   6
    destroy: 6
    before:  7
    >>> 7
      TickObject(8): trigger: 7 execution: 7
    after:   7
    before:  8
    after:   8
    console

As illustrated in the example, `executionAsyncId()` and `execution` each specify the value of the current execution context; which is delineated by calls to `before` and `after`.

Only using `execution` to graph resource allocation results in the following:

      root(1)
         ^
         |
    TickObject(6)
         ^
         |
     Timeout(7)
    console

The `TCPSERVERWRAP` is not part of this graph, even though it was the reason for `console.log()` being called. This is because binding to a port without a host name is a synchronous operation, but to maintain a completely asynchronous API the user's callback is placed in a `process.nextTick()`. Which is why `TickObject` is present in the output and is a 'parent' for `.listen()` callback.

The graph only shows when a resource was created, not why, so to track the why use `triggerAsyncId`. Which can be represented with the following graph:

     bootstrap(1)
         |
         ˅
    TCPSERVERWRAP(5)
         |
         ˅
     TickObject(6)
         |
         ˅
      Timeout(7)
    console

##### `before(asyncId)`#

- `asyncId` `<number>`

When an asynchronous operation is initiated (such as a TCP server receiving a new connection) or completes (such as writing data to disk) a callback is called to notify the user. The `before` callback is called just before said callback is executed. `asyncId` is the unique identifier assigned to the resource about to execute the callback.

The `before` callback will be called 0 to N times. The `before` callback will typically be called 0 times if the asynchronous operation was cancelled or, for example, if no connections are received by a TCP server. Persistent asynchronous resources like a TCP server will typically call the `before` callback multiple times, while other operations like `fs.open()` will call it only once.

##### `after(asyncId)`#

- `asyncId` `<number>`

Called immediately after the callback specified in `before` is completed.

If an uncaught exception occurs during execution of the callback, then `after` will run after the `'uncaughtException'` event is emitted or a `domain`'s handler runs.

##### `destroy(asyncId)`#

- `asyncId` `<number>`

Called after the resource corresponding to `asyncId` is destroyed. It is also called asynchronously from the embedder API `emitDestroy()`.

Some resources depend on garbage collection for cleanup, so if a reference is made to the `resource` object passed to `init` it is possible that `destroy` will never be called, causing a memory leak in the application. If the resource does not depend on garbage collection, then this will not be an issue.

Using the destroy hook results in additional overhead because it enables tracking of `Promise` instances via the garbage collector.

##### `promiseResolve(asyncId)`#

Added in: v8.6.0

- `asyncId` `<number>`

Called when the `resolve` function passed to the `Promise` constructor is invoked (either directly or through other means of resolving a promise).

`resolve()` does not do any observable synchronous work.

The `Promise` is not necessarily fulfilled or rejected at this point if the `Promise` was resolved by assuming the state of another `Promise`.

    new Promise((resolve) => resolve(true)).then((a) => {});
    js

calls the following callbacks:

    init for PROMISE with id 5, trigger id: 1
      promise resolve 5      # corresponds to resolve(true)
    init for PROMISE with id 6, trigger id: 5  # the Promise returned by then()
      before 6               # the then() callback is entered
      promise resolve 6      # the then() callback resolves the promise by returning
      after 6
    text

#### `async_hooks.executionAsyncResource()`#

Added in: v13.9.0, v12.17.0

- Returns: `<Object>` The resource representing the current execution. Useful to store data within the resource.

Resource objects returned by `executionAsyncResource()` are most often internal Node.js handle objects with undocumented APIs. Using any functions or properties on the object is likely to crash your application and should be avoided.

Using `executionAsyncResource()` in the top-level execution context will return an empty object as there is no handle or request object to use, but having an object representing the top-level can be helpful.

    import { open } from 'node:fs';
    import { executionAsyncId, executionAsyncResource } from 'node:async_hooks';

    console.log(executionAsyncId(), executionAsyncResource());  // 1 {}
    open(new URL(import.meta.url), 'r', (err, fd) => {
      console.log(executionAsyncId(), executionAsyncResource());  // 7 FSReqWrap
    });
    const { open } = require('node:fs');
    const { executionAsyncId, executionAsyncResource } = require('node:async_hooks');

    console.log(executionAsyncId(), executionAsyncResource());  // 1 {}
    open(__filename, 'r', (err, fd) => {
      console.log(executionAsyncId(), executionAsyncResource());  // 7 FSReqWrap
    });
    javascript

This can be used to implement continuation local storage without the use of a tracking `Map` to store the metadata:

    import { createServer } from 'node:http';
    import {
      executionAsyncId,
      executionAsyncResource,
      createHook,
    } from 'node:async_hooks';
    const sym = Symbol('state'); // Private symbol to avoid pollution

    createHook({
      init(asyncId, type, triggerAsyncId, resource) {
        const cr = executionAsyncResource();
        if (cr) {
          resource[sym] = cr[sym];
        }
      },
    }).enable();

    const server = createServer((req, res) => {
      executionAsyncResource()[sym] = { state: req.url };
      setTimeout(function() {
        res.end(JSON.stringify(executionAsyncResource()[sym]));
      }, 100);
    }).listen(3000);
    const { createServer } = require('node:http');
    const {
      executionAsyncId,
      executionAsyncResource,
      createHook,
    } = require('node:async_hooks');
    const sym = Symbol('state'); // Private symbol to avoid pollution

    createHook({
      init(asyncId, type, triggerAsyncId, resource) {
        const cr = executionAsyncResource();
        if (cr) {
          resource[sym] = cr[sym];
        }
      },
    }).enable();

    const server = createServer((req, res) => {
      executionAsyncResource()[sym] = { state: req.url };
      setTimeout(function() {
        res.end(JSON.stringify(executionAsyncResource()[sym]));
      }, 100);
    }).listen(3000);
    javascript

#### `async_hooks.executionAsyncId()`#

Added in: v8.1.0
History

Version | Changes
--- | ---
v8.2.0 | Renamed from `currentId`.

- Returns: `<number>` The `asyncId` of the current execution context. Useful to track when something calls.

    import { executionAsyncId } from 'node:async_hooks';
    import fs from 'node:fs';

    console.log(executionAsyncId());  // 1 - bootstrap
    const path = '.';
    fs.open(path, 'r', (err, fd) => {
      console.log(executionAsyncId());  // 6 - open()
    });
    const async_hooks = require('node:async_hooks');
    const fs = require('node:fs');

    console.log(async_hooks.executionAsyncId());  // 1 - bootstrap
    const path = '.';
    fs.open(path, 'r', (err, fd) => {
      console.log(async_hooks.executionAsyncId());  // 6 - open()
    });
    javascript

The ID returned from `executionAsyncId()` is related to execution timing, not causality (which is covered by `triggerAsyncId()`):

    const server = net.createServer((conn) => {
      // Returns the ID of the server, not of the new connection, because the
      // callback runs in the execution scope of the server's MakeCallback().
      async_hooks.executionAsyncId();

    }).listen(port, () => {
      // Returns the ID of a TickObject (process.nextTick()) because all
      // callbacks passed to .listen() are wrapped in a nextTick().
      async_hooks.executionAsyncId();
    });
    js

Promise contexts may not get precise `executionAsyncIds` by default. See the section on promise execution tracking.

#### `async_hooks.triggerAsyncId()`#

- Returns: `<number>` The ID of the resource responsible for calling the callback that is currently being executed.

    const server = net.createServer((conn) => {
      // The resource that caused (or triggered) this callback to be called
      // was that of the new connection. Thus the return value of triggerAsyncId()
      // is the asyncId of "conn".
      async_hooks.triggerAsyncId();

    }).listen(port, () => {
      // Even though all callbacks passed to .listen() are wrapped in a nextTick()
      // the callback itself exists because the call to the server's .listen()
      // was made. So the return value would be the ID of the server.
      async_hooks.triggerAsyncId();
    });
    js

Promise contexts may not get valid `triggerAsyncId`s by default. See the section on promise execution tracking.

#### `async_hooks.asyncWrapProviders`#

Added in: v17.2.0, v16.14.0

- Returns: A map of provider types to the corresponding numeric id. This map contains all the event types that might be emitted by the `async_hooks.init()` event.

This feature suppresses the deprecated usage of `process.binding('async_wrap').Providers`. See: DEP0111

### Promise execution tracking#

By default, promise executions are not assigned `asyncId`s due to the relatively expensive nature of the promise introspection API provided by V8. This means that programs using promises or `async`/`await` will not get correct execution and trigger ids for promise callback contexts by default.

    import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';

    Promise.resolve(1729).then(() => {
      console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
    });
    // produces:
    // eid 1 tid 0
    const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');

    Promise.resolve(1729).then(() => {
      console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
    });
    // produces:
    // eid 1 tid 0
    javascript

Observe that the `then()` callback claims to have executed in the context of the outer scope even though there was an asynchronous hop involved. Also, the `triggerAsyncId` value is `0`, which means that we are missing context about the resource that caused (triggered) the `then()` callback to be executed.

Installing async hooks via `async_hooks.createHook` enables promise execution tracking:

    import { createHook, executionAsyncId, triggerAsyncId } from 'node:async_hooks';
    createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
    Promise.resolve(1729).then(() => {
      console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
    });
    // produces:
    // eid 7 tid 6
    const { createHook, executionAsyncId, triggerAsyncId } = require('node:async_hooks');

    createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
    Promise.resolve(1729).then(() => {
      console.log(`eid ${executionAsyncId()} tid ${triggerAsyncId()}`);
    });
    // produces:
    // eid 7 tid 6
    javascript

In this example, adding any actual hook function enabled the tracking of promises. There are two promises in the example above; the promise created by `Promise.resolve()` and the promise returned by the call to `then()`. In the example above, the first promise got the `asyncId` `6` and the latter got `asyncId` `7`. During the execution of the `then()` callback, we are executing in the context of promise with `asyncId` `7`. This promise was triggered by async resource `6`.

Another subtlety with promises is that `before` and `after` callbacks are run only on chained promises. That means promises not created by `then()`/`catch()` will not have the `before` and `after` callbacks fired on them. For more details see the details of the V8 PromiseHooks API.

#### Disabling promise execution tracking#

Tracking promise execution can cause a significant performance overhead. To opt out of promise tracking, set `trackPromises` to `false`:

    const { createHook } = require('node:async_hooks');
    const { writeSync } = require('node:fs');
    createHook({
      init(asyncId, type, triggerAsyncId, resource) {
        // This init hook does not get called when trackPromises is set to false.
        writeSync(1, `init hook triggered for ${type}\n`);
      },
      trackPromises: false,  // Do not track promises.
    }).enable();
    Promise.resolve(1729);
    import { createHook } from 'node:async_hooks';
    import { writeSync } from 'node:fs';

    createHook({
      init(asyncId, type, triggerAsyncId, resource) {
        // This init hook does not get called when trackPromises is set to false.
        writeSync(1, `init hook triggered for ${type}\n`);
      },
      trackPromises: false,  // Do not track promises.
    }).enable();
    Promise.resolve(1729);
    javascript

### JavaScript embedder API#

Library developers that handle their own asynchronous resources performing tasks like I/O, connection pooling, or managing callback queues may use the `AsyncResource` JavaScript API so that all the appropriate callbacks are called.

#### Class: `AsyncResource`#

The documentation for this class has moved `AsyncResource`.

### Class: `AsyncLocalStorage`#

The documentation for this class has moved `AsyncLocalStorage`.

## Asynchronous context tracking#

Source Code: lib/async_hooks.js

Stability: 2 - Stable

### Introduction#

These classes are used to associate state and propagate it throughout callbacks and promise chains. They allow storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages.

The `AsyncLocalStorage` and `AsyncResource` classes are part of the `node:async_hooks` module:

    import { AsyncLocalStorage, AsyncResource } from 'node:async_hooks';
    const { AsyncLocalStorage, AsyncResource } = require('node:async_hooks');
    javascript

### Class: `AsyncLocalStorage`#

Added in: v13.10.0, v12.17.0
History

Version | Changes
--- | ---
v16.4.0 | AsyncLocalStorage is now Stable. Previously, it had been Experimental.

This class creates stores that stay coherent through asynchronous operations.

While you can create your own implementation on top of the `node:async_hooks` module, `AsyncLocalStorage` should be preferred as it is a performant and memory safe implementation that involves significant optimizations that are non-obvious to implement.

The following example uses `AsyncLocalStorage` to build a simple logger that assigns IDs to incoming HTTP requests and includes them in messages logged within each request.

    import http from 'node:http';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const asyncLocalStorage = new AsyncLocalStorage();

    function logWithId(msg) {
      const id = asyncLocalStorage.getStore();
      console.log(`${id !== undefined ? id : '-'}:`, msg);
    }

    let idSeq = 0;
    http.createServer((req, res) => {
      asyncLocalStorage.run(idSeq++, () => {
        logWithId('start');
        // Imagine any chain of async operations here
        setImmediate(() => {
          logWithId('finish');
          res.end();
        });
      });
    }).listen(8080);

    http.get('http://localhost:8080');
    http.get('http://localhost:8080');
    // Prints:
    //   0: start
    //   0: finish
    //   1: start
    //   1: finish
    const http = require('node:http');
    const { AsyncLocalStorage } = require('node:async_hooks');

    const asyncLocalStorage = new AsyncLocalStorage();

    function logWithId(msg) {
      const id = asyncLocalStorage.getStore();
      console.log(`${id !== undefined ? id : '-'}:`, msg);
    }

    let idSeq = 0;
    http.createServer((req, res) => {
      asyncLocalStorage.run(idSeq++, () => {
        logWithId('start');
        // Imagine any chain of async operations here
        setImmediate(() => {
          logWithId('finish');
          res.end();
        });
      });
    }).listen(8080);

    http.get('http://localhost:8080');
    http.get('http://localhost:8080');
    // Prints:
    //   0: start
    //   0: finish
    //   1: start
    //   1: finish
    javascript

Each instance of `AsyncLocalStorage` maintains an independent storage context. Multiple instances can safely exist simultaneously without risk of interfering with each other's data.

#### `new AsyncLocalStorage([options])`#

Added in: v13.10.0, v12.17.0
History

Version | Changes
--- | ---
v24.0.0 | Add `defaultValue` and `name` options.
v19.7.0, v18.16.0 | Removed experimental onPropagate option.
v19.2.0, v18.13.0 | Add option onPropagate.

- `options` `<Object>`
  - `defaultValue` `<any>` The default value to be used when no store is provided.
  - `name` `<string>` A name for the `AsyncLocalStorage` value.

Creates a new instance of `AsyncLocalStorage`. Store is only provided within a `run()` call or after an `enterWith()` call.

#### Static method: `AsyncLocalStorage.bind(fn)`#

Added in: v19.8.0, v18.16.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

- `fn` `<Function>` The function to bind to the current execution context.
- Returns: `<Function>` A new function that calls `fn` within the captured execution context.

Binds the given function to the current execution context.

#### Static method: `AsyncLocalStorage.snapshot()`#

Added in: v19.8.0, v18.16.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

- Returns: `<Function>` A new function with the signature `(fn: (...args) : R, ...args) : R`.

Captures the current execution context and returns a function that accepts a function as an argument. Whenever the returned function is called, it calls the function passed to it within the captured context.

    const asyncLocalStorage = new AsyncLocalStorage();
    const runInAsyncScope = asyncLocalStorage.run(123, () => AsyncLocalStorage.snapshot());
    const result = asyncLocalStorage.run(321, () => runInAsyncScope(() => asyncLocalStorage.getStore()));
    console.log(result);  // returns 123
    js

AsyncLocalStorage.snapshot() can replace the use of AsyncResource for simple async context tracking purposes, for example:

    class Foo {
      #runInAsyncScope = AsyncLocalStorage.snapshot();

      get() { return this.#runInAsyncScope(() => asyncLocalStorage.getStore()); }
    }

    const foo = asyncLocalStorage.run(123, () => new Foo());
    console.log(asyncLocalStorage.run(321, () => foo.get())); // returns 123
    js

#### `asyncLocalStorage.disable()`#

Added in: v13.10.0, v12.17.0

Stability: 1 - Experimental

Disables the instance of `AsyncLocalStorage`. All subsequent calls to `asyncLocalStorage.getStore()` will return `undefined` until `asyncLocalStorage.run()` or `asyncLocalStorage.enterWith()` is called again.

When calling `asyncLocalStorage.disable()`, all current contexts linked to the instance will be exited.

Calling `asyncLocalStorage.disable()` is required before the `asyncLocalStorage` can be garbage collected. This does not apply to stores provided by the `asyncLocalStorage`, as those objects are garbage collected along with the corresponding async resources.

Use this method when the `asyncLocalStorage` is not in use anymore in the current process.

#### `asyncLocalStorage.getStore()`#

Added in: v13.10.0, v12.17.0

- Returns: `<any>`

Returns the current store. If called outside of an asynchronous context initialized by calling `asyncLocalStorage.run()` or `asyncLocalStorage.enterWith()`, it returns `undefined`.

#### `asyncLocalStorage.enterWith(store)`#

Added in: v13.11.0, v12.17.0

Stability: 1 - Experimental

- `store` `<any>`

Transitions into the context for the remainder of the current synchronous execution and then persists the store through any following asynchronous calls.

Example:

    const store = { id: 1 };
    // Replaces previous store with the given store object
    asyncLocalStorage.enterWith(store);
    asyncLocalStorage.getStore(); // Returns the store object
    someAsyncOperation(() => {
      asyncLocalStorage.getStore(); // Returns the same object
    });
    js

This transition will continue for the entire synchronous execution. This means that if, for example, the context is entered within an event handler subsequent event handlers will also run within that context unless specifically bound to another context with an `AsyncResource`. That is why `run()` should be preferred over `enterWith()` unless there are strong reasons to use the latter method.

    const store = { id: 1 };

    emitter.on('my-event', () => {
      asyncLocalStorage.enterWith(store);
    });
    emitter.on('my-event', () => {
      asyncLocalStorage.getStore(); // Returns the same object
    });

    asyncLocalStorage.getStore(); // Returns undefined
    emitter.emit('my-event');
    asyncLocalStorage.getStore(); // Returns the same object
    js

#### `asyncLocalStorage.name`#

Added in: v24.0.0

- Type: `<string>`

The name of the `AsyncLocalStorage` instance if provided.

#### `asyncLocalStorage.run(store, callback[, ...args])`#

Added in: v13.10.0, v12.17.0

- `store` `<any>`
- `callback` `<Function>`
- `...args` `<any>`

Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback.

The optional `args` are passed to the callback function.

If the callback function throws an error, the error is thrown by `run()` too. The stacktrace is not impacted by this call and the context is exited.

Example:

    const store = { id: 2 };
    try {
      asyncLocalStorage.run(store, () => {
        asyncLocalStorage.getStore(); // Returns the store object
        setTimeout(() => {
          asyncLocalStorage.getStore(); // Returns the store object
        }, 200);
        throw new Error();
      });
    } catch (e) {
      asyncLocalStorage.getStore(); // Returns undefined
      // The error will be caught here
    }
    js

#### `asyncLocalStorage.exit(callback[, ...args])`#

Added in: v13.10.0, v12.17.0

Stability: 1 - Experimental

- `callback` `<Function>`
- `...args` `<any>`

Runs a function synchronously outside of a context and returns its return value. The store is not accessible within the callback function or the asynchronous operations created within the callback. Any `getStore()` call done within the callback function will always return `undefined`.

The optional `args` are passed to the callback function.

If the callback function throws an error, the error is thrown by `exit()` too. The stacktrace is not impacted by this call and the context is re-entered.

Example:

    // Within a call to run
    try {
      asyncLocalStorage.getStore(); // Returns the store object or value
      asyncLocalStorage.exit(() => {
        asyncLocalStorage.getStore(); // Returns undefined
        throw new Error();
      });
    } catch (e) {
      asyncLocalStorage.getStore(); // Returns the same object or value
      // The error will be caught here
    }
    js

#### `asyncLocalStorage.withScope(store)`#

Added in: v25.9.0

Stability: 1 - Experimental

- `store` `<any>`
- Returns: `<RunScope>`

Creates a disposable scope that enters the given store and automatically restores the previous store value when the scope is disposed. This method is designed to work with JavaScript's explicit resource management (`using` syntax).

Example:

    import { AsyncLocalStorage } from 'node:async_hooks';

    const asyncLocalStorage = new AsyncLocalStorage();

    {
      using _ = asyncLocalStorage.withScope('my-store');
      console.log(asyncLocalStorage.getStore()); // Prints: my-store
    }

    console.log(asyncLocalStorage.getStore()); // Prints: undefined
    const { AsyncLocalStorage } = require('node:async_hooks');

    const asyncLocalStorage = new AsyncLocalStorage();

    {
      using _ = asyncLocalStorage.withScope('my-store');
      console.log(asyncLocalStorage.getStore()); // Prints: my-store
    }

    console.log(asyncLocalStorage.getStore()); // Prints: undefined
    javascript

The `withScope()` method is particularly useful for managing context in synchronous code where you want to ensure the previous store value is restored when exiting a block, even if an error is thrown.

    import { AsyncLocalStorage } from 'node:async_hooks';

    const asyncLocalStorage = new AsyncLocalStorage();

    try {
      using _ = asyncLocalStorage.withScope('my-store');
      console.log(asyncLocalStorage.getStore()); // Prints: my-store
      throw new Error('test');
    } catch (e) {
      // Store is automatically restored even after error
      console.log(asyncLocalStorage.getStore()); // Prints: undefined
    }
    const { AsyncLocalStorage } = require('node:async_hooks');

    const asyncLocalStorage = new AsyncLocalStorage();

    try {
      using _ = asyncLocalStorage.withScope('my-store');
      console.log(asyncLocalStorage.getStore()); // Prints: my-store
      throw new Error('test');
    } catch (e) {
      // Store is automatically restored even after error
      console.log(asyncLocalStorage.getStore()); // Prints: undefined
    }
    javascript

Important: When using `withScope()` in async functions before the first `await`, be aware that the scope change will affect the caller's context. The synchronous portion of an async function (before the first `await`) runs immediately when called, and when it reaches the first `await`, it returns the promise to the caller. At that point, the scope change becomes visible in the caller's context and will persist in subsequent synchronous code until something else changes the scope value. For async operations, prefer using `run()` which properly isolates context across async boundaries.

    import { AsyncLocalStorage } from 'node:async_hooks';

    const asyncLocalStorage = new AsyncLocalStorage();

    async function example() {
      using _ = asyncLocalStorage.withScope('my-store');
      console.log(asyncLocalStorage.getStore()); // Prints: my-store
      await someAsyncOperation(); // Function pauses here and returns promise
      console.log(asyncLocalStorage.getStore()); // Prints: my-store
    }

    // Calling without await
    example(); // Synchronous portion runs, then pauses at first await
    // After the promise is returned, the scope 'my-store' is now active in caller!
    console.log(asyncLocalStorage.getStore()); // Prints: my-store (unexpected!)
    mjs

#### Usage with `async/await`#

If, within an async function, only one `await` call is to run within a context, the following pattern should be used:

    async function fn() {
      await asyncLocalStorage.run(new Map(), () => {
        asyncLocalStorage.getStore().set('key', value);
        return foo(); // The return value of foo will be awaited
      });
    }
    js

In this example, the store is only available in the callback function and the functions called by `foo`. Outside of `run`, calling `getStore` will return `undefined`.

#### Troubleshooting: Context loss#

In most cases, `AsyncLocalStorage` works without issues. In rare situations, the current store is lost in one of the asynchronous operations.

If your code is callback-based, it is enough to promisify it with `util.promisify()` so it starts working with native promises.

If you need to use a callback-based API or your code assumes a custom thenable implementation, use the `AsyncResource` class to associate the asynchronous operation with the correct execution context. Find the function call responsible for the context loss by logging the content of `asyncLocalStorage.getStore()` after the calls you suspect are responsible for the loss. When the code logs `undefined`, the last callback called is probably responsible for the context loss.

### Class: `RunScope`#

Added in: v25.9.0

Stability: 1 - Experimental

A disposable scope returned by `asyncLocalStorage.withScope()` that automatically restores the previous store value when disposed. This class implements the Explicit Resource Management protocol and is designed to work with JavaScript's `using` syntax.

The scope automatically restores the previous store value when the `using` block exits, whether through normal completion or by throwing an error.

#### `scope.dispose()`#

Added in: v25.9.0

Explicitly ends the scope and restores the previous store value. This method is idempotent: calling it multiple times has the same effect as calling it once.

The `[Symbol.dispose]()` method defers to `dispose()`.

If `withScope()` is called without the `using` keyword, `dispose()` must be called manually to restore the previous store value. Forgetting to call `dispose()` will cause the store value to persist for the remainder of the current execution context:

    import { AsyncLocalStorage } from 'node:async_hooks';

    const storage = new AsyncLocalStorage();

    // Without using, the scope must be disposed manually
    const scope = storage.withScope('my-store');
    // storage.getStore() === 'my-store' here

    scope.dispose(); // Restore previous value
    // storage.getStore() === undefined here
    const { AsyncLocalStorage } = require('node:async_hooks');

    const storage = new AsyncLocalStorage();

    // Without using, the scope must be disposed manually
    const scope = storage.withScope('my-store');
    // storage.getStore() === 'my-store' here

    scope.dispose(); // Restore previous value
    // storage.getStore() === undefined here
    javascript

### Class: `AsyncResource`#

History

Version | Changes
--- | ---
v16.4.0 | AsyncResource is now Stable. Previously, it had been Experimental.

The class `AsyncResource` is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources.

The `init` hook will trigger when an `AsyncResource` is instantiated.

The following is an overview of the `AsyncResource` API.

    import { AsyncResource, executionAsyncId } from 'node:async_hooks';

    // AsyncResource() is meant to be extended. Instantiating a
    // new AsyncResource() also triggers init. If triggerAsyncId is omitted then
    // async_hook.executionAsyncId() is used.
    const asyncResource = new AsyncResource(
      type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
    );

    // Run a function in the execution context of the resource. This will
    // * establish the context of the resource
    // * trigger the AsyncHooks before callbacks
    // * call the provided function `fn` with the supplied arguments
    // * trigger the AsyncHooks after callbacks
    // * restore the original execution context
    asyncResource.runInAsyncScope(fn, thisArg, ...args);

    // Call AsyncHooks destroy callbacks.
    asyncResource.emitDestroy();

    // Return the unique ID assigned to the AsyncResource instance.
    asyncResource.asyncId();

    // Return the trigger ID for the AsyncResource instance.
    asyncResource.triggerAsyncId();
    const { AsyncResource, executionAsyncId } = require('node:async_hooks');

    // AsyncResource() is meant to be extended. Instantiating a
    // new AsyncResource() also triggers init. If triggerAsyncId is omitted then
    // async_hook.executionAsyncId() is used.
    const asyncResource = new AsyncResource(
      type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false },
    );

    // Run a function in the execution context of the resource. This will
    // * establish the context of the resource
    // * trigger the AsyncHooks before callbacks
    // * call the provided function `fn` with the supplied arguments
    // * trigger the AsyncHooks after callbacks
    // * restore the original execution context
    asyncResource.runInAsyncScope(fn, thisArg, ...args);

    // Call AsyncHooks destroy callbacks.
    asyncResource.emitDestroy();

    // Return the unique ID assigned to the AsyncResource instance.
    asyncResource.asyncId();

    // Return the trigger ID for the AsyncResource instance.
    asyncResource.triggerAsyncId();
    javascript

#### `new AsyncResource(type[, options])`#

- `type` `<string>` The type of async event.
- `options` `<Object>`
  - `triggerAsyncId` `<number>` The ID of the execution context that created this async event. Default: `executionAsyncId()`.
  - `requireManualDestroy` `<boolean>` If set to `true`, disables `emitDestroy` when the object is garbage collected. This usually does not need to be set (even if `emitDestroy` is called manually), unless the resource's `asyncId` is retrieved and the sensitive API's `emitDestroy` is called with it. When set to `false`, the `emitDestroy` call on garbage collection will only take place if there is at least one active `destroy` hook. Default: `false`.

Example usage:

    class DBQuery extends AsyncResource {
      constructor(db) {
        super('DBQuery');
        this.db = db;
      }

      getInfo(query, callback) {
        this.db.get(query, (err, data) => {
          this.runInAsyncScope(callback, null, err, data);
        });
      }

      close() {
        this.db = null;
        this.emitDestroy();
      }
    }
    js

#### Static method: `AsyncResource.bind(fn[, type[, thisArg]])`#

Added in: v14.8.0, v12.19.0
History

Version | Changes
--- | ---
v20.0.0 | The `asyncResource` property added to the bound function has been deprecated and will be removed in a future version.
v17.8.0, v16.15.0 | Changed the default when `thisArg` is undefined to use `this` from the caller.
v16.0.0 | Added optional thisArg.

- `fn` `<Function>` The function to bind to the current execution context.
- `type` `<string>` An optional name to associate with the underlying `AsyncResource`.
- `thisArg` `<any>`

Binds the given function to the current execution context.

#### `asyncResource.bind(fn[, thisArg])`#

Added in: v14.8.0, v12.19.0
History

Version | Changes
--- | ---
v20.0.0 | The `asyncResource` property added to the bound function has been deprecated and will be removed in a future version.
v17.8.0, v16.15.0 | Changed the default when `thisArg` is undefined to use `this` from the caller.
v16.0.0 | Added optional thisArg.

- `fn` `<Function>` The function to bind to the current `AsyncResource`.
- `thisArg` `<any>`

Binds the given function to execute to this `AsyncResource`'s scope.

#### `asyncResource.runInAsyncScope(fn[, thisArg, ...args])`#

Added in: v9.6.0

- `fn` `<Function>` The function to call in the execution context of this async resource.
- `thisArg` `<any>` The receiver to be used for the function call.
- `...args` `<any>` Optional arguments to pass to the function.

Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context.

#### `asyncResource.emitDestroy()`#

- Returns: `<AsyncResource>` A reference to `asyncResource`.

Call all `destroy` hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the `destroy` hooks will never be called.

#### `asyncResource.asyncId()`#

- Returns: `<number>` The unique `asyncId` assigned to the resource.

#### `asyncResource.triggerAsyncId()`#

- Returns: `<number>` The same `triggerAsyncId` that is passed to the `AsyncResource` constructor.

#### Using `AsyncResource` for a `Worker` thread pool#

The following example shows how to use the `AsyncResource` class to properly provide async tracking for a `Worker` pool. Other resource pools, such as database connection pools, can follow a similar model.

Assuming that the task is adding two numbers, using a file named `task_processor.js` with the following content:

    import { parentPort } from 'node:worker_threads';
    parentPort.on('message', (task) => {
      parentPort.postMessage(task.a + task.b);
    });
    const { parentPort } = require('node:worker_threads');
    parentPort.on('message', (task) => {
      parentPort.postMessage(task.a + task.b);
    });
    javascript

a Worker pool around it could use the following structure:

    import { AsyncResource } from 'node:async_hooks';
    import { EventEmitter } from 'node:events';
    import { Worker } from 'node:worker_threads';

    const kTaskInfo = Symbol('kTaskInfo');
    const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

    class WorkerPoolTaskInfo extends AsyncResource {
      constructor(callback) {
        super('WorkerPoolTaskInfo');
        this.callback = callback;
      }

      done(err, result) {
        this.runInAsyncScope(this.callback, null, err, result);
        this.emitDestroy();  // `TaskInfo`s are used only once.
      }
    }

    export default class WorkerPool extends EventEmitter {
      constructor(numThreads) {
        super();
        this.numThreads = numThreads;
        this.workers = [];
        this.freeWorkers = [];
        this.tasks = [];

        for (let i = 0; i < numThreads; i++)
          this.addNewWorker();

        // Any time the kWorkerFreedEvent is emitted, dispatch
        // the next task pending in the queue, if any.
        this.on(kWorkerFreedEvent, () => {
          if (this.tasks.length > 0) {
            const { task, callback } = this.tasks.shift();
            this.runTask(task, callback);
          }
        });
      }

      addNewWorker() {
        const worker = new Worker(new URL('task_processor.js', import.meta.url));
        worker.on('message', (result) => {
          // In case of success: Call the callback that was passed to `runTask`,
          // remove the `TaskInfo` associated with the Worker, and mark it as free
          // again.
          worker[kTaskInfo].done(null, result);
          worker[kTaskInfo] = null;
          this.freeWorkers.push(worker);
          this.emit(kWorkerFreedEvent);
        });
        worker.on('error', (err) => {
          // In case of an uncaught exception: Call the callback that was passed to
          // `runTask` with the error.
          if (worker[kTaskInfo])
            worker[kTaskInfo].done(err, null);
          else
            this.emit('error', err);
          // Remove the worker from the list and start a new Worker to replace the
          // current one.
          this.workers.splice(this.workers.indexOf(worker), 1);
          this.addNewWorker();
        });
        this.workers.push(worker);
        this.freeWorkers.push(worker);
        this.emit(kWorkerFreedEvent);
      }

      runTask(task, callback) {
        if (this.freeWorkers.length === 0) {
          // No free threads, wait until a worker thread becomes free.
          this.tasks.push({ task, callback });
          return;
        }

        const worker = this.freeWorkers.pop();
        worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
        worker.postMessage(task);
      }

      close() {
        for (const worker of this.workers) worker.terminate();
      }
    }
    const { AsyncResource } = require('node:async_hooks');
    const { EventEmitter } = require('node:events');
    const path = require('node:path');
    const { Worker } = require('node:worker_threads');

    const kTaskInfo = Symbol('kTaskInfo');
    const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

    class WorkerPoolTaskInfo extends AsyncResource {
      constructor(callback) {
        super('WorkerPoolTaskInfo');
        this.callback = callback;
      }

      done(err, result) {
        this.runInAsyncScope(this.callback, null, err, result);
        this.emitDestroy();  // `TaskInfo`s are used only once.
      }
    }

    class WorkerPool extends EventEmitter {
      constructor(numThreads) {
        super();
        this.numThreads = numThreads;
        this.workers = [];
        this.freeWorkers = [];
        this.tasks = [];

        for (let i = 0; i < numThreads; i++)
          this.addNewWorker();

        // Any time the kWorkerFreedEvent is emitted, dispatch
        // the next task pending in the queue, if any.
        this.on(kWorkerFreedEvent, () => {
          if (this.tasks.length > 0) {
            const { task, callback } = this.tasks.shift();
            this.runTask(task, callback);
          }
        });
      }

      addNewWorker() {
        const worker = new Worker(path.resolve(__dirname, 'task_processor.js'));
        worker.on('message', (result) => {
          // In case of success: Call the callback that was passed to `runTask`,
          // remove the `TaskInfo` associated with the Worker, and mark it as free
          // again.
          worker[kTaskInfo].done(null, result);
          worker[kTaskInfo] = null;
          this.freeWorkers.push(worker);
          this.emit(kWorkerFreedEvent);
        });
        worker.on('error', (err) => {
          // In case of an uncaught exception: Call the callback that was passed to
          // `runTask` with the error.
          if (worker[kTaskInfo])
            worker[kTaskInfo].done(err, null);
          else
            this.emit('error', err);
          // Remove the worker from the list and start a new Worker to replace the
          // current one.
          this.workers.splice(this.workers.indexOf(worker), 1);
          this.addNewWorker();
        });
        this.workers.push(worker);
        this.freeWorkers.push(worker);
        this.emit(kWorkerFreedEvent);
      }

      runTask(task, callback) {
        if (this.freeWorkers.length === 0) {
          // No free threads, wait until a worker thread becomes free.
          this.tasks.push({ task, callback });
          return;
        }

        const worker = this.freeWorkers.pop();
        worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
        worker.postMessage(task);
      }

      close() {
        for (const worker of this.workers) worker.terminate();
      }
    }

    module.exports = WorkerPool;
    javascript

Without the explicit tracking added by the `WorkerPoolTaskInfo` objects, it would appear that the callbacks are associated with the individual `Worker` objects. However, the creation of the `Worker`s is not associated with the creation of the tasks and does not provide information about when tasks were scheduled.

This pool could be used as follows:

    import WorkerPool from './worker_pool.js';
    import os from 'node:os';

    const pool = new WorkerPool(os.availableParallelism());

    let finished = 0;
    for (let i = 0; i < 10; i++) {
      pool.runTask({ a: 42, b: 100 }, (err, result) => {
        console.log(i, err, result);
        if (++finished === 10)
          pool.close();
      });
    }
    const WorkerPool = require('./worker_pool.js');
    const os = require('node:os');

    const pool = new WorkerPool(os.availableParallelism());

    let finished = 0;
    for (let i = 0; i < 10; i++) {
      pool.runTask({ a: 42, b: 100 }, (err, result) => {
        console.log(i, err, result);
        if (++finished === 10)
          pool.close();
      });
    }
    javascript

#### Integrating `AsyncResource` with `EventEmitter`#

Event listeners triggered by an `EventEmitter` may be run in a different execution context than the one that was active when `eventEmitter.on()` was called.

The following example shows how to use the `AsyncResource` class to properly associate an event listener with the correct execution context. The same approach can be applied to a `Stream` or a similar event-driven class.

    import { createServer } from 'node:http';
    import { AsyncResource, executionAsyncId } from 'node:async_hooks';

    const server = createServer((req, res) => {
      req.on('close', AsyncResource.bind(() => {
        // Execution context is bound to the current outer scope.
      }));
      req.on('close', () => {
        // Execution context is bound to the scope that caused 'close' to emit.
      });
      res.end();
    }).listen(3000);
    const { createServer } = require('node:http');
    const { AsyncResource, executionAsyncId } = require('node:async_hooks');

    const server = createServer((req, res) => {
      req.on('close', AsyncResource.bind(() => {
        // Execution context is bound to the current outer scope.
      }));
      req.on('close', () => {
        // Execution context is bound to the scope that caused 'close' to emit.
      });
      res.end();
    }).listen(3000);
    javascript

## Buffer#

Source Code: lib/buffer.js

Stability: 2 - Stable

`Buffer` objects are used to represent a fixed-length sequence of bytes. Many Node.js APIs support `Buffer`s.

The `Buffer` class is a subclass of JavaScript's `<Uint8Array>` class and extends it with methods that cover additional use cases. Node.js APIs accept plain `<Uint8Array>`s wherever `Buffer`s are supported as well.

While the `Buffer` class is available within the global scope, it is still recommended to explicitly reference it via an import or require statement.

    import { Buffer } from 'node:buffer';

    // Creates a zero-filled Buffer of length 10.
    const buf1 = Buffer.alloc(10);

    // Creates a Buffer of length 10,
    // filled with bytes which all have the value `1`.
    const buf2 = Buffer.alloc(10, 1);

    // Creates an uninitialized buffer of length 10.
    // This is faster than calling Buffer.alloc() but the returned
    // Buffer instance might contain old data that needs to be
    // overwritten using fill(), write(), or other functions that fill the Buffer's
    // contents.
    const buf3 = Buffer.allocUnsafe(10);

    // Creates a Buffer containing the bytes [1, 2, 3].
    const buf4 = Buffer.from([1, 2, 3]);

    // Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
    // are all truncated using `(value & 255)` to fit into the range 0–255.
    const buf5 = Buffer.from([257, 257.5, -255, '1']);

    // Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
    // [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
    // [116, 195, 169, 115, 116] (in decimal notation)
    const buf6 = Buffer.from('tést');

    // Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
    const buf7 = Buffer.from('tést', 'latin1');
    const { Buffer } = require('node:buffer');

    // Creates a zero-filled Buffer of length 10.
    const buf1 = Buffer.alloc(10);

    // Creates a Buffer of length 10,
    // filled with bytes which all have the value `1`.
    const buf2 = Buffer.alloc(10, 1);

    // Creates an uninitialized buffer of length 10.
    // This is faster than calling Buffer.alloc() but the returned
    // Buffer instance might contain old data that needs to be
    // overwritten using fill(), write(), or other functions that fill the Buffer's
    // contents.
    const buf3 = Buffer.allocUnsafe(10);

    // Creates a Buffer containing the bytes [1, 2, 3].
    const buf4 = Buffer.from([1, 2, 3]);

    // Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
    // are all truncated using `(value & 255)` to fit into the range 0–255.
    const buf5 = Buffer.from([257, 257.5, -255, '1']);

    // Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
    // [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
    // [116, 195, 169, 115, 116] (in decimal notation)
    const buf6 = Buffer.from('tést');

    // Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
    const buf7 = Buffer.from('tést', 'latin1');
    javascript

### Buffers and character encodings#

History

Version | Changes
--- | ---
v15.7.0, v14.18.0 | Introduced `base64url` encoding.
v6.4.0 | Introduced `latin1` as an alias for `binary`.
v5.0.0 | Removed the deprecated `raw` and `raws` encodings.

When converting between `Buffer`s and strings, a character encoding may be specified. If no character encoding is specified, UTF-8 will be used as the default.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('hello world', 'utf8');

    console.log(buf.toString('hex'));
    // Prints: 68656c6c6f20776f726c64
    console.log(buf.toString('base64'));
    // Prints: aGVsbG8gd29ybGQ=

    console.log(Buffer.from('fhqwhgads', 'utf8'));
    // Prints: <Buffer 66 68 71 77 68 67 61 64 73>
    console.log(Buffer.from('fhqwhgads', 'utf16le'));
    // Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('hello world', 'utf8');

    console.log(buf.toString('hex'));
    // Prints: 68656c6c6f20776f726c64
    console.log(buf.toString('base64'));
    // Prints: aGVsbG8gd29ybGQ=

    console.log(Buffer.from('fhqwhgads', 'utf8'));
    // Prints: <Buffer 66 68 71 77 68 67 61 64 73>
    console.log(Buffer.from('fhqwhgads', 'utf16le'));
    // Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>
    javascript

Node.js buffers accept all case variations of encoding strings that they receive. For example, UTF-8 can be specified as `'utf8'`, `'UTF8'`, or `'uTf8'`.

The character encodings currently supported by Node.js are the following:

- `'utf8'` (alias: `'utf-8'`): Multi-byte encoded Unicode characters. Many web pages and other document formats use UTF-8. This is the default character encoding. When decoding a `Buffer` into a string that does not exclusively contain valid UTF-8 data, the Unicode replacement character `U+FFFD` � will be used to represent those errors.

- `'utf16le'` (alias: `'utf-16le'`): Multi-byte encoded Unicode characters. Unlike `'utf8'`, each character in the string will be encoded using either 2 or 4 bytes. Node.js only supports the little-endian variant of UTF-16.

- `'latin1'`: Latin-1 stands for ISO-8859-1. This character encoding only supports the Unicode characters from `U+0000` to `U+00FF`. Each character is encoded using a single byte. Characters that do not fit into that range are truncated and will be mapped to characters in that range.

Converting a `Buffer` into a string using one of the above is referred to as decoding, and converting a string into a `Buffer` is referred to as encoding.

Node.js also supports the following binary-to-text encodings. For binary-to-text encodings, the naming convention is reversed: Converting a `Buffer` into a string is typically referred to as encoding, and converting a string into a `Buffer` as decoding.

- `'base64'`: Base64 encoding. When creating a `Buffer` from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC 4648, Section 5. Whitespace characters such as spaces, tabs, and new lines contained within the base64-encoded string are ignored.

- `'base64url'`: base64url encoding as specified in RFC 4648, Section 5. When creating a `Buffer` from a string, this encoding will also correctly accept regular base64-encoded strings. When encoding a `Buffer` to a string, this encoding will omit padding.

- `'hex'`: Encode each byte as two hexadecimal characters. Data truncation may occur when decoding strings that do not exclusively consist of an even number of hexadecimal characters. See below for an example.

The following legacy character encodings are also supported:

- `'ascii'`: For 7-bit ASCII data only. When encoding a string into a `Buffer`, this is equivalent to using `'latin1'`. When decoding a `Buffer` into a string, using this encoding will additionally unset the highest bit of each byte before decoding as `'latin1'`. Generally, there should be no reason to use this encoding, as `'utf8'` (or, if the data is known to always be ASCII-only, `'latin1'`) will be a better choice when encoding or decoding ASCII-only text. It is only provided for legacy compatibility.

- `'binary'`: Alias for `'latin1'`. The name of this encoding can be very misleading, as all of the encodings listed here convert between strings and binary data. For converting between strings and `Buffer`s, typically `'utf8'` is the right choice.

- `'ucs2'`, `'ucs-2'`: Aliases of `'utf16le'`. UCS-2 used to refer to a variant of UTF-16 that did not support characters that had code points larger than U+FFFF. In Node.js, these code points are always supported.

    import { Buffer } from 'node:buffer';

    Buffer.from('1ag123', 'hex');
    // Prints <Buffer 1a>, data truncated when first non-hexadecimal value
    // ('g') encountered.

    Buffer.from('1a7', 'hex');
    // Prints <Buffer 1a>, data truncated when data ends in single digit ('7').

    Buffer.from('1634', 'hex');
    // Prints <Buffer 16 34>, all data represented.
    const { Buffer } = require('node:buffer');

    Buffer.from('1ag123', 'hex');
    // Prints <Buffer 1a>, data truncated when first non-hexadecimal value
    // ('g') encountered.

    Buffer.from('1a7', 'hex');
    // Prints <Buffer 1a>, data truncated when data ends in single digit ('7').

    Buffer.from('1634', 'hex');
    // Prints <Buffer 16 34>, all data represented.
    javascript

Modern Web browsers follow the WHATWG Encoding Standard which aliases both `'latin1'` and `'ISO-8859-1'` to `'win-1252'`. This means that while doing something like `http.get()`, if the returned charset is one of those listed in the WHATWG specification it is possible that the server actually returned `'win-1252'`-encoded data, and using `'latin1'` encoding may incorrectly decode the characters.

### Buffers and TypedArrays#

History

Version | Changes
--- | ---
v3.0.0 | The `Buffer` class now inherits from `Uint8Array`.

`Buffer` instances are also JavaScript `<Uint8Array>` and `<TypedArray>` instances. All `<TypedArray>` methods and properties are available on `Buffer`s. There are, however, subtle incompatibilities between the `Buffer` API and the `<TypedArray>` API.

In particular:

- While `TypedArray.prototype.slice()` creates a copy of part of the `TypedArray`, `Buffer.prototype.slice()` creates a view over the existing `Buffer` without copying. This behavior can be surprising, and only exists for legacy compatibility. `TypedArray.prototype.subarray()` can be used to achieve the behavior of `Buffer.prototype.slice()` on both `Buffer`s and other `TypedArray`s and should be preferred.
- `buf.toString()` is incompatible with its `TypedArray` equivalent.
- A number of methods, e.g. `buf.indexOf()`, support additional arguments.

There are two ways to create new `<TypedArray>` instances from a `Buffer`:

- Passing a `Buffer` to a `<TypedArray>` constructor will copy the `Buffer`'s contents, interpreted as an array of integers, and not as a byte sequence of the target type.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, 2, 3, 4]);
    const uint32array = new Uint32Array(buf);

    console.log(uint32array);

    // Prints: Uint32Array(4) [ 1, 2, 3, 4 ]
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, 2, 3, 4]);
    const uint32array = new Uint32Array(buf);

    console.log(uint32array);

    // Prints: Uint32Array(4) [ 1, 2, 3, 4 ]
    javascript

- Passing the `Buffer`'s underlying `<ArrayBuffer>` will create a `<TypedArray>` that shares its memory with the `Buffer`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('hello', 'utf16le');
    const uint16array = new Uint16Array(
      buf.buffer,
      buf.byteOffset,
      buf.length / Uint16Array.BYTES_PER_ELEMENT);

    console.log(uint16array);

    // Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('hello', 'utf16le');
    const uint16array = new Uint16Array(
      buf.buffer,
      buf.byteOffset,
      buf.length / Uint16Array.BYTES_PER_ELEMENT);

    console.log(uint16array);

    // Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]
    javascript

It is possible to create a new `Buffer` that shares the same allocated memory as a `<TypedArray>` instance by using the `TypedArray` object's `.buffer` property in the same way. `Buffer.from()` behaves like `new Uint8Array()` in this context.

    import { Buffer } from 'node:buffer';

    const arr = new Uint16Array(2);

    arr[0] = 5000;
    arr[1] = 4000;

    // Copies the contents of `arr`.
    const buf1 = Buffer.from(arr);

    // Shares memory with `arr`.
    const buf2 = Buffer.from(arr.buffer);

    console.log(buf1);
    // Prints: <Buffer 88 a0>
    console.log(buf2);
    // Prints: <Buffer 88 13 a0 0f>

    arr[1] = 6000;

    console.log(buf1);
    // Prints: <Buffer 88 a0>
    console.log(buf2);
    // Prints: <Buffer 88 13 70 17>
    const { Buffer } = require('node:buffer');

    const arr = new Uint16Array(2);

    arr[0] = 5000;
    arr[1] = 4000;

    // Copies the contents of `arr`.
    const buf1 = Buffer.from(arr);

    // Shares memory with `arr`.
    const buf2 = Buffer.from(arr.buffer);

    console.log(buf1);
    // Prints: <Buffer 88 a0>
    console.log(buf2);
    // Prints: <Buffer 88 13 a0 0f>

    arr[1] = 6000;

    console.log(buf1);
    // Prints: <Buffer 88 a0>
    console.log(buf2);
    // Prints: <Buffer 88 13 70 17>
    javascript

When creating a `Buffer` using a `<TypedArray>`'s `.buffer`, it is possible to use only a portion of the underlying `<ArrayBuffer>` by passing in `byteOffset` and `length` parameters.

    import { Buffer } from 'node:buffer';

    const arr = new Uint16Array(20);
    const buf = Buffer.from(arr.buffer, 0, 16);

    console.log(buf.length);
    // Prints: 16
    const { Buffer } = require('node:buffer');

    const arr = new Uint16Array(20);
    const buf = Buffer.from(arr.buffer, 0, 16);

    console.log(buf.length);
    // Prints: 16
    javascript

The `Buffer.from()` and `TypedArray.from()` have different signatures and implementations. Specifically, the `<TypedArray>` variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

- `TypedArray.from(source[, mapFn[, thisArg]])`

The `Buffer.from()` method, however, does not support the use of a mapping function:

- `Buffer.from(array)`
- `Buffer.from(buffer)`
- `Buffer.from(arrayBuffer[, byteOffset[, length]])`
- `Buffer.from(string[, encoding])`

#### Buffer methods are callable with `Uint8Array` instances#

All methods on the Buffer prototype are callable with a `Uint8Array` instance.

    const { toString, write } = Buffer.prototype;

    const uint8array = new Uint8Array(5);

    write.call(uint8array, 'hello', 0, 5, 'utf8'); // 5
    // <Uint8Array 68 65 6c 6c 6f>

    toString.call(uint8array, 'utf8'); // 'hello'
    js

### Buffers and iteration#

`Buffer` instances can be iterated over using `for..of` syntax:

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, 2, 3]);

    for (const b of buf) {
      console.log(b);
    }
    // Prints:
    //   1
    //   2
    //   3
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, 2, 3]);

    for (const b of buf) {
      console.log(b);
    }
    // Prints:
    //   1
    //   2
    //   3
    javascript

Additionally, the `buf.values()`, `buf.keys()`, and `buf.entries()` methods can be used to create iterators.

### Class: `Blob`#

Added in: v15.7.0, v14.18.0
History

Version | Changes
--- | ---
v18.0.0, v16.17.0 | No longer experimental.

A `<Blob>` encapsulates immutable, raw data that can be safely shared across multiple worker threads.

#### `new buffer.Blob([sources[, options]])`#

Added in: v15.7.0, v14.18.0
History

Version | Changes
--- | ---
v16.7.0 | Added the standard `endings` option to replace line-endings, and removed the non-standard `encoding` option.

- `sources` `<string>`[] | `<ArrayBuffer>`[] | `<TypedArray>`[] | `<DataView>`[] | `<Blob>`[] An array of string, `<ArrayBuffer>`, `<TypedArray>`, `<DataView>`, or `<Blob>` objects, or any mix of such objects, that will be stored within the `Blob`.
- `options` `<Object>`
  - `endings` `<string>` One of either `'transparent'` or `'native'`. When set to `'native'`, line endings in string source parts will be converted to the platform native line-ending as specified by `require('node:os').EOL`.
  - `type` `<string>` The Blob content-type. The intent is for `type` to convey the MIME media type of the data, however no validation of the type format is performed.

Creates a new `Blob` object containing a concatenation of the given sources.

`<ArrayBuffer>`, `<TypedArray>`, `<DataView>`, and `<Buffer>` sources are copied into the 'Blob' and can therefore be safely modified after the 'Blob' is created.

String sources are encoded as UTF-8 byte sequences and copied into the Blob. Unmatched surrogate pairs within each string part will be replaced by Unicode U+FFFD replacement characters.

#### `blob.arrayBuffer()`#

Added in: v15.7.0, v14.18.0

- Returns: `<Promise>`

Returns a promise that fulfills with an `<ArrayBuffer>` containing a copy of the `Blob` data.

#### `blob.bytes()`#

Added in: v22.3.0, v20.16.0

The `blob.bytes()` method returns the byte of the `Blob` object as a `Promise<Uint8Array>`.

    const blob = new Blob(['hello']);
    blob.bytes().then((bytes) => {
      console.log(bytes); // Outputs: Uint8Array(5) [ 104, 101, 108, 108, 111 ]
    });
    js

#### `blob.size`#

Added in: v15.7.0, v14.18.0

The total size of the `Blob` in bytes.

#### `blob.slice([start[, end[, type]]])`#

Added in: v15.7.0, v14.18.0

- `start` `<number>` The starting index.
- `end` `<number>` The ending index.
- `type` `<string>` The content-type for the new `Blob`

Creates and returns a new `Blob` containing a subset of this `Blob` objects data. The original `Blob` is not altered.

#### `blob.stream()`#

Added in: v16.7.0

- Returns: `<ReadableStream>`

Returns a new `ReadableStream` that allows the content of the `Blob` to be read.

#### `blob.text()`#

Added in: v15.7.0, v14.18.0

- Returns: `<Promise>`

Returns a promise that fulfills with the contents of the `Blob` decoded as a UTF-8 string.

#### `blob.textStream()`#

Added in: v26.5.0

- Returns: `<ReadableStream>`

Returns a new `ReadableStream` that allows the content of the `Blob` to be read as a stream of UTF-8 decoded strings. It is equivalent to piping `blob.stream()` through a `TextDecoderStream` set up with UTF-8.

#### `blob.type`#

Added in: v15.7.0, v14.18.0

- Type: `<string>`

The content-type of the `Blob`.

#### `Blob` objects and `MessageChannel`#

Once a `<Blob>` object is created, it can be sent via `MessagePort` to multiple destinations without transferring or immediately copying the data. The data contained by the `Blob` is copied only when the `arrayBuffer()` or `text()` methods are called.

    import { Blob } from 'node:buffer';
    import { setTimeout as delay } from 'node:timers/promises';

    const blob = new Blob(['hello there']);

    const mc1 = new MessageChannel();
    const mc2 = new MessageChannel();

    mc1.port1.onmessage = async ({ data }) => {
      console.log(await data.arrayBuffer());
      mc1.port1.close();
    };

    mc2.port1.onmessage = async ({ data }) => {
      await delay(1000);
      console.log(await data.arrayBuffer());
      mc2.port1.close();
    };

    mc1.port2.postMessage(blob);
    mc2.port2.postMessage(blob);

    // The Blob is still usable after posting.
    blob.text().then(console.log);
    const { Blob } = require('node:buffer');
    const { setTimeout: delay } = require('node:timers/promises');

    const blob = new Blob(['hello there']);

    const mc1 = new MessageChannel();
    const mc2 = new MessageChannel();

    mc1.port1.onmessage = async ({ data }) => {
      console.log(await data.arrayBuffer());
      mc1.port1.close();
    };

    mc2.port1.onmessage = async ({ data }) => {
      await delay(1000);
      console.log(await data.arrayBuffer());
      mc2.port1.close();
    };

    mc1.port2.postMessage(blob);
    mc2.port2.postMessage(blob);

    // The Blob is still usable after posting.
    blob.text().then(console.log);
    javascript

### Class: `Buffer`#

The `Buffer` class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

#### Static method: `Buffer.alloc(size[, fill[, encoding]])`#

Added in: v5.10.0
History

Version | Changes
--- | ---
v20.0.0 | Throw ERR_INVALID_ARG_TYPE or ERR_OUT_OF_RANGE instead of ERR_INVALID_ARG_VALUE for invalid input arguments.
v15.0.0 | Throw ERR_INVALID_ARG_VALUE instead of ERR_INVALID_OPT_VALUE for invalid input arguments.
v10.0.0 | Attempting to fill a non-zero length buffer with a zero length buffer triggers a thrown exception.
v10.0.0 | Specifying an invalid string for `fill` triggers a thrown exception.
v8.9.3 | Specifying an invalid string for `fill` now results in a zero-filled buffer.

- `size` `<integer>` The desired length of the new `Buffer`.
- `fill` `<string>` | `<Buffer>` | `<Uint8Array>` | `<integer>` A value to pre-fill the new `Buffer` with. Default: `0`.
- `encoding` `<string>` If `fill` is a string, this is its encoding. Default: `'utf8'`.
- Returns: `<Buffer>`

Allocates a new `Buffer` of `size` bytes. If `fill` is `undefined`, the `Buffer` will be zero-filled.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.alloc(5);

    console.log(buf);
    // Prints: <Buffer 00 00 00 00 00>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.alloc(5);

    console.log(buf);
    // Prints: <Buffer 00 00 00 00 00>
    javascript

If `size` is larger than `buffer.constants.MAX_LENGTH` or smaller than 0, `ERR_OUT_OF_RANGE` is thrown.

If `fill` is specified, the allocated `Buffer` will be initialized by calling `buf.fill(fill)`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.alloc(5, 'a');

    console.log(buf);
    // Prints: <Buffer 61 61 61 61 61>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.alloc(5, 'a');

    console.log(buf);
    // Prints: <Buffer 61 61 61 61 61>
    javascript

If both `fill` and `encoding` are specified, the allocated `Buffer` will be initialized by calling `buf.fill(fill, encoding)`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

    console.log(buf);
    // Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

    console.log(buf);
    // Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
    javascript

Calling `Buffer.alloc()` can be measurably slower than the alternative `Buffer.allocUnsafe()` but ensures that the newly created `Buffer` instance contents will never contain sensitive data from previous allocations, including data that might not have been allocated for `Buffer`s.

A `TypeError` will be thrown if `size` is not a number.

#### Static method: `Buffer.allocUnsafe(size)`#

Added in: v5.10.0
History

Version | Changes
--- | ---
v20.0.0 | Throw ERR_INVALID_ARG_TYPE or ERR_OUT_OF_RANGE instead of ERR_INVALID_ARG_VALUE for invalid input arguments.
v15.0.0 | Throw ERR_INVALID_ARG_VALUE instead of ERR_INVALID_OPT_VALUE for invalid input arguments.
v7.0.0 | Passing a negative `size` will now throw an error.

- `size` `<integer>` The desired length of the new `Buffer`.
- Returns: `<Buffer>`

Allocates a new `Buffer` of `size` bytes. If `size` is larger than `buffer.constants.MAX_LENGTH` or smaller than 0, `ERR_OUT_OF_RANGE` is thrown.

The underlying memory for `Buffer` instances created in this way is not initialized. The contents of the newly created `Buffer` are unknown and may contain sensitive data. Use `Buffer.alloc()` instead to initialize `Buffer` instances with zeroes.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(10);

    console.log(buf);
    // Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>

    buf.fill(0);

    console.log(buf);
    // Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(10);

    console.log(buf);
    // Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>

    buf.fill(0);

    console.log(buf);
    // Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
    javascript

A `TypeError` will be thrown if `size` is not a number.

The `Buffer` module pre-allocates an internal `Buffer` instance of size `Buffer.poolSize` that is used as a pool for the fast allocation of new `Buffer` instances created using `Buffer.allocUnsafe()`, `Buffer.from(array)`, `Buffer.from(string)`, and `Buffer.concat()` only when `size` is less than `Buffer.poolSize >>> 1` (floor of `Buffer.poolSize` divided by two).

Use of this pre-allocated internal memory pool is a key difference between calling `Buffer.alloc(size, fill)` vs. `Buffer.allocUnsafe(size).fill(fill)`. Specifically, `Buffer.alloc(size, fill)` will never use the internal `Buffer` pool, while `Buffer.allocUnsafe(size).fill(fill)` will use the internal `Buffer` pool if `size` is less than or equal to half `Buffer.poolSize`. The difference is subtle but can be important when an application requires the additional performance that `Buffer.allocUnsafe()` provides.

#### Static method: `Buffer.allocUnsafeSlow(size)`#

Added in: v5.12.0
History

Version | Changes
--- | ---
v20.0.0 | Throw ERR_INVALID_ARG_TYPE or ERR_OUT_OF_RANGE instead of ERR_INVALID_ARG_VALUE for invalid input arguments.
v15.0.0 | Throw ERR_INVALID_ARG_VALUE instead of ERR_INVALID_OPT_VALUE for invalid input arguments.

- `size` `<integer>` The desired length of the new `Buffer`.
- Returns: `<Buffer>`

Allocates a new `Buffer` of `size` bytes. If `size` is larger than `buffer.constants.MAX_LENGTH` or smaller than 0, `ERR_OUT_OF_RANGE` is thrown. A zero-length `Buffer` is created if `size` is 0.

The underlying memory for `Buffer` instances created in this way is not initialized. The contents of the newly created `Buffer` are unknown and may contain sensitive data. Use `buf.fill(0)` to initialize such `Buffer` instances with zeroes.

When using `Buffer.allocUnsafe()` to allocate new `Buffer` instances, allocations less than `Buffer.poolSize >>> 1` (32KiB when default poolSize is used) are sliced from a single pre-allocated `Buffer`. This allows applications to avoid the garbage collection overhead of creating many individually allocated `Buffer` instances. This approach improves both performance and memory usage by eliminating the need to track and clean up as many individual `ArrayBuffer` objects.

However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled `Buffer` instance using `Buffer.allocUnsafeSlow()` and then copying out the relevant bits.

    import { Buffer } from 'node:buffer';

    // Need to keep around a few small chunks of memory.
    const store = [];

    socket.on('readable', () => {
      let data;
      while (null !== (data = readable.read())) {
        // Allocate for retained data.
        const sb = Buffer.allocUnsafeSlow(10);

        // Copy the data into the new allocation.
        data.copy(sb, 0, 0, 10);

        store.push(sb);
      }
    });
    const { Buffer } = require('node:buffer');

    // Need to keep around a few small chunks of memory.
    const store = [];

    socket.on('readable', () => {
      let data;
      while (null !== (data = readable.read())) {
        // Allocate for retained data.
        const sb = Buffer.allocUnsafeSlow(10);

        // Copy the data into the new allocation.
        data.copy(sb, 0, 0, 10);

        store.push(sb);
      }
    });
    javascript

A `TypeError` will be thrown if `size` is not a number.

#### Static method: `Buffer.byteLength(string[, encoding])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v7.0.0 | Passing invalid input will now throw an error.
v5.10.0 | The `string` parameter can now be any `TypedArray`, `DataView` or `ArrayBuffer`.

- `string` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<SharedArrayBuffer>` A value to calculate the length of.
- `encoding` `<string>` If `string` is a string, this is its encoding. Default: `'utf8'`.
- Returns: `<integer>` The number of bytes contained within `string`.

Returns the byte length of a string when encoded using `encoding`. This is not the same as `String.prototype.length`, which does not account for the encoding that is used to convert the string into bytes.

For `'base64'`, `'base64url'`, and `'hex'`, this function assumes valid input. For strings that contain non-base64/hex-encoded data (e.g. whitespace), the return value might be greater than the length of a `Buffer` created from the string.

    import { Buffer } from 'node:buffer';

    const str = '\u00bd + \u00bc = \u00be';

    console.log(`${str}: ${str.length} characters, ` +
                `${Buffer.byteLength(str, 'utf8')} bytes`);
    // Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
    const { Buffer } = require('node:buffer');

    const str = '\u00bd + \u00bc = \u00be';

    console.log(`${str}: ${str.length} characters, ` +
                `${Buffer.byteLength(str, 'utf8')} bytes`);
    // Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
    javascript

When `string` is a `<Buffer>` | `<DataView>` | `<TypedArray>` | `<ArrayBuffer>` | `<SharedArrayBuffer>`, the byte length as reported by `.byteLength` is returned.

#### Static method: `Buffer.compare(buf1, buf2)`#

Added in: v0.11.13
History

Version | Changes
--- | ---
v8.0.0 | The arguments can now be `Uint8Array`s.

- `buf1` `<Buffer>` | `<Uint8Array>`
- `buf2` `<Buffer>` | `<Uint8Array>`
- Returns: `<integer>` Either `-1`, `0`, or `1`, depending on the result of the comparison. See `buf.compare()` for details.

Compares `buf1` to `buf2`, typically for the purpose of sorting arrays of `Buffer` instances. This is equivalent to calling `buf1.compare(buf2)`.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from('1234');
    const buf2 = Buffer.from('0123');
    const arr = [buf1, buf2];

    console.log(arr.sort(Buffer.compare));
    // Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
    // (This result is equal to: [buf2, buf1].)
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from('1234');
    const buf2 = Buffer.from('0123');
    const arr = [buf1, buf2];

    console.log(arr.sort(Buffer.compare));
    // Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
    // (This result is equal to: [buf2, buf1].)
    javascript

#### Static method: `Buffer.concat(list[, totalLength])`#

Added in: v0.7.11
History

Version | Changes
--- | ---
v8.0.0 | The elements of `list` can now be `Uint8Array`s.

- `list` `<Buffer>`[] | `<Uint8Array>`[] List of `Buffer` or `<Uint8Array>` instances to concatenate.
- `totalLength` `<integer>` Total length of the `Buffer` instances in `list` when concatenated.
- Returns: `<Buffer>`

Returns a new `Buffer` which is the result of concatenating all the `Buffer` instances in the `list` together.

If the list has no items, or if the `totalLength` is 0, then a new zero-length `Buffer` is returned.

If `totalLength` is not provided, it is calculated from the `Buffer` instances in `list` by adding their lengths.

If `totalLength` is provided, it must be an unsigned integer. If the combined length of the `Buffer`s in `list` exceeds `totalLength`, the result is truncated to `totalLength`. If the combined length of the `Buffer`s in `list` is less than `totalLength`, the remaining space is filled with zeros.

    import { Buffer } from 'node:buffer';

    // Create a single `Buffer` from a list of three `Buffer` instances.

    const buf1 = Buffer.alloc(10);
    const buf2 = Buffer.alloc(14);
    const buf3 = Buffer.alloc(18);
    const totalLength = buf1.length + buf2.length + buf3.length;

    console.log(totalLength);
    // Prints: 42

    const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

    console.log(bufA);
    // Prints: <Buffer 00 00 00 00 ...>
    console.log(bufA.length);
    // Prints: 42
    const { Buffer } = require('node:buffer');

    // Create a single `Buffer` from a list of three `Buffer` instances.

    const buf1 = Buffer.alloc(10);
    const buf2 = Buffer.alloc(14);
    const buf3 = Buffer.alloc(18);
    const totalLength = buf1.length + buf2.length + buf3.length;

    console.log(totalLength);
    // Prints: 42

    const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

    console.log(bufA);
    // Prints: <Buffer 00 00 00 00 ...>
    console.log(bufA.length);
    // Prints: 42
    javascript

`Buffer.concat()` may also use the internal `Buffer` pool like `Buffer.allocUnsafe()` does.

#### Static method: `Buffer.copyBytesFrom(view[, offset[, length]])`#

Added in: v19.8.0, v18.16.0

- `view` `<TypedArray>` The `<TypedArray>` to copy.
- `offset` `<integer>` The starting offset within `view`. Default: `0`.
- `length` `<integer>` The number of elements from `view` to copy. Default: `view.length - offset`.
- Returns: `<Buffer>`

Copies the underlying memory of `view` into a new `Buffer`.

    const u16 = new Uint16Array([0, 0xffff]);
    const buf = Buffer.copyBytesFrom(u16, 1, 1);
    u16[1] = 0;
    console.log(buf.length); // 2
    console.log(buf[0]); // 255
    console.log(buf[1]); // 255
    js

#### Static method: `Buffer.from(array)`#

Added in: v5.10.0

- `array` `<integer>`[]
- Returns: `<Buffer>`

Allocates a new `Buffer` using an `array` of bytes in the range `0` – `255`. Array entries outside that range will be truncated to fit into it.

    import { Buffer } from 'node:buffer';

    // Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
    const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
    const { Buffer } = require('node:buffer');

    // Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
    const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
    javascript

If `array` is an `Array`-like object (that is, one with a `length` property of type `number`), it is treated as if it is an array, unless it is a `Buffer` or a `Uint8Array`. This means all other `TypedArray` variants get treated as an `Array`. To create a `Buffer` from the bytes backing a `TypedArray`, use `Buffer.copyBytesFrom()`.

A `TypeError` will be thrown if `array` is not an `Array` or another type appropriate for `Buffer.from()` variants.

`Buffer.from(array)` and `Buffer.from(string)` may also use the internal `Buffer` pool like `Buffer.allocUnsafe()` does.

#### Static method: `Buffer.from(arrayBuffer[, byteOffset[, length]])`#

Added in: v5.10.0

- `arrayBuffer` `<ArrayBuffer>` | `<SharedArrayBuffer>` An `<ArrayBuffer>`, `<SharedArrayBuffer>`, for example the `.buffer` property of a `<TypedArray>`.
- `byteOffset` `<integer>` Index of first byte to expose. Default: `0`.
- `length` `<integer>` Number of bytes to expose. Default: `arrayBuffer.byteLength - byteOffset`.
- Returns: `<Buffer>`

This creates a view of the `<ArrayBuffer>` without copying the underlying memory. For example, when passed a reference to the `.buffer` property of a `<TypedArray>` instance, the newly created `Buffer` will share the same allocated memory as the `<TypedArray>`'s underlying `ArrayBuffer`.

    import { Buffer } from 'node:buffer';

    const arr = new Uint16Array(2);

    arr[0] = 5000;
    arr[1] = 4000;

    // Shares memory with `arr`.
    const buf = Buffer.from(arr.buffer);

    console.log(buf);
    // Prints: <Buffer 88 13 a0 0f>

    // Changing the original Uint16Array changes the Buffer also.
    arr[1] = 6000;

    console.log(buf);
    // Prints: <Buffer 88 13 70 17>
    const { Buffer } = require('node:buffer');

    const arr = new Uint16Array(2);

    arr[0] = 5000;
    arr[1] = 4000;

    // Shares memory with `arr`.
    const buf = Buffer.from(arr.buffer);

    console.log(buf);
    // Prints: <Buffer 88 13 a0 0f>

    // Changing the original Uint16Array changes the Buffer also.
    arr[1] = 6000;

    console.log(buf);
    // Prints: <Buffer 88 13 70 17>
    javascript

The optional `byteOffset` and `length` arguments specify a memory range within the `arrayBuffer` that will be shared by the `Buffer`.

    import { Buffer } from 'node:buffer';

    const ab = new ArrayBuffer(10);
    const buf = Buffer.from(ab, 0, 2);

    console.log(buf.length);
    // Prints: 2
    const { Buffer } = require('node:buffer');

    const ab = new ArrayBuffer(10);
    const buf = Buffer.from(ab, 0, 2);

    console.log(buf.length);
    // Prints: 2
    javascript

A `TypeError` will be thrown if `arrayBuffer` is not an `<ArrayBuffer>` or a `<SharedArrayBuffer>` or another type appropriate for `Buffer.from()` variants.

It is important to remember that a backing `ArrayBuffer` can cover a range of memory that extends beyond the bounds of a `TypedArray` view. A new `Buffer` created using the `buffer` property of a `TypedArray` may extend beyond the range of the `TypedArray`:

    import { Buffer } from 'node:buffer';

    const arrA = Uint8Array.from([0x63, 0x64, 0x65, 0x66]); // 4 elements
    const arrB = new Uint8Array(arrA.buffer, 1, 2); // 2 elements
    console.log(arrA.buffer === arrB.buffer); // true

    const buf = Buffer.from(arrB.buffer);
    console.log(buf);
    // Prints: <Buffer 63 64 65 66>
    const { Buffer } = require('node:buffer');

    const arrA = Uint8Array.from([0x63, 0x64, 0x65, 0x66]); // 4 elements
    const arrB = new Uint8Array(arrA.buffer, 1, 2); // 2 elements
    console.log(arrA.buffer === arrB.buffer); // true

    const buf = Buffer.from(arrB.buffer);
    console.log(buf);
    // Prints: <Buffer 63 64 65 66>
    javascript

#### Static method: `Buffer.from(buffer)`#

Added in: v5.10.0

- `buffer` `<Buffer>` | `<Uint8Array>` An existing `Buffer` or `<Uint8Array>` from which to copy data.
- Returns: `<Buffer>`

Copies the passed `buffer` data onto a new `Buffer` instance.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from('buffer');
    const buf2 = Buffer.from(buf1);

    buf1[0] = 0x61;

    console.log(buf1.toString());
    // Prints: auffer
    console.log(buf2.toString());
    // Prints: buffer
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from('buffer');
    const buf2 = Buffer.from(buf1);

    buf1[0] = 0x61;

    console.log(buf1.toString());
    // Prints: auffer
    console.log(buf2.toString());
    // Prints: buffer
    javascript

A `TypeError` will be thrown if `buffer` is not a `Buffer` or another type appropriate for `Buffer.from()` variants.

#### Static method: `Buffer.from(object[, offsetOrEncoding[, length]])`#

Added in: v8.2.0

- `object` `<Object>` An object supporting `Symbol.toPrimitive` or `valueOf()`.
- `offsetOrEncoding` `<integer>` | `<string>` A byte-offset or encoding.
- `length` `<integer>` A length.
- Returns: `<Buffer>`

For objects whose `valueOf()` function returns a value not strictly equal to `object`, returns `Buffer.from(object.valueOf(), offsetOrEncoding, length)`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from(new String('this is a test'));
    // Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from(new String('this is a test'));
    // Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
    javascript

For objects that support `Symbol.toPrimitive`, returns `Buffer.from(object[Symbol.toPrimitive]('string'), offsetOrEncoding)`.

    import { Buffer } from 'node:buffer';

    class Foo {
      [Symbol.toPrimitive]() {
        return 'this is a test';
      }
    }

    const buf = Buffer.from(new Foo(), 'utf8');
    // Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
    const { Buffer } = require('node:buffer');

    class Foo {
      [Symbol.toPrimitive]() {
        return 'this is a test';
      }
    }

    const buf = Buffer.from(new Foo(), 'utf8');
    // Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
    javascript

A `TypeError` will be thrown if `object` does not have the mentioned methods or is not of another type appropriate for `Buffer.from()` variants.

#### Static method: `Buffer.from(string[, encoding])`#

Added in: v5.10.0

- `string` `<string>` A string to encode.
- `encoding` `<string>` The encoding of `string`. Default: `'utf8'`.
- Returns: `<Buffer>`

Creates a new `Buffer` containing `string`. The `encoding` parameter identifies the character encoding to be used when converting `string` into bytes.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from('this is a tést');
    const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

    console.log(buf1.toString());
    // Prints: this is a tést
    console.log(buf2.toString());
    // Prints: this is a tést
    console.log(buf1.toString('latin1'));
    // Prints: this is a tÃ©st
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from('this is a tést');
    const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

    console.log(buf1.toString());
    // Prints: this is a tést
    console.log(buf2.toString());
    // Prints: this is a tést
    console.log(buf1.toString('latin1'));
    // Prints: this is a tÃ©st
    javascript

A `TypeError` will be thrown if `string` is not a string or another type appropriate for `Buffer.from()` variants.

`Buffer.from(string)` may also use the internal `Buffer` pool like `Buffer.allocUnsafe()` does.

#### Static method: `Buffer.isBuffer(obj)`#

Added in: v0.1.101

- `obj` `<Object>`
- Returns: `<boolean>`

Returns `true` if `obj` is a `Buffer`, `false` otherwise.

    import { Buffer } from 'node:buffer';

    Buffer.isBuffer(Buffer.alloc(10)); // true
    Buffer.isBuffer(Buffer.from('foo')); // true
    Buffer.isBuffer('a string'); // false
    Buffer.isBuffer([]); // false
    Buffer.isBuffer(new Uint8Array(1024)); // false
    const { Buffer } = require('node:buffer');

    Buffer.isBuffer(Buffer.alloc(10)); // true
    Buffer.isBuffer(Buffer.from('foo')); // true
    Buffer.isBuffer('a string'); // false
    Buffer.isBuffer([]); // false
    Buffer.isBuffer(new Uint8Array(1024)); // false
    javascript

#### Static method: `Buffer.isEncoding(encoding)`#

Added in: v0.9.1

- `encoding` `<string>` A character encoding name to check.
- Returns: `<boolean>`

Returns `true` if `encoding` is the name of a supported character encoding, or `false` otherwise.

    import { Buffer } from 'node:buffer';

    console.log(Buffer.isEncoding('utf8'));
    // Prints: true

    console.log(Buffer.isEncoding('hex'));
    // Prints: true

    console.log(Buffer.isEncoding('utf/8'));
    // Prints: false

    console.log(Buffer.isEncoding(''));
    // Prints: false
    const { Buffer } = require('node:buffer');

    console.log(Buffer.isEncoding('utf8'));
    // Prints: true

    console.log(Buffer.isEncoding('hex'));
    // Prints: true

    console.log(Buffer.isEncoding('utf/8'));
    // Prints: false

    console.log(Buffer.isEncoding(''));
    // Prints: false
    javascript

#### `Buffer.poolSize`#

Added in: v0.11.3
History

Version | Changes
--- | ---
v26.3.0 | Default raised from 8192 to 65536.

- Type: `<integer>` Default: `65536`

This is the size (in bytes) of pre-allocated internal `Buffer` instances used for pooling. This value may be modified.

#### `buf[index]`#

- `index` `<integer>`

The index operator `[index]` can be used to get and set the octet at position `index` in `buf`. The values refer to individual bytes, so the legal value range is between `0x00` and `0xFF` (hex) or `0` and `255` (decimal).

This operator is inherited from `Uint8Array`, so its behavior on out-of-bounds access is the same as `Uint8Array`. In other words, `buf[index]` returns `undefined` when `index` is negative or greater or equal to `buf.length`, and `buf[index] = value` does not modify the buffer if `index` is negative or `>= buf.length`.

    import { Buffer } from 'node:buffer';

    // Copy an ASCII string into a `Buffer` one byte at a time.
    // (This only works for ASCII-only strings. In general, one should use
    // `Buffer.from()` to perform this conversion.)

    const str = 'Node.js';
    const buf = Buffer.allocUnsafe(str.length);

    for (let i = 0; i < str.length; i++) {
      buf[i] = str.charCodeAt(i);
    }

    console.log(buf.toString('utf8'));
    // Prints: Node.js
    const { Buffer } = require('node:buffer');

    // Copy an ASCII string into a `Buffer` one byte at a time.
    // (This only works for ASCII-only strings. In general, one should use
    // `Buffer.from()` to perform this conversion.)

    const str = 'Node.js';
    const buf = Buffer.allocUnsafe(str.length);

    for (let i = 0; i < str.length; i++) {
      buf[i] = str.charCodeAt(i);
    }

    console.log(buf.toString('utf8'));
    // Prints: Node.js
    javascript

#### `buf.buffer`#

- Type: `<ArrayBuffer>` The underlying `ArrayBuffer` object based on which this `Buffer` object is created.

This `ArrayBuffer` is not guaranteed to correspond exactly to the original `Buffer`. See the notes on `buf.byteOffset` for details.

    import { Buffer } from 'node:buffer';

    const arrayBuffer = new ArrayBuffer(16);
    const buffer = Buffer.from(arrayBuffer);

    console.log(buffer.buffer === arrayBuffer);
    // Prints: true
    const { Buffer } = require('node:buffer');

    const arrayBuffer = new ArrayBuffer(16);
    const buffer = Buffer.from(arrayBuffer);

    console.log(buffer.buffer === arrayBuffer);
    // Prints: true
    javascript

#### `buf.byteOffset`#

- Type: `<integer>` The `byteOffset` of the `Buffer`'s underlying `ArrayBuffer` object.

When setting `byteOffset` in `Buffer.from(ArrayBuffer, byteOffset, length)`, or sometimes when allocating a `Buffer` smaller than `Buffer.poolSize`, the buffer does not start from a zero offset on the underlying `ArrayBuffer`.

This can cause problems when accessing the underlying `ArrayBuffer` directly using `buf.buffer`, as other parts of the `ArrayBuffer` may be unrelated to the `Buffer` object itself.

A common issue when creating a `TypedArray` object that shares its memory with a `Buffer` is that in this case one needs to specify the `byteOffset` correctly:

    import { Buffer } from 'node:buffer';

    // Create a buffer smaller than `Buffer.poolSize`.
    const nodeBuffer = Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

    // When casting the Node.js Buffer to an Int8Array, use the byteOffset
    // to refer only to the part of `nodeBuffer.buffer` that contains the memory
    // for `nodeBuffer`.
    new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);
    const { Buffer } = require('node:buffer');

    // Create a buffer smaller than `Buffer.poolSize`.
    const nodeBuffer = Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

    // When casting the Node.js Buffer to an Int8Array, use the byteOffset
    // to refer only to the part of `nodeBuffer.buffer` that contains the memory
    // for `nodeBuffer`.
    new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);
    javascript

#### `buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])`#

Added in: v0.11.13
History

Version | Changes
--- | ---
v8.0.0 | The `target` parameter can now be a `Uint8Array`.
v5.11.0 | Additional parameters for specifying offsets are supported now.

- `target` `<Buffer>` | `<Uint8Array>` A `Buffer` or `<Uint8Array>` with which to compare `buf`.
- `targetStart` `<integer>` The offset within `target` at which to begin comparison. Default: `0`.
- `targetEnd` `<integer>` The offset within `target` at which to end comparison (not inclusive). Default: `target.length`.
- `sourceStart` `<integer>` The offset within `buf` at which to begin comparison. Default: `0`.
- `sourceEnd` `<integer>` The offset within `buf` at which to end comparison (not inclusive). Default: `buf.length`.
- Returns: `<integer>`

Compares `buf` with `target` and returns a number indicating whether `buf` comes before, after, or is the same as `target` in sort order. Comparison is based on the actual sequence of bytes in each `Buffer`.

- `0` is returned if `target` is the same as `buf`
- `1` is returned if `target` should come before `buf` when sorted.
- `-1` is returned if `target` should come after `buf` when sorted.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from('ABC');
    const buf2 = Buffer.from('BCD');
    const buf3 = Buffer.from('ABCD');

    console.log(buf1.compare(buf1));
    // Prints: 0
    console.log(buf1.compare(buf2));
    // Prints: -1
    console.log(buf1.compare(buf3));
    // Prints: -1
    console.log(buf2.compare(buf1));
    // Prints: 1
    console.log(buf2.compare(buf3));
    // Prints: 1
    console.log([buf1, buf2, buf3].sort(Buffer.compare));
    // Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
    // (This result is equal to: [buf1, buf3, buf2].)
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from('ABC');
    const buf2 = Buffer.from('BCD');
    const buf3 = Buffer.from('ABCD');

    console.log(buf1.compare(buf1));
    // Prints: 0
    console.log(buf1.compare(buf2));
    // Prints: -1
    console.log(buf1.compare(buf3));
    // Prints: -1
    console.log(buf2.compare(buf1));
    // Prints: 1
    console.log(buf2.compare(buf3));
    // Prints: 1
    console.log([buf1, buf2, buf3].sort(Buffer.compare));
    // Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
    // (This result is equal to: [buf1, buf3, buf2].)
    javascript

The optional `targetStart`, `targetEnd`, `sourceStart`, and `sourceEnd` arguments can be used to limit the comparison to specific ranges within `target` and `buf` respectively.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
    const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

    console.log(buf1.compare(buf2, 5, 9, 0, 4));
    // Prints: 0
    console.log(buf1.compare(buf2, 0, 6, 4));
    // Prints: -1
    console.log(buf1.compare(buf2, 5, 6, 5));
    // Prints: 1
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
    const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

    console.log(buf1.compare(buf2, 5, 9, 0, 4));
    // Prints: 0
    console.log(buf1.compare(buf2, 0, 6, 4));
    // Prints: -1
    console.log(buf1.compare(buf2, 5, 6, 5));
    // Prints: 1
    javascript

`ERR_OUT_OF_RANGE` is thrown if `targetStart < 0`, `sourceStart < 0`, `targetEnd > target.byteLength`, or `sourceEnd > source.byteLength`.

#### `buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])`#

Added in: v0.1.90

- `target` `<Buffer>` | `<Uint8Array>` A `Buffer` or `<Uint8Array>` to copy into.
- `targetStart` `<integer>` The offset within `target` at which to begin writing. Default: `0`.
- `sourceStart` `<integer>` The offset within `buf` from which to begin copying. Default: `0`.
- `sourceEnd` `<integer>` The offset within `buf` at which to stop copying (not inclusive). Default: `buf.length`.
- Returns: `<integer>` The number of bytes copied.

Copies data from a region of `buf` to a region in `target`, even if the `target` memory region overlaps with `buf`.

`TypedArray.prototype.set()` performs the same operation, and is available for all TypedArrays, including Node.js `Buffer`s, although it takes different function arguments.

    import { Buffer } from 'node:buffer';

    // Create two `Buffer` instances.
    const buf1 = Buffer.allocUnsafe(26);
    const buf2 = Buffer.allocUnsafe(26).fill('!');

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf1[i] = i + 97;
    }

    // Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
    buf1.copy(buf2, 8, 16, 20);
    // This is equivalent to:
    // buf2.set(buf1.subarray(16, 20), 8);

    console.log(buf2.toString('ascii', 0, 25));
    // Prints: !!!!!!!!qrst!!!!!!!!!!!!!
    const { Buffer } = require('node:buffer');

    // Create two `Buffer` instances.
    const buf1 = Buffer.allocUnsafe(26);
    const buf2 = Buffer.allocUnsafe(26).fill('!');

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf1[i] = i + 97;
    }

    // Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
    buf1.copy(buf2, 8, 16, 20);
    // This is equivalent to:
    // buf2.set(buf1.subarray(16, 20), 8);

    console.log(buf2.toString('ascii', 0, 25));
    // Prints: !!!!!!!!qrst!!!!!!!!!!!!!
    javascript

    import { Buffer } from 'node:buffer';

    // Create a `Buffer` and copy data from one region to an overlapping region
    // within the same `Buffer`.

    const buf = Buffer.allocUnsafe(26);

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf[i] = i + 97;
    }

    buf.copy(buf, 0, 4, 10);

    console.log(buf.toString());
    // Prints: efghijghijklmnopqrstuvwxyz
    const { Buffer } = require('node:buffer');

    // Create a `Buffer` and copy data from one region to an overlapping region
    // within the same `Buffer`.

    const buf = Buffer.allocUnsafe(26);

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf[i] = i + 97;
    }

    buf.copy(buf, 0, 4, 10);

    console.log(buf.toString());
    // Prints: efghijghijklmnopqrstuvwxyz
    javascript

#### `buf.entries()`#

Added in: v1.1.0

- Returns: `<Iterator>`

Creates and returns an iterator of `[index, byte]` pairs from the contents of `buf`.

    import { Buffer } from 'node:buffer';

    // Log the entire contents of a `Buffer`.

    const buf = Buffer.from('buffer');

    for (const pair of buf.entries()) {
      console.log(pair);
    }
    // Prints:
    //   [0, 98]
    //   [1, 117]
    //   [2, 102]
    //   [3, 102]
    //   [4, 101]
    //   [5, 114]
    const { Buffer } = require('node:buffer');

    // Log the entire contents of a `Buffer`.

    const buf = Buffer.from('buffer');

    for (const pair of buf.entries()) {
      console.log(pair);
    }
    // Prints:
    //   [0, 98]
    //   [1, 117]
    //   [2, 102]
    //   [3, 102]
    //   [4, 101]
    //   [5, 114]
    javascript

#### `buf.equals(otherBuffer)`#

Added in: v0.11.13
History

Version | Changes
--- | ---
v8.0.0 | The arguments can now be `Uint8Array`s.

- `otherBuffer` `<Buffer>` | `<Uint8Array>` A `Buffer` or `<Uint8Array>` with which to compare `buf`.
- Returns: `<boolean>`

Returns `true` if both `buf` and `otherBuffer` have exactly the same bytes, `false` otherwise. Equivalent to `buf.compare(otherBuffer) === 0`.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from('ABC');
    const buf2 = Buffer.from('414243', 'hex');
    const buf3 = Buffer.from('ABCD');

    console.log(buf1.equals(buf2));
    // Prints: true
    console.log(buf1.equals(buf3));
    // Prints: false
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from('ABC');
    const buf2 = Buffer.from('414243', 'hex');
    const buf3 = Buffer.from('ABCD');

    console.log(buf1.equals(buf2));
    // Prints: true
    console.log(buf1.equals(buf3));
    // Prints: false
    javascript

#### `buf.fill(value[, offset[, end]][, encoding])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v11.0.0 | Throws `ERR_OUT_OF_RANGE` instead of `ERR_INDEX_OUT_OF_RANGE`.
v10.0.0 | Negative `end` values throw an `ERR_INDEX_OUT_OF_RANGE` error.
v10.0.0 | Attempting to fill a non-zero length buffer with a zero length buffer triggers a thrown exception.
v10.0.0 | Specifying an invalid string for `value` triggers a thrown exception.
v5.7.0 | The `encoding` parameter is supported now.

- `value` `<string>` | `<Buffer>` | `<Uint8Array>` | `<integer>` The value with which to fill `buf`. Empty value (string, Uint8Array, Buffer) is coerced to `0`.
- `offset` `<integer>` Number of bytes to skip before starting to fill `buf`. Default: `0`.
- `end` `<integer>` Where to stop filling `buf` (not inclusive). Default: `buf.length`.
- `encoding` `<string>` The encoding for `value` if `value` is a string. Default: `'utf8'`.
- Returns: `<Buffer>` A reference to `buf`.

Fills `buf` with the specified `value`. If the `offset` and `end` are not given, the entire `buf` will be filled:

    import { Buffer } from 'node:buffer';

    // Fill a `Buffer` with the ASCII character 'h'.

    const b = Buffer.allocUnsafe(50).fill('h');

    console.log(b.toString());
    // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

    // Fill a buffer with empty string
    const c = Buffer.allocUnsafe(5).fill('');

    console.log(c.fill(''));
    // Prints: <Buffer 00 00 00 00 00>
    const { Buffer } = require('node:buffer');

    // Fill a `Buffer` with the ASCII character 'h'.

    const b = Buffer.allocUnsafe(50).fill('h');

    console.log(b.toString());
    // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

    // Fill a buffer with empty string
    const c = Buffer.allocUnsafe(5).fill('');

    console.log(c.fill(''));
    // Prints: <Buffer 00 00 00 00 00>
    javascript

`value` is coerced to a `uint32` value if it is not a string, `Buffer`, or integer. If the resulting integer is greater than `255` (decimal), `buf` will be filled with `value & 255`.

If the final write of a `fill()` operation falls on a multi-byte character, then only the bytes of that character that fit into `buf` are written:

    import { Buffer } from 'node:buffer';

    // Fill a `Buffer` with character that takes up two bytes in UTF-8.

    console.log(Buffer.allocUnsafe(5).fill('\u0222'));
    // Prints: <Buffer c8 a2 c8 a2 c8>
    const { Buffer } = require('node:buffer');

    // Fill a `Buffer` with character that takes up two bytes in UTF-8.

    console.log(Buffer.allocUnsafe(5).fill('\u0222'));
    // Prints: <Buffer c8 a2 c8 a2 c8>
    javascript

If `value` contains invalid characters, it is truncated; if no valid fill data remains, an exception is thrown:

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(5);

    console.log(buf.fill('a'));
    // Prints: <Buffer 61 61 61 61 61>
    console.log(buf.fill('aazz', 'hex'));
    // Prints: <Buffer aa aa aa aa aa>
    console.log(buf.fill('zz', 'hex'));
    // Throws an exception.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(5);

    console.log(buf.fill('a'));
    // Prints: <Buffer 61 61 61 61 61>
    console.log(buf.fill('aazz', 'hex'));
    // Prints: <Buffer aa aa aa aa aa>
    console.log(buf.fill('zz', 'hex'));
    // Throws an exception.
    javascript

#### `buf.includes(value[, start[, end]][, encoding])`#

Added in: v5.3.0
History

Version | Changes
--- | ---
v26.1.0 | Added the `end` parameter.
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.

- `value` `<string>` | `<Buffer>` | `<Uint8Array>` | `<integer>` What to search for.
- `start` `<integer>` Where to begin searching in `buf`. If negative, then offset is calculated from the end of `buf`. Default: `0`.
- `end` `<integer>` Where to stop searching in `buf` (exclusive). Default: `buf.length`.
- `encoding` `<string>` If `value` is a string, this is its encoding. Default: `'utf8'`.
- Returns: `<boolean>` `true` if `value` was found in `buf`, `false` otherwise.

Equivalent to `buf.indexOf() !== -1`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('this is a buffer');

    console.log(buf.includes('this'));
    // Prints: true
    console.log(buf.includes('is'));
    // Prints: true
    console.log(buf.includes(Buffer.from('a buffer')));
    // Prints: true
    console.log(buf.includes(97));
    // Prints: true (97 is the decimal ASCII value for 'a')
    console.log(buf.includes(Buffer.from('a buffer example')));
    // Prints: false
    console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
    // Prints: true
    console.log(buf.includes('this', 4));
    // Prints: false
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('this is a buffer');

    console.log(buf.includes('this'));
    // Prints: true
    console.log(buf.includes('is'));
    // Prints: true
    console.log(buf.includes(Buffer.from('a buffer')));
    // Prints: true
    console.log(buf.includes(97));
    // Prints: true (97 is the decimal ASCII value for 'a')
    console.log(buf.includes(Buffer.from('a buffer example')));
    // Prints: false
    console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
    // Prints: true
    console.log(buf.includes('this', 4));
    // Prints: false
    javascript

#### `buf.indexOf(value[, start[, end]][, encoding])`#

Added in: v1.5.0
History

Version | Changes
--- | ---
v26.1.0 | Added the `end` parameter.
v8.0.0 | The `value` can now be a `Uint8Array`.
v5.7.0, v4.4.0 | When `encoding` is being passed, the `byteOffset` parameter is no longer required.

- `value` `<string>` | `<Buffer>` | `<Uint8Array>` | `<integer>` What to search for.
- `start` `<integer>` Where to begin searching in `buf`. If negative, then offset is calculated from the end of `buf`. Default: `0`.
- `end` `<integer>` Where to stop searching in `buf` (exclusive). Default: `buf.length`.
- `encoding` `<string>` If `value` is a string, this is the encoding used to determine the binary representation of the string that will be searched for in `buf`. Default: `'utf8'`.
- Returns: `<integer>` The index of the first occurrence of `value` in `buf`, or `-1` if `buf` does not contain `value`.

If `value` is:

- a string, `value` is interpreted according to the character encoding in `encoding`.
- a `Buffer` or `<Uint8Array>`, `value` will be used in its entirety. To compare a partial `Buffer`, use `buf.subarray`.
- a number, `value` will be interpreted as an unsigned 8-bit integer value between `0` and `255`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('this is a buffer');

    console.log(buf.indexOf('this'));
    // Prints: 0
    console.log(buf.indexOf('is'));
    // Prints: 2
    console.log(buf.indexOf(Buffer.from('a buffer')));
    // Prints: 8
    console.log(buf.indexOf(97));
    // Prints: 8 (97 is the decimal ASCII value for 'a')
    console.log(buf.indexOf(Buffer.from('a buffer example')));
    // Prints: -1
    console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
    // Prints: 8

    const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

    console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
    // Prints: 4
    console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
    // Prints: 6
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('this is a buffer');

    console.log(buf.indexOf('this'));
    // Prints: 0
    console.log(buf.indexOf('is'));
    // Prints: 2
    console.log(buf.indexOf(Buffer.from('a buffer')));
    // Prints: 8
    console.log(buf.indexOf(97));
    // Prints: 8 (97 is the decimal ASCII value for 'a')
    console.log(buf.indexOf(Buffer.from('a buffer example')));
    // Prints: -1
    console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
    // Prints: 8

    const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

    console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
    // Prints: 4
    console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
    // Prints: 6
    javascript

If `value` is not a string, number, or `Buffer`, this method will throw a `TypeError`. If `value` is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If `byteOffset` is not a number, it will be coerced to a number. If the result of coercion is `NaN` or `0`, then the entire buffer will be searched. This behavior matches `String.prototype.indexOf()`.

    import { Buffer } from 'node:buffer';

    const b = Buffer.from('abcdef');

    // Passing a value that's a number, but not a valid byte.
    // Prints: 2, equivalent to searching for 99 or 'c'.
    console.log(b.indexOf(99.9));
    console.log(b.indexOf(256 + 99));

    // Passing a byteOffset that coerces to NaN or 0.
    // Prints: 1, searching the whole buffer.
    console.log(b.indexOf('b', undefined));
    console.log(b.indexOf('b', {}));
    console.log(b.indexOf('b', null));
    console.log(b.indexOf('b', []));
    const { Buffer } = require('node:buffer');

    const b = Buffer.from('abcdef');

    // Passing a value that's a number, but not a valid byte.
    // Prints: 2, equivalent to searching for 99 or 'c'.
    console.log(b.indexOf(99.9));
    console.log(b.indexOf(256 + 99));

    // Passing a byteOffset that coerces to NaN or 0.
    // Prints: 1, searching the whole buffer.
    console.log(b.indexOf('b', undefined));
    console.log(b.indexOf('b', {}));
    console.log(b.indexOf('b', null));
    console.log(b.indexOf('b', []));
    javascript

If `value` is an empty string or empty `Buffer` and `byteOffset` is less than `buf.length`, `byteOffset` will be returned. If `value` is empty and `byteOffset` is at least `buf.length`, `buf.length` will be returned.

#### `buf.keys()`#

Added in: v1.1.0

- Returns: `<Iterator>`

Creates and returns an iterator of `buf` keys (indexes).

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('buffer');

    for (const key of buf.keys()) {
      console.log(key);
    }
    // Prints:
    //   0
    //   1
    //   2
    //   3
    //   4
    //   5
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('buffer');

    for (const key of buf.keys()) {
      console.log(key);
    }
    // Prints:
    //   0
    //   1
    //   2
    //   3
    //   4
    //   5
    javascript

#### `buf.lastIndexOf(value[, start[, end]][, encoding])`#

Added in: v6.0.0
History

Version | Changes
--- | ---
v26.1.0 | Added the `end` parameter.
v8.0.0 | The `value` can now be a `Uint8Array`.

- `value` `<string>` | `<Buffer>` | `<Uint8Array>` | `<integer>` What to search for.
- `start` `<integer>` Where to begin searching in `buf`. If negative, then offset is calculated from the end of `buf`. Default: `buf.length - 1`.
- `end` `<integer>` Where to stop searching in `buf` (exclusive). Default: `buf.length`.
- `encoding` `<string>` If `value` is a string, this is the encoding used to determine the binary representation of the string that will be searched for in `buf`. Default: `'utf8'`.
- Returns: `<integer>` The index of the last occurrence of `value` in `buf`, or `-1` if `buf` does not contain `value`.

Identical to `buf.indexOf()`, except the last occurrence of `value` is found rather than the first occurrence.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('this buffer is a buffer');

    console.log(buf.lastIndexOf('this'));
    // Prints: 0
    console.log(buf.lastIndexOf('buffer'));
    // Prints: 17
    console.log(buf.lastIndexOf(Buffer.from('buffer')));
    // Prints: 17
    console.log(buf.lastIndexOf(97));
    // Prints: 15 (97 is the decimal ASCII value for 'a')
    console.log(buf.lastIndexOf(Buffer.from('yolo')));
    // Prints: -1
    console.log(buf.lastIndexOf('buffer', 5));
    // Prints: 5
    console.log(buf.lastIndexOf('buffer', 4));
    // Prints: -1

    const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

    console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
    // Prints: 6
    console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
    // Prints: 4
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('this buffer is a buffer');

    console.log(buf.lastIndexOf('this'));
    // Prints: 0
    console.log(buf.lastIndexOf('buffer'));
    // Prints: 17
    console.log(buf.lastIndexOf(Buffer.from('buffer')));
    // Prints: 17
    console.log(buf.lastIndexOf(97));
    // Prints: 15 (97 is the decimal ASCII value for 'a')
    console.log(buf.lastIndexOf(Buffer.from('yolo')));
    // Prints: -1
    console.log(buf.lastIndexOf('buffer', 5));
    // Prints: 5
    console.log(buf.lastIndexOf('buffer', 4));
    // Prints: -1

    const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

    console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
    // Prints: 6
    console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
    // Prints: 4
    javascript

If `value` is not a string, number, or `Buffer`, this method will throw a `TypeError`. If `value` is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If `byteOffset` is not a number, it will be coerced to a number. Any arguments that coerce to `NaN`, like `{}` or `undefined`, will search the whole buffer. This behavior matches `String.prototype.lastIndexOf()`.

    import { Buffer } from 'node:buffer';

    const b = Buffer.from('abcdef');

    // Passing a value that's a number, but not a valid byte.
    // Prints: 2, equivalent to searching for 99 or 'c'.
    console.log(b.lastIndexOf(99.9));
    console.log(b.lastIndexOf(256 + 99));

    // Passing a byteOffset that coerces to NaN.
    // Prints: 1, searching the whole buffer.
    console.log(b.lastIndexOf('b', undefined));
    console.log(b.lastIndexOf('b', {}));

    // Passing a byteOffset that coerces to 0.
    // Prints: -1, equivalent to passing 0.
    console.log(b.lastIndexOf('b', null));
    console.log(b.lastIndexOf('b', []));
    const { Buffer } = require('node:buffer');

    const b = Buffer.from('abcdef');

    // Passing a value that's a number, but not a valid byte.
    // Prints: 2, equivalent to searching for 99 or 'c'.
    console.log(b.lastIndexOf(99.9));
    console.log(b.lastIndexOf(256 + 99));

    // Passing a byteOffset that coerces to NaN.
    // Prints: 1, searching the whole buffer.
    console.log(b.lastIndexOf('b', undefined));
    console.log(b.lastIndexOf('b', {}));

    // Passing a byteOffset that coerces to 0.
    // Prints: -1, equivalent to passing 0.
    console.log(b.lastIndexOf('b', null));
    console.log(b.lastIndexOf('b', []));
    javascript

If `value` is an empty string or empty `Buffer`, `byteOffset` will be returned.

#### `buf.length`#

Added in: v0.1.90

- Type: `<integer>`

Returns the number of bytes in `buf`.

    import { Buffer } from 'node:buffer';

    // Create a `Buffer` and write a shorter string to it using UTF-8.

    const buf = Buffer.alloc(1234);

    console.log(buf.length);
    // Prints: 1234

    buf.write('some string', 0, 'utf8');

    console.log(buf.length);
    // Prints: 1234
    const { Buffer } = require('node:buffer');

    // Create a `Buffer` and write a shorter string to it using UTF-8.

    const buf = Buffer.alloc(1234);

    console.log(buf.length);
    // Prints: 1234

    buf.write('some string', 0, 'utf8');

    console.log(buf.length);
    // Prints: 1234
    javascript

#### `buf.parent`#

Deprecated in: v8.0.0

Stability: 0 - Deprecated: Use `buf.buffer` instead.

The `buf.parent` property is a deprecated alias for `buf.buffer`.

#### `buf.readBigInt64BE([offset])`#

Added in: v12.0.0, v10.20.0

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<bigint>`

Reads a signed, big-endian 64-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

#### `buf.readBigInt64LE([offset])`#

Added in: v12.0.0, v10.20.0

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<bigint>`

Reads a signed, little-endian 64-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

#### `buf.readBigUInt64BE([offset])`#

Added in: v12.0.0, v10.20.0
History

Version | Changes
--- | ---
v14.10.0, v12.19.0 | This function is also available as `buf.readBigUint64BE()`.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<bigint>`

Reads an unsigned, big-endian 64-bit integer from `buf` at the specified `offset`.

This function is also available under the `readBigUint64BE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

    console.log(buf.readBigUInt64BE(0));
    // Prints: 4294967295n
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

    console.log(buf.readBigUInt64BE(0));
    // Prints: 4294967295n
    javascript

#### `buf.readBigUInt64LE([offset])`#

Added in: v12.0.0, v10.20.0
History

Version | Changes
--- | ---
v14.10.0, v12.19.0 | This function is also available as `buf.readBigUint64LE()`.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<bigint>`

Reads an unsigned, little-endian 64-bit integer from `buf` at the specified `offset`.

This function is also available under the `readBigUint64LE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

    console.log(buf.readBigUInt64LE(0));
    // Prints: 18446744069414584320n
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

    console.log(buf.readBigUInt64LE(0));
    // Prints: 18446744069414584320n
    javascript

#### `buf.readDoubleBE([offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<number>`

Reads a 64-bit, big-endian double from `buf` at the specified `offset`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

    console.log(buf.readDoubleBE(0));
    // Prints: 8.20788039913184e-304
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

    console.log(buf.readDoubleBE(0));
    // Prints: 8.20788039913184e-304
    javascript

#### `buf.readDoubleLE([offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<number>`

Reads a 64-bit, little-endian double from `buf` at the specified `offset`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

    console.log(buf.readDoubleLE(0));
    // Prints: 5.447603722011605e-270
    console.log(buf.readDoubleLE(1));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

    console.log(buf.readDoubleLE(0));
    // Prints: 5.447603722011605e-270
    console.log(buf.readDoubleLE(1));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readFloatBE([offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<number>`

Reads a 32-bit, big-endian float from `buf` at the specified `offset`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, 2, 3, 4]);

    console.log(buf.readFloatBE(0));
    // Prints: 2.387939260590663e-38
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, 2, 3, 4]);

    console.log(buf.readFloatBE(0));
    // Prints: 2.387939260590663e-38
    javascript

#### `buf.readFloatLE([offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<number>`

Reads a 32-bit, little-endian float from `buf` at the specified `offset`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, 2, 3, 4]);

    console.log(buf.readFloatLE(0));
    // Prints: 1.539989614439558e-36
    console.log(buf.readFloatLE(1));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, 2, 3, 4]);

    console.log(buf.readFloatLE(0));
    // Prints: 1.539989614439558e-36
    console.log(buf.readFloatLE(1));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readInt8([offset])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 1`. Default: `0`.
- Returns: `<integer>`

Reads a signed 8-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([-1, 5]);

    console.log(buf.readInt8(0));
    // Prints: -1
    console.log(buf.readInt8(1));
    // Prints: 5
    console.log(buf.readInt8(2));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([-1, 5]);

    console.log(buf.readInt8(0));
    // Prints: -1
    console.log(buf.readInt8(1));
    // Prints: 5
    console.log(buf.readInt8(2));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readInt16BE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>`

Reads a signed, big-endian 16-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0, 5]);

    console.log(buf.readInt16BE(0));
    // Prints: 5
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0, 5]);

    console.log(buf.readInt16BE(0));
    // Prints: 5
    javascript

#### `buf.readInt16LE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>`

Reads a signed, little-endian 16-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0, 5]);

    console.log(buf.readInt16LE(0));
    // Prints: 1280
    console.log(buf.readInt16LE(1));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0, 5]);

    console.log(buf.readInt16LE(0));
    // Prints: 1280
    console.log(buf.readInt16LE(1));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readInt32BE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>`

Reads a signed, big-endian 32-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0, 0, 0, 5]);

    console.log(buf.readInt32BE(0));
    // Prints: 5
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0, 0, 0, 5]);

    console.log(buf.readInt32BE(0));
    // Prints: 5
    javascript

#### `buf.readInt32LE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>`

Reads a signed, little-endian 32-bit integer from `buf` at the specified `offset`.

Integers read from a `Buffer` are interpreted as two's complement signed values.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0, 0, 0, 5]);

    console.log(buf.readInt32LE(0));
    // Prints: 83886080
    console.log(buf.readInt32LE(1));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0, 0, 0, 5]);

    console.log(buf.readInt32LE(0));
    // Prints: 83886080
    console.log(buf.readInt32LE(1));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readIntBE(offset, byteLength)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to read. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>`

Reads `byteLength` number of bytes from `buf` at the specified `offset` and interprets the result as a big-endian, two's complement signed value supporting up to 48 bits of accuracy.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readIntBE(0, 6).toString(16));
    // Prints: 1234567890ab
    console.log(buf.readIntBE(1, 6).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    console.log(buf.readIntBE(1, 0).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readIntBE(0, 6).toString(16));
    // Prints: 1234567890ab
    console.log(buf.readIntBE(1, 6).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    console.log(buf.readIntBE(1, 0).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readIntLE(offset, byteLength)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to read. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>`

Reads `byteLength` number of bytes from `buf` at the specified `offset` and interprets the result as a little-endian, two's complement signed value supporting up to 48 bits of accuracy.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readIntLE(0, 6).toString(16));
    // Prints: -546f87a9cbee
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readIntLE(0, 6).toString(16));
    // Prints: -546f87a9cbee
    javascript

#### `buf.readUInt8([offset])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.readUint8()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 1`. Default: `0`.
- Returns: `<integer>`

Reads an unsigned 8-bit integer from `buf` at the specified `offset`.

This function is also available under the `readUint8` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([1, -2]);

    console.log(buf.readUInt8(0));
    // Prints: 1
    console.log(buf.readUInt8(1));
    // Prints: 254
    console.log(buf.readUInt8(2));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([1, -2]);

    console.log(buf.readUInt8(0));
    // Prints: 1
    console.log(buf.readUInt8(1));
    // Prints: 254
    console.log(buf.readUInt8(2));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readUInt16BE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.readUint16BE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>`

Reads an unsigned, big-endian 16-bit integer from `buf` at the specified `offset`.

This function is also available under the `readUint16BE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56]);

    console.log(buf.readUInt16BE(0).toString(16));
    // Prints: 1234
    console.log(buf.readUInt16BE(1).toString(16));
    // Prints: 3456
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56]);

    console.log(buf.readUInt16BE(0).toString(16));
    // Prints: 1234
    console.log(buf.readUInt16BE(1).toString(16));
    // Prints: 3456
    javascript

#### `buf.readUInt16LE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.readUint16LE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>`

Reads an unsigned, little-endian 16-bit integer from `buf` at the specified `offset`.

This function is also available under the `readUint16LE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56]);

    console.log(buf.readUInt16LE(0).toString(16));
    // Prints: 3412
    console.log(buf.readUInt16LE(1).toString(16));
    // Prints: 5634
    console.log(buf.readUInt16LE(2).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56]);

    console.log(buf.readUInt16LE(0).toString(16));
    // Prints: 3412
    console.log(buf.readUInt16LE(1).toString(16));
    // Prints: 5634
    console.log(buf.readUInt16LE(2).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readUInt32BE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.readUint32BE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>`

Reads an unsigned, big-endian 32-bit integer from `buf` at the specified `offset`.

This function is also available under the `readUint32BE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

    console.log(buf.readUInt32BE(0).toString(16));
    // Prints: 12345678
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

    console.log(buf.readUInt32BE(0).toString(16));
    // Prints: 12345678
    javascript

#### `buf.readUInt32LE([offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.readUint32LE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>`

Reads an unsigned, little-endian 32-bit integer from `buf` at the specified `offset`.

This function is also available under the `readUint32LE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

    console.log(buf.readUInt32LE(0).toString(16));
    // Prints: 78563412
    console.log(buf.readUInt32LE(1).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

    console.log(buf.readUInt32LE(0).toString(16));
    // Prints: 78563412
    console.log(buf.readUInt32LE(1).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readUIntBE(offset, byteLength)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.
v14.9.0, v12.19.0 | This function is also available as `buf.readUintBE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to read. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>`

Reads `byteLength` number of bytes from `buf` at the specified `offset` and interprets the result as an unsigned big-endian integer supporting up to 48 bits of accuracy.

This function is also available under the `readUintBE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readUIntBE(0, 6).toString(16));
    // Prints: 1234567890ab
    console.log(buf.readUIntBE(1, 6).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readUIntBE(0, 6).toString(16));
    // Prints: 1234567890ab
    console.log(buf.readUIntBE(1, 6).toString(16));
    // Throws ERR_OUT_OF_RANGE.
    javascript

#### `buf.readUIntLE(offset, byteLength)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.
v14.9.0, v12.19.0 | This function is also available as `buf.readUintLE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `offset` `<integer>` Number of bytes to skip before starting to read. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to read. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>`

Reads `byteLength` number of bytes from `buf` at the specified `offset` and interprets the result as an unsigned, little-endian integer supporting up to 48 bits of accuracy.

This function is also available under the `readUintLE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readUIntLE(0, 6).toString(16));
    // Prints: ab9078563412
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

    console.log(buf.readUIntLE(0, 6).toString(16));
    // Prints: ab9078563412
    javascript

#### `buf.subarray([start[, end]])`#

Added in: v3.0.0

- `start` `<integer>` Where the new `Buffer` will start. Default: `0`.
- `end` `<integer>` Where the new `Buffer` will end (not inclusive). Default: `buf.length`.
- Returns: `<Buffer>`

Returns a new `Buffer` that references the same memory as the original, but offset and cropped by the `start` and `end` indexes.

Specifying `end` greater than `buf.length` will return the same result as that of `end` equal to `buf.length`.

This method is inherited from `TypedArray.prototype.subarray()`.

Modifying the new `Buffer` slice will modify the memory in the original `Buffer` because the allocated memory of the two objects overlap.

    import { Buffer } from 'node:buffer';

    // Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
    // from the original `Buffer`.

    const buf1 = Buffer.allocUnsafe(26);

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf1[i] = i + 97;
    }

    const buf2 = buf1.subarray(0, 3);

    console.log(buf2.toString('ascii', 0, buf2.length));
    // Prints: abc

    buf1[0] = 33;

    console.log(buf2.toString('ascii', 0, buf2.length));
    // Prints: !bc
    const { Buffer } = require('node:buffer');

    // Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
    // from the original `Buffer`.

    const buf1 = Buffer.allocUnsafe(26);

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf1[i] = i + 97;
    }

    const buf2 = buf1.subarray(0, 3);

    console.log(buf2.toString('ascii', 0, buf2.length));
    // Prints: abc

    buf1[0] = 33;

    console.log(buf2.toString('ascii', 0, buf2.length));
    // Prints: !bc
    javascript

Specifying negative indexes causes the slice to be generated relative to the end of `buf` rather than the beginning.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('buffer');

    console.log(buf.subarray(-6, -1).toString());
    // Prints: buffe
    // (Equivalent to buf.subarray(0, 5).)

    console.log(buf.subarray(-6, -2).toString());
    // Prints: buff
    // (Equivalent to buf.subarray(0, 4).)

    console.log(buf.subarray(-5, -2).toString());
    // Prints: uff
    // (Equivalent to buf.subarray(1, 4).)
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('buffer');

    console.log(buf.subarray(-6, -1).toString());
    // Prints: buffe
    // (Equivalent to buf.subarray(0, 5).)

    console.log(buf.subarray(-6, -2).toString());
    // Prints: buff
    // (Equivalent to buf.subarray(0, 4).)

    console.log(buf.subarray(-5, -2).toString());
    // Prints: uff
    // (Equivalent to buf.subarray(1, 4).)
    javascript

#### `buf.slice([start[, end]])`#

Added in: v0.3.0
History

Version | Changes
--- | ---
v17.5.0, v16.15.0 | The buf.slice() method has been deprecated.
v7.1.0, v6.9.2 | Coercing the offsets to integers now handles values outside the 32-bit integer range properly.
v7.0.0 | All offsets are now coerced to integers before doing any calculations with them.

Stability: 0 - Deprecated: Use `buf.subarray` instead.

- `start` `<integer>` Where the new `Buffer` will start. Default: `0`.
- `end` `<integer>` Where the new `Buffer` will end (not inclusive). Default: `buf.length`.
- Returns: `<Buffer>`

Returns a new `Buffer` that references the same memory as the original, but offset and cropped by the `start` and `end` indexes.

This method is not compatible with the `Uint8Array.prototype.slice()`, which is a superclass of `Buffer`. To copy the slice, use `Uint8Array.prototype.slice()`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('buffer');

    const copiedBuf = Uint8Array.prototype.slice.call(buf);
    copiedBuf[0]++;
    console.log(copiedBuf.toString());
    // Prints: cuffer

    console.log(buf.toString());
    // Prints: buffer

    // With buf.slice(), the original buffer is modified.
    const notReallyCopiedBuf = buf.slice();
    notReallyCopiedBuf[0]++;
    console.log(notReallyCopiedBuf.toString());
    // Prints: cuffer
    console.log(buf.toString());
    // Also prints: cuffer (!)
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('buffer');

    const copiedBuf = Uint8Array.prototype.slice.call(buf);
    copiedBuf[0]++;
    console.log(copiedBuf.toString());
    // Prints: cuffer

    console.log(buf.toString());
    // Prints: buffer

    // With buf.slice(), the original buffer is modified.
    const notReallyCopiedBuf = buf.slice();
    notReallyCopiedBuf[0]++;
    console.log(notReallyCopiedBuf.toString());
    // Prints: cuffer
    console.log(buf.toString());
    // Also prints: cuffer (!)
    javascript

#### `buf.swap16()`#

Added in: v5.10.0

- Returns: `<Buffer>` A reference to `buf`.

Interprets `buf` as an array of unsigned 16-bit integers and swaps the byte order in-place. Throws `ERR_INVALID_BUFFER_SIZE` if `buf.length` is not a multiple of 2.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

    console.log(buf1);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>

    buf1.swap16();

    console.log(buf1);
    // Prints: <Buffer 02 01 04 03 06 05 08 07>

    const buf2 = Buffer.from([0x1, 0x2, 0x3]);

    buf2.swap16();
    // Throws ERR_INVALID_BUFFER_SIZE.
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

    console.log(buf1);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>

    buf1.swap16();

    console.log(buf1);
    // Prints: <Buffer 02 01 04 03 06 05 08 07>

    const buf2 = Buffer.from([0x1, 0x2, 0x3]);

    buf2.swap16();
    // Throws ERR_INVALID_BUFFER_SIZE.
    javascript

One convenient use of `buf.swap16()` is to perform a fast in-place conversion between UTF-16 little-endian and UTF-16 big-endian:

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
    buf.swap16(); // Convert to big-endian UTF-16 text.
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
    buf.swap16(); // Convert to big-endian UTF-16 text.
    javascript

#### `buf.swap32()`#

Added in: v5.10.0

- Returns: `<Buffer>` A reference to `buf`.

Interprets `buf` as an array of unsigned 32-bit integers and swaps the byte order in-place. Throws `ERR_INVALID_BUFFER_SIZE` if `buf.length` is not a multiple of 4.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

    console.log(buf1);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>

    buf1.swap32();

    console.log(buf1);
    // Prints: <Buffer 04 03 02 01 08 07 06 05>

    const buf2 = Buffer.from([0x1, 0x2, 0x3]);

    buf2.swap32();
    // Throws ERR_INVALID_BUFFER_SIZE.
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

    console.log(buf1);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>

    buf1.swap32();

    console.log(buf1);
    // Prints: <Buffer 04 03 02 01 08 07 06 05>

    const buf2 = Buffer.from([0x1, 0x2, 0x3]);

    buf2.swap32();
    // Throws ERR_INVALID_BUFFER_SIZE.
    javascript

#### `buf.swap64()`#

Added in: v6.3.0

- Returns: `<Buffer>` A reference to `buf`.

Interprets `buf` as an array of 64-bit numbers and swaps byte order in-place. Throws `ERR_INVALID_BUFFER_SIZE` if `buf.length` is not a multiple of 8.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

    console.log(buf1);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>

    buf1.swap64();

    console.log(buf1);
    // Prints: <Buffer 08 07 06 05 04 03 02 01>

    const buf2 = Buffer.from([0x1, 0x2, 0x3]);

    buf2.swap64();
    // Throws ERR_INVALID_BUFFER_SIZE.
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

    console.log(buf1);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>

    buf1.swap64();

    console.log(buf1);
    // Prints: <Buffer 08 07 06 05 04 03 02 01>

    const buf2 = Buffer.from([0x1, 0x2, 0x3]);

    buf2.swap64();
    // Throws ERR_INVALID_BUFFER_SIZE.
    javascript

#### `buf.toJSON()`#

Added in: v0.9.2

- Returns: `<Object>`

Returns a JSON representation of `buf`. `JSON.stringify()` implicitly calls this function when stringifying a `Buffer` instance.

`Buffer.from()` accepts objects in the format returned from this method. In particular, `Buffer.from(buf.toJSON())` works like `Buffer.from(buf)`.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
    const json = JSON.stringify(buf);

    console.log(json);
    // Prints: {"type":"Buffer","data":[1,2,3,4,5]}

    const copy = JSON.parse(json, (key, value) => {
      return value && value.type === 'Buffer' ?
        Buffer.from(value) :
        value;
    });

    console.log(copy);
    // Prints: <Buffer 01 02 03 04 05>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
    const json = JSON.stringify(buf);

    console.log(json);
    // Prints: {"type":"Buffer","data":[1,2,3,4,5]}

    const copy = JSON.parse(json, (key, value) => {
      return value && value.type === 'Buffer' ?
        Buffer.from(value) :
        value;
    });

    console.log(copy);
    // Prints: <Buffer 01 02 03 04 05>
    javascript

#### `buf.toString([encoding[, start[, end]]])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.

- `encoding` `<string>` The character encoding to use. Default: `'utf8'`.
- `start` `<integer>` The byte offset to start decoding at. Default: `0`.
- `end` `<integer>` The byte offset to stop decoding at (not inclusive). Default: `buf.length`.
- Returns: `<string>`

Decodes `buf` to a string according to the specified character encoding in `encoding`. `start` and `end` may be passed to decode only a subset of `buf`.

If `encoding` is `'utf8'` and a byte sequence in the input is not valid UTF-8, then each invalid byte is replaced with the replacement character `U+FFFD`.

The maximum length of a string instance (in UTF-16 code units) is available as `buffer.constants.MAX_STRING_LENGTH`.

    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.allocUnsafe(26);

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf1[i] = i + 97;
    }

    console.log(buf1.toString('utf8'));
    // Prints: abcdefghijklmnopqrstuvwxyz
    console.log(buf1.toString('utf8', 0, 5));
    // Prints: abcde

    const buf2 = Buffer.from('tést');

    console.log(buf2.toString('hex'));
    // Prints: 74c3a97374
    console.log(buf2.toString('utf8', 0, 3));
    // Prints: té
    console.log(buf2.toString(undefined, 0, 3));
    // Prints: té
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.allocUnsafe(26);

    for (let i = 0; i < 26; i++) {
      // 97 is the decimal ASCII value for 'a'.
      buf1[i] = i + 97;
    }

    console.log(buf1.toString('utf8'));
    // Prints: abcdefghijklmnopqrstuvwxyz
    console.log(buf1.toString('utf8', 0, 5));
    // Prints: abcde

    const buf2 = Buffer.from('tést');

    console.log(buf2.toString('hex'));
    // Prints: 74c3a97374
    console.log(buf2.toString('utf8', 0, 3));
    // Prints: té
    console.log(buf2.toString(undefined, 0, 3));
    // Prints: té
    javascript

#### `buf.values()`#

Added in: v1.1.0

- Returns: `<Iterator>`

Creates and returns an iterator for `buf` values (bytes). This function is called automatically when a `Buffer` is used in a `for..of` statement.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.from('buffer');

    for (const value of buf.values()) {
      console.log(value);
    }
    // Prints:
    //   98
    //   117
    //   102
    //   102
    //   101
    //   114

    for (const value of buf) {
      console.log(value);
    }
    // Prints:
    //   98
    //   117
    //   102
    //   102
    //   101
    //   114
    const { Buffer } = require('node:buffer');

    const buf = Buffer.from('buffer');

    for (const value of buf.values()) {
      console.log(value);
    }
    // Prints:
    //   98
    //   117
    //   102
    //   102
    //   101
    //   114

    for (const value of buf) {
      console.log(value);
    }
    // Prints:
    //   98
    //   117
    //   102
    //   102
    //   101
    //   114
    javascript

#### `buf.write(string[, offset[, length]][, encoding])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | supports Uint8Array as `this` value.

- `string` `<string>` String to write to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write `string`. Default: `0`.
- `length` `<integer>` Maximum number of bytes to write (written bytes will not exceed `buf.length - offset`). Default: `buf.length - offset`.
- `encoding` `<string>` The character encoding of `string`. Default: `'utf8'`.
- Returns: `<integer>` Number of bytes written.

Writes `string` to `buf` at `offset` according to the character encoding in `encoding`. The `length` parameter is the number of bytes to write. If `buf` did not contain enough space to fit the entire string, only part of `string` will be written. However, partially encoded characters will not be written.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.alloc(256);

    const len = buf.write('\u00bd + \u00bc = \u00be', 0);

    console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
    // Prints: 12 bytes: ½ + ¼ = ¾

    const buffer = Buffer.alloc(10);

    const length = buffer.write('abcd', 8);

    console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
    // Prints: 2 bytes : ab
    const { Buffer } = require('node:buffer');

    const buf = Buffer.alloc(256);

    const len = buf.write('\u00bd + \u00bc = \u00be', 0);

    console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
    // Prints: 12 bytes: ½ + ¼ = ¾

    const buffer = Buffer.alloc(10);

    const length = buffer.write('abcd', 8);

    console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
    // Prints: 2 bytes : ab
    javascript

#### `buf.writeBigInt64BE(value[, offset])`#

Added in: v12.0.0, v10.20.0

- `value` `<bigint>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian.

`value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigInt64BE(0x0102030405060708n, 0);

    console.log(buf);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigInt64BE(0x0102030405060708n, 0);

    console.log(buf);
    // Prints: <Buffer 01 02 03 04 05 06 07 08>
    javascript

#### `buf.writeBigInt64LE(value[, offset])`#

Added in: v12.0.0, v10.20.0

- `value` `<bigint>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian.

`value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigInt64LE(0x0102030405060708n, 0);

    console.log(buf);
    // Prints: <Buffer 08 07 06 05 04 03 02 01>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigInt64LE(0x0102030405060708n, 0);

    console.log(buf);
    // Prints: <Buffer 08 07 06 05 04 03 02 01>
    javascript

#### `buf.writeBigUInt64BE(value[, offset])`#

Added in: v12.0.0, v10.20.0
History

Version | Changes
--- | ---
v14.10.0, v12.19.0 | This function is also available as `buf.writeBigUint64BE()`.

- `value` `<bigint>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian.

This function is also available under the `writeBigUint64BE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigUInt64BE(0xdecafafecacefaden, 0);

    console.log(buf);
    // Prints: <Buffer de ca fa fe ca ce fa de>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigUInt64BE(0xdecafafecacefaden, 0);

    console.log(buf);
    // Prints: <Buffer de ca fa fe ca ce fa de>
    javascript

#### `buf.writeBigUInt64LE(value[, offset])`#

Added in: v12.0.0, v10.20.0
History

Version | Changes
--- | ---
v14.10.0, v12.19.0 | This function is also available as `buf.writeBigUint64LE()`.

- `value` `<bigint>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy: `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigUInt64LE(0xdecafafecacefaden, 0);

    console.log(buf);
    // Prints: <Buffer de fa ce ca fe fa ca de>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(8);

    buf.writeBigUInt64LE(0xdecafafecacefaden, 0);

    console.log(buf);
    // Prints: <Buffer de fa ce ca fe fa ca de>
    javascript

This function is also available under the `writeBigUint64LE` alias.

#### `buf.writeDoubleBE(value[, offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<number>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian. The `value` must be a JavaScript number. Behavior is undefined when `value` is anything other than a JavaScript number.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(8);

    buf.writeDoubleBE(123.456, 0);

    console.log(buf);
    // Prints: <Buffer 40 5e dd 2f 1a 9f be 77>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(8);

    buf.writeDoubleBE(123.456, 0);

    console.log(buf);
    // Prints: <Buffer 40 5e dd 2f 1a 9f be 77>
    javascript

#### `buf.writeDoubleLE(value[, offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<number>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 8`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian. The `value` must be a JavaScript number. Behavior is undefined when `value` is anything other than a JavaScript number.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(8);

    buf.writeDoubleLE(123.456, 0);

    console.log(buf);
    // Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(8);

    buf.writeDoubleLE(123.456, 0);

    console.log(buf);
    // Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>
    javascript

#### `buf.writeFloatBE(value[, offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<number>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian. Behavior is undefined when `value` is anything other than a JavaScript number.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeFloatBE(0xcafebabe, 0);

    console.log(buf);
    // Prints: <Buffer 4f 4a fe bb>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeFloatBE(0xcafebabe, 0);

    console.log(buf);
    // Prints: <Buffer 4f 4a fe bb>
    javascript

#### `buf.writeFloatLE(value[, offset])`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<number>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian. Behavior is undefined when `value` is anything other than a JavaScript number.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeFloatLE(0xcafebabe, 0);

    console.log(buf);
    // Prints: <Buffer bb fe 4a 4f>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeFloatLE(0xcafebabe, 0);

    console.log(buf);
    // Prints: <Buffer bb fe 4a 4f>
    javascript

#### `buf.writeInt8(value[, offset])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 1`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset`. `value` must be a valid signed 8-bit integer. Behavior is undefined when `value` is anything other than a signed 8-bit integer.

`value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(2);

    buf.writeInt8(2, 0);
    buf.writeInt8(-2, 1);

    console.log(buf);
    // Prints: <Buffer 02 fe>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(2);

    buf.writeInt8(2, 0);
    buf.writeInt8(-2, 1);

    console.log(buf);
    // Prints: <Buffer 02 fe>
    javascript

#### `buf.writeInt16BE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian. The `value` must be a valid signed 16-bit integer. Behavior is undefined when `value` is anything other than a signed 16-bit integer.

The `value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(2);

    buf.writeInt16BE(0x0102, 0);

    console.log(buf);
    // Prints: <Buffer 01 02>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(2);

    buf.writeInt16BE(0x0102, 0);

    console.log(buf);
    // Prints: <Buffer 01 02>
    javascript

#### `buf.writeInt16LE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian. The `value` must be a valid signed 16-bit integer. Behavior is undefined when `value` is anything other than a signed 16-bit integer.

The `value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(2);

    buf.writeInt16LE(0x0304, 0);

    console.log(buf);
    // Prints: <Buffer 04 03>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(2);

    buf.writeInt16LE(0x0304, 0);

    console.log(buf);
    // Prints: <Buffer 04 03>
    javascript

#### `buf.writeInt32BE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian. The `value` must be a valid signed 32-bit integer. Behavior is undefined when `value` is anything other than a signed 32-bit integer.

The `value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeInt32BE(0x01020304, 0);

    console.log(buf);
    // Prints: <Buffer 01 02 03 04>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeInt32BE(0x01020304, 0);

    console.log(buf);
    // Prints: <Buffer 01 02 03 04>
    javascript

#### `buf.writeInt32LE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian. The `value` must be a valid signed 32-bit integer. Behavior is undefined when `value` is anything other than a signed 32-bit integer.

The `value` is interpreted and written as a two's complement signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeInt32LE(0x05060708, 0);

    console.log(buf);
    // Prints: <Buffer 08 07 06 05>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeInt32LE(0x05060708, 0);

    console.log(buf);
    // Prints: <Buffer 08 07 06 05>
    javascript

#### `buf.writeIntBE(value, offset, byteLength)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to write. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `byteLength` bytes of `value` to `buf` at the specified `offset` as big-endian. Supports up to 48 bits of accuracy. Behavior is undefined when `value` is anything other than a signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(6);

    buf.writeIntBE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer 12 34 56 78 90 ab>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(6);

    buf.writeIntBE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer 12 34 56 78 90 ab>
    javascript

#### `buf.writeIntLE(value, offset, byteLength)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to write. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `byteLength` bytes of `value` to `buf` at the specified `offset` as little-endian. Supports up to 48 bits of accuracy. Behavior is undefined when `value` is anything other than a signed integer.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(6);

    buf.writeIntLE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer ab 90 78 56 34 12>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(6);

    buf.writeIntLE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer ab 90 78 56 34 12>
    javascript

#### `buf.writeUInt8(value[, offset])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUint8()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 1`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset`. `value` must be a valid unsigned 8-bit integer. Behavior is undefined when `value` is anything other than an unsigned 8-bit integer.

This function is also available under the `writeUint8` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt8(0x3, 0);
    buf.writeUInt8(0x4, 1);
    buf.writeUInt8(0x23, 2);
    buf.writeUInt8(0x42, 3);

    console.log(buf);
    // Prints: <Buffer 03 04 23 42>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt8(0x3, 0);
    buf.writeUInt8(0x4, 1);
    buf.writeUInt8(0x23, 2);
    buf.writeUInt8(0x42, 3);

    console.log(buf);
    // Prints: <Buffer 03 04 23 42>
    javascript

#### `buf.writeUInt16BE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUint16BE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian. The `value` must be a valid unsigned 16-bit integer. Behavior is undefined when `value` is anything other than an unsigned 16-bit integer.

This function is also available under the `writeUint16BE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt16BE(0xdead, 0);
    buf.writeUInt16BE(0xbeef, 2);

    console.log(buf);
    // Prints: <Buffer de ad be ef>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt16BE(0xdead, 0);
    buf.writeUInt16BE(0xbeef, 2);

    console.log(buf);
    // Prints: <Buffer de ad be ef>
    javascript

#### `buf.writeUInt16LE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUint16LE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 2`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian. The `value` must be a valid unsigned 16-bit integer. Behavior is undefined when `value` is anything other than an unsigned 16-bit integer.

This function is also available under the `writeUint16LE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt16LE(0xdead, 0);
    buf.writeUInt16LE(0xbeef, 2);

    console.log(buf);
    // Prints: <Buffer ad de ef be>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt16LE(0xdead, 0);
    buf.writeUInt16LE(0xbeef, 2);

    console.log(buf);
    // Prints: <Buffer ad de ef be>
    javascript

#### `buf.writeUInt32BE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUint32BE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as big-endian. The `value` must be a valid unsigned 32-bit integer. Behavior is undefined when `value` is anything other than an unsigned 32-bit integer.

This function is also available under the `writeUint32BE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt32BE(0xfeedface, 0);

    console.log(buf);
    // Prints: <Buffer fe ed fa ce>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt32BE(0xfeedface, 0);

    console.log(buf);
    // Prints: <Buffer fe ed fa ce>
    javascript

#### `buf.writeUInt32LE(value[, offset])`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUint32LE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - 4`. Default: `0`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `value` to `buf` at the specified `offset` as little-endian. The `value` must be a valid unsigned 32-bit integer. Behavior is undefined when `value` is anything other than an unsigned 32-bit integer.

This function is also available under the `writeUint32LE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt32LE(0xfeedface, 0);

    console.log(buf);
    // Prints: <Buffer ce fa ed fe>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(4);

    buf.writeUInt32LE(0xfeedface, 0);

    console.log(buf);
    // Prints: <Buffer ce fa ed fe>
    javascript

#### `buf.writeUIntBE(value, offset, byteLength)`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUintBE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to write. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `byteLength` bytes of `value` to `buf` at the specified `offset` as big-endian. Supports up to 48 bits of accuracy. Behavior is undefined when `value` is anything other than an unsigned integer.

This function is also available under the `writeUintBE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(6);

    buf.writeUIntBE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer 12 34 56 78 90 ab>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(6);

    buf.writeUIntBE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer 12 34 56 78 90 ab>
    javascript

#### `buf.writeUIntLE(value, offset, byteLength)`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v14.9.0, v12.19.0 | This function is also available as `buf.writeUintLE()`.
v10.0.0 | Removed `noAssert` and no implicit coercion of the offset and `byteLength` to `uint32` anymore.

- `value` `<integer>` Number to be written to `buf`.
- `offset` `<integer>` Number of bytes to skip before starting to write. Must satisfy `0 <= offset <= buf.length - byteLength`.
- `byteLength` `<integer>` Number of bytes to write. Must satisfy `0 < byteLength <= 6`.
- Returns: `<integer>` `offset` plus the number of bytes written.

Writes `byteLength` bytes of `value` to `buf` at the specified `offset` as little-endian. Supports up to 48 bits of accuracy. Behavior is undefined when `value` is anything other than an unsigned integer.

This function is also available under the `writeUintLE` alias.

    import { Buffer } from 'node:buffer';

    const buf = Buffer.allocUnsafe(6);

    buf.writeUIntLE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer ab 90 78 56 34 12>
    const { Buffer } = require('node:buffer');

    const buf = Buffer.allocUnsafe(6);

    buf.writeUIntLE(0x1234567890ab, 0, 6);

    console.log(buf);
    // Prints: <Buffer ab 90 78 56 34 12>
    javascript

#### `new Buffer(array)`#

Deprecated in: v6.0.0
History

Version | Changes
--- | ---
v10.0.0 | Calling this constructor emits a deprecation warning when run from code outside the `node_modules` directory.
v7.2.1 | Calling this constructor no longer emits a deprecation warning.
v7.0.0 | Calling this constructor emits a deprecation warning now.

Stability: 0 - Deprecated: Use `Buffer.from(array)` instead.

- `array` `<integer>`[] An array of bytes to copy from.

See `Buffer.from(array)`.

#### `new Buffer(arrayBuffer[, byteOffset[, length]])`#

Added in: v3.0.0Deprecated in: v6.0.0
History

Version | Changes
--- | ---
v10.0.0 | Calling this constructor emits a deprecation warning when run from code outside the `node_modules` directory.
v7.2.1 | Calling this constructor no longer emits a deprecation warning.
v7.0.0 | Calling this constructor emits a deprecation warning now.
v6.0.0 | The `byteOffset` and `length` parameters are supported now.

Stability: 0 - Deprecated: Use `Buffer.from(arrayBuffer[, byteOffset[, length]])` instead.

- `arrayBuffer` `<ArrayBuffer>` | `<SharedArrayBuffer>` An `<ArrayBuffer>`, `<SharedArrayBuffer>` or the `.buffer` property of a `<TypedArray>`.
- `byteOffset` `<integer>` Index of first byte to expose. Default: `0`.
- `length` `<integer>` Number of bytes to expose. Default: `arrayBuffer.byteLength - byteOffset`.

See `Buffer.from(arrayBuffer[, byteOffset[, length]])`.

#### `new Buffer(buffer)`#

Deprecated in: v6.0.0
History

Version | Changes
--- | ---
v10.0.0 | Calling this constructor emits a deprecation warning when run from code outside the `node_modules` directory.
v7.2.1 | Calling this constructor no longer emits a deprecation warning.
v7.0.0 | Calling this constructor emits a deprecation warning now.

Stability: 0 - Deprecated: Use `Buffer.from(buffer)` instead.

- `buffer` `<Buffer>` | `<Uint8Array>` An existing `Buffer` or `<Uint8Array>` from which to copy data.

See `Buffer.from(buffer)`.

#### `new Buffer(size)`#

Deprecated in: v6.0.0
History

Version | Changes
--- | ---
v10.0.0 | Calling this constructor emits a deprecation warning when run from code outside the `node_modules` directory.
v8.0.0 | The `new Buffer(size)` will return zero-filled memory by default.
v7.2.1 | Calling this constructor no longer emits a deprecation warning.
v7.0.0 | Calling this constructor emits a deprecation warning now.

Stability: 0 - Deprecated: Use `Buffer.alloc()` instead (also see `Buffer.allocUnsafe()`).

- `size` `<integer>` The desired length of the new `Buffer`.

See `Buffer.alloc()` and `Buffer.allocUnsafe()`. This variant of the constructor is equivalent to `Buffer.alloc()`.

#### `new Buffer(string[, encoding])`#

Deprecated in: v6.0.0
History

Version | Changes
--- | ---
v10.0.0 | Calling this constructor emits a deprecation warning when run from code outside the `node_modules` directory.
v7.2.1 | Calling this constructor no longer emits a deprecation warning.
v7.0.0 | Calling this constructor emits a deprecation warning now.

Stability: 0 - Deprecated: Use `Buffer.from(string[, encoding])` instead.

- `string` `<string>` String to encode.
- `encoding` `<string>` The encoding of `string`. Default: `'utf8'`.

See `Buffer.from(string[, encoding])`.

### Class: `File`#

Added in: v19.2.0, v18.13.0
History

Version | Changes
--- | ---
v23.0.0 | Makes File instances cloneable.
v20.0.0 | No longer experimental.

- Extends: `<Blob>`

A `<File>` provides information about files.

#### `new buffer.File(sources, fileName[, options])`#

Added in: v19.2.0, v18.13.0

- `sources` `<string>`[] | `<ArrayBuffer>`[] | `<TypedArray>`[] | `<DataView>`[] | `<Blob>`[] | `<File>`[] An array of string, `<ArrayBuffer>`, `<TypedArray>`, `<DataView>`, `<File>`, or `<Blob>` objects, or any mix of such objects, that will be stored within the `File`.
- `fileName` `<string>` The name of the file.
- `options` `<Object>`
  - `endings` `<string>` One of either `'transparent'` or `'native'`. When set to `'native'`, line endings in string source parts will be converted to the platform native line-ending as specified by `require('node:os').EOL`.
  - `type` `<string>` The File content-type.
  - `lastModified` `<number>` The last modified date of the file. Default: `Date.now()`.

#### `file.name`#

Added in: v19.2.0, v18.13.0

- Type: `<string>`

The name of the `File`.

#### `file.lastModified`#

Added in: v19.2.0, v18.13.0

- Type: `<number>`

The last modified date of the `File`.

### `node:buffer` module APIs#

While, the `Buffer` object is available as a global, there are additional `Buffer`-related APIs that are available only via the `node:buffer` module accessed using `require('node:buffer')`.

#### `buffer.atob(data)`#

Added in: v15.13.0, v14.17.0

Stability: 3 - Legacy. Use `Buffer.from(data, 'base64')` instead.

- `data` `<any>` The Base64-encoded input string.

Decodes a string of Base64-encoded data into bytes, and encodes those bytes into a string using Latin-1 (ISO-8859-1).

The `data` may be any JavaScript-value that can be coerced into a string.

This function is only provided for compatibility with legacy web platform APIs and should never be used in new code, because they use strings to represent binary data and predate the introduction of typed arrays in JavaScript. For code running using Node.js APIs, converting between base64-encoded strings and binary data should be performed using `Buffer.from(str, 'base64')` and `buf.toString('base64')`.

An automated migration is available (source:

    npx codemod@latest @nodejs/buffer-atob-btoa
    bash

#### `buffer.btoa(data)`#

Added in: v15.13.0, v14.17.0

Stability: 3 - Legacy. Use `buf.toString('base64')` instead.

- `data` `<any>` An ASCII (Latin1) string.

Decodes a string into bytes using Latin-1 (ISO-8859), and encodes those bytes into a string using Base64.

The `data` may be any JavaScript-value that can be coerced into a string.

This function is only provided for compatibility with legacy web platform APIs and should never be used in new code, because they use strings to represent binary data and predate the introduction of typed arrays in JavaScript. For code running using Node.js APIs, converting between base64-encoded strings and binary data should be performed using `Buffer.from(str, 'base64')` and `buf.toString('base64')`.

An automated migration is available (source:

    npx codemod@latest @nodejs/buffer-atob-btoa
    bash

#### `buffer.isAscii(input)`#

Added in: v19.6.0, v18.15.0

- `input` `<Buffer>` | `<ArrayBuffer>` | `<TypedArray>` The input to validate.
- Returns: `<boolean>`

This function returns `true` if `input` contains only valid ASCII-encoded data, including the case in which `input` is empty.

Throws if the `input` is a detached array buffer.

#### `buffer.isUtf8(input)`#

Added in: v19.4.0, v18.14.0

- `input` `<Buffer>` | `<ArrayBuffer>` | `<TypedArray>` The input to validate.
- Returns: `<boolean>`

This function returns `true` if `input` contains only valid UTF-8-encoded data, including the case in which `input` is empty.

Throws if the `input` is a detached array buffer.

#### `buffer.INSPECT_MAX_BYTES`#

Added in: v0.5.4

- Type: `<integer>` Default: `50`

Returns the maximum number of bytes that will be returned when `buf.inspect()` is called. This can be overridden by user modules. See `util.inspect()` for more details on `buf.inspect()` behavior.

#### `buffer.kMaxLength`#

Added in: v3.0.0

- Type: `<integer>` The largest size allowed for a single `Buffer` instance.

An alias for `buffer.constants.MAX_LENGTH`.

#### `buffer.kStringMaxLength`#

Added in: v3.0.0

- Type: `<integer>` The largest length allowed for a single `string` instance.

An alias for `buffer.constants.MAX_STRING_LENGTH`.

#### `buffer.resolveObjectURL(id)`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `id` `<string>` A `'blob:nodedata:...` URL string returned by a prior call to `URL.createObjectURL()`.
- Returns: `<Blob>`

Resolves a `'blob:nodedata:...'` an associated `<Blob>` object registered using a prior call to `URL.createObjectURL()`.

#### `buffer.transcode(source, fromEnc, toEnc)`#

Added in: v7.1.0
History

Version | Changes
--- | ---
v8.0.0 | The `source` parameter can now be a `Uint8Array`.

- `source` `<Buffer>` | `<Uint8Array>` A `Buffer` or `Uint8Array` instance.
- `fromEnc` `<string>` The current encoding.
- `toEnc` `<string>` To target encoding.
- Returns: `<Buffer>`

Re-encodes the given `Buffer` or `Uint8Array` instance from one character encoding to another. Returns a new `Buffer` instance.

Throws if the `fromEnc` or `toEnc` specify invalid character encodings or if conversion from `fromEnc` to `toEnc` is not permitted.

Encodings supported by `buffer.transcode()` are: `'ascii'`, `'utf8'`, `'utf16le'`, `'ucs2'`, `'latin1'`, and `'binary'`.

The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:

    import { Buffer, transcode } from 'node:buffer';

    const newBuf = transcode(Buffer.from('€'), 'utf8', 'ascii');
    console.log(newBuf.toString('ascii'));
    // Prints: '?'
    const { Buffer, transcode } = require('node:buffer');

    const newBuf = transcode(Buffer.from('€'), 'utf8', 'ascii');
    console.log(newBuf.toString('ascii'));
    // Prints: '?'
    javascript

Because the Euro (`€`) sign is not representable in US-ASCII, it is replaced with `?` in the transcoded `Buffer`.

#### Buffer constants#

Added in: v8.2.0

##### `buffer.constants.MAX_LENGTH`#

Added in: v8.2.0
History

Version | Changes
--- | ---
v22.0.0 | Value is changed to 253 - 1 on 64-bit architectures, and 231 - 1 on 32-bit architectures.
v15.0.0 | Value is changed to 232 on 64-bit architectures.
v14.0.0 | Value is changed from 231 - 1 to 232 - 1 on 64-bit architectures.

- Type: `<integer>` The largest size allowed for a single `Buffer` instance.

On 32-bit architectures, this value is equal to 231 - 1 (about 2 GiB).

On 64-bit architectures, this value is equal to `Number.MAX_SAFE_INTEGER` (253 - 1, about 8 PiB).

It reflects `v8::Uint8Array::kMaxLength` under the hood.

This value is also available as `buffer.kMaxLength`.

##### `buffer.constants.MAX_STRING_LENGTH`#

Added in: v8.2.0

- Type: `<integer>` The largest length allowed for a single `string` instance.

Represents the largest `length` that a `string` primitive can have, counted in UTF-16 code units.

This value may depend on the JS engine that is being used.

### `Buffer.from()`, `Buffer.alloc()`, and `Buffer.allocUnsafe()`#

In versions of Node.js prior to 6.0.0, `Buffer` instances were created using the `Buffer` constructor function, which allocates the returned `Buffer` differently based on what arguments are provided:

- Passing a number as the first argument to `Buffer()` (e.g. `new Buffer(10)`) allocates a new `Buffer` object of the specified size. Prior to Node.js 8.0.0, the memory allocated for such `Buffer` instances is not initialized and can contain sensitive data. Such `Buffer` instances must be subsequently initialized by using either `buf.fill(0)` or by writing to the entire `Buffer` before reading data from the `Buffer`. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized `Buffer` versus creating a slower-but-safer `Buffer`. Since Node.js 8.0.0, `Buffer(num)` and `new Buffer(num)` return a `Buffer` with initialized memory.
- Passing a string, array, or `Buffer` as the first argument copies the passed object's data into the `Buffer`.
- Passing an `<ArrayBuffer>` or a `<SharedArrayBuffer>` returns a `Buffer` that shares allocated memory with the given array buffer.

Because the behavior of `new Buffer()` is different depending on the type of the first argument, security and reliability issues can be inadvertently introduced into applications when argument validation or `Buffer` initialization is not performed.

For example, if an attacker can cause an application to receive a number where a string is expected, the application may call `new Buffer(100)` instead of `new Buffer("100")`, leading it to allocate a 100 byte buffer instead of allocating a 3 byte buffer with content `"100"`. This is commonly possible using JSON API calls. Since JSON distinguishes between numeric and string types, it allows injection of numbers where a naively written application that does not validate its input sufficiently might expect to always receive a string. Before Node.js 8.0.0, the 100 byte buffer might contain arbitrary pre-existing in-memory data, so may be used to expose in-memory secrets to a remote attacker. Since Node.js 8.0.0, exposure of memory cannot occur because the data is zero-filled. However, other attacks are still possible, such as causing very large buffers to be allocated by the server, leading to performance degradation or crashing on memory exhaustion.

To make the creation of `Buffer` instances more reliable and less error-prone, the various forms of the `new Buffer()` constructor have been deprecated and replaced by separate `Buffer.from()`, `Buffer.alloc()`, and `Buffer.allocUnsafe()` methods.

Developers should migrate all existing uses of the `new Buffer()` constructors to one of these new APIs.

- `Buffer.from(array)` returns a new `Buffer` that contains a copy of the provided octets.
- `Buffer.from(arrayBuffer[, byteOffset[, length]])` returns a new `Buffer` that shares the same allocated memory as the given `<ArrayBuffer>`.
- `Buffer.from(buffer)` returns a new `Buffer` that contains a copy of the contents of the given `Buffer`.
- `Buffer.from(string[, encoding])` returns a new `Buffer` that contains a copy of the provided string.
- `Buffer.alloc(size[, fill[, encoding]])` returns a new initialized `Buffer` of the specified size. This method is slower than `Buffer.allocUnsafe(size)` but guarantees that newly created `Buffer` instances never contain old data that is potentially sensitive. A `TypeError` will be thrown if `size` is not a number.
- `Buffer.allocUnsafe(size)` and `Buffer.allocUnsafeSlow(size)` each return a new uninitialized `Buffer` of the specified `size`. Because the `Buffer` is uninitialized, the allocated segment of memory might contain old data that is potentially sensitive.

`Buffer` instances returned by `Buffer.allocUnsafe()`, `Buffer.from(string)`, `Buffer.concat()` and `Buffer.from(array)` may be allocated off a shared internal memory pool if `size` is less than or equal to half `Buffer.poolSize`. Instances returned by `Buffer.allocUnsafeSlow()` never use the shared internal memory pool.

#### The `--zero-fill-buffers` command-line option#

Added in: v5.10.0

Node.js can be started using the `--zero-fill-buffers` command-line option to cause all newly-allocated `Buffer` instances to be zero-filled upon creation by default. Without the option, buffers created with `Buffer.allocUnsafe()` and `Buffer.allocUnsafeSlow()` are not zero-filled. Use of this flag can have a measurable negative impact on performance. Use the `--zero-fill-buffers` option only when necessary to enforce that newly allocated `Buffer` instances cannot contain old data that is potentially sensitive.

    $ node --zero-fill-buffers
    > Buffer.allocUnsafe(5);
    <Buffer 00 00 00 00 00>
    console

#### What makes `Buffer.allocUnsafe()` and `Buffer.allocUnsafeSlow()` "unsafe"?#

When calling `Buffer.allocUnsafe()` and `Buffer.allocUnsafeSlow()`, the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a `Buffer` created by `Buffer.allocUnsafe()` without completely overwriting the memory can allow this old data to be leaked when the `Buffer` memory is read.

While there are clear performance advantages to using `Buffer.allocUnsafe()`, extra care must be taken in order to avoid introducing security vulnerabilities into an application.

## C++ addons#

Addons are dynamically-linked shared objects that can be loaded via the `require()` function as ordinary Node.js modules. Addons provide a foreign function interface between JavaScript and native code.

There are three options for implementing addons:

- Node-API (recommended)
- `nan` (Native Abstractions for Node.js)
- direct use of internal V8, libuv, and Node.js libraries

The rest of this document focuses on the latter, requiring knowledge of multiple components and APIs:

- V8: the C++ library Node.js uses to provide the JavaScript implementation. It provides the mechanisms for creating objects, calling functions, etc. The V8's API is documented mostly in the `v8.h` header file (`deps/v8/include/v8.h` in the Node.js source tree), and is also available online.

- `libuv`: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the file system, sockets, timers, and system events. libuv also provides a threading abstraction similar to POSIX threads for more sophisticated asynchronous addons that need to move beyond the standard event loop. Addon authors should avoid blocking the event loop with I/O or other time-intensive tasks by offloading work via libuv to non-blocking system operations, worker threads, or a custom use of libuv threads.

- Internal Node.js libraries: Node.js itself exports C++ APIs that addons can use, the most important of which is the `node::ObjectWrap` class.

- Other statically linked libraries (including OpenSSL): These other libraries are located in the `deps/` directory in the Node.js source tree. Only the libuv, OpenSSL, V8, and zlib symbols are purposefully re-exported by Node.js and may be used to various extents by addons. See Linking to libraries included with Node.js for additional information.

All of the following examples are available for download and may be used as the starting-point for an addon.

### Hello world#

This "Hello world" example is a simple addon, written in C++, that is the equivalent of the following JavaScript code:

    module.exports.hello = () => 'world';
    js

First, create the file `hello.cc`:

    // hello.cc
    #include <node.h>

    namespace demo {

    using v8::FunctionCallbackInfo;
    using v8::Isolate;
    using v8::Local;
    using v8::NewStringType;
    using v8::Object;
    using v8::String;
    using v8::Value;

    void Method(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      args.GetReturnValue().Set(String::NewFromUtf8(
          isolate, "world", NewStringType::kNormal).ToLocalChecked());
    }

    void Initialize(Local<Object> exports) {
      NODE_SET_METHOD(exports, "hello", Method);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize) // N.B.: no semi-colon, this is not a function

    }  // namespace demo
    cpp

On most platforms, the following `Makefile` can get us started:

    NODEJS_DEV_ROOT ?= $(shell dirname "$(command -v node)")/..
    CXXFLAGS = -std=c++23 -I$(NODEJS_DEV_ROOT)/include/node -fPIC -shared -Wl,-undefined,dynamic_lookup

    hello.node: hello.cc
    	$(CXX) $(CXXFLAGS) -o $@ $<
    bash

Then running the following commands will compile and run the code:

    $ make
    $ node -p 'require("./hello.node").hello()'
    world
    console

To integrate with the npm ecosystem, see the Building section.

#### Context-aware addons#

Addons defined with `NODE_MODULE()` cannot be loaded in multiple contexts or multiple threads at the same time.

There are environments in which Node.js addons may need to be loaded multiple times in multiple contexts. For example, the Electron runtime runs multiple instances of Node.js in a single process. Each instance will have its own `require()` cache, and thus each instance will need a native addon to behave correctly when loaded via `require()`. This means that the addon must support multiple initializations.

A context-aware addon can be constructed by using the macro `NODE_MODULE_INITIALIZER`, which expands to the name of a function which Node.js will expect to find when it loads an addon. An addon can thus be initialized as in the following example:

    using namespace v8;

    extern "C" NODE_MODULE_EXPORT void
    NODE_MODULE_INITIALIZER(Local<Object> exports,
                            Local<Value> module,
                            Local<Context> context) {
      /* Perform addon initialization steps here. */
    }
    cpp

Another option is to use the macro `NODE_MODULE_INIT()`, which will also construct a context-aware addon. Unlike `NODE_MODULE()`, which is used to construct an addon around a given addon initializer function, `NODE_MODULE_INIT()` serves as the declaration of such an initializer to be followed by a function body.

The following three variables may be used inside the function body following an invocation of `NODE_MODULE_INIT()`:

- `Local<Object> exports`,
- `Local<Value> module`, and
- `Local<Context> context`

Building a context-aware addon requires careful management of global static data to ensure stability and correctness. Since the addon may be loaded multiple times, potentially even from different threads, any global static data stored in the addon must be properly protected, and must not contain any persistent references to JavaScript objects. The reason for this is that JavaScript objects are only valid in one context, and will likely cause a crash when accessed from the wrong context or from a different thread than the one on which they were created.

The context-aware addon can be structured to avoid global static data by performing the following steps:

- Define a class which will hold per-addon-instance data and which has a static member of the form

      static void DeleteInstance(void* data) {
        // Cast `data` to an instance of the class and delete it.
      }
      cpp

- Heap-allocate an instance of this class in the addon initializer. This can be accomplished using the `new` keyword.
- Call `node::AddEnvironmentCleanupHook()`, passing it the above-created instance and a pointer to `DeleteInstance()`. This will ensure the instance is deleted when the environment is torn down.
- Store the instance of the class in a `v8::External`, and
- Pass the `v8::External` to all methods exposed to JavaScript by passing it to `v8::FunctionTemplate::New()` or `v8::Function::New()` which creates the native-backed JavaScript functions. The third parameter of `v8::FunctionTemplate::New()` or `v8::Function::New()` accepts the `v8::External` and makes it available in the native callback using the `v8::FunctionCallbackInfo::Data()` method.

This will ensure that the per-addon-instance data reaches each binding that can be called from JavaScript. The per-addon-instance data must also be passed into any asynchronous callbacks the addon may create.

The following example illustrates the implementation of a context-aware addon:

    #include <node.h>

    using namespace v8;

    class AddonData {
     public:
      explicit AddonData(Isolate* isolate):
          call_count(0) {
        // Ensure this per-addon-instance data is deleted at environment cleanup.
        node::AddEnvironmentCleanupHook(isolate, DeleteInstance, this);
      }

      // Per-addon data.
      int call_count;

      static void DeleteInstance(void* data) {
        delete static_cast<AddonData*>(data);
      }
    };

    static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) {
      // Retrieve the per-addon-instance data.
      AddonData* data =
          reinterpret_cast<AddonData*>(info.Data().As<External>()->Value());
      data->call_count++;
      info.GetReturnValue().Set((double)data->call_count);
    }

    // Initialize this addon to be context-aware.
    NODE_MODULE_INIT(/* exports, module, context */) {
      Isolate* isolate = Isolate::GetCurrent();

      // Create a new instance of `AddonData` for this instance of the addon and
      // tie its life cycle to that of the Node.js environment.
      AddonData* data = new AddonData(isolate);

      // Wrap the data in a `v8::External` so we can pass it to the method we
      // expose.
      Local<External> external = External::New(isolate, data);

      // Expose the method `Method` to JavaScript, and make sure it receives the
      // per-addon-instance data we created above by passing `external` as the
      // third parameter to the `FunctionTemplate` constructor.
      exports->Set(context,
                   String::NewFromUtf8(isolate, "method").ToLocalChecked(),
                   FunctionTemplate::New(isolate, Method, external)
                      ->GetFunction(context).ToLocalChecked()).FromJust();
    }
    cpp

##### Worker support#

History

Version | Changes
--- | ---
v14.8.0, v12.19.0 | Cleanup hooks may now be asynchronous.

In order to be loaded from multiple Node.js environments, such as a main thread and a Worker thread, an add-on needs to either:

- Be a Node-API addon.
- Be declared as context-aware using `NODE_MODULE_INIT()` as described above.

In order to support `Worker` threads, addons need to clean up any resources they may have allocated when such a thread exits. This can be achieved through the usage of the `AddEnvironmentCleanupHook()` function:

    void AddEnvironmentCleanupHook(v8::Isolate* isolate,
                                   void (*fun)(void* arg),
                                   void* arg);
    cpp

This function adds a hook that will run before a given Node.js instance shuts down. If necessary, such hooks can be removed before they are run using `RemoveEnvironmentCleanupHook()`, which has the same signature. Callbacks are run in last-in first-out order.

If necessary, there is an additional pair of `AddEnvironmentCleanupHook()` and `RemoveEnvironmentCleanupHook()` overloads, where the cleanup hook takes a callback function. This can be used for shutting down asynchronous resources, such as any libuv handles registered by the addon.

The following `addon.cc` uses `AddEnvironmentCleanupHook`:

    // addon.cc
    #include <node.h>
    #include <assert.h>
    #include <stdlib.h>

    using node::AddEnvironmentCleanupHook;
    using v8::HandleScope;
    using v8::Isolate;
    using v8::Local;
    using v8::Object;

    // Note: In a real-world application, do not rely on static/global data.
    static char cookie[] = "yum yum";
    static int cleanup_cb1_called = 0;
    static int cleanup_cb2_called = 0;

    static void cleanup_cb1(void* arg) {
      Isolate* isolate = static_cast<Isolate*>(arg);
      HandleScope scope(isolate);
      Local<Object> obj = Object::New(isolate);
      assert(!obj.IsEmpty());  // assert VM is still alive
      assert(obj->IsObject());
      cleanup_cb1_called++;
    }

    static void cleanup_cb2(void* arg) {
      assert(arg == static_cast<void*>(cookie));
      cleanup_cb2_called++;
    }

    static void sanity_check(void*) {
      assert(cleanup_cb1_called == 1);
      assert(cleanup_cb2_called == 1);
    }

    // Initialize this addon to be context-aware.
    NODE_MODULE_INIT(/* exports, module, context */) {
      Isolate* isolate = Isolate::GetCurrent();

      AddEnvironmentCleanupHook(isolate, sanity_check, nullptr);
      AddEnvironmentCleanupHook(isolate, cleanup_cb2, cookie);
      AddEnvironmentCleanupHook(isolate, cleanup_cb1, isolate);
    }
    cpp

Test in JavaScript by running:

    // test.js
    require('./build/Release/addon');
    js

#### Building#

Once the source code has been written, it must be compiled into the binary `addon.node` file. To do so, create a file called `binding.gyp` in the top-level of the project describing the build configuration of the module using a JSON-like format. This file is used by `node-gyp`, a tool written specifically to compile Node.js addons.

    {
      "targets": [
        {
          "target_name": "addon",
          "sources": [ "hello.cc" ]
        }
      ]
    }
    json

A version of the `node-gyp` utility is bundled and distributed with Node.js as part of `npm`. This version is not made directly available for developers to use and is intended only to support the ability to use the `npm install` command to compile and install addons. Developers who wish to use `node-gyp` directly can install it using the command `npm install -g node-gyp`. See the `node-gyp` installation instructions for more information, including platform-specific requirements.

Once the `binding.gyp` file has been created, use `node-gyp configure` to generate the appropriate project build files for the current platform. This will generate either a `Makefile` (on Unix platforms) or a `vcxproj` file (on Windows) in the `build/` directory.

Next, invoke the `node-gyp build` command to generate the compiled `addon.node` file. This will be put into the `build/Release/` directory.

When using `npm install` to install a Node.js addon, npm uses its own bundled version of `node-gyp` to perform this same set of actions, generating a compiled version of the addon for the user's platform on demand.

Once built, the binary addon can be used from within Node.js by pointing `require()` to the built `addon.node` module:

    // hello.js
    const addon = require('./build/Release/addon');

    console.log(addon.hello());
    // Prints: 'world'
    js

Because the exact path to the compiled addon binary can vary depending on how it is compiled (i.e. sometimes it may be in `./build/Debug/`), addons can use the bindings package to load the compiled module.

While the `bindings` package implementation is more sophisticated in how it locates addon modules, it is essentially using a `try…catch` pattern similar to:

    try {
      return require('./build/Release/addon.node');
    } catch (err) {
      return require('./build/Debug/addon.node');
    }
    js

#### Linking to libraries included with Node.js#

Node.js uses statically linked libraries such as V8, libuv, and OpenSSL. All addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate `#include <...>` statements (e.g. `#include <v8.h>`) and `node-gyp` will locate the appropriate headers automatically. However, there are a few caveats to be aware of:

- When `node-gyp` runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.

- `node-gyp` can be run using the `--nodedir` flag pointing at a local Node.js source image. Using this option, the addon will have access to the full set of dependencies.

#### Loading addons using `require()`#

The filename extension of the compiled addon binary is `.node` (as opposed to `.dll` or `.so`). The `require()` function is written to look for files with the `.node` file extension and initialize those as dynamically-linked libraries.

When calling `require()`, the `.node` extension can usually be omitted and Node.js will still find and initialize the addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file `addon.js` in the same directory as the binary `addon.node`, then `require('addon')` will give precedence to the `addon.js` file and load it instead.

#### Loading addons using `import`#

Added in: v23.6.0, v22.20.0

Stability: 1.0 - Early development

You can use the `--experimental-addon-modules` flag to enable support for both static `import` and dynamic `import()` to load binary addons.

If we reuse the Hello World example from earlier, you could do:

    // hello.mjs
    import myAddon from './hello.node';
    // N.B.: import {hello} from './hello.node' would not work

    console.log(myAddon.hello());
    mjs

    $ node --experimental-addon-modules hello.mjs
    world
    console

### Native abstractions for Node.js#

Each of the examples illustrated in this document directly use the Node.js and V8 APIs for implementing addons. The V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do to ensure stability of the V8 APIs.

The Native Abstractions for Node.js (or `nan`) provide a set of tools that addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the `nan` examples for an illustration of how it can be used.

### Node-API#

Stability: 2 - Stable

See C/C++ addons with Node-API.

### Addon examples#

The following are some example addons intended to help developers get started. The examples use the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

Each of these examples uses the following `binding.gyp` file:

    {
      "targets": [
        {
          "target_name": "addon",
          "sources": [ "addon.cc" ]
        }
      ]
    }
    json

In cases where there is more than one `.cc` file, simply add the additional filename to the `sources` array:

    "sources": ["addon.cc", "myexample.cc"]
    json

Once the `binding.gyp` file is ready, the example addons can be configured and built using `node-gyp`:

    node-gyp configure build
    bash

#### Function arguments#

Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.

The following example illustrates how to read function arguments passed from JavaScript and how to return a result:

    // addon.cc
    #include <node.h>

    namespace demo {

    using v8::Exception;
    using v8::FunctionCallbackInfo;
    using v8::Isolate;
    using v8::Local;
    using v8::Number;
    using v8::Object;
    using v8::String;
    using v8::Value;

    // This is the implementation of the "add" method
    // Input arguments are passed using the
    // const FunctionCallbackInfo<Value>& args struct
    void Add(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();

      // Check the number of arguments passed.
      if (args.Length() < 2) {
        // Throw an Error that is passed back to JavaScript
        isolate->ThrowException(Exception::TypeError(
            String::NewFromUtf8(isolate,
                                "Wrong number of arguments").ToLocalChecked()));
        return;
      }

      // Check the argument types
      if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
        isolate->ThrowException(Exception::TypeError(
            String::NewFromUtf8(isolate,
                                "Wrong arguments").ToLocalChecked()));
        return;
      }

      // Perform the operation
      double value =
          args[0].As<Number>()->Value() + args[1].As<Number>()->Value();
      Local<Number> num = Number::New(isolate, value);

      // Set the return value (using the passed in
      // FunctionCallbackInfo<Value>&)
      args.GetReturnValue().Set(num);
    }

    void Init(Local<Object> exports) {
      NODE_SET_METHOD(exports, "add", Add);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

    }  // namespace demo
    cpp

Once compiled, the example addon can be required and used from within Node.js:

    // test.js
    const addon = require('./build/Release/addon');

    console.log('This should be eight:', addon.add(3, 5));
    js

#### Callbacks#

It is common practice within addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:

    // addon.cc
    #include <node.h>

    namespace demo {

    using v8::Context;
    using v8::Function;
    using v8::FunctionCallbackInfo;
    using v8::Isolate;
    using v8::Local;
    using v8::Null;
    using v8::Object;
    using v8::String;
    using v8::Value;

    void RunCallback(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      Local<Context> context = isolate->GetCurrentContext();
      Local<Function> cb = Local<Function>::Cast(args[0]);
      const unsigned argc = 1;
      Local<Value> argv[argc] = {
          String::NewFromUtf8(isolate,
                              "hello world").ToLocalChecked() };
      cb->Call(context, Null(isolate), argc, argv).ToLocalChecked();
    }

    void Init(Local<Object> exports, Local<Object> module) {
      NODE_SET_METHOD(module, "exports", RunCallback);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

    }  // namespace demo
    cpp

This example uses a two-argument form of `Init()` that receives the full `module` object as the second argument. This allows the addon to completely overwrite `exports` with a single function instead of adding the function as a property of `exports`.

To test it, run the following JavaScript:

    // test.js
    const addon = require('./build/Release/addon');

    addon((msg) => {
      console.log(msg);
    // Prints: 'hello world'
    });
    js

In this example, the callback function is invoked synchronously.

#### Object factory#

Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property `msg` that echoes the string passed to `createObject()`:

    // addon.cc
    #include <node.h>

    namespace demo {

    using v8::Context;
    using v8::FunctionCallbackInfo;
    using v8::Isolate;
    using v8::Local;
    using v8::Object;
    using v8::String;
    using v8::Value;

    void CreateObject(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      Local<Context> context = isolate->GetCurrentContext();

      Local<Object> obj = Object::New(isolate);
      obj->Set(context,
               String::NewFromUtf8(isolate,
                                   "msg").ToLocalChecked(),
                                   args[0]->ToString(context).ToLocalChecked())
               .FromJust();

      args.GetReturnValue().Set(obj);
    }

    void Init(Local<Object> exports, Local<Object> module) {
      NODE_SET_METHOD(module, "exports", CreateObject);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

    }  // namespace demo
    cpp

To test it in JavaScript:

    // test.js
    const addon = require('./build/Release/addon');

    const obj1 = addon('hello');
    const obj2 = addon('world');
    console.log(obj1.msg, obj2.msg);
    // Prints: 'hello world'
    js

#### Function factory#

Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:

    // addon.cc
    #include <node.h>

    namespace demo {

    using v8::Context;
    using v8::Function;
    using v8::FunctionCallbackInfo;
    using v8::FunctionTemplate;
    using v8::Isolate;
    using v8::Local;
    using v8::Object;
    using v8::String;
    using v8::Value;

    void MyFunction(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      args.GetReturnValue().Set(String::NewFromUtf8(
          isolate, "hello world").ToLocalChecked());
    }

    void CreateFunction(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();

      Local<Context> context = isolate->GetCurrentContext();
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
      Local<Function> fn = tpl->GetFunction(context).ToLocalChecked();

      // omit this to make it anonymous
      fn->SetName(String::NewFromUtf8(
          isolate, "theFunction").ToLocalChecked());

      args.GetReturnValue().Set(fn);
    }

    void Init(Local<Object> exports, Local<Object> module) {
      NODE_SET_METHOD(module, "exports", CreateFunction);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

    }  // namespace demo
    cpp

To test:

    // test.js
    const addon = require('./build/Release/addon');

    const fn = addon();
    console.log(fn());
    // Prints: 'hello world'
    js

#### Wrapping C++ objects#

It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript `new` operator:

    // addon.cc
    #include <node.h>
    #include "myobject.h"

    namespace demo {

    using v8::Local;
    using v8::Object;

    void InitAll(Local<Object> exports) {
      MyObject::Init(exports);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

    }  // namespace demo
    cpp

Then, in `myobject.h`, the wrapper class inherits from `node::ObjectWrap`:

    // myobject.h
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>
    #include <node_object_wrap.h>

    namespace demo {

    class MyObject : public node::ObjectWrap {
     public:
      static void Init(v8::Local<v8::Object> exports);

     private:
      explicit MyObject(double value = 0);
      ~MyObject();

      static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
      static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);

      double value_;
    };

    }  // namespace demo

    #endif
    cpp

In `myobject.cc`, implement the various methods that are to be exposed. In the following code, the method `plusOne()` is exposed by adding it to the constructor's prototype:

    // myobject.cc
    #include "myobject.h"

    namespace demo {

    using v8::Context;
    using v8::Function;
    using v8::FunctionCallbackInfo;
    using v8::FunctionTemplate;
    using v8::Isolate;
    using v8::Local;
    using v8::Number;
    using v8::Object;
    using v8::ObjectTemplate;
    using v8::String;
    using v8::Value;

    MyObject::MyObject(double value) : value_(value) {
    }

    MyObject::~MyObject() {
    }

    void MyObject::Init(Local<Object> exports) {
      Isolate* isolate = Isolate::GetCurrent();
      Local<Context> context = isolate->GetCurrentContext();

      Local<ObjectTemplate> addon_data_tpl = ObjectTemplate::New(isolate);
      addon_data_tpl->SetInternalFieldCount(1);  // 1 field for the MyObject::New()
      Local<Object> addon_data =
          addon_data_tpl->NewInstance(context).ToLocalChecked();

      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New, addon_data);
      tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      // Prototype
      NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

      Local<Function> constructor = tpl->GetFunction(context).ToLocalChecked();
      addon_data->SetInternalField(0, constructor);
      exports->Set(context, String::NewFromUtf8(
          isolate, "MyObject").ToLocalChecked(),
          constructor).FromJust();
    }

    void MyObject::New(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      Local<Context> context = isolate->GetCurrentContext();

      if (args.IsConstructCall()) {
        // Invoked as constructor: `new MyObject(...)`
        double value = args[0]->IsUndefined() ?
            0 : args[0]->NumberValue(context).FromMaybe(0);
        MyObject* obj = new MyObject(value);
        obj->Wrap(args.This());
        args.GetReturnValue().Set(args.This());
      } else {
        // Invoked as plain function `MyObject(...)`, turn into construct call.
        const int argc = 1;
        Local<Value> argv[argc] = { args[0] };
        Local<Function> cons =
            args.Data().As<Object>()->GetInternalField(0)
                .As<Value>().As<Function>();
        Local<Object> result =
            cons->NewInstance(context, argc, argv).ToLocalChecked();
        args.GetReturnValue().Set(result);
      }
    }

    void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();

      MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.This());
      obj->value_ += 1;

      args.GetReturnValue().Set(Number::New(isolate, obj->value_));
    }

    }  // namespace demo
    cpp

To build this example, the `myobject.cc` file must be added to the `binding.gyp`:

    {
      "targets": [
        {
          "target_name": "addon",
          "sources": [
            "addon.cc",
            "myobject.cc"
          ]
        }
      ]
    }
    json

Test it with:

    // test.js
    const addon = require('./build/Release/addon');

    const obj = new addon.MyObject(10);
    console.log(obj.plusOne());
    // Prints: 11
    console.log(obj.plusOne());
    // Prints: 12
    console.log(obj.plusOne());
    // Prints: 13
    js

The destructor for a wrapper object will run when the object is garbage-collected. For destructor testing, there are command-line flags that can be used to make it possible to force garbage collection. These flags are provided by the underlying V8 JavaScript engine. They are subject to change or removal at any time. They are not documented by Node.js or V8, and they should never be used outside of testing.

During shutdown of the process or worker threads, destructors are not called by the JS engine. Therefore it's the responsibility of the user to track these objects and ensure proper destruction to avoid resource leaks.

#### Factory of wrapped objects#

Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript `new` operator:

    const obj = addon.createObject();
    // instead of:
    // const obj = new addon.Object();
    js

First, the `createObject()` method is implemented in `addon.cc`:

    // addon.cc
    #include <node.h>
    #include "myobject.h"

    namespace demo {

    using v8::FunctionCallbackInfo;
    using v8::Isolate;
    using v8::Local;
    using v8::Object;
    using v8::String;
    using v8::Value;

    void CreateObject(const FunctionCallbackInfo<Value>& args) {
      MyObject::NewInstance(args);
    }

    void InitAll(Local<Object> exports, Local<Object> module) {
      MyObject::Init();

      NODE_SET_METHOD(module, "exports", CreateObject);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

    }  // namespace demo
    cpp

In `myobject.h`, the static method `NewInstance()` is added to handle instantiating the object. This method takes the place of using `new` in JavaScript:

    // myobject.h
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>
    #include <node_object_wrap.h>

    namespace demo {

    class MyObject : public node::ObjectWrap {
     public:
      static void Init();
      static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);

     private:
      explicit MyObject(double value = 0);
      ~MyObject();

      static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
      static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
      static v8::Global<v8::Function> constructor;
      double value_;
    };

    }  // namespace demo

    #endif
    cpp

The implementation in `myobject.cc` is similar to the previous example:

    // myobject.cc
    #include <node.h>
    #include "myobject.h"

    namespace demo {

    using node::AddEnvironmentCleanupHook;
    using v8::Context;
    using v8::Function;
    using v8::FunctionCallbackInfo;
    using v8::FunctionTemplate;
    using v8::Global;
    using v8::Isolate;
    using v8::Local;
    using v8::Number;
    using v8::Object;
    using v8::String;
    using v8::Value;

    // Warning! This is not thread-safe, this addon cannot be used for worker
    // threads.
    Global<Function> MyObject::constructor;

    MyObject::MyObject(double value) : value_(value) {
    }

    MyObject::~MyObject() {
    }

    void MyObject::Init() {
      Isolate* isolate = Isolate::GetCurrent();
      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
      tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      // Prototype
      NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

      Local<Context> context = isolate->GetCurrentContext();
      constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());

      AddEnvironmentCleanupHook(isolate, [](void*) {
        constructor.Reset();
      }, nullptr);
    }

    void MyObject::New(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      Local<Context> context = isolate->GetCurrentContext();

      if (args.IsConstructCall()) {
        // Invoked as constructor: `new MyObject(...)`
        double value = args[0]->IsUndefined() ?
            0 : args[0]->NumberValue(context).FromMaybe(0);
        MyObject* obj = new MyObject(value);
        obj->Wrap(args.This());
        args.GetReturnValue().Set(args.This());
      } else {
        // Invoked as plain function `MyObject(...)`, turn into construct call.
        const int argc = 1;
        Local<Value> argv[argc] = { args[0] };
        Local<Function> cons = Local<Function>::New(isolate, constructor);
        Local<Object> instance =
            cons->NewInstance(context, argc, argv).ToLocalChecked();
        args.GetReturnValue().Set(instance);
      }
    }

    void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();

      const unsigned argc = 1;
      Local<Value> argv[argc] = { args[0] };
      Local<Function> cons = Local<Function>::New(isolate, constructor);
      Local<Context> context = isolate->GetCurrentContext();
      Local<Object> instance =
          cons->NewInstance(context, argc, argv).ToLocalChecked();

      args.GetReturnValue().Set(instance);
    }

    void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();

      MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.This());
      obj->value_ += 1;

      args.GetReturnValue().Set(Number::New(isolate, obj->value_));
    }

    }  // namespace demo
    cpp

Once again, to build this example, the `myobject.cc` file must be added to the `binding.gyp`:

    {
      "targets": [
        {
          "target_name": "addon",
          "sources": [
            "addon.cc",
            "myobject.cc"
          ]
        }
      ]
    }
    json

Test it with:

    // test.js
    const createObject = require('./build/Release/addon');

    const obj = createObject(10);
    console.log(obj.plusOne());
    // Prints: 11
    console.log(obj.plusOne());
    // Prints: 12
    console.log(obj.plusOne());
    // Prints: 13

    const obj2 = createObject(20);
    console.log(obj2.plusOne());
    // Prints: 21
    console.log(obj2.plusOne());
    // Prints: 22
    console.log(obj2.plusOne());
    // Prints: 23
    js

#### Passing wrapped objects around#

In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function `node::ObjectWrap::Unwrap`. The following example shows a function `add()` that can take two `MyObject` objects as input arguments:

    // addon.cc
    #include <node.h>
    #include <node_object_wrap.h>
    #include "myobject.h"

    namespace demo {

    using v8::Context;
    using v8::FunctionCallbackInfo;
    using v8::Isolate;
    using v8::Local;
    using v8::Number;
    using v8::Object;
    using v8::String;
    using v8::Value;

    void CreateObject(const FunctionCallbackInfo<Value>& args) {
      MyObject::NewInstance(args);
    }

    void Add(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      Local<Context> context = isolate->GetCurrentContext();

      MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
          args[0]->ToObject(context).ToLocalChecked());
      MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
          args[1]->ToObject(context).ToLocalChecked());

      double sum = obj1->value() + obj2->value();
      args.GetReturnValue().Set(Number::New(isolate, sum));
    }

    void InitAll(Local<Object> exports) {
      MyObject::Init();

      NODE_SET_METHOD(exports, "createObject", CreateObject);
      NODE_SET_METHOD(exports, "add", Add);
    }

    NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

    }  // namespace demo
    cpp

In `myobject.h`, a new public method is added to allow access to private values after unwrapping the object.

    // myobject.h
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>
    #include <node_object_wrap.h>

    namespace demo {

    class MyObject : public node::ObjectWrap {
     public:
      static void Init();
      static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
      inline double value() const { return value_; }

     private:
      explicit MyObject(double value = 0);
      ~MyObject();

      static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
      static v8::Global<v8::Function> constructor;
      double value_;
    };

    }  // namespace demo

    #endif
    cpp

The implementation of `myobject.cc` remains similar to the previous version:

    // myobject.cc
    #include <node.h>
    #include "myobject.h"

    namespace demo {

    using node::AddEnvironmentCleanupHook;
    using v8::Context;
    using v8::Function;
    using v8::FunctionCallbackInfo;
    using v8::FunctionTemplate;
    using v8::Global;
    using v8::Isolate;
    using v8::Local;
    using v8::Object;
    using v8::String;
    using v8::Value;

    // Warning! This is not thread-safe, this addon cannot be used for worker
    // threads.
    Global<Function> MyObject::constructor;

    MyObject::MyObject(double value) : value_(value) {
    }

    MyObject::~MyObject() {
    }

    void MyObject::Init() {
      Isolate* isolate = Isolate::GetCurrent();
      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
      tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject").ToLocalChecked());
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      Local<Context> context = isolate->GetCurrentContext();
      constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());

      AddEnvironmentCleanupHook(isolate, [](void*) {
        constructor.Reset();
      }, nullptr);
    }

    void MyObject::New(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();
      Local<Context> context = isolate->GetCurrentContext();

      if (args.IsConstructCall()) {
        // Invoked as constructor: `new MyObject(...)`
        double value = args[0]->IsUndefined() ?
            0 : args[0]->NumberValue(context).FromMaybe(0);
        MyObject* obj = new MyObject(value);
        obj->Wrap(args.This());
        args.GetReturnValue().Set(args.This());
      } else {
        // Invoked as plain function `MyObject(...)`, turn into construct call.
        const int argc = 1;
        Local<Value> argv[argc] = { args[0] };
        Local<Function> cons = Local<Function>::New(isolate, constructor);
        Local<Object> instance =
            cons->NewInstance(context, argc, argv).ToLocalChecked();
        args.GetReturnValue().Set(instance);
      }
    }

    void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
      Isolate* isolate = args.GetIsolate();

      const unsigned argc = 1;
      Local<Value> argv[argc] = { args[0] };
      Local<Function> cons = Local<Function>::New(isolate, constructor);
      Local<Context> context = isolate->GetCurrentContext();
      Local<Object> instance =
          cons->NewInstance(context, argc, argv).ToLocalChecked();

      args.GetReturnValue().Set(instance);
    }

    }  // namespace demo
    cpp

Test it with:

    // test.js
    const addon = require('./build/Release/addon');

    const obj1 = addon.createObject(10);
    const obj2 = addon.createObject(20);
    const result = addon.add(obj1, obj2);

    console.log(result);
    // Prints: 30
    js

## C++ embedder API#

Node.js provides a number of C++ APIs that can be used to execute JavaScript in a Node.js environment from other C++ software.

The documentation for these APIs can be found in src/node.h in the Node.js source tree. In addition to the APIs exposed by Node.js, some required concepts are provided by the V8 embedder API.

Because using Node.js as an embedded library is different from writing code that is executed by Node.js, breaking changes do not follow typical Node.js deprecation policy and may occur on each semver-major release without prior warning.

### Example embedding application#

The following sections will provide an overview over how to use these APIs to create an application from scratch that will perform the equivalent of `node -e <code>`, i.e. that will take a piece of JavaScript and run it in a Node.js-specific environment.

The full code can be found in the Node.js source tree.

#### Setting up a per-process state#

Node.js requires some per-process state management in order to run:

- Arguments parsing for Node.js CLI options,
- V8 per-process requirements, such as a `v8::Platform` instance.

The following example shows how these can be set up. Some class names are from the `node` and `v8` C++ namespaces, respectively.

    int main(int argc, char** argv) {
      argv = uv_setup_args(argc, argv);
      std::vector<std::string> args(argv, argv + argc);
      // Parse Node.js CLI options, and print any errors that have occurred while
      // trying to parse them.
      std::unique_ptr<node::InitializationResult> result =
          node::InitializeOncePerProcess(args, {
            node::ProcessInitializationFlags::kNoInitializeV8,
            node::ProcessInitializationFlags::kNoInitializeNodeV8Platform
          });

      for (const std::string& error : result->errors())
        fprintf(stderr, "%s: %s\n", args[0].c_str(), error.c_str());
      if (result->early_return() != 0) {
        return result->exit_code();
      }

      // Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way
      // to create a v8::Platform instance that Node.js can use when creating
      // Worker threads. When no `MultiIsolatePlatform` instance is present,
      // Worker threads are disabled.
      std::unique_ptr<MultiIsolatePlatform> platform =
          MultiIsolatePlatform::Create(4);
      V8::InitializePlatform(platform.get());
      V8::Initialize();

      // See below for the contents of this function.
      int ret = RunNodeInstance(
          platform.get(), result->args(), result->exec_args());

      V8::Dispose();
      V8::DisposePlatform();

      node::TearDownOncePerProcess();
      return ret;
    }
    cpp

#### Setting up a per-instance state#

History

Version | Changes
--- | ---
v15.0.0 | The `CommonEnvironmentSetup` and `SpinEventLoop` utilities were added.

Node.js has a concept of a “Node.js instance”, that is commonly being referred to as `node::Environment`. Each `node::Environment` is associated with:

- Exactly one `v8::Isolate`, i.e. one JS Engine instance,
- Exactly one `uv_loop_t`, i.e. one event loop,
- A number of `v8::Context`s, but exactly one main `v8::Context`, and
- One `node::IsolateData` instance that contains information that could be shared by multiple `node::Environment`s. The embedder should make sure that `node::IsolateData` is shared only among `node::Environment`s that use the same `v8::Isolate`, Node.js does not perform this check.

In order to set up a `v8::Isolate`, an `v8::ArrayBuffer::Allocator` needs to be provided. One possible choice is the default Node.js allocator, which can be created through `node::ArrayBufferAllocator::Create()`. Using the Node.js allocator allows minor performance optimizations when addons use the Node.js C++ `Buffer` API, and is required in order to track `ArrayBuffer` memory in `process.memoryUsage()`.

Additionally, each `v8::Isolate` that is used for a Node.js instance needs to be registered and unregistered with the `MultiIsolatePlatform` instance, if one is being used, in order for the platform to know which event loop to use for tasks scheduled by the `v8::Isolate`.

The `node::NewIsolate()` helper function creates a `v8::Isolate`, sets it up with some Node.js-specific hooks (e.g. the Node.js error handler), and registers it with the platform automatically.

    int RunNodeInstance(MultiIsolatePlatform* platform,
                        const std::vector<std::string>& args,
                        const std::vector<std::string>& exec_args) {
      int exit_code = 0;

      // Set up a libuv event loop, v8::Isolate, and Node.js Environment.
      std::vector<std::string> errors;
      std::unique_ptr<CommonEnvironmentSetup> setup =
          CommonEnvironmentSetup::Create(platform, &errors, args, exec_args);
      if (!setup) {
        for (const std::string& err : errors)
          fprintf(stderr, "%s: %s\n", args[0].c_str(), err.c_str());
        return 1;
      }

      Isolate* isolate = setup->isolate();
      Environment* env = setup->env();

      {
        Locker locker(isolate);
        Isolate::Scope isolate_scope(isolate);
        HandleScope handle_scope(isolate);
        // The v8::Context needs to be entered when node::CreateEnvironment() and
        // node::LoadEnvironment() are being called.
        Context::Scope context_scope(setup->context());

        // Set up the Node.js instance for execution, and run code inside of it.
        // There is also a variant that takes a callback and provides it with
        // the `require` and `process` objects, so that it can manually compile
        // and run scripts as needed.
        // The `require` function inside this script does *not* access the file
        // system, and can only load built-in Node.js modules.
        // `module.createRequire()` is being used to create one that is able to
        // load files from the disk, and uses the standard CommonJS file loader
        // instead of the internal-only `require` function.
        MaybeLocal<Value> loadenv_ret = node::LoadEnvironment(
            env,
            "const publicRequire ="
            "  require('node:module').createRequire(process.cwd() + '/');"
            "globalThis.require = publicRequire;"
            "require('node:vm').runInThisContext(process.argv[1]);");

        if (loadenv_ret.IsEmpty())  // There has been a JS exception.
          return 1;

        exit_code = node::SpinEventLoop(env).FromMaybe(1);

        // node::Stop() can be used to explicitly stop the event loop and keep
        // further JavaScript from running. It can be called from any thread,
        // and will act like worker.terminate() if called from another thread.
        node::Stop(env);
      }

      return exit_code;
    }
    cpp

## Child process#

Source Code: lib/child_process.js

Stability: 2 - Stable

The `node:child_process` module provides the ability to spawn subprocesses in a manner that is similar, but not identical, to `popen(3)`. This capability is primarily provided by the `child_process.spawn()` function:

    const { spawn } = require('node:child_process');
    const ls = spawn('ls', ['-lh', '/usr']);

    ls.stdout.on('data', (data) => {
      console.log(`stdout: ${data}`);
    });

    ls.stderr.on('data', (data) => {
      console.error(`stderr: ${data}`);
    });

    ls.on('close', (code) => {
      console.log(`child process exited with code ${code}`);
    });
    import { spawn } from 'node:child_process';
    import { once } from 'node:events';
    const ls = spawn('ls', ['-lh', '/usr']);

    ls.stdout.on('data', (data) => {
      console.log(`stdout: ${data}`);
    });

    ls.stderr.on('data', (data) => {
      console.error(`stderr: ${data}`);
    });

    const [code] = await once(ls, 'close');
    console.log(`child process exited with code ${code}`);
    javascript

By default, pipes for `stdin`, `stdout`, and `stderr` are established between the parent Node.js process and the spawned subprocess. These pipes have limited (and platform-specific) capacity. If the subprocess writes to stdout in excess of that limit without the output being captured, the subprocess blocks, waiting for the pipe buffer to accept more data. This is identical to the behavior of pipes in the shell. Use the `{ stdio: 'ignore' }` option if the output will not be consumed.

The command lookup is performed using the `options.env.PATH` environment variable if `env` is in the `options` object. Otherwise, `process.env.PATH` is used. If `options.env` is set without `PATH`, lookup on Unix is performed on a default search path search of `/usr/bin:/bin` (see your operating system's manual for execvpe/execvp), on Windows the current processes environment variable `PATH` is used.

On Windows, environment variables are case-insensitive. Node.js lexicographically sorts the `env` keys and uses the first one that case-insensitively matches. Only first (in lexicographic order) entry will be passed to the subprocess. This might lead to issues on Windows when passing objects to the `env` option that have multiple variants of the same key, such as `PATH` and `Path`.

The `child_process.spawn()` method spawns the child process asynchronously, without blocking the Node.js event loop. The `child_process.spawnSync()` function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated.

For convenience, the `node:child_process` module provides a handful of synchronous and asynchronous alternatives to `child_process.spawn()` and `child_process.spawnSync()`. Each of these alternatives are implemented on top of `child_process.spawn()` or `child_process.spawnSync()`.

- `child_process.exec()`: spawns a shell and runs a command within that shell, passing the `stdout` and `stderr` to a callback function when complete.
- `child_process.execFile()`: similar to `child_process.exec()` except that it spawns the command directly without first spawning a shell by default.
- `child_process.fork()`: spawns a new Node.js process and invokes a specified module with an IPC communication channel established that allows sending messages between parent and child.
- `child_process.execSync()`: a synchronous version of `child_process.exec()` that will block the Node.js event loop.
- `child_process.execFileSync()`: a synchronous version of `child_process.execFile()` that will block the Node.js event loop.

For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.

### Asynchronous process creation#

The `child_process.spawn()`, `child_process.fork()`, `child_process.exec()`, and `child_process.execFile()` methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs.

Each of the methods returns a `ChildProcess` instance. These objects implement the Node.js `EventEmitter` API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process.

The `child_process.exec()` and `child_process.execFile()` methods additionally allow for an optional `callback` function to be specified that is invoked when the child process terminates.

#### Spawning `.bat` and `.cmd` files on Windows#

The importance of the distinction between `child_process.exec()` and `child_process.execFile()` can vary based on platform. On Unix-type operating systems (Unix, Linux, macOS) `child_process.execFile()` can be more efficient because it does not spawn a shell by default. On Windows, however, `.bat` and `.cmd` files are not executable on their own without a terminal, and therefore cannot be launched using `child_process.execFile()`. When running on Windows, `.bat` and `.cmd` files can be invoked by:

- using `child_process.spawn()` with the `shell` option set (not recommended, see DEP0190), or
- using `child_process.exec()`, or
- spawning `cmd.exe` and passing the `.bat` or `.cmd` file as an argument (which is what `child_process.exec()` does internally).

In any case, if the script filename contains spaces, it needs to be quoted.

    const { exec, spawn } = require('node:child_process');

    exec('my.bat', (err, stdout, stderr) => { /* ... */ });

    // Or, spawning cmd.exe directly:
    const bat = spawn('cmd.exe', ['/c', 'my.bat']);

    // If the script filename contains spaces, it needs to be quoted
    exec('"my script.cmd" a b', (err, stdout, stderr) => { /* ... */ });
    import { exec, spawn } from 'node:child_process';

    exec('my.bat', (err, stdout, stderr) => { /* ... */ });

    // Or, spawning cmd.exe directly:
    const bat = spawn('cmd.exe', ['/c', 'my.bat']);

    // If the script filename contains spaces, it needs to be quoted
    exec('"my script.cmd" a b', (err, stdout, stderr) => { /* ... */ });
    javascript

#### `child_process.exec(command[, options][, callback])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v15.4.0 | AbortSignal support was added.
v8.8.0 | The `windowsHide` option is supported now.

- `command` `<string>` The command to run, with space-separated arguments.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process. Default: `process.cwd()`.
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `encoding` `<string>` Default: `'utf8'`
  - `shell` `<string>` Shell to execute the command with. See Shell requirements and Default Windows shell. Default: `'/bin/sh'` on Unix, `process.env.ComSpec` on Windows.
  - `signal` `<AbortSignal>` allows aborting the child process using an AbortSignal.
  - `timeout` `<number>` Default: `0`
  - `maxBuffer` `<number>` Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at `maxBuffer` and Unicode. Default: `1024 * 1024`.
  - `killSignal` `<string>` | `<integer>` Default: `'SIGTERM'`
  - `uid` `<number>` Sets the user identity of the process (see `setuid(2)`).
  - `gid` `<number>` Sets the group identity of the process (see `setgid(2)`).
  - `windowsHide` `<boolean>` Hide the subprocess console window that would normally be created on Windows systems. Default: `false`.
- `callback` `<Function>` called with the output when process terminates.
  - `error` `<Error>`
  - `stdout` `<string>` | `<Buffer>`
  - `stderr` `<string>` | `<Buffer>`
- Returns: `<ChildProcess>`

Spawns a shell then executes the `command` within that shell, buffering any generated output. The `command` string passed to the exec function is processed directly by the shell and special characters (vary based on shell) need to be dealt with accordingly:

    const { exec } = require('node:child_process');

    exec('"/path/to/test file/test.sh" arg1 arg2');
    // Double quotes are used so that the space in the path is not interpreted as
    // a delimiter of multiple arguments.

    exec('echo "The \\$HOME variable is $HOME"');
    // The $HOME variable is escaped in the first instance, but not in the second.
    import { exec } from 'node:child_process';

    exec('"/path/to/test file/test.sh" arg1 arg2');
    // Double quotes are used so that the space in the path is not interpreted as
    // a delimiter of multiple arguments.

    exec('echo "The \\$HOME variable is $HOME"');
    // The $HOME variable is escaped in the first instance, but not in the second.
    javascript

Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

If a `callback` function is provided, it is called with the arguments `(error, stdout, stderr)`. On success, `error` will be `null`. On error, `error` will be an instance of `Error`. The `error.code` property will be the exit code of the process. By convention, any exit code other than `0` indicates an error. `error.signal` will be the signal that terminated the process.

The `stdout` and `stderr` arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The `encoding` option can be used to specify the character encoding used to decode the stdout and stderr output. If `encoding` is `'buffer'`, or an unrecognized character encoding, `Buffer` objects will be passed to the callback instead.

    const { exec } = require('node:child_process');
    exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => {
      if (error) {
        console.error(`exec error: ${error}`);
        return;
      }
      console.log(`stdout: ${stdout}`);
      console.error(`stderr: ${stderr}`);
    });
    import { exec } from 'node:child_process';
    exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => {
      if (error) {
        console.error(`exec error: ${error}`);
        return;
      }
      console.log(`stdout: ${stdout}`);
      console.error(`stderr: ${stderr}`);
    });
    javascript

If `timeout` is greater than `0`, the parent process will send the signal identified by the `killSignal` property (the default is `'SIGTERM'`) if the child process runs longer than `timeout` milliseconds.

Unlike the `exec(3)` POSIX system call, `child_process.exec()` does not replace the existing process and uses a shell to execute the command.

If this method is invoked as its `util.promisify()`ed version, it returns a `Promise` for an `Object` with `stdout` and `stderr` properties. The returned `ChildProcess` instance is attached to the `Promise` as a `child` property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same `error` object given in the callback, but with two additional properties `stdout` and `stderr`.

    const util = require('node:util');
    const exec = util.promisify(require('node:child_process').exec);

    async function lsExample() {
      const { stdout, stderr } = await exec('ls');
      console.log('stdout:', stdout);
      console.error('stderr:', stderr);
    }
    lsExample();
    import { promisify } from 'node:util';
    import child_process from 'node:child_process';
    const exec = promisify(child_process.exec);

    async function lsExample() {
      const { stdout, stderr } = await exec('ls');
      console.log('stdout:', stdout);
      console.error('stderr:', stderr);
    }
    lsExample();
    javascript

If the `signal` option is enabled, calling `.abort()` on the corresponding `AbortController` is similar to calling `.kill()` on the child process except the error passed to the callback will be an `AbortError`:

    const { exec } = require('node:child_process');
    const controller = new AbortController();
    const { signal } = controller;
    const child = exec('grep ssh', { signal }, (error) => {
      console.error(error); // an AbortError
    });
    controller.abort();
    import { exec } from 'node:child_process';
    const controller = new AbortController();
    const { signal } = controller;
    const child = exec('grep ssh', { signal }, (error) => {
      console.error(error); // an AbortError
    });
    controller.abort();
    javascript

#### `child_process.execFile(file[, args][, options][, callback])`#

Added in: v0.1.91
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Passing `args` when `shell` is set to `true` is deprecated.
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v15.4.0, v14.17.0 | AbortSignal support was added.
v8.8.0 | The `windowsHide` option is supported now.

- `file` `<string>` The name or path of the executable file to run.
- `args` `<string>`[] List of string arguments.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process.
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `encoding` `<string>` Default: `'utf8'`
  - `timeout` `<number>` Default: `0`
  - `maxBuffer` `<number>` Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at `maxBuffer` and Unicode. Default: `1024 * 1024`.
  - `killSignal` `<string>` | `<integer>` Default: `'SIGTERM'`
  - `uid` `<number>` Sets the user identity of the process (see `setuid(2)`).
  - `gid` `<number>` Sets the group identity of the process (see `setgid(2)`).
  - `windowsHide` `<boolean>` Hide the subprocess console window that would normally be created on Windows systems. Default: `false`.
  - `windowsVerbatimArguments` `<boolean>` No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: `false`.
  - `shell` `<boolean>` | `<string>` If `true`, runs `command` inside of a shell. Uses `'/bin/sh'` on Unix, and `process.env.ComSpec` on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: `false` (no shell).
  - `signal` `<AbortSignal>` allows aborting the child process using an AbortSignal.
- `callback` `<Function>` Called with the output when process terminates.
  - `error` `<Error>`
  - `stdout` `<string>` | `<Buffer>`
  - `stderr` `<string>` | `<Buffer>`
- Returns: `<ChildProcess>`

The `child_process.execFile()` function is similar to `child_process.exec()` except that it does not spawn a shell by default. Rather, the specified executable `file` is spawned directly as a new process making it slightly more efficient than `child_process.exec()`.

The same options as `child_process.exec()` are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported.

    const { execFile } = require('node:child_process');
    const child = execFile('node', ['--version'], (error, stdout, stderr) => {
      if (error) {
        throw error;
      }
      console.log(stdout);
    });
    import { execFile } from 'node:child_process';
    const child = execFile('node', ['--version'], (error, stdout, stderr) => {
      if (error) {
        throw error;
      }
      console.log(stdout);
    });
    javascript

The `stdout` and `stderr` arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The `encoding` option can be used to specify the character encoding used to decode the stdout and stderr output. If `encoding` is `'buffer'`, or an unrecognized character encoding, `Buffer` objects will be passed to the callback instead.

If this method is invoked as its `util.promisify()`ed version, it returns a `Promise` for an `Object` with `stdout` and `stderr` properties. The returned `ChildProcess` instance is attached to the `Promise` as a `child` property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same `error` object given in the callback, but with two additional properties `stdout` and `stderr`.

    const util = require('node:util');
    const execFile = util.promisify(require('node:child_process').execFile);
    async function getVersion() {
      const { stdout } = await execFile('node', ['--version']);
      console.log(stdout);
    }
    getVersion();
    import { promisify } from 'node:util';
    import child_process from 'node:child_process';
    const execFile = promisify(child_process.execFile);
    async function getVersion() {
      const { stdout } = await execFile('node', ['--version']);
      console.log(stdout);
    }
    getVersion();
    javascript

If the `shell` option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

If the `signal` option is enabled, calling `.abort()` on the corresponding `AbortController` is similar to calling `.kill()` on the child process except the error passed to the callback will be an `AbortError`:

    const { execFile } = require('node:child_process');
    const controller = new AbortController();
    const { signal } = controller;
    const child = execFile('node', ['--version'], { signal }, (error) => {
      console.error(error); // an AbortError
    });
    controller.abort();
    import { execFile } from 'node:child_process';
    const controller = new AbortController();
    const { signal } = controller;
    const child = execFile('node', ['--version'], { signal }, (error) => {
      console.error(error); // an AbortError
    });
    controller.abort();
    javascript

#### `child_process.fork(modulePath[, args][, options])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v17.4.0, v16.14.0 | The `modulePath` parameter can be a WHATWG `URL` object using `file:` protocol.
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v15.13.0, v14.18.0 | timeout was added.
v15.11.0, v14.18.0 | killSignal for AbortSignal was added.
v15.6.0, v14.17.0 | AbortSignal support was added.
v13.2.0, v12.16.0 | The `serialization` option is supported now.
v8.0.0 | The `stdio` option can now be a string.
v6.4.0 | The `stdio` option is supported now.

- `modulePath` `<string>` | `<URL>` The module to run in the child.
- `args` `<string>`[] List of string arguments.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process.
  - `detached` `<boolean>` Prepare child process to run independently of its parent process. Specific behavior depends on the platform (see `options.detached`).
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `execPath` `<string>` Executable used to create the child process.
  - `execArgv` `<string>`[] List of string arguments passed to the executable. Default: `process.execArgv`.
  - `gid` `<number>` Sets the group identity of the process (see `setgid(2)`).
  - `serialization` `<string>` Specify the kind of serialization used for sending messages between processes. Possible values are `'json'` and `'advanced'`. See Advanced serialization for more details. Default: `'json'`.
  - `signal` `<AbortSignal>` Allows closing the child process using an AbortSignal.
  - `killSignal` `<string>` | `<integer>` The signal value to be used when the spawned process will be killed by timeout or abort signal. Default: `'SIGTERM'`.
  - `silent` `<boolean>` If `true`, stdin, stdout, and stderr of the child process will be piped to the parent process, otherwise they will be inherited from the parent process, see the `'pipe'` and `'inherit'` options for `child_process.spawn()`'s `stdio` for more details. Default: `false`.
  - `stdio` `<Array>` | `<string>` See `child_process.spawn()`'s `stdio`. When this option is provided, it overrides `silent`. If the array variant is used, it must contain exactly one item with value `'ipc'` or an error will be thrown. For instance `[0, 1, 2, 'ipc']`.
  - `uid` `<number>` Sets the user identity of the process (see `setuid(2)`).
  - `windowsVerbatimArguments` `<boolean>` No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: `false`.
  - `timeout` `<number>` In milliseconds the maximum amount of time the process is allowed to run. Default: `undefined`.
- Returns: `<ChildProcess>`

The `child_process.fork()` method is a special case of `child_process.spawn()` used specifically to spawn new Node.js processes. Like `child_process.spawn()`, a `ChildProcess` object is returned. The returned `ChildProcess` will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See `subprocess.send()` for details.

Keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.

By default, `child_process.fork()` will spawn new Node.js instances using the `process.execPath` of the parent process. The `execPath` property in the `options` object allows for an alternative execution path to be used.

Node.js processes launched with a custom `execPath` will communicate with the parent process using the file descriptor (fd) identified using the environment variable `NODE_CHANNEL_FD` on the child process.

Unlike the `fork(2)` POSIX system call, `child_process.fork()` does not clone the current process.

The `shell` option available in `child_process.spawn()` is not supported by `child_process.fork()` and will be ignored if set.

If the `signal` option is enabled, calling `.abort()` on the corresponding `AbortController` is similar to calling `.kill()` on the child process except the error passed to the callback will be an `AbortError`:

    const { fork } = require('node:child_process');

    if (process.argv[2] === 'child') {
      setTimeout(() => {
        console.log(`Hello from ${process.argv[2]}!`);
      }, 1_000);
    } else {
      const controller = new AbortController();
      const { signal } = controller;
      const child = fork(__filename, ['child'], { signal });
      child.on('error', (err) => {
        // This will be called with err being an AbortError if the controller aborts
      });
      controller.abort(); // Stops the child process
    }
    import { fork } from 'node:child_process';
    import process from 'node:process';

    if (process.argv[2] === 'child') {
      setTimeout(() => {
        console.log(`Hello from ${process.argv[2]}!`);
      }, 1_000);
    } else {
      const controller = new AbortController();
      const { signal } = controller;
      const child = fork(import.meta.url, ['child'], { signal });
      child.on('error', (err) => {
        // This will be called with err being an AbortError if the controller aborts
      });
      controller.abort(); // Stops the child process
    }
    javascript

#### `child_process.spawn(command[, args][, options])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Passing `args` when `shell` is set to `true` is deprecated.
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v15.13.0, v14.18.0 | timeout was added.
v15.11.0, v14.18.0 | killSignal for AbortSignal was added.
v15.5.0, v14.17.0 | AbortSignal support was added.
v13.2.0, v12.16.0 | The `serialization` option is supported now.
v8.8.0 | The `windowsHide` option is supported now.
v6.4.0 | The `argv0` option is supported now.
v5.7.0 | The `shell` option is supported now.

- `command` `<string>` The command to run.
- `args` `<string>`[] List of string arguments.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process.
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `argv0` `<string>` Explicitly set the value of `argv[0]` sent to the child process. This will be set to `command` if not specified.
  - `stdio` `<Array>` | `<string>` Child's stdio configuration (see `options.stdio`).
  - `detached` `<boolean>` Prepare child process to run independently of its parent process. Specific behavior depends on the platform (see `options.detached`).
  - `uid` `<number>` Sets the user identity of the process (see `setuid(2)`).
  - `gid` `<number>` Sets the group identity of the process (see `setgid(2)`).
  - `serialization` `<string>` Specify the kind of serialization used for sending messages between processes. Possible values are `'json'` and `'advanced'`. See Advanced serialization for more details. Default: `'json'`.
  - `shell` `<boolean>` | `<string>` If `true`, runs `command` inside of a shell. Uses `'/bin/sh'` on Unix, and `process.env.ComSpec` on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: `false` (no shell).
  - `windowsVerbatimArguments` `<boolean>` No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to `true` automatically when `shell` is specified and is CMD. Default: `false`.
  - `windowsHide` `<boolean>` Hide the subprocess console window that would normally be created on Windows systems. Default: `false`.
  - `signal` `<AbortSignal>` allows aborting the child process using an AbortSignal.
  - `timeout` `<number>` In milliseconds the maximum amount of time the process is allowed to run. Default: `undefined`.
  - `killSignal` `<string>` | `<integer>` The signal value to be used when the spawned process will be killed by timeout or abort signal. Default: `'SIGTERM'`.
- Returns: `<ChildProcess>`

The `child_process.spawn()` method spawns a new process using the given `command`, with command-line arguments in `args`. If omitted, `args` defaults to an empty array.

If the `shell` option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

A third argument may be used to specify additional options, with these defaults:

    const defaults = {
      cwd: undefined,
      env: process.env,
    };
    js

Use `cwd` to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory. If given, but the path does not exist, the child process emits an `ENOENT` error and exits immediately. `ENOENT` is also emitted when the command does not exist.

Use `env` to specify environment variables that will be visible to the new process, the default is `process.env`.

`undefined` values in `env` will be ignored.

Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:

    const { spawn } = require('node:child_process');
    const ls = spawn('ls', ['-lh', '/usr']);

    ls.stdout.on('data', (data) => {
      console.log(`stdout: ${data}`);
    });

    ls.stderr.on('data', (data) => {
      console.error(`stderr: ${data}`);
    });

    ls.on('close', (code) => {
      console.log(`child process exited with code ${code}`);
    });
    import { spawn } from 'node:child_process';
    import { once } from 'node:events';
    const ls = spawn('ls', ['-lh', '/usr']);

    ls.stdout.on('data', (data) => {
      console.log(`stdout: ${data}`);
    });

    ls.stderr.on('data', (data) => {
      console.error(`stderr: ${data}`);
    });

    const [code] = await once(ls, 'close');
    console.log(`child process exited with code ${code}`);
    javascript

Example: A very elaborate way to run `ps ax | grep ssh`

    const { spawn } = require('node:child_process');
    const ps = spawn('ps', ['ax']);
    const grep = spawn('grep', ['ssh']);

    ps.stdout.on('data', (data) => {
      grep.stdin.write(data);
    });

    ps.stderr.on('data', (data) => {
      console.error(`ps stderr: ${data}`);
    });

    ps.on('close', (code) => {
      if (code !== 0) {
        console.log(`ps process exited with code ${code}`);
      }
      grep.stdin.end();
    });

    grep.stdout.on('data', (data) => {
      console.log(data.toString());
    });

    grep.stderr.on('data', (data) => {
      console.error(`grep stderr: ${data}`);
    });

    grep.on('close', (code) => {
      if (code !== 0) {
        console.log(`grep process exited with code ${code}`);
      }
    });
    import { spawn } from 'node:child_process';
    const ps = spawn('ps', ['ax']);
    const grep = spawn('grep', ['ssh']);

    ps.stdout.on('data', (data) => {
      grep.stdin.write(data);
    });

    ps.stderr.on('data', (data) => {
      console.error(`ps stderr: ${data}`);
    });

    ps.on('close', (code) => {
      if (code !== 0) {
        console.log(`ps process exited with code ${code}`);
      }
      grep.stdin.end();
    });

    grep.stdout.on('data', (data) => {
      console.log(data.toString());
    });

    grep.stderr.on('data', (data) => {
      console.error(`grep stderr: ${data}`);
    });

    grep.on('close', (code) => {
      if (code !== 0) {
        console.log(`grep process exited with code ${code}`);
      }
    });
    javascript

Example of checking for failed `spawn`:

    const { spawn } = require('node:child_process');
    const subprocess = spawn('bad_command');

    subprocess.on('error', (err) => {
      console.error('Failed to start subprocess.');
    });
    import { spawn } from 'node:child_process';
    const subprocess = spawn('bad_command');

    subprocess.on('error', (err) => {
      console.error('Failed to start subprocess.');
    });
    javascript

Certain platforms (macOS, Linux) will use the value of `argv[0]` for the process title while others (Windows, SunOS) will use `command`.

Node.js overwrites `argv[0]` with `process.execPath` on startup, so `process.argv[0]` in a Node.js child process will not match the `argv0` parameter passed to `spawn` from the parent. Retrieve it with the `process.argv0` property instead.

If the `signal` option is enabled, calling `.abort()` on the corresponding `AbortController` is similar to calling `.kill()` on the child process except the error passed to the callback will be an `AbortError`:

    const { spawn } = require('node:child_process');
    const controller = new AbortController();
    const { signal } = controller;
    const grep = spawn('grep', ['ssh'], { signal });
    grep.on('error', (err) => {
      // This will be called with err being an AbortError if the controller aborts
    });
    controller.abort(); // Stops the child process
    import { spawn } from 'node:child_process';
    const controller = new AbortController();
    const { signal } = controller;
    const grep = spawn('grep', ['ssh'], { signal });
    grep.on('error', (err) => {
      // This will be called with err being an AbortError if the controller aborts
    });
    controller.abort(); // Stops the child process
    javascript

##### `options.detached`#

Added in: v0.7.10

On Windows, setting `options.detached` to `true` makes it possible for the child process to continue running after the parent exits. The child process will have its own console window. Once enabled for a child process, it cannot be disabled.

On non-Windows platforms, if `options.detached` is set to `true`, the child process will be made the leader of a new process group and session. Child processes may continue running after the parent exits regardless of whether they are detached or not. See `setsid(2)` for more information.

By default, the parent will wait for the detached child process to exit. To prevent the parent process from waiting for a given `subprocess` to exit, use the `subprocess.unref()` method. Doing so will cause the parent process' event loop to not include the child process in its reference count, allowing the parent process to exit independently of the child process, unless there is an established IPC channel between the child and the parent processes.

When using the `detached` option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a `stdio` configuration that is not connected to the parent. If the parent process' `stdio` is inherited, the child process will remain attached to the controlling terminal.

Example of a long-running process, by detaching and also ignoring its parent `stdio` file descriptors, in order to ignore the parent's termination:

    const { spawn } = require('node:child_process');

    const subprocess = spawn(process.argv[0], ['child_program.js'], {
      detached: true,
      stdio: 'ignore',
    });

    subprocess.unref();
    import { spawn } from 'node:child_process';
    import process from 'node:process';

    const subprocess = spawn(process.argv[0], ['child_program.js'], {
      detached: true,
      stdio: 'ignore',
    });

    subprocess.unref();
    javascript

Alternatively one can redirect the child process' output into files:

    const { openSync } = require('node:fs');
    const { spawn } = require('node:child_process');
    const out = openSync('./out.log', 'a');
    const err = openSync('./out.log', 'a');

    const subprocess = spawn('prg', [], {
      detached: true,
      stdio: [ 'ignore', out, err ],
    });

    subprocess.unref();
    import { openSync } from 'node:fs';
    import { spawn } from 'node:child_process';
    const out = openSync('./out.log', 'a');
    const err = openSync('./out.log', 'a');

    const subprocess = spawn('prg', [], {
      detached: true,
      stdio: [ 'ignore', out, err ],
    });

    subprocess.unref();
    javascript

##### `options.stdio`#

Added in: v0.7.10
History

Version | Changes
--- | ---
v15.6.0, v14.18.0 | Added the `overlapped` stdio flag.
v3.3.1 | The value `0` is now accepted as a file descriptor.

The `options.stdio` option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding `subprocess.stdin`, `subprocess.stdout`, and `subprocess.stderr` streams on the `ChildProcess` object. This is equivalent to setting the `options.stdio` equal to `['pipe', 'pipe', 'pipe']`.

For convenience, `options.stdio` may be one of the following strings:

- `'pipe'`: equivalent to `['pipe', 'pipe', 'pipe']` (the default)
- `'overlapped'`: equivalent to `['overlapped', 'overlapped', 'overlapped']`
- `'ignore'`: equivalent to `['ignore', 'ignore', 'ignore']`
- `'inherit'`: equivalent to `['inherit', 'inherit', 'inherit']` or `[0, 1, 2]`

Otherwise, the value of `options.stdio` is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and child. The value is one of the following:

1. `'pipe'`: Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the `child_process` object as `subprocess.stdio[fd]`. Pipes created for fds 0, 1, and 2 are also available as `subprocess.stdin`, `subprocess.stdout` and `subprocess.stderr`, respectively. These are not actual Unix pipes and therefore the child process can not use them by their descriptor files, e.g. `/dev/fd/2` or `/dev/stdout`.

2. `'overlapped'`: Same as `'pipe'` except that the `FILE_FLAG_OVERLAPPED` flag is set on the handle. This is necessary for overlapped I/O on the child process's stdio handles. See the docs for more details. This is exactly the same as `'pipe'` on non-Windows systems.

3. `'ipc'`: Create an IPC channel for passing messages/file descriptors between parent and child. A `ChildProcess` may have at most one IPC stdio file descriptor. Setting this option enables the `subprocess.send()` method. If the child process is a Node.js instance, the presence of an IPC channel will enable `process.send()` and `process.disconnect()` methods, as well as `'disconnect'` and `'message'` events within the child process.

   Accessing the IPC channel fd in any way other than `process.send()` or using the IPC channel with a child process that is not a Node.js instance is not supported.

4. `'ignore'`: Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0, 1, and 2 for the processes it spawns, setting the fd to `'ignore'` will cause Node.js to open `/dev/null` and attach it to the child's fd.

5. `'inherit'`: Pass through the corresponding stdio stream to/from the parent process. In the first three positions, this is equivalent to `process.stdin`, `process.stdout`, and `process.stderr`, respectively. In any other position, equivalent to `'ignore'`.

6. `<Stream>` object: Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the `stdio` array. The stream must have an underlying descriptor (file streams do not start until the `'open'` event has occurred). NOTE: While it is technically possible to pass `stdin` as a writable or `stdout`/`stderr` as readable, it is not recommended. Readable and writable streams are designed with distinct behaviors, and using them incorrectly (e.g., passing a readable stream where a writable stream is expected) can lead to unexpected results or errors. This practice is discouraged as it may result in undefined behavior or dropped callbacks if the stream encounters errors. Always ensure that `stdin` is used as readable and `stdout`/`stderr` as writable to maintain the intended flow of data between the parent and child processes.

7. Positive integer: The integer value is interpreted as a file descriptor that is open in the parent process. It is shared with the child process, similar to how `<Stream>` objects can be shared. Passing sockets is not supported on Windows.

8. `null`, `undefined`: Use default value. For stdio fds 0, 1, and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is `'ignore'`.

    const { spawn } = require('node:child_process');

    // Child will use parent's stdios.
    spawn('prg', [], { stdio: 'inherit' });

    // Spawn child sharing only stderr.
    spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

    // Open an extra fd=4, to interact with programs presenting a
    // startd-style interface.
    spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
    import { spawn } from 'node:child_process';
    import process from 'node:process';

    // Child will use parent's stdios.
    spawn('prg', [], { stdio: 'inherit' });

    // Spawn child sharing only stderr.
    spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

    // Open an extra fd=4, to interact with programs presenting a
    // startd-style interface.
    spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
    javascript

It is worth noting that when an IPC channel is established between the parent and child processes, and the child process is a Node.js instance, the child process is launched with the IPC channel unreferenced (using `unref()`) until the child process registers an event handler for the `'disconnect'` event or the `'message'` event. This allows the child process to exit normally without the process being held open by the open IPC channel. See also: `child_process.exec()` and `child_process.fork()`.

### Synchronous process creation#

The `child_process.spawnSync()`, `child_process.execSync()`, and `child_process.execFileSync()` methods are synchronous and will block the Node.js event loop, pausing execution of any additional code until the spawned process exits.

Blocking calls like these are mostly useful for simplifying general-purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.

#### `child_process.execFileSync(file[, args][, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v10.10.0 | The `input` option can now be any `TypedArray` or a `DataView`.
v8.8.0 | The `windowsHide` option is supported now.
v8.0.0 | The `input` option can now be a `Uint8Array`.
v6.2.1, v4.5.0 | The `encoding` option can now explicitly be set to `buffer`.

- `file` `<string>` The name or path of the executable file to run.
- `args` `<string>`[] List of string arguments.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process.
  - `input` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The value which will be passed as stdin to the spawned process. If `stdio[0]` is set to `'pipe'`, Supplying this value will override `stdio[0]`.
  - `stdio` `<string>` | `<Array>` Child's stdio configuration. See `child_process.spawn()`'s `stdio`. `stderr` by default will be output to the parent process' stderr unless `stdio` is specified. Default: `'pipe'`.
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `uid` `<number>` Sets the user identity of the process (see `setuid(2)`).
  - `gid` `<number>` Sets the group identity of the process (see `setgid(2)`).
  - `timeout` `<number>` In milliseconds the maximum amount of time the process is allowed to run. Default: `undefined`.
  - `killSignal` `<string>` | `<integer>` The signal value to be used when the spawned process will be killed. Default: `'SIGTERM'`.
  - `maxBuffer` `<number>` Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated. See caveat at `maxBuffer` and Unicode. Default: `1024 * 1024`.
  - `encoding` `<string>` The encoding used for all stdio inputs and outputs. Default: `'buffer'`.
  - `windowsHide` `<boolean>` Hide the subprocess console window that would normally be created on Windows systems. Default: `false`.
  - `shell` `<boolean>` | `<string>` If `true`, runs `command` inside of a shell. Uses `'/bin/sh'` on Unix, and `process.env.ComSpec` on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: `false` (no shell).
- Returns: `<Buffer>` | `<string>` The stdout from the command.

The `child_process.execFileSync()` method is generally identical to `child_process.execFile()` with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and `killSignal` is sent, the method won't return until the process has completely exited.

If the child process intercepts and handles the `SIGTERM` signal and does not exit, the parent process will still wait until the child process has exited.

If the process times out or has a non-zero exit code, this method will throw an `Error` that will include the full result of the underlying `child_process.spawnSync()`.

If the `shell` option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

    const { execFileSync } = require('node:child_process');

    try {
      const stdout = execFileSync('my-script.sh', ['my-arg'], {
        // Capture stdout and stderr from child process. Overrides the
        // default behavior of streaming child stderr to the parent stderr
        stdio: 'pipe',

        // Use utf8 encoding for stdio pipes
        encoding: 'utf8',
      });

      console.log(stdout);
    } catch (err) {
      if (err.code) {
        // Spawning child process failed
        console.error(err.code);
      } else {
        // Child was spawned but exited with non-zero exit code
        // Error contains any stdout and stderr from the child
        const { stdout, stderr } = err;

        console.error({ stdout, stderr });
      }
    }
    import { execFileSync } from 'node:child_process';

    try {
      const stdout = execFileSync('my-script.sh', ['my-arg'], {
        // Capture stdout and stderr from child process. Overrides the
        // default behavior of streaming child stderr to the parent stderr
        stdio: 'pipe',

        // Use utf8 encoding for stdio pipes
        encoding: 'utf8',
      });

      console.log(stdout);
    } catch (err) {
      if (err.code) {
        // Spawning child process failed
        console.error(err.code);
      } else {
        // Child was spawned but exited with non-zero exit code
        // Error contains any stdout and stderr from the child
        const { stdout, stderr } = err;

        console.error({ stdout, stderr });
      }
    }
    javascript

#### `child_process.execSync(command[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v10.10.0 | The `input` option can now be any `TypedArray` or a `DataView`.
v8.8.0 | The `windowsHide` option is supported now.
v8.0.0 | The `input` option can now be a `Uint8Array`.

- `command` `<string>` The command to run.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process.
  - `input` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The value which will be passed as stdin to the spawned process. If `stdio[0]` is set to `'pipe'`, Supplying this value will override `stdio[0]`.
  - `stdio` `<string>` | `<Array>` Child's stdio configuration. See `child_process.spawn()`'s `stdio`. `stderr` by default will be output to the parent process' stderr unless `stdio` is specified. Default: `'pipe'`.
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `shell` `<string>` Shell to execute the command with. See Shell requirements and Default Windows shell. Default: `'/bin/sh'` on Unix, `process.env.ComSpec` on Windows.
  - `uid` `<number>` Sets the user identity of the process. (See `setuid(2)`).
  - `gid` `<number>` Sets the group identity of the process. (See `setgid(2)`).
  - `timeout` `<number>` In milliseconds the maximum amount of time the process is allowed to run. Default: `undefined`.
  - `killSignal` `<string>` | `<integer>` The signal value to be used when the spawned process will be killed. Default: `'SIGTERM'`.
  - `maxBuffer` `<number>` Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at `maxBuffer` and Unicode. Default: `1024 * 1024`.
  - `encoding` `<string>` The encoding used for all stdio inputs and outputs. Default: `'buffer'`.
  - `windowsHide` `<boolean>` Hide the subprocess console window that would normally be created on Windows systems. Default: `false`.
- Returns: `<Buffer>` | `<string>` The stdout from the command.

The `child_process.execSync()` method is generally identical to `child_process.exec()` with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and `killSignal` is sent, the method won't return until the process has completely exited. If the child process intercepts and handles the `SIGTERM` signal and doesn't exit, the parent process will wait until the child process has exited.

If the process times out or has a non-zero exit code, this method will throw. The `Error` object will contain the entire result from `child_process.spawnSync()`.

Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

#### `child_process.spawnSync(command[, args][, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v16.4.0, v14.18.0 | The `cwd` option can be a WHATWG `URL` object using `file:` protocol.
v10.10.0 | The `input` option can now be any `TypedArray` or a `DataView`.
v8.8.0 | The `windowsHide` option is supported now.
v8.0.0 | The `input` option can now be a `Uint8Array`.
v6.2.1, v4.5.0 | The `encoding` option can now explicitly be set to `buffer`.
v5.7.0 | The `shell` option is supported now.

- `command` `<string>` The command to run.
- `args` `<string>`[] List of string arguments.
- `options` `<Object>`
  - `cwd` `<string>` | `<URL>` Current working directory of the child process.
  - `input` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The value which will be passed as stdin to the spawned process. If `stdio[0]` is set to `'pipe'`, Supplying this value will override `stdio[0]`.
  - `argv0` `<string>` Explicitly set the value of `argv[0]` sent to the child process. This will be set to `command` if not specified.
  - `stdio` `<string>` | `<Array>` Child's stdio configuration. See `child_process.spawn()`'s `stdio`. Default: `'pipe'`.
  - `env` `<Object>` Environment key-value pairs. Default: `process.env`.
  - `uid` `<number>` Sets the user identity of the process (see `setuid(2)`).
  - `gid` `<number>` Sets the group identity of the process (see `setgid(2)`).
  - `timeout` `<number>` In milliseconds the maximum amount of time the process is allowed to run. Default: `undefined`.
  - `killSignal` `<string>` | `<integer>` The signal value to be used when the spawned process will be killed. Default: `'SIGTERM'`.
  - `maxBuffer` `<number>` Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at `maxBuffer` and Unicode. Default: `1024 * 1024`.
  - `encoding` `<string>` The encoding used for all stdio inputs and outputs. Default: `'buffer'`.
  - `shell` `<boolean>` | `<string>` If `true`, runs `command` inside of a shell. Uses `'/bin/sh'` on Unix, and `process.env.ComSpec` on Windows. A different shell can be specified as a string. See Shell requirements and Default Windows shell. Default: `false` (no shell).
  - `windowsVerbatimArguments` `<boolean>` No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to `true` automatically when `shell` is specified and is CMD. Default: `false`.
  - `windowsHide` `<boolean>` Hide the subprocess console window that would normally be created on Windows systems. Default: `false`.
- Returns: `<Object>`
  - `pid` `<number>` Pid of the child process.
  - `output` `<Array>` Array of results from stdio output.
  - `stdout` `<Buffer>` | `<string>` The contents of `output[1]`.
  - `stderr` `<Buffer>` | `<string>` The contents of `output[2]`.
  - `status` `<number>` | `<null>` The exit code of the subprocess, or `null` if the subprocess terminated due to a signal.
  - `signal` `<string>` | `<null>` The signal used to kill the subprocess, or `null` if the subprocess did not terminate due to a signal.
  - `error` `<Error>` The error object if the child process failed or timed out.

The `child_process.spawnSync()` method is generally identical to `child_process.spawn()` with the exception that the function will not return until the child process has fully closed. When a timeout has been encountered and `killSignal` is sent, the method won't return until the process has completely exited. If the process intercepts and handles the `SIGTERM` signal and doesn't exit, the parent process will wait until the child process has exited.

If the `shell` option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

### Class: `ChildProcess`#

Added in: v2.2.0

- Extends: `<EventEmitter>`

Instances of the `ChildProcess` represent spawned child processes.

Instances of `ChildProcess` are not intended to be created directly. Rather, use the `child_process.spawn()`, `child_process.exec()`, `child_process.execFile()`, or `child_process.fork()` methods to create instances of `ChildProcess`.

#### Event: `'close'`#

Added in: v0.7.7

- `code` `<number>` The exit code if the child process exited on its own, or `null` if the child process terminated due to a signal.
- `signal` `<string>` The signal by which the child process was terminated, or `null` if the child process did not terminated due to a signal.

The `'close'` event is emitted after a process has ended and the stdio streams of a child process have been closed. This is distinct from the `'exit'` event, since multiple processes might share the same stdio streams. The `'close'` event will always emit after `'exit'` was already emitted, or `'error'` if the child process failed to spawn.

If the process exited, `code` is the final exit code of the process, otherwise `null`. If the process terminated due to receipt of a signal, `signal` is the string name of the signal, otherwise `null`. One of the two will always be non-`null`.

    const { spawn } = require('node:child_process');
    const ls = spawn('ls', ['-lh', '/usr']);

    ls.stdout.on('data', (data) => {
      console.log(`stdout: ${data}`);
    });

    ls.on('close', (code) => {
      console.log(`child process close all stdio with code ${code}`);
    });

    ls.on('exit', (code) => {
      console.log(`child process exited with code ${code}`);
    });
    import { spawn } from 'node:child_process';
    import { once } from 'node:events';
    const ls = spawn('ls', ['-lh', '/usr']);

    ls.stdout.on('data', (data) => {
      console.log(`stdout: ${data}`);
    });

    ls.on('close', (code) => {
      console.log(`child process close all stdio with code ${code}`);
    });

    ls.on('exit', (code) => {
      console.log(`child process exited with code ${code}`);
    });

    const [code] = await once(ls, 'close');
    console.log(`child process close all stdio with code ${code}`);
    javascript

#### Event: `'disconnect'`#

Added in: v0.7.2

The `'disconnect'` event is emitted after calling the `subprocess.disconnect()` method in parent process or `process.disconnect()` in child process. After disconnecting it is no longer possible to send or receive messages, and the `subprocess.connected` property is `false`.

#### Event: `'error'`#

- `err` `<Error>` The error.

The `'error'` event is emitted whenever:

- The process could not be spawned.
- The process could not be killed.
- Sending a message to the child process failed.
- The child process was aborted via the `signal` option.

The `'exit'` event may or may not fire after an error has occurred. When listening to both the `'exit'` and `'error'` events, guard against accidentally invoking handler functions multiple times.

See also `subprocess.kill()` and `subprocess.send()`.

#### Event: `'exit'`#

Added in: v0.1.90

- `code` `<number>` The exit code if the child process exited on its own, or `null` if the child process terminated due to a signal.
- `signal` `<string>` The signal by which the child process was terminated, or `null` if the child process did not terminated due to a signal.

The `'exit'` event is emitted after the child process ends. If the process exited, `code` is the final exit code of the process, otherwise `null`. If the process terminated due to receipt of a signal, `signal` is the string name of the signal, otherwise `null`. One of the two will always be non-`null`.

When the `'exit'` event is triggered, child process stdio streams might still be open.

Node.js establishes signal handlers for `SIGINT` and `SIGTERM` and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See `waitpid(2)`.

When `code` is `null` due to signal termination, you can use `util.convertProcessSignalToExitCode()` to convert the signal to a POSIX exit code.

#### Event: `'message'`#

Added in: v0.5.9

- `message` `<Object>` A parsed JSON object or primitive value.
- `sendHandle` `<Handle>` | `<undefined>` `undefined` or a `net.Socket`, `net.Server`, or `dgram.Socket` object.

The `'message'` event is triggered when a child process uses `process.send()` to send messages.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the `serialization` option was set to `'advanced'` used when spawning the child process, the `message` argument can contain data that JSON is not able to represent. See Advanced serialization for more details.

#### Event: `'spawn'`#

Added in: v15.1.0, v14.17.0

The `'spawn'` event is emitted once the child process has spawned successfully. If the child process does not spawn successfully, the `'spawn'` event is not emitted and the `'error'` event is emitted instead.

If emitted, the `'spawn'` event comes before all other events and before any data is received via `stdout` or `stderr`.

The `'spawn'` event will fire regardless of whether an error occurs within the spawned process. For example, if `bash some-command` spawns successfully, the `'spawn'` event will fire, though `bash` may fail to spawn `some-command`. This caveat also applies when using `{ shell: true }`.

#### `subprocess.channel`#

Added in: v7.1.0
History

Version | Changes
--- | ---
v14.0.0 | The object no longer accidentally exposes native C++ bindings.

- Type: `<Object>` A pipe representing the IPC channel to the child process.

The `subprocess.channel` property is a reference to the child's IPC channel. If no IPC channel exists, this property is `undefined`.

##### `subprocess.channel.ref()`#

Added in: v7.1.0

This method makes the IPC channel keep the event loop of the parent process running if `.unref()` has been called before.

##### `subprocess.channel.unref()`#

Added in: v7.1.0

This method makes the IPC channel not keep the event loop of the parent process running, and lets it finish even while the channel is open.

#### `subprocess.connected`#

Added in: v0.7.2

- Type: `<boolean>` Set to `false` after `subprocess.disconnect()` is called.

The `subprocess.connected` property indicates whether it is still possible to send and receive messages from a child process. When `subprocess.connected` is `false`, it is no longer possible to send or receive messages.

#### `subprocess.disconnect()`#

Added in: v0.7.2

Closes the IPC channel between parent and child processes, allowing the child process to exit gracefully once there are no other connections keeping it alive. After calling this method the `subprocess.connected` and `process.connected` properties in both the parent and child processes (respectively) will be set to `false`, and it will be no longer possible to pass messages between the processes.

The `'disconnect'` event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling `subprocess.disconnect()`.

When the child process is a Node.js instance (e.g. spawned using `child_process.fork()`), the `process.disconnect()` method can be invoked within the child process to close the IPC channel as well.

#### `subprocess.exitCode`#

- Type: `<integer>`

The `subprocess.exitCode` property indicates the exit code of the child process. If the child process is still running, the field will be `null`.

When the child process is terminated by a signal, `subprocess.exitCode` will be `null` and `subprocess.signalCode` will be set. To get the corresponding POSIX exit code, use `util.convertProcessSignalToExitCode(subprocess.signalCode)`.

#### `subprocess.kill([signal])`#

Added in: v0.1.90

- `signal` `<number>` | `<string>`
- Returns: `<boolean>`

The `subprocess.kill()` method sends a signal to the child process. If no argument is given, the process will be sent the `'SIGTERM'` signal. See `signal(7)` for a list of available signals. This function returns `true` if `kill(2)` succeeds, and `false` otherwise.

    const { spawn } = require('node:child_process');
    const grep = spawn('grep', ['ssh']);

    grep.on('close', (code, signal) => {
      console.log(
        `child process terminated due to receipt of signal ${signal}`);
    });

    // Send SIGHUP to process.
    grep.kill('SIGHUP');
    import { spawn } from 'node:child_process';
    const grep = spawn('grep', ['ssh']);

    grep.on('close', (code, signal) => {
      console.log(
        `child process terminated due to receipt of signal ${signal}`);
    });

    // Send SIGHUP to process.
    grep.kill('SIGHUP');
    javascript

The `ChildProcess` object may emit an `'error'` event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results.

While the function is called `kill`, the signal delivered to the child process may not actually terminate the process.

See `kill(2)` for reference.

On Windows, where POSIX signals do not exist, the `signal` argument will be ignored except for `'SIGKILL'`, `'SIGTERM'`, `'SIGINT'` and `'SIGQUIT'`, and the process will always be killed forcefully and abruptly (similar to `'SIGKILL'`). See Signal Events for more details.

On Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with the use of the `shell` option of `ChildProcess`:

    const { spawn } = require('node:child_process');

    const subprocess = spawn(
      'sh',
      [
        '-c',
        `node -e "setInterval(() => {
          console.log(process.pid, 'is alive')
        }, 500);"`,
      ], {
        stdio: ['inherit', 'inherit', 'inherit'],
      },
    );

    setTimeout(() => {
      subprocess.kill(); // Does not terminate the Node.js process in the shell.
    }, 2000);
    import { spawn } from 'node:child_process';

    const subprocess = spawn(
      'sh',
      [
        '-c',
        `node -e "setInterval(() => {
          console.log(process.pid, 'is alive')
        }, 500);"`,
      ], {
        stdio: ['inherit', 'inherit', 'inherit'],
      },
    );

    setTimeout(() => {
      subprocess.kill(); // Does not terminate the Node.js process in the shell.
    }, 2000);
    javascript

#### `subprocess[Symbol.dispose]()`#

Added in: v20.5.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `subprocess.kill()` with `'SIGTERM'`.

#### `subprocess.killed`#

Added in: v0.5.10

- Type: `<boolean>` Set to `true` after `subprocess.kill()` is used to successfully send a signal to the child process.

The `subprocess.killed` property indicates whether the child process successfully received a signal from `subprocess.kill()`. The `killed` property does not indicate that the child process has been terminated.

#### `subprocess.pid`#

Added in: v0.1.90

- Type: `<integer>` | `<undefined>`

Returns the process identifier (PID) of the child process. If the child process fails to spawn due to errors, then the value is `undefined` and `error` is emitted.

    const { spawn } = require('node:child_process');
    const grep = spawn('grep', ['ssh']);

    console.log(`Spawned child pid: ${grep.pid}`);
    grep.stdin.end();
    import { spawn } from 'node:child_process';
    const grep = spawn('grep', ['ssh']);

    console.log(`Spawned child pid: ${grep.pid}`);
    grep.stdin.end();
    javascript

#### `subprocess.ref()`#

Added in: v0.7.10

Calling `subprocess.ref()` after making a call to `subprocess.unref()` will restore the removed reference count for the child process, forcing the parent process to wait for the child process to exit before exiting itself.

    const { spawn } = require('node:child_process');

    const subprocess = spawn(process.argv[0], ['child_program.js'], {
      detached: true,
      stdio: 'ignore',
    });

    subprocess.unref();
    subprocess.ref();
    import { spawn } from 'node:child_process';
    import process from 'node:process';

    const subprocess = spawn(process.argv[0], ['child_program.js'], {
      detached: true,
      stdio: 'ignore',
    });

    subprocess.unref();
    subprocess.ref();
    javascript

#### `subprocess.send(message[, sendHandle[, options]][, callback])`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v5.8.0 | The `options` parameter, and the `keepOpen` option in particular, is supported now.
v5.0.0 | This method returns a boolean for flow control now.
v4.0.0 | The `callback` parameter is supported now.

- `message` `<Object>`
- `sendHandle` `<Handle>` | `<undefined>` `undefined`, or a `net.Socket`, `net.Server`, or `dgram.Socket` object.
- `options` `<Object>` The `options` argument, if present, is an object used to parameterize the sending of certain types of handles. `options` supports the following properties:
  - `keepOpen` `<boolean>` A value that can be used when passing instances of `net.Socket`. When `true`, the socket is kept open in the sending process. Default: `false`.
- `callback` `<Function>`
- Returns: `<boolean>`

When an IPC channel has been established between the parent and child processes ( i.e. when using `child_process.fork()`), the `subprocess.send()` method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the `'message'` event.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

For example, in the parent script:

    const { fork } = require('node:child_process');
    const forkedProcess = fork(`${__dirname}/sub.js`);

    forkedProcess.on('message', (message) => {
      console.log('PARENT got message:', message);
    });

    // Causes the child to print: CHILD got message: { hello: 'world' }
    forkedProcess.send({ hello: 'world' });
    import { fork } from 'node:child_process';
    const forkedProcess = fork(`${import.meta.dirname}/sub.js`);

    forkedProcess.on('message', (message) => {
      console.log('PARENT got message:', message);
    });

    // Causes the child to print: CHILD got message: { hello: 'world' }
    forkedProcess.send({ hello: 'world' });
    javascript

And then the child script, `'sub.js'` might look like this:

    process.on('message', (message) => {
      console.log('CHILD got message:', message);
    });

    // Causes the parent to print: PARENT got message: { foo: 'bar', baz: null }
    process.send({ foo: 'bar', baz: NaN });
    js

Child Node.js processes will have a `process.send()` method of their own that allows the child process to send messages back to the parent process.

There is a special case when sending a `{cmd: 'NODE_foo'}` message. Messages containing a `NODE_` prefix in the `cmd` property are reserved for use within Node.js core and will not be emitted in the child's `'message'` event. Rather, such messages are emitted using the `'internalMessage'` event and are consumed internally by Node.js. Applications should avoid using such messages or listening for `'internalMessage'` events as it is subject to change without notice.

The optional `sendHandle` argument that may be passed to `subprocess.send()` is for passing a TCP server or socket object to the child process. The child process will receive the object as the second argument passed to the callback function registered on the `'message'` event. Any data that is received and buffered in the socket will not be sent to the child. Sending IPC sockets is not supported on Windows.

The optional `callback` is a function that is invoked after the message is sent but before the child process may have received it. The function is called with a single argument: `null` on success, or an `Error` object on failure.

If no `callback` function is provided and the message cannot be sent, an `'error'` event will be emitted by the `ChildProcess` object. This can happen, for instance, when the child process has already exited.

`subprocess.send()` will return `false` if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns `true`. The `callback` function can be used to implement flow control.

##### Example: sending a server object#

The `sendHandle` argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

    const { fork } = require('node:child_process');
    const { createServer } = require('node:net');

    const subprocess = fork('subprocess.js');

    // Open up the server object and send the handle.
    const server = createServer();
    server.on('connection', (socket) => {
      socket.end('handled by parent');
    });
    server.listen(1337, () => {
      subprocess.send('server', server);
    });
    import { fork } from 'node:child_process';
    import { createServer } from 'node:net';

    const subprocess = fork('subprocess.js');

    // Open up the server object and send the handle.
    const server = createServer();
    server.on('connection', (socket) => {
      socket.end('handled by parent');
    });
    server.listen(1337, () => {
      subprocess.send('server', server);
    });
    javascript

The child process would then receive the server object as:

    process.on('message', (m, server) => {
      if (m === 'server') {
        server.on('connection', (socket) => {
          socket.end('handled by child');
        });
      }
    });
    js

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the `node:net` module, `node:dgram` module servers use exactly the same workflow with the exceptions of listening on a `'message'` event instead of `'connection'` and using `server.bind()` instead of `server.listen()`. This is, however, only supported on Unix platforms.

##### Example: sending a socket object#

Similarly, the `sendHandler` argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

    const { fork } = require('node:child_process');
    const { createServer } = require('node:net');

    const normal = fork('subprocess.js', ['normal']);
    const special = fork('subprocess.js', ['special']);

    // Open up the server and send sockets to child. Use pauseOnConnect to prevent
    // the sockets from being read before they are sent to the child process.
    const server = createServer({ pauseOnConnect: true });
    server.on('connection', (socket) => {

      // If this is special priority...
      if (socket.remoteAddress === '74.125.127.100') {
        special.send('socket', socket);
        return;
      }
      // This is normal priority.
      normal.send('socket', socket);
    });
    server.listen(1337);
    import { fork } from 'node:child_process';
    import { createServer } from 'node:net';

    const normal = fork('subprocess.js', ['normal']);
    const special = fork('subprocess.js', ['special']);

    // Open up the server and send sockets to child. Use pauseOnConnect to prevent
    // the sockets from being read before they are sent to the child process.
    const server = createServer({ pauseOnConnect: true });
    server.on('connection', (socket) => {

      // If this is special priority...
      if (socket.remoteAddress === '74.125.127.100') {
        special.send('socket', socket);
        return;
      }
      // This is normal priority.
      normal.send('socket', socket);
    });
    server.listen(1337);
    javascript

The `subprocess.js` would receive the socket handle as the second argument passed to the event callback function:

    process.on('message', (m, socket) => {
      if (m === 'socket') {
        if (socket) {
          // Check that the client socket exists.
          // It is possible for the socket to be closed between the time it is
          // sent and the time it is received in the child process.
          socket.end(`Request handled with ${process.argv[2]} priority`);
        }
      }
    });
    js

Do not use `.maxConnections` on a socket that has been passed to a subprocess. The parent cannot track when the socket is destroyed.

Any `'message'` handlers in the subprocess should verify that `socket` exists, as the connection may have been closed during the time it takes to send the connection to the child.

#### `subprocess.signalCode`#

- Type: `<string>` | `<null>`

The `subprocess.signalCode` property indicates the signal received by the child process if any, else `null`.

When the child process is terminated by a signal, `subprocess.exitCode` will be `null`. To get the corresponding POSIX exit code, use `util.convertProcessSignalToExitCode(subprocess.signalCode)`.

#### `subprocess.spawnargs`#

- Type: `<Array>`

The `subprocess.spawnargs` property represents the full list of command-line arguments the child process was launched with.

#### `subprocess.spawnfile`#

- Type: `<string>`

The `subprocess.spawnfile` property indicates the executable file name of the child process that is launched.

For `child_process.fork()`, its value will be equal to `process.execPath`. For `child_process.spawn()`, its value will be the name of the executable file. For `child_process.exec()`, its value will be the name of the shell in which the child process is launched.

#### `subprocess.stderr`#

Added in: v0.1.90

- Type: `<stream.Readable>` | `<null>` | `<undefined>`

A `Readable Stream` that represents the child process's `stderr`.

If the child process was spawned with `stdio[2]` set to anything other than `'pipe'`, then this will be `null`.

`subprocess.stderr` is an alias for `subprocess.stdio[2]`. Both properties will refer to the same value.

The `subprocess.stderr` property can be `null` or `undefined` if the child process could not be successfully spawned.

#### `subprocess.stdin`#

Added in: v0.1.90

- Type: `<stream.Writable>` | `<null>` | `<undefined>`

A `Writable Stream` that represents the child process's `stdin`.

If a child process waits to read all of its input, the child process will not continue until this stream has been closed via `end()`.

If the child process was spawned with `stdio[0]` set to anything other than `'pipe'`, then this will be `null`.

`subprocess.stdin` is an alias for `subprocess.stdio[0]`. Both properties will refer to the same value.

The `subprocess.stdin` property can be `null` or `undefined` if the child process could not be successfully spawned.

#### `subprocess.stdio`#

Added in: v0.7.10

- Type: `<Array>`

A sparse array of pipes to the child process, corresponding with positions in the `stdio` option passed to `child_process.spawn()` that have been set to the value `'pipe'`. `subprocess.stdio[0]`, `subprocess.stdio[1]`, and `subprocess.stdio[2]` are also available as `subprocess.stdin`, `subprocess.stdout`, and `subprocess.stderr`, respectively.

In the following example, only the child's fd `1` (stdout) is configured as a pipe, so only the parent's `subprocess.stdio[1]` is a stream, all other values in the array are `null`.

    const assert = require('node:assert');
    const fs = require('node:fs');
    const child_process = require('node:child_process');

    const subprocess = child_process.spawn('ls', {
      stdio: [
        0, // Use parent's stdin for child.
        'pipe', // Pipe child's stdout to parent.
        fs.openSync('err.out', 'w'), // Direct child's stderr to a file.
      ],
    });

    assert.strictEqual(subprocess.stdio[0], null);
    assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

    assert(subprocess.stdout);
    assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

    assert.strictEqual(subprocess.stdio[2], null);
    assert.strictEqual(subprocess.stdio[2], subprocess.stderr);
    import assert from 'node:assert';
    import fs from 'node:fs';
    import child_process from 'node:child_process';

    const subprocess = child_process.spawn('ls', {
      stdio: [
        0, // Use parent's stdin for child.
        'pipe', // Pipe child's stdout to parent.
        fs.openSync('err.out', 'w'), // Direct child's stderr to a file.
      ],
    });

    assert.strictEqual(subprocess.stdio[0], null);
    assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

    assert(subprocess.stdout);
    assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

    assert.strictEqual(subprocess.stdio[2], null);
    assert.strictEqual(subprocess.stdio[2], subprocess.stderr);
    javascript

The `subprocess.stdio` property can be `undefined` if the child process could not be successfully spawned.

#### `subprocess.stdout`#

Added in: v0.1.90

- Type: `<stream.Readable>` | `<null>` | `<undefined>`

A `Readable Stream` that represents the child process's `stdout`.

If the child process was spawned with `stdio[1]` set to anything other than `'pipe'`, then this will be `null`.

`subprocess.stdout` is an alias for `subprocess.stdio[1]`. Both properties will refer to the same value.

    const { spawn } = require('node:child_process');

    const subprocess = spawn('ls');

    subprocess.stdout.on('data', (data) => {
      console.log(`Received chunk ${data}`);
    });
    import { spawn } from 'node:child_process';

    const subprocess = spawn('ls');

    subprocess.stdout.on('data', (data) => {
      console.log(`Received chunk ${data}`);
    });
    javascript

The `subprocess.stdout` property can be `null` or `undefined` if the child process could not be successfully spawned.

#### `subprocess.unref()`#

Added in: v0.7.10

By default, the parent process will wait for the detached child process to exit. To prevent the parent process from waiting for a given `subprocess` to exit, use the `subprocess.unref()` method. Doing so will cause the parent's event loop to not include the child process in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent processes.

    const { spawn } = require('node:child_process');

    const subprocess = spawn(process.argv[0], ['child_program.js'], {
      detached: true,
      stdio: 'ignore',
    });

    subprocess.unref();
    import { spawn } from 'node:child_process';
    import process from 'node:process';

    const subprocess = spawn(process.argv[0], ['child_program.js'], {
      detached: true,
      stdio: 'ignore',
    });

    subprocess.unref();
    javascript

### `maxBuffer` and Unicode#

The `maxBuffer` option specifies the largest number of bytes allowed on `stdout` or `stderr`. If this value is exceeded, then the child process is terminated. This impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, `console.log('中文测试')` will send 13 UTF-8 encoded bytes to `stdout` although there are only 4 characters.

### Shell requirements#

The shell should understand the `-c` switch. If the shell is `'cmd.exe'`, it should understand the `/d /s /c` switches and command-line parsing should be compatible.

### Default Windows shell#

Although Microsoft specifies `%COMSPEC%` must contain the path to `'cmd.exe'` in the root environment, child processes are not always subject to the same requirement. Thus, in `child_process` functions where a shell can be spawned, `'cmd.exe'` is used as a fallback if `process.env.ComSpec` is unavailable.

### Advanced serialization#

Added in: v13.2.0, v12.16.0

Child processes support a serialization mechanism for IPC that is based on the serialization API of the `node:v8` module, based on the HTML structured clone algorithm. This is generally more powerful and supports more built-in JavaScript object types, such as `BigInt`, `Map` and `Set`, `ArrayBuffer` and `TypedArray`, `Buffer`, `Error`, `RegExp` etc.

However, this format is not a full superset of JSON, and e.g. properties set on objects of such built-in types will not be passed on through the serialization step. Additionally, performance may not be equivalent to that of JSON, depending on the structure of the passed data. Therefore, this feature requires opting in by setting the `serialization` option to `'advanced'` when calling `child_process.spawn()` or `child_process.fork()`.

## Cluster#

Source Code: lib/cluster.js

Stability: 2 - Stable

Clusters of Node.js processes can be used to run multiple instances of Node.js that can distribute workloads among their application threads. When process isolation is not needed, use the `worker_threads` module instead, which allows running multiple application threads within a single Node.js instance.

The cluster module allows easy creation of child processes that all share server ports.

    import cluster from 'node:cluster';
    import http from 'node:http';
    import { availableParallelism } from 'node:os';
    import process from 'node:process';

    const numCPUs = availableParallelism();

    if (cluster.isPrimary) {
      console.log(`Primary ${process.pid} is running`);

      // Fork workers.
      for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      cluster.on('exit', (worker, code, signal) => {
        console.log(`worker ${worker.process.pid} died`);
      });
    } else {
      // Workers can share any TCP connection
      // In this case it is an HTTP server
      http.createServer((req, res) => {
        res.writeHead(200);
        res.end('hello world\n');
      }).listen(8000);

      console.log(`Worker ${process.pid} started`);
    }
    const cluster = require('node:cluster');
    const http = require('node:http');
    const numCPUs = require('node:os').availableParallelism();

    if (cluster.isPrimary) {
      console.log(`Primary ${process.pid} is running`);

      // Fork workers.
      for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      cluster.on('exit', (worker, code, signal) => {
        console.log(`worker ${worker.process.pid} died`);
      });
    } else {
      // Workers can share any TCP connection
      // In this case it is an HTTP server
      http.createServer((req, res) => {
        res.writeHead(200);
        res.end('hello world\n');
      }).listen(8000);

      console.log(`Worker ${process.pid} started`);
    }
    javascript

Running Node.js will now share port 8000 between the workers:

    $ node server.js
    Primary 3596 is running
    Worker 4324 started
    Worker 4520 started
    Worker 6056 started
    Worker 5644 started
    console

On Windows, it is not yet possible to set up a named pipe server in a worker.

### How it works#

The worker processes are spawned using the `child_process.fork()` method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows) is the round-robin approach, where the primary process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the primary process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because `server.listen()` hands off most of the work to the primary process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

1. `server.listen({fd: 7})` Because the message is passed to the primary, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
2. `server.listen(handle)` Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the primary process.
3. `server.listen(0)` Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do `listen(0)`. In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID.

Node.js does not provide routing logic. It is therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs.

Although a primary use case for the `node:cluster` module is networking, it can also be used for other use cases requiring worker processes.

### Class: `Worker`#

Added in: v0.7.0

- Extends: `<EventEmitter>`

A `Worker` object contains all public information and method about a worker. In the primary it can be obtained using `cluster.workers`. In a worker it can be obtained using `cluster.worker`.

#### Event: `'disconnect'`#

Added in: v0.7.7

Similar to the `cluster.on('disconnect')` event, but specific to this worker.

    cluster.fork().on('disconnect', () => {
      // Worker has disconnected
    });
    js

#### Event: `'error'`#

Added in: v0.7.3

This event is the same as the one provided by `child_process.fork()`.

Within a worker, `process.on('error')` may also be used.

#### Event: `'exit'`#

Added in: v0.11.2

- `code` `<number>` The exit code, if it exited normally.
- `signal` `<string>` The name of the signal (e.g. `'SIGHUP'`) that caused the process to be killed.

Similar to the `cluster.on('exit')` event, but specific to this worker.

    import cluster from 'node:cluster';

    if (cluster.isPrimary) {
      const worker = cluster.fork();
      worker.on('exit', (code, signal) => {
        if (signal) {
          console.log(`worker was killed by signal: ${signal}`);
        } else if (code !== 0) {
          console.log(`worker exited with error code: ${code}`);
        } else {
          console.log('worker success!');
        }
      });
    }
    const cluster = require('node:cluster');

    if (cluster.isPrimary) {
      const worker = cluster.fork();
      worker.on('exit', (code, signal) => {
        if (signal) {
          console.log(`worker was killed by signal: ${signal}`);
        } else if (code !== 0) {
          console.log(`worker exited with error code: ${code}`);
        } else {
          console.log('worker success!');
        }
      });
    }
    javascript

#### Event: `'listening'`#

Added in: v0.7.0

- `address` `<Object>`

Similar to the `cluster.on('listening')` event, but specific to this worker.

    cluster.fork().on('listening', (address) => {
      // Worker is listening
    });
    cluster.fork().on('listening', (address) => {
      // Worker is listening
    });
    javascript

It is not emitted in the worker.

#### Event: `'message'`#

Added in: v0.7.0

- `message` `<Object>`
- `handle` `<undefined>` | `<Object>`

Similar to the `'message'` event of `cluster`, but specific to this worker.

Within a worker, `process.on('message')` may also be used.

See `process` event: `'message'`.

Here is an example using the message system. It keeps a count in the primary process of the number of HTTP requests received by the workers:

    import cluster from 'node:cluster';
    import http from 'node:http';
    import { availableParallelism } from 'node:os';
    import process from 'node:process';

    if (cluster.isPrimary) {

      // Keep track of http requests
      let numReqs = 0;
      setInterval(() => {
        console.log(`numReqs = ${numReqs}`);
      }, 1000);

      // Count requests
      function messageHandler(msg) {
        if (msg.cmd && msg.cmd === 'notifyRequest') {
          numReqs += 1;
        }
      }

      // Start workers and listen for messages containing notifyRequest
      const numCPUs = availableParallelism();
      for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      for (const id in cluster.workers) {
        cluster.workers[id].on('message', messageHandler);
      }

    } else {

      // Worker processes have a http server.
      http.Server((req, res) => {
        res.writeHead(200);
        res.end('hello world\n');

        // Notify primary about the request
        process.send({ cmd: 'notifyRequest' });
      }).listen(8000);
    }
    const cluster = require('node:cluster');
    const http = require('node:http');
    const numCPUs = require('node:os').availableParallelism();

    if (cluster.isPrimary) {

      // Keep track of http requests
      let numReqs = 0;
      setInterval(() => {
        console.log(`numReqs = ${numReqs}`);
      }, 1000);

      // Count requests
      function messageHandler(msg) {
        if (msg.cmd && msg.cmd === 'notifyRequest') {
          numReqs += 1;
        }
      }

      // Start workers and listen for messages containing notifyRequest
      for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      for (const id in cluster.workers) {
        cluster.workers[id].on('message', messageHandler);
      }

    } else {

      // Worker processes have a http server.
      http.Server((req, res) => {
        res.writeHead(200);
        res.end('hello world\n');

        // Notify primary about the request
        process.send({ cmd: 'notifyRequest' });
      }).listen(8000);
    }
    javascript

#### Event: `'online'`#

Added in: v0.7.0

Similar to the `cluster.on('online')` event, but specific to this worker.

    cluster.fork().on('online', () => {
      // Worker is online
    });
    js

It is not emitted in the worker.

#### `worker.disconnect()`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v7.3.0 | This method now returns a reference to `worker`.

- Returns: `<cluster.Worker>` A reference to `worker`.

In a worker, this function will close all servers, wait for the `'close'` event on those servers, and then disconnect the IPC channel.

In the primary, an internal message is sent to the worker causing it to call `.disconnect()` on itself.

Causes `.exitedAfterDisconnect` to be set.

After a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see `server.close()`, the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

In a worker, `process.disconnect` exists, but it is not this function; it is `disconnect()`.

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the `'disconnect'` event has not been emitted after some time.

    if (cluster.isPrimary) {
      const worker = cluster.fork();
      let timeout;

      worker.on('listening', (address) => {
        worker.send('shutdown');
        worker.disconnect();
        timeout = setTimeout(() => {
          worker.kill();
        }, 2000);
      });

      worker.on('disconnect', () => {
        clearTimeout(timeout);
      });

    } else if (cluster.isWorker) {
      const net = require('node:net');
      const server = net.createServer((socket) => {
        // Connections never end
      });

      server.listen(8000);

      process.on('message', (msg) => {
        if (msg === 'shutdown') {
          // Initiate graceful close of any connections to server
        }
      });
    }
    js

#### `worker.exitedAfterDisconnect`#

Added in: v6.0.0

- Type: `<boolean>`

This property is `true` if the worker exited due to `.disconnect()`. If the worker exited any other way, it is `false`. If the worker has not exited, it is `undefined`.

The boolean `worker.exitedAfterDisconnect` allows distinguishing between voluntary and accidental exit, the primary may choose not to respawn a worker based on this value.

    cluster.on('exit', (worker, code, signal) => {
      if (worker.exitedAfterDisconnect === true) {
        console.log('Oh, it was just voluntary – no need to worry');
      }
    });

    // kill worker
    worker.kill();
    js

#### `worker.id`#

Added in: v0.8.0

- Type: `<integer>`

Each new worker is given its own unique id, this id is stored in the `id`.

While a worker is alive, this is the key that indexes it in `cluster.workers`.

#### `worker.isConnected()`#

Added in: v0.11.14

This function returns `true` if the worker is connected to its primary via its IPC channel, `false` otherwise. A worker is connected to its primary after it has been created. It is disconnected after the `'disconnect'` event is emitted.

#### `worker.isDead()`#

Added in: v0.11.14

This function returns `true` if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns `false`.

    import cluster from 'node:cluster';
    import http from 'node:http';
    import { availableParallelism } from 'node:os';
    import process from 'node:process';

    const numCPUs = availableParallelism();

    if (cluster.isPrimary) {
      console.log(`Primary ${process.pid} is running`);

      // Fork workers.
      for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      cluster.on('fork', (worker) => {
        console.log('worker is dead:', worker.isDead());
      });

      cluster.on('exit', (worker, code, signal) => {
        console.log('worker is dead:', worker.isDead());
      });
    } else {
      // Workers can share any TCP connection. In this case, it is an HTTP server.
      http.createServer((req, res) => {
        res.writeHead(200);
        res.end(`Current process\n ${process.pid}`);
        process.kill(process.pid);
      }).listen(8000);
    }
    const cluster = require('node:cluster');
    const http = require('node:http');
    const numCPUs = require('node:os').availableParallelism();

    if (cluster.isPrimary) {
      console.log(`Primary ${process.pid} is running`);

      // Fork workers.
      for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
      }

      cluster.on('fork', (worker) => {
        console.log('worker is dead:', worker.isDead());
      });

      cluster.on('exit', (worker, code, signal) => {
        console.log('worker is dead:', worker.isDead());
      });
    } else {
      // Workers can share any TCP connection. In this case, it is an HTTP server.
      http.createServer((req, res) => {
        res.writeHead(200);
        res.end(`Current process\n ${process.pid}`);
        process.kill(process.pid);
      }).listen(8000);
    }
    javascript

#### `worker.kill([signal])`#

Added in: v0.9.12

- `signal` `<string>` Name of the kill signal to send to the worker process. Default: `'SIGTERM'`

This function will kill the worker. In the primary worker, it does this by disconnecting the `worker.process`, and once disconnected, killing with `signal`. In the worker, it does it by killing the process with `signal`.

The `kill()` function kills the worker process without waiting for a graceful disconnect, it has the same behavior as `worker.process.kill()`.

This method is aliased as `worker.destroy()` for backwards compatibility.

In a worker, `process.kill()` exists, but it is not this function; it is `kill()`.

#### `worker.process`#

Added in: v0.7.0

- Type: `<ChildProcess>`

All workers are created using `child_process.fork()`, the returned object from this function is stored as `.process`. In a worker, the global `process` is stored.

See: Child Process module.

Workers will call `process.exit(0)` if the `'disconnect'` event occurs on `process` and `.exitedAfterDisconnect` is not `true`. This protects against accidental disconnection.

#### `worker.send(message[, sendHandle[, options]][, callback])`#

Added in: v0.7.0
History

Version | Changes
--- | ---
v4.0.0 | The `callback` parameter is supported now.

- `message` `<Object>`
- `sendHandle` `<Handle>`
- `options` `<Object>` The `options` argument, if present, is an object used to parameterize the sending of certain types of handles. `options` supports the following properties:
  - `keepOpen` `<boolean>` A value that can be used when passing instances of `net.Socket`. When `true`, the socket is kept open in the sending process. Default: `false`.
- `callback` `<Function>`
- Returns: `<boolean>`

Send a message to a worker or primary, optionally with a handle.

In the primary, this sends a message to a specific worker. It is identical to `ChildProcess.send()`.

In a worker, this sends a message to the primary. It is identical to `process.send()`.

This example will echo back all messages from the primary:

    if (cluster.isPrimary) {
      const worker = cluster.fork();
      worker.send('hi there');

    } else if (cluster.isWorker) {
      process.on('message', (msg) => {
        process.send(msg);
      });
    }
    js

### Event: `'disconnect'`#

Added in: v0.7.9

- `worker` `<cluster.Worker>`

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with `worker.disconnect()`).

There may be a delay between the `'disconnect'` and `'exit'` events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

    cluster.on('disconnect', (worker) => {
      console.log(`The worker #${worker.id} has disconnected`);
    });
    js

### Event: `'exit'`#

Added in: v0.7.9

- `worker` `<cluster.Worker>`
- `code` `<number>` The exit code, if it exited normally.
- `signal` `<string>` The name of the signal (e.g. `'SIGHUP'`) that caused the process to be killed.

When any of the workers die the cluster module will emit the `'exit'` event.

This can be used to restart the worker by calling `.fork()` again.

    cluster.on('exit', (worker, code, signal) => {
      console.log('worker %d died (%s). restarting...',
                  worker.process.pid, signal || code);
      cluster.fork();
    });
    js

See `child_process` event: `'exit'`.

### Event: `'fork'`#

Added in: v0.7.0

- `worker` `<cluster.Worker>`

When a new worker is forked the cluster module will emit a `'fork'` event. This can be used to log worker activity, and create a custom timeout.

    const timeouts = [];
    function errorMsg() {
      console.error('Something must be wrong with the connection ...');
    }

    cluster.on('fork', (worker) => {
      timeouts[worker.id] = setTimeout(errorMsg, 2000);
    });
    cluster.on('listening', (worker, address) => {
      clearTimeout(timeouts[worker.id]);
    });
    cluster.on('exit', (worker, code, signal) => {
      clearTimeout(timeouts[worker.id]);
      errorMsg();
    });
    js

### Event: `'listening'`#

Added in: v0.7.0

- `worker` `<cluster.Worker>`
- `address` `<Object>`

After calling `listen()` from a worker, when the `'listening'` event is emitted on the server, a `'listening'` event will also be emitted on `cluster` in the primary.

The event handler is executed with two arguments, the `worker` contains the worker object and the `address` object contains the following connection properties: `address`, `port`, and `addressType`. This is very useful if the worker is listening on more than one address.

    cluster.on('listening', (worker, address) => {
      console.log(
        `A worker is now connected to ${address.address}:${address.port}`);
    });
    js

The `addressType` is one of:

- `4` (TCPv4)
- `6` (TCPv6)
- `-1` (Unix domain socket)
- `'udp4'` or `'udp6'` (UDPv4 or UDPv6)

### Event: `'message'`#

Added in: v2.5.0
History

Version | Changes
--- | ---
v6.0.0 | The `worker` parameter is passed now; see below for details.

- `worker` `<cluster.Worker>`
- `message` `<Object>`
- `handle` `<undefined>` | `<Object>`

Emitted when the cluster primary receives a message from any worker.

See `child_process` event: `'message'`.

### Event: `'online'`#

Added in: v0.7.0

- `worker` `<cluster.Worker>`

After forking a new worker, the worker should respond with an online message. When the primary receives an online message it will emit this event. The difference between `'fork'` and `'online'` is that fork is emitted when the primary forks a worker, and `'online'` is emitted when the worker is running.

    cluster.on('online', (worker) => {
      console.log('Yay, the worker responded after it was forked');
    });
    js

### Event: `'setup'`#

Added in: v0.7.1

- `settings` `<Object>`

Emitted every time `.setupPrimary()` is called.

The `settings` object is the `cluster.settings` object at the time `.setupPrimary()` was called and is advisory only, since multiple calls to `.setupPrimary()` can be made in a single tick.

If accuracy is important, use `cluster.settings`.

### `cluster.disconnect([callback])`#

Added in: v0.7.7

- `callback` `<Function>` Called when all workers are disconnected and handles are closed.

Calls `.disconnect()` on each worker in `cluster.workers`.

When they are disconnected all internal handles will be closed, allowing the primary process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the primary process.

### `cluster.fork([env])`#

Added in: v0.6.0

- `env` `<Object>` Key/value pairs to add to worker process environment.
- Returns: `<cluster.Worker>`

Spawn a new worker process.

This can only be called from the primary process.

### `cluster.isMaster`#

Added in: v0.8.1Deprecated in: v16.0.0

Stability: 0 - Deprecated

Deprecated alias for `cluster.isPrimary`.

### `cluster.isPrimary`#

Added in: v16.0.0

- Type: `<boolean>`

True if the process is a primary. This is determined by the `process.env.NODE_UNIQUE_ID`. If `process.env.NODE_UNIQUE_ID` is undefined, then `isPrimary` is `true`.

### `cluster.isWorker`#

Added in: v0.6.0

- Type: `<boolean>`

True if the process is not a primary (it is the negation of `cluster.isPrimary`).

### `cluster.schedulingPolicy`#

Added in: v0.11.2

The scheduling policy, either `cluster.SCHED_RR` for round-robin or `cluster.SCHED_NONE` to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or `.setupPrimary()` is called, whichever comes first.

`SCHED_RR` is the default on all operating systems except Windows. Windows will change to `SCHED_RR` once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

`cluster.schedulingPolicy` can also be set through the `NODE_CLUSTER_SCHED_POLICY` environment variable. Valid values are `'rr'` and `'none'`.

### `cluster.settings`#

Added in: v0.7.1
History

Version | Changes
--- | ---
v13.2.0, v12.16.0 | The `serialization` option is supported now.
v9.5.0 | The `cwd` option is supported now.
v9.4.0 | The `windowsHide` option is supported now.
v8.2.0 | The `inspectPort` option is supported now.
v6.4.0 | The `stdio` option is supported now.

- Type: `<Object>`
  - `execArgv` `<string>`[] List of string arguments passed to the Node.js executable. Default: `process.execArgv`.
  - `exec` `<string>` File path to worker file. Default: `process.argv[1]`.
  - `args` `<string>`[] String arguments passed to worker. Default: `process.argv.slice(2)`.
  - `cwd` `<string>` Current working directory of the worker process. Default: `undefined` (inherits from parent process).
  - `serialization` `<string>` Specify the kind of serialization used for sending messages between processes. Possible values are `'json'` and `'advanced'`. See Advanced serialization for `child_process` for more details. Default: `false`.
  - `silent` `<boolean>` Whether or not to send output to parent's stdio. Default: `false`.
  - `stdio` `<Array>` Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an `'ipc'` entry. When this option is provided, it overrides `silent`. See `child_process.spawn()`'s `stdio`.
  - `uid` `<number>` Sets the user identity of the process. (See `setuid(2)`.)
  - `gid` `<number>` Sets the group identity of the process. (See `setgid(2)`.)
  - `inspectPort` `<number>` | `<Function>` Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the primary's `process.debugPort`.
  - `windowsHide` `<boolean>` Hide the forked processes console window that would normally be created on Windows systems. Default: `false`.

After calling `.setupPrimary()` (or `.fork()`) this settings object will contain the settings, including the default values.

This object is not intended to be changed or set manually.

### `cluster.setupMaster([settings])`#

Added in: v0.7.1Deprecated in: v16.0.0
History

Version | Changes
--- | ---
v6.4.0 | The `stdio` option is supported now.

Stability: 0 - Deprecated

Deprecated alias for `.setupPrimary()`.

### `cluster.setupPrimary([settings])`#

Added in: v16.0.0

- `settings` `<Object>` See `cluster.settings`.

`setupPrimary` is used to change the default 'fork' behavior. Once called, the settings will be present in `cluster.settings`.

Any settings changes only affect future calls to `.fork()` and have no effect on workers that are already running.

The only attribute of a worker that cannot be set via `.setupPrimary()` is the `env` passed to `.fork()`.

The defaults above apply to the first call only; the defaults for later calls are the current values at the time of `cluster.setupPrimary()` is called.

    import cluster from 'node:cluster';

    cluster.setupPrimary({
      exec: 'worker.js',
      args: ['--use', 'https'],
      silent: true,
    });
    cluster.fork(); // https worker
    cluster.setupPrimary({
      exec: 'worker.js',
      args: ['--use', 'http'],
    });
    cluster.fork(); // http worker
    const cluster = require('node:cluster');

    cluster.setupPrimary({
      exec: 'worker.js',
      args: ['--use', 'https'],
      silent: true,
    });
    cluster.fork(); // https worker
    cluster.setupPrimary({
      exec: 'worker.js',
      args: ['--use', 'http'],
    });
    cluster.fork(); // http worker
    javascript

This can only be called from the primary process.

### `cluster.worker`#

Added in: v0.7.0

- Type: `<Object>`

A reference to the current worker object. Not available in the primary process.

    import cluster from 'node:cluster';

    if (cluster.isPrimary) {
      console.log('I am primary');
      cluster.fork();
      cluster.fork();
    } else if (cluster.isWorker) {
      console.log(`I am worker #${cluster.worker.id}`);
    }
    const cluster = require('node:cluster');

    if (cluster.isPrimary) {
      console.log('I am primary');
      cluster.fork();
      cluster.fork();
    } else if (cluster.isWorker) {
      console.log(`I am worker #${cluster.worker.id}`);
    }
    javascript

### `cluster.workers`#

Added in: v0.7.0

- Type: `<Object>`

A hash that stores the active worker objects, keyed by `id` field. This makes it easy to loop through all the workers. It is only available in the primary process.

A worker is removed from `cluster.workers` after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the `cluster.workers` list happens before the last `'disconnect'` or `'exit'` event is emitted.

    import cluster from 'node:cluster';

    for (const worker of Object.values(cluster.workers)) {
      worker.send('big announcement to all workers');
    }
    const cluster = require('node:cluster');

    for (const worker of Object.values(cluster.workers)) {
      worker.send('big announcement to all workers');
    }
    javascript

## Command-line API#

Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.

To view this documentation as a manual page in a terminal, run `man node`.

### Synopsis#

`node [options] [V8 options] [<program-entry-point> | -e "script" | -] [--] [arguments]`

`node inspect [<program-entry-point> | -e "script" | <host>:<port>] …`

`node --v8-options`

Execute without arguments to start the REPL.

For more info about `node inspect`, see the debugger documentation.

### Program entry point#

The program entry point is a specifier-like string. If the string is not an absolute path, it's resolved as a relative path from the current working directory. That entry point string is then resolved as if it's been requested by `require()` from the current working directory. If no corresponding file is found, an error is thrown.

By default, the resolved path is also loaded as if it's been requested by `require()`, unless one of the conditions below apply—then it's loaded as if it's been requested by `import()`:

- The program was started with a command-line flag that forces the entry point to be loaded with ECMAScript module loader, such as `--import`.
- The file has an `.mjs`, `.mts` or `.wasm` extension.
- The file does not have a `.cjs` extension, and the nearest parent `package.json` file contains a top-level `"type"` field with a value of `"module"`.

See module resolution and loading for more details.

### Options#

History

Version | Changes
--- | ---
v10.12.0 | Underscores instead of dashes are now allowed for Node.js options as well, in addition to V8 options.

Stability: 2 - Stable

All options, including V8 options, allow words to be separated by both dashes (`-`) or underscores (`_`). For example, `--pending-deprecation` is equivalent to `--pending_deprecation`.

If an option that takes a single value (such as `--max-http-header-size`) is passed more than once, then the last passed value is used. Options from the command line take precedence over options passed through the `NODE_OPTIONS` environment variable.

#### `-`#

Added in: v8.0.0

Alias for stdin. Analogous to the use of `-` in other command-line utilities, meaning that the script is read from stdin, and the rest of the options are passed to that script.

#### `--`#

Added in: v6.11.0

Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument is used as a script filename.

#### `--abort-on-uncaught-exception`#

Added in: v0.10.8

Aborting instead of exiting causes a core file to be generated for post-mortem analysis using a debugger (such as `lldb`, `gdb`, and `mdb`).

If this flag is passed, the behavior can still be set to not abort through `process.setUncaughtExceptionCaptureCallback()` (and through usage of the `node:domain` module that uses it).

#### `--allow-addons`#

Added in: v21.6.0, v20.12.0

Stability: 1.1 - Active development

When using the Permission Model, the process will not be able to use native addons by default. Attempts to do so will throw an `ERR_DLOPEN_DISABLED` unless the user explicitly passes the `--allow-addons` flag when starting Node.js.

Example:

    // Attempt to require an native addon
    require('nodejs-addon-example');
    cjs

    $ node --permission --allow-fs-read=* index.js
    node:internal/modules/cjs/loader:1319
      return process.dlopen(module, path.toNamespacedPath(filename));
                     ^

    Error: Cannot load native addon because loading addons is disabled.
        at Module._extensions..node (node:internal/modules/cjs/loader:1319:18)
        at Module.load (node:internal/modules/cjs/loader:1091:32)
        at Module._load (node:internal/modules/cjs/loader:938:12)
        at Module.require (node:internal/modules/cjs/loader:1115:19)
        at require (node:internal/modules/helpers:130:18)
        at Object.<anonymous> (/home/index.js:1:15)
        at Module._compile (node:internal/modules/cjs/loader:1233:14)
        at Module._extensions..js (node:internal/modules/cjs/loader:1287:10)
        at Module.load (node:internal/modules/cjs/loader:1091:32)
        at Module._load (node:internal/modules/cjs/loader:938:12) {
      code: 'ERR_DLOPEN_DISABLED'
    }
    console

#### `--allow-child-process`#

Added in: v20.0.0
History

Version | Changes
--- | ---
v24.4.0, v22.18.0 | When spawning process with the permission model enabled. The flags are inherit to the child Node.js process through NODE_OPTIONS environment variable.

Stability: 1.1 - Active development

When using the Permission Model, the process will not be able to spawn any child process by default. Attempts to do so will throw an `ERR_ACCESS_DENIED` unless the user explicitly passes the `--allow-child-process` flag when starting Node.js.

Example:

    const childProcess = require('node:child_process');
    // Attempt to bypass the permission
    childProcess.spawn('node', ['-e', 'require("fs").writeFileSync("/new-file", "example")']);
    js

    $ node --permission --allow-fs-read=* index.js
    node:internal/child_process:388
      const err = this._handle.spawn(options);
                               ^
    Error: Access to this API has been restricted
        at ChildProcess.spawn (node:internal/child_process:388:28)
        at node:internal/main/run_main_module:17:47 {
      code: 'ERR_ACCESS_DENIED',
      permission: 'ChildProcess'
    }
    console

The `child_process.fork()` API inherits the execution arguments from the parent process. This means that if Node.js is started with the Permission Model enabled and the `--allow-child-process` flag is set, any child process created using `child_process.fork()` will automatically receive all relevant Permission Model flags.

This behavior also applies to `child_process.spawn()`, but in that case, the flags are propagated via the `NODE_OPTIONS` environment variable rather than directly through the process arguments.

#### `--allow-ffi`#

Added in: v26.1.0

Stability: 1.1 - Active development

When using the Permission Model, the process will not be able to use FFI APIs by default. Attempts to use FFI APIs will throw an `ERR_ACCESS_DENIED` exception unless the user explicitly passes the `--allow-ffi` flag when starting Node.js. The `node:ffi` module also requires the `--experimental-ffi` flag and is only available in builds with FFI support.

Example:

    const { DynamicLibrary } = require('node:ffi');
    const lib = new DynamicLibrary('mylib.so');
    js

    $ node --permission --experimental-ffi index.js
    Error: Access to this API has been restricted. Use --allow-ffi to manage permissions.
        at node:internal/main/run_main_module:17:47 {
      code: 'ERR_ACCESS_DENIED',
      permission: 'FFI'
    }
    console

#### `--allow-fs-read`#

Added in: v20.0.0
History

Version | Changes
--- | ---
v24.2.0, v22.17.0 | Entrypoints of your application are allowed to be read implicitly.
v23.5.0, v22.13.0 | Permission Model and --allow-fs flags are stable.
v20.7.0 | Paths delimited by comma (`,`) are no longer allowed.

This flag configures file system read permissions using the Permission Model.

The valid arguments for the `--allow-fs-read` flag are:

- `*` - To allow all `FileSystemRead` operations.
- Multiple paths can be allowed using multiple `--allow-fs-read` flags. Example `--allow-fs-read=/folder1/ --allow-fs-read=/folder2/`

Examples can be found in the File System Permissions documentation.

The initializer module and custom `--require` modules has a implicit read permission.

    $ node --permission -r custom-require.js -r custom-require-2.js index.js
    console

- The `custom-require.js`, `custom-require-2.js`, and `index.js` will be by default in the allowed read list.

    process.permission.has('fs.read', 'index.js'); // true
    process.permission.has('fs.read', 'custom-require.js'); // true
    process.permission.has('fs.read', 'custom-require-2.js'); // true
    js

#### `--allow-fs-write`#

Added in: v20.0.0
History

Version | Changes
--- | ---
v23.5.0, v22.13.0 | Permission Model and --allow-fs flags are stable.
v20.7.0 | Paths delimited by comma (`,`) are no longer allowed.

This flag configures file system write permissions using the Permission Model.

The valid arguments for the `--allow-fs-write` flag are:

- `*` - To allow all `FileSystemWrite` operations.
- Multiple paths can be allowed using multiple `--allow-fs-write` flags. Example `--allow-fs-write=/folder1/ --allow-fs-write=/folder2/`

Paths delimited by comma (`,`) are no longer allowed. When passing a single flag with a comma a warning will be displayed.

Examples can be found in the File System Permissions documentation.

#### `--allow-inspector`#

Added in: v25.0.0, v24.12.0

Stability: 1.0 - Early development

When using the Permission Model, the process will not be able to connect through inspector protocol.

Attempts to do so will throw an `ERR_ACCESS_DENIED` unless the user explicitly passes the `--allow-inspector` flag when starting Node.js.

Example:

    const { Session } = require('node:inspector/promises');

    const session = new Session();
    session.connect();
    js

    $ node --permission index.js
    Error: connect ERR_ACCESS_DENIED Access to this API has been restricted. Use --allow-inspector to manage permissions.
      code: 'ERR_ACCESS_DENIED',
    }
    console

#### `--allow-net`#

Added in: v25.0.0

Stability: 1.1 - Active development

When using the Permission Model, the process will not be able to access network by default. Attempts to do so will throw an `ERR_ACCESS_DENIED` unless the user explicitly passes the `--allow-net` flag when starting Node.js.

Example:

    const http = require('node:http');
    // Attempt to bypass the permission
    const req = http.get('http://example.com', () => {});

    req.on('error', (err) => {
      console.log('err', err);
    });
    js

    $ node --permission index.js
    Error: connect ERR_ACCESS_DENIED Access to this API has been restricted. Use --allow-net to manage permissions.
      code: 'ERR_ACCESS_DENIED',
    }
    console

#### `--allow-openssl-store`#

Added in: v26.7.0

Stability: 1.1 - Active development

When using the Permission Model, the process will not be able to use OpenSSL STORE loaders by default, for example to load a private key from a `<URL>` passed to `crypto.createPrivateKey()`. Attempts to do so will throw an `ERR_ACCESS_DENIED` unless the user explicitly passes the `--allow-openssl-store` flag. This permission can be dropped at runtime via `permission.drop()`.

This flag grants broad authority to configured OpenSSL STORE loaders. A loader may access files, devices, tokens, or the network. Access performed by a loader is not constrained by the `fs.read`, `fs.write`, or `net` permission scopes.

#### `--allow-wasi`#

Added in: v22.3.0, v20.16.0

Stability: 1.1 - Active development

When using the Permission Model, the process will not be capable of creating any WASI instances by default. For security reasons, the call will throw an `ERR_ACCESS_DENIED` unless the user explicitly passes the flag `--allow-wasi` in the main Node.js process.

Example:

    const { WASI } = require('node:wasi');
    // Attempt to bypass the permission
    new WASI({
      version: 'preview1',
      // Attempt to mount the whole filesystem
      preopens: {
        '/': '/',
      },
    });
    js

    $ node --permission --allow-fs-read=* index.js

    Error: Access to this API has been restricted
        at node:internal/main/run_main_module:30:49 {
      code: 'ERR_ACCESS_DENIED',
      permission: 'WASI',
    }
    console

#### `--allow-worker`#

Added in: v20.0.0

Stability: 1.1 - Active development

When using the Permission Model, the process will not be able to create any worker threads by default. For security reasons, the call will throw an `ERR_ACCESS_DENIED` unless the user explicitly pass the flag `--allow-worker` in the main Node.js process.

Example:

    const { Worker } = require('node:worker_threads');
    // Attempt to bypass the permission
    new Worker(__filename);
    js

    $ node --permission --allow-fs-read=* index.js

    Error: Access to this API has been restricted
        at node:internal/main/run_main_module:17:47 {
      code: 'ERR_ACCESS_DENIED',
      permission: 'WorkerThreads'
    }
    console

#### `--build-sea=config`#

Added in: v25.5.0

Stability: 1.1 - Active development

Generates a single executable application from a JSON configuration file. The argument must be a path to the configuration file. If the path is not absolute, it is resolved relative to the current working directory.

For configuration fields, cross-platform notes, and asset APIs, see the single executable application documentation.

#### `--build-snapshot`#

Added in: v18.8.0
History

Version | Changes
--- | ---
v25.4.0, v24.13.1 | The snapshot building process is no longer experimental.

Generates a snapshot blob when the process exits and writes it to disk, which can be loaded later with `--snapshot-blob`.

When building the snapshot, if `--snapshot-blob` is not specified, the generated blob will be written, by default, to `snapshot.blob` in the current working directory. Otherwise it will be written to the path specified by `--snapshot-blob`.

    $ echo "globalThis.foo = 'I am from the snapshot'" > snapshot.js

    # Run snapshot.js to initialize the application and snapshot the
    # state of it into snapshot.blob.
    $ node --snapshot-blob snapshot.blob --build-snapshot snapshot.js

    $ echo "console.log(globalThis.foo)" > index.js

    # Load the generated snapshot and start the application from index.js.
    $ node --snapshot-blob snapshot.blob index.js
    I am from the snapshot
    console

The `v8.startupSnapshot` API can be used to specify an entry point at snapshot building time, thus avoiding the need of an additional entry script at deserialization time:

    $ echo "require('v8').startupSnapshot.setDeserializeMainFunction(() => console.log('I am from the snapshot'))" > snapshot.js
    $ node --snapshot-blob snapshot.blob --build-snapshot snapshot.js
    $ node --snapshot-blob snapshot.blob
    I am from the snapshot
    console

For more information, check out the `v8.startupSnapshot` API documentation.

The snapshot currently only supports loading a single entrypoint during the snapshot building process, which can load built-in modules, but not additional user-land modules. Users can bundle their applications into a single script with their bundler of choice before building a snapshot.

As it's complicated to ensure the serializablility of all built-in modules, which are also growing over time, only a subset of the built-in modules are well tested to be serializable during the snapshot building process. The Node.js core test suite checks that a few fairly complex applications can be snapshotted. The list of built-in modules being captured by the built-in snapshot of Node.js is considered supported. When the snapshot builder encounters a built-in module that cannot be serialized, it may crash the snapshot building process. In that case a typical workaround would be to delay loading that module until runtime, using either `v8.startupSnapshot.setDeserializeMainFunction()` or `v8.startupSnapshot.addDeserializeCallback()`. If serialization for an additional module during the snapshot building process is needed, please file a request in the Node.js issue tracker and link to it in the tracking issue for user-land snapshots.

#### `--build-snapshot-config`#

Added in: v21.6.0, v20.12.0
History

Version | Changes
--- | ---
v25.4.0, v24.13.1 | The snapshot building process is no longer experimental.

Specifies the path to a JSON configuration file which configures snapshot creation behavior.

The following options are currently supported:

- `builder` `<string>` Required. Provides the name to the script that is executed before building the snapshot, as if `--build-snapshot` had been passed with `builder` as the main script name.
- `withoutCodeCache` `<boolean>` Optional. Including the code cache reduces the time spent on compiling functions included in the snapshot at the expense of a bigger snapshot size and potentially breaking portability of the snapshot.

When using this flag, additional script files provided on the command line will not be executed and instead be interpreted as regular command line arguments.

#### `-c`, `--check`#

Added in: v5.0.0, v4.2.0
History

Version | Changes
--- | ---
v10.0.0 | The `--require` option is now supported when checking a file.

Syntax check the script without executing.

#### `--completion-bash`#

Added in: v10.12.0

Print source-able bash completion script for Node.js.

    node --completion-bash > node_bash_completion
    source node_bash_completion
    bash

#### `-C condition`, `--conditions=condition`#

Added in: v14.9.0, v12.19.0
History

Version | Changes
--- | ---
v22.9.0, v20.18.0 | The flag is no longer experimental.

Provide custom conditional exports resolution conditions.

Any number of custom string condition names are permitted.

The default Node.js conditions of `"node"`, `"default"`, `"import"`, and `"require"` will always apply as defined.

For example, to run a module with "development" resolutions:

    node -C development app.js
    bash

#### `--cpu-prof`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--cpu-prof` flags are now stable.

Starts the V8 CPU profiler on start up, and writes the CPU profile to disk before exit.

If `--cpu-prof-dir` is not specified, the generated profile is placed in the current working directory.

If `--cpu-prof-name` is not specified, the generated profile is named `CPU.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.cpuprofile`.

    $ node --cpu-prof index.js
    $ ls *.cpuprofile
    CPU.20190409.202950.15293.0.0.cpuprofile
    console

If `--cpu-prof-name` is specified, the provided value is used as a template for the file name. The following placeholder is supported and will be substituted at runtime:

- `${pid}` — the current process ID

    $ node --cpu-prof --cpu-prof-name 'CPU.${pid}.cpuprofile' index.js
    $ ls *.cpuprofile
    CPU.15293.cpuprofile
    console

#### `--cpu-prof-dir`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--cpu-prof` flags are now stable.

Specify the directory where the CPU profiles generated by `--cpu-prof` will be placed.

The default value is controlled by the `--diagnostic-dir` command-line option.

#### `--cpu-prof-interval`#

Added in: v12.2.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--cpu-prof` flags are now stable.

Specify the sampling interval in microseconds for the CPU profiles generated by `--cpu-prof`. The default is 1000 microseconds.

#### `--cpu-prof-name`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--cpu-prof` flags are now stable.

Specify the file name of the CPU profile generated by `--cpu-prof`.

#### `--diagnostic-dir=directory`#

Set the directory to which all diagnostic output files are written. Defaults to current working directory.

Affects the default output directory of:

- `--cpu-prof-dir`
- `--heap-prof-dir`
- `--redirect-warnings`

#### `--disable-proto=mode`#

Added in: v13.12.0, v12.17.0

Disable the `Object.prototype.__proto__` property. If `mode` is `delete`, the property is removed entirely. If `mode` is `throw`, accesses to the property throw an exception with the code `ERR_PROTO_ACCESS`.

#### `--disable-sigusr1`#

Added in: v23.7.0, v22.14.0
History

Version | Changes
--- | ---
v24.8.0, v22.20.0 | The option is no longer experimental.

Disable the ability of starting a debugging session by sending a `SIGUSR1` signal to the process.

#### `--disable-warning=code-or-type`#

Added in: v21.3.0, v20.11.0
History

Version | Changes
--- | ---
v26.7.0 | The `--disable-warning` flag is now stable.

Stability: 2 - Stable

Disable specific process warnings by `code` or `type`.

Warnings emitted from `process.emitWarning()` may contain a `code` and a `type`. This option will not-emit warnings that have a matching `code` or `type`.

List of deprecation warnings.

The Node.js core warning types are: `DeprecationWarning` and `ExperimentalWarning`

For example, the following script will not emit DEP0025 `require('node:sys')` when executed with `node --disable-warning=DEP0025`:

    import sys from 'node:sys';
    const sys = require('node:sys');
    javascript

For example, the following script will emit the DEP0025 `require('node:sys')`, but not any Experimental Warnings (such as ExperimentalWarning: `vm.measureMemory` is an experimental feature in <=v21) when executed with `node --disable-warning=ExperimentalWarning`:

    import sys from 'node:sys';
    import vm from 'node:vm';

    vm.measureMemory();
    const sys = require('node:sys');
    const vm = require('node:vm');

    vm.measureMemory();
    javascript

#### `--disable-wasm-trap-handler`#

Added in: v22.2.0, v20.15.0
History

Version | Changes
--- | ---
v26.0.0 | Node.js now automatically disables the trap handler when there is not enough virtual memory available at startup to allocate one cage.

Node.js enables V8's trap-handler-based WebAssembly bound checks on 64-bit platforms, which significantly improves WebAssembly performance by eliminating the need for inline bound checks. This optimization requires allocating a large virtual memory cage per WebAssembly memory instance (currently typically 8GB for 32-bit WebAssembly memory, 16GB for 64-bit WebAssembly memory) to trap out-of-bound accesses. On most 64-bit platforms, the virtual memory address space is usually large enough (around 128TB) to accommodate typical WebAssembly usages, but if the machine has manual limits on virtual memory (e.g. through `ulimit -v`), WebAssembly memory allocation is more likely to fail with `WebAssembly.Memory(): could not allocate memory`.

At startup, Node.js automatically checks whether there is enough virtual memory available to allocate at least one cage, and if not, the trap-handler optimization is automatically disabled so that WebAssembly can still run using inline bound checks (with less optimal performance). But if the application needs to create many WebAssembly memory instances and the machine still configures a relatively high limit on virtual memory, allocation of WebAssembly memory instances may still fail more quickly than expected due to the raised virtual memory usage.

`--disable-wasm-trap-handler` fully disables this optimization so that WebAssembly memory instances always use inline bound checks instead of reserving large virtual memory cages. This allows more instances to be created when the virtual memory address space available to the Node.js process is limited.

#### `--disallow-code-generation-from-strings`#

Added in: v9.8.0

Make built-in language features like `eval` and `new Function` that generate code from strings throw an exception instead. This does not affect the Node.js `node:vm` module.

#### `--dns-result-order=order`#

Added in: v16.4.0, v14.18.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `ipv6first` is supported now.
v17.0.0 | Changed default value to `verbatim`.

Set the default value of `order` in `dns.lookup()` and `dnsPromises.lookup()`. The value could be:

- `ipv4first`: sets default `order` to `ipv4first`.
- `ipv6first`: sets default `order` to `ipv6first`.
- `verbatim`: sets default `order` to `verbatim`.

The default is `verbatim` and `dns.setDefaultResultOrder()` have higher priority than `--dns-result-order`.

#### `--enable-fips`#

Added in: v6.0.0

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built against FIPS-compatible OpenSSL.)

#### `--enable-source-maps`#

Added in: v12.12.0
History

Version | Changes
--- | ---
v15.11.0, v14.18.0 | This API is no longer experimental.

Enable Source Map support for stack traces.

When using a transpiler, such as TypeScript, stack traces thrown by an application reference the transpiled code, not the original source position. `--enable-source-maps` enables caching of Source Maps and makes a best effort to report stack traces relative to the original source file.

Overriding `Error.prepareStackTrace` may prevent `--enable-source-maps` from modifying the stack trace. Call and return the results of the original `Error.prepareStackTrace` in the overriding function to modify the stack trace with source maps.

    const originalPrepareStackTrace = Error.prepareStackTrace;
    Error.prepareStackTrace = (error, trace) => {
      // Modify error and trace and format stack trace with
      // original Error.prepareStackTrace.
      return originalPrepareStackTrace(error, trace);
    };
    js

Note, enabling source maps can introduce latency to your application when `Error.stack` is accessed. If you access `Error.stack` frequently in your application, take into account the performance implications of `--enable-source-maps`.

#### `--entry-url`#

Added in: v23.0.0, v22.10.0

Stability: 1 - Experimental

When present, Node.js will interpret the entry point as a URL, rather than a path.

Follows ECMAScript module resolution rules.

Any query parameter or hash in the URL will be accessible via `import.meta.url`.

    node --entry-url 'file:///path/to/file.js?queryparams=work#and-hashes-too'
    node --entry-url 'file.ts?query#hash'
    node --entry-url 'data:text/javascript,console.log("Hello")'
    bash

#### `--env-file-if-exists=file`#

Added in: v22.9.0
History

Version | Changes
--- | ---
v24.10.0, v22.21.0 | The `--env-file-if-exists` flag is no longer experimental.

Behavior is the same as `--env-file`, but an error is not thrown if the file does not exist.

#### `--env-file=file`#

Added in: v20.6.0
History

Version | Changes
--- | ---
v24.10.0, v22.21.0 | The `--env-file` flag is no longer experimental.
v21.7.0, v20.12.0 | Add support to multi-line values.

Loads environment variables from a file relative to the current directory, making them available to applications on `process.env`. The environment variables which configure Node.js, such as `NODE_OPTIONS`, are parsed and applied. If the same variable is defined in the environment and in the file, the value from the environment takes precedence.

You can pass multiple `--env-file` arguments. Subsequent files override pre-existing variables defined in previous files.

An error is thrown if the file does not exist.

    node --env-file=.env --env-file=.development.env index.js
    bash

The format of the file should be one line per key-value pair of environment variable name and value separated by `=`:

    PORT=3000
    text

Any text after a `#` is treated as a comment:

    # This is a comment
    PORT=3000 # This is also a comment
    text

Values can start and end with the following quotes: ```, `"` or `'`. They are omitted from the values.

    USERNAME="nodejs" # will result in `nodejs` as the value.
    text

Multi-line values are supported:

    MULTI_LINE="THIS IS
    A MULTILINE"
    # will result in `THIS IS\nA MULTILINE` as the value.
    text

Export keyword before a key is ignored:

    export USERNAME="nodejs" # will result in `nodejs` as the value.
    text

If you want to load environment variables from a file that may not exist, you can use the `--env-file-if-exists` flag instead.

#### `-e`, `--eval "script"`#

Added in: v0.5.2
History

Version | Changes
--- | ---
v22.6.0 | Eval now supports experimental type-stripping.
v5.11.0 | Built-in libraries are now available as predefined variables.

Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in `script`.

If `script` starts with `-`, pass it using `=` (for example, `node --print --eval=-42`) so it is parsed as the value of `--eval`.

On Windows, using `cmd.exe` a single quote will not work correctly because it only recognizes double `"` for quoting. In Powershell or Git bash, both `'` and `"` are usable.

It is possible to run code containing inline types unless the `--no-strip-types` flag is provided.

#### `--experimental-addon-modules`#

Added in: v23.6.0, v22.20.0
History

Version | Changes
--- | ---
v26.5.0 | This is enabled by default.

Stability: 1.2 - Release candidate

Enable experimental import support for `.node` addons.

#### `--experimental-config-file=path`, `--experimental-config-file`#

Added in: v23.10.0, v22.16.0
History

Version | Changes
--- | ---
v26.7.0 | Marked as release candidate.

Stability: 1.2 - Release candidate

If present, Node.js will look for a configuration file at the specified path. If the path is not specified, Node.js will look for a `node.config.json` file in the current working directory. To specify a custom path, use the `--experimental-config-file=path` form. The space-separated `--experimental-config-file path` form is not supported. The alias `--experimental-default-config-file` is equivalent to `--experimental-config-file` without an argument. Node.js will read the configuration file and apply the settings. The configuration file should be a JSON file with the following structure. `vX.Y.Z` in the `$schema` must be replaced with the version of Node.js you are using or `latest-vX.x` for the latest version of that major release line.

    {
      "$schema": "https://nodejs.org/dist/vX.Y.Z/docs/node-config-schema.json",
      "nodeOptions": {
        "import": [
          "amaro/strip"
        ],
        "watch-path": "src",
        "watch-preserve-output": true
      },
      "test": {
        "test-isolation": "process"
      },
      "watch": {
        "watch-preserve-output": true
      }
    }
    json

The configuration file supports namespace-specific options:

- The `nodeOptions` field contains CLI flags that are allowed in `NODE_OPTIONS`.

- Namespace fields like `test`, `watch`, and `permission` contain configuration specific to that subsystem.

The configuration file can target a specific Node.js major version with `nodeVersion`:

    {
      "nodeVersion": 25,
      "nodeOptions": {
        "watch-path": "src"
      }
    }
    json

To keep multiple version-specific configurations in the same file, use the `configs` array. Node.js will use the first entry whose `nodeVersion` matches the current Node.js major version:

    {
      "$schema": "https://nodejs.org/dist/latest-v26.x/docs/node-config-schema.json",
      "configs": [
        {
          "nodeVersion": 25,
          "config": {
            "$schema": "https://nodejs.org/dist/latest-v25.x/docs/node-config-schema.json",
            "nodeOptions": {
              "watch-path": "src"
            }
          }
        }
      ]
    }
    json

When `configs` is used, the top level may only contain `$schema` and `configs`. Each `configs` item must define an integer `nodeVersion` and an object `config`. A single top-level config does not require `nodeVersion`, but if present it must match the current Node.js major version.

When a namespace is present in the configuration file, Node.js automatically enables the corresponding flag (e.g., `--test`, `--watch`, `--permission`). This allows you to configure subsystem-specific options without explicitly passing the flag on the command line.

For example:

    {
      "test": {
        "test-isolation": "process"
      }
    }
    json

is equivalent to:

    node --test --test-isolation=process
    bash

To disable the automatic flag while still using namespace options, you can explicitly set the flag to `false` within the namespace:

    {
      "test": {
        "test": false,
        "test-isolation": "process"
      }
    }
    json

No-op flags are not supported. Not all V8 flags are currently supported.

It is possible to use the official JSON schema to validate the configuration file, which may vary depending on the Node.js version. Each key in the configuration file corresponds to a flag that can be passed as a command-line argument. The value of the key is the value that would be passed to the flag.

For example, the configuration file above is equivalent to the following command-line arguments:

    node --import amaro/strip --watch-path=src --watch-preserve-output --test-isolation=process
    bash

The priority in configuration is as follows:

1. NODE_OPTIONS and command-line options
2. Dotenv NODE_OPTIONS
3. Configuration file

Values in the configuration file will not override the values in the environment variables, command-line options, or the `NODE_OPTIONS` env file parsed by the `--env-file` flag.

Keys cannot be duplicated within the same or different namespaces.

The configuration parser will throw an error if the configuration file contains unknown keys or keys that cannot be used in a namespace.

Node.js will not sanitize or perform validation on the user-provided configuration, so NEVER use untrusted configuration files.

#### `--experimental-default-config-file`#

Added in: v23.10.0, v22.16.0

Stability: 1.0 - Early development

This flag is an alias for `--experimental-config-file` without an argument. If present, Node.js will look for a `node.config.json` file in the current working directory and load it as a configuration file.

#### `--experimental-eventsource`#

Added in: v22.3.0, v20.18.0

Enable exposition of EventSource Web API on the global scope.

#### `--experimental-ffi`#

Added in: v26.1.0

Stability: 1 - Experimental

Enable the experimental `node:ffi` module.

This flag is only available in builds with FFI support.

#### `--experimental-import-meta-resolve`#

Added in: v13.9.0, v12.16.2
History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | synchronous import.meta.resolve made available by default, with the flag retained for enabling the experimental second argument as previously supported.

Enable experimental `import.meta.resolve()` parent URL support, which allows passing a second `parentURL` argument for contextual resolution.

Previously gated the entire `import.meta.resolve` feature.

#### `--experimental-import-text`#

Added in: v26.5.0

Stability: 1.0 - Early development

Enable experimental support for importing modules with `with { type: 'text' }`.

#### `--experimental-inspector-network-resource`#

Added in: v24.5.0, v22.19.0

Stability: 1.1 - Active Development

Enable experimental support for inspector network resources.

#### `--experimental-loader=module`#

Added in: v8.8.0
History

Version | Changes
--- | ---
v23.6.1, v22.13.1, v20.18.2 | Using this feature with the permission model enabled requires passing `--allow-worker`.
v12.11.1 | This flag was renamed from `--loader` to `--experimental-loader`.

> This flag is discouraged and may be removed in a future version of Node.js. Please use `--import` with `register()` instead.

Specify the `module` containing exported asynchronous module customization hooks. `module` may be any string accepted as an `import` specifier.

This feature requires `--allow-worker` if used with the Permission Model.

#### `--experimental-network-inspection`#

Added in: v22.6.0, v20.18.0

Stability: 1 - Experimental

Enable experimental support for the network inspection with Chrome DevTools.

#### `--experimental-package-map=<path>`#

Added in: v26.4.0

Stability: 1 - Experimental

Enable experimental package map resolution. The `path` argument specifies the location of a JSON configuration file that defines package resolution mappings.

    node --experimental-package-map=./package-map.json app.js
    bash

When enabled, bare specifier resolution consults the package map for resolution. This allows explicit control over which packages can import which dependencies.

See Package maps for details on the configuration file format and resolution algorithm.

#### `--experimental-print-required-tla`#

Added in: v22.0.0, v20.17.0
History

Version | Changes
--- | ---
v26.5.0 | Print the top-level awaits without evaluating the modules.

If the ES module graph cannot be `require()`'d because it contains any top-level `await`, this flag allows Node.js to locate and print their locations.

#### `--experimental-quic`#

Added in: v25.0.0

Stability: 1.1 - Active development

Enable experimental support for the QUIC protocol.

#### `--experimental-sea-config`#

Added in: v20.0.0

Stability: 1 - Experimental

Use this flag to generate a blob that can be injected into the Node.js binary to produce a single executable application. See the documentation about this configuration for details.

#### `--experimental-shadow-realm`#

Added in: v19.0.0, v18.13.0

Use this flag to enable ShadowRealm support.

#### `--experimental-storage-inspection`#

Added in: v25.5.0

Stability: 1.1 - Active Development

Enable experimental support for storage inspection

#### `--experimental-stream-iter`#

Added in: v25.9.0

Stability: 1 - Experimental

Enable the experimental `node:stream/iter` module.

#### `--experimental-test-coverage`#

Added in: v19.7.0, v18.15.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | This option can be used with `--test`.

When used in conjunction with the `node:test` module, a code coverage report is generated as part of the test runner output. If no tests are run, a coverage report is not generated. See the documentation on collecting code coverage from tests for more details.

#### `--experimental-test-module-mocks`#

Added in: v22.3.0, v20.18.0
History

Version | Changes
--- | ---
v23.6.1, v22.13.1, v20.18.2 | Using this feature with the permission model enabled requires passing `--allow-worker`.

Stability: 1.0 - Early development

Enable module mocking in the test runner.

This feature requires `--allow-worker` if used with the Permission Model.

#### `--experimental-test-tag-filter=<tag>`#

Added in: v26.2.0

Stability: 1.0 - Early development

Run only tests whose tag set contains `<tag>`. Tests declare tags via the `tags` option on `test()`, `it()`, `suite()`, or `describe()`; tags inherit from suites to nested tests by union. Filtering is case-insensitive.

The flag may be specified more than once; tests must contain every filter value to run. See Test tags for details on declaring and inheriting tags.

#### `--experimental-vfs`#

Added in: v26.4.0

Stability: 1 - Experimental

Enable the experimental `node:vfs` module.

#### `--experimental-vm-modules`#

Added in: v9.6.0

Enable experimental ES Module support in the `node:vm` module.

#### `--experimental-wasi-unstable-preview1`#

Added in: v13.3.0, v12.16.0
History

Version | Changes
--- | ---
v20.0.0, v18.17.0 | This option is no longer required as WASI is enabled by default, but can still be passed.
v13.6.0 | changed from `--experimental-wasi-unstable-preview0` to `--experimental-wasi-unstable-preview1`.

Enable experimental WebAssembly System Interface (WASI) support.

#### `--experimental-worker-inspection`#

Added in: v24.1.0, v22.17.0

Stability: 1.1 - Active Development

Enable experimental support for the worker inspection with Chrome DevTools.

#### `--expose-gc`#

Added in: v22.3.0, v20.18.0

Stability: 1 - Experimental. This flag is inherited from V8 and is subject to change upstream.

This flag will expose the gc extension from V8.

    if (globalThis.gc) {
      globalThis.gc();
    }
    js

#### `--force-context-aware`#

Added in: v12.12.0

Disable loading native addons that are not context-aware.

#### `--force-fips`#

Added in: v6.0.0

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as `--enable-fips`.)

#### `--force-node-api-uncaught-exceptions-policy`#

Added in: v18.3.0, v16.17.0

Enforces `uncaughtException` event on Node-API asynchronous callbacks.

To prevent from an existing add-on from crashing the process, this flag is not enabled by default. In the future, this flag will be enabled by default to enforce the correct behavior.

#### `--frozen-intrinsics`#

Added in: v11.12.0

Stability: 1 - Experimental

Enable experimental frozen intrinsics like `Array` and `Object`.

Only the root context is supported. There is no guarantee that `globalThis.Array` is indeed the default intrinsic reference. Code may break under this flag.

To allow polyfills to be added, `--require` and `--import` both run before freezing intrinsics.

#### `--heap-prof`#

Added in: v12.4.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--heap-prof` flags are now stable.

Starts the V8 heap profiler on start up, and writes the heap profile to disk before exit.

If `--heap-prof-dir` is not specified, the generated profile is placed in the current working directory.

If `--heap-prof-name` is not specified, the generated profile is named `Heap.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.heapprofile`.

    $ node --heap-prof index.js
    $ ls *.heapprofile
    Heap.20190409.202950.15293.0.001.heapprofile
    console

#### `--heap-prof-dir`#

Added in: v12.4.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--heap-prof` flags are now stable.

Specify the directory where the heap profiles generated by `--heap-prof` will be placed.

The default value is controlled by the `--diagnostic-dir` command-line option.

#### `--heap-prof-interval`#

Added in: v12.4.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--heap-prof` flags are now stable.

Specify the average sampling interval in bytes for the heap profiles generated by `--heap-prof`. The default is 512 * 1024 bytes.

#### `--heap-prof-name`#

Added in: v12.4.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `--heap-prof` flags are now stable.

Specify the file name of the heap profile generated by `--heap-prof`.

#### `--heapsnapshot-near-heap-limit=max_count`#

Added in: v15.1.0, v14.18.0
History

Version | Changes
--- | ---
v25.4.0, v24.13.1, v22.22.1 | The flag is no longer experimental.

Writes a V8 heap snapshot to disk when the V8 heap usage is approaching the heap limit. `count` should be a non-negative integer (in which case Node.js will write no more than `max_count` snapshots to disk).

When generating snapshots, garbage collection may be triggered and bring the heap usage down. Therefore multiple snapshots may be written to disk before the Node.js instance finally runs out of memory. These heap snapshots can be compared to determine what objects are being allocated during the time consecutive snapshots are taken. It's not guaranteed that Node.js will write exactly `max_count` snapshots to disk, but it will try its best to generate at least one and up to `max_count` snapshots before the Node.js instance runs out of memory when `max_count` is greater than `0`.

Generating V8 snapshots takes time and memory (both memory managed by the V8 heap and native memory outside the V8 heap). The bigger the heap is, the more resources it needs. Node.js will adjust the V8 heap to accommodate the additional V8 heap memory overhead, and try its best to avoid using up all the memory available to the process. When the process uses more memory than the system deems appropriate, the process may be terminated abruptly by the system, depending on the system configuration.

    $ node --max-old-space-size=100 --heapsnapshot-near-heap-limit=3 index.js
    Wrote snapshot to Heap.20200430.100036.49580.0.001.heapsnapshot
    Wrote snapshot to Heap.20200430.100037.49580.0.002.heapsnapshot
    Wrote snapshot to Heap.20200430.100038.49580.0.003.heapsnapshot

    <--- Last few GCs --->

    [49580:0x110000000]     4826 ms: Mark-sweep 130.6 (147.8) -> 130.5 (147.8) MB, 27.4 / 0.0 ms  (average mu = 0.126, current mu = 0.034) allocation failure scavenge might not succeed
    [49580:0x110000000]     4845 ms: Mark-sweep 130.6 (147.8) -> 130.6 (147.8) MB, 18.8 / 0.0 ms  (average mu = 0.088, current mu = 0.031) allocation failure scavenge might not succeed

    <--- JS stacktrace --->

    FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
    ....
    console

#### `--heapsnapshot-signal=signal`#

Added in: v12.0.0

Enables a signal handler that causes the Node.js process to write a heap dump when the specified signal is received. `signal` must be a valid signal name. Disabled by default.

    $ node --heapsnapshot-signal=SIGUSR2 index.js &
    $ ps aux
    USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
    node         1  5.5  6.1 787252 247004 ?       Ssl  16:43   0:02 node --heapsnapshot-signal=SIGUSR2 index.js
    $ kill -USR2 1
    $ ls
    Heap.20190718.133405.15554.0.001.heapsnapshot
    console

#### `-h`, `--help`#

Added in: v0.1.3

Print node command-line options. The output of this option is less detailed than this document.

#### `--icu-data-dir=file`#

Added in: v0.11.15

Specify ICU data load path. (Overrides `NODE_ICU_DATA`.)

#### `--import=module`#

Added in: v19.0.0, v18.18.0

Stability: 1 - Experimental

Preload the specified module at startup. If the flag is provided several times, each module will be executed sequentially in the order they appear, starting with the ones provided in `NODE_OPTIONS`.

Follows ECMAScript module resolution rules. Use `--require` to load a CommonJS module. Modules preloaded with `--require` will run before modules preloaded with `--import`.

Modules are preloaded into the main thread as well as any worker threads, forked processes, or clustered processes.

#### `--input-type=type`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v23.6.0, v22.18.0 | Add support for `-typescript` values.
v22.7.0, v20.19.0 | ESM syntax detection is enabled by default.

This configures Node.js to interpret `--eval` or `STDIN` input as CommonJS or as an ES module. Valid values are `"commonjs"`, `"module"`, `"module-typescript"` and `"commonjs-typescript"`. The `"-typescript"` values are not available with the flag `--no-strip-types`. The default is no value, or `"commonjs"` if `--no-experimental-detect-module` is passed.

If `--input-type` is not provided, Node.js will try to detect the syntax with the following steps:

1. Run the input as CommonJS.
2. If step 1 fails, run the input as an ES module.
3. If step 2 fails with a SyntaxError, strip the types.
4. If step 3 fails with an error code `ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX` or `ERR_INVALID_TYPESCRIPT_SYNTAX`, throw the error from step 2, including the TypeScript error in the message, else run as CommonJS.
5. If step 4 fails, run the input as an ES module.

To avoid the delay of multiple syntax detection passes, the `--input-type=type` flag can be used to specify how the `--eval` input should be interpreted.

The REPL does not support this option. Usage of `--input-type=module` with `--print` will throw an error, as `--print` does not support ES module syntax.

#### `--insecure-http-parser`#

Added in: v13.4.0, v12.15.0, v10.19.0

Enable leniency flags on the HTTP parser. This may allow interoperability with non-conformant HTTP implementations.

When enabled, the parser will accept the following:

- Invalid HTTP headers values.
- Invalid HTTP versions.
- Allow message containing both `Transfer-Encoding` and `Content-Length` headers.
- Allow extra data after message when `Connection: close` is present.
- Allow extra transfer encodings after `chunked` has been provided.
- Allow `\n` to be used as token separator instead of `\r\n`.
- Allow `\r\n` not to be provided after a chunk.
- Allow spaces to be present after a chunk size and before `\r\n`.

All the above will expose your application to request smuggling or poisoning attack. Avoid using this option.

#### `--inspect-brk[=[host:]port]`#

Added in: v7.6.0

Activate inspector on `host:port` and break at start of user script. Default `host:port` is `127.0.0.1:9229`. If port `0` is specified, a random available port will be used.

See V8 Inspector integration for Node.js for further explanation on Node.js debugger.

See the security warning below regarding the `host` parameter usage.

#### `--inspect-port=[host:]port`#

Added in: v7.6.0

Set the `host:port` to be used when the inspector is activated. Useful when activating the inspector by sending the `SIGUSR1` signal. Except when `--disable-sigusr1` is passed.

Default host is `127.0.0.1`. If port `0` is specified, a random available port will be used.

See the security warning below regarding the `host` parameter usage.

#### `--inspect-publish-uid=stderr,http`#

Specify ways of the inspector web socket url exposure.

By default inspector websocket url is available in stderr and under `/json/list` endpoint on `http://host:port/json/list`.

#### `--inspect-wait[=[host:]port]`#

Added in: v22.2.0, v20.15.0

Activate inspector on `host:port` and wait for debugger to be attached. Default `host:port` is `127.0.0.1:9229`. If port `0` is specified, a random available port will be used.

See V8 Inspector integration for Node.js for further explanation on Node.js debugger.

See the security warning below regarding the `host` parameter usage.

#### `--inspect[=[host:]port]`#

Added in: v6.3.0

Activate inspector on `host:port`. Default is `127.0.0.1:9229`. If port `0` is specified, a random available port will be used.

V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug and profile Node.js instances. The tools attach to Node.js instances via a tcp port and communicate using the Chrome DevTools Protocol. See V8 Inspector integration for Node.js for further explanation on Node.js debugger.

##### Warning: binding inspector to a public IP:port combination is insecure#

Binding the inspector to a public IP (including `0.0.0.0`) with an open port is insecure, as it allows external hosts to connect to the inspector and perform a remote code execution attack.

If specifying a host, make sure that either:

- The host is not accessible from public networks.
- A firewall disallows unwanted connections on the port.

More specifically, `--inspect=0.0.0.0` is insecure if the port (`9229` by default) is not firewall-protected.

See the debugging security implications section for more information.

#### `-i`, `--interactive`#

Added in: v0.7.7

Opens the REPL even if stdin does not appear to be a terminal.

#### `--jitless`#

Added in: v12.0.0

Stability: 1 - Experimental. This flag is inherited from V8 and is subject to change upstream.

Disable runtime allocation of executable memory. This may be required on some platforms for security reasons. It can also reduce attack surface on other platforms, but the performance impact may be severe.

#### `--localstorage-file=file`#

Added in: v22.4.0

Stability: 1.2 - Release candidate.

The file used to store `localStorage` data. If the file does not exist, it is created the first time `localStorage` is accessed. The same file may be shared between multiple Node.js processes concurrently.

#### `--max-http-header-size=size`#

Added in: v11.6.0, v10.15.0
History

Version | Changes
--- | ---
v13.13.0 | Change maximum default size of HTTP headers from 8 KiB to 16 KiB.

Specify the maximum size, in bytes, of HTTP headers. Defaults to 16 KiB.

#### `--max-old-space-size-percentage=percentage`#

Sets the maximum memory size of V8's old memory section as a percentage of available system memory. This flag takes precedence over `--max-old-space-size` when both are specified.

The `percentage` parameter must be a number greater than 0 and up to 100, representing the percentage of available system memory to allocate to the V8 heap.

Note: This flag utilizes `--max-old-space-size`, which may be unreliable on 32-bit platforms due to integer overflow issues.

    # Using 50% of available system memory
    node --max-old-space-size-percentage=50 index.js

    # Using 75% of available system memory
    node --max-old-space-size-percentage=75 index.js
    bash

#### `--network-family-autoselection-attempt-timeout`#

Added in: v22.1.0, v20.13.0

Sets the default value for the network family autoselection attempt timeout. For more information, see `net.getDefaultAutoSelectFamilyAttemptTimeout()`.

#### `--no-addons`#

Added in: v16.10.0, v14.19.0

Disable the `node-addons` exports condition as well as disable loading native addons. When `--no-addons` is specified, calling `process.dlopen` or requiring a native C++ addon will fail and throw an exception.

#### `--no-async-context-frame`#

Added in: v24.0.0

Disables the use of `AsyncLocalStorage` backed by `AsyncContextFrame` and uses the prior implementation which relied on async_hooks. The previous model is retained for compatibility with Electron and for cases where the context flow may differ. However, if a difference in flow is found please report it.

#### `--no-deprecation`#

Added in: v0.8.0

Silence deprecation warnings.

#### `--no-experimental-detect-module`#

Added in: v21.1.0, v20.10.0
History

Version | Changes
--- | ---
v22.7.0, v20.19.0 | Syntax detection is enabled by default.

Disable using syntax detection to determine module type.

#### `--no-experimental-global-navigator`#

Added in: v21.2.0

Stability: 1 - Experimental

Disable exposition of Navigator API on the global scope.

#### `--no-experimental-repl-await`#

Added in: v16.6.0

Use this flag to disable top-level await in REPL.

#### `--no-experimental-require-module`#

Added in: v22.0.0, v20.17.0
History

Version | Changes
--- | ---
v25.4.0 | The flag was renamed from `--no-experimental-require-module` to `--no-require-module`, with the former marked as legacy.
v23.0.0, v22.12.0, v20.19.0 | This is now false by default.

Stability: 3 - Legacy: Use `--no-require-module` instead.

Legacy alias for `--no-require-module`.

#### `--no-experimental-sqlite`#

Added in: v22.5.0
History

Version | Changes
--- | ---
v23.4.0, v22.13.0 | SQLite is unflagged but still experimental.

Disable the experimental `node:sqlite` module.

#### `--no-experimental-websocket`#

Added in: v22.0.0

Disable exposition of `<WebSocket>` on the global scope.

#### `--no-experimental-webstorage`#

Added in: v22.4.0
History

Version | Changes
--- | ---
v25.0.0 | The feature is now enabled by default.

Stability: 1.2 - Release candidate.

Disable `Web Storage` support.

#### `--no-extra-info-on-fatal-exception`#

Added in: v17.0.0

Hide extra information on fatal exception that causes exit.

#### `--no-force-async-hooks-checks`#

Added in: v9.0.0

Disables runtime checks for `async_hooks`. These will still be enabled dynamically when `async_hooks` is enabled.

#### `--no-global-search-paths`#

Added in: v16.10.0

Do not search modules from global paths like `$HOME/.node_modules` and `$NODE_PATH`.

#### `--no-network-family-autoselection`#

Added in: v19.4.0
History

Version | Changes
--- | ---
v20.0.0 | The flag was renamed from `--no-enable-network-family-autoselection` to `--no-network-family-autoselection`. The old name can still work as an alias.

Disables the family autoselection algorithm unless connection options explicitly enables it.

#### `--no-require-module`#

Added in: v22.0.0, v20.17.0
History

Version | Changes
--- | ---
v25.4.0 | This flag is no longer experimental.
v25.4.0 | This flag was renamed from `--no-experimental-require-module` to `--no-require-module`.
v23.0.0, v22.12.0, v20.19.0 | This is now false by default.

Disable support for loading a synchronous ES module graph in `require()`.

See Loading ECMAScript modules using `require()`.

#### `--no-strip-types`#

Added in: v22.6.0
History

Version | Changes
--- | ---
v25.2.0, v24.12.0 | Type stripping is now stable, the flag was renamed from `--no-experimental-strip-types` to `--no-strip-types`.
v23.6.0, v22.18.0 | Type stripping is enabled by default.

Disable type-stripping for TypeScript files. For more information, see the TypeScript type-stripping documentation.

#### `--no-warnings`#

Added in: v6.0.0

Silence all process warnings (including deprecations).

#### `--node-memory-debug`#

Added in: v15.0.0, v14.18.0

Enable extra debug checks for memory leaks in Node.js internals. This is usually only useful for developers debugging Node.js itself.

#### `--openssl-config=file`#

Added in: v6.9.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built against FIPS-enabled OpenSSL.

#### `--openssl-legacy-provider`#

Added in: v17.0.0, v16.17.0

Enable OpenSSL 3.0 legacy provider. For more information please see OSSL_PROVIDER-legacy.

#### `--openssl-shared-config`#

Added in: v18.5.0, v16.17.0, v14.21.0

Enable OpenSSL default configuration section, `openssl_conf` to be read from the OpenSSL configuration file. The default configuration file is named `openssl.cnf` but this can be changed using the environment variable `OPENSSL_CONF`, or by using the command line option `--openssl-config`. The location of the default OpenSSL configuration file depends on how OpenSSL is being linked to Node.js. Sharing the OpenSSL configuration may have unwanted implications and it is recommended to use a configuration section specific to Node.js which is `nodejs_conf` and is default when this option is not used.

#### `--pending-deprecation`#

Added in: v8.0.0

Emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the `--pending-deprecation` command-line flag, or the `NODE_PENDING_DEPRECATION=1` environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage.

#### `--permission`#

Added in: v20.0.0
History

Version | Changes
--- | ---
v23.5.0, v22.13.0 | Permission Model is now stable.

Enable the Permission Model for current process. When enabled, the following permissions are restricted:

> See also `--permission-audit` for an audit-only mode that logs violations without denying access.

- File System - manageable through `--allow-fs-read`, `--allow-fs-write` flags
- Network - manageable through `--allow-net` flag
- Child Process - manageable through `--allow-child-process` flag
- Worker Threads - manageable through `--allow-worker` flag
- WASI - manageable through `--allow-wasi` flag
- Addons - manageable through `--allow-addons` flag
- FFI - manageable through `--allow-ffi` flag
- OpenSSL STORE loaders - manageable through `--allow-openssl-store` flag

#### `--permission-audit`#

Added in: v25.8.0

Enable audit mode for the permission model. When enabled, permission checks are performed but access is not denied — no `ERR_ACCESS_DENIED` error is thrown. Instead, each permission violation is published through the `node:diagnostics_channel` module, and execution continues normally.

This flag does not require `--permission` to be specified. The `--allow-*` flags are not needed in audit mode, since no access is denied.

Audit mode is useful for discovering what permissions your application requires before deploying with `--permission`. See the Permission Model documentation for the list of diagnostics channel names and the message format.

If both `--permission` and `--permission-audit` are specified, `--permission` takes precedence and the Permission Model runs in enforce mode.

#### `--preserve-symlinks`#

Added in: v6.3.0

Instructs the module loader to preserve symbolic links when resolving and caching modules.

By default, when Node.js loads a module from a path that is symbolically linked to a different on-disk location, Node.js will dereference the link and use the actual on-disk "real path" of the module as both an identifier and as a root path to locate other dependency modules. In most cases, this default behavior is acceptable. However, when using symbolically linked peer dependencies, as illustrated in the example below, the default behavior causes an exception to be thrown if `moduleA` attempts to require `moduleB` as a peer dependency:

    {appDir}
     ├── app
     │   ├── index.js
     │   └── node_modules
     │       ├── moduleA -> {appDir}/moduleA
     │       └── moduleB
     │           ├── index.js
     │           └── package.json
     └── moduleA
         ├── index.js
         └── package.json
    text

The `--preserve-symlinks` command-line flag instructs Node.js to use the symlink path for modules as opposed to the real path, allowing symbolically linked peer dependencies to be found.

Note, however, that using `--preserve-symlinks` can have other side effects. Specifically, symbolically linked native modules can fail to load if those are linked from more than one location in the dependency tree (Node.js would see those as two separate modules and would attempt to load the module multiple times, causing an exception to be thrown).

The `--preserve-symlinks` flag does not apply to the main module, which allows `node --preserve-symlinks node_module/.bin/<foo>` to work. To apply the same behavior for the main module, also use `--preserve-symlinks-main`.

#### `--preserve-symlinks-main`#

Added in: v10.2.0

Instructs the module loader to preserve symbolic links when resolving and caching the main module (`require.main`).

This flag exists so that the main module can be opted-in to the same behavior that `--preserve-symlinks` gives to all other imports; they are separate flags, however, for backward compatibility with older Node.js versions.

`--preserve-symlinks-main` does not imply `--preserve-symlinks`; use `--preserve-symlinks-main` in addition to `--preserve-symlinks` when it is not desirable to follow symlinks before resolving relative paths.

See `--preserve-symlinks` for more information.

#### `-p`, `--print "script"`#

Added in: v0.6.4
History

Version | Changes
--- | ---
v5.11.0 | Built-in libraries are now available as predefined variables.

Identical to `-e` but prints the result.

#### `--prof`#

Added in: v2.0.0

Generate V8 profiler output.

#### `--prof-process`#

Added in: v5.2.0

Process V8 profiler output generated using the V8 option `--prof`.

#### `--redirect-warnings=file`#

Added in: v8.0.0

Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.

The `file` name may be an absolute path. If it is not, the default directory it will be written to is controlled by the `--diagnostic-dir` command-line option.

#### `--report-compact`#

Added in: v13.12.0, v12.17.0

Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

#### `--report-dir=directory`, `--report-directory=directory`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This option is no longer experimental.
v12.0.0 | Changed from `--diagnostic-report-directory` to `--report-directory`.

Location at which the report will be generated.

#### `--report-exclude-env`#

Added in: v23.3.0, v22.13.0

When `--report-exclude-env` is passed the diagnostic report generated will not contain the `environmentVariables` data.

#### `--report-exclude-network`#

Added in: v22.0.0, v20.13.0

Exclude `header.networkInterfaces` from the diagnostic report. By default this is not set and the network interfaces are included.

#### `--report-filename=filename`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This option is no longer experimental.
v12.0.0 | changed from `--diagnostic-report-filename` to `--report-filename`.

Name of the file to which the report will be written.

If the filename is set to `'stdout'` or `'stderr'`, the report is written to the stdout or stderr of the process respectively.

#### `--report-on-fatalerror`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v14.0.0, v13.14.0, v12.17.0 | This option is no longer experimental.
v12.0.0 | changed from `--diagnostic-report-on-fatalerror` to `--report-on-fatalerror`.

Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime such as out of memory) that lead to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.

#### `--report-on-signal`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This option is no longer experimental.
v12.0.0 | changed from `--diagnostic-report-on-signal` to `--report-on-signal`.

Enables report to be generated upon receiving the specified (or predefined) signal to the running Node.js process. The signal to trigger the report is specified through `--report-signal`.

#### `--report-signal=signal`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This option is no longer experimental.
v12.0.0 | changed from `--diagnostic-report-signal` to `--report-signal`.

Sets or resets the signal for report generation (not supported on Windows). Default signal is `SIGUSR2`.

#### `--report-uncaught-exception`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v18.8.0, v16.18.0 | Report is not generated if the uncaught exception is handled.
v13.12.0, v12.17.0 | This option is no longer experimental.
v12.0.0 | changed from `--diagnostic-report-uncaught-exception` to `--report-uncaught-exception`.

Enables report to be generated when the process exits due to an uncaught exception. Useful when inspecting the JavaScript stack in conjunction with native stack and other runtime environment data.

#### `-r`, `--require module`#

Added in: v1.6.0
History

Version | Changes
--- | ---
v23.0.0, v22.12.0, v20.19.0 | This option also supports ECMAScript module.

Preload the specified module at startup.

Follows `require()`'s module resolution rules. `module` may be either a path to a file, or a node module name.

Modules preloaded with `--require` will run before modules preloaded with `--import`.

Modules are preloaded into the main thread as well as any worker threads, forked processes, or clustered processes.

#### `--run`#

Added in: v22.0.0
History

Version | Changes
--- | ---
v22.3.0 | NODE_RUN_SCRIPT_NAME environment variable is added.
v22.3.0 | NODE_RUN_PACKAGE_JSON_PATH environment variable is added.
v22.3.0 | Traverses up to the root directory and finds a `package.json` file to run the command from, and updates `PATH` environment variable accordingly.

This runs a specified command from a package.json's `"scripts"` object. If a missing `"command"` is provided, it will list the available scripts.

`--run` will traverse up to the root directory and finds a `package.json` file to run the command from.

`--run` prepends `./node_modules/.bin` for each ancestor of the current directory, to the `PATH` in order to execute the binaries from different folders where multiple `node_modules` directories are present, if `ancestor-folder/node_modules/.bin` is a directory.

`--run` executes the command in the directory containing the related `package.json`.

For example, the following command will run the `test` script of the `package.json` in the current folder:

    $ node --run test
    console

You can also pass arguments to the command. Any argument after `--` will be appended to the script:

    $ node --run test -- --verbose
    console

##### Intentional limitations#

`node --run` is not meant to match the behaviors of `npm run` or of the `run` commands of other package managers. The Node.js implementation is intentionally more limited, in order to focus on top performance for the most common use cases. Some features of other `run` implementations that are intentionally excluded are:

- Running `pre` or `post` scripts in addition to the specified script.
- Defining package manager-specific environment variables.

##### Environment variables#

The following environment variables are set when running a script with `--run`:

- `NODE_RUN_SCRIPT_NAME`: The name of the script being run. For example, if `--run` is used to run `test`, the value of this variable will be `test`.
- `NODE_RUN_PACKAGE_JSON_PATH`: The path to the `package.json` that is being processed.

Environment variables loaded from a file with `--env-file` are not applied to the command executed by `--run`.

#### `--secure-heap-min=n`#

Added in: v15.6.0

When using `--secure-heap`, the `--secure-heap-min` flag specifies the minimum allocation from the secure heap. The minimum value is `2`. The maximum value is the lesser of `--secure-heap` or `2147483647`. The value given must be a power of two.

#### `--secure-heap=n`#

Added in: v15.6.0

Initializes an OpenSSL secure heap of `n` bytes. When initialized, the secure heap is used for selected types of allocations within OpenSSL during key generation and other operations. This is useful, for instance, to prevent sensitive information from leaking due to pointer overruns or underruns.

The secure heap is a fixed size and cannot be resized at runtime so, if used, it is important to select a large enough heap to cover all application uses.

The heap size given must be a power of two. Any value less than 2 will disable the secure heap.

The secure heap is disabled by default.

The secure heap is not available on Windows.

See `CRYPTO_secure_malloc_init` for more details.

#### `--snapshot-blob=path`#

Added in: v18.8.0

Stability: 1 - Experimental

When used with `--build-snapshot`, `--snapshot-blob` specifies the path where the generated snapshot blob is written to. If not specified, the generated blob is written to `snapshot.blob` in the current working directory.

When used without `--build-snapshot`, `--snapshot-blob` specifies the path to the blob that is used to restore the application state.

When loading a snapshot, Node.js checks that:

1. The version, architecture, and platform of the running Node.js binary are exactly the same as that of the binary that generates the snapshot.
2. The V8 flags and CPU features are compatible with that of the binary that generates the snapshot.

If they don't match, Node.js refuses to load the snapshot and exits with status code 1.

#### `--test`#

Added in: v18.1.0, v16.17.0
History

Version | Changes
--- | ---
v20.0.0 | The test runner is now stable.
v19.2.0, v18.13.0 | Test runner now supports running in watch mode.

Starts the Node.js command line test runner. This flag cannot be combined with `--watch-path`, `--check`, `--eval`, `--interactive`, or the inspector. See the documentation on running tests from the command line for more details.

#### `--test-concurrency`#

Added in: v21.0.0, v20.10.0, v18.19.0

The maximum number of test files that the test runner CLI will execute concurrently. If `--test-isolation` is set to `'none'`, this flag is ignored and concurrency is one. Otherwise, concurrency defaults to `os.availableParallelism() - 1`.

#### `--test-coverage-branches=threshold`#

Added in: v22.8.0

Stability: 1 - Experimental

Require a minimum percent of covered branches. If code coverage does not reach the threshold specified, the process will exit with code `1`.

#### `--test-coverage-exclude`#

Added in: v22.5.0

Stability: 1 - Experimental

Excludes specific files from code coverage using a glob pattern, which can match both absolute and relative file paths.

This option may be specified multiple times to exclude multiple glob patterns.

If both `--test-coverage-exclude` and `--test-coverage-include` are provided, files must meet both criteria to be included in the coverage report.

By default all the matching test files are excluded from the coverage report. Specifying this option will override the default behavior.

#### `--test-coverage-functions=threshold`#

Added in: v22.8.0

Stability: 1 - Experimental

Require a minimum percent of covered functions. If code coverage does not reach the threshold specified, the process will exit with code `1`.

#### `--test-coverage-include`#

Added in: v22.5.0

Stability: 1 - Experimental

Includes specific files in code coverage using a glob pattern, which can match both absolute and relative file paths.

This option may be specified multiple times to include multiple glob patterns.

If both `--test-coverage-exclude` and `--test-coverage-include` are provided, files must meet both criteria to be included in the coverage report.

#### `--test-coverage-include-all`#

Added in: v26.7.0

Stability: 1 - Experimental

Includes source files that were never loaded by the test run in the coverage report, where they are reported as having zero coverage.

Candidate files are searched for in the current working directory, and are subject to the same `--test-coverage-include` and `--test-coverage-exclude` filtering as the rest of the report.

#### `--test-coverage-lines=threshold`#

Added in: v22.8.0

Stability: 1 - Experimental

Require a minimum percent of covered lines. If code coverage does not reach the threshold specified, the process will exit with code `1`.

#### `--test-force-exit`#

Added in: v22.0.0, v20.14.0

Configures the test runner to exit the process once all known tests have finished executing even if the event loop would otherwise remain active.

#### `--test-global-setup=module`#

Added in: v24.0.0

Stability: 1.0 - Early development

Specify a module that will be evaluated before all tests are executed and can be used to setup global state or fixtures for tests.

See the documentation on global setup and teardown for more details.

#### `--test-isolation=mode`#

Added in: v22.8.0
History

Version | Changes
--- | ---
v23.6.0 | This flag was renamed from `--experimental-test-isolation` to `--test-isolation`.

Configures the type of test isolation used in the test runner. When `mode` is `'process'`, each test file is run in a separate child process. When `mode` is `'none'`, all test files run in the same process as the test runner. The default isolation mode is `'process'`. This flag is ignored if the `--test` flag is not present. See the test runner execution model section for more information.

#### `--test-name-pattern`#

Added in: v18.11.0
History

Version | Changes
--- | ---
v20.0.0 | The test runner is now stable.

A regular expression that configures the test runner to only execute tests whose name matches the provided pattern. See the documentation on filtering tests by name for more details.

If both `--test-name-pattern` and `--test-skip-pattern` are supplied, tests must satisfy both requirements in order to be executed.

#### `--test-only`#

Added in: v18.0.0, v16.17.0
History

Version | Changes
--- | ---
v20.0.0 | The test runner is now stable.

Configures the test runner to only execute top level tests that have the `only` option set. This flag is not necessary when test isolation is disabled.

#### `--test-random-seed`#

Added in: v26.1.0

Set the seed used to randomize test execution order. This applies to both test file execution order and queued tests within each file. Providing this flag enables randomization implicitly, even without `--test-randomize`.

The value must be an integer between `0` and `4294967295`.

This flag cannot be used with `--watch` or `--test-rerun-failures`.

#### `--test-randomize`#

Added in: v26.1.0

Randomize test execution order. This applies to both test file execution order and queued tests within each file. This can help detect tests that rely on shared state or execution order.

The seed used for randomization is printed in the test summary and can be reused with `--test-random-seed`.

For detailed behavior and examples, see randomizing tests execution order.

This flag cannot be used with `--watch` or `--test-rerun-failures`.

#### `--test-reporter`#

Added in: v19.6.0, v18.15.0
History

Version | Changes
--- | ---
v20.0.0 | The test runner is now stable.

A test reporter to use when running tests. See the documentation on test reporters for more details.

#### `--test-reporter-destination`#

Added in: v19.6.0, v18.15.0
History

Version | Changes
--- | ---
v20.0.0 | The test runner is now stable.

The destination for the corresponding test reporter. See the documentation on test reporters for more details.

#### `--test-rerun-failures`#

Added in: v24.7.0

A path to a file allowing the test runner to persist the state of the test suite between runs. The test runner will use this file to determine which tests have already succeeded or failed, allowing for re-running of failed tests without having to re-run the entire test suite. The test runner will create this file if it does not exist. See the documentation on test reruns for more details.

#### `--test-shard`#

Added in: v20.5.0, v18.19.0

Test suite shard to execute in a format of `<index>/<total>`, where

- `index` is a positive integer, index of divided parts.
- `total` is a positive integer, total of divided part.

This command will divide all tests files into `total` equal parts, and will run only those that happen to be in an `index` part.

For example, to split your tests suite into three parts, use this:

    node --test --test-shard=1/3
    node --test --test-shard=2/3
    node --test --test-shard=3/3
    bash

#### `--test-skip-pattern`#

Added in: v22.1.0

A regular expression that configures the test runner to skip tests whose name matches the provided pattern. See the documentation on filtering tests by name for more details.

If both `--test-name-pattern` and `--test-skip-pattern` are supplied, tests must satisfy both requirements in order to be executed.

#### `--test-timeout`#

Added in: v21.2.0, v20.11.0

A number of milliseconds the test execution will fail after. If unspecified, subtests inherit this value from their parent. The default value is `Infinity`.

#### `--test-update-snapshots`#

Added in: v22.3.0
History

Version | Changes
--- | ---
v23.4.0, v22.13.0 | Snapshot testing is no longer experimental.

Regenerates the snapshot files used by the test runner for snapshot testing.

#### `--throw-deprecation`#

Added in: v0.11.14

Throw errors for deprecations.

#### `--title=title`#

Added in: v10.7.0

Set `process.title` on startup.

#### `--tls-cipher-list=list`#

Added in: v4.0.0

Specify an alternative default TLS cipher list. Requires Node.js to be built with crypto support (default).

#### `--tls-keylog=file`#

Added in: v13.2.0, v12.16.0

Log TLS key material to a file. The key material is in NSS `SSLKEYLOGFILE` format and can be used by software (such as Wireshark) to decrypt the TLS traffic.

#### `--tls-max-v1.2`#

Added in: v12.0.0, v10.20.0

Set `tls.DEFAULT_MAX_VERSION` to 'TLSv1.2'. Use to disable support for TLSv1.3.

#### `--tls-max-v1.3`#

Added in: v12.0.0

Set default `tls.DEFAULT_MAX_VERSION` to 'TLSv1.3'. Use to enable support for TLSv1.3.

#### `--tls-min-v1.0`#

Added in: v12.0.0, v10.20.0

Set default `tls.DEFAULT_MIN_VERSION` to 'TLSv1'. Use for compatibility with old TLS clients or servers.

#### `--tls-min-v1.1`#

Added in: v12.0.0, v10.20.0

Set default `tls.DEFAULT_MIN_VERSION` to 'TLSv1.1'. Use for compatibility with old TLS clients or servers.

#### `--tls-min-v1.2`#

Added in: v12.2.0, v10.20.0

Set default `tls.DEFAULT_MIN_VERSION` to 'TLSv1.2'. This is the default for 12.x and later, but the option is supported for compatibility with older Node.js versions.

#### `--tls-min-v1.3`#

Added in: v12.0.0

Set default `tls.DEFAULT_MIN_VERSION` to 'TLSv1.3'. Use to disable support for TLSv1.2, which is not as secure as TLSv1.3.

#### `--trace-deprecation`#

Added in: v0.8.0

Print stack traces for deprecations.

#### `--trace-env`#

Added in: v23.4.0, v22.13.0

Print information about any access to environment variables done in the current Node.js instance to stderr, including:

- The environment variable reads that Node.js does internally.
- Writes in the form of `process.env.KEY = "SOME VALUE"`.
- Reads in the form of `process.env.KEY`.
- Definitions in the form of `Object.defineProperty(process.env, 'KEY', {...})`.
- Queries in the form of `Object.hasOwn(process.env, 'KEY')`, `process.env.hasOwnProperty('KEY')` or `'KEY' in process.env`.
- Deletions in the form of `delete process.env.KEY`.
- Enumerations inf the form of `...process.env` or `Object.keys(process.env)`.

Only the names of the environment variables being accessed are printed. The values are not printed.

To print the stack trace of the access, use `--trace-env-js-stack` and/or `--trace-env-native-stack`.

#### `--trace-env-js-stack`#

Added in: v23.4.0, v22.13.0

In addition to what `--trace-env` does, this prints the JavaScript stack trace of the access.

#### `--trace-env-native-stack`#

Added in: v23.4.0, v22.13.0

In addition to what `--trace-env` does, this prints the native stack trace of the access.

#### `--trace-event-categories`#

Added in: v7.7.0

A comma separated list of categories that should be traced when trace event tracing is enabled using `--trace-events-enabled`.

#### `--trace-event-file-pattern`#

Added in: v9.8.0

Template string specifying the filepath for the trace event data, it supports `${rotation}` and `${pid}`.

#### `--trace-events-enabled`#

Added in: v7.7.0

Enables the collection of trace event tracing information.

#### `--trace-exit`#

Added in: v13.5.0, v12.16.0

Prints a stack trace whenever an environment is exited proactively, i.e. invoking `process.exit()`.

#### `--trace-require-module=mode`#

Added in: v23.5.0, v22.13.0, v20.19.0

Prints information about usage of Loading ECMAScript modules using `require()`.

When `mode` is `all`, all usage is printed. When `mode` is `no-node-modules`, usage from the `node_modules` folder is excluded.

#### `--trace-sigint`#

Added in: v13.9.0, v12.17.0

Prints a stack trace on SIGINT.

#### `--trace-sync-io`#

Added in: v2.1.0

Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.

#### `--trace-tls`#

Added in: v12.2.0

Prints TLS packet trace information to `stderr`. This can be used to debug TLS connection problems.

#### `--trace-uncaught`#

Added in: v13.1.0

Print stack traces for uncaught exceptions; usually, the stack trace associated with the creation of an `Error` is printed, whereas this makes Node.js also print the stack trace associated with throwing the value (which does not need to be an `Error` instance).

Enabling this option may affect garbage collection behavior negatively.

#### `--trace-warnings`#

Added in: v6.0.0

Print stack traces for process warnings (including deprecations).

#### `--track-heap-objects`#

Added in: v2.4.0

Track heap object allocations for heap snapshots.

#### `--unhandled-rejections=mode`#

Added in: v12.0.0, v10.17.0
History

Version | Changes
--- | ---
v15.0.0 | Changed default mode to `throw`. Previously, a warning was emitted.

Using this flag allows to change what should happen when an unhandled rejection occurs. One of the following modes can be chosen:

- `throw`: Emit `unhandledRejection`. If this hook is not set, raise the unhandled rejection as an uncaught exception. This is the default.
- `strict`: Raise the unhandled rejection as an uncaught exception. If the exception is handled, `unhandledRejection` is emitted.
- `warn`: Always trigger a warning, no matter if the `unhandledRejection` hook is set or not but do not print the deprecation warning.
- `warn-with-error-code`: Emit `unhandledRejection`. If this hook is not set, trigger a warning, and set the process exit code to 1.
- `none`: Silence all warnings.

If a rejection happens during the command line entry point's ES module static loading phase, it will always raise it as an uncaught exception.

#### `--use-bundled-ca`, `--use-openssl-ca`#

Added in: v6.11.0

Use bundled Mozilla CA store as supplied by current Node.js version or use OpenSSL's default CA store. The default store is selectable at build-time.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.

Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environment variables.

See `SSL_CERT_DIR` and `SSL_CERT_FILE`.

#### `--use-env-proxy`#

Added in: v24.5.0, v22.21.0

Stability: 1.1 - Active Development

When enabled, Node.js parses the `HTTP_PROXY`, `HTTPS_PROXY` and `NO_PROXY` environment variables during startup, and routes requests through the specified proxy.

Use this only with proxies that are trusted and authorized for the deployment. Proxy support is intended for reaching external networks through authorized proxy servers, for example when a firewall requires one. It is not for hiding traffic or evading network policy. See Built-in Proxy Support.

This is equivalent to setting the `NODE_USE_ENV_PROXY=1` environment variable. When both are set, `--use-env-proxy` takes precedence.

#### `--use-largepages=mode`#

Added in: v13.6.0, v12.17.0

Re-map the Node.js static code to large memory pages at startup. If supported on the target system, this will cause the Node.js static code to be moved onto 2 MiB pages instead of 4 KiB pages.

The following values are valid for `mode`:

- `off`: No mapping will be attempted. This is the default.
- `on`: If supported by the OS, mapping will be attempted. Failure to map will be ignored and a message will be printed to standard error.
- `silent`: If supported by the OS, mapping will be attempted. Failure to map will be ignored and will not be reported.

#### `--use-system-ca`#

Added in: v23.8.0
History

Version | Changes
--- | ---
v23.9.0 | Added support on non-Windows and non-macOS.

Node.js uses the trusted CA certificates present in the system store along with the `--use-bundled-ca` option and the `NODE_EXTRA_CA_CERTS` environment variable. On platforms other than Windows and macOS, this loads certificates from the directory and file trusted by OpenSSL, similar to `--use-openssl-ca`, with the difference being that it caches the certificates after first load.

On Windows and macOS, the certificate trust policy is similar to Chromium's policy for locally trusted certificates, but with some differences:

On macOS, the following settings are respected:

- Default and System Keychains
  - Trust:
    - Any certificate where the “When using this certificate” flag is set to “Always Trust” or
    - Any certificate where the “Secure Sockets Layer (SSL)” flag is set to “Always Trust”.
  - The certificate must also be valid, with "X.509 Basic Policy" set to “Always Trust”.

On Windows, the following settings are respected:

- Local Machine (accessed via `certlm.msc`)
  - Trust:
    - Trusted Root Certification Authorities
    - Trusted People
    - Enterprise Trust -> Enterprise -> Trusted Root Certification Authorities
    - Enterprise Trust -> Enterprise -> Trusted People
    - Enterprise Trust -> Group Policy -> Trusted Root Certification Authorities
    - Enterprise Trust -> Group Policy -> Trusted People
- Current User (accessed via `certmgr.msc`)
  - Trust:
    - Trusted Root Certification Authorities
    - Enterprise Trust -> Group Policy -> Trusted Root Certification Authorities

On Windows and macOS, Node.js would check that the user settings for the trusted certificates do not forbid them for TLS server authentication before using them.

Node.js currently does not support distrust/revocation of certificates from another source based on system settings.

On other systems, Node.js loads certificates from the default certificate file (typically `/etc/ssl/cert.pem`) and default certificate directory (typically `/etc/ssl/certs`) that the version of OpenSSL that Node.js links to respects. This typically works with the convention on major Linux distributions and other Unix-like systems. If the overriding OpenSSL environment variables (typically `SSL_CERT_FILE` and `SSL_CERT_DIR`, depending on the configuration of the OpenSSL that Node.js links to) are set, the specified paths will be used to load certificates instead. These environment variables can be used as workarounds if the conventional paths used by the version of OpenSSL Node.js links to are not consistent with the system configuration that the users have for some reason.

#### `--v8-options`#

Added in: v0.1.3

Print V8 command-line options.

#### `--v8-pool-size=num`#

Added in: v5.10.0

Set V8's thread pool size which will be used to allocate background jobs.

If set to `0` then Node.js will choose an appropriate size of the thread pool based on an estimate of the amount of parallelism.

The amount of parallelism refers to the number of computations that can be carried out simultaneously in a given machine. In general, it's the same as the amount of CPUs, but it may diverge in environments such as VMs or containers.

#### `-v`, `--version`#

Added in: v0.1.3

Print node's version.

#### `--watch`#

Added in: v18.11.0, v16.19.0
History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | Watch mode is now stable.
v19.2.0, v18.13.0 | Test runner now supports running in watch mode.

Starts Node.js in watch mode. When in watch mode, changes in the watched files cause the Node.js process to restart. By default, watch mode will watch the entry point and any required or imported module. Use `--watch-path` to specify what paths to watch.

This flag cannot be combined with `--check`, `--eval`, `--interactive`, or the REPL.

Note: The `--watch` flag requires a file path as an argument and is incompatible with `--run` or inline script input, as `--run` takes precedence and ignores watch mode. If no file is provided, Node.js will exit with status code `9`.

    node --watch index.js
    bash

#### `--watch-kill-signal`#

Added in: v24.4.0, v22.18.0

Stability: 1.1 - Active Development

Customizes the signal sent to the process on watch mode restarts.

    node --watch --watch-kill-signal SIGINT test.js
    bash

#### `--watch-path`#

Added in: v18.11.0, v16.19.0
History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | Watch mode is now stable.

Starts Node.js in watch mode and specifies what paths to watch. When in watch mode, changes in the watched paths cause the Node.js process to restart. This will turn off watching of required or imported modules, even when used in combination with `--watch`.

This flag cannot be combined with `--check`, `--eval`, `--interactive`, `--test`, or the REPL.

Note: Using `--watch-path` implicitly enables `--watch`, which requires a file path and is incompatible with `--run`, as `--run` takes precedence and ignores watch mode.

    node --watch-path=./src --watch-path=./tests index.js
    bash

This option is only supported on macOS and Windows. An `ERR_FEATURE_UNAVAILABLE_ON_PLATFORM` exception will be thrown when the option is used on a platform that does not support it.

#### `--watch-preserve-output`#

Added in: v19.3.0, v18.13.0

Disable the clearing of the console when watch mode restarts the process.

    node --watch --watch-preserve-output test.js
    bash

#### `--zero-fill-buffers`#

Added in: v6.0.0

Automatically zero-fills all newly allocated `Buffer` instances.

### Environment variables#

Stability: 2 - Stable

#### `FORCE_COLOR=[1, 2, 3]`#

The `FORCE_COLOR` environment variable is used to enable ANSI colorized output. The value may be:

- `1`, `true`, or the empty string `''` indicate 16-color support,
- `2` to indicate 256-color support, or
- `3` to indicate 16 million-color support.

When `FORCE_COLOR` is used and set to a supported value, both the `NO_COLOR`, and `NODE_DISABLE_COLORS` environment variables are ignored.

Any other value will result in colorized output being disabled.

#### `NODE_COMPILE_CACHE=dir`#

Added in: v22.1.0
History

Version | Changes
--- | ---
v25.4.0 | This feature is no longer experimental.

Enable the module compile cache for the Node.js instance. See the documentation of module compile cache for details.

#### `NODE_COMPILE_CACHE_PORTABLE=1`#

When set to 1, the module compile cache can be reused across different directory locations as long as the module layout relative to the cache directory remains the same.

#### `NODE_DEBUG=module[,…]`#

Added in: v0.1.32

`','`-separated list of core modules that should print debug information.

#### `NODE_DEBUG_NATIVE=module[,…]`#

`','`-separated list of core C++ modules that should print debug information.

#### `NODE_DISABLE_COLORS=1`#

Added in: v0.3.0

When set, colors will not be used in the REPL.

#### `NODE_DISABLE_COMPILE_CACHE=1`#

Added in: v22.8.0

Stability: 1.1 - Active Development

Disable the module compile cache for the Node.js instance. See the documentation of module compile cache for details.

#### `NODE_EXTRA_CA_CERTS=file`#

Added in: v7.3.0

When set, the well known "root" CAs (like VeriSign) will be extended with the extra certificates in `file`. The file should consist of one or more trusted certificates in PEM format. A message will be emitted (once) with `process.emitWarning()` if the file is missing or malformed, but any errors are otherwise ignored.

Neither the well known nor extra certificates are used when the `ca` options property is explicitly specified for a TLS or HTTPS client or server.

This environment variable is ignored when `node` runs as setuid root or has Linux file capabilities set.

The `NODE_EXTRA_CA_CERTS` environment variable is only read when the Node.js process is first launched. Changing the value at runtime using `process.env.NODE_EXTRA_CA_CERTS` has no effect on the current process.

#### `NODE_ICU_DATA=file`#

Added in: v0.11.15

Data path for ICU (`Intl` object) data. Will extend linked-in data when compiled with small-icu support.

#### `NODE_NO_WARNINGS=1`#

Added in: v6.11.0

When set to `1`, process warnings are silenced.

#### `NODE_OPTIONS=options...`#

Added in: v8.0.0

A space-separated list of command-line options. `options...` are interpreted before command-line options, so command-line options will override or compound after anything in `options...`. Node.js will exit with an error if an option that is not allowed in the environment is used, such as `-p` or a script file.

If an option value contains a space, it can be escaped using double quotes:

    NODE_OPTIONS='--require "./my path/file.js"'
    bash

A singleton flag passed as a command-line option will override the same flag passed into `NODE_OPTIONS`:

    # The inspector will be available on port 5555
    NODE_OPTIONS='--inspect=localhost:4444' node --inspect=localhost:5555
    bash

A flag that can be passed multiple times will be treated as if its `NODE_OPTIONS` instances were passed first, and then its command-line instances afterwards:

    NODE_OPTIONS='--require "./a.js"' node --require "./b.js"
    # is equivalent to:
    node --require "./a.js" --require "./b.js"
    bash

Node.js options that are allowed are in the following list. If an option supports both --XX and --no-XX variants, they are both supported but only one is included in the list below.

- `--allow-addons`
- `--allow-child-process`
- `--allow-ffi`
- `--allow-fs-read`
- `--allow-fs-write`
- `--allow-inspector`
- `--allow-net`
- `--allow-openssl-store`
- `--allow-wasi`
- `--allow-worker`
- `--conditions`, `-C`
- `--cpu-prof-dir`
- `--cpu-prof-interval`
- `--cpu-prof-name`
- `--cpu-prof`
- `--diagnostic-dir`
- `--disable-proto`
- `--disable-sigusr1`
- `--disable-warning`
- `--disable-wasm-trap-handler`
- `--dns-result-order`
- `--enable-fips`
- `--enable-network-family-autoselection`
- `--enable-source-maps`
- `--entry-url`
- `--experimental-abortcontroller`
- `--experimental-addon-modules`
- `--experimental-detect-module`
- `--experimental-eventsource`
- `--experimental-ffi`
- `--experimental-import-meta-resolve`
- `--experimental-import-text`
- `--experimental-json-modules`
- `--experimental-loader`
- `--experimental-modules`
- `--experimental-package-map`
- `--experimental-print-required-tla`
- `--experimental-quic`
- `--experimental-require-module`
- `--experimental-shadow-realm`
- `--experimental-specifier-resolution`
- `--experimental-stream-iter`
- `--experimental-test-isolation`
- `--experimental-top-level-await`
- `--experimental-vfs`
- `--experimental-vm-modules`
- `--experimental-wasi-unstable-preview1`
- `--force-context-aware`
- `--force-fips`
- `--force-node-api-uncaught-exceptions-policy`
- `--frozen-intrinsics`
- `--heap-prof-dir`
- `--heap-prof-interval`
- `--heap-prof-name`
- `--heap-prof`
- `--heapsnapshot-near-heap-limit`
- `--heapsnapshot-signal`
- `--http-parser`
- `--icu-data-dir`
- `--import`
- `--input-type`
- `--insecure-http-parser`
- `--inspect-brk`
- `--inspect-port`, `--debug-port`
- `--inspect-publish-uid`
- `--inspect-wait`
- `--inspect`
- `--localstorage-file`
- `--max-http-header-size`
- `--max-old-space-size-percentage`
- `--network-family-autoselection-attempt-timeout`
- `--no-addons`
- `--no-async-context-frame`
- `--no-deprecation`
- `--no-experimental-global-navigator`
- `--no-experimental-repl-await`
- `--no-experimental-sqlite`
- `--no-experimental-strip-types`
- `--no-experimental-websocket`
- `--no-experimental-webstorage`
- `--no-extra-info-on-fatal-exception`
- `--no-force-async-hooks-checks`
- `--no-global-search-paths`
- `--no-network-family-autoselection`
- `--no-strip-types`
- `--no-warnings`
- `--no-webstorage`
- `--node-memory-debug`
- `--openssl-config`
- `--openssl-legacy-provider`
- `--openssl-shared-config`
- `--pending-deprecation`
- `--permission-audit`
- `--permission`
- `--preserve-symlinks-main`
- `--preserve-symlinks`
- `--prof-process`
- `--redirect-warnings`
- `--report-compact`
- `--report-dir`, `--report-directory`
- `--report-exclude-env`
- `--report-exclude-network`
- `--report-filename`
- `--report-on-fatalerror`
- `--report-on-signal`
- `--report-signal`
- `--report-uncaught-exception`
- `--require-module`
- `--require`, `-r`
- `--secure-heap-min`
- `--secure-heap`
- `--snapshot-blob`
- `--test-coverage-branches`
- `--test-coverage-exclude`
- `--test-coverage-functions`
- `--test-coverage-include-all`
- `--test-coverage-include`
- `--test-coverage-lines`
- `--test-global-setup`
- `--test-isolation`
- `--test-name-pattern`
- `--test-only`
- `--test-random-seed`
- `--test-randomize`
- `--test-reporter-destination`
- `--test-reporter`
- `--test-rerun-failures`
- `--test-shard`
- `--test-skip-pattern`
- `--throw-deprecation`
- `--title`
- `--tls-cipher-list`
- `--tls-keylog`
- `--tls-max-v1.2`
- `--tls-max-v1.3`
- `--tls-min-v1.0`
- `--tls-min-v1.1`
- `--tls-min-v1.2`
- `--tls-min-v1.3`
- `--trace-deprecation`
- `--trace-env-js-stack`
- `--trace-env-native-stack`
- `--trace-env`
- `--trace-event-categories`
- `--trace-event-file-pattern`
- `--trace-events-enabled`
- `--trace-exit`
- `--trace-require-module`
- `--trace-sigint`
- `--trace-sync-io`
- `--trace-tls`
- `--trace-uncaught`
- `--trace-warnings`
- `--track-heap-objects`
- `--unhandled-rejections`
- `--use-bundled-ca`
- `--use-env-proxy`
- `--use-largepages`
- `--use-openssl-ca`
- `--use-system-ca`
- `--v8-pool-size`
- `--watch-kill-signal`
- `--watch-path`
- `--watch-preserve-output`
- `--watch`
- `--zero-fill-buffers`

V8 options that are allowed are:

- `--abort-on-uncaught-exception`
- `--disallow-code-generation-from-strings`
- `--enable-etw-stack-walking`
- `--expose-gc`
- `--interpreted-frames-native-stack`
- `--jitless`
- `--max-heap-size`
- `--max-old-space-size`
- `--max-semi-space-size`
- `--perf-basic-prof-only-functions`
- `--perf-basic-prof`
- `--perf-prof-unwinding-info`
- `--perf-prof`
- `--stack-trace-limit`

`--perf-basic-prof-only-functions`, `--perf-basic-prof`, `--perf-prof-unwinding-info`, and `--perf-prof` are only available on Linux.

`--enable-etw-stack-walking` is only available on Windows.

#### `NODE_PATH=path[:…]`#

Added in: v0.1.32

`':'`-separated list of directories prefixed to the module search path.

On Windows, this is a `';'`-separated list instead.

#### `NODE_PENDING_DEPRECATION=1`#

Added in: v8.0.0

When set to `1`, emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the `--pending-deprecation` command-line flag, or the `NODE_PENDING_DEPRECATION=1` environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage.

#### `NODE_PENDING_PIPE_INSTANCES=instances`#

Set the number of pending pipe instance handles when the pipe server is waiting for connections. This setting applies to Windows only.

#### `NODE_PRESERVE_SYMLINKS=1`#

Added in: v7.1.0

When set to `1`, instructs the module loader to preserve symbolic links when resolving and caching modules.

#### `NODE_REDIRECT_WARNINGS=file`#

Added in: v8.0.0

When set, process warnings will be emitted to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. This is equivalent to using the `--redirect-warnings=file` command-line flag.

#### `NODE_REPL_EXTERNAL_MODULE=file`#

Added in: v13.0.0, v12.16.0
History

Version | Changes
--- | ---
v22.3.0, v20.16.0 | Remove the possibility to use this env var with kDisableNodeOptionsEnv for embedders.

Path to a Node.js module which will be loaded in place of the built-in REPL. Overriding this value to an empty string (`''`) will use the built-in REPL.

#### `NODE_REPL_HISTORY=file`#

Added in: v3.0.0

Path to the file used to store the persistent REPL history. The default path is `~/.node_repl_history`, which is overridden by this variable. Setting the value to an empty string (`''` or `' '`) disables persistent REPL history.

#### `NODE_SKIP_PLATFORM_CHECK=value`#

Added in: v14.5.0

If `value` equals `'1'`, the check for a supported platform is skipped during Node.js startup. Node.js might not execute correctly. Any issues encountered on unsupported platforms will not be fixed.

#### `NODE_TEST_CONTEXT=value`#

If `value` equals `'child'`, test reporter options will be overridden and test output will be sent to stdout in the TAP format. If any other value is provided, Node.js makes no guarantees about the reporter format used or its stability.

#### `NODE_TLS_REJECT_UNAUTHORIZED=value`#

If `value` equals `'0'`, certificate validation is disabled for TLS connections. This makes TLS, and HTTPS by extension, insecure. The use of this environment variable is strongly discouraged.

#### `NODE_USE_ENV_PROXY=1`#

Added in: v24.0.0, v22.21.0

Stability: 1.1 - Active Development

When enabled, Node.js parses the `HTTP_PROXY`, `HTTPS_PROXY` and `NO_PROXY` environment variables during startup, and routes requests through the specified proxy.

Use this only with proxies that are trusted and authorized for the deployment. Proxy support is intended for reaching external networks through authorized proxy servers, for example when a firewall requires one. It is not for hiding traffic or evading network policy. See Built-in Proxy Support.

This can also be enabled using the `--use-env-proxy` command-line flag. When both are set, `--use-env-proxy` takes precedence.

#### `NODE_USE_SYSTEM_CA=1`#

Added in: v24.6.0, v22.19.0

Node.js uses the trusted CA certificates present in the system store along with the `--use-bundled-ca` option and the `NODE_EXTRA_CA_CERTS` environment variable.

This can also be enabled using the `--use-system-ca` command-line flag. When both are set, `--use-system-ca` takes precedence.

#### `NODE_V8_COVERAGE=dir`#

When set, Node.js will begin outputting V8 JavaScript code coverage and Source Map data to the directory provided as an argument (coverage information is written as JSON to files with a `coverage` prefix).

`NODE_V8_COVERAGE` will automatically propagate to subprocesses, making it easier to instrument applications that call the `child_process.spawn()` family of functions. `NODE_V8_COVERAGE` can be set to an empty string, to prevent propagation.

##### Coverage output#

Coverage is output as an array of ScriptCoverage objects on the top-level key `result`:

    {
      "result": [
        {
          "scriptId": "67",
          "url": "internal/tty.js",
          "functions": []
        }
      ]
    }
    json

##### Source map cache#

Stability: 1 - Experimental

If found, source map data is appended to the top-level key `source-map-cache` on the JSON coverage object.

`source-map-cache` is an object with keys representing the files source maps were extracted from, and values which include the raw source-map URL (in the key `url`), the parsed Source Map v3 information (in the key `data`), and the line lengths of the source file (in the key `lineLengths`).

    {
      "result": [
        {
          "scriptId": "68",
          "url": "file:///absolute/path/to/source.js",
          "functions": []
        }
      ],
      "source-map-cache": {
        "file:///absolute/path/to/source.js": {
          "url": "./path-to-map.json",
          "data": {
            "version": 3,
            "sources": [
              "file:///absolute/path/to/original.js"
            ],
            "names": [
              "Foo",
              "console",
              "info"
            ],
            "mappings": "MAAMA,IACJC,YAAaC",
            "sourceRoot": "./"
          },
          "lineLengths": [
            13,
            62,
            38,
            27
          ]
        }
      }
    }
    json

#### `NO_COLOR=<any>`#

`NO_COLOR` is an alias for `NODE_DISABLE_COLORS`. The value of the environment variable is arbitrary.

#### `OPENSSL_CONF=file`#

Added in: v6.11.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with `./configure --openssl-fips`.

If the `--openssl-config` command-line option is used, the environment variable is ignored.

#### `SSL_CERT_DIR=dir`#

Added in: v7.7.0

If `--use-openssl-ca` is enabled, or if `--use-system-ca` is enabled on platforms other than macOS and Windows, this overrides and sets OpenSSL's directory containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

#### `SSL_CERT_FILE=file`#

Added in: v7.7.0

If `--use-openssl-ca` is enabled, or if `--use-system-ca` is enabled on platforms other than macOS and Windows, this overrides and sets OpenSSL's file containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

#### `TZ`#

Added in: v0.0.1
History

Version | Changes
--- | ---
v16.2.0 | Changing the TZ variable using process.env.TZ = changes the timezone on Windows as well.
v13.0.0 | Changing the TZ variable using process.env.TZ = changes the timezone on POSIX systems.

The `TZ` environment variable is used to specify the timezone configuration.

While Node.js does not support all of the various ways that `TZ` is handled in other environments, it does support basic timezone IDs (such as `'Etc/UTC'`, `'Europe/Paris'`, or `'America/New_York'`). It may support a few other abbreviations or aliases, but these are strongly discouraged and not guaranteed.

    $ TZ=Europe/Dublin node -pe "new Date().toString()"
    Wed May 12 2021 20:30:48 GMT+0100 (Irish Standard Time)
    console

#### `UV_THREADPOOL_SIZE=size`#

Set the number of threads used in libuv's threadpool to `size` threads.

Asynchronous system APIs are used by Node.js whenever possible, but where they do not exist, libuv's threadpool is used to create asynchronous node APIs based on synchronous system APIs. Node.js APIs that use the threadpool are:

- all `fs` APIs, other than the file watcher APIs and those that are explicitly synchronous
- asynchronous crypto APIs such as `crypto.pbkdf2()`, `crypto.scrypt()`, `crypto.randomBytes()`, `crypto.randomFill()`, `crypto.generateKeyPair()`
- `dns.lookup()`
- all `zlib` APIs, other than those that are explicitly synchronous

Because libuv's threadpool has a fixed size, it means that if for whatever reason any of these APIs takes a long time, other (seemingly unrelated) APIs that run in libuv's threadpool will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the `'UV_THREADPOOL_SIZE'` environment variable to a value greater than `4` (its current default value). However, setting this from inside the process using `process.env.UV_THREADPOOL_SIZE=size` is not guaranteed to work as the threadpool would have been created as part of the runtime initialisation much before user code is run. For more information, see the libuv threadpool documentation.

### Useful V8 options#

V8 has its own set of CLI options. Any V8 CLI option that is provided to `node` will be passed on to V8 to handle. V8's options have no stability guarantee. The V8 team themselves don't consider them to be part of their formal API, and reserve the right to change them at any time. Likewise, they are not covered by the Node.js stability guarantees. Many of the V8 options are of interest only to V8 developers. Despite this, there is a small set of V8 options that are widely applicable to Node.js, and they are documented here:

#### `--abort-on-uncaught-exception`#

#### `--disallow-code-generation-from-strings`#

#### `--enable-etw-stack-walking`#

#### `--expose-gc`#

#### `--harmony-shadow-realm`#

#### `--heap-snapshot-on-oom`#

#### `--interpreted-frames-native-stack`#

#### `--jitless`#

#### `--max-heap-size`#

Specifies the maximum heap size (in megabytes) for the process.

This option is typically used to limit the amount of memory the process can use for its JavaScript heap.

#### `--max-old-space-size=SIZE` (in MiB)#

Sets the max memory size of V8's old memory section. As memory consumption approaches the limit, V8 will spend more time on garbage collection in an effort to free unused memory.

On a machine with 2 GiB of memory, consider setting this to 1536 (1.5 GiB) to leave some memory for other uses and avoid swapping.

    node --max-old-space-size=1536 index.js
    bash

#### `--max-semi-space-size=SIZE` (in MiB)#

Sets the maximum semi-space size for V8's scavenge garbage collector in MiB (mebibytes). Increasing the max size of a semi-space may improve throughput for Node.js at the cost of more memory consumption.

Since the young generation size of the V8 heap is three times (see `YoungGenerationSizeFromSemiSpaceSize` in V8) the size of the semi-space, an increase of 1 MiB to semi-space applies to each of the three individual semi-spaces and causes the heap size to increase by 3 MiB. The throughput improvement depends on your workload (see #42511).

The default value depends on the memory limit. For example, on 64-bit systems with a memory limit of 512 MiB, the max size of a semi-space defaults to 1 MiB. For memory limits up to and including 2GiB, the default max size of a semi-space will be less than 16 MiB on 64-bit systems.

To get the best configuration for your application, you should try different max-semi-space-size values when running benchmarks for your application.

For example, benchmark on a 64-bit systems:

    for MiB in 16 32 64 128; do
        node --max-semi-space-size=$MiB index.js
    done
    bash

#### `--perf-basic-prof`#

#### `--perf-basic-prof-only-functions`#

#### `--perf-prof`#

#### `--perf-prof-unwinding-info`#

#### `--prof`#

#### `--security-revert`#

#### `--stack-trace-limit=limit`#

The maximum number of stack frames to collect in an error's stack trace. Setting it to 0 disables stack trace collection. The default value is 10.

    node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
    bash

## V8#

Source Code: lib/v8.js

The `node:v8` module exposes APIs that are specific to the version of V8 built into the Node.js binary. It can be accessed using:

    import v8 from 'node:v8';
    const v8 = require('node:v8');
    javascript

### `v8.cachedDataVersionTag()`#

Added in: v8.0.0

- Returns: `<integer>`

Returns an integer representing a version tag derived from the V8 version, command-line flags, and detected CPU features. This is useful for determining whether a `vm.Script` `cachedData` buffer is compatible with this instance of V8.

    console.log(v8.cachedDataVersionTag()); // 3947234607
    // The value returned by v8.cachedDataVersionTag() is derived from the V8
    // version, command-line flags, and detected CPU features. Test that the value
    // does indeed update when flags are toggled.
    v8.setFlagsFromString('--allow_natives_syntax');
    console.log(v8.cachedDataVersionTag()); // 183726201
    js

### `v8.getHeapCodeStatistics()`#

Added in: v12.8.0

- Returns: `<Object>`

Get statistics about code and its metadata in the heap, see V8 `GetHeapCodeAndMetadataStatistics` API. Returns an object with the following properties:

- `code_and_metadata_size` `<number>`
- `bytecode_and_metadata_size` `<number>`
- `external_script_source_size` `<number>`
- `cpu_profiler_metadata_size` `<number>`

    {
      "code_and_metadata_size": 212208,
      "bytecode_and_metadata_size": 161368,
      "external_script_source_size": 1410794,
      "cpu_profiler_metadata_size": 0
    }
    json

### `v8.getHeapSnapshot([options])`#

Added in: v11.13.0
History

Version | Changes
--- | ---
v19.1.0 | Support options to configure the heap snapshot.

- `options` `<Object>`

  - `exposeInternals` `<boolean>` If true, expose internals in the heap snapshot. Default: `false`.
  - `exposeNumericValues` `<boolean>` If true, expose numeric values in artificial fields. Default: `false`.

- Returns: `<stream.Readable>` A Readable containing the V8 heap snapshot.

Generates a snapshot of the current V8 heap and returns a Readable Stream that may be used to read the JSON serialized representation. This JSON stream format is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine. Therefore, the schema may change from one version of V8 to the next.

Creating a heap snapshot requires memory about twice the size of the heap at the time the snapshot is created. This results in the risk of OOM killers terminating the process.

Generating a snapshot is a synchronous operation which blocks the event loop for a duration depending on the heap size.

    // Print heap snapshot to the console
    import { getHeapSnapshot } from 'node:v8';
    import process from 'node:process';
    const stream = getHeapSnapshot();
    stream.pipe(process.stdout);
    mjs

    // Print heap snapshot to the console
    const v8 = require('node:v8');
    const stream = v8.getHeapSnapshot();
    stream.pipe(process.stdout);
    js

### `v8.getHeapSpaceStatistics()`#

Added in: v6.0.0
History

Version | Changes
--- | ---
v7.5.0 | Support values exceeding the 32-bit unsigned integer range.

- Returns: `<Object>`[]

Returns statistics about the V8 heap spaces, i.e. the segments which make up the V8 heap. Neither the ordering of heap spaces, nor the availability of a heap space can be guaranteed as the statistics are provided via the V8 `GetHeapSpaceStatistics` function and may change from one V8 version to the next.

The value returned is an array of objects containing the following properties:

- `space_name` `<string>`
- `space_size` `<number>`
- `space_used_size` `<number>`
- `space_available_size` `<number>`
- `physical_space_size` `<number>`

    [
      {
        "space_name": "new_space",
        "space_size": 2063872,
        "space_used_size": 951112,
        "space_available_size": 80824,
        "physical_space_size": 2063872
      },
      {
        "space_name": "old_space",
        "space_size": 3090560,
        "space_used_size": 2493792,
        "space_available_size": 0,
        "physical_space_size": 3090560
      },
      {
        "space_name": "code_space",
        "space_size": 1260160,
        "space_used_size": 644256,
        "space_available_size": 960,
        "physical_space_size": 1260160
      },
      {
        "space_name": "map_space",
        "space_size": 1094160,
        "space_used_size": 201608,
        "space_available_size": 0,
        "physical_space_size": 1094160
      },
      {
        "space_name": "large_object_space",
        "space_size": 0,
        "space_used_size": 0,
        "space_available_size": 1490980608,
        "physical_space_size": 0
      }
    ]
    json

### `v8.getHeapStatistics()`#

Added in: v1.0.0
History

Version | Changes
--- | ---
v7.5.0 | Support values exceeding the 32-bit unsigned integer range.
v7.2.0 | Added `malloced_memory`, `peak_malloced_memory`, and `does_zap_garbage`.

- Returns: `<Object>`

Returns an object with the following properties:

- `total_heap_size` `<number>`
- `total_heap_size_executable` `<number>`
- `total_physical_size` `<number>`
- `total_available_size` `<number>`
- `used_heap_size` `<number>`
- `heap_size_limit` `<number>`
- `malloced_memory` `<number>`
- `peak_malloced_memory` `<number>`
- `does_zap_garbage` `<number>`
- `number_of_native_contexts` `<number>`
- `number_of_detached_contexts` `<number>`
- `total_global_handles_size` `<number>`
- `used_global_handles_size` `<number>`
- `external_memory` `<number>`
- `total_allocated_bytes` `<number>`

`total_heap_size` The value of total_heap_size is the number of bytes V8 has allocated for the heap. This can grow if used_heap needs more memory.

`total_heap_size_executable` The value of total_heap_size_executable is the portion of the heap that can contain executable code, in bytes. This includes memory used by JIT-compiled code and any memory that must be kept executable.

`total_physical_size` The value of total_physical_size is the actual physical memory used by the V8 heap, in bytes. This is the amount of memory that is committed (or in use) rather than reserved.

`total_available_size` The value of total_available_size is the number of bytes of memory available to the V8 heap. This value represents how much more memory V8 can use before it exceeds the heap limit.

`used_heap_size` The value of used_heap_size is number of bytes currently being used by V8’s JavaScript objects. This is the actual memory in use and does not include memory that has been allocated but not yet used.

`heap_size_limit` The value of heap_size_limit is the maximum size of the V8 heap, in bytes (either the default limit, determined by system resources, or the value passed to the `--max_old_space_size` option).

`malloced_memory` The value of malloced_memory is the number of bytes allocated through `malloc` by V8.

`peak_malloced_memory` The value of peak_malloced_memory is the peak number of bytes allocated through `malloc` by V8 during the lifetime of the process.

`does_zap_garbage` is a 0/1 boolean, which signifies whether the `--zap_code_space` option is enabled or not. This makes V8 overwrite heap garbage with a bit pattern. The RSS footprint (resident set size) gets bigger because it continuously touches all heap pages and that makes them less likely to get swapped out by the operating system.

`number_of_native_contexts` The value of native_context is the number of the top-level contexts currently active. Increase of this number over time indicates a memory leak.

`number_of_detached_contexts` The value of detached_context is the number of contexts that were detached and not yet garbage collected. This number being non-zero indicates a potential memory leak.

`total_global_handles_size` The value of total_global_handles_size is the total memory size of V8 global handles.

`used_global_handles_size` The value of used_global_handles_size is the used memory size of V8 global handles.

`external_memory` The value of external_memory is the memory size of array buffers and external strings.

`total_allocated_bytes` The value of total allocated bytes since the Isolate creation

    {
      "total_heap_size": 7326976,
      "total_heap_size_executable": 4194304,
      "total_physical_size": 7326976,
      "total_available_size": 1152656,
      "used_heap_size": 3476208,
      "heap_size_limit": 1535115264,
      "malloced_memory": 16384,
      "peak_malloced_memory": 1127496,
      "does_zap_garbage": 0,
      "number_of_native_contexts": 1,
      "number_of_detached_contexts": 0,
      "total_global_handles_size": 8192,
      "used_global_handles_size": 3296,
      "external_memory": 318824
    }
    json

### `v8.getCppHeapStatistics([detailLevel])`#

Retrieves CppHeap statistics regarding memory consumption and utilization using the V8 `CollectStatistics()` function which may change from one V8 version to the next.

- `detailLevel` `<string>` | `<undefined>`: Default: `'detailed'`. Specifies the level of detail in the returned statistics. Accepted values are:
  - `'brief'`: Brief statistics contain only the top-level allocated and used memory statistics for the entire heap.
  - `'detailed'`: Detailed statistics also contain a break down per space and page, as well as freelist statistics and object type histograms.

It returns an object with a structure similar to the `cppgc::HeapStatistics` object. See the V8 documentation for more information about the properties of the object.

    // Detailed
    ({
      committed_size_bytes: 131072,
      resident_size_bytes: 131072,
      used_size_bytes: 152,
      space_statistics: [
        {
          name: 'NormalPageSpace0',
          committed_size_bytes: 0,
          resident_size_bytes: 0,
          used_size_bytes: 0,
          page_stats: [{}],
          free_list_stats: {},
        },
        {
          name: 'NormalPageSpace1',
          committed_size_bytes: 131072,
          resident_size_bytes: 131072,
          used_size_bytes: 152,
          page_stats: [{}],
          free_list_stats: {},
        },
        {
          name: 'NormalPageSpace2',
          committed_size_bytes: 0,
          resident_size_bytes: 0,
          used_size_bytes: 0,
          page_stats: [{}],
          free_list_stats: {},
        },
        {
          name: 'NormalPageSpace3',
          committed_size_bytes: 0,
          resident_size_bytes: 0,
          used_size_bytes: 0,
          page_stats: [{}],
          free_list_stats: {},
        },
        {
          name: 'LargePageSpace',
          committed_size_bytes: 0,
          resident_size_bytes: 0,
          used_size_bytes: 0,
          page_stats: [{}],
          free_list_stats: {},
        },
      ],
      type_names: [],
      detail_level: 'detailed',
    });
    js

    // Brief
    ({
      committed_size_bytes: 131072,
      resident_size_bytes: 131072,
      used_size_bytes: 128864,
      space_statistics: [],
      type_names: [],
      detail_level: 'brief',
    });
    js

### `v8.queryObjects(ctor[, options])`#

Added in: v22.0.0, v20.13.0
History

Version | Changes
--- | ---
v25.4.0, v24.13.1 | This API is no longer experimental.

- `ctor` `<Function>` The constructor that can be used to search on the prototype chain in order to filter target objects in the heap.
- `options` `<undefined>` | `<Object>`
  - `format` `<string>` If it's `'count'`, the count of matched objects is returned. If it's `'summary'`, an array with summary strings of the matched objects is returned.
- Returns: `<number>` | `<string>`[]

This is similar to the `queryObjects()` console API provided by the Chromium DevTools console. It can be used to search for objects that have the matching constructor on its prototype chain in the heap after a full garbage collection, which can be useful for memory leak regression tests. To avoid surprising results, users should avoid using this API on constructors whose implementation they don't control, or on constructors that can be invoked by other parties in the application.

To avoid accidental leaks, this API does not return raw references to the objects found. By default, it returns the count of the objects found. If `options.format` is `'summary'`, it returns an array containing brief string representations for each object. The visibility provided in this API is similar to what the heap snapshot provides, while users can save the cost of serialization and parsing and directly filter the target objects during the search.

Only objects created in the current execution context are included in the results.

    const { queryObjects } = require('node:v8');
    class A { foo = 'bar'; }
    console.log(queryObjects(A)); // 0
    const a = new A();
    console.log(queryObjects(A)); // 1
    // [ "A { foo: 'bar' }" ]
    console.log(queryObjects(A, { format: 'summary' }));

    class B extends A { bar = 'qux'; }
    const b = new B();
    console.log(queryObjects(B)); // 1
    // [ "B { foo: 'bar', bar: 'qux' }" ]
    console.log(queryObjects(B, { format: 'summary' }));

    // Note that, when there are child classes inheriting from a constructor,
    // the constructor also shows up in the prototype chain of the child
    // classes's prototype, so the child classes's prototype would also be
    // included in the result.
    console.log(queryObjects(A));  // 3
    // [ "B { foo: 'bar', bar: 'qux' }", 'A {}', "A { foo: 'bar' }" ]
    console.log(queryObjects(A, { format: 'summary' }));
    import { queryObjects } from 'node:v8';
    class A { foo = 'bar'; }
    console.log(queryObjects(A)); // 0
    const a = new A();
    console.log(queryObjects(A)); // 1
    // [ "A { foo: 'bar' }" ]
    console.log(queryObjects(A, { format: 'summary' }));

    class B extends A { bar = 'qux'; }
    const b = new B();
    console.log(queryObjects(B)); // 1
    // [ "B { foo: 'bar', bar: 'qux' }" ]
    console.log(queryObjects(B, { format: 'summary' }));

    // Note that, when there are child classes inheriting from a constructor,
    // the constructor also shows up in the prototype chain of the child
    // classes's prototype, so the child classes's prototype would also be
    // included in the result.
    console.log(queryObjects(A));  // 3
    // [ "B { foo: 'bar', bar: 'qux' }", 'A {}', "A { foo: 'bar' }" ]
    console.log(queryObjects(A, { format: 'summary' }));
    javascript

### `v8.setFlagsFromString(flags)`#

Added in: v1.0.0

- `flags` `<string>`

The `v8.setFlagsFromString()` method can be used to programmatically set V8 command-line flags. This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

The V8 options available for a version of Node.js may be determined by running `node --v8-options`.

Usage:

    import { setFlagsFromString } from 'node:v8';
    import { setInterval } from 'node:timers';

    // setFlagsFromString to trace garbage collection events
    setFlagsFromString('--trace-gc');

    // Trigger GC events by using some memory
    let arrays = [];
    const interval = setInterval(() => {
      for (let i = 0; i < 500; i++) {
        arrays.push(new Array(10000).fill(Math.random()));
      }

      if (arrays.length > 5000) {
        arrays = arrays.slice(-1000);
      }

      console.log(`\n* Created ${arrays.length} arrays\n`);
    }, 100);

    // setFlagsFromString to stop tracing GC events after 1.5 seconds
    setTimeout(() => {
      setFlagsFromString('--notrace-gc');
      console.log('\nStopped tracing!\n');
    }, 1500);

    // Stop triggering GC events altogether after 2.5 seconds
    setTimeout(() => {
      clearInterval(interval);
    }, 2500);
    const { setFlagsFromString } = require('node:v8');
    const { setInterval } = require('node:timers');

    // setFlagsFromString to trace garbage collection events
    setFlagsFromString('--trace-gc');

    // Trigger GC events by using some memory
    let arrays = [];
    const interval = setInterval(() => {
      for (let i = 0; i < 500; i++) {
        arrays.push(new Array(10000).fill(Math.random()));
      }

      if (arrays.length > 5000) {
        arrays = arrays.slice(-1000);
      }

      console.log(`\n* Created ${arrays.length} arrays\n`);
    }, 100);

    // setFlagsFromString to stop tracing GC events after 1.5 seconds
    setTimeout(() => {
      console.log('\nStopped tracing!\n');
      setFlagsFromString('--notrace-gc');
    }, 1500);

    // Stop triggering GC events altogether after 2.5 seconds
    setTimeout(() => {
      clearInterval(interval);
    }, 2500);
    javascript

### `v8.stopCoverage()`#

Added in: v15.1.0, v14.18.0, v12.22.0

The `v8.stopCoverage()` method allows the user to stop the coverage collection started by `NODE_V8_COVERAGE`, so that V8 can release the execution count records and optimize code. This can be used in conjunction with `v8.takeCoverage()` if the user wants to collect the coverage on demand.

### `v8.takeCoverage()`#

Added in: v15.1.0, v14.18.0, v12.22.0

The `v8.takeCoverage()` method allows the user to write the coverage started by `NODE_V8_COVERAGE` to disk on demand. This method can be invoked multiple times during the lifetime of the process. Each time the execution counter will be reset and a new coverage report will be written to the directory specified by `NODE_V8_COVERAGE`.

When the process is about to exit, one last coverage will still be written to disk unless `v8.stopCoverage()` is invoked before the process exits.

### `v8.writeHeapSnapshot([filename[,options]])`#

Added in: v11.13.0
History

Version | Changes
--- | ---
v19.1.0 | Support options to configure the heap snapshot.
v18.0.0 | An exception will now be thrown if the file could not be written.
v18.0.0 | Make the returned error codes consistent across all platforms.

- `filename` `<string>` The file path where the V8 heap snapshot is to be saved. If not specified, a file name with the pattern `'Heap-${yyyymmdd}-${hhmmss}-${pid}-${thread_id}.heapsnapshot'` will be generated, where `{pid}` will be the PID of the Node.js process, `{thread_id}` will be `0` when `writeHeapSnapshot()` is called from the main Node.js thread or the id of a worker thread.
- `options` `<Object>`
  - `exposeInternals` `<boolean>` If true, expose internals in the heap snapshot. Default: `false`.
  - `exposeNumericValues` `<boolean>` If true, expose numeric values in artificial fields. Default: `false`.
- Returns: `<string>` The filename where the snapshot was saved.

Generates a snapshot of the current V8 heap and writes it to a JSON file. This file is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine, and may change from one version of V8 to the next.

A heap snapshot is specific to a single V8 isolate. When using worker threads, a heap snapshot generated from the main thread will not contain any information about the workers, and vice versa.

Creating a heap snapshot requires memory about twice the size of the heap at the time the snapshot is created. This results in the risk of OOM killers terminating the process.

Generating a snapshot is a synchronous operation which blocks the event loop for a duration depending on the heap size.

    import { writeHeapSnapshot } from 'node:v8';
    import { Worker, isMainThread, parentPort } from 'node:worker_threads';
    import { fileURLToPath } from 'node:url';

    if (isMainThread) {
      const __filename = fileURLToPath(import.meta.url);
      const worker = new Worker(__filename);

      worker.once('message', (filename) => {
        console.log(`worker heapdump: ${filename}`);
        // Now get a heapdump for the main thread.
        console.log(`main thread heapdump: ${writeHeapSnapshot()}`);
      });

      // Tell the worker to create a heapdump.
      worker.postMessage('heapdump');
    } else {
      parentPort.once('message', (message) => {
        if (message === 'heapdump') {
          // Generate a heapdump for the worker
          // and return the filename to the parent.
          parentPort.postMessage(writeHeapSnapshot());
        }
      });
    }
    const { writeHeapSnapshot } = require('node:v8');
    const { Worker, isMainThread, parentPort } = require('node:worker_threads');

    if (isMainThread) {
      const worker = new Worker(__filename);

      worker.once('message', (filename) => {
        console.log(`worker heapdump: ${filename}`);
        // Now get a heapdump for the main thread.
        console.log(`main thread heapdump: ${writeHeapSnapshot()}`);
      });

      // Tell the worker to create a heapdump.
      worker.postMessage('heapdump');
    } else {
      parentPort.once('message', (message) => {
        if (message === 'heapdump') {
          // Generate a heapdump for the worker
          // and return the filename to the parent.
          parentPort.postMessage(writeHeapSnapshot());
        }
      });
    }
    javascript

### `v8.setHeapSnapshotNearHeapLimit(limit)`#

Added in: v18.10.0, v16.18.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `limit` `<integer>`

The API is a no-op if `--heapsnapshot-near-heap-limit` is already set from the command line or the API is called more than once. `limit` must be a positive integer. See `--heapsnapshot-near-heap-limit` for more information.

### Serialization API#

The serialization API provides means of serializing JavaScript values in a way that is compatible with the HTML structured clone algorithm.

The format is backward-compatible (i.e. safe to store to disk). Equal JavaScript values may result in different serialized output.

#### `v8.serialize(value)`#

Added in: v8.0.0

- `value` `<any>`
- Returns: `<Buffer>`

Uses a `DefaultSerializer` to serialize `value` into a buffer.

`ERR_BUFFER_TOO_LARGE` will be thrown when trying to serialize a huge object which requires buffer larger than `buffer.constants.MAX_LENGTH`.

#### `v8.deserialize(buffer)`#

Added in: v8.0.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A buffer returned by `serialize()`.

Uses a `DefaultDeserializer` with default options to read a JS value from a buffer.

#### Class: `v8.Serializer`#

Added in: v8.0.0

##### `new Serializer()`#

Creates a new `Serializer` object.

##### `serializer.writeHeader()`#

Writes out a header, which includes the serialization format version.

##### `serializer.writeValue(value)`#

- `value` `<any>`

Serializes a JavaScript value and adds the serialized representation to the internal buffer.

This throws an error if `value` cannot be serialized.

##### `serializer.releaseBuffer()`#

- Returns: `<Buffer>`

Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.

##### `serializer.transferArrayBuffer(id, arrayBuffer)`#

- `id` `<integer>` A 32-bit unsigned integer.
- `arrayBuffer` `<ArrayBuffer>` An `ArrayBuffer` instance.

Marks an `ArrayBuffer` as having its contents transferred out of band. Pass the corresponding `ArrayBuffer` in the deserializing context to `deserializer.transferArrayBuffer()`.

##### `serializer.writeUint32(value)`#

- `value` `<integer>`

Write a raw 32-bit unsigned integer. For use inside of a custom `serializer._writeHostObject()`.

##### `serializer.writeUint64(hi, lo)`#

- `hi` `<integer>`
- `lo` `<integer>`

Write a raw 64-bit unsigned integer, split into high and low 32-bit parts. For use inside of a custom `serializer._writeHostObject()`.

##### `serializer.writeDouble(value)`#

- `value` `<number>`

Write a JS `number` value. For use inside of a custom `serializer._writeHostObject()`.

##### `serializer.writeRawBytes(buffer)`#

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>`

Write raw bytes into the serializer's internal buffer. The deserializer will require a way to compute the length of the buffer. For use inside of a custom `serializer._writeHostObject()`.

##### `serializer._writeHostObject(object)`#

- `object` `<Object>`

This method is called to write some kind of host object, i.e. an object created by native C++ bindings. If it is not possible to serialize `object`, a suitable exception should be thrown.

This method is not present on the `Serializer` class itself but can be provided by subclasses.

##### `serializer._getDataCloneError(message)`#

- `message` `<string>`

This method is called to generate error objects that will be thrown when an object can not be cloned.

This method defaults to the `Error` constructor and can be overridden on subclasses.

##### `serializer._getSharedArrayBufferId(sharedArrayBuffer)`#

- `sharedArrayBuffer` `<SharedArrayBuffer>`

This method is called when the serializer is going to serialize a `SharedArrayBuffer` object. It must return an unsigned 32-bit integer ID for the object, using the same ID if this `SharedArrayBuffer` has already been serialized. When deserializing, this ID will be passed to `deserializer.transferArrayBuffer()`.

If the object cannot be serialized, an exception should be thrown.

This method is not present on the `Serializer` class itself but can be provided by subclasses.

##### `serializer._setTreatArrayBufferViewsAsHostObjects(flag)`#

- `flag` `<boolean>` Default: `false`

Indicate whether to treat `TypedArray` and `DataView` objects as host objects, i.e. pass them to `serializer._writeHostObject()`.

#### Class: `v8.Deserializer`#

Added in: v8.0.0

##### `new Deserializer(buffer)`#

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` A buffer returned by `serializer.releaseBuffer()`.

Creates a new `Deserializer` object.

##### `deserializer.readHeader()`#

Reads and validates a header (including the format version). May, for example, reject an invalid or unsupported wire format. In that case, an `Error` is thrown.

##### `deserializer.readValue()`#

Deserializes a JavaScript value from the buffer and returns it.

##### `deserializer.transferArrayBuffer(id, arrayBuffer)`#

- `id` `<integer>` A 32-bit unsigned integer.
- `arrayBuffer` `<ArrayBuffer>` | `<SharedArrayBuffer>` An `ArrayBuffer` instance.

Marks an `ArrayBuffer` as having its contents transferred out of band. Pass the corresponding `ArrayBuffer` in the serializing context to `serializer.transferArrayBuffer()` (or return the `id` from `serializer._getSharedArrayBufferId()` in the case of `SharedArrayBuffer`s).

##### `deserializer.getWireFormatVersion()`#

- Returns: `<integer>`

Reads the underlying wire format version. Likely mostly to be useful to legacy code reading old wire format versions. May not be called before `.readHeader()`.

##### `deserializer.readUint32()`#

- Returns: `<integer>`

Read a raw 32-bit unsigned integer and return it. For use inside of a custom `deserializer._readHostObject()`.

##### `deserializer.readUint64()`#

- Returns: `<integer>`[]

Read a raw 64-bit unsigned integer and return it as an array `[hi, lo]` with two 32-bit unsigned integer entries. For use inside of a custom `deserializer._readHostObject()`.

##### `deserializer.readDouble()`#

- Returns: `<number>`

Read a JS `number` value. For use inside of a custom `deserializer._readHostObject()`.

##### `deserializer.readRawBytes(length)`#

- `length` `<integer>`
- Returns: `<Buffer>`

Read raw bytes from the deserializer's internal buffer. The `length` parameter must correspond to the length of the buffer that was passed to `serializer.writeRawBytes()`. For use inside of a custom `deserializer._readHostObject()`.

##### `deserializer._readHostObject()`#

This method is called to read some kind of host object, i.e. an object that is created by native C++ bindings. If it is not possible to deserialize the data, a suitable exception should be thrown.

This method is not present on the `Deserializer` class itself but can be provided by subclasses.

#### Class: `v8.DefaultSerializer`#

Added in: v8.0.0

A subclass of `Serializer` that serializes `TypedArray` (in particular `Buffer`) and `DataView` objects as host objects, and only stores the part of their underlying `ArrayBuffer`s that they are referring to.

#### Class: `v8.DefaultDeserializer`#

Added in: v8.0.0

A subclass of `Deserializer` corresponding to the format written by `DefaultSerializer`.

### Promise hooks#

The `promiseHooks` interface can be used to track promise lifecycle events. To track all async activity, see `async_hooks` which internally uses this module to produce promise lifecycle events in addition to events for other async resources. For request context management, see `AsyncLocalStorage`.

    import { promiseHooks } from 'node:v8';

    // There are four lifecycle events produced by promises:

    // The `init` event represents the creation of a promise. This could be a
    // direct creation such as with `new Promise(...)` or a continuation such
    // as `then()` or `catch()`. It also happens whenever an async function is
    // called or does an `await`. If a continuation promise is created, the
    // `parent` will be the promise it is a continuation from.
    function init(promise, parent) {
      console.log('a promise was created', { promise, parent });
    }

    // The `settled` event happens when a promise receives a resolution or
    // rejection value. This may happen synchronously such as when using
    // `Promise.resolve()` on non-promise input.
    function settled(promise) {
      console.log('a promise resolved or rejected', { promise });
    }

    // The `before` event runs immediately before a `then()` or `catch()` handler
    // runs or an `await` resumes execution.
    function before(promise) {
      console.log('a promise is about to call a then handler', { promise });
    }

    // The `after` event runs immediately after a `then()` handler runs or when
    // an `await` begins after resuming from another.
    function after(promise) {
      console.log('a promise is done calling a then handler', { promise });
    }

    // Lifecycle hooks may be started and stopped individually
    const stopWatchingInits = promiseHooks.onInit(init);
    const stopWatchingSettleds = promiseHooks.onSettled(settled);
    const stopWatchingBefores = promiseHooks.onBefore(before);
    const stopWatchingAfters = promiseHooks.onAfter(after);

    // Or they may be started and stopped in groups
    const stopHookSet = promiseHooks.createHook({
      init,
      settled,
      before,
      after,
    });

    // Trigger the hooks by using promises
    const promiseLog = (word) => Promise.resolve(word).then(console.log);
    promiseLog('Hello');
    promiseLog('World');

    // To stop a hook, call the function returned at its creation.
    stopWatchingInits();
    stopWatchingSettleds();
    stopWatchingBefores();
    stopWatchingAfters();
    stopHookSet();
    const { promiseHooks } = require('node:v8');

    // There are four lifecycle events produced by promises:

    // The `init` event represents the creation of a promise. This could be a
    // direct creation such as with `new Promise(...)` or a continuation such
    // as `then()` or `catch()`. It also happens whenever an async function is
    // called or does an `await`. If a continuation promise is created, the
    // `parent` will be the promise it is a continuation from.
    function init(promise, parent) {
      console.log('a promise was created', { promise, parent });
    }

    // The `settled` event happens when a promise receives a resolution or
    // rejection value. This may happen synchronously such as when using
    // `Promise.resolve()` on non-promise input.
    function settled(promise) {
      console.log('a promise resolved or rejected', { promise });
    }

    // The `before` event runs immediately before a `then()` or `catch()` handler
    // runs or an `await` resumes execution.
    function before(promise) {
      console.log('a promise is about to call a then handler', { promise });
    }

    // The `after` event runs immediately after a `then()` handler runs or when
    // an `await` begins after resuming from another.
    function after(promise) {
      console.log('a promise is done calling a then handler', { promise });
    }

    // Lifecycle hooks may be started and stopped individually
    const stopWatchingInits = promiseHooks.onInit(init);
    const stopWatchingSettleds = promiseHooks.onSettled(settled);
    const stopWatchingBefores = promiseHooks.onBefore(before);
    const stopWatchingAfters = promiseHooks.onAfter(after);

    // Or they may be started and stopped in groups
    const stopHookSet = promiseHooks.createHook({
      init,
      settled,
      before,
      after,
    });

    // Trigger the hooks by using promises
    const promisePrint = (word) => Promise.resolve(word).then(console.log);
    promisePrint('Hello');
    promisePrint('World');

    // To stop a hook, call the function returned at its creation.
    stopWatchingInits();
    stopWatchingSettleds();
    stopWatchingBefores();
    stopWatchingAfters();
    stopHookSet();
    javascript

#### `promiseHooks.onInit(init)`#

Added in: v17.1.0, v16.14.0

- `init` `<Function>` The `init` callback to call when a promise is created.
- Returns: `<Function>` Call to stop the hook.

The `init` hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

    import { promiseHooks } from 'node:v8';

    const stop = promiseHooks.onInit((promise, parent) => {});
    const { promiseHooks } = require('node:v8');

    const stop = promiseHooks.onInit((promise, parent) => {});
    javascript

#### `promiseHooks.onSettled(settled)`#

Added in: v17.1.0, v16.14.0

- `settled` `<Function>` The `settled` callback to call when a promise is resolved or rejected.
- Returns: `<Function>` Call to stop the hook.

The `settled` hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

    import { promiseHooks } from 'node:v8';

    const stop = promiseHooks.onSettled((promise) => {});
    const { promiseHooks } = require('node:v8');

    const stop = promiseHooks.onSettled((promise) => {});
    javascript

#### `promiseHooks.onBefore(before)`#

Added in: v17.1.0, v16.14.0

- `before` `<Function>` The `before` callback to call before a promise continuation executes.
- Returns: `<Function>` Call to stop the hook.

The `before` hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

    import { promiseHooks } from 'node:v8';

    const stop = promiseHooks.onBefore((promise) => {});
    const { promiseHooks } = require('node:v8');

    const stop = promiseHooks.onBefore((promise) => {});
    javascript

#### `promiseHooks.onAfter(after)`#

Added in: v17.1.0, v16.14.0

- `after` `<Function>` The `after` callback to call after a promise continuation executes.
- Returns: `<Function>` Call to stop the hook.

The `after` hook must be a plain function. Providing an async function will throw as it would produce an infinite microtask loop.

    import { promiseHooks } from 'node:v8';

    const stop = promiseHooks.onAfter((promise) => {});
    const { promiseHooks } = require('node:v8');

    const stop = promiseHooks.onAfter((promise) => {});
    javascript

#### `promiseHooks.createHook(callbacks)`#

Added in: v17.1.0, v16.14.0

- `callbacks` `<Object>` The Hook Callbacks to register
  - `init` `<Function>` The `init` callback.
  - `before` `<Function>` The `before` callback.
  - `after` `<Function>` The `after` callback.
  - `settled` `<Function>` The `settled` callback.
- Returns: `<Function>` Used for disabling hooks

The hook callbacks must be plain functions. Providing async functions will throw as it would produce an infinite microtask loop.

Registers functions to be called for different lifetime events of each promise.

The callbacks `init()`/`before()`/`after()`/`settled()` are called for the respective events during a promise's lifetime.

All callbacks are optional. For example, if only promise creation needs to be tracked, then only the `init` callback needs to be passed. The specifics of all functions that can be passed to `callbacks` is in the Hook Callbacks section.

    import { promiseHooks } from 'node:v8';

    const stopAll = promiseHooks.createHook({
      init(promise, parent) {},
    });
    const { promiseHooks } = require('node:v8');

    const stopAll = promiseHooks.createHook({
      init(promise, parent) {},
    });
    javascript

#### Hook callbacks#

Key events in the lifetime of a promise have been categorized into four areas: creation of a promise, before/after a continuation handler is called or around an await, and when the promise resolves or rejects.

While these hooks are similar to those of `async_hooks` they lack a `destroy` hook. Other types of async resources typically represent sockets or file descriptors which have a distinct "closed" state to express the `destroy` lifecycle event while promises remain usable for as long as code can still reach them. Garbage collection tracking is used to make promises fit into the `async_hooks` event model, however this tracking is very expensive and they may not necessarily ever even be garbage collected.

Because promises are asynchronous resources whose lifecycle is tracked via the promise hooks mechanism, the `init()`, `before()`, `after()`, and `settled()` callbacks must not be async functions as they create more promises which would produce an infinite loop.

While this API is used to feed promise events into `async_hooks`, the ordering between the two is undefined. Both APIs are multi-tenant and therefore could produce events in any order relative to each other.

##### `init(promise, parent)`#

- `promise` `<Promise>` The promise being created.
- `parent` `<Promise>` The promise continued from, if applicable.

Called when a promise is constructed. This does not mean that corresponding `before`/`after` events will occur, only that the possibility exists. This will happen if a promise is created without ever getting a continuation.

##### `before(promise)`#

- `promise` `<Promise>`

Called before a promise continuation executes. This can be in the form of `then()`, `catch()`, or `finally()` handlers or an `await` resuming.

The `before` callback will be called 0 to N times. The `before` callback will typically be called 0 times if no continuation was ever made for the promise. The `before` callback may be called many times in the case where many continuations have been made from the same promise.

##### `after(promise)`#

- `promise` `<Promise>`

Called immediately after a promise continuation executes. This may be after a `then()`, `catch()`, or `finally()` handler or before an `await` after another `await`.

##### `settled(promise)`#

- `promise` `<Promise>`

Called when the promise receives a resolution or rejection value. This may occur synchronously in the case of `Promise.resolve()` or `Promise.reject()`.

### Startup Snapshot API#

Added in: v18.6.0, v16.17.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

The `v8.startupSnapshot` interface can be used to add serialization and deserialization hooks for custom startup snapshots.

    $ node --snapshot-blob snapshot.blob --build-snapshot entry.js
    # This launches a process with the snapshot
    $ node --snapshot-blob snapshot.blob
    console

In the example above, `entry.js` can use methods from the `v8.startupSnapshot` interface to specify how to save information for custom objects in the snapshot during serialization and how the information can be used to synchronize these objects during deserialization of the snapshot. For example, if the `entry.js` contains the following script:

    const fs = require('node:fs');
    const zlib = require('node:zlib');
    const path = require('node:path');
    const assert = require('node:assert');

    const v8 = require('node:v8');

    class BookShelf {
      storage = new Map();

      // Reading a series of files from directory and store them into storage.
      constructor(directory, books) {
        for (const book of books) {
          this.storage.set(book, fs.readFileSync(path.join(directory, book)));
        }
      }

      static compressAll(shelf) {
        for (const [ book, content ] of shelf.storage) {
          shelf.storage.set(book, zlib.gzipSync(content));
        }
      }

      static decompressAll(shelf) {
        for (const [ book, content ] of shelf.storage) {
          shelf.storage.set(book, zlib.gunzipSync(content));
        }
      }
    }

    // __dirname here is where the snapshot script is placed
    // during snapshot building time.
    const shelf = new BookShelf(__dirname, [
      'book1.en_US.txt',
      'book1.es_ES.txt',
      'book2.zh_CN.txt',
    ]);

    assert(v8.startupSnapshot.isBuildingSnapshot());
    // On snapshot serialization, compress the books to reduce size.
    v8.startupSnapshot.addSerializeCallback(BookShelf.compressAll, shelf);
    // On snapshot deserialization, decompress the books.
    v8.startupSnapshot.addDeserializeCallback(BookShelf.decompressAll, shelf);
    v8.startupSnapshot.setDeserializeMainFunction((shelf) => {
      // process.env and process.argv are refreshed during snapshot
      // deserialization.
      const lang = process.env.BOOK_LANG || 'en_US';
      const book = process.argv[1];
      const name = `${book}.${lang}.txt`;
      console.log(shelf.storage.get(name));
    }, shelf);
    cjs

The resulted binary will get print the data deserialized from the snapshot during start up, using the refreshed `process.env` and `process.argv` of the launched process:

    $ BOOK_LANG=es_ES node --snapshot-blob snapshot.blob book1
    # Prints content of book1.es_ES.txt deserialized from the snapshot.
    console

Currently the application deserialized from a user-land snapshot cannot be snapshotted again, so these APIs are only available to applications that are not deserialized from a user-land snapshot.

#### `v8.startupSnapshot.addSerializeCallback(callback[, data])`#

Added in: v18.6.0, v16.17.0

- `callback` `<Function>` Callback to be invoked before serialization.
- `data` `<any>` Optional data that will be passed to the `callback` when it gets called.

Add a callback that will be called when the Node.js instance is about to get serialized into a snapshot and exit. This can be used to release resources that should not or cannot be serialized or to convert user data into a form more suitable for serialization.

Callbacks are run in the order in which they are added.

#### `v8.startupSnapshot.addDeserializeCallback(callback[, data])`#

Added in: v18.6.0, v16.17.0

- `callback` `<Function>` Callback to be invoked after the snapshot is deserialized.
- `data` `<any>` Optional data that will be passed to the `callback` when it gets called.

Add a callback that will be called when the Node.js instance is deserialized from a snapshot. The `callback` and the `data` (if provided) will be serialized into the snapshot, they can be used to re-initialize the state of the application or to re-acquire resources that the application needs when the application is restarted from the snapshot.

Callbacks are run in the order in which they are added.

#### `v8.startupSnapshot.setDeserializeMainFunction(callback[, data])`#

Added in: v18.6.0, v16.17.0

- `callback` `<Function>` Callback to be invoked as the entry point after the snapshot is deserialized.
- `data` `<any>` Optional data that will be passed to the `callback` when it gets called.

This sets the entry point of the Node.js application when it is deserialized from a snapshot. This can be called only once in the snapshot building script. If called, the deserialized application no longer needs an additional entry point script to start up and will simply invoke the callback along with the deserialized data (if provided), otherwise an entry point script still needs to be provided to the deserialized application.

#### `v8.startupSnapshot.isBuildingSnapshot()`#

Added in: v18.6.0, v16.17.0

- Returns: `<boolean>`

Returns true if the Node.js instance is run to build a snapshot.

### Class: `v8.GCProfiler`#

Added in: v19.6.0, v18.15.0

This API collects GC data in current thread.

#### `new v8.GCProfiler()`#

Added in: v19.6.0, v18.15.0

Create a new instance of the `v8.GCProfiler` class. This API supports `using` syntax.

#### `profiler.start()`#

Added in: v19.6.0, v18.15.0

Start collecting GC data.

#### `profiler.stop()`#

Added in: v19.6.0, v18.15.0

Stop collecting GC data and return an object. The content of object is as follows.

    {
      "version": 1,
      "startTime": 1674059033862,
      "statistics": [
        {
          "gcType": "Scavenge",
          "beforeGC": {
            "heapStatistics": {
              "totalHeapSize": 5005312,
              "totalHeapSizeExecutable": 524288,
              "totalPhysicalSize": 5226496,
              "totalAvailableSize": 4341325216,
              "totalGlobalHandlesSize": 8192,
              "usedGlobalHandlesSize": 2112,
              "usedHeapSize": 4883840,
              "heapSizeLimit": 4345298944,
              "mallocedMemory": 254128,
              "externalMemory": 225138,
              "peakMallocedMemory": 181760
            },
            "heapSpaceStatistics": [
              {
                "spaceName": "read_only_space",
                "spaceSize": 0,
                "spaceUsedSize": 0,
                "spaceAvailableSize": 0,
                "physicalSpaceSize": 0
              }
            ]
          },
          "cost": 1574.14,
          "afterGC": {
            "heapStatistics": {
              "totalHeapSize": 6053888,
              "totalHeapSizeExecutable": 524288,
              "totalPhysicalSize": 5500928,
              "totalAvailableSize": 4341101384,
              "totalGlobalHandlesSize": 8192,
              "usedGlobalHandlesSize": 2112,
              "usedHeapSize": 4059096,
              "heapSizeLimit": 4345298944,
              "mallocedMemory": 254128,
              "externalMemory": 225138,
              "peakMallocedMemory": 181760
            },
            "heapSpaceStatistics": [
              {
                "spaceName": "read_only_space",
                "spaceSize": 0,
                "spaceUsedSize": 0,
                "spaceAvailableSize": 0,
                "physicalSpaceSize": 0
              }
            ]
          }
        }
      ],
      "endTime": 1674059036865
    }
    json

Here's an example.

    import { GCProfiler } from 'node:v8';
    const profiler = new GCProfiler();
    profiler.start();
    setTimeout(() => {
      console.log(profiler.stop());
    }, 1000);
    const { GCProfiler } = require('node:v8');
    const profiler = new GCProfiler();
    profiler.start();
    setTimeout(() => {
      console.log(profiler.stop());
    }, 1000);
    javascript

#### `profiler[Symbol.dispose]()`#

Added in: v25.5.0, v24.13.1

Stop collecting GC data, and discard the profile.

### Class: `SyncCPUProfileHandle`#

Added in: v25.0.0, v24.12.0

#### `syncCpuProfileHandle.stop()`#

Added in: v25.0.0, v24.12.0

- Returns: `<string>`

Stopping collecting the profile and return the profile data.

#### `syncCpuProfileHandle[Symbol.dispose]()`#

Added in: v25.0.0, v24.12.0

Stopping collecting the profile and the profile will be discarded.

### Class: `SyncHeapProfileHandle`#

Added in: v26.1.0

Stability: 1 - Experimental

#### `syncHeapProfileHandle.stop()`#

Added in: v26.1.0

- Returns: `<string>`

Stopping collecting the profile and return the profile data.

#### `syncHeapProfileHandle[Symbol.dispose]()`#

Added in: v26.1.0

Stopping collecting the profile and the profile will be discarded.

### Class: `CPUProfileHandle`#

Added in: v24.8.0

#### `cpuProfileHandle.stop()`#

Added in: v24.8.0

- Returns: `<Promise>`

Stopping collecting the profile, then return a Promise that fulfills with an error or the profile data.

#### `cpuProfileHandle[Symbol.asyncDispose]()`#

Added in: v24.8.0

- Returns: `<Promise>`

Stopping collecting the profile and the profile will be discarded.

### Class: `HeapProfileHandle`#

Added in: v24.9.0

#### `heapProfileHandle.stop()`#

Added in: v24.9.0

- Returns: `<Promise>`

Stopping collecting the profile, then return a Promise that fulfills with an error or the profile data.

#### `heapProfileHandle[Symbol.asyncDispose]()`#

Added in: v24.9.0

- Returns: `<Promise>`

Stopping collecting the profile and the profile will be discarded.

### `v8.isStringOneByteRepresentation(content)`#

Added in: v23.10.0, v22.15.0

- `content` `<string>`
- Returns: `<boolean>`

V8 only supports `Latin-1/ISO-8859-1` and `UTF16` as the underlying representation of a string. If the `content` uses `Latin-1/ISO-8859-1` as the underlying representation, this function will return true; otherwise, it returns false.

If this method returns false, that does not mean that the string contains some characters not in `Latin-1/ISO-8859-1`. Sometimes a `Latin-1` string may also be represented as `UTF16`.

    import { isStringOneByteRepresentation } from 'node:v8';
    import { Buffer } from 'node:buffer';

    const Encoding = {
      latin1: 1,
      utf16le: 2,
    };
    const buffer = Buffer.alloc(100);
    function writeString(input) {
      if (isStringOneByteRepresentation(input)) {
        console.log(`input: '${input}'`);
        buffer.writeUint8(Encoding.latin1);
        buffer.writeUint32LE(input.length, 1);
        buffer.write(input, 5, 'latin1');
        console.log(`decoded: '${buffer.toString('latin1', 5, 5 + input.length)}'\n`);
      } else {
        console.log(`input: '${input}'`);
        buffer.writeUint8(Encoding.utf16le);
        buffer.writeUint32LE(input.length * 2, 1);
        buffer.write(input, 5, 'utf16le');
        console.log(`decoded: '${buffer.toString('utf16le', 5, 5 + input.length * 2)}'`);
      }
    }
    writeString('hello');
    writeString('你好');
    const { isStringOneByteRepresentation } = require('node:v8');
    const { Buffer } = require('node:buffer');

    const Encoding = {
      latin1: 1,
      utf16le: 2,
    };
    const buffer = Buffer.alloc(100);
    function writeString(input) {
      if (isStringOneByteRepresentation(input)) {
        console.log(`input: '${input}'`);
        buffer.writeUint8(Encoding.latin1);
        buffer.writeUint32LE(input.length, 1);
        buffer.write(input, 5, 'latin1');
        console.log(`decoded: '${buffer.toString('latin1', 5, 5 + input.length)}'\n`);
      } else {
        console.log(`input: '${input}'`);
        buffer.writeUint8(Encoding.utf16le);
        buffer.writeUint32LE(input.length * 2, 1);
        buffer.write(input, 5, 'utf16le');
        console.log(`decoded: '${buffer.toString('utf16le', 5, 5 + input.length * 2)}'`);
      }
    }
    writeString('hello');
    writeString('你好');
    javascript

### `v8.startCpuProfile([options])`#

Added in: v25.0.0, v24.12.0

- `options` `<Object>`
  - `sampleInterval` `<number>` Requested sampling interval in milliseconds. Default: `0`.
  - `maxBufferSize` `<integer>` Maximum number of samples to keep before older entries are discarded. Default: `4294967295`.
- Returns: `<SyncCPUProfileHandle>`

Starting a CPU profile then return a `SyncCPUProfileHandle` object. This API supports `using` syntax.

    const handle = v8.startCpuProfile({ sampleInterval: 1, maxBufferSize: 10_000 });
    const profile = handle.stop();
    console.log(profile);
    cjs

### `v8.startHeapProfile([options])`#

Added in: v26.1.0

- `options` `<Object>`
  - `sampleInterval` `<number>` The average sampling interval in bytes. Default: `524288` (512 KiB).
  - `stackDepth` `<integer>` The maximum stack depth for samples. Default: `16`.
  - `forceGC` `<boolean>` Force garbage collection before taking the profile. Default: `false`.
  - `includeObjectsCollectedByMajorGC` `<boolean>` Include objects collected by major GC. Default: `false`.
  - `includeObjectsCollectedByMinorGC` `<boolean>` Include objects collected by minor GC. Default: `false`.
- Returns: `<SyncHeapProfileHandle>`

Starting a heap profile then return a `SyncHeapProfileHandle` object. This API supports `using` syntax.

    const v8 = require('node:v8');

    const handle = v8.startHeapProfile();
    const profile = handle.stop();
    console.log(profile);
    import v8 from 'node:v8';

    const handle = v8.startHeapProfile();
    const profile = handle.stop();
    console.log(profile);
    javascript

With custom parameters:

    const v8 = require('node:v8');

    const handle = v8.startHeapProfile({
      sampleInterval: 1024,
      stackDepth: 8,
      forceGC: true,
      includeObjectsCollectedByMajorGC: true,
    });
    const profile = handle.stop();
    console.log(profile);
    import v8 from 'node:v8';

    const handle = v8.startHeapProfile({
      sampleInterval: 1024,
      stackDepth: 8,
      forceGC: true,
      includeObjectsCollectedByMajorGC: true,
    });
    const profile = handle.stop();
    console.log(profile);
    javascript

## Virtual File System#

Source Code: lib/vfs.js Added in: v26.4.0

Stability: 1 - Experimental

The `node:vfs` module provides a virtual file system with a `node:fs`-like API. It is useful for tests, fixtures, embedded assets, and other scenarios where you need a self-contained file system without touching the actual file-system.

To access it:

    import vfs from 'node:vfs';
    const vfs = require('node:vfs');
    javascript

This module is only available under the `node:` scheme, and only when Node.js is started with the `--experimental-vfs` flag.

### Security#

The VFS API is not a sandbox, permission system, or access-control mechanism. It does not isolate untrusted code from the host file system or from other Node.js capabilities. Code that can access a `VirtualFileSystem` instance, mount it, select its provider, or pass paths to it is trusted application code.

Mounting a VFS only redirects supported `node:fs` calls whose resolved paths are under the mount point. It does not prevent code from using other paths or other Node.js APIs to access resources available to the process. `RealFSProvider` maps VFS paths under its configured root and rejects paths that resolve outside that root, but that check is not a security boundary. Do not rely on VFS to run untrusted code; use operating-system-level isolation, such as separate users, containers, or platform sandboxes, when a security boundary is required.

### Basic usage#

    const vfs = require('node:vfs');

    const myVfs = vfs.create();
    myVfs.mkdirSync('/dir', { recursive: true });
    myVfs.writeFileSync('/dir/hello.txt', 'Hello, VFS!');

    console.log(myVfs.readFileSync('/dir/hello.txt', 'utf8')); // 'Hello, VFS!'
    cjs

`vfs.create()` returns a `VirtualFileSystem` instance backed by a `MemoryProvider` by default. The instance exposes synchronous, callback-based, and promise-based file system methods that mirror the shape of the `node:fs` API. All paths are POSIX-style and absolute (starting with `/`).

### `vfs.create([provider][, options])`#

Added in: v26.4.0

- `provider` `<VirtualProvider>` The provider to use. Default: `new MemoryProvider()`.
- `options` `<Object>`
  - `emitExperimentalWarning` `<boolean>` Whether to emit the experimental warning when the instance is created. Default: `true`.
- Returns: `<VirtualFileSystem>`

Convenience factory equivalent to `new VirtualFileSystem(provider, options)`.

    const vfs = require('node:vfs');

    // Default in-memory provider
    const memoryVfs = vfs.create();

    // Explicit provider
    const realVfs = vfs.create(new vfs.RealFSProvider('/tmp/vfs-root'));
    cjs

### Class: `VirtualFileSystem`#

Added in: v26.4.0

A `VirtualFileSystem` wraps a `VirtualProvider` and exposes a `node:fs`-like API. Each instance maintains its own file tree.

#### `new VirtualFileSystem([provider][, options])`#

Added in: v26.4.0

- `provider` `<VirtualProvider>` The provider to use. Default: `new MemoryProvider()`.
- `options` `<Object>`
  - `emitExperimentalWarning` `<boolean>` Whether to emit the experimental warning. Default: `true`.

#### `vfs.provider`#

Added in: v26.4.0

- `<VirtualProvider>`

The provider backing this VFS instance.

#### `vfs.readonly`#

Added in: v26.4.0

- `<boolean>`

`true` when the underlying provider is read-only.

#### APIs#

`VirtualFileSystem` implements the following methods, with the same signatures as their `node:fs` counterparts:

##### Synchronous API#

- `existsSync(path)`
- `statSync(path[, options])`
- `lstatSync(path[, options])`
- `readFileSync(path[, options])`
- `writeFileSync(path, data[, options])`
- `appendFileSync(path, data[, options])`
- `readdirSync(path[, options])`
- `mkdirSync(path[, options])`
- `rmdirSync(path)`
- `unlinkSync(path)`
- `renameSync(oldPath, newPath)`
- `copyFileSync(src, dest[, mode])`
- `realpathSync(path[, options])`
- `readlinkSync(path[, options])`
- `symlinkSync(target, path[, type])`
- `accessSync(path[, mode])`
- `rmSync(path[, options])`
- `truncateSync(path[, len])`
- `ftruncateSync(fd[, len])`
- `linkSync(existingPath, newPath)`
- `chmodSync(path, mode)`
- `chownSync(path, uid, gid)`
- `lchownSync(path, uid, gid)`
- `utimesSync(path, atime, mtime)`
- `lutimesSync(path, atime, mtime)`
- `mkdtempSync(prefix)`
- `opendirSync(path[, options])`
- `openAsBlob(path[, options])`
- File-descriptor ops: `openSync`, `closeSync`, `readSync`, `writeSync`, `fstatSync`
- Streams: `createReadStream`, `createWriteStream`
- Watchers: `watch`, `watchFile`, `unwatchFile`

##### Callback API#

`readFile`, `writeFile`, `stat`, `lstat`, `readdir`, `realpath`, `readlink`, `access`, `open`, `close`, `read`, `write`, `rm`, `fstat`, `truncate`, `ftruncate`, `link`, `mkdtemp`, `opendir`. Each takes a Node.js-style callback `(err, ...result) => {}`.

##### Promise API#

`vfs.promises` exposes the promise-based variants:

    const vfs = require('node:vfs');

    async function example() {
      const myVfs = vfs.create();
      await myVfs.promises.writeFile('/file.txt', 'hello');
      const data = await myVfs.promises.readFile('/file.txt', 'utf8');
      return data;
    }
    example();
    cjs

The promise namespace mirrors `fs.promises` and includes `readFile`, `writeFile`, `appendFile`, `stat`, `lstat`, `readdir`, `mkdir`, `rmdir`, `unlink`, `rename`, `copyFile`, `realpath`, `readlink`, `symlink`, `access`, `rm`, `truncate`, `link`, `mkdtemp`, `chmod`, `chown`, `lchown`, `utimes`, `lutimes`, `open`, `lchmod`, and `watch`.

### Class: `VirtualProvider`#

Added in: v26.4.0

The base class for all VFS providers. Subclasses implement the essential primitives (such as `open`, `stat`, `readdir`, `mkdir`, `rmdir`, `unlink`, `rename`, etc.) and inherit default implementations of the derived methods (such as `readFile`, `writeFile`, `exists`, `copyFile`, `access`, etc.).

#### Capability flags#

- `provider.readonly` `<boolean>` Default: `false`.
- `provider.supportsSymlinks` `<boolean>` Default: `false`.
- `provider.supportsWatch` `<boolean>` Default: `false`.

#### Creating custom providers#

    const { VirtualProvider } = require('node:vfs');

    class StaticProvider extends VirtualProvider {
      get readonly() { return true; }

      statSync(path) { /* ... */ }
      openSync(path, flags) { /* ... */ }
      readdirSync(path, options) { /* ... */ }
      // ...
    }
    cjs

The base class throws `ERR_METHOD_NOT_IMPLEMENTED` for any primitive that has not been overridden, and rejects writes from a `readonly` provider with `EROFS`.

### Class: `MemoryProvider`#

Added in: v26.4.0

The default in-memory provider. Stores files, directories, and symbolic links in a `Map`-backed tree, supports symlinks (`supportsSymlinks === true`), and supports watching (`supportsWatch === true`).

#### `memoryProvider.setReadOnly()`#

Added in: v26.4.0

Locks the provider into read-only mode. Subsequent writes through any `VirtualFileSystem` using this provider throw `EROFS`. There is no way to revert the provider to writable.

    const vfs = require('node:vfs');

    const provider = new vfs.MemoryProvider();
    const myVfs = vfs.create(provider);
    myVfs.writeFileSync('/seed.txt', 'initial');

    provider.setReadOnly();

    myVfs.writeFileSync('/x.txt', 'fail'); // throws EROFS
    cjs

### Class: `RealFSProvider`#

Added in: v26.4.0

A provider that wraps a directory (i.e. one on the actual file system) and exposes its contents through the VFS API. All VFS paths are resolved relative to the root and verified to stay inside it; symbolic links resolving outside the root are rejected. This path mapping is not a sandbox or access-control mechanism.

#### `new RealFSProvider(rootPath)`#

Added in: v26.4.0

- `rootPath` `<string>` The absolute file-system path to use as the root. Must be a non-empty string.

    const vfs = require('node:vfs');

    const realVfs = vfs.create(new vfs.RealFSProvider('/tmp/vfs-root'));
    realVfs.writeFileSync('/file.txt', 'hello'); // writes /tmp/vfs-root/file.txt
    cjs

#### `realFSProvider.rootPath`#

Added in: v26.4.0

- `<string>`

The resolved absolute path used as the root.

### Implementation details#

#### `Stats` objects#

VFS `Stats` objects are real instances of `fs.Stats` (or `fs.BigIntStats` when `{ bigint: true }` is requested). Their fields use synthetic but stable values:

- `dev` is `4085` (the VFS device id).
- `ino` is monotonically increasing per process.
- `blksize` is `4096`.
- `blocks` is `Math.ceil(size / 512)`.
- Times default to the moment the entry was created/last modified.

## VM (executing JavaScript)#

Source Code: lib/vm.js

Stability: 2 - Stable

The `node:vm` module enables compiling and running code within V8 Virtual Machine contexts.

The `node:vm` module is not a security mechanism. Do not use it to run untrusted code.

JavaScript code can be compiled and run immediately or compiled, saved, and run later.

A common use case is to run the code in a different V8 Context. This means invoked code has a different global object than the invoking code.

One can provide the context by contextifying an object. The invoked code treats any property in the context like a global variable. Any changes to global variables caused by the invoked code are reflected in the context object.

    import { createContext, runInContext } from 'node:vm';

    const x = 1;

    const context = { x: 2 };
    createContext(context); // Contextify the object.

    const code = 'x += 40; var y = 17;';
    // `x` and `y` are global variables in the context.
    // Initially, x has the value 2 because that is the value of context.x.
    runInContext(code, context);

    console.log(context.x); // 42
    console.log(context.y); // 17

    console.log(x); // 1; y is not defined
    const { createContext, runInContext } = require('node:vm');

    const x = 1;

    const context = { x: 2 };
    createContext(context); // Contextify the object.

    const code = 'x += 40; var y = 17;';
    // `x` and `y` are global variables in the context.
    // Initially, x has the value 2 because that is the value of context.x.
    runInContext(code, context);

    console.log(context.x); // 42
    console.log(context.y); // 17

    console.log(x); // 1; y is not defined
    javascript

### Class: `vm.Script`#

Added in: v0.3.1

Instances of the `vm.Script` class contain precompiled scripts that can be executed in specific contexts.

#### `new vm.Script(code[, options])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v21.7.0, v20.12.0 | Added support for `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
v17.0.0, v16.12.0 | Added support for import attributes to the `importModuleDynamically` parameter.
v10.6.0 | The `produceCachedData` is deprecated in favour of `script.createCachedData()`.
v5.7.0 | The `cachedData` and `produceCachedData` options are supported now.

- `code` `<string>` The JavaScript code to compile.
- `options` `<Object>` | `<string>`
  - `filename` `<string>` Specifies the filename used in stack traces produced by this script. Default: `'evalmachine.<anonymous>'`.
  - `lineOffset` `<number>` Specifies the line number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `columnOffset` `<number>` Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `cachedData` `<Buffer>` | `<TypedArray>` | `<DataView>` Provides an optional `Buffer` or `TypedArray`, or `DataView` with V8's code cache data for the supplied source. When supplied, the `cachedDataRejected` value will be set to either `true` or `false` depending on acceptance of the data by V8.
  - `produceCachedData` `<boolean>` When `true` and no `cachedData` is present, V8 will attempt to produce code cache data for `code`. Upon success, a `Buffer` with V8's code cache data will be produced and stored in the `cachedData` property of the returned `vm.Script` instance. The `cachedDataProduced` value will be set to either `true` or `false` depending on whether code cache data is produced successfully. This option is deprecated in favor of `script.createCachedData()`. Default: `false`.
  - `importModuleDynamically` `<Function>` | `<vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER>` Used to specify how the modules should be loaded during the evaluation of this script when `import()` is called. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.

If `options` is a string, then it specifies the filename.

Creating a new `vm.Script` object compiles `code` but does not run it. The compiled `vm.Script` can be run later multiple times. The `code` is not bound to any global object; rather, it is bound before each run, just for that run.

#### `script.cachedDataRejected`#

Added in: v5.7.0

- Type: `<boolean>` | `<undefined>`

When `cachedData` is supplied to create the `vm.Script`, this value will be set to either `true` or `false` depending on acceptance of the data by V8. Otherwise the value is `undefined`.

#### `script.createCachedData()`#

Added in: v10.6.0

- Returns: `<Buffer>`

Creates a code cache that can be used with the `Script` constructor's `cachedData` option. Returns a `Buffer`. This method may be called at any time and any number of times.

The code cache of the `Script` doesn't contain any JavaScript observable states. The code cache is safe to be saved along side the script source and used to construct new `Script` instances multiple times.

Functions in the `Script` source can be marked as lazily compiled and they are not compiled at construction of the `Script`. These functions are going to be compiled when they are invoked the first time. The code cache serializes the metadata that V8 currently knows about the `Script` that it can use to speed up future compilations.

    const script = new vm.Script(`
    function add(a, b) {
      return a + b;
    }

    const x = add(1, 2);
    `);

    const cacheWithoutAdd = script.createCachedData();
    // In `cacheWithoutAdd` the function `add()` is marked for full compilation
    // upon invocation.

    script.runInThisContext();

    const cacheWithAdd = script.createCachedData();
    // `cacheWithAdd` contains fully compiled function `add()`.
    js

#### `script.runInContext(contextifiedObject[, options])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v6.3.0 | The `breakOnSigint` option is supported now.

- `contextifiedObject` `<Object>` A contextified object as returned by the `vm.createContext()` method.
- `options` `<Object>`
  - `displayErrors` `<boolean>` When `true`, if an `Error` occurs while compiling the `code`, the line of code causing the error is attached to the stack trace. Default: `true`.
  - `timeout` `<integer>` Specifies the number of milliseconds to execute `code` before terminating execution. If execution is terminated, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
- Returns: `<any>` the result of the very last statement executed in the script.

Runs the compiled code contained by the `vm.Script` object within the given `contextifiedObject` and returns the result. Running code does not have access to local scope.

The following example compiles code that increments a global variable, sets the value of another global variable, then execute the code multiple times. The globals are contained in the `context` object.

    import { createContext, Script } from 'node:vm';

    const context = {
      animal: 'cat',
      count: 2,
    };

    const script = new Script('count += 1; name = "kitty";');

    createContext(context);
    for (let i = 0; i < 10; ++i) {
      script.runInContext(context);
    }

    console.log(context);
    // Prints: { animal: 'cat', count: 12, name: 'kitty' }
    const { createContext, Script } = require('node:vm');

    const context = {
      animal: 'cat',
      count: 2,
    };

    const script = new Script('count += 1; name = "kitty";');

    createContext(context);
    for (let i = 0; i < 10; ++i) {
      script.runInContext(context);
    }

    console.log(context);
    // Prints: { animal: 'cat', count: 12, name: 'kitty' }
    javascript

Using the `timeout` or `breakOnSigint` options will result in new event loops and corresponding threads being started, which have a non-zero performance overhead.

#### `script.runInNewContext([contextObject[, options]])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v22.8.0, v20.18.0 | The `contextObject` argument now accepts `vm.constants.DONT_CONTEXTIFY`.
v14.6.0 | The `microtaskMode` option is supported now.
v10.0.0 | The `contextCodeGeneration` option is supported now.
v6.3.0 | The `breakOnSigint` option is supported now.

- `contextObject` `<Object>` | `<vm.constants.DONT_CONTEXTIFY>` | `<undefined>` Either `vm.constants.DONT_CONTEXTIFY` or an object that will be contextified. If `undefined`, an empty contextified object will be created for backwards compatibility.
- `options` `<Object>`
  - `displayErrors` `<boolean>` When `true`, if an `Error` occurs while compiling the `code`, the line of code causing the error is attached to the stack trace. Default: `true`.
  - `timeout` `<integer>` Specifies the number of milliseconds to execute `code` before terminating execution. If execution is terminated, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
  - `contextName` `<string>` Human-readable name of the newly created context. Default: `'VM Context i'`, where `i` is an ascending numerical index of the created context.
  - `contextOrigin` `<string>` Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the `url.origin` property of a `URL` object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: `''`.
  - `contextCodeGeneration` `<Object>`
    - `strings` `<boolean>` If set to false any calls to `eval` or function constructors (`Function`, `GeneratorFunction`, etc) will throw an `EvalError`. Default: `true`.
    - `wasm` `<boolean>` If set to false any attempt to compile a WebAssembly module will throw a `WebAssembly.CompileError`. Default: `true`.
  - `microtaskMode` `<string>` If set to `afterEvaluate`, microtasks (tasks scheduled through `Promise`s and `async function`s) will be run immediately after the script has run. They are included in the `timeout` and `breakOnSigint` scopes in that case.
- Returns: `<any>` the result of the very last statement executed in the script.

This method is a shortcut to `script.runInContext(vm.createContext(options), options)`. It does several things at once:

1. Creates a new context.
2. If `contextObject` is an object, contextifies it with the new context. If `contextObject` is undefined, creates a new object and contextifies it. If `contextObject` is `vm.constants.DONT_CONTEXTIFY`, don't contextify anything.
3. Runs the compiled code contained by the `vm.Script` object within the created context. The code does not have access to the scope in which this method is called.
4. Returns the result.

The following example compiles code that sets a global variable, then executes the code multiple times in different contexts. The globals are set on and contained within each individual `context`.

    import { constants, Script } from 'node:vm';

    const script = new Script('globalVar = "set"');

    const contexts = [{}, {}, {}];
    contexts.forEach((context) => {
      script.runInNewContext(context);
    });

    console.log(contexts);
    // Prints: [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

    // This would throw if the context is created from a contextified object.
    // constants.DONT_CONTEXTIFY allows creating contexts with ordinary
    // global objects that can be frozen.
    const freezeScript = new Script('Object.freeze(globalThis); globalThis;');
    const frozenContext = freezeScript.runInNewContext(constants.DONT_CONTEXTIFY);
    const { constants, Script } = require('node:vm');

    const script = new Script('globalVar = "set"');

    const contexts = [{}, {}, {}];
    contexts.forEach((context) => {
      script.runInNewContext(context);
    });

    console.log(contexts);
    // Prints: [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

    // This would throw if the context is created from a contextified object.
    // constants.DONT_CONTEXTIFY allows creating contexts with ordinary
    // global objects that can be frozen.
    const freezeScript = new Script('Object.freeze(globalThis); globalThis;');
    const frozenContext = freezeScript.runInNewContext(constants.DONT_CONTEXTIFY);
    javascript

#### `script.runInThisContext([options])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v6.3.0 | The `breakOnSigint` option is supported now.

- `options` `<Object>`
  - `displayErrors` `<boolean>` When `true`, if an `Error` occurs while compiling the `code`, the line of code causing the error is attached to the stack trace. Default: `true`.
  - `timeout` `<integer>` Specifies the number of milliseconds to execute `code` before terminating execution. If execution is terminated, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
- Returns: `<any>` the result of the very last statement executed in the script.

Runs the compiled code contained by the `vm.Script` within the context of the current `global` object. Running code does not have access to local scope, but does have access to the current `global` object.

The following example compiles code that increments a `global` variable then executes that code multiple times:

    import { Script } from 'node:vm';

    global.globalVar = 0;

    const script = new Script('globalVar += 1', { filename: 'myfile.vm' });

    for (let i = 0; i < 1000; ++i) {
      script.runInThisContext();
    }

    console.log(globalVar);

    // 1000
    const { Script } = require('node:vm');

    global.globalVar = 0;

    const script = new Script('globalVar += 1', { filename: 'myfile.vm' });

    for (let i = 0; i < 1000; ++i) {
      script.runInThisContext();
    }

    console.log(globalVar);

    // 1000
    javascript

#### `script.sourceMapURL`#

Added in: v19.1.0, v18.13.0

- Type: `<string>` | `<undefined>`

When the script is compiled from a source that contains a source map magic comment, this property will be set to the URL of the source map.

    import vm from 'node:vm';

    const script = new vm.Script(`
    function myFunc() {}
    //# sourceMappingURL=sourcemap.json
    `);

    console.log(script.sourceMapURL);
    // Prints: sourcemap.json
    const vm = require('node:vm');

    const script = new vm.Script(`
    function myFunc() {}
    //# sourceMappingURL=sourcemap.json
    `);

    console.log(script.sourceMapURL);
    // Prints: sourcemap.json
    javascript

### Class: `vm.Module`#

Added in: v13.0.0, v12.16.0

Stability: 1 - Experimental

This feature is only available with the `--experimental-vm-modules` command flag enabled.

The `vm.Module` class provides a low-level interface for using ECMAScript modules in VM contexts. It is the counterpart of the `vm.Script` class that closely mirrors Module Records as defined in the ECMAScript specification.

Unlike `vm.Script` however, every `vm.Module` object is bound to a context from its creation.

Using a `vm.Module` object requires three distinct steps: creation/parsing, linking, and evaluation. These three steps are illustrated in the following example.

This implementation lies at a lower level than the ECMAScript Module loader. There is also no way to interact with the Loader yet, though support is planned.

    import vm from 'node:vm';

    const contextifiedObject = vm.createContext({
      secret: 42,
      print: console.log,
    });

    // Step 1
    //
    // Create a Module by constructing a new `vm.SourceTextModule` object. This
    // parses the provided source text, throwing a `SyntaxError` if anything goes
    // wrong. By default, a Module is created in the top context. But here, we
    // specify `contextifiedObject` as the context this Module belongs to.
    //
    // Here, we attempt to obtain the default export from the module "foo", and
    // put it into local binding "secret".

    const rootModule = new vm.SourceTextModule(`
      import s from 'foo';
      s;
      print(s);
    `, { context: contextifiedObject });

    // Step 2
    //
    // "Link" the imported dependencies of this Module to it.
    //
    // Obtain the requested dependencies of a SourceTextModule by
    // `sourceTextModule.moduleRequests` and resolve them.
    //
    // Even top-level Modules without dependencies must be explicitly linked. The
    // array passed to `sourceTextModule.linkRequests(modules)` can be
    // empty, however.
    //
    // Note: This is a contrived example in that the resolveAndLinkDependencies
    // creates a new "foo" module every time it is called. In a full-fledged
    // module system, a cache would probably be used to avoid duplicated modules.

    const moduleMap = new Map([
      ['root', rootModule],
    ]);

    function resolveAndLinkDependencies(module) {
      const requestedModules = module.moduleRequests.map((request) => {
        // In a full-fledged module system, the resolveAndLinkDependencies would
        // resolve the module with the module cache key `[specifier, attributes]`.
        // In this example, we just use the specifier as the key.
        const specifier = request.specifier;

        let requestedModule = moduleMap.get(specifier);
        if (requestedModule === undefined) {
          requestedModule = new vm.SourceTextModule(`
            // The "secret" variable refers to the global variable we added to
            // "contextifiedObject" when creating the context.
            export default secret;
          `, { context: module.context });
          moduleMap.set(specifier, requestedModule);
          // Resolve the dependencies of the new module as well.
          resolveAndLinkDependencies(requestedModule);
        }

        return requestedModule;
      });

      module.linkRequests(requestedModules);
    }

    resolveAndLinkDependencies(rootModule);
    rootModule.instantiate();

    // Step 3
    //
    // Evaluate the Module. The evaluate() method returns a promise which will
    // resolve after the module has finished evaluating.

    // Prints 42.
    await rootModule.evaluate();
    const vm = require('node:vm');

    const contextifiedObject = vm.createContext({
      secret: 42,
      print: console.log,
    });

    (async () => {
      // Step 1
      //
      // Create a Module by constructing a new `vm.SourceTextModule` object. This
      // parses the provided source text, throwing a `SyntaxError` if anything goes
      // wrong. By default, a Module is created in the top context. But here, we
      // specify `contextifiedObject` as the context this Module belongs to.
      //
      // Here, we attempt to obtain the default export from the module "foo", and
      // put it into local binding "secret".

      const rootModule = new vm.SourceTextModule(`
        import s from 'foo';
        s;
        print(s);
      `, { context: contextifiedObject });

      // Step 2
      //
      // "Link" the imported dependencies of this Module to it.
      //
      // Obtain the requested dependencies of a SourceTextModule by
      // `sourceTextModule.moduleRequests` and resolve them.
      //
      // Even top-level Modules without dependencies must be explicitly linked. The
      // array passed to `sourceTextModule.linkRequests(modules)` can be
      // empty, however.
      //
      // Note: This is a contrived example in that the resolveAndLinkDependencies
      // creates a new "foo" module every time it is called. In a full-fledged
      // module system, a cache would probably be used to avoid duplicated modules.

      const moduleMap = new Map([
        ['root', rootModule],
      ]);

      function resolveAndLinkDependencies(module) {
        const requestedModules = module.moduleRequests.map((request) => {
          // In a full-fledged module system, the resolveAndLinkDependencies would
          // resolve the module with the module cache key `[specifier, attributes]`.
          // In this example, we just use the specifier as the key.
          const specifier = request.specifier;

          let requestedModule = moduleMap.get(specifier);
          if (requestedModule === undefined) {
            requestedModule = new vm.SourceTextModule(`
              // The "secret" variable refers to the global variable we added to
              // "contextifiedObject" when creating the context.
              export default secret;
            `, { context: module.context });
            moduleMap.set(specifier, requestedModule);
            // Resolve the dependencies of the new module as well.
            resolveAndLinkDependencies(requestedModule);
          }

          return requestedModule;
        });

        module.linkRequests(requestedModules);
      }

      resolveAndLinkDependencies(rootModule);
      rootModule.instantiate();

      // Step 3
      //
      // Evaluate the Module. The evaluate() method returns a promise which will
      // resolve after the module has finished evaluating.

      // Prints 42.
      await rootModule.evaluate();
    })();
    javascript

#### `module.error`#

- Type: `<any>`

If the `module.status` is `'errored'`, this property contains the exception thrown by the module during evaluation. If the status is anything else, accessing this property will result in a thrown exception.

The value `undefined` cannot be used for cases where there is not a thrown exception due to possible ambiguity with `throw undefined;`.

Corresponds to the `[[EvaluationError]]` field of Cyclic Module Records in the ECMAScript specification.

#### `module.evaluate([options])`#

- `options` `<Object>`
  - `timeout` `<integer>` Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
- Returns: `<Promise>` Fulfills with `undefined` upon success.

Evaluate the module and its dependencies. Corresponds to the Evaluate() concrete method field of Cyclic Module Records in the ECMAScript specification.

If the module is a `vm.SourceTextModule`, `evaluate()` must be called after the module has been instantiated; otherwise `evaluate()` will return a rejected promise.

For a `vm.SourceTextModule`, the promise returned by `evaluate()` may be fulfilled either synchronously or asynchronously:

1. If the `vm.SourceTextModule` has no top-level `await` in itself or any of its dependencies, the promise will be fulfilled synchronously after the module and all its dependencies have been evaluated.
   1. If the evaluation succeeds, the promise will be synchronously resolved to `undefined`.
   2. If the evaluation results in an exception, the promise will be synchronously rejected with the exception that causes the evaluation to fail, which is the same as `module.error`.
2. If the `vm.SourceTextModule` has top-level `await` in itself or any of its dependencies, the promise will be fulfilled asynchronously after the module and all its dependencies have been evaluated.
   1. If the evaluation succeeds, the promise will be asynchronously resolved to `undefined`.
   2. If the evaluation results in an exception, the promise will be asynchronously rejected with the exception that causes the evaluation to fail.

If the module is a `vm.SyntheticModule`, `evaluate()` always returns a promise that fulfills synchronously, see the specification of Evaluate() of a Synthetic Module Record:

1. If the `evaluateCallback` passed to its constructor throws an exception synchronously, `evaluate()` returns a promise that will be synchronously rejected with that exception.
2. If the `evaluateCallback` does not throw an exception, `evaluate()` returns a promise that will be synchronously resolved to `undefined`.

The `evaluateCallback` of a `vm.SyntheticModule` is executed synchronously within the `evaluate()` call, and its return value is discarded. This means if `evaluateCallback` is an asynchronous function, the promise returned by `evaluate()` will not reflect its asynchronous behavior, and any rejections from an asynchronous `evaluateCallback` will be lost.

`evaluate()` could also be called again after the module has already been evaluated, in which case:

1. If the initial evaluation ended in success (`module.status` is `'evaluated'`), it will do nothing and return a promise that resolves to `undefined`.
2. If the initial evaluation resulted in an exception (`module.status` is `'errored'`), it will re-reject the exception that the initial evaluation resulted in.

This method cannot be called while the module is being evaluated (`module.status` is `'evaluating'`).

#### `module.identifier`#

- Type: `<string>`

The identifier of the current module, as set in the constructor.

#### `module.link(linker)`#

History

Version | Changes
--- | ---
v21.1.0, v20.10.0, v18.19.0 | The option `extra.assert` is renamed to `extra.attributes`. The former name is still provided for backward compatibility.

- `linker` `<Function>`

  - `specifier` `<string>` The specifier of the requested module:

        import foo from 'foo';
        //              ^^^^^ the module specifier
        mjs

  - `referencingModule` `<vm.Module>` The `Module` object `link()` is called on.

  - `extra` `<Object>`

    - `attributes` `<Object>` The data from the attribute:

          import foo from 'foo' with { name: 'value' };
          //                         ^^^^^^^^^^^^^^^^^ the attribute
          mjs

      Per ECMA-262, hosts are expected to trigger an error if an unsupported attribute is present.
    - `assert` `<Object>` Alias for `extra.attributes`.

  - Returns: `<vm.Module>` | `<Promise>`

- Returns: `<Promise>`

Link module dependencies. This method must be called before evaluation, and can only be called once per module.

Use `sourceTextModule.linkRequests(modules)` and `sourceTextModule.instantiate()` to link modules either synchronously or asynchronously.

The function is expected to return a `Module` object or a `Promise` that eventually resolves to a `Module` object. The returned `Module` must satisfy the following two invariants:

- It must belong to the same context as the parent `Module`.
- Its `status` must not be `'errored'`.

If the returned `Module`'s `status` is `'unlinked'`, this method will be recursively called on the returned `Module` with the same provided `linker` function.

`link()` returns a `Promise` that will either get resolved when all linking instances resolve to a valid `Module`, or rejected if the linker function either throws an exception or returns an invalid `Module`.

The linker function roughly corresponds to the implementation-defined HostResolveImportedModule abstract operation in the ECMAScript specification, with a few key differences:

- The linker function is allowed to be asynchronous while HostResolveImportedModule is synchronous.

The actual HostResolveImportedModule implementation used during module linking is one that returns the modules linked during linking. Since at that point all modules would have been fully linked already, the HostResolveImportedModule implementation is fully synchronous per specification.

Corresponds to the Link() concrete method field of Cyclic Module Records in the ECMAScript specification.

#### `module.namespace`#

- Type: `<Object>`

The namespace object of the module. This is only available after linking (`module.link()`) has completed.

Corresponds to the GetModuleNamespace abstract operation in the ECMAScript specification.

#### `module.status`#

- Type: `<string>`

The current status of the module. Will be one of:

- `'unlinked'`: `module.link()` has not yet been called.

- `'linking'`: `module.link()` has been called, but not all Promises returned by the linker function have been resolved yet.

- `'linked'`: The module has been linked successfully, and all of its dependencies are linked, but `module.evaluate()` has not yet been called.

- `'evaluating'`: The module is being evaluated through a `module.evaluate()` on itself or a parent module.

- `'evaluated'`: The module has been successfully evaluated.

- `'errored'`: The module has been evaluated, but an exception was thrown.

Other than `'errored'`, this status string corresponds to the specification's Cyclic Module Record's `[[Status]]` field. `'errored'` corresponds to `'evaluated'` in the specification, but with `[[EvaluationError]]` set to a value that is not `undefined`.

### Class: `vm.SourceTextModule`#

Added in: v9.6.0

Stability: 1 - Experimental

This feature is only available with the `--experimental-vm-modules` command flag enabled.

- Extends: `<vm.Module>`

The `vm.SourceTextModule` class provides the Source Text Module Record as defined in the ECMAScript specification.

#### `new vm.SourceTextModule(code[, options])`#

History

Version | Changes
--- | ---
v17.0.0, v16.12.0 | Added support for import attributes to the `importModuleDynamically` parameter.

- `code` `<string>` JavaScript Module code to parse
- `options`
  - `identifier` `<string>` String used in stack traces. Default: `'vm:module(i)'` where `i` is a context-specific ascending index.
  - `cachedData` `<Buffer>` | `<TypedArray>` | `<DataView>` Provides an optional `Buffer` or `TypedArray`, or `DataView` with V8's code cache data for the supplied source. The `code` must be the same as the module from which this `cachedData` was created.
  - `context` `<Object>` The contextified object as returned by the `vm.createContext()` method, to compile and evaluate this `Module` in. If no context is specified, the module is evaluated in the current execution context.
  - `lineOffset` `<integer>` Specifies the line number offset that is displayed in stack traces produced by this `Module`. Default: `0`.
  - `columnOffset` `<integer>` Specifies the first-line column number offset that is displayed in stack traces produced by this `Module`. Default: `0`.
  - `initializeImportMeta` `<Function>` Called during evaluation of this `Module` to initialize the `import.meta`.
    - `meta` `<import.meta>`
    - `module` `<vm.SourceTextModule>`
  - `importModuleDynamically` `<Function>` Used to specify the how the modules should be loaded during the evaluation of this module when `import()` is called. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.

Creates a new `SourceTextModule` instance.

Properties assigned to the `import.meta` object that are objects may allow the module to access information outside the specified `context`. Use `vm.runInContext()` to create objects in a specific context.

    import vm from 'node:vm';

    const contextifiedObject = vm.createContext({ secret: 42 });

    const module = new vm.SourceTextModule(
      'Object.getPrototypeOf(import.meta.prop).secret = secret;',
      {
        context: contextifiedObject,
        initializeImportMeta(meta) {
          // Note: this object is created in the top context. As such,
          // Object.getPrototypeOf(import.meta.prop) points to the
          // Object.prototype in the top context rather than that in
          // the contextified object.
          meta.prop = {};
        },
      });
    // The module has an empty `moduleRequests` array.
    module.linkRequests([]);
    module.instantiate();
    await module.evaluate();

    // Now, Object.prototype.secret will be equal to 42.
    //
    // To fix this problem, replace
    //     meta.prop = {};
    // above with
    //     meta.prop = vm.runInContext('({})', contextifiedObject);
    const vm = require('node:vm');
    const contextifiedObject = vm.createContext({ secret: 42 });
    (async () => {
      const module = new vm.SourceTextModule(
        'Object.getPrototypeOf(import.meta.prop).secret = secret;',
        {
          context: contextifiedObject,
          initializeImportMeta(meta) {
            // Note: this object is created in the top context. As such,
            // Object.getPrototypeOf(import.meta.prop) points to the
            // Object.prototype in the top context rather than that in
            // the contextified object.
            meta.prop = {};
          },
        });
      // The module has an empty `moduleRequests` array.
      module.linkRequests([]);
      module.instantiate();
      await module.evaluate();
      // Now, Object.prototype.secret will be equal to 42.
      //
      // To fix this problem, replace
      //     meta.prop = {};
      // above with
      //     meta.prop = vm.runInContext('({})', contextifiedObject);
    })();
    javascript

#### `sourceTextModule.createCachedData()`#

Added in: v13.7.0, v12.17.0

- Returns: `<Buffer>`

Creates a code cache that can be used with the `SourceTextModule` constructor's `cachedData` option. Returns a `Buffer`. This method may be called any number of times before the module has been evaluated.

The code cache of the `SourceTextModule` doesn't contain any JavaScript observable states. The code cache is safe to be saved along side the script source and used to construct new `SourceTextModule` instances multiple times.

Functions in the `SourceTextModule` source can be marked as lazily compiled and they are not compiled at construction of the `SourceTextModule`. These functions are going to be compiled when they are invoked the first time. The code cache serializes the metadata that V8 currently knows about the `SourceTextModule` that it can use to speed up future compilations.

    // Create an initial module
    const module = new vm.SourceTextModule('const a = 1;');

    // Create cached data from this module
    const cachedData = module.createCachedData();

    // Create a new module using the cached data. The code must be the same.
    const module2 = new vm.SourceTextModule('const a = 1;', { cachedData });
    js

#### `sourceTextModule.dependencySpecifiers`#

History

Version | Changes
--- | ---
v24.4.0, v22.20.0 | This is deprecated in favour of `sourceTextModule.moduleRequests`.

Stability: 0 - Deprecated: Use `sourceTextModule.moduleRequests` instead.

- Type: `<string>`[]

The specifiers of all dependencies of this module. The returned array is frozen to disallow any changes to it.

Corresponds to the `[[RequestedModules]]` field of Cyclic Module Records in the ECMAScript specification.

#### `sourceTextModule.hasAsyncGraph()`#

Added in: v24.9.0

- Returns: `<boolean>`

Iterates over the dependency graph and returns `true` if any module in its dependencies or this module itself contains top-level `await` expressions, otherwise returns `false`.

The search may be slow if the graph is big enough.

This requires the module to be instantiated first. If the module is not instantiated yet, an error will be thrown.

#### `sourceTextModule.hasTopLevelAwait()`#

Added in: v24.9.0

- Returns: `<boolean>`

Returns whether the module itself contains any top-level `await` expressions.

This corresponds to the field `[[HasTLA]]` in Cyclic Module Record in the ECMAScript specification.

#### `sourceTextModule.instantiate()`#

Added in: v24.8.0, v22.21.0

- Returns: `<undefined>`

Instantiate the module with the linked requested modules.

This resolves the imported bindings of the module, including re-exported binding names. When there are any bindings that cannot be resolved, an error would be thrown synchronously.

If the requested modules include cyclic dependencies, the `sourceTextModule.linkRequests(modules)` method must be called on all modules in the cycle before calling this method.

#### `sourceTextModule.linkRequests(modules)`#

Added in: v24.8.0, v22.21.0

- `modules` `<vm.Module>`[] Array of `vm.Module` objects that this module depends on. The order of the modules in the array is the order of `sourceTextModule.moduleRequests`.
- Returns: `<undefined>`

Link module dependencies. This method must be called before evaluation, and can only be called once per module.

The order of the module instances in the `modules` array should correspond to the order of `sourceTextModule.moduleRequests` being resolved. If two module requests have the same specifier and import attributes, they must be resolved with the same module instance or an `ERR_MODULE_LINK_MISMATCH` would be thrown. For example, when linking requests for this module:

    import foo from 'foo';
    import source Foo from 'foo';
    mjs

The `modules` array must contain two references to the same instance, because the two module requests are identical but in two phases.

If the module has no dependencies, the `modules` array can be empty.

Users can use `sourceTextModule.moduleRequests` to implement the host-defined HostLoadImportedModule abstract operation in the ECMAScript specification, and using `sourceTextModule.linkRequests()` to invoke specification defined FinishLoadingImportedModule, on the module with all dependencies in a batch.

It's up to the creator of the `SourceTextModule` to determine if the resolution of the dependencies is synchronous or asynchronous.

After each module in the `modules` array is linked, call `sourceTextModule.instantiate()`.

#### `sourceTextModule.moduleRequests`#

Added in: v24.4.0, v22.20.0

- Type: `<ModuleRequest>`[] Dependencies of this module.

The requested import dependencies of this module. The returned array is frozen to disallow any changes to it.

For example, given a source text:

    import foo from 'foo';
    import fooAlias from 'foo';
    import bar from './bar.js';
    import withAttrs from '../with-attrs.ts' with { arbitraryAttr: 'attr-val' };
    import source Module from 'wasm-mod.wasm';
    mjs

The value of the `sourceTextModule.moduleRequests` will be:

    [
      {
        specifier: 'foo',
        attributes: {},
        phase: 'evaluation',
      },
      {
        specifier: 'foo',
        attributes: {},
        phase: 'evaluation',
      },
      {
        specifier: './bar.js',
        attributes: {},
        phase: 'evaluation',
      },
      {
        specifier: '../with-attrs.ts',
        attributes: { arbitraryAttr: 'attr-val' },
        phase: 'evaluation',
      },
      {
        specifier: 'wasm-mod.wasm',
        attributes: {},
        phase: 'source',
      },
    ];
    js

### Class: `vm.SyntheticModule`#

Added in: v13.0.0, v12.16.0

Stability: 1 - Experimental

This feature is only available with the `--experimental-vm-modules` command flag enabled.

- Extends: `<vm.Module>`

The `vm.SyntheticModule` class provides the Synthetic Module Record as defined in the WebIDL specification. The purpose of synthetic modules is to provide a generic interface for exposing non-JavaScript sources to ECMAScript module graphs.

    import { SyntheticModule } from 'node:vm';

    const source = '{ "a": 1 }';
    const syntheticModule = new SyntheticModule(['default'], function() {
      const obj = JSON.parse(source);
      this.setExport('default', obj);
    });

    // Use `syntheticModule` in linking
    (async () => {
      await syntheticModule.link(() => {});
      await syntheticModule.evaluate();

      console.log('Default export:', syntheticModule.namespace.default);
    })();
    const { SyntheticModule } = require('node:vm');

    const source = '{ "a": 1 }';
    const syntheticModule = new SyntheticModule(['default'], function() {
      const obj = JSON.parse(source);
      this.setExport('default', obj);
    });

    // Use `syntheticModule` in linking
    (async () => {
      await syntheticModule.link(() => {});
      await syntheticModule.evaluate();

      console.log('Default export:', syntheticModule.namespace.default);
    })();
    javascript

#### `new vm.SyntheticModule(exportNames, evaluateCallback[, options])`#

Added in: v13.0.0, v12.16.0

- `exportNames` `<string>`[] Array of names that will be exported from the module.
- `evaluateCallback` `<Function>` Called when the module is evaluated.
- `options`
  - `identifier` `<string>` String used in stack traces. Default: `'vm:module(i)'` where `i` is a context-specific ascending index.
  - `context` `<Object>` The contextified object as returned by the `vm.createContext()` method, to compile and evaluate this `Module` in.

Creates a new `SyntheticModule` instance.

Objects assigned to the exports of this instance may allow importers of the module to access information outside the specified `context`. Use `vm.runInContext()` to create objects in a specific context.

#### `syntheticModule.setExport(name, value)`#

Added in: v13.0.0, v12.16.0
History

Version | Changes
--- | ---
v24.8.0, v22.21.0 | No longer need to call `syntheticModule.link()` before calling this method.

- `name` `<string>` Name of the export to set.
- `value` `<any>` The value to set the export to.

This method sets the module export binding slots with the given value.

    import vm from 'node:vm';

    const m = new vm.SyntheticModule(['x'], () => {
      m.setExport('x', 1);
    });

    await m.evaluate();

    assert.strictEqual(m.namespace.x, 1);
    const vm = require('node:vm');
    (async () => {
      const m = new vm.SyntheticModule(['x'], () => {
        m.setExport('x', 1);
      });
      await m.evaluate();
      assert.strictEqual(m.namespace.x, 1);
    })();
    javascript

### Type: `ModuleRequest`#

Added in: v24.4.0, v22.20.0

- Type: `<Object>`
  - `specifier` `<string>` The specifier of the requested module.
  - `attributes` `<Object>` The `"with"` value passed to the WithClause in a ImportDeclaration, or an empty object if no value was provided.
  - `phase` `<string>` The phase of the requested module (`"source"` or `"evaluation"`).

A `ModuleRequest` represents the request to import a module with given import attributes and phase.

### `vm.compileFunction(code[, params[, options]])`#

Added in: v10.10.0
History

Version | Changes
--- | ---
v21.7.0, v20.12.0 | Added support for `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
v19.6.0, v18.15.0 | The return value now includes `cachedDataRejected` with the same semantics as the `vm.Script` version if the `cachedData` option was passed.
v17.0.0, v16.12.0 | Added support for import attributes to the `importModuleDynamically` parameter.
v15.9.0 | Added `importModuleDynamically` option again.
v14.3.0 | Removal of `importModuleDynamically` due to compatibility issues.
v14.1.0, v13.14.0 | The `importModuleDynamically` option is now supported.

- `code` `<string>` The body of the function to compile.
- `params` `<string>`[] An array of strings containing all parameters for the function.
- `options` `<Object>`
  - `filename` `<string>` Specifies the filename used in stack traces produced by this script. Default: `''`.
  - `lineOffset` `<number>` Specifies the line number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `columnOffset` `<number>` Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `cachedData` `<Buffer>` | `<TypedArray>` | `<DataView>` Provides an optional `Buffer` or `TypedArray`, or `DataView` with V8's code cache data for the supplied source. This must be produced by a prior call to `vm.compileFunction()` with the same `code` and `params`.
  - `produceCachedData` `<boolean>` Specifies whether to produce new cache data. Default: `false`.
  - `parsingContext` `<Object>` The contextified object in which the said function should be compiled in.
  - `contextExtensions` `<Object>`[] An array containing a collection of context extensions (objects wrapping the current scope) to be applied while compiling. Default: `[]`.
  - `importModuleDynamically` `<Function>` | `<vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER>` Used to specify the how the modules should be loaded during the evaluation of this function when `import()` is called. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.
- Returns: `<Function>`

Compiles the given code into the provided context (if no context is supplied, the current context is used), and returns it wrapped inside a function with the given `params`.

### `vm.constants`#

Added in: v21.7.0, v20.12.0

- Type: `<Object>`

Returns an object containing commonly used constants for VM operations.

#### `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`#

Added in: v21.7.0, v20.12.0

Stability: 1.1 - Active development

A constant that can be used as the `importModuleDynamically` option to `vm.Script` and `vm.compileFunction()` so that Node.js uses the default ESM loader from the main context to load the requested module.

For detailed information, see Support of dynamic `import()` in compilation APIs.

### `vm.createContext([contextObject[, options]])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v22.8.0, v20.18.0 | The `contextObject` argument now accepts `vm.constants.DONT_CONTEXTIFY`.
v21.7.0, v20.12.0 | Added support for `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
v21.2.0, v20.11.0 | The `importModuleDynamically` option is supported now.
v14.6.0 | The `microtaskMode` option is supported now.
v10.0.0 | The first argument can no longer be a function.
v10.0.0 | The `codeGeneration` option is supported now.

- `contextObject` `<Object>` | `<vm.constants.DONT_CONTEXTIFY>` | `<undefined>` Either `vm.constants.DONT_CONTEXTIFY` or an object that will be contextified. If `undefined`, an empty contextified object will be created for backwards compatibility.
- `options` `<Object>`
  - `name` `<string>` Human-readable name of the newly created context. Default: `'VM Context i'`, where `i` is an ascending numerical index of the created context.
  - `origin` `<string>` Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the `url.origin` property of a `URL` object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: `''`.
  - `codeGeneration` `<Object>`
    - `strings` `<boolean>` If set to false any calls to `eval` or function constructors (`Function`, `GeneratorFunction`, etc) will throw an `EvalError`. Default: `true`.
    - `wasm` `<boolean>` If set to false any attempt to compile a WebAssembly module will throw a `WebAssembly.CompileError`. Default: `true`.
  - `microtaskMode` `<string>` If set to `afterEvaluate`, microtasks (tasks scheduled through `Promise`s and `async function`s) will be run immediately after a script has run through `script.runInContext()`. They are included in the `timeout` and `breakOnSigint` scopes in that case.
  - `importModuleDynamically` `<Function>` | `<vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER>` Used to specify the how the modules should be loaded when `import()` is called in this context without a referrer script or module. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.
- Returns: `<Object>` contextified object.

If the given `contextObject` is an object, the `vm.createContext()` method will prepare that object and return a reference to it so that it can be used in calls to `vm.runInContext()` or `script.runInContext()`. Inside such scripts, the global object will be wrapped by the `contextObject`, retaining all of its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, global variables will remain unchanged.

    import { createContext, runInContext } from 'node:vm';

    global.globalVar = 3;

    const context = { globalVar: 1 };
    createContext(context);

    runInContext('globalVar *= 2;', context);

    console.log(context);
    // Prints: { globalVar: 2 }

    console.log(global.globalVar);
    // Prints: 3
    const { createContext, runInContext } = require('node:vm');

    global.globalVar = 3;

    const context = { globalVar: 1 };
    createContext(context);

    runInContext('globalVar *= 2;', context);

    console.log(context);
    // Prints: { globalVar: 2 }

    console.log(global.globalVar);
    // Prints: 3
    javascript

If `contextObject` is omitted (or passed explicitly as `undefined`), a new, empty contextified object will be returned.

When the global object in the newly created context is contextified, it has some quirks compared to ordinary global objects. For example, it cannot be frozen. To create a context without the contextifying quirks, pass `vm.constants.DONT_CONTEXTIFY` as the `contextObject` argument. See the documentation of `vm.constants.DONT_CONTEXTIFY` for details.

The `vm.createContext()` method is primarily useful for creating a single context that can be used to run multiple scripts. For instance, if emulating a web browser, the method can be used to create a single context representing a window's global object, then run all `<script>` tags together within that context.

The provided `name` and `origin` of the context are made visible through the Inspector API.

### `vm.isContext(object)`#

Added in: v0.11.7

- `object` `<Object>`
- Returns: `<boolean>`

Returns `true` if the given `object` object has been contextified using `vm.createContext()`, or if it's the global object of a context created using `vm.constants.DONT_CONTEXTIFY`.

### `vm.measureMemory([options])`#

Added in: v13.10.0

Stability: 1 - Experimental

Measure the memory known to V8 and used by all contexts known to the current V8 isolate, or the main context.

- `options` `<Object>` Optional.
  - `mode` `<string>` Either `'summary'` or `'detailed'`. In summary mode, only the memory measured for the main context will be returned. In detailed mode, the memory measured for all contexts known to the current V8 isolate will be returned. Default: `'summary'`
  - `execution` `<string>` Either `'default'` or `'eager'`. With default execution, the promise will not resolve until after the next scheduled garbage collection starts, which may take a while (or never if the program exits before the next GC). With eager execution, the GC will be started right away to measure the memory. Default: `'default'`
- Returns: `<Promise>` If the memory is successfully measured, the promise will resolve with an object containing information about the memory usage. Otherwise it will be rejected with an `ERR_CONTEXT_NOT_INITIALIZED` error.

The format of the object that the returned Promise may resolve with is specific to the V8 engine and may change from one version of V8 to the next.

The returned result is different from the statistics returned by `v8.getHeapSpaceStatistics()` in that `vm.measureMemory()` measure the memory reachable by each V8 specific contexts in the current instance of the V8 engine, while the result of `v8.getHeapSpaceStatistics()` measure the memory occupied by each heap space in the current V8 instance.

    import { createContext, measureMemory } from 'node:vm';
    // Measure the memory used by the main context.
    measureMemory({ mode: 'summary' })
      // This is the same as vm.measureMemory()
      .then((result) => {
        // The current format is:
        // {
        //   total: { jsMemoryEstimate: 1601828, jsMemoryRange: [1601828, 5275288] },
        //   WebAssembly: { code: 0, metadata: 33962 },
        // }
        console.log(result);
      });

    const context = createContext({ a: 1 });
    measureMemory({ mode: 'detailed', execution: 'eager' }).then((result) => {
      // Reference the context here so that it won't be GC'ed
      // until the measurement is complete.
      console.log('Context:', context.a);
      // {
      //   total: { jsMemoryEstimate: 1767100, jsMemoryRange: [1767100, 5440560] },
      //   WebAssembly: { code: 0, metadata: 33962 },
      //   current: { jsMemoryEstimate: 1601828, jsMemoryRange: [1601828, 5275288] },
      //   other: [{ jsMemoryEstimate: 165272, jsMemoryRange: [Array] }],
      // }
      console.log(result);
    });
    const { createContext, measureMemory } = require('node:vm');
    // Measure the memory used by the main context.
    measureMemory({ mode: 'summary' })
      // This is the same as vm.measureMemory()
      .then((result) => {
        // The current format is:
        // {
        //   total: { jsMemoryEstimate: 1601828, jsMemoryRange: [1601828, 5275288] },
        //   WebAssembly: { code: 0, metadata: 33962 },
        // }
        console.log(result);
      });

    const context = createContext({ a: 1 });
    measureMemory({ mode: 'detailed', execution: 'eager' }).then((result) => {
      // Reference the context here so that it won't be GC'ed
      // until the measurement is complete.
      console.log('Context:', context.a);
      // {
      //   total: { jsMemoryEstimate: 1767100, jsMemoryRange: [1767100, 5440560] },
      //   WebAssembly: { code: 0, metadata: 33962 },
      //   current: { jsMemoryEstimate: 1601828, jsMemoryRange: [1601828, 5275288] },
      //   other: [{ jsMemoryEstimate: 165272, jsMemoryRange: [Array] }],
      // }
      console.log(result);
    });
    javascript

### `vm.runInContext(code, contextifiedObject[, options])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v21.7.0, v20.12.0 | Added support for `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
v17.0.0, v16.12.0 | Added support for import attributes to the `importModuleDynamically` parameter.
v6.3.0 | The `breakOnSigint` option is supported now.

- `code` `<string>` The JavaScript code to compile and run.
- `contextifiedObject` `<Object>` The contextified object that will be used as the `global` when the `code` is compiled and run.
- `options` `<Object>` | `<string>`
  - `filename` `<string>` Specifies the filename used in stack traces produced by this script. Default: `'evalmachine.<anonymous>'`.
  - `lineOffset` `<number>` Specifies the line number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `columnOffset` `<number>` Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `displayErrors` `<boolean>` When `true`, if an `Error` occurs while compiling the `code`, the line of code causing the error is attached to the stack trace. Default: `true`.
  - `timeout` `<integer>` Specifies the number of milliseconds to execute `code` before terminating execution. If execution is terminated, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
  - `cachedData` `<Buffer>` | `<TypedArray>` | `<DataView>` Provides an optional `Buffer` or `TypedArray`, or `DataView` with V8's code cache data for the supplied source.
  - `importModuleDynamically` `<Function>` | `<vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER>` Used to specify the how the modules should be loaded during the evaluation of this script when `import()` is called. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.

The `vm.runInContext()` method compiles `code`, runs it within the context of the `contextifiedObject`, then returns the result. Running code does not have access to the local scope. The `contextifiedObject` object must have been previously contextified using the `vm.createContext()` method.

If `options` is a string, then it specifies the filename.

The following example compiles and executes different scripts using a single contextified object:

    import { createContext, runInContext } from 'node:vm';

    const contextObject = { globalVar: 1 };
    createContext(contextObject);

    for (let i = 0; i < 10; ++i) {
      runInContext('globalVar *= 2;', contextObject);
    }
    console.log(contextObject);
    // Prints: { globalVar: 1024 }
    const { createContext, runInContext } = require('node:vm');

    const contextObject = { globalVar: 1 };
    createContext(contextObject);

    for (let i = 0; i < 10; ++i) {
      runInContext('globalVar *= 2;', contextObject);
    }
    console.log(contextObject);
    // Prints: { globalVar: 1024 }
    javascript

### `vm.runInNewContext(code[, contextObject[, options]])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v22.8.0, v20.18.0 | The `contextObject` argument now accepts `vm.constants.DONT_CONTEXTIFY`.
v21.7.0, v20.12.0 | Added support for `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
v17.0.0, v16.12.0 | Added support for import attributes to the `importModuleDynamically` parameter.
v14.6.0 | The `microtaskMode` option is supported now.
v10.0.0 | The `contextCodeGeneration` option is supported now.
v6.3.0 | The `breakOnSigint` option is supported now.

- `code` `<string>` The JavaScript code to compile and run.
- `contextObject` `<Object>` | `<vm.constants.DONT_CONTEXTIFY>` | `<undefined>` Either `vm.constants.DONT_CONTEXTIFY` or an object that will be contextified. If `undefined`, an empty contextified object will be created for backwards compatibility.
- `options` `<Object>` | `<string>`
  - `filename` `<string>` Specifies the filename used in stack traces produced by this script. Default: `'evalmachine.<anonymous>'`.
  - `lineOffset` `<number>` Specifies the line number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `columnOffset` `<number>` Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `displayErrors` `<boolean>` When `true`, if an `Error` occurs while compiling the `code`, the line of code causing the error is attached to the stack trace. Default: `true`.
  - `timeout` `<integer>` Specifies the number of milliseconds to execute `code` before terminating execution. If execution is terminated, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
  - `contextName` `<string>` Human-readable name of the newly created context. Default: `'VM Context i'`, where `i` is an ascending numerical index of the created context.
  - `contextOrigin` `<string>` Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the `url.origin` property of a `URL` object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: `''`.
  - `contextCodeGeneration` `<Object>`
    - `strings` `<boolean>` If set to false any calls to `eval` or function constructors (`Function`, `GeneratorFunction`, etc) will throw an `EvalError`. Default: `true`.
    - `wasm` `<boolean>` If set to false any attempt to compile a WebAssembly module will throw a `WebAssembly.CompileError`. Default: `true`.
  - `cachedData` `<Buffer>` | `<TypedArray>` | `<DataView>` Provides an optional `Buffer` or `TypedArray`, or `DataView` with V8's code cache data for the supplied source.
  - `importModuleDynamically` `<Function>` | `<vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER>` Used to specify the how the modules should be loaded during the evaluation of this script when `import()` is called. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.
  - `microtaskMode` `<string>` If set to `afterEvaluate`, microtasks (tasks scheduled through `Promise`s and `async function`s) will be run immediately after the script has run. They are included in the `timeout` and `breakOnSigint` scopes in that case.
- Returns: `<any>` the result of the very last statement executed in the script.

This method is a shortcut to `(new vm.Script(code, options)).runInContext(vm.createContext(options), options)`. If `options` is a string, then it specifies the filename.

It does several things at once:

1. Creates a new context.
2. If `contextObject` is an object, contextifies it with the new context. If `contextObject` is undefined, creates a new object and contextifies it. If `contextObject` is `vm.constants.DONT_CONTEXTIFY`, don't contextify anything.
3. Compiles the code as a `vm.Script`
4. Runs the compiled code within the created context. The code does not have access to the scope in which this method is called.
5. Returns the result.

The following example compiles and executes code that increments a global variable and sets a new one. These globals are contained in the `contextObject`.

    import { runInNewContext, constants } from 'node:vm';

    const contextObject = {
      animal: 'cat',
      count: 2,
    };

    runInNewContext('count += 1; name = "kitty"', contextObject);
    console.log(contextObject);
    // Prints: { animal: 'cat', count: 3, name: 'kitty' }

    // This would throw if the context is created from a contextified object.
    // vm.constants.DONT_CONTEXTIFY allows creating contexts with ordinary global objects that
    // can be frozen.
    const frozenContext = runInNewContext(
      'Object.freeze(globalThis); globalThis;',
      constants.DONT_CONTEXTIFY,
    );
    const { runInNewContext, constants } = require('node:vm');

    const contextObject = {
      animal: 'cat',
      count: 2,
    };

    runInNewContext('count += 1; name = "kitty"', contextObject);
    console.log(contextObject);
    // Prints: { animal: 'cat', count: 3, name: 'kitty' }

    // This would throw if the context is created from a contextified object.
    // vm.constants.DONT_CONTEXTIFY allows creating contexts with ordinary global objects that
    // can be frozen.
    const frozenContext = runInNewContext(
      'Object.freeze(globalThis); globalThis;',
      constants.DONT_CONTEXTIFY,
    );
    javascript

### `vm.runInThisContext(code[, options])`#

Added in: v0.3.1
History

Version | Changes
--- | ---
v21.7.0, v20.12.0 | Added support for `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`.
v17.0.0, v16.12.0 | Added support for import attributes to the `importModuleDynamically` parameter.
v6.3.0 | The `breakOnSigint` option is supported now.

- `code` `<string>` The JavaScript code to compile and run.
- `options` `<Object>` | `<string>`
  - `filename` `<string>` Specifies the filename used in stack traces produced by this script. Default: `'evalmachine.<anonymous>'`.
  - `lineOffset` `<number>` Specifies the line number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `columnOffset` `<number>` Specifies the first-line column number offset that is displayed in stack traces produced by this script. Default: `0`.
  - `displayErrors` `<boolean>` When `true`, if an `Error` occurs while compiling the `code`, the line of code causing the error is attached to the stack trace. Default: `true`.
  - `timeout` `<integer>` Specifies the number of milliseconds to execute `code` before terminating execution. If execution is terminated, an `Error` will be thrown. This value must be a strictly positive integer.
  - `breakOnSigint` `<boolean>` If `true`, receiving `SIGINT` (`Ctrl`+`C`) will terminate execution and throw an `Error`. Existing handlers for the event that have been attached via `process.on('SIGINT')` are disabled during script execution, but continue to work after that. Default: `false`.
  - `cachedData` `<Buffer>` | `<TypedArray>` | `<DataView>` Provides an optional `Buffer` or `TypedArray`, or `DataView` with V8's code cache data for the supplied source.
  - `importModuleDynamically` `<Function>` | `<vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER>` Used to specify the how the modules should be loaded during the evaluation of this script when `import()` is called. This option is part of the experimental modules API. We do not recommend using it in a production environment. For detailed information, see Support of dynamic `import()` in compilation APIs.
- Returns: `<any>` the result of the very last statement executed in the script.

`vm.runInThisContext()` compiles `code`, runs it within the context of the current `global` and returns the result. Running code does not have access to local scope, but does have access to the current `global` object.

If `options` is a string, then it specifies the filename.

The following example illustrates using both `vm.runInThisContext()` and the JavaScript `eval()` function to run the same code:

    import { runInThisContext } from 'node:vm';
    let localVar = 'initial value';

    const vmResult = runInThisContext('localVar = "vm";');
    console.log(`vmResult: '${vmResult}', localVar: '${localVar}'`);
    // Prints: vmResult: 'vm', localVar: 'initial value'

    const evalResult = eval('localVar = "eval";');
    console.log(`evalResult: '${evalResult}', localVar: '${localVar}'`);
    // Prints: evalResult: 'eval', localVar: 'eval'
    const { runInThisContext } = require('node:vm');
    let localVar = 'initial value';

    const vmResult = runInThisContext('localVar = "vm";');
    console.log(`vmResult: '${vmResult}', localVar: '${localVar}'`);
    // Prints: vmResult: 'vm', localVar: 'initial value'

    const evalResult = eval('localVar = "eval";');
    console.log(`evalResult: '${evalResult}', localVar: '${localVar}'`);
    // Prints: evalResult: 'eval', localVar: 'eval'
    javascript

Because `vm.runInThisContext()` does not have access to the local scope, `localVar` is unchanged. In contrast, a direct `eval()` call does have access to the local scope, so the value `localVar` is changed. In this way `vm.runInThisContext()` is much like an indirect `eval()` call, e.g. `(0,eval)('code')`.

### Example: Running an HTTP server within a VM#

When using either `script.runInThisContext()` or `vm.runInThisContext()`, the code is executed within the current V8 global context. The code passed to this VM context will have its own isolated scope.

In order to run a simple web server using the `node:http` module the code passed to the context must either call `require('node:http')` on its own, or have a reference to the `node:http` module passed to it. For instance:

    import { runInThisContext } from 'node:vm';
    import { createRequire } from 'node:module';

    const require = createRequire(import.meta.url);

    const code = `
    ((require) => {
      const { createServer } = require('node:http');

      createServer((request, response) => {
        response.writeHead(200, { 'Content-Type': 'text/plain' });
        response.end('Hello World\\n');
      }).listen(8124);

      console.log('Server running at http://127.0.0.1:8124/');
    })`;

    runInThisContext(code)(require);
    const { runInThisContext } = require('node:vm');

    const code = `
    ((require) => {
      const { createServer } = require('node:http');

      createServer((request, response) => {
        response.writeHead(200, { 'Content-Type': 'text/plain' });
        response.end('Hello World\\n');
      }).listen(8124);

      console.log('Server running at http://127.0.0.1:8124/');
    })`;

    runInThisContext(code)(require);
    javascript

The `require()` in the above case shares the state with the context it is passed from. This may introduce risks when untrusted code is executed, e.g. altering objects in the context in unwanted ways.

### What does it mean to "contextify" an object?#

All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:

> In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

When the method `vm.createContext()` is called with an object, the `contextObject` argument will be used to wrap the global object of a new instance of a V8 Context (if `contextObject` is `undefined`, a new object will be created from the current context before its contextified). This V8 Context provides the `code` run using the `node:vm` module's methods with an isolated global environment within which it can operate. The process of creating the V8 Context and associating it with the `contextObject` in the outer context is what this document refers to as "contextifying" the object.

The contextifying would introduce some quirks to the `globalThis` value in the context. For example, it cannot be frozen, and it is not reference equal to the `contextObject` in the outer context.

    import { createContext, runInContext } from 'node:vm';

    // An undefined `contextObject` option makes the global object contextified.
    const context = createContext();
    console.log(runInContext('globalThis', context) === context);  // false
    // A contextified global object cannot be frozen.
    try {
      runInContext('Object.freeze(globalThis);', context);
    } catch (e) {
      console.log(`${e.constructor.name}: ${e.message}`); // TypeError: Cannot freeze
    }
    console.log(runInContext('globalThis.foo = 1; foo;', context));  // 1
    const { createContext, runInContext } = require('node:vm');

    // An undefined `contextObject` option makes the global object contextified.
    const context = createContext();
    console.log(runInContext('globalThis', context) === context);  // false
    // A contextified global object cannot be frozen.
    try {
      runInContext('Object.freeze(globalThis);', context);
    } catch (e) {
      console.log(`${e.constructor.name}: ${e.message}`); // TypeError: Cannot freeze
    }
    console.log(runInContext('globalThis.foo = 1; foo;', context));  // 1
    javascript

To create a context with an ordinary global object and get access to a global proxy in the outer context with fewer quirks, specify `vm.constants.DONT_CONTEXTIFY` as the `contextObject` argument.

#### `vm.constants.DONT_CONTEXTIFY`#

This constant, when used as the `contextObject` argument in vm APIs, instructs Node.js to create a context without wrapping its global object with another object in a Node.js-specific manner. As a result, the `globalThis` value inside the new context would behave more closely to an ordinary one.

    import { createContext, runInContext, constants } from 'node:vm';

    // Use vm.constants.DONT_CONTEXTIFY to freeze the global object.
    const context = createContext(constants.DONT_CONTEXTIFY);
    runInContext('Object.freeze(globalThis);', context);
    try {
      runInContext('bar = 1; bar;', context);
    } catch (e) {
      console.log(`${e.constructor.name}: ${e.message}`); // ReferenceError: bar is not defined
    }
    const { createContext, runInContext, constants } = require('node:vm');

    // Use vm.constants.DONT_CONTEXTIFY to freeze the global object.
    const context = createContext(constants.DONT_CONTEXTIFY);
    runInContext('Object.freeze(globalThis);', context);
    try {
      runInContext('bar = 1; bar;', context);
    } catch (e) {
      console.log(`${e.constructor.name}: ${e.message}`); // ReferenceError: bar is not defined
    }
    javascript

When `vm.constants.DONT_CONTEXTIFY` is used as the `contextObject` argument to `vm.createContext()`, the returned object is a proxy-like object to the global object in the newly created context with fewer Node.js-specific quirks. It is reference equal to the `globalThis` value in the new context, can be modified from outside the context, and can be used to access built-ins in the new context directly.

    import { createContext, runInContext, constants } from 'node:vm';

    const context = createContext(constants.DONT_CONTEXTIFY);

    // Returned object is reference equal to globalThis in the new context.
    console.log(runInContext('globalThis', context) === context);  // true

    // Can be used to access globals in the new context directly.
    console.log(context.Array);  // [Function: Array]
    runInContext('foo = 1;', context);
    console.log(context.foo);  // 1
    context.bar = 1;
    console.log(runInContext('bar;', context));  // 1

    // Can be frozen and it affects the inner context.
    Object.freeze(context);
    try {
      runInContext('baz = 1; baz;', context);
    } catch (e) {
      console.log(`${e.constructor.name}: ${e.message}`); // ReferenceError: baz is not defined
    }
    const { createContext, runInContext, constants } = require('node:vm');

    const context = createContext(constants.DONT_CONTEXTIFY);

    // Returned object is reference equal to globalThis in the new context.
    console.log(runInContext('globalThis', context) === context);  // true

    // Can be used to access globals in the new context directly.
    console.log(context.Array);  // [Function: Array]
    runInContext('foo = 1;', context);
    console.log(context.foo);  // 1
    context.bar = 1;
    console.log(runInContext('bar;', context));  // 1

    // Can be frozen and it affects the inner context.
    Object.freeze(context);
    try {
      runInContext('baz = 1; baz;', context);
    } catch (e) {
      console.log(`${e.constructor.name}: ${e.message}`); // ReferenceError: baz is not defined
    }
    javascript

### Timeout interactions with asynchronous tasks and Promises#

`Promise`s and `async function`s can schedule tasks run by the JavaScript engine asynchronously. By default, these tasks are run after all JavaScript functions on the current stack are done executing. This allows escaping the functionality of the `timeout` and `breakOnSigint` options.

For example, the following code executed by `vm.runInNewContext()` with a timeout of 5 milliseconds schedules an infinite loop to run after a promise resolves. The scheduled loop is never interrupted by the timeout:

    import { runInNewContext } from 'node:vm';

    function loop() {
      console.log('entering loop');
      while (1) console.log(Date.now());
    }

    runInNewContext(
      'Promise.resolve().then(() => loop());',
      { loop, console },
      { timeout: 5 },
    );
    // This is printed *before* 'entering infinite loop' (!)
    console.log('done executing');
    const { runInNewContext } = require('node:vm');

    function loop() {
      console.log('entering loop');
      while (1) console.log(Date.now());
    }

    runInNewContext(
      'Promise.resolve().then(() => loop());',
      { loop, console },
      { timeout: 5 },
    );
    // This is printed *before* 'entering infinite loop' (!)
    console.log('done executing');
    javascript

This can be addressed by passing `microtaskMode: 'afterEvaluate'` to the code that creates the `Context`:

    import { runInNewContext } from 'node:vm';

    function loop() {
      while (1) console.log(Date.now());
    }

    runInNewContext(
      'Promise.resolve().then(() => loop());',
      { loop, console },
      { timeout: 5, microtaskMode: 'afterEvaluate' },
    );
    const { runInNewContext } = require('node:vm');

    function loop() {
      while (1) console.log(Date.now());
    }

    runInNewContext(
      'Promise.resolve().then(() => loop());',
      { loop, console },
      { timeout: 5, microtaskMode: 'afterEvaluate' },
    );
    javascript

In this case, the microtask scheduled through `promise.then()` will be run before returning from `vm.runInNewContext()`, and will be interrupted by the `timeout` functionality. This applies only to code running in a `vm.Context`, so e.g. `vm.runInThisContext()` does not take this option.

Promise callbacks are entered into the microtask queue of the context in which they were created. For example, if `() => loop()` is replaced with just `loop` in the above example, then `loop` will be pushed into the global microtask queue, because it is a function from the outer (main) context, and thus will also be able to escape the timeout.

If asynchronous scheduling functions such as `process.nextTick()`, `queueMicrotask()`, `setTimeout()`, `setImmediate()`, etc. are made available inside a `vm.Context`, functions passed to them will be added to global queues, which are shared by all contexts. Therefore, callbacks passed to those functions are not controllable through the timeout either.

#### When `microtaskMode` is `'afterEvaluate'`, beware sharing Promises between Contexts#

In `'afterEvaluate'` mode, the `Context` has its own microtask queue, separate from the global microtask queue used by the outer (main) context. While this mode is necessary to enforce `timeout` and enable `breakOnSigint` with asynchronous tasks, it also makes sharing promises between contexts challenging.

In the example below, a promise is created in the inner context and shared with the outer context. When the outer context `await` on the promise, the execution flow of the outer context is disrupted in a surprising way: the log statement is never executed.

    import { createContext, runInContext } from 'node:vm';

    const inner_context = createContext({}, { microtaskMode: 'afterEvaluate' });

    // runInContext() returns a Promise created in the inner context.
    const inner_promise = runInContext('Promise.resolve()', inner_context);

    // As part of performing `await`, the JavaScript runtime must enqueue a task
    // on the microtask queue of the context where `inner_promise` was created.
    // A task is added on the inner microtask queue, but **it will not be run
    // automatically**: this task will remain pending indefinitely.
    //
    // Since the outer microtask queue is empty, execution in the outer module
    // falls through, and the log statement below is never executed.
    await inner_promise;

    console.log('this will NOT be printed');
    const { createContext, runInContext } = require('node:vm');

    // runInContext() returns a Promise created in the inner context.
    const inner_context = createContext({}, { microtaskMode: 'afterEvaluate' });

    (async () => {
      const inner_promise = runInContext('Promise.resolve()', inner_context);

      // As part of performing `await`, the JavaScript runtime must enqueue a task
      // on the microtask queue of the context where `inner_promise` was created.
      // A task is added on the inner microtask queue, but **it will not be run
      // automatically**: this task will remain pending indefinitely.
      //
      // Since the outer microtask queue is empty, execution in the outer module
      // falls through, and the log statement below is never executed.
      await inner_promise;

      console.log('this will NOT be printed');
    })();
    javascript

To successfully share promises between contexts with different microtask queues, it is necessary to ensure that tasks on the inner microtask queue will be run whenever the outer context enqueues a task on the inner microtask queue.

The tasks on the microtask queue of a given context are run whenever `runInContext()` or `SourceTextModule.evaluate()` are invoked on a script or module using this context. In our example, the normal execution flow can be restored by scheduling a second call to `runInContext()` before `await inner_promise`.

    // Schedule `runInContext()` to manually drain the inner context microtask
    // queue; it will run after the `await` statement below.
    setImmediate(() => {
      vm.runInContext('', context);
    });

    await inner_promise;

    console.log('OK');
    mjs

Note: Strictly speaking, in this mode, `node:vm` departs from the letter of the ECMAScript specification for enqueuing jobs, by allowing asynchronous tasks from different contexts to run in a different order than they were enqueued.

### Support of dynamic `import()` in compilation APIs#

The following APIs support an `importModuleDynamically` option to enable dynamic `import()` in code compiled by the vm module.

- `new vm.Script`
- `vm.compileFunction()`
- `new vm.SourceTextModule`
- `vm.runInThisContext()`
- `vm.runInContext()`
- `vm.runInNewContext()`
- `vm.createContext()`

This option is still part of the experimental modules API. We do not recommend using it in a production environment.

#### When the `importModuleDynamically` option is not specified or undefined#

If this option is not specified, or if it's `undefined`, code containing `import()` can still be compiled by the vm APIs, but when the compiled code is executed and it actually calls `import()`, the result will reject with `ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING`.

#### When `importModuleDynamically` is `vm.constants.USE_MAIN_CONTEXT_DEFAULT_LOADER`#

This option is currently not supported for `vm.SourceTextModule`.

With this option, when an `import()` is initiated in the compiled code, Node.js would use the default ESM loader from the main context to load the requested module and return it to the code being executed.

This gives access to Node.js built-in modules such as `fs` or `http` to the code being compiled. If the code is executed in a different context, be aware that the objects created by modules loaded from the main context are still from the main context and not `instanceof` built-in classes in the new context.

    const { Script, constants } = require('node:vm');
    const script = new Script(
      'import("node:fs").then(({readFile}) => readFile instanceof Function)',
      { importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER });

    // false: URL loaded from the main context is not an instance of the Function
    // class in the new context.
    script.runInNewContext().then(console.log);
    import { Script, constants } from 'node:vm';

    const script = new Script(
      'import("node:fs").then(({readFile}) => readFile instanceof Function)',
      { importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER });

    // false: URL loaded from the main context is not an instance of the Function
    // class in the new context.
    script.runInNewContext().then(console.log);
    javascript

This option also allows the script or function to load user modules:

    import { Script, constants } from 'node:vm';
    import { resolve } from 'node:path';
    import { writeFileSync } from 'node:fs';

    // Write test.js and test.txt to the directory where the current script
    // being run is located.
    writeFileSync(resolve(import.meta.dirname, 'test.mjs'),
                  'export const filename = "./test.json";');
    writeFileSync(resolve(import.meta.dirname, 'test.json'),
                  '{"hello": "world"}');

    // Compile a script that loads test.mjs and then test.json
    // as if the script is placed in the same directory.
    const script = new Script(
      `(async function() {
        const { filename } = await import('./test.mjs');
        return import(filename, { with: { type: 'json' } })
      })();`,
      {
        filename: resolve(import.meta.dirname, 'test-with-default.js'),
        importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER,
      });

    // { default: { hello: 'world' } }
    script.runInThisContext().then(console.log);
    const { Script, constants } = require('node:vm');
    const { resolve } = require('node:path');
    const { writeFileSync } = require('node:fs');

    // Write test.js and test.txt to the directory where the current script
    // being run is located.
    writeFileSync(resolve(__dirname, 'test.mjs'),
                  'export const filename = "./test.json";');
    writeFileSync(resolve(__dirname, 'test.json'),
                  '{"hello": "world"}');

    // Compile a script that loads test.mjs and then test.json
    // as if the script is placed in the same directory.
    const script = new Script(
      `(async function() {
        const { filename } = await import('./test.mjs');
        return import(filename, { with: { type: 'json' } })
      })();`,
      {
        filename: resolve(__dirname, 'test-with-default.js'),
        importModuleDynamically: constants.USE_MAIN_CONTEXT_DEFAULT_LOADER,
      });

    // { default: { hello: 'world' } }
    script.runInThisContext().then(console.log);
    javascript

There are a few caveats with loading user modules using the default loader from the main context:

1. The module being resolved would be relative to the `filename` option passed to `vm.Script` or `vm.compileFunction()`. The resolution can work with a `filename` that's either an absolute path or a URL string. If `filename` is a string that's neither an absolute path or a URL, or if it's undefined, the resolution will be relative to the current working directory of the process. In the case of `vm.createContext()`, the resolution is always relative to the current working directory since this option is only used when there isn't a referrer script or module.
2. For any given `filename` that resolves to a specific path, once the process manages to load a particular module from that path, the result may be cached, and subsequent load of the same module from the same path would return the same thing. If the `filename` is a URL string, the cache would not be hit if it has different search parameters. For `filename`s that are not URL strings, there is currently no way to bypass the caching behavior.

#### When `importModuleDynamically` is a function#

When `importModuleDynamically` is a function, it will be invoked when `import()` is called in the compiled code for users to customize how the requested module should be compiled and evaluated. Currently, the Node.js instance must be launched with the `--experimental-vm-modules` flag for this option to work. If the flag isn't set, this callback will be ignored. If the code evaluated actually calls to `import()`, the result will reject with `ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING_FLAG`.

The callback `importModuleDynamically(specifier, referrer, importAttributes)` has the following signature:

- `specifier` `<string>` specifier passed to `import()`
- `referrer` `<vm.Script>` | `<Function>` | `<vm.SourceTextModule>` | `<Object>` The referrer is the compiled `vm.Script` for `new vm.Script`, `vm.runInThisContext`, `vm.runInContext` and `vm.runInNewContext`. It's the compiled `Function` for `vm.compileFunction`, the compiled `vm.SourceTextModule` for `new vm.SourceTextModule`, and the context `Object` for `vm.createContext()`.
- `importAttributes` `<Object>` The `"with"` value passed to the `optionsExpression` optional parameter, or an empty object if no value was provided.
- `phase` `<string>` The phase of the dynamic import (`"source"` or `"evaluation"`).
- Returns: `<Module Namespace Object>` | `<vm.Module>` Returning a `vm.Module` is recommended in order to take advantage of error tracking, and to avoid issues with namespaces that contain `then` function exports.

    // This script must be run with --experimental-vm-modules.
    import { Script, SyntheticModule } from 'node:vm';

    const script = new Script('import("foo.json", { with: { type: "json" } })', {
      async importModuleDynamically(specifier, referrer, importAttributes) {
        console.log(specifier);  // 'foo.json'
        console.log(referrer);   // The compiled script
        console.log(importAttributes);  // { type: 'json' }
        const m = new SyntheticModule(['bar'], () => { });
        await m.link(() => { });
        m.setExport('bar', { hello: 'world' });
        return m;
      },
    });
    const result = await script.runInThisContext();
    console.log(result);  //  { bar: { hello: 'world' } }
    // This script must be run with --experimental-vm-modules.
    const { Script, SyntheticModule } = require('node:vm');

    (async function main() {
      const script = new Script('import("foo.json", { with: { type: "json" } })', {
        async importModuleDynamically(specifier, referrer, importAttributes) {
          console.log(specifier);  // 'foo.json'
          console.log(referrer);   // The compiled script
          console.log(importAttributes);  // { type: 'json' }
          const m = new SyntheticModule(['bar'], () => { });
          await m.link(() => { });
          m.setExport('bar', { hello: 'world' });
          return m;
        },
      });
      const result = await script.runInThisContext();
      console.log(result);  //  { bar: { hello: 'world' } }
    })();
    javascript

## Web Crypto API#

History

Version | Changes
--- | ---
v25.9.0 | TurboSHAKE and KangarooTwelve algorithms are now supported.
v24.8.0 | KMAC algorithms are now supported.
v24.8.0 | Argon2 algorithms are now supported.
v24.7.0 | AES-OCB algorithm is now supported.
v24.7.0 | ML-KEM algorithms are now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.
v24.7.0 | SHA-3 algorithms are now supported.
v24.7.0 | SHAKE algorithms are now supported.
v24.7.0 | ML-DSA algorithms are now supported.
v23.5.0, v22.13.0, v20.19.3 | Algorithms `Ed25519` and `X25519` are now stable.
v20.0.0, v18.17.0 | Arguments are now coerced and validated as per their WebIDL definitions like in other Web Crypto API implementations.
v19.0.0 | No longer experimental except for the `Ed25519`, `Ed448`, `X25519`, and `X448` algorithms.
v18.4.0, v16.17.0 | Removed proprietary `'node.keyObject'` import/export format.
v18.4.0, v16.17.0 | Removed proprietary `'NODE-DSA'`, `'NODE-DH'`, and `'NODE-SCRYPT'` algorithms.
v18.4.0, v16.17.0 | Added `'Ed25519'`, `'Ed448'`, `'X25519'`, and `'X448'` algorithms.
v18.4.0, v16.17.0 | Removed proprietary `'NODE-ED25519'` and `'NODE-ED448'` algorithms.
v18.4.0, v16.17.0 | Removed proprietary `'NODE-X25519'` and `'NODE-X448'` named curves from the `'ECDH'` algorithm.

Stability: 2 - Stable

Node.js provides an implementation of the Web Crypto API standard.

Use `globalThis.crypto` or `require('node:crypto').webcrypto` to access this module.

    const { subtle } = globalThis.crypto;

    (async function() {

      const key = await subtle.generateKey({
        name: 'HMAC',
        hash: 'SHA-256',
        length: 256,
      }, true, ['sign', 'verify']);

      const enc = new TextEncoder();
      const message = enc.encode('I love cupcakes');

      const digest = await subtle.sign({
        name: 'HMAC',
      }, key, message);

    })();
    js

### Modern Algorithms in the Web Cryptography API#

Stability: 1.1 - Active development

Node.js provides an implementation of the following features from the Modern Algorithms in the Web Cryptography API WICG proposal:

Algorithms:

- `'AES-OCB'`1
- `'Argon2d'`2
- `'Argon2i'`2
- `'Argon2id'`2
- `'ChaCha20-Poly1305'`
- `'cSHAKE128'`
- `'cSHAKE256'`
- `'KMAC128'`1
- `'KMAC256'`1
- `'KT128'`
- `'KT256'`
- `'ML-DSA-44'`3
- `'ML-DSA-65'`3
- `'ML-DSA-87'`3
- `'ML-KEM-512'`3
- `'ML-KEM-768'`3
- `'ML-KEM-1024'`3
- `'SHA3-256'`
- `'SHA3-384'`
- `'SHA3-512'`
- `'TurboSHAKE128'`
- `'TurboSHAKE256'`

Key Formats:

- `'raw-public'`
- `'raw-secret'`
- `'raw-seed'`

Methods:

- `subtle.decapsulateBits()`
- `subtle.decapsulateKey()`
- `subtle.encapsulateBits()`
- `subtle.encapsulateKey()`
- `subtle.getPublicKey()`
- `SubtleCrypto.supports()`

### Secure Curves in the Web Cryptography API#

Stability: 1.1 - Active development

Node.js provides an implementation of the following features from the Secure Curves in the Web Cryptography API WICG proposal:

Algorithms:

- `'Ed448'`
- `'X448'`

### Examples#

#### Generating keys#

The `<SubtleCrypto>` class can be used to generate symmetric (secret) keys or asymmetric key pairs (public key and private key).

##### AES keys#

    const { subtle } = globalThis.crypto;

    async function generateAesKey(length = 256) {
      const key = await subtle.generateKey({
        name: 'AES-CBC',
        length,
      }, true, ['encrypt', 'decrypt']);

      return key;
    }
    js

##### ECDSA key pairs#

    const { subtle } = globalThis.crypto;

    async function generateEcKey(namedCurve = 'P-521') {
      const {
        publicKey,
        privateKey,
      } = await subtle.generateKey({
        name: 'ECDSA',
        namedCurve,
      }, true, ['sign', 'verify']);

      return { publicKey, privateKey };
    }
    js

##### Ed25519/X25519 key pairs#

    const { subtle } = globalThis.crypto;

    async function generateEd25519Key() {
      return subtle.generateKey({
        name: 'Ed25519',
      }, true, ['sign', 'verify']);
    }

    async function generateX25519Key() {
      return subtle.generateKey({
        name: 'X25519',
      }, true, ['deriveKey']);
    }
    js

##### HMAC keys#

    const { subtle } = globalThis.crypto;

    async function generateHmacKey(hash = 'SHA-256') {
      const key = await subtle.generateKey({
        name: 'HMAC',
        hash,
      }, true, ['sign', 'verify']);

      return key;
    }
    js

##### RSA key pairs#

    const { subtle } = globalThis.crypto;
    const publicExponent = new Uint8Array([1, 0, 1]);

    async function generateRsaKey(modulusLength = 2048, hash = 'SHA-256') {
      const {
        publicKey,
        privateKey,
      } = await subtle.generateKey({
        name: 'RSASSA-PKCS1-v1_5',
        modulusLength,
        publicExponent,
        hash,
      }, true, ['sign', 'verify']);

      return { publicKey, privateKey };
    }
    js

#### Encryption and decryption#

    const crypto = globalThis.crypto;

    async function aesEncrypt(plaintext) {
      const ec = new TextEncoder();
      const key = await generateAesKey();
      const iv = crypto.getRandomValues(new Uint8Array(16));

      const ciphertext = await crypto.subtle.encrypt({
        name: 'AES-CBC',
        iv,
      }, key, ec.encode(plaintext));

      return {
        key,
        iv,
        ciphertext,
      };
    }

    async function aesDecrypt(ciphertext, key, iv) {
      const dec = new TextDecoder();
      const plaintext = await crypto.subtle.decrypt({
        name: 'AES-CBC',
        iv,
      }, key, ciphertext);

      return dec.decode(plaintext);
    }
    js

#### Exporting and importing keys#

    const { subtle } = globalThis.crypto;

    async function generateAndExportHmacKey(format = 'jwk', hash = 'SHA-512') {
      const key = await subtle.generateKey({
        name: 'HMAC',
        hash,
      }, true, ['sign', 'verify']);

      return subtle.exportKey(format, key);
    }

    async function importHmacKey(keyData, format = 'jwk', hash = 'SHA-512') {
      const key = await subtle.importKey(format, keyData, {
        name: 'HMAC',
        hash,
      }, true, ['sign', 'verify']);

      return key;
    }
    js

#### Wrapping and unwrapping keys#

    const { subtle } = globalThis.crypto;

    async function generateAndWrapHmacKey(format = 'jwk', hash = 'SHA-512') {
      const [
        key,
        wrappingKey,
      ] = await Promise.all([
        subtle.generateKey({
          name: 'HMAC', hash,
        }, true, ['sign', 'verify']),
        subtle.generateKey({
          name: 'AES-KW',
          length: 256,
        }, true, ['wrapKey', 'unwrapKey']),
      ]);

      const wrappedKey = await subtle.wrapKey(format, key, wrappingKey, 'AES-KW');

      return { wrappedKey, wrappingKey };
    }

    async function unwrapHmacKey(
      wrappedKey,
      wrappingKey,
      format = 'jwk',
      hash = 'SHA-512') {

      const key = await subtle.unwrapKey(
        format,
        wrappedKey,
        wrappingKey,
        'AES-KW',
        { name: 'HMAC', hash },
        true,
        ['sign', 'verify']);

      return key;
    }
    js

#### Sign and verify#

    const { subtle } = globalThis.crypto;

    async function sign(key, data) {
      const ec = new TextEncoder();
      const signature =
        await subtle.sign('RSASSA-PKCS1-v1_5', key, ec.encode(data));
      return signature;
    }

    async function verify(key, signature, data) {
      const ec = new TextEncoder();
      const verified =
        await subtle.verify(
          'RSASSA-PKCS1-v1_5',
          key,
          signature,
          ec.encode(data));
      return verified;
    }
    js

#### Deriving bits and keys#

    const { subtle } = globalThis.crypto;

    async function pbkdf2(pass, salt, iterations = 1000, length = 256) {
      const ec = new TextEncoder();
      const key = await subtle.importKey(
        'raw',
        ec.encode(pass),
        'PBKDF2',
        false,
        ['deriveBits']);
      const bits = await subtle.deriveBits({
        name: 'PBKDF2',
        hash: 'SHA-512',
        salt: ec.encode(salt),
        iterations,
      }, key, length);
      return bits;
    }

    async function pbkdf2Key(pass, salt, iterations = 1000, length = 256) {
      const ec = new TextEncoder();
      const keyMaterial = await subtle.importKey(
        'raw',
        ec.encode(pass),
        'PBKDF2',
        false,
        ['deriveKey']);
      const key = await subtle.deriveKey({
        name: 'PBKDF2',
        hash: 'SHA-512',
        salt: ec.encode(salt),
        iterations,
      }, keyMaterial, {
        name: 'AES-GCM',
        length,
      }, true, ['encrypt', 'decrypt']);
      return key;
    }
    js

#### Digest#

    const { subtle } = globalThis.crypto;

    async function digest(data, algorithm = 'SHA-512') {
      const ec = new TextEncoder();
      const digest = await subtle.digest(algorithm, ec.encode(data));
      return digest;
    }
    js

#### Checking for runtime algorithm support#

`SubtleCrypto.supports()` allows feature detection in Web Crypto API, which can be used to detect whether a given algorithm identifier (including its parameters) is supported for the given operation.

This example derives a key from a password using Argon2, if available, or PBKDF2, otherwise; and then encrypts and decrypts some text with it using AES-OCB, if available, and AES-GCM, otherwise.

    const { SubtleCrypto, crypto } = globalThis;

    const password = 'correct horse battery staple';
    const derivationAlg =
      SubtleCrypto.supports?.('importKey', 'Argon2id') ?
        'Argon2id' :
        'PBKDF2';
    const encryptionAlg =
      SubtleCrypto.supports?.('importKey', 'AES-OCB') ?
        'AES-OCB' :
        'AES-GCM';
    const passwordKey = await crypto.subtle.importKey(
      derivationAlg === 'Argon2id' ? 'raw-secret' : 'raw',
      new TextEncoder().encode(password),
      derivationAlg,
      false,
      ['deriveKey'],
    );
    const nonce = crypto.getRandomValues(new Uint8Array(16));
    const derivationParams =
      derivationAlg === 'Argon2id' ?
        {
          nonce,
          parallelism: 4,
          memory: 2 ** 21,
          passes: 1,
        } :
        {
          salt: nonce,
          iterations: 100_000,
          hash: 'SHA-256',
        };
    const key = await crypto.subtle.deriveKey(
      {
        name: derivationAlg,
        ...derivationParams,
      },
      passwordKey,
      {
        name: encryptionAlg,
        length: 256,
      },
      false,
      ['encrypt', 'decrypt'],
    );
    const plaintext = 'Hello, world!';
    const iv = crypto.getRandomValues(new Uint8Array(12));
    const encrypted = await crypto.subtle.encrypt(
      { name: encryptionAlg, iv },
      key,
      new TextEncoder().encode(plaintext),
    );
    const decrypted = new TextDecoder().decode(await crypto.subtle.decrypt(
      { name: encryptionAlg, iv },
      key,
      encrypted,
    ));
    mjs

### Algorithm support#

The following sections detail the algorithms supported by the Node.js Web Crypto API implementation and the APIs supported for each:

#### Key Management APIs#

- `subtle.generateKey()`, `subtle.exportKey()`, and `subtle.importKey()` support `'AES-CBC'`, `'AES-CTR'`, `'AES-GCM'`, `'AES-KW'`, `'AES-OCB'`, `'ChaCha20-Poly1305'`4, `'HMAC'`, `'KMAC128'`4, and `'KMAC256'`4.
- `subtle.importKey()` supports `'Argon2d'`, `'Argon2i'`, `'Argon2id'`, `'HKDF'`, and `'PBKDF2'`.
- `subtle.generateKey()`, `subtle.exportKey()`, `subtle.importKey()`, and `subtle.getPublicKey()` support `'ECDH'`, `'ECDSA'`, `'Ed25519'`, `'Ed448'`5, `'ML-DSA-44'`4, `'ML-DSA-65'`4, `'ML-DSA-87'`4, `'ML-KEM-512'`4, `'ML-KEM-768'`4, `'ML-KEM-1024'`4, `'RSA-OAEP'`, `'RSA-PSS'`, `'RSASSA-PKCS1-v1_5'`, `'X25519'`, and `'X448'`5.

#### Crypto Operation APIs#

- `subtle.encrypt()` and `subtle.decrypt()` support `'AES-CBC'`, `'AES-CTR'`, `'AES-GCM'`, `'AES-OCB'`, `'ChaCha20-Poly1305'`4, and `'RSA-OAEP'`.
- `subtle.sign()` and `subtle.verify()` support `'ECDSA'`, `'Ed25519'`, `'Ed448'`5, `'HMAC'`, `'KMAC128'`4, `'KMAC256'`4, `'ML-DSA-44'`4, `'ML-DSA-65'`4, `'ML-DSA-87'`4, `'RSA-PSS'`, and `'RSASSA-PKCS1-v1_5'`.
- `subtle.deriveBits()` and `subtle.deriveKey()` support `'Argon2d'`, `'Argon2i'`, `'Argon2id'`, `'ECDH'`, `'HKDF'`, `'PBKDF2'`, `'X25519'`, and `'X448'`5.
- `subtle.wrapKey()` and `subtle.unwrapKey()` support `'AES-CBC'`, `'AES-CTR'`, `'AES-GCM'`, `'AES-KW'`, `'AES-OCB'`, `'ChaCha20-Poly1305'`4, and `'RSA-OAEP'`.
- `subtle.encapsulateBits()`, `subtle.decapsulateBits()`, `subtle.encapsulateKey()`, and `subtle.decapsulateKey()` support `'ML-KEM-512'`4, `'ML-KEM-768'`4, and `'ML-KEM-1024'`4.
- `subtle.digest()` supports `'cSHAKE128'`4, `'cSHAKE256'`4, `'KT128'`4, `'KT256'`4, `'SHA-1'`, `'SHA-256'`, `'SHA-384'`, `'SHA-512'`, `'SHA3-256'`4, `'SHA3-384'`4, `'SHA3-512'`4, `'TurboSHAKE128'`4, and `'TurboSHAKE256'`4.

#### Key Formats#

The following list describes the formats supported by `subtle.importKey()` and `subtle.exportKey()`.

- `'AES-CBC'`, `'AES-CTR'`, `'AES-GCM'`, `'AES-KW'`, and `'HMAC'` can be imported and exported using `'jwk'`, `'raw'`, and `'raw-secret'`4.
- `'AES-OCB'`4, `'ChaCha20-Poly1305'`4, `'KMAC128'`4, and `'KMAC256'`4 can be imported and exported using `'jwk'` and `'raw-secret'`4.
- `'Argon2d'`4, `'Argon2i'`4, and `'Argon2id'`4 can be imported using `'raw-secret'`4; export is not supported.
- `'ECDH'`, `'ECDSA'`, `'Ed25519'`, `'Ed448'`5, `'X25519'`, and `'X448'`5 can be imported and exported using `'spki'`, `'pkcs8'`, `'jwk'`, `'raw'`, and `'raw-public'`4.
- `'HKDF'` and `'PBKDF2'` can be imported using `'raw'` and `'raw-secret'`4; export is not supported.
- `'ML-DSA-44'`4, `'ML-DSA-65'`4, `'ML-DSA-87'`4, `'ML-KEM-512'`4, `'ML-KEM-768'`4, and `'ML-KEM-1024'`4 can be imported and exported using `'spki'`, `'pkcs8'`, `'jwk'`, `'raw-public'`4, and `'raw-seed'`4.
- `'RSA-OAEP'`, `'RSA-PSS'`, and `'RSASSA-PKCS1-v1_5'` can be imported and exported using `'spki'`, `'pkcs8'`, and `'jwk'`.

### Class: `Crypto`#

Added in: v15.0.0

`globalThis.crypto` is an instance of the `Crypto` class. `Crypto` is a singleton that provides access to the remainder of the crypto API.

#### `crypto.subtle`#

Added in: v15.0.0

- Type: `<SubtleCrypto>`

Provides access to the `SubtleCrypto` API.

#### `crypto.getRandomValues(typedArray)`#

Added in: v15.0.0

- `typedArray` `<Buffer>` | `<TypedArray>`
- Returns: `<Buffer>` | `<TypedArray>`

Generates cryptographically strong random values. The given `typedArray` is filled with random values, and a reference to `typedArray` is returned.

The given `typedArray` must be an integer-based instance of `<TypedArray>`, i.e. `Float32Array` and `Float64Array` are not accepted.

An error will be thrown if the given `typedArray` is larger than 65,536 bytes.

#### `crypto.randomUUID()`#

Added in: v16.7.0

- Returns: `<string>`

Generates a random RFC 4122 version 4 UUID. The UUID is generated using a cryptographic pseudorandom number generator.

### Class: `CryptoKey`#

Added in: v15.0.0

#### `cryptoKey.algorithm`#

Added in: v15.0.0

- Type: `<KeyAlgorithm>` | `<RsaHashedKeyAlgorithm>` | `<EcKeyAlgorithm>` | `<AesKeyAlgorithm>` | `<HmacKeyAlgorithm>` | `<KmacKeyAlgorithm>`

An object detailing the algorithm for which the key can be used along with additional algorithm-specific parameters.

Read-only.

#### `cryptoKey.extractable`#

Added in: v15.0.0

- Type: `<boolean>`

When `true`, the `<CryptoKey>` can be extracted using either `subtle.exportKey()` or `subtle.wrapKey()`.

Read-only.

#### `cryptoKey.type`#

Added in: v15.0.0

- Type: `<string>` One of `'secret'`, `'private'`, or `'public'`.

A string identifying whether the key is a symmetric (`'secret'`) or asymmetric (`'private'` or `'public'`) key.

#### `cryptoKey.usages`#

Added in: v15.0.0

- Type: `<string>`[]

An array of strings identifying the operations for which the key may be used.

The possible usages are:

- `'encrypt'` - Enable using the key with `subtle.encrypt()`
- `'decrypt'` - Enable using the key with `subtle.decrypt()`
- `'sign'` - Enable using the key with `subtle.sign()`
- `'verify'` - Enable using the key with `subtle.verify()`
- `'deriveKey'` - Enable using the key with `subtle.deriveKey()`
- `'deriveBits'` - Enable using the key with `subtle.deriveBits()`
- `'encapsulateBits'` - Enable using the key with `subtle.encapsulateBits()`
- `'decapsulateBits'` - Enable using the key with `subtle.decapsulateBits()`
- `'encapsulateKey'` - Enable using the key with `subtle.encapsulateKey()`
- `'decapsulateKey'` - Enable using the key with `subtle.decapsulateKey()`
- `'wrapKey'` - Enable using the key with `subtle.wrapKey()`
- `'unwrapKey'` - Enable using the key with `subtle.unwrapKey()`

Valid key usages depend on the key algorithm (identified by `cryptokey.algorithm.name`). See Crypto operation APIs for the operations supported by each key algorithm.

### Class: `CryptoKeyPair`#

Added in: v15.0.0

The `CryptoKeyPair` is a simple dictionary object with `publicKey` and `privateKey` properties, representing an asymmetric key pair.

#### `cryptoKeyPair.privateKey`#

Added in: v15.0.0

- Type: `<CryptoKey>` A `<CryptoKey>` whose `type` will be `'private'`.

#### `cryptoKeyPair.publicKey`#

Added in: v15.0.0

- Type: `<CryptoKey>` A `<CryptoKey>` whose `type` will be `'public'`.

### Class: `SubtleCrypto`#

Added in: v15.0.0

#### Static method: `SubtleCrypto.supports(operation, algorithm[, lengthOrAdditionalAlgorithm])`#

Added in: v24.7.0

Stability: 1.1 - Active development

- `operation` `<string>` "encrypt", "decrypt", "sign", "verify", "digest", "generateKey", "deriveKey", "deriveBits", "importKey", "exportKey", "getPublicKey", "wrapKey", "unwrapKey", "encapsulateBits", "encapsulateKey", "decapsulateBits", or "decapsulateKey"
- `algorithm` `<string>` | `<Algorithm>`
- `lengthOrAdditionalAlgorithm` `<null>` | `<number>` | `<string>` | `<Algorithm>` | `<undefined>` Depending on the operation this is either ignored, the value of the length argument when operation is "deriveBits", the algorithm of key to be derived when operation is "deriveKey", the algorithm of key to be exported before wrapping when operation is "wrapKey", the algorithm of key to be imported after unwrapping when operation is "unwrapKey", or the algorithm of key to be imported after en/decapsulating a key when operation is "encapsulateKey" or "decapsulateKey". Default: `null` when operation is "deriveBits", `undefined` otherwise.
- Returns: `<boolean>` Indicating whether the implementation supports the given operation

Allows feature detection in Web Crypto API, which can be used to detect whether a given algorithm identifier (including its parameters) is supported for the given operation.

See Checking for runtime algorithm support for an example use of this method.

#### `subtle.decapsulateBits(decapsulationAlgorithm, decapsulationKey, ciphertext)`#

Added in: v24.7.0

Stability: 1.1 - Active development

- `decapsulationAlgorithm` `<string>` | `<Algorithm>`
- `decapsulationKey` `<CryptoKey>`
- `ciphertext` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- Returns: `<Promise>` Fulfills with `<ArrayBuffer>` upon success.

A message recipient uses their asymmetric private key to decrypt an "encapsulated key" (ciphertext), thereby recovering a temporary symmetric key (represented as `<ArrayBuffer>`) which is then used to decrypt a message.

The algorithms currently supported include:

- `'ML-KEM-512'`4
- `'ML-KEM-768'`4
- `'ML-KEM-1024'`4

#### `subtle.decapsulateKey(decapsulationAlgorithm, decapsulationKey, ciphertext, sharedKeyAlgorithm, extractable, keyUsages)`#

Added in: v24.7.0

Stability: 1.1 - Active development

- `decapsulationAlgorithm` `<string>` | `<Algorithm>`
- `decapsulationKey` `<CryptoKey>`
- `ciphertext` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- `sharedKeyAlgorithm` `<string>` | `<Algorithm>` | `<HmacImportParams>` | `<AesDerivedKeyParams>` | `<KmacImportParams>`
- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with `<CryptoKey>` upon success.

A message recipient uses their asymmetric private key to decrypt an "encapsulated key" (ciphertext), thereby recovering a temporary symmetric key (represented as `<CryptoKey>`) which is then used to decrypt a message.

The algorithms currently supported include:

- `'ML-KEM-512'`4
- `'ML-KEM-768'`4
- `'ML-KEM-1024'`4

#### `subtle.decrypt(algorithm, key, data)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | AES-OCB algorithm is now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.

- `algorithm` `<RsaOaepParams>` | `<AesCtrParams>` | `<AesCbcParams>` | `<AeadParams>`
- `key` `<CryptoKey>`
- `data` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` upon success.

Using the method and parameters specified in `algorithm` and the keying material provided by `key`, this method attempts to decipher the provided `data`. If successful, the returned promise will be resolved with an `<ArrayBuffer>` containing the plaintext result.

The algorithms currently supported include:

- `'AES-CBC'`
- `'AES-CTR'`
- `'AES-GCM'`
- `'AES-OCB'`4
- `'ChaCha20-Poly1305'`4
- `'RSA-OAEP'`

#### `subtle.deriveBits(algorithm, baseKey[, length])`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.8.0 | Argon2 algorithms are now supported.
v22.5.0, v20.17.0, v18.20.5 | The length parameter is now optional for `'ECDH'`, `'X25519'`, and `'X448'`.
v18.4.0, v16.17.0 | Added `'X25519'`, and `'X448'` algorithms.

- `algorithm` `<EcdhKeyDeriveParams>` | `<HkdfParams>` | `<Pbkdf2Params>` | `<Argon2Params>`
- `baseKey` `<CryptoKey>`
- `length` `<number>` | `<null>` Default: `null`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` upon success.

Using the method and parameters specified in `algorithm` and the keying material provided by `baseKey`, this method attempts to generate `length` bits.

When `length` is not provided or `null` the maximum number of bits for a given algorithm is generated. This is allowed for the `'ECDH'`, `'X25519'`, and `'X448'`5 algorithms, for other algorithms `length` is required to be a number.

If successful, the returned promise will be resolved with an `<ArrayBuffer>` containing the generated data.

The algorithms currently supported include:

- `'Argon2d'`4
- `'Argon2i'`4
- `'Argon2id'`4
- `'ECDH'`
- `'HKDF'`
- `'PBKDF2'`
- `'X25519'`
- `'X448'`5

#### `subtle.deriveKey(algorithm, baseKey, derivedKeyType, extractable, keyUsages)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.8.0 | Argon2 algorithms are now supported.
v18.4.0, v16.17.0 | Added `'X25519'`, and `'X448'` algorithms.

- `algorithm` `<EcdhKeyDeriveParams>` | `<HkdfParams>` | `<Pbkdf2Params>` | `<Argon2Params>`
- `baseKey` `<CryptoKey>`
- `derivedKeyType` `<string>` | `<Algorithm>` | `<HmacImportParams>` | `<AesDerivedKeyParams>` | `<KmacImportParams>`
- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with a `<CryptoKey>` upon success.

Using the method and parameters specified in `algorithm`, and the keying material provided by `baseKey`, this method attempts to generate a new `<CryptoKey>` based on the method and parameters in `derivedKeyType`.

Calling this method is equivalent to calling `subtle.deriveBits()` to generate raw keying material, then passing the result into the `subtle.importKey()` method using the `derivedKeyType`, `extractable`, and `keyUsages` parameters as input.

The algorithms currently supported include:

- `'Argon2d'`4
- `'Argon2i'`4
- `'Argon2id'`4
- `'ECDH'`
- `'HKDF'`
- `'PBKDF2'`
- `'X25519'`
- `'X448'`5

#### `subtle.digest(algorithm, data)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v25.9.0 | TurboSHAKE and KangarooTwelve algorithms are now supported.
v24.7.0 | SHA-3 algorithms are now supported.
v24.7.0 | SHAKE algorithms are now supported.

- `algorithm` `<string>` | `<Algorithm>` | `<CShakeParams>` | `<TurboShakeParams>` | `<KangarooTwelveParams>`
- `data` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` upon success.

Using the method identified by `algorithm`, this method attempts to generate a digest of `data`. If successful, the returned promise is resolved with an `<ArrayBuffer>` containing the computed digest.

If `algorithm` is provided as a `<string>`, it must be one of:

- `'cSHAKE128'`4
- `'cSHAKE256'`4
- `'KT128'`4
- `'KT256'`4
- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4
- `'TurboSHAKE128'`4
- `'TurboSHAKE256'`4

If `algorithm` is provided as an `<Object>`, it must have a `name` property whose value is one of the above.

#### `subtle.encapsulateBits(encapsulationAlgorithm, encapsulationKey)`#

Added in: v24.7.0

Stability: 1.1 - Active development

- `encapsulationAlgorithm` `<string>` | `<Algorithm>`
- `encapsulationKey` `<CryptoKey>`
- Returns: `<Promise>` Fulfills with `<EncapsulatedBits>` upon success.

Uses a message recipient's asymmetric public key to encrypt a temporary symmetric key. This encrypted key is the "encapsulated key" represented as `<EncapsulatedBits>`.

The algorithms currently supported include:

- `'ML-KEM-512'`4
- `'ML-KEM-768'`4
- `'ML-KEM-1024'`4

#### `subtle.encapsulateKey(encapsulationAlgorithm, encapsulationKey, sharedKeyAlgorithm, extractable, keyUsages)`#

Added in: v24.7.0

Stability: 1.1 - Active development

- `encapsulationAlgorithm` `<string>` | `<Algorithm>`
- `encapsulationKey` `<CryptoKey>`
- `sharedKeyAlgorithm` `<string>` | `<Algorithm>` | `<HmacImportParams>` | `<AesDerivedKeyParams>` | `<KmacImportParams>`
- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with `<EncapsulatedKey>` upon success.

Uses a message recipient's asymmetric public key to encrypt a temporary symmetric key. This encrypted key is the "encapsulated key" represented as `<EncapsulatedKey>`.

The algorithms currently supported include:

- `'ML-KEM-512'`4
- `'ML-KEM-768'`4
- `'ML-KEM-1024'`4

#### `subtle.encrypt(algorithm, key, data)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | AES-OCB algorithm is now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.

- `algorithm` `<RsaOaepParams>` | `<AesCtrParams>` | `<AesCbcParams>` | `<AeadParams>`
- `key` `<CryptoKey>`
- `data` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` upon success.

Using the method and parameters specified by `algorithm` and the keying material provided by `key`, this method attempts to encipher `data`. If successful, the returned promise is resolved with an `<ArrayBuffer>` containing the encrypted result.

The algorithms currently supported include:

- `'AES-CBC'`
- `'AES-CTR'`
- `'AES-GCM'`
- `'AES-OCB'`4
- `'ChaCha20-Poly1305'`4
- `'RSA-OAEP'`

#### `subtle.exportKey(format, key)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v26.1.0 | Added JWK format support for ML-KEM key types.
v24.8.0 | KMAC algorithms are now supported.
v24.7.0 | ML-KEM algorithms are now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.
v24.7.0 | ML-DSA algorithms are now supported.
v18.4.0, v16.17.0 | Added `'Ed25519'`, `'Ed448'`, `'X25519'`, and `'X448'` algorithms.
v15.9.0 | Removed `'NODE-DSA'` JWK export.

- `format` `<string>` Must be one of `'raw'`, `'pkcs8'`, `'spki'`, `'jwk'`, `'raw-secret'`4, `'raw-public'`4, or `'raw-seed'`4.
- `key` `<CryptoKey>`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` | `<Object>` upon success.

Exports the given key into the specified format, if supported.

If the `<CryptoKey>` is not extractable, the returned promise will reject.

When `format` is either `'pkcs8'` or `'spki'` and the export is successful, the returned promise will be resolved with an `<ArrayBuffer>` containing the exported key data.

When `format` is `'jwk'` and the export is successful, the returned promise will be resolved with a JavaScript object conforming to the JSON Web Key specification.

See Key formats for the formats supported by each algorithm.

#### `subtle.getPublicKey(key, keyUsages)`#

Added in: v24.7.0

Stability: 1.1 - Active development

- `key` `<CryptoKey>` A private key from which to derive the corresponding public key.
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with a `<CryptoKey>` upon success.

Derives the public key from a given private key.

#### `subtle.generateKey(algorithm, extractable, keyUsages)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.8.0 | KMAC algorithms are now supported.
v24.7.0 | ML-KEM algorithms are now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.
v24.7.0 | ML-DSA algorithms are now supported.

- `algorithm` `<string>` | `<Algorithm>` | `<RsaHashedKeyGenParams>` | `<EcKeyGenParams>` | `<HmacKeyGenParams>` | `<AesKeyGenParams>` | `<KmacKeyGenParams>`

- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with a `<CryptoKey>` | `<CryptoKeyPair>` upon success.

Using the parameters provided in `algorithm`, this method attempts to generate new keying material. Depending on the algorithm used either a single `<CryptoKey>` or a `<CryptoKeyPair>` is generated.

The `<CryptoKeyPair>` (public and private key) generating algorithms supported include:

- `'ECDH'`
- `'ECDSA'`
- `'Ed25519'`
- `'Ed448'`5
- `'ML-DSA-44'`4
- `'ML-DSA-65'`4
- `'ML-DSA-87'`4
- `'ML-KEM-512'`4
- `'ML-KEM-768'`4
- `'ML-KEM-1024'`4
- `'RSA-OAEP'`
- `'RSA-PSS'`
- `'RSASSA-PKCS1-v1_5'`
- `'X25519'`
- `'X448'`5

The `<CryptoKey>` (secret key) generating algorithms supported include:

- `'AES-CBC'`
- `'AES-CTR'`
- `'AES-GCM'`
- `'AES-KW'`
- `'AES-OCB'`4
- `'ChaCha20-Poly1305'`4
- `'HMAC'`
- `'KMAC128'`4
- `'KMAC256'`4

#### `subtle.importKey(format, keyData, algorithm, extractable, keyUsages)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v26.1.0 | Added JWK format support for ML-KEM key types.
v25.9.0 | Importing ML-DSA and ML-KEM PKCS#8 keys without a seed is no longer supported.
v24.8.0 | KMAC algorithms are now supported.
v24.7.0 | ML-KEM algorithms are now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.
v24.7.0 | ML-DSA algorithms are now supported.
v18.4.0, v16.17.0 | Added `'Ed25519'`, `'Ed448'`, `'X25519'`, and `'X448'` algorithms.
v15.9.0 | Removed `'NODE-DSA'` JWK import.

- `format` `<string>` Must be one of `'raw'`, `'pkcs8'`, `'spki'`, `'jwk'`, `'raw-secret'`4, `'raw-public'`4, or `'raw-seed'`4.
- `keyData` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<Object>`

- `algorithm` `<string>` | `<Algorithm>` | `<RsaHashedImportParams>` | `<EcKeyImportParams>` | `<HmacImportParams>` | `<KmacImportParams>`

- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with a `<CryptoKey>` upon success.

This method attempts to interpret the provided `keyData` as the given `format` to create a `<CryptoKey>` instance using the provided `algorithm`, `extractable`, and `keyUsages` arguments. If the import is successful, the returned promise will be resolved with a `<CryptoKey>` representation of the key material.

If importing KDF algorithm keys, `extractable` must be `false`.

See Key formats for the algorithms and formats currently supported.

#### `subtle.sign(algorithm, key, data)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.8.0 | KMAC algorithms are now supported.
v24.7.0 | ML-DSA algorithms are now supported.
v18.4.0, v16.17.0 | Added `'Ed25519'`, and `'Ed448'` algorithms.

- `algorithm` `<string>` | `<Algorithm>` | `<RsaPssParams>` | `<EcdsaParams>` | `<ContextParams>` | `<KmacParams>`
- `key` `<CryptoKey>`
- `data` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` upon success.

Using the method and parameters given by `algorithm` and the keying material provided by `key`, this method attempts to generate a cryptographic signature of `data`. If successful, the returned promise is resolved with an `<ArrayBuffer>` containing the generated signature.

The algorithms currently supported include:

- `'ECDSA'`
- `'Ed25519'`
- `'Ed448'`5
- `'HMAC'`
- `'KMAC128'`4
- `'KMAC256'`4
- `'ML-DSA-44'`4
- `'ML-DSA-65'`4
- `'ML-DSA-87'`4
- `'RSA-PSS'`
- `'RSASSA-PKCS1-v1_5'`

#### `subtle.unwrapKey(format, wrappedKey, unwrappingKey, unwrapAlgorithm, unwrappedKeyAlgorithm, extractable, keyUsages)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | AES-OCB algorithm is now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.

- `format` `<string>` Must be one of `'raw'`, `'pkcs8'`, `'spki'`, `'jwk'`, `'raw-secret'`4, `'raw-public'`4, or `'raw-seed'`4.
- `wrappedKey` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- `unwrappingKey` `<CryptoKey>`

- `unwrapAlgorithm` `<string>` | `<Algorithm>` | `<RsaOaepParams>` | `<AesCtrParams>` | `<AesCbcParams>` | `<AeadParams>`
- `unwrappedKeyAlgorithm` `<string>` | `<Algorithm>` | `<RsaHashedImportParams>` | `<EcKeyImportParams>` | `<HmacImportParams>` | `<KmacImportParams>`

- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<Promise>` Fulfills with a `<CryptoKey>` upon success.

In cryptography, "wrapping a key" refers to exporting and then encrypting the keying material. This method attempts to decrypt a wrapped key and create a `<CryptoKey>` instance. It is equivalent to calling `subtle.decrypt()` first on the encrypted key data (using the `wrappedKey`, `unwrapAlgorithm`, and `unwrappingKey` arguments as input) then passing the results to the `subtle.importKey()` method using the `unwrappedKeyAlgorithm`, `extractable`, and `keyUsages` arguments as inputs. If successful, the returned promise is resolved with a `<CryptoKey>` object.

The wrapping algorithms currently supported include:

- `'AES-CBC'`
- `'AES-CTR'`
- `'AES-GCM'`
- `'AES-KW'`
- `'AES-OCB'`4
- `'ChaCha20-Poly1305'`4
- `'RSA-OAEP'`

The unwrapped key algorithms supported include:

- `'AES-CBC'`
- `'AES-CTR'`
- `'AES-GCM'`
- `'AES-KW'`
- `'AES-OCB'`4
- `'ChaCha20-Poly1305'`4
- `'ECDH'`
- `'ECDSA'`
- `'Ed25519'`
- `'Ed448'`5
- `'HMAC'`
- `'KMAC128'`4
- `'KMAC256'`4
- `'ML-DSA-44'`4
- `'ML-DSA-65'`4
- `'ML-DSA-87'`4
- `'ML-KEM-512'`4
- `'ML-KEM-768'`4
- `'ML-KEM-1024'`4
- `'RSA-OAEP'`
- `'RSA-PSS'`
- `'RSASSA-PKCS1-v1_5'`
- `'X25519'`
- `'X448'`5

#### `subtle.verify(algorithm, key, signature, data)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.8.0 | KMAC algorithms are now supported.
v24.7.0 | ML-DSA algorithms are now supported.
v18.4.0, v16.17.0 | Added `'Ed25519'`, and `'Ed448'` algorithms.

- `algorithm` `<string>` | `<Algorithm>` | `<RsaPssParams>` | `<EcdsaParams>` | `<ContextParams>` | `<KmacParams>`
- `key` `<CryptoKey>`
- `signature` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- `data` `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`
- Returns: `<Promise>` Fulfills with a `<boolean>` upon success.

Using the method and parameters given in `algorithm` and the keying material provided by `key`, this method attempts to verify that `signature` is a valid cryptographic signature of `data`. The returned promise is resolved with either `true` or `false`.

The algorithms currently supported include:

- `'ECDSA'`
- `'Ed25519'`
- `'Ed448'`5
- `'HMAC'`
- `'KMAC128'`4
- `'KMAC256'`4
- `'ML-DSA-44'`4
- `'ML-DSA-65'`4
- `'ML-DSA-87'`4
- `'RSA-PSS'`
- `'RSASSA-PKCS1-v1_5'`

#### `subtle.wrapKey(format, key, wrappingKey, wrapAlgorithm)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | AES-OCB algorithm is now supported.
v24.7.0 | ChaCha20-Poly1305 algorithm is now supported.

- `format` `<string>` Must be one of `'raw'`, `'pkcs8'`, `'spki'`, `'jwk'`, `'raw-secret'`4, `'raw-public'`4, or `'raw-seed'`4.
- `key` `<CryptoKey>`
- `wrappingKey` `<CryptoKey>`
- `wrapAlgorithm` `<string>` | `<Algorithm>` | `<RsaOaepParams>` | `<AesCtrParams>` | `<AesCbcParams>` | `<AeadParams>`
- Returns: `<Promise>` Fulfills with an `<ArrayBuffer>` upon success.

In cryptography, "wrapping a key" refers to exporting and then encrypting the keying material. This method exports the keying material into the format identified by `format`, then encrypts it using the method and parameters specified by `wrapAlgorithm` and the keying material provided by `wrappingKey`. It is the equivalent to calling `subtle.exportKey()` using `format` and `key` as the arguments, then passing the result to the `subtle.encrypt()` method using `wrappingKey` and `wrapAlgorithm` as inputs. If successful, the returned promise will be resolved with an `<ArrayBuffer>` containing the encrypted key data.

The wrapping algorithms currently supported include:

- `'AES-CBC'`
- `'AES-CTR'`
- `'AES-GCM'`
- `'AES-KW'`
- `'AES-OCB'`4
- `'ChaCha20-Poly1305'`4
- `'RSA-OAEP'`

### Algorithm parameters#

The algorithm parameter objects define the methods and parameters used by the various `<SubtleCrypto>` methods. While described here as "classes", they are simple JavaScript dictionary objects.

#### Class: `Algorithm`#

Added in: v15.0.0

##### `Algorithm.name`#

Added in: v15.0.0

- Type: `<string>`

#### Class: `AeadParams`#

Added in: v15.0.0

##### `aeadParams.additionalData`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<undefined>`

Extra input that is not encrypted but is included in the authentication of the data. The use of `additionalData` is optional.

##### `aeadParams.iv`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

The initialization vector must be unique for every encryption operation using a given key.

##### `aeadParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'AES-GCM'`, `'AES-OCB'`, or `'ChaCha20-Poly1305'`.

##### `aeadParams.tagLength`#

Added in: v15.0.0

- Type: `<number>` The size in bits of the generated authentication tag.

#### Class: `AesDerivedKeyParams`#

Added in: v15.0.0

##### `aesDerivedKeyParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'AES-CBC'`, `'AES-CTR'`, `'AES-GCM'`, `'AES-OCB'`, or `'AES-KW'`

##### `aesDerivedKeyParams.length`#

Added in: v15.0.0

- Type: `<number>`

The length of the AES key to be derived. This must be either `128`, `192`, or `256`.

#### Class: `AesCbcParams`#

Added in: v15.0.0

##### `aesCbcParams.iv`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

Provides the initialization vector. It must be exactly 16-bytes in length and should be unpredictable and cryptographically random.

##### `aesCbcParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'AES-CBC'`.

#### Class: `AesCtrParams`#

Added in: v15.0.0

##### `aesCtrParams.counter`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

The initial value of the counter block. This must be exactly 16 bytes long.

The `AES-CTR` method uses the rightmost `length` bits of the block as the counter and the remaining bits as the nonce.

##### `aesCtrParams.length`#

Added in: v15.0.0

- Type: `<number>` The number of bits in the `aesCtrParams.counter` that are to be used as the counter.

##### `aesCtrParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'AES-CTR'`.

#### Class: `AesKeyAlgorithm`#

Added in: v15.0.0

##### `aesKeyAlgorithm.length`#

Added in: v15.0.0

- Type: `<number>`

The length of the AES key in bits.

##### `aesKeyAlgorithm.name`#

Added in: v15.0.0

- Type: `<string>`

#### Class: `AesKeyGenParams`#

Added in: v15.0.0

##### `aesKeyGenParams.length`#

Added in: v15.0.0

- Type: `<number>`

The length of the AES key to be generated. This must be either `128`, `192`, or `256`.

##### `aesKeyGenParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'AES-CBC'`, `'AES-CTR'`, `'AES-GCM'`, or `'AES-KW'`

#### Class: `Argon2Params`#

Added in: v24.8.0

##### `argon2Params.associatedData`#

Added in: v24.8.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

Represents the optional associated data.

##### `argon2Params.memory`#

Added in: v24.8.0

- Type: `<number>`

Represents the memory size in kibibytes. It must be at least 8 times the degree of parallelism.

##### `argon2Params.name`#

Added in: v24.8.0

- Type: `<string>` Must be one of `'Argon2d'`, `'Argon2i'`, or `'Argon2id'`.

##### `argon2Params.nonce`#

Added in: v24.8.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

Represents the nonce, which is a salt for password hashing applications.

##### `argon2Params.parallelism`#

Added in: v24.8.0

- Type: `<number>`

Represents the degree of parallelism.

##### `argon2Params.passes`#

Added in: v24.8.0

- Type: `<number>`

Represents the number of passes.

##### `argon2Params.secretValue`#

Added in: v24.8.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

Represents the optional secret value.

##### `argon2Params.version`#

Added in: v24.8.0

- Type: `<number>`

Represents the Argon2 version number. The default and currently only defined version is `19` (`0x13`).

#### Class: `ContextParams`#

Added in: v24.7.0

##### `contextParams.name`#

Added in: v24.7.0

- Type: `<string>` Must be `'Ed448'`5, `'ML-DSA-44'`4, `'ML-DSA-65'`4, or `'ML-DSA-87'`4.

##### `contextParams.context`#

Added in: v24.7.0
History

Version | Changes
--- | ---
v24.8.0 | Non-empty context is now supported.

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<undefined>`

The `context` member represents the optional context data to associate with the message.

#### Class: `CShakeParams`#

Added in: v24.7.0
History

Version | Changes
--- | ---
v25.9.0 | Renamed `cShakeParams.length` to `cShakeParams.outputLength`.

##### `cShakeParams.name`#

Added in: v24.7.0

- Type: `<string>` Must be `'cSHAKE128'`4 or `'cSHAKE256'`4.

##### `cShakeParams.outputLength`#

Added in: v25.9.0

- Type: `<number>` represents the requested output length in bits.

##### `cShakeParams.functionName`#

Added in: v24.7.0
History

Version | Changes
--- | ---
v26.4.0 | Named cSHAKE variants are now accepted.

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<undefined>`

The `functionName` member represents the NIST function-name byte string used to domain-separate functions built on top of cSHAKE. Accepted values are:

- empty or `undefined`, in which case cSHAKE is equivalent to plain SHAKE
- the ASCII byte sequence `'KMAC'`
- the ASCII byte sequence `'TupleHash'`
- the ASCII byte sequence `'ParallelHash'`

##### `cShakeParams.customization`#

Added in: v24.7.0
History

Version | Changes
--- | ---
v26.4.0 | Non-empty customization is now supported.

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<undefined>`

The `customization` member represents the customization data. Accepted values are:

- empty or `undefined`, in which case cSHAKE is equivalent to plain SHAKE
- up to 512 bytes of arbitrary data

#### Class: `EcdhKeyDeriveParams`#

Added in: v15.0.0

##### `ecdhKeyDeriveParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'ECDH'`, `'X25519'`, or `'X448'`5.

##### `ecdhKeyDeriveParams.public`#

Added in: v15.0.0

- Type: `<CryptoKey>`

ECDH key derivation operates by taking as input one party's private key and another party's public key -- using both to generate a common shared secret. The `ecdhKeyDeriveParams.public` property is set to the other party's public key.

#### Class: `EcdsaParams`#

Added in: v15.0.0

##### `ecdsaParams.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `ecdsaParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'ECDSA'`.

#### Class: `EcKeyAlgorithm`#

Added in: v15.0.0

##### `ecKeyAlgorithm.name`#

Added in: v15.0.0

- Type: `<string>`

##### `ecKeyAlgorithm.namedCurve`#

Added in: v15.0.0

- Type: `<string>`

#### Class: `EcKeyGenParams`#

Added in: v15.0.0

##### `ecKeyGenParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'ECDSA'` or `'ECDH'`.

##### `ecKeyGenParams.namedCurve`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'P-256'`, `'P-384'`, `'P-521'`.

#### Class: `EcKeyImportParams`#

Added in: v15.0.0

##### `ecKeyImportParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'ECDSA'` or `'ECDH'`.

##### `ecKeyImportParams.namedCurve`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'P-256'`, `'P-384'`, `'P-521'`.

#### Class: `EncapsulatedBits`#

Added in: v24.7.0

A temporary symmetric secret key (represented as `<ArrayBuffer>`) for message encryption and the ciphertext (that can be transmitted to the message recipient along with the message) encrypted by this shared key. The recipient uses their private key to determine what the shared key is which then allows them to decrypt the message.

##### `encapsulatedBits.ciphertext`#

Added in: v24.7.0

- Type: `<ArrayBuffer>`

##### `encapsulatedBits.sharedKey`#

Added in: v24.7.0

- Type: `<ArrayBuffer>`

#### Class: `EncapsulatedKey`#

Added in: v24.7.0

A temporary symmetric secret key (represented as `<CryptoKey>`) for message encryption and the ciphertext (that can be transmitted to the message recipient along with the message) encrypted by this shared key. The recipient uses their private key to determine what the shared key is which then allows them to decrypt the message.

##### `encapsulatedKey.ciphertext`#

Added in: v24.7.0

- Type: `<ArrayBuffer>`

##### `encapsulatedKey.sharedKey`#

Added in: v24.7.0

- Type: `<CryptoKey>`

#### Class: `HkdfParams`#

Added in: v15.0.0

##### `hkdfParams.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `hkdfParams.info`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

Provides application-specific contextual input to the HKDF algorithm. This can be zero-length but must be provided.

##### `hkdfParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'HKDF'`.

##### `hkdfParams.salt`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

The salt value significantly improves the strength of the HKDF algorithm. It should be random or pseudorandom and should be the same length as the output of the digest function (for instance, if using `'SHA-256'` as the digest, the salt should be 256-bits of random data).

#### Class: `HmacImportParams`#

Added in: v15.0.0

##### `hmacImportParams.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `hmacImportParams.length`#

Added in: v15.0.0

- Type: `<number>`

The optional number of bits in the HMAC key. This is optional and should be omitted for most cases.

##### `hmacImportParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'HMAC'`.

#### Class: `HmacKeyAlgorithm`#

Added in: v15.0.0

##### `hmacKeyAlgorithm.hash`#

Added in: v15.0.0

- Type: `<Algorithm>`

##### `hmacKeyAlgorithm.length`#

Added in: v15.0.0

- Type: `<number>`

The length of the HMAC key in bits.

##### `hmacKeyAlgorithm.name`#

Added in: v15.0.0

- Type: `<string>`

#### Class: `HmacKeyGenParams`#

Added in: v15.0.0

##### `hmacKeyGenParams.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `hmacKeyGenParams.length`#

Added in: v15.0.0

- Type: `<number>`

The number of bits to generate for the HMAC key. If omitted, the length will be determined by the hash algorithm used. This is optional and should be omitted for most cases.

##### `hmacKeyGenParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'HMAC'`.

#### Class: `KeyAlgorithm`#

Added in: v15.0.0

##### `keyAlgorithm.name`#

Added in: v15.0.0

- Type: `<string>`

#### Class: `KangarooTwelveParams`#

Added in: v25.9.0

##### `kangarooTwelveParams.customization`#

Added in: v25.9.0
History

Version | Changes
--- | ---
v26.6.0 | Limit customization to 512 bytes.

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<undefined>`

The optional customization string for KangarooTwelve. It must not exceed 512 bytes.

##### `kangarooTwelveParams.name`#

Added in: v25.9.0

- Type: `<string>` Must be `'KT128'`4 or `'KT256'`4.

##### `kangarooTwelveParams.outputLength`#

Added in: v25.9.0

- Type: `<number>` represents the requested output length in bits.

#### Class: `KmacImportParams`#

Added in: v24.8.0

##### `kmacImportParams.length`#

Added in: v24.8.0

- Type: `<number>`

The optional number of bits in the KMAC key. This is optional and should be omitted for most cases.

##### `kmacImportParams.name`#

Added in: v24.8.0

- Type: `<string>` Must be `'KMAC128'` or `'KMAC256'`.

#### Class: `KmacKeyAlgorithm`#

Added in: v24.8.0

##### `kmacKeyAlgorithm.length`#

Added in: v24.8.0

- Type: `<number>`

The length of the KMAC key in bits.

##### `kmacKeyAlgorithm.name`#

Added in: v24.8.0

- Type: `<string>`

#### Class: `KmacKeyGenParams`#

Added in: v24.8.0

##### `kmacKeyGenParams.length`#

Added in: v24.8.0

- Type: `<number>`

The number of bits to generate for the KMAC key. If omitted, the length will be determined by the KMAC algorithm used. This is optional and should be omitted for most cases.

##### `kmacKeyGenParams.name`#

Added in: v24.8.0

- Type: `<string>` Must be `'KMAC128'` or `'KMAC256'`.

#### Class: `KmacParams`#

Added in: v24.8.0
History

Version | Changes
--- | ---
v25.9.0 | Renamed `kmacParams.length` to `kmacParams.outputLength`.

##### `kmacParams.algorithm`#

Added in: v24.8.0

- Type: `<string>` Must be `'KMAC128'` or `'KMAC256'`.

##### `kmacParams.outputLength`#

Added in: v25.9.0

- Type: `<number>` represents the requested output length in bits.

##### `kmacParams.customization`#

Added in: v24.8.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>` | `<undefined>`

The `customization` member represents the optional customization string.

#### Class: `Pbkdf2Params`#

Added in: v15.0.0

##### `pbkdf2Params.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `pbkdf2Params.iterations`#

Added in: v15.0.0

- Type: `<number>`

The number of iterations the PBKDF2 algorithm should make when deriving bits.

##### `pbkdf2Params.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'PBKDF2'`.

##### `pbkdf2Params.salt`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

Should be at least 16 random or pseudorandom bytes.

#### Class: `RsaHashedImportParams`#

Added in: v15.0.0

##### `rsaHashedImportParams.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `rsaHashedImportParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'RSASSA-PKCS1-v1_5'`, `'RSA-PSS'`, or `'RSA-OAEP'`.

#### Class: `RsaHashedKeyAlgorithm`#

Added in: v15.0.0

##### `rsaHashedKeyAlgorithm.hash`#

Added in: v15.0.0

- Type: `<Algorithm>`

##### `rsaHashedKeyAlgorithm.modulusLength`#

Added in: v15.0.0

- Type: `<number>`

The length in bits of the RSA modulus.

##### `rsaHashedKeyAlgorithm.name`#

Added in: v15.0.0

- Type: `<string>`

##### `rsaHashedKeyAlgorithm.publicExponent`#

Added in: v15.0.0

- Type: `<Uint8Array>`

The RSA public exponent.

#### Class: `RsaHashedKeyGenParams`#

Added in: v15.0.0

##### `rsaHashedKeyGenParams.hash`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v24.7.0 | SHA-3 algorithms are now supported.

- Type: `<string>` | `<Algorithm>`

If represented as a `<string>`, the value must be one of:

- `'SHA-1'`
- `'SHA-256'`
- `'SHA-384'`
- `'SHA-512'`
- `'SHA3-256'`4
- `'SHA3-384'`4
- `'SHA3-512'`4

If represented as an `<Algorithm>`, the object's `name` property must be one of the above listed values.

##### `rsaHashedKeyGenParams.modulusLength`#

Added in: v15.0.0

- Type: `<number>`

The length in bits of the RSA modulus. As a best practice, this should be at least `2048`.

##### `rsaHashedKeyGenParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be one of `'RSASSA-PKCS1-v1_5'`, `'RSA-PSS'`, or `'RSA-OAEP'`.

##### `rsaHashedKeyGenParams.publicExponent`#

Added in: v15.0.0

- Type: `<Uint8Array>`

The RSA public exponent. This must be a `<Uint8Array>` containing a big-endian, unsigned integer that must fit within 32-bits. The `<Uint8Array>` may contain an arbitrary number of leading zero-bits. The value must be a prime number. Unless there is reason to use a different value, use `new Uint8Array([1, 0, 1])` (65537) as the public exponent.

#### Class: `RsaOaepParams`#

Added in: v15.0.0

##### `rsaOaepParams.label`#

Added in: v15.0.0

- Type: `<ArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<Buffer>`

An additional collection of bytes that will not be encrypted, but will be bound to the generated ciphertext.

The `rsaOaepParams.label` parameter is optional.

##### `rsaOaepParams.name`#

Added in: v15.0.0

- Type: `<string>` must be `'RSA-OAEP'`.

#### Class: `RsaPssParams`#

Added in: v15.0.0

##### `rsaPssParams.name`#

Added in: v15.0.0

- Type: `<string>` Must be `'RSA-PSS'`.

##### `rsaPssParams.saltLength`#

Added in: v15.0.0

- Type: `<number>`

The length (in bytes) of the random salt to use.

#### Class: `TurboShakeParams`#

Added in: v25.9.0

##### `turboShakeParams.domainSeparation`#

Added in: v25.9.0

- Type: `<number>` | `<undefined>`

The optional domain separation byte (0x01-0x7f). Defaults to `0x1f`.

##### `turboShakeParams.name`#

Added in: v25.9.0

- Type: `<string>` Must be `'TurboSHAKE128'`4 or `'TurboSHAKE256'`4.

##### `turboShakeParams.outputLength`#

Added in: v25.9.0

- Type: `<number>` represents the requested output length in bits.

## Footnotes

1. Requires OpenSSL >= 3.0 ↩ ↩2 ↩3

2. Requires OpenSSL >= 3.2 ↩ ↩2 ↩3

3. Requires OpenSSL >= 3.5 ↩ ↩2 ↩3 ↩4 ↩5 ↩6

4. See Modern Algorithms in the Web Cryptography API ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13 ↩14 ↩15 ↩16 ↩17 ↩18 ↩19 ↩20 ↩21 ↩22 ↩23 ↩24 ↩25 ↩26 ↩27 ↩28 ↩29 ↩30 ↩31 ↩32 ↩33 ↩34 ↩35 ↩36 ↩37 ↩38 ↩39 ↩40 ↩41 ↩42 ↩43 ↩44 ↩45 ↩46 ↩47 ↩48 ↩49 ↩50 ↩51 ↩52 ↩53 ↩54 ↩55 ↩56 ↩57 ↩58 ↩59 ↩60 ↩61 ↩62 ↩63 ↩64 ↩65 ↩66 ↩67 ↩68 ↩69 ↩70 ↩71 ↩72 ↩73 ↩74 ↩75 ↩76 ↩77 ↩78 ↩79 ↩80 ↩81 ↩82 ↩83 ↩84 ↩85 ↩86 ↩87 ↩88 ↩89 ↩90 ↩91 ↩92 ↩93 ↩94 ↩95 ↩96 ↩97 ↩98 ↩99 ↩100 ↩101 ↩102 ↩103 ↩104 ↩105 ↩106 ↩107 ↩108 ↩109 ↩110 ↩111 ↩112 ↩113 ↩114 ↩115 ↩116 ↩117 ↩118 ↩119 ↩120 ↩121 ↩122 ↩123 ↩124 ↩125 ↩126 ↩127 ↩128 ↩129 ↩130 ↩131 ↩132 ↩133 ↩134 ↩135 ↩136 ↩137 ↩138 ↩139 ↩140 ↩141 ↩142 ↩143 ↩144 ↩145 ↩146 ↩147 ↩148 ↩149 ↩150 ↩151 ↩152 ↩153 ↩154 ↩155

5. See Secure Curves in the Web Cryptography API ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13 ↩14 ↩15 ↩16 ↩17

## Web Streams API#

Added in: v16.5.0
History

Version | Changes
--- | ---
v21.0.0 | No longer experimental.
v18.0.0 | Use of this API no longer emit a runtime warning.

Stability: 2 - Stable

An implementation of the WHATWG Streams Standard.

### Overview#

The WHATWG Streams Standard (or "web streams") defines an API for handling streaming data. It is similar to the Node.js Streams API but emerged later and has become the "standard" API for streaming data across many JavaScript environments.

There are three primary types of objects:

- `ReadableStream` - Represents a source of streaming data.
- `WritableStream` - Represents a destination for streaming data.
- `TransformStream` - Represents an algorithm for transforming streaming data.

#### Example `ReadableStream`#

This example creates a simple `ReadableStream` that pushes the current `performance.now()` timestamp once every second forever. An async iterable is used to read the data from the stream.

    import {
      ReadableStream,
    } from 'node:stream/web';

    import {
      setInterval as every,
    } from 'node:timers/promises';

    import {
      performance,
    } from 'node:perf_hooks';

    const SECOND = 1000;

    const stream = new ReadableStream({
      async start(controller) {
        for await (const _ of every(SECOND))
          controller.enqueue(performance.now());
      },
    });

    for await (const value of stream)
      console.log(value);
    const {
      ReadableStream,
    } = require('node:stream/web');

    const {
      setInterval: every,
    } = require('node:timers/promises');

    const {
      performance,
    } = require('node:perf_hooks');

    const SECOND = 1000;

    const stream = new ReadableStream({
      async start(controller) {
        for await (const _ of every(SECOND))
          controller.enqueue(performance.now());
      },
    });

    (async () => {
      for await (const value of stream)
        console.log(value);
    })();
    javascript

#### Node.js streams interoperability#

Node.js streams can be converted to web streams and vice versa via the `toWeb` and `fromWeb` methods present on `stream.Readable`, `stream.Writable` and `stream.Duplex` objects.

For more details refer to the relevant documentation:

- `stream.Readable.toWeb`
- `stream.Readable.fromWeb`
- `stream.Writable.toWeb`
- `stream.Writable.fromWeb`
- `stream.Duplex.toWeb`
- `stream.Duplex.fromWeb`

### API#

#### `ReadableStreamTee(stream[, cloneForBranch2])`#

Added in: v26.5.0

Stability: 1 - Experimental

- `stream` `<ReadableStream>`
- `cloneForBranch2` `<boolean>` When `true`, chunks enqueued into the second branch are cloned from chunks enqueued into the first branch. Default: `false`.
- Returns: `<ReadableStream>`[] Two `<ReadableStream>` branches.

Runs the WHATWG `ReadableStreamTee` abstract operation on `stream`.

This differs from `readableStream.tee()` only when `cloneForBranch2` is `true`. The `tee()` method always passes `false`, while other web platform specifications, such as Fetch body cloning, pass `true` so that the second branch receives cloned chunks and consumption of one branch cannot mutate chunks seen by the other.

#### Class: `ReadableStream`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new ReadableStream([underlyingSource [, strategy]])`#

Added in: v16.5.0

- `underlyingSource` `<Object>`
  - `start` `<Function>` A user-defined function that is invoked immediately when the `ReadableStream` is created.
    - `controller` `<ReadableStreamDefaultController>` | `<ReadableByteStreamController>`
    - Returns: `undefined` or a promise fulfilled with `undefined`.
  - `pull` `<Function>` A user-defined function that is called repeatedly when the `ReadableStream` internal queue is not full. The operation may be sync or async. If async, the function will not be called again until the previously returned promise is fulfilled.
    - `controller` `<ReadableStreamDefaultController>` | `<ReadableByteStreamController>`
    - Returns: A promise fulfilled with `undefined`.
  - `cancel` `<Function>` A user-defined function that is called when the `ReadableStream` is canceled.
    - `reason` `<any>`
    - Returns: A promise fulfilled with `undefined`.
  - `type` `<string>` Must be `'bytes'` or `undefined`.
  - `autoAllocateChunkSize` `<number>` Used only when `type` is equal to `'bytes'`. When set to a non-zero value a view buffer is automatically allocated to `ReadableByteStreamController.byobRequest`. When not set one must use stream's internal queues to transfer data via the default reader `ReadableStreamDefaultReader`.
- `strategy` `<Object>`
  - `highWaterMark` `<number>` The maximum internal queue size before backpressure is applied.
  - `size` `<Function>` A user-defined function used to identify the size of each chunk of data.
    - `chunk` `<any>`
    - Returns: `<number>`

##### `readableStream.locked`#

Added in: v16.5.0

- Type: `<boolean>` Set to `true` if there is an active reader for this `<ReadableStream>`.

The `readableStream.locked` property is `false` by default, and is switched to `true` while there is an active reader consuming the stream's data.

##### `readableStream.cancel([reason])`#

Added in: v16.5.0

- `reason` `<any>`
- Returns: A promise fulfilled with `undefined` once cancelation has been completed.

##### `readableStream.getReader([options])`#

Added in: v16.5.0

- `options` `<Object>`
  - `mode` `<string>` `'byob'` or `undefined`
- Returns: `<ReadableStreamDefaultReader>` | `<ReadableStreamBYOBReader>`

    import { ReadableStream } from 'node:stream/web';

    const stream = new ReadableStream();

    const reader = stream.getReader();

    console.log(await reader.read());
    const { ReadableStream } = require('node:stream/web');

    const stream = new ReadableStream();

    const reader = stream.getReader();

    reader.read().then(console.log);
    javascript

Causes the `readableStream.locked` to be `true`.

##### `readableStream.pipeThrough(transform[, options])`#

Added in: v16.5.0

- `transform` `<Object>`
  - `readable` `<ReadableStream>` The `ReadableStream` to which `transform.writable` will push the potentially modified data it receives from this `ReadableStream`.
  - `writable` `<WritableStream>` The `WritableStream` to which this `ReadableStream`'s data will be written.
- `options` `<Object>`
  - `preventAbort` `<boolean>` When `true`, errors in this `ReadableStream` will not cause `transform.writable` to be aborted.
  - `preventCancel` `<boolean>` When `true`, errors in the destination `transform.writable` do not cause this `ReadableStream` to be canceled.
  - `preventClose` `<boolean>` When `true`, closing this `ReadableStream` does not cause `transform.writable` to be closed.
  - `signal` `<AbortSignal>` Allows the transfer of data to be canceled using an `<AbortController>`.
- Returns: `<ReadableStream>` From `transform.readable`.

Connects this `<ReadableStream>` to the pair of `<ReadableStream>` and `<WritableStream>` provided in the `transform` argument such that the data from this `<ReadableStream>` is written in to `transform.writable`, possibly transformed, then pushed to `transform.readable`. Once the pipeline is configured, `transform.readable` is returned.

Causes the `readableStream.locked` to be `true` while the pipe operation is active.

    import {
      ReadableStream,
      TransformStream,
    } from 'node:stream/web';

    const stream = new ReadableStream({
      start(controller) {
        controller.enqueue('a');
      },
    });

    const transform = new TransformStream({
      transform(chunk, controller) {
        controller.enqueue(chunk.toUpperCase());
      },
    });

    const transformedStream = stream.pipeThrough(transform);

    for await (const chunk of transformedStream)
      console.log(chunk);
      // Prints: A
    const {
      ReadableStream,
      TransformStream,
    } = require('node:stream/web');

    const stream = new ReadableStream({
      start(controller) {
        controller.enqueue('a');
      },
    });

    const transform = new TransformStream({
      transform(chunk, controller) {
        controller.enqueue(chunk.toUpperCase());
      },
    });

    const transformedStream = stream.pipeThrough(transform);

    (async () => {
      for await (const chunk of transformedStream)
        console.log(chunk);
        // Prints: A
    })();
    javascript

##### `readableStream.pipeTo(destination[, options])`#

Added in: v16.5.0

- `destination` `<WritableStream>` A `<WritableStream>` to which this `ReadableStream`'s data will be written.
- `options` `<Object>`
  - `preventAbort` `<boolean>` When `true`, errors in this `ReadableStream` will not cause `destination` to be aborted.
  - `preventCancel` `<boolean>` When `true`, errors in the `destination` will not cause this `ReadableStream` to be canceled.
  - `preventClose` `<boolean>` When `true`, closing this `ReadableStream` does not cause `destination` to be closed.
  - `signal` `<AbortSignal>` Allows the transfer of data to be canceled using an `<AbortController>`.
- Returns: A promise fulfilled with `undefined`

Causes the `readableStream.locked` to be `true` while the pipe operation is active.

##### `readableStream.tee()`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.10.0, v16.18.0 | Support teeing a readable byte stream.

- Returns: `<ReadableStream>`[]

Returns a pair of new `<ReadableStream>` instances to which this `ReadableStream`'s data will be forwarded. Each will receive the same data.

Causes the `readableStream.locked` to be `true`.

##### `readableStream.values([options])`#

Added in: v16.5.0

- `options` `<Object>`
  - `preventCancel` `<boolean>` When `true`, prevents the `<ReadableStream>` from being closed when the async iterator abruptly terminates. Default: `false`.

Creates and returns an async iterator usable for consuming this `ReadableStream`'s data.

Causes the `readableStream.locked` to be `true` while the async iterator is active.

    import { Buffer } from 'node:buffer';

    const stream = new ReadableStream(getSomeSource());

    for await (const chunk of stream.values({ preventCancel: true }))
      console.log(Buffer.from(chunk).toString());
    mjs

##### Async Iteration#

The `<ReadableStream>` object supports the async iterator protocol using `for await` syntax.

    import { Buffer } from 'node:buffer';

    const stream = new ReadableStream(getSomeSource());

    for await (const chunk of stream)
      console.log(Buffer.from(chunk).toString());
    mjs

The async iterator will consume the `<ReadableStream>` until it terminates.

By default, if the async iterator exits early (via either a `break`, `return`, or a `throw`), the `<ReadableStream>` will be closed. To prevent automatic closing of the `<ReadableStream>`, use the `readableStream.values()` method to acquire the async iterator and set the `preventCancel` option to `true`.

The `<ReadableStream>` must not be locked (that is, it must not have an existing active reader). During the async iteration, the `<ReadableStream>` will be locked.

##### Transferring with `postMessage()`#

A `<ReadableStream>` instance can be transferred using a `<MessagePort>`.

    const stream = new ReadableStream(getReadableSourceSomehow());

    const { port1, port2 } = new MessageChannel();

    port1.onmessage = ({ data }) => {
      data.getReader().read().then((chunk) => {
        console.log(chunk);
      });
    };

    port2.postMessage(stream, [stream]);
    js

#### `ReadableStream.from(iterable)`#

Added in: v20.6.0

- `iterable` `<Iterable>` Object implementing the `Symbol.asyncIterator` or `Symbol.iterator` iterable protocol.

A utility method that creates a new `<ReadableStream>` from an iterable.

    import { ReadableStream } from 'node:stream/web';

    async function* asyncIterableGenerator() {
      yield 'a';
      yield 'b';
      yield 'c';
    }

    const stream = ReadableStream.from(asyncIterableGenerator());

    for await (const chunk of stream)
      console.log(chunk); // Prints: 'a', 'b', 'c'
    const { ReadableStream } = require('node:stream/web');

    async function* asyncIterableGenerator() {
      yield 'a';
      yield 'b';
      yield 'c';
    }

    (async () => {
      const stream = ReadableStream.from(asyncIterableGenerator());

      for await (const chunk of stream)
        console.log(chunk); // Prints: 'a', 'b', 'c'
    })();
    javascript

To pipe the resulting `<ReadableStream>` into a `<WritableStream>` the `<Iterable>` should yield a sequence of `<Buffer>`, `<TypedArray>`, or `<DataView>` objects.

    import { ReadableStream } from 'node:stream/web';
    import { Buffer } from 'node:buffer';

    async function* asyncIterableGenerator() {
      yield Buffer.from('a');
      yield Buffer.from('b');
      yield Buffer.from('c');
    }

    const stream = ReadableStream.from(asyncIterableGenerator());

    await stream.pipeTo(createWritableStreamSomehow());
    const { ReadableStream } = require('node:stream/web');
    const { Buffer } = require('node:buffer');

    async function* asyncIterableGenerator() {
      yield Buffer.from('a');
      yield Buffer.from('b');
      yield Buffer.from('c');
    }

    const stream = ReadableStream.from(asyncIterableGenerator());

    (async () => {
      await stream.pipeTo(createWritableStreamSomehow());
    })();
    javascript

#### Class: `ReadableStreamDefaultReader`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

By default, calling `readableStream.getReader()` with no arguments will return an instance of `ReadableStreamDefaultReader`. The default reader treats the chunks of data passed through the stream as opaque values, which allows the `<ReadableStream>` to work with generally any JavaScript value.

##### `new ReadableStreamDefaultReader(stream)`#

Added in: v16.5.0

- `stream` `<ReadableStream>`

Creates a new `<ReadableStreamDefaultReader>` that is locked to the given `<ReadableStream>`.

##### `readableStreamDefaultReader.cancel([reason])`#

Added in: v16.5.0

- `reason` `<any>`
- Returns: A promise fulfilled with `undefined`.

Cancels the `<ReadableStream>` and returns a promise that is fulfilled when the underlying stream has been canceled.

##### `readableStreamDefaultReader.closed`#

Added in: v16.5.0

- Type: `<Promise>` Fulfilled with `undefined` when the associated `<ReadableStream>` is closed or rejected if the stream errors or the reader's lock is released before the stream finishes closing.

##### `readableStreamDefaultReader.read()`#

Added in: v16.5.0

- Returns: A promise fulfilled with an object:
  - `value` `<any>`
  - `done` `<boolean>`

Requests the next chunk of data from the underlying `<ReadableStream>` and returns a promise that is fulfilled with the data once it is available.

##### `readableStreamDefaultReader.releaseLock()`#

Added in: v16.5.0

Releases this reader's lock on the underlying `<ReadableStream>`.

#### Class: `ReadableStreamBYOBReader`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

The `ReadableStreamBYOBReader` is an alternative consumer for byte-oriented `<ReadableStream>`s (those that are created with `underlyingSource.type` set equal to `'bytes'` when the `ReadableStream` was created).

The `BYOB` is short for "bring your own buffer". This is a pattern that allows for more efficient reading of byte-oriented data that avoids extraneous copying.

    import {
      open,
    } from 'node:fs/promises';

    import {
      ReadableStream,
    } from 'node:stream/web';

    import { Buffer } from 'node:buffer';

    class Source {
      type = 'bytes';
      autoAllocateChunkSize = 1024;

      async start(controller) {
        this.file = await open(new URL(import.meta.url));
        this.controller = controller;
      }

      async pull(controller) {
        const view = controller.byobRequest?.view;
        const {
          bytesRead,
        } = await this.file.read({
          buffer: view,
          offset: view.byteOffset,
          length: view.byteLength,
        });

        if (bytesRead === 0) {
          await this.file.close();
          this.controller.close();
        }
        controller.byobRequest.respond(bytesRead);
      }
    }

    const stream = new ReadableStream(new Source());

    async function read(stream) {
      const reader = stream.getReader({ mode: 'byob' });

      const chunks = [];
      let result;
      do {
        result = await reader.read(Buffer.alloc(100));
        if (result.value !== undefined)
          chunks.push(Buffer.from(result.value));
      } while (!result.done);

      return Buffer.concat(chunks);
    }

    const data = await read(stream);
    console.log(Buffer.from(data).toString());
    mjs

##### `new ReadableStreamBYOBReader(stream)`#

Added in: v16.5.0

- `stream` `<ReadableStream>`

Creates a new `ReadableStreamBYOBReader` that is locked to the given `<ReadableStream>`.

##### `readableStreamBYOBReader.cancel([reason])`#

Added in: v16.5.0

- `reason` `<any>`
- Returns: A promise fulfilled with `undefined`.

Cancels the `<ReadableStream>` and returns a promise that is fulfilled when the underlying stream has been canceled.

##### `readableStreamBYOBReader.closed`#

Added in: v16.5.0

- Type: `<Promise>` Fulfilled with `undefined` when the associated `<ReadableStream>` is closed or rejected if the stream errors or the reader's lock is released before the stream finishes closing.

##### `readableStreamBYOBReader.read(view[, options])`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v21.7.0, v20.17.0 | Added `min` option.

- `view` `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>`
  - `min` `<number>` When set, the returned promise will only be fulfilled as soon as `min` number of elements are available. When not set, the promise fulfills when at least one element is available.
- Returns: A promise fulfilled with an object:
  - `value` `<TypedArray>` | `<DataView>`
  - `done` `<boolean>`

Requests the next chunk of data from the underlying `<ReadableStream>` and returns a promise that is fulfilled with the data once it is available.

Do not pass a pooled `<Buffer>` object instance in to this method. Pooled `Buffer` objects are created using `Buffer.allocUnsafe()`, or `Buffer.from()`, or are often returned by various `node:fs` module callbacks. These types of `Buffer`s use a shared underlying `<ArrayBuffer>` object that contains all of the data from all of the pooled `Buffer` instances. When a `Buffer`, `<TypedArray>`, or `<DataView>` is passed in to `readableStreamBYOBReader.read()`, the view's underlying `ArrayBuffer` is detached, invalidating all existing views that may exist on that `ArrayBuffer`. This can have disastrous consequences for your application.

##### `readableStreamBYOBReader.releaseLock()`#

Added in: v16.5.0

Releases this reader's lock on the underlying `<ReadableStream>`.

#### Class: `ReadableStreamDefaultController`#

Added in: v16.5.0

Every `<ReadableStream>` has a controller that is responsible for the internal state and management of the stream's queue. The `ReadableStreamDefaultController` is the default controller implementation for `ReadableStream`s that are not byte-oriented.

##### `readableStreamDefaultController.close()`#

Added in: v16.5.0

Closes the `<ReadableStream>` to which this controller is associated.

##### `readableStreamDefaultController.desiredSize`#

Added in: v16.5.0

- Type: `<number>`

Returns the amount of data remaining to fill the `<ReadableStream>`'s queue.

##### `readableStreamDefaultController.enqueue([chunk])`#

Added in: v16.5.0

- `chunk` `<any>`

Appends a new chunk of data to the `<ReadableStream>`'s queue.

##### `readableStreamDefaultController.error([error])`#

Added in: v16.5.0

- `error` `<any>`

Signals an error that causes the `<ReadableStream>` to error and close.

#### Class: `ReadableByteStreamController`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.10.0 | Support handling a BYOB pull request from a released reader.

Every `<ReadableStream>` has a controller that is responsible for the internal state and management of the stream's queue. The `ReadableByteStreamController` is for byte-oriented `ReadableStream`s.

##### `readableByteStreamController.byobRequest`#

Added in: v16.5.0

- Type: `<ReadableStreamBYOBRequest>`

##### `readableByteStreamController.close()`#

Added in: v16.5.0

Closes the `<ReadableStream>` to which this controller is associated.

##### `readableByteStreamController.desiredSize`#

Added in: v16.5.0

- Type: `<number>`

Returns the amount of data remaining to fill the `<ReadableStream>`'s queue.

##### `readableByteStreamController.enqueue(chunk)`#

Added in: v16.5.0

- `chunk` `<Buffer>` | `<TypedArray>` | `<DataView>`

Appends a new chunk of data to the `<ReadableStream>`'s queue.

##### `readableByteStreamController.error([error])`#

Added in: v16.5.0

- `error` `<any>`

Signals an error that causes the `<ReadableStream>` to error and close.

#### Class: `ReadableStreamBYOBRequest`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

When using `ReadableByteStreamController` in byte-oriented streams, and when using the `ReadableStreamBYOBReader`, the `readableByteStreamController.byobRequest` property provides access to a `ReadableStreamBYOBRequest` instance that represents the current read request. The object is used to gain access to the `ArrayBuffer`/`TypedArray` that has been provided for the read request to fill, and provides methods for signaling that the data has been provided.

##### `readableStreamBYOBRequest.respond(bytesWritten)`#

Added in: v16.5.0

- `bytesWritten` `<number>`

Signals that a `bytesWritten` number of bytes have been written to `readableStreamBYOBRequest.view`.

##### `readableStreamBYOBRequest.respondWithNewView(view)`#

Added in: v16.5.0

- `view` `<Buffer>` | `<TypedArray>` | `<DataView>`

Signals that the request has been fulfilled with bytes written to a new `Buffer`, `TypedArray`, or `DataView`.

##### `readableStreamBYOBRequest.view`#

Added in: v16.5.0

- Type: `<Buffer>` | `<TypedArray>` | `<DataView>`

#### Class: `WritableStream`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

The `WritableStream` is a destination to which stream data is sent.

    import {
      WritableStream,
    } from 'node:stream/web';

    const stream = new WritableStream({
      write(chunk) {
        console.log(chunk);
      },
    });

    await stream.getWriter().write('Hello World');
    mjs

##### `new WritableStream([underlyingSink[, strategy]])`#

Added in: v16.5.0

- `underlyingSink` `<Object>`
  - `start` `<Function>` A user-defined function that is invoked immediately when the `WritableStream` is created.
    - `controller` `<WritableStreamDefaultController>`
    - Returns: `undefined` or a promise fulfilled with `undefined`.
  - `write` `<Function>` A user-defined function that is invoked when a chunk of data has been written to the `WritableStream`.
    - `chunk` `<any>`
    - `controller` `<WritableStreamDefaultController>`
    - Returns: A promise fulfilled with `undefined`.
  - `close` `<Function>` A user-defined function that is called when the `WritableStream` is closed.
    - Returns: A promise fulfilled with `undefined`.
  - `abort` `<Function>` A user-defined function that is called to abruptly close the `WritableStream`.
    - `reason` `<any>`
    - Returns: A promise fulfilled with `undefined`.
  - `type` `<any>` The `type` option is reserved for future use and must be undefined.
- `strategy` `<Object>`
  - `highWaterMark` `<number>` The maximum internal queue size before backpressure is applied.
  - `size` `<Function>` A user-defined function used to identify the size of each chunk of data.
    - `chunk` `<any>`
    - Returns: `<number>`

##### `writableStream.abort([reason])`#

Added in: v16.5.0

- `reason` `<any>`
- Returns: A promise fulfilled with `undefined`.

Abruptly terminates the `WritableStream`. All queued writes will be canceled with their associated promises rejected.

##### `writableStream.close()`#

Added in: v16.5.0

- Returns: A promise fulfilled with `undefined`.

Closes the `WritableStream` when no additional writes are expected.

##### `writableStream.getWriter()`#

Added in: v16.5.0

- Returns: `<WritableStreamDefaultWriter>`

Creates and returns a new writer instance that can be used to write data into the `WritableStream`.

##### `writableStream.locked`#

Added in: v16.5.0

- Type: `<boolean>`

The `writableStream.locked` property is `false` by default, and is switched to `true` while there is an active writer attached to this `WritableStream`.

##### Transferring with postMessage()#

A `<WritableStream>` instance can be transferred using a `<MessagePort>`.

    const stream = new WritableStream(getWritableSinkSomehow());

    const { port1, port2 } = new MessageChannel();

    port1.onmessage = ({ data }) => {
      data.getWriter().write('hello');
    };

    port2.postMessage(stream, [stream]);
    js

#### Class: `WritableStreamDefaultWriter`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new WritableStreamDefaultWriter(stream)`#

Added in: v16.5.0

- `stream` `<WritableStream>`

Creates a new `WritableStreamDefaultWriter` that is locked to the given `WritableStream`.

##### `writableStreamDefaultWriter.abort([reason])`#

Added in: v16.5.0

- `reason` `<any>`
- Returns: A promise fulfilled with `undefined`.

Abruptly terminates the `WritableStream`. All queued writes will be canceled with their associated promises rejected.

##### `writableStreamDefaultWriter.close()`#

Added in: v16.5.0

- Returns: A promise fulfilled with `undefined`.

Closes the `WritableStream` when no additional writes are expected.

##### `writableStreamDefaultWriter.closed`#

Added in: v16.5.0

- Type: `<Promise>` Fulfilled with `undefined` when the associated `<WritableStream>` is closed or rejected if the stream errors or the writer's lock is released before the stream finishes closing.

##### `writableStreamDefaultWriter.desiredSize`#

Added in: v16.5.0

- Type: `<number>`

The amount of data required to fill the `<WritableStream>`'s queue.

##### `writableStreamDefaultWriter.ready`#

Added in: v16.5.0

- Type: `<Promise>` Fulfilled with `undefined` when the writer is ready to be used.

##### `writableStreamDefaultWriter.releaseLock()`#

Added in: v16.5.0

Releases this writer's lock on the underlying `<ReadableStream>`.

##### `writableStreamDefaultWriter.write([chunk])`#

Added in: v16.5.0

- `chunk` `<any>`
- Returns: A promise fulfilled with `undefined`.

Appends a new chunk of data to the `<WritableStream>`'s queue.

#### Class: `WritableStreamDefaultController`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

The `WritableStreamDefaultController` manages the `<WritableStream>`'s internal state.

##### `writableStreamDefaultController.error([error])`#

Added in: v16.5.0

- `error` `<any>`

Called by user-code to signal that an error has occurred while processing the `WritableStream` data. When called, the `<WritableStream>` will be aborted, with currently pending writes canceled.

##### `writableStreamDefaultController.signal`#

- Type: `<AbortSignal>` An `AbortSignal` that can be used to cancel pending write or close operations when a `<WritableStream>` is aborted.

#### Class: `TransformStream`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

A `TransformStream` consists of a `<ReadableStream>` and a `<WritableStream>` that are connected such that the data written to the `WritableStream` is received, and potentially transformed, before being pushed into the `ReadableStream`'s queue.

    import {
      TransformStream,
    } from 'node:stream/web';

    const transform = new TransformStream({
      transform(chunk, controller) {
        controller.enqueue(chunk.toUpperCase());
      },
    });

    await Promise.all([
      transform.writable.getWriter().write('A'),
      transform.readable.getReader().read(),
    ]);
    mjs

##### `new TransformStream([transformer[, writableStrategy[, readableStrategy]]])`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v21.5.0, v20.14.0 | Supports the `cancel` transformer callback.

- `transformer` `<Object>`
  - `start` `<Function>` A user-defined function that is invoked immediately when the `TransformStream` is created.
    - `controller` `<TransformStreamDefaultController>`
    - Returns: `undefined` or a promise fulfilled with `undefined`
  - `transform` `<Function>` A user-defined function that receives, and potentially modifies, a chunk of data written to `transformStream.writable`, before forwarding that on to `transformStream.readable`.
    - `chunk` `<any>`
    - `controller` `<TransformStreamDefaultController>`
    - Returns: A promise fulfilled with `undefined`.
  - `flush` `<Function>` A user-defined function that is called immediately before the writable side of the `TransformStream` is closed, signaling the end of the transformation process.
    - `controller` `<TransformStreamDefaultController>`
    - Returns: A promise fulfilled with `undefined`.
  - `cancel` `<Function>` A user-defined function that is called when either the readable side of the `TransformStream` is canceled or the writable side is aborted.
    - `reason` `<any>`
    - Returns: A promise fulfilled with `undefined`.
  - `readableType` `<any>` the `readableType` option is reserved for future use and must be `undefined`.
  - `writableType` `<any>` the `writableType` option is reserved for future use and must be `undefined`.
- `writableStrategy` `<Object>`
  - `highWaterMark` `<number>` The maximum internal queue size before backpressure is applied.
  - `size` `<Function>` A user-defined function used to identify the size of each chunk of data.
    - `chunk` `<any>`
    - Returns: `<number>`
- `readableStrategy` `<Object>`
  - `highWaterMark` `<number>` The maximum internal queue size before backpressure is applied.
  - `size` `<Function>` A user-defined function used to identify the size of each chunk of data.
    - `chunk` `<any>`
    - Returns: `<number>`

##### `transformStream.readable`#

Added in: v16.5.0

- Type: `<ReadableStream>`

##### `transformStream.writable`#

Added in: v16.5.0

- Type: `<WritableStream>`

##### Transferring with postMessage()#

A `<TransformStream>` instance can be transferred using a `<MessagePort>`.

    const stream = new TransformStream();

    const { port1, port2 } = new MessageChannel();

    port1.onmessage = ({ data }) => {
      const { writable, readable } = data;
      // ...
    };

    port2.postMessage(stream, [stream]);
    js

#### Class: `TransformStreamDefaultController`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

The `TransformStreamDefaultController` manages the internal state of the `TransformStream`.

##### `transformStreamDefaultController.desiredSize`#

Added in: v16.5.0

- Type: `<number>`

The amount of data required to fill the readable side's queue.

##### `transformStreamDefaultController.enqueue([chunk])`#

Added in: v16.5.0

- `chunk` `<any>`

Appends a chunk of data to the readable side's queue.

##### `transformStreamDefaultController.error([reason])`#

Added in: v16.5.0

- `reason` `<any>`

Signals to both the readable and writable side that an error has occurred while processing the transform data, causing both sides to be abruptly closed.

##### `transformStreamDefaultController.terminate()`#

Added in: v16.5.0

Closes the readable side of the transport and causes the writable side to be abruptly closed with an error.

#### Class: `ByteLengthQueuingStrategy`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new ByteLengthQueuingStrategy(init)`#

Added in: v16.5.0

- `init` `<Object>`
  - `highWaterMark` `<number>`

##### `byteLengthQueuingStrategy.highWaterMark`#

Added in: v16.5.0

- Type: `<number>`

##### `byteLengthQueuingStrategy.size`#

Added in: v16.5.0

- Type: `<Function>`
  - `chunk` `<any>`
  - Returns: `<number>`

#### Class: `CountQueuingStrategy`#

Added in: v16.5.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new CountQueuingStrategy(init)`#

Added in: v16.5.0

- `init` `<Object>`
  - `highWaterMark` `<number>`

##### `countQueuingStrategy.highWaterMark`#

Added in: v16.5.0

- Type: `<number>`

##### `countQueuingStrategy.size`#

Added in: v16.5.0

- Type: `<Function>`
  - `chunk` `<any>`
  - Returns: `<number>`

#### Class: `TextEncoderStream`#

Added in: v16.6.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new TextEncoderStream()`#

Added in: v16.6.0

Creates a new `TextEncoderStream` instance.

##### `textEncoderStream.encoding`#

Added in: v16.6.0

- Type: `<string>`

The encoding supported by the `TextEncoderStream` instance.

##### `textEncoderStream.readable`#

Added in: v16.6.0

- Type: `<ReadableStream>`

##### `textEncoderStream.writable`#

Added in: v16.6.0

- Type: `<WritableStream>`

#### Class: `TextDecoderStream`#

Added in: v16.6.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new TextDecoderStream([encoding[, options]])`#

Added in: v16.6.0

- `encoding` `<string>` Identifies the `encoding` that this `TextDecoder` instance supports. Default: `'utf-8'`.
- `options` `<Object>`
  - `fatal` `<boolean>` `true` if decoding failures are fatal.
  - `ignoreBOM` `<boolean>` When `true`, the `TextDecoderStream` will include the byte order mark in the decoded result. When `false`, the byte order mark will be removed from the output. This option is only used when `encoding` is `'utf-8'`, `'utf-16be'`, or `'utf-16le'`. Default: `false`.

Creates a new `TextDecoderStream` instance.

##### `textDecoderStream.encoding`#

Added in: v16.6.0

- Type: `<string>`

The encoding supported by the `TextDecoderStream` instance.

##### `textDecoderStream.fatal`#

Added in: v16.6.0

- Type: `<boolean>`

The value will be `true` if decoding errors result in a `TypeError` being thrown.

##### `textDecoderStream.ignoreBOM`#

Added in: v16.6.0

- Type: `<boolean>`

The value will be `true` if the decoding result will include the byte order mark.

##### `textDecoderStream.readable`#

Added in: v16.6.0

- Type: `<ReadableStream>`

##### `textDecoderStream.writable`#

Added in: v16.6.0

- Type: `<WritableStream>`

#### Class: `CompressionStream`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new CompressionStream(format)`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.7.0, v22.20.0 | format now accepts `brotli` value.
v21.2.0, v20.12.0 | format now accepts `deflate-raw` value.

- `format` `<string>` One of `'deflate'`, `'deflate-raw'`, `'gzip'`, or `'brotli'`.

##### `compressionStream.readable`#

Added in: v17.0.0

- Type: `<ReadableStream>`

##### `compressionStream.writable`#

Added in: v17.0.0

- Type: `<WritableStream>`

#### Class: `DecompressionStream`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v18.0.0 | This class is now exposed on the global object.

##### `new DecompressionStream(format)`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.7.0, v22.20.0 | format now accepts `brotli` value.
v21.2.0, v20.12.0 | format now accepts `deflate-raw` value.

- `format` `<string>` One of `'deflate'`, `'deflate-raw'`, `'gzip'`, or `'brotli'`.

##### `decompressionStream.readable`#

Added in: v17.0.0

- Type: `<ReadableStream>`

##### `decompressionStream.writable`#

Added in: v17.0.0

- Type: `<WritableStream>`

#### Utility Consumers#

Added in: v16.7.0

The utility consumer functions provide common options for consuming streams.

They are accessed using:

    import {
      arrayBuffer,
      blob,
      buffer,
      json,
      text,
    } from 'node:stream/consumers';
    const {
      arrayBuffer,
      blob,
      buffer,
      json,
      text,
    } = require('node:stream/consumers');
    javascript

##### `streamConsumers.arrayBuffer(stream)`#

Added in: v16.7.0

- `stream` `<ReadableStream>` | `<stream.Readable>` | `<AsyncIterator>`
- Returns: `<Promise>` Fulfills with an `ArrayBuffer` containing the full contents of the stream.

    import { arrayBuffer } from 'node:stream/consumers';
    import { Readable } from 'node:stream';
    import { TextEncoder } from 'node:util';

    const encoder = new TextEncoder();
    const dataArray = encoder.encode('hello world from consumers!');

    const readable = Readable.from(dataArray);
    const data = await arrayBuffer(readable);
    console.log(`from readable: ${data.byteLength}`);
    // Prints: from readable: 76
    const { arrayBuffer } = require('node:stream/consumers');
    const { Readable } = require('node:stream');
    const { TextEncoder } = require('node:util');

    const encoder = new TextEncoder();
    const dataArray = encoder.encode('hello world from consumers!');
    const readable = Readable.from(dataArray);
    arrayBuffer(readable).then((data) => {
      console.log(`from readable: ${data.byteLength}`);
      // Prints: from readable: 76
    });
    javascript

##### `streamConsumers.blob(stream)`#

Added in: v16.7.0

- `stream` `<ReadableStream>` | `<stream.Readable>` | `<AsyncIterator>`
- Returns: `<Promise>` Fulfills with a `<Blob>` containing the full contents of the stream.

    import { blob } from 'node:stream/consumers';

    const dataBlob = new Blob(['hello world from consumers!']);

    const readable = dataBlob.stream();
    const data = await blob(readable);
    console.log(`from readable: ${data.size}`);
    // Prints: from readable: 27
    const { blob } = require('node:stream/consumers');

    const dataBlob = new Blob(['hello world from consumers!']);

    const readable = dataBlob.stream();
    blob(readable).then((data) => {
      console.log(`from readable: ${data.size}`);
      // Prints: from readable: 27
    });
    javascript

##### `streamConsumers.buffer(stream)`#

Added in: v16.7.0

- `stream` `<ReadableStream>` | `<stream.Readable>` | `<AsyncIterator>`
- Returns: `<Promise>` Fulfills with a `<Buffer>` containing the full contents of the stream.

    import { buffer } from 'node:stream/consumers';
    import { Readable } from 'node:stream';
    import { Buffer } from 'node:buffer';

    const dataBuffer = Buffer.from('hello world from consumers!');

    const readable = Readable.from(dataBuffer);
    const data = await buffer(readable);
    console.log(`from readable: ${data.length}`);
    // Prints: from readable: 27
    const { buffer } = require('node:stream/consumers');
    const { Readable } = require('node:stream');
    const { Buffer } = require('node:buffer');

    const dataBuffer = Buffer.from('hello world from consumers!');

    const readable = Readable.from(dataBuffer);
    buffer(readable).then((data) => {
      console.log(`from readable: ${data.length}`);
      // Prints: from readable: 27
    });
    javascript

##### `streamConsumers.bytes(stream)`#

Added in: v25.6.0, v24.14.0

- `stream` `<ReadableStream>` | `<stream.Readable>` | `<AsyncIterator>`
- Returns: `<Promise>` Fulfills with a `<Uint8Array>` containing the full contents of the stream.

    import { bytes } from 'node:stream/consumers';
    import { Readable } from 'node:stream';
    import { Buffer } from 'node:buffer';

    const dataBuffer = Buffer.from('hello world from consumers!');

    const readable = Readable.from(dataBuffer);
    const data = await bytes(readable);
    console.log(`from readable: ${data.length}`);
    // Prints: from readable: 27
    const { bytes } = require('node:stream/consumers');
    const { Readable } = require('node:stream');
    const { Buffer } = require('node:buffer');

    const dataBuffer = Buffer.from('hello world from consumers!');

    const readable = Readable.from(dataBuffer);
    bytes(readable).then((data) => {
      console.log(`from readable: ${data.length}`);
      // Prints: from readable: 27
    });
    javascript

##### `streamConsumers.json(stream)`#

Added in: v16.7.0

- `stream` `<ReadableStream>` | `<stream.Readable>` | `<AsyncIterator>`
- Returns: `<Promise>` Fulfills with the contents of the stream parsed as a UTF-8 encoded string that is then passed through `JSON.parse()`.

    import { json } from 'node:stream/consumers';
    import { Readable } from 'node:stream';

    const items = Array.from(
      {
        length: 100,
      },
      () => ({
        message: 'hello world from consumers!',
      }),
    );

    const readable = Readable.from(JSON.stringify(items));
    const data = await json(readable);
    console.log(`from readable: ${data.length}`);
    // Prints: from readable: 100
    const { json } = require('node:stream/consumers');
    const { Readable } = require('node:stream');

    const items = Array.from(
      {
        length: 100,
      },
      () => ({
        message: 'hello world from consumers!',
      }),
    );

    const readable = Readable.from(JSON.stringify(items));
    json(readable).then((data) => {
      console.log(`from readable: ${data.length}`);
      // Prints: from readable: 100
    });
    javascript

##### `streamConsumers.text(stream)`#

Added in: v16.7.0

- `stream` `<ReadableStream>` | `<stream.Readable>` | `<AsyncIterator>`
- Returns: `<Promise>` Fulfills with the contents of the stream parsed as a UTF-8 encoded string.

    import { text } from 'node:stream/consumers';
    import { Readable } from 'node:stream';

    const readable = Readable.from('Hello world from consumers!');
    const data = await text(readable);
    console.log(`from readable: ${data.length}`);
    // Prints: from readable: 27
    const { text } = require('node:stream/consumers');
    const { Readable } = require('node:stream');

    const readable = Readable.from('Hello world from consumers!');
    text(readable).then((data) => {
      console.log(`from readable: ${data.length}`);
      // Prints: from readable: 27
    });
    javascript

## WebAssembly System Interface (WASI)#

Source Code: lib/wasi.js

Stability: 1 - Experimental

The `node:wasi` module does not currently provide the comprehensive file system security properties provided by some WASI runtimes. Full support for secure file system sandboxing may or may not be implemented in future. In the mean time, do not rely on it to run untrusted code.

The WASI API provides an implementation of the WebAssembly System Interface specification. WASI gives WebAssembly applications access to the underlying operating system via a collection of POSIX-like functions.

    import { readFile } from 'node:fs/promises';
    import { WASI } from 'node:wasi';
    import { argv, env } from 'node:process';

    const wasi = new WASI({
      version: 'preview1',
      args: argv,
      env,
      preopens: {
        '/local': '/some/real/path/that/wasm/can/access',
      },
    });

    const wasm = await WebAssembly.compile(
      await readFile(new URL('./demo.wasm', import.meta.url)),
    );
    const instance = await WebAssembly.instantiate(wasm, wasi.getImportObject());

    wasi.start(instance);
    const { readFile } = require('node:fs/promises');
    const { WASI } = require('node:wasi');
    const { argv, env } = require('node:process');
    const { join } = require('node:path');

    const wasi = new WASI({
      version: 'preview1',
      args: argv,
      env,
      preopens: {
        '/local': '/some/real/path/that/wasm/can/access',
      },
    });

    (async () => {
      const wasm = await WebAssembly.compile(
        await readFile(join(__dirname, 'demo.wasm')),
      );
      const instance = await WebAssembly.instantiate(wasm, wasi.getImportObject());

      wasi.start(instance);
    })();
    javascript

To run the above example, create a new WebAssembly text format file named `demo.wat`:

    (module
        ;; Import the required fd_write WASI function which will write the given io vectors to stdout
        ;; The function signature for fd_write is:
        ;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
        (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))

        (memory 1)
        (export "memory" (memory 0))

        ;; Write 'hello world\n' to memory at an offset of 8 bytes
        ;; Note the trailing newline which is required for the text to appear
        (data (i32.const 8) "hello world\n")

        (func $main (export "_start")
            ;; Creating a new io vector within linear memory
            (i32.store (i32.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
            (i32.store (i32.const 4) (i32.const 12))  ;; iov.iov_len - The length of the 'hello world\n' string

            (call $fd_write
                (i32.const 1) ;; file_descriptor - 1 for stdout
                (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
                (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
                (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
            )
            drop ;; Discard the number of bytes written from the top of the stack
        )
    )
    text

Use wabt to compile `.wat` to `.wasm`

    wat2wasm demo.wat
    bash

### Security#

Added in: v21.2.0, v20.11.0
History

Version | Changes
--- | ---
v21.2.0, v20.11.0 | Clarify WASI security properties.

WASI provides a capabilities-based model through which applications are provided their own custom `env`, `preopens`, `stdin`, `stdout`, `stderr`, and `exit` capabilities.

The current Node.js threat model does not provide secure sandboxing as is present in some WASI runtimes.

While the capability features are supported, they do not form a security model in Node.js. For example, the file system sandboxing can be escaped with various techniques. The project is exploring whether these security guarantees could be added in future.

### Class: `WASI`#

Added in: v13.3.0, v12.16.0

The `WASI` class provides the WASI system call API and additional convenience methods for working with WASI-based applications. Each `WASI` instance represents a distinct environment.

#### `new WASI([options])`#

Added in: v13.3.0, v12.16.0
History

Version | Changes
--- | ---
v20.1.0 | default value of returnOnExit changed to true.
v20.0.0 | The version option is now required and has no default value.
v19.8.0 | version field added to options.

- `options` `<Object>`
  - `args` `<Array>` An array of strings that the WebAssembly application will see as command-line arguments. The first argument is the virtual path to the WASI command itself. Default: `[]`.
  - `env` `<Object>` An object similar to `process.env` that the WebAssembly application will see as its environment. Default: `{}`.
  - `preopens` `<Object>` This object represents the WebAssembly application's local directory structure. The string keys of `preopens` are treated as directories within the file system. The corresponding values in `preopens` are the real paths to those directories on the host machine.
  - `returnOnExit` `<boolean>` By default, when WASI applications call `__wasi_proc_exit()` `wasi.start()` will return with the exit code specified rather than terminating the process. Setting this option to `false` will cause the Node.js process to exit with the specified exit code instead. Default: `true`.
  - `stdin` `<integer>` The file descriptor used as standard input in the WebAssembly application. Default: `0`.
  - `stdout` `<integer>` The file descriptor used as standard output in the WebAssembly application. Default: `1`.
  - `stderr` `<integer>` The file descriptor used as standard error in the WebAssembly application. Default: `2`.
  - `version` `<string>` The version of WASI requested. Currently the only supported versions are `unstable` and `preview1`. This option is mandatory.

#### `wasi.getImportObject()`#

Added in: v19.8.0

Return an import object that can be passed to `WebAssembly.instantiate()` if no other WASM imports are needed beyond those provided by WASI.

If version `unstable` was passed into the constructor it will return:

    { wasi_unstable: wasi.wasiImport }
    json

If version `preview1` was passed into the constructor or no version was specified it will return:

    { wasi_snapshot_preview1: wasi.wasiImport }
    json

#### `wasi.start(instance)`#

Added in: v13.3.0, v12.16.0

- `instance` `<WebAssembly.Instance>`

Attempt to begin execution of `instance` as a WASI command by invoking its `_start()` export. If `instance` does not contain a `_start()` export, or if `instance` contains an `_initialize()` export, then an exception is thrown.

`start()` requires that `instance` exports a `WebAssembly.Memory` named `memory`. If `instance` does not have a `memory` export an exception is thrown.

If `start()` is called more than once, an exception is thrown.

#### `wasi.initialize(instance)`#

Added in: v14.6.0, v12.19.0

- `instance` `<WebAssembly.Instance>`

Attempt to initialize `instance` as a WASI reactor by invoking its `_initialize()` export, if it is present. If `instance` contains a `_start()` export, then an exception is thrown.

`initialize()` requires that `instance` exports a `WebAssembly.Memory` named `memory`. If `instance` does not have a `memory` export an exception is thrown.

If `initialize()` is called more than once, an exception is thrown.

#### `wasi.finalizeBindings(instance[, options])`#

Added in: v24.4.0

- `instance` `<WebAssembly.Instance>`
- `options` `<Object>`
  - `memory` `<WebAssembly.Memory>` Default: `instance.exports.memory`.

Set up WASI host bindings to `instance` without calling `initialize()` or `start()`. This method is useful when the WASI module is instantiated in child threads for sharing the memory across threads.

`finalizeBindings()` requires that either `instance` exports a `WebAssembly.Memory` named `memory` or user specify a `WebAssembly.Memory` object in `options.memory`. If the `memory` is invalid an exception is thrown.

`start()` and `initialize()` will call `finalizeBindings()` internally. If `finalizeBindings()` is called more than once, an exception is thrown.

#### `wasi.wasiImport`#

Added in: v13.3.0, v12.16.0

- Type: `<Object>`

`wasiImport` is an object that implements the WASI system call API. This object should be passed as the `wasi_snapshot_preview1` import during the instantiation of a `WebAssembly.Instance`.

## Worker threads#

Source Code: lib/worker_threads.js Added in: v10.5.0
History

Version | Changes
--- | ---
v12.11.0 | This API is no longer experimental.
v11.7.0 | This API is no longer behind the `--experimental-worker` CLI flag.

Stability: 2 - Stable

The `node:worker_threads` module enables the use of threads that execute JavaScript in parallel. To access it:

    import worker_threads from 'node:worker_threads';
    const worker_threads = require('node:worker_threads');
    javascript

Workers (threads) are useful for performing CPU-intensive JavaScript operations. They do not help much with I/O-intensive work. The Node.js built-in asynchronous I/O operations are more efficient than Workers can be.

Unlike `child_process` or `cluster`, `worker_threads` can share memory. They do so by transferring `ArrayBuffer` instances or sharing `SharedArrayBuffer` instances.

    import {
      Worker,
      isMainThread,
      parentPort,
      workerData,
    } from 'node:worker_threads';

    if (!isMainThread) {
      const { parse } = await import('some-js-parsing-library');
      const script = workerData;
      parentPort.postMessage(parse(script));
    }

    export default function parseJSAsync(script) {
      return new Promise((resolve, reject) => {
        const worker = new Worker(new URL(import.meta.url), {
          workerData: script,
        });
        worker.on('message', resolve);
        worker.once('error', reject);
        worker.once('exit', (code) => {
          if (code !== 0)
            reject(new Error(`Worker stopped with exit code ${code}`));
        });
      });
    };
    const {
      Worker,
      isMainThread,
      parentPort,
      workerData,
    } = require('node:worker_threads');

    if (isMainThread) {
      module.exports = function parseJSAsync(script) {
        return new Promise((resolve, reject) => {
          const worker = new Worker(__filename, {
            workerData: script,
          });
          worker.on('message', resolve);
          worker.once('error', reject);
          worker.once('exit', (code) => {
            if (code !== 0)
              reject(new Error(`Worker stopped with exit code ${code}`));
          });
        });
      };
    } else {
      const { parse } = require('some-js-parsing-library');
      const script = workerData;
      parentPort.postMessage(parse(script));
    }
    javascript

The above example spawns a Worker thread for each `parseJSAsync()` call. In practice, use a pool of Workers for these kinds of tasks. Otherwise, the overhead of creating Workers would likely exceed their benefit.

When implementing a worker pool, use the `AsyncResource` API to inform diagnostic tools (e.g. to provide asynchronous stack traces) about the correlation between tasks and their outcomes. See "Using `AsyncResource` for a `Worker` thread pool" in the `async_hooks` documentation for an example implementation.

Worker threads inherit non-process-specific options by default. Refer to `Worker constructor options` to know how to customize worker thread options, specifically `argv` and `execArgv` options.

### `worker_threads.getEnvironmentData(key)`#

Added in: v15.12.0, v14.18.0
History

Version | Changes
--- | ---
v17.5.0, v16.15.0 | No longer experimental.

- `key` `<any>` Any arbitrary, cloneable JavaScript value that can be used as a `<Map>` key.
- Returns: `<any>`

Within a worker thread, `worker.getEnvironmentData()` returns a clone of data passed to the spawning thread's `worker.setEnvironmentData()`. Every new `Worker` receives its own copy of the environment data automatically.

    import {
      Worker,
      isMainThread,
      setEnvironmentData,
      getEnvironmentData,
    } from 'node:worker_threads';

    if (isMainThread) {
      setEnvironmentData('Hello', 'World!');
      const worker = new Worker(new URL(import.meta.url));
    } else {
      console.log(getEnvironmentData('Hello'));  // Prints 'World!'.
    }
    const {
      Worker,
      isMainThread,
      setEnvironmentData,
      getEnvironmentData,
    } = require('node:worker_threads');

    if (isMainThread) {
      setEnvironmentData('Hello', 'World!');
      const worker = new Worker(__filename);
    } else {
      console.log(getEnvironmentData('Hello'));  // Prints 'World!'.
    }
    javascript

### `worker_threads.isInternalThread`#

Added in: v23.7.0, v22.14.0

- Type: `<boolean>`

Is `true` if this code is running inside of an internal `Worker` thread (e.g the loader thread).

    // loader.js
    import { isInternalThread } from 'node:worker_threads';
    console.log(isInternalThread);  // true
    // loader.js
    const { isInternalThread } = require('node:worker_threads');
    console.log(isInternalThread);  // true
    javascript

    // main.js
    import { isInternalThread } from 'node:worker_threads';
    console.log(isInternalThread);  // false
    // main.js
    const { isInternalThread } = require('node:worker_threads');
    console.log(isInternalThread);  // false
    javascript

### `worker_threads.isMainThread`#

Added in: v10.5.0

- Type: `<boolean>`

Is `true` if this code is not running inside of a `Worker` thread.

    import { Worker, isMainThread } from 'node:worker_threads';

    if (isMainThread) {
      // This re-loads the current file inside a Worker instance.
      new Worker(new URL(import.meta.url));
    } else {
      console.log('Inside Worker!');
      console.log(isMainThread);  // Prints 'false'.
    }
    const { Worker, isMainThread } = require('node:worker_threads');

    if (isMainThread) {
      // This re-loads the current file inside a Worker instance.
      new Worker(__filename);
    } else {
      console.log('Inside Worker!');
      console.log(isMainThread);  // Prints 'false'.
    }
    javascript

### `worker_threads.markAsUntransferable(object)`#

Added in: v14.5.0, v12.19.0

- `object` `<any>` Any arbitrary JavaScript value.

Mark an object as not transferable. If `object` occurs in the transfer list of a `port.postMessage()` call, an error is thrown. This is a no-op if `object` is a primitive value.

In particular, this makes sense for objects that can be cloned, rather than transferred, and which are used by other objects on the sending side. For example, Node.js marks the `ArrayBuffer`s it uses for its `Buffer` pool with this. `ArrayBuffer.prototype.transfer()` is disallowed on such array buffer instances.

This operation cannot be undone.

    import { MessageChannel, markAsUntransferable } from 'node:worker_threads';

    const pooledBuffer = new ArrayBuffer(8);
    const typedArray1 = new Uint8Array(pooledBuffer);
    const typedArray2 = new Float64Array(pooledBuffer);

    markAsUntransferable(pooledBuffer);

    const { port1 } = new MessageChannel();
    try {
      // This will throw an error, because pooledBuffer is not transferable.
      port1.postMessage(typedArray1, [ typedArray1.buffer ]);
    } catch (error) {
      // error.name === 'DataCloneError'
    }

    // The following line prints the contents of typedArray1 -- it still owns
    // its memory and has not been transferred. Without
    // `markAsUntransferable()`, this would print an empty Uint8Array and the
    // postMessage call would have succeeded.
    // typedArray2 is intact as well.
    console.log(typedArray1);
    console.log(typedArray2);
    const { MessageChannel, markAsUntransferable } = require('node:worker_threads');

    const pooledBuffer = new ArrayBuffer(8);
    const typedArray1 = new Uint8Array(pooledBuffer);
    const typedArray2 = new Float64Array(pooledBuffer);

    markAsUntransferable(pooledBuffer);

    const { port1 } = new MessageChannel();
    try {
      // This will throw an error, because pooledBuffer is not transferable.
      port1.postMessage(typedArray1, [ typedArray1.buffer ]);
    } catch (error) {
      // error.name === 'DataCloneError'
    }

    // The following line prints the contents of typedArray1 -- it still owns
    // its memory and has not been transferred. Without
    // `markAsUntransferable()`, this would print an empty Uint8Array and the
    // postMessage call would have succeeded.
    // typedArray2 is intact as well.
    console.log(typedArray1);
    console.log(typedArray2);
    javascript

There is no equivalent to this API in browsers.

### `worker_threads.isMarkedAsUntransferable(object)`#

Added in: v21.0.0

- `object` `<any>` Any JavaScript value.
- Returns: `<boolean>`

Check if an object is marked as not transferable with `markAsUntransferable()`.

    import { markAsUntransferable, isMarkedAsUntransferable } from 'node:worker_threads';

    const pooledBuffer = new ArrayBuffer(8);
    markAsUntransferable(pooledBuffer);

    isMarkedAsUntransferable(pooledBuffer);  // Returns true.
    const { markAsUntransferable, isMarkedAsUntransferable } = require('node:worker_threads');

    const pooledBuffer = new ArrayBuffer(8);
    markAsUntransferable(pooledBuffer);

    isMarkedAsUntransferable(pooledBuffer);  // Returns true.
    javascript

There is no equivalent to this API in browsers.

### `worker_threads.markAsUncloneable(object)`#

Added in: v23.0.0, v22.10.0

- `object` `<any>` Any arbitrary JavaScript value.

Mark an object as not cloneable. If `object` is used as `message` in a `port.postMessage()` call, an error is thrown. This is a no-op if `object` is a primitive value.

This has no effect on `ArrayBuffer`, or any `Buffer` like objects.

This operation cannot be undone.

    import { markAsUncloneable } from 'node:worker_threads';

    const anyObject = { foo: 'bar' };
    markAsUncloneable(anyObject);
    const { port1 } = new MessageChannel();
    try {
      // This will throw an error, because anyObject is not cloneable.
      port1.postMessage(anyObject);
    } catch (error) {
      // error.name === 'DataCloneError'
    }
    const { markAsUncloneable } = require('node:worker_threads');

    const anyObject = { foo: 'bar' };
    markAsUncloneable(anyObject);
    const { port1 } = new MessageChannel();
    try {
      // This will throw an error, because anyObject is not cloneable.
      port1.postMessage(anyObject);
    } catch (error) {
      // error.name === 'DataCloneError'
    }
    javascript

There is no equivalent to this API in browsers.

### `worker_threads.moveMessagePortToContext(port, contextifiedSandbox)`#

Added in: v11.13.0

- `port` `<MessagePort>` The message port to transfer.

- `contextifiedSandbox` `<Object>` A contextified object as returned by the `vm.createContext()` method.

- Returns: `<MessagePort>`

Transfer a `MessagePort` to a different `vm` Context. The original `port` object is rendered unusable, and the returned `MessagePort` instance takes its place.

The returned `MessagePort` is an object in the target context and inherits from its global `Object` class. Objects passed to the `port.onmessage()` listener are also created in the target context and inherit from its global `Object` class.

However, the created `MessagePort` no longer inherits from `<EventTarget>`, and only `port.onmessage()` can be used to receive events using it.

### `worker_threads.parentPort`#

Added in: v10.5.0

- Type: `<null>` | `<MessagePort>`

If this thread is a `Worker`, this is a `MessagePort` allowing communication with the parent thread. Messages sent using `parentPort.postMessage()` are available in the parent thread using `worker.on('message')`, and messages sent from the parent thread using `worker.postMessage()` are available in this thread using `parentPort.on('message')`.

    import { Worker, isMainThread, parentPort } from 'node:worker_threads';

    if (isMainThread) {
      const worker = new Worker(new URL(import.meta.url));
      worker.once('message', (message) => {
        console.log(message);  // Prints 'Hello, world!'.
      });
      worker.postMessage('Hello, world!');
    } else {
      // When a message from the parent thread is received, send it back:
      parentPort.once('message', (message) => {
        parentPort.postMessage(message);
      });
    }
    const { Worker, isMainThread, parentPort } = require('node:worker_threads');

    if (isMainThread) {
      const worker = new Worker(__filename);
      worker.once('message', (message) => {
        console.log(message);  // Prints 'Hello, world!'.
      });
      worker.postMessage('Hello, world!');
    } else {
      // When a message from the parent thread is received, send it back:
      parentPort.once('message', (message) => {
        parentPort.postMessage(message);
      });
    }
    javascript

### `worker_threads.postMessageToThread(threadId, value[, transferList][, timeout])`#

Added in: v22.5.0, v20.19.0

Stability: 1.1 - Active development

- `threadId` `<number>` The target thread ID. If the thread ID is invalid, a `ERR_WORKER_MESSAGING_FAILED` error will be thrown. If the target thread ID is the current thread ID, a `ERR_WORKER_MESSAGING_SAME_THREAD` error will be thrown.
- `value` `<any>` The value to send.
- `transferList` `<Object>`[] If one or more `MessagePort`-like objects are passed in `value`, a `transferList` is required for those items or `ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST` is thrown. See `port.postMessage()` for more information.
- `timeout` `<number>` Time to wait for the message to be delivered in milliseconds. By default it's `undefined`, which means wait forever. If the operation times out, a `ERR_WORKER_MESSAGING_TIMEOUT` error is thrown.
- Returns: `<Promise>` A promise which is fulfilled if the message was successfully processed by destination thread.

Sends a value to another worker, identified by its thread ID.

If the target thread has no listener for the `workerMessage` event, then the operation will throw a `ERR_WORKER_MESSAGING_FAILED` error.

If the target thread threw an error while processing the `workerMessage` event, then the operation will throw a `ERR_WORKER_MESSAGING_ERRORED` error.

This method should be used when the target thread is not the direct parent or child of the current thread. If the two threads are parent-children, use the `require('node:worker_threads').parentPort.postMessage()` and the `worker.postMessage()` to let the threads communicate.

The example below shows the use of `postMessageToThread`: it creates 10 nested threads, the last one will try to communicate with the main thread.

    import process from 'node:process';
    import {
      postMessageToThread,
      threadId,
      workerData,
      Worker,
    } from 'node:worker_threads';

    const channel = new BroadcastChannel('sync');
    const level = workerData?.level ?? 0;

    if (level < 10) {
      const worker = new Worker(new URL(import.meta.url), {
        workerData: { level: level + 1 },
      });
    }

    if (level === 0) {
      process.on('workerMessage', (value, source) => {
        console.log(`${source} -> ${threadId}:`, value);
        postMessageToThread(source, { message: 'pong' });
      });
    } else if (level === 10) {
      process.on('workerMessage', (value, source) => {
        console.log(`${source} -> ${threadId}:`, value);
        channel.postMessage('done');
        channel.close();
      });

      await postMessageToThread(0, { message: 'ping' });
    }

    channel.onmessage = channel.close;
    const {
      postMessageToThread,
      threadId,
      workerData,
      Worker,
    } = require('node:worker_threads');

    const channel = new BroadcastChannel('sync');
    const level = workerData?.level ?? 0;

    if (level < 10) {
      const worker = new Worker(__filename, {
        workerData: { level: level + 1 },
      });
    }

    if (level === 0) {
      process.on('workerMessage', (value, source) => {
        console.log(`${source} -> ${threadId}:`, value);
        postMessageToThread(source, { message: 'pong' });
      });
    } else if (level === 10) {
      process.on('workerMessage', (value, source) => {
        console.log(`${source} -> ${threadId}:`, value);
        channel.postMessage('done');
        channel.close();
      });

      postMessageToThread(0, { message: 'ping' });
    }

    channel.onmessage = channel.close;
    javascript

### `worker_threads.receiveMessageOnPort(port)`#

Added in: v12.3.0
History

Version | Changes
--- | ---
v15.12.0 | The port argument can also refer to a `BroadcastChannel` now.

- `port` `<MessagePort>` | `<BroadcastChannel>`

- Returns: `<Object>` | `<undefined>`

Receive a single message from a given `MessagePort`. If no message is available, `undefined` is returned, otherwise an object with a single `message` property that contains the message payload, corresponding to the oldest message in the `MessagePort`'s queue.

    import { MessageChannel, receiveMessageOnPort } from 'node:worker_threads';
    const { port1, port2 } = new MessageChannel();
    port1.postMessage({ hello: 'world' });

    console.log(receiveMessageOnPort(port2));
    // Prints: { message: { hello: 'world' } }
    console.log(receiveMessageOnPort(port2));
    // Prints: undefined
    const { MessageChannel, receiveMessageOnPort } = require('node:worker_threads');
    const { port1, port2 } = new MessageChannel();
    port1.postMessage({ hello: 'world' });

    console.log(receiveMessageOnPort(port2));
    // Prints: { message: { hello: 'world' } }
    console.log(receiveMessageOnPort(port2));
    // Prints: undefined
    javascript

When this function is used, no `'message'` event is emitted and the `onmessage` listener is not invoked.

### `worker_threads.resourceLimits`#

Added in: v13.2.0, v12.16.0

- Type: `<Object>`
  - `maxYoungGenerationSizeMb` `<number>`
  - `maxOldGenerationSizeMb` `<number>`
  - `codeRangeSizeMb` `<number>`
  - `stackSizeMb` `<number>`

Provides the set of JS engine resource constraints inside this Worker thread. If the `resourceLimits` option was passed to the `Worker` constructor, this matches its values.

If this is used in the main thread, its value is an empty object.

### `worker_threads.SHARE_ENV`#

Added in: v11.14.0

- Type: `<symbol>`

A special value that can be passed as the `env` option of the `Worker` constructor, to indicate that the current thread and the Worker thread should share read and write access to the same set of environment variables.

    import process from 'node:process';
    import { Worker, SHARE_ENV } from 'node:worker_threads';
    new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
      .once('exit', () => {
        console.log(process.env.SET_IN_WORKER);  // Prints 'foo'.
      });
    const { Worker, SHARE_ENV } = require('node:worker_threads');
    new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
      .once('exit', () => {
        console.log(process.env.SET_IN_WORKER);  // Prints 'foo'.
      });
    javascript

### `worker_threads.setEnvironmentData(key[, value])`#

Added in: v15.12.0, v14.18.0
History

Version | Changes
--- | ---
v17.5.0, v16.15.0 | No longer experimental.

- `key` `<any>` Any arbitrary, cloneable JavaScript value that can be used as a `<Map>` key.
- `value` `<any>` Any arbitrary, cloneable JavaScript value that will be cloned and passed automatically to all new `Worker` instances. If `value` is passed as `undefined`, any previously set value for the `key` will be deleted.

The `worker.setEnvironmentData()` API sets the content of `worker.getEnvironmentData()` in the current thread and all new `Worker` instances spawned from the current context.

### `worker_threads.threadId`#

Added in: v10.5.0

- Type: `<integer>`

An integer identifier for the current thread. On the corresponding worker object (if there is any), it is available as `worker.threadId`. This value is unique for each `Worker` instance inside a single process.

### `worker_threads.threadName`#

Added in: v24.6.0, v22.20.0

- `<string>` | `<null>`

A string identifier for the current thread or null if the thread is not running. On the corresponding worker object (if there is any), it is available as `worker.threadName`.

### `worker_threads.workerData`#

Added in: v10.5.0

An arbitrary JavaScript value that contains a clone of the data passed to this thread's `Worker` constructor.

The data is cloned as if using `postMessage()`, according to the HTML structured clone algorithm.

    import { Worker, isMainThread, workerData } from 'node:worker_threads';

    if (isMainThread) {
      const worker = new Worker(new URL(import.meta.url), { workerData: 'Hello, world!' });
    } else {
      console.log(workerData);  // Prints 'Hello, world!'.
    }
    const { Worker, isMainThread, workerData } = require('node:worker_threads');

    if (isMainThread) {
      const worker = new Worker(__filename, { workerData: 'Hello, world!' });
    } else {
      console.log(workerData);  // Prints 'Hello, world!'.
    }
    javascript

### `worker_threads.locks`#

Added in: v24.5.0

Stability: 1 - Experimental

- `<LockManager>`

An instance of a `LockManager` that can be used to coordinate access to resources that may be shared across multiple threads within the same process. The API mirrors the semantics of the browser `LockManager`

#### Class: `Lock`#

Added in: v24.5.0

The `Lock` interface provides information about a lock that has been granted via `locks.request()`

##### `lock.name`#

Added in: v24.5.0

- `<string>`

The name of the lock.

##### `lock.mode`#

Added in: v24.5.0

- `<string>`

The mode of the lock. Either `shared` or `exclusive`.

#### Class: `LockManager`#

Added in: v24.5.0

The `LockManager` interface provides methods for requesting and introspecting locks. To obtain a `LockManager` instance use

    import { locks } from 'node:worker_threads';
    const { locks } = require('node:worker_threads');
    javascript

This implementation matches the browser `LockManager` API.

##### `locks.request(name[, options], callback)`#

Added in: v24.5.0

- `name` `<string>`
- `options` `<Object>`
  - `mode` `<string>` Either `'exclusive'` or `'shared'`. Default: `'exclusive'`.
  - `ifAvailable` `<boolean>` If `true`, the request will only be granted if the lock is not already held. If it cannot be granted, `callback` will be invoked with `null` instead of a `Lock` instance. Default: `false`.
  - `steal` `<boolean>` If `true`, any existing locks with the same name are released and the request is granted immediately, pre-empting any queued requests. Default: `false`.
  - `signal` `<AbortSignal>` that can be used to abort a pending (but not yet granted) lock request.
- `callback` `<Function>` Invoked once the lock is granted (or immediately with `null` if `ifAvailable` is `true` and the lock is unavailable). The lock is released automatically when the function returns, or—if the function returns a promise—when that promise settles.
- Returns: `<Promise>` Resolves once the lock has been released.

    import { locks } from 'node:worker_threads';

    await locks.request('my_resource', async (lock) => {
      // The lock has been acquired.
    });
    // The lock has been released here.
    const { locks } = require('node:worker_threads');

    locks.request('my_resource', async (lock) => {
      // The lock has been acquired.
    }).then(() => {
      // The lock has been released here.
    });
    javascript

##### `locks.query()`#

Added in: v24.5.0

- Returns: `<Promise>`

Resolves with a `LockManagerSnapshot` describing the currently held and pending locks for the current process.

    import { locks } from 'node:worker_threads';

    const snapshot = await locks.query();
    for (const lock of snapshot.held) {
      console.log(`held lock: name ${lock.name}, mode ${lock.mode}`);
    }
    for (const pending of snapshot.pending) {
      console.log(`pending lock: name ${pending.name}, mode ${pending.mode}`);
    }
    const { locks } = require('node:worker_threads');

    locks.query().then((snapshot) => {
      for (const lock of snapshot.held) {
        console.log(`held lock: name ${lock.name}, mode ${lock.mode}`);
      }
      for (const pending of snapshot.pending) {
        console.log(`pending lock: name ${pending.name}, mode ${pending.mode}`);
      }
    });
    javascript

### Class: `BroadcastChannel extends EventTarget`#

Added in: v15.4.0
History

Version | Changes
--- | ---
v18.0.0 | No longer experimental.

Instances of `BroadcastChannel` allow asynchronous one-to-many communication with all other `BroadcastChannel` instances bound to the same channel name.

    import {
      isMainThread,
      BroadcastChannel,
      Worker,
    } from 'node:worker_threads';

    const bc = new BroadcastChannel('hello');

    if (isMainThread) {
      let c = 0;
      bc.onmessage = (event) => {
        console.log(event.data);
        if (++c === 10) bc.close();
      };
      for (let n = 0; n < 10; n++)
        new Worker(new URL(import.meta.url));
    } else {
      bc.postMessage('hello from every worker');
      bc.close();
    }
    const {
      isMainThread,
      BroadcastChannel,
      Worker,
    } = require('node:worker_threads');

    const bc = new BroadcastChannel('hello');

    if (isMainThread) {
      let c = 0;
      bc.onmessage = (event) => {
        console.log(event.data);
        if (++c === 10) bc.close();
      };
      for (let n = 0; n < 10; n++)
        new Worker(__filename);
    } else {
      bc.postMessage('hello from every worker');
      bc.close();
    }
    javascript

#### `new BroadcastChannel(name)`#

Added in: v15.4.0

- `name` `<any>` The name of the channel to connect to. Any JavaScript value that can be converted to a string using ``${name}`` is permitted.

#### `broadcastChannel.close()`#

Added in: v15.4.0

Closes the `BroadcastChannel` connection.

#### `broadcastChannel.onmessage`#

Added in: v15.4.0

- Type: `<Function>` Invoked with a single `MessageEvent` argument when a message is received.

#### `broadcastChannel.onmessageerror`#

Added in: v15.4.0

- Type: `<Function>` Invoked with a received message cannot be deserialized.

#### `broadcastChannel.postMessage(message)`#

Added in: v15.4.0

- `message` `<any>` Any cloneable JavaScript value.

#### `broadcastChannel.ref()`#

Added in: v15.4.0

Opposite of `unref()`. Calling `ref()` on a previously `unref()`ed BroadcastChannel does not let the program exit if it's the only active handle left (the default behavior). If the port is `ref()`ed, calling `ref()` again has no effect.

#### `broadcastChannel.unref()`#

Added in: v15.4.0

Calling `unref()` on a BroadcastChannel allows the thread to exit if this is the only active handle in the event system. If the BroadcastChannel is already `unref()`ed calling `unref()` again has no effect.

### Class: `MessageChannel`#

Added in: v10.5.0

Instances of the `worker.MessageChannel` class represent an asynchronous, two-way communications channel. The `MessageChannel` has no methods of its own. `new MessageChannel()` yields an object with `port1` and `port2` properties, which refer to linked `MessagePort` instances.

    import { MessageChannel } from 'node:worker_threads';

    const { port1, port2 } = new MessageChannel();
    port1.on('message', (message) => console.log('received', message));
    port2.postMessage({ foo: 'bar' });
    // Prints: received { foo: 'bar' } from the `port1.on('message')` listener
    const { MessageChannel } = require('node:worker_threads');

    const { port1, port2 } = new MessageChannel();
    port1.on('message', (message) => console.log('received', message));
    port2.postMessage({ foo: 'bar' });
    // Prints: received { foo: 'bar' } from the `port1.on('message')` listener
    javascript

### Class: `MessagePort`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v14.7.0 | This class now inherits from `EventTarget` rather than from `EventEmitter`.

- Extends: `<EventTarget>`

Instances of the `worker.MessagePort` class represent one end of an asynchronous, two-way communications channel. It can be used to transfer structured data, memory regions and other `MessagePort`s between different `Worker`s.

This implementation matches browser `MessagePort`s.

#### Event: `'close'`#

Added in: v10.5.0

The `'close'` event is emitted once either side of the channel has been disconnected.

    import { MessageChannel } from 'node:worker_threads';
    const { port1, port2 } = new MessageChannel();

    // Prints:
    //   foobar
    //   closed!
    port2.on('message', (message) => console.log(message));
    port2.once('close', () => console.log('closed!'));

    port1.postMessage('foobar');
    port1.close();
    const { MessageChannel } = require('node:worker_threads');
    const { port1, port2 } = new MessageChannel();

    // Prints:
    //   foobar
    //   closed!
    port2.on('message', (message) => console.log(message));
    port2.once('close', () => console.log('closed!'));

    port1.postMessage('foobar');
    port1.close();
    javascript

#### Event: `'message'`#

Added in: v10.5.0

- `value` `<any>` The transmitted value

The `'message'` event is emitted for any incoming message, containing the cloned input of `port.postMessage()`.

Listeners on this event receive a clone of the `value` parameter as passed to `postMessage()` and no further arguments.

#### Event: `'messageerror'`#

Added in: v14.5.0, v12.19.0

- `error` `<Error>` An Error object

The `'messageerror'` event is emitted when deserializing a message failed.

Currently, this event is emitted when there is an error occurring while instantiating the posted JS object on the receiving end. Such situations are rare, but can happen, for instance, when certain Node.js API objects are received in a `vm.Context` (where Node.js APIs are currently unavailable).

#### `port.close()`#

Added in: v10.5.0

Disables further sending of messages on either side of the connection. This method can be called when no further communication will happen over this `MessagePort`.

The `'close'` event is emitted on both `MessagePort` instances that are part of the channel.

#### `port.postMessage(value[, transferList])`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v21.0.0 | An error is thrown when an untransferable object is in the transfer list.
v15.14.0, v14.18.0 | Add 'BlockList' to the list of cloneable types.
v15.9.0, v14.18.0 | Add 'Histogram' types to the list of cloneable types.
v15.6.0 | Added `X509Certificate` to the list of cloneable types.
v15.0.0 | Added `CryptoKey` to the list of cloneable types.
v14.5.0, v12.19.0 | Added `KeyObject` to the list of cloneable types.
v14.5.0, v12.19.0 | Added `FileHandle` to the list of transferable types.

- `value` `<any>`
- `transferList` `<Object>`[]

Sends a JavaScript value to the receiving side of this channel. `value` is transferred in a way which is compatible with the HTML structured clone algorithm.

In particular, the significant differences to `JSON` are:

- `value` may contain circular references.
- `value` may contain instances of builtin JS types such as `RegExp`s, `BigInt`s, `Map`s, `Set`s, etc.
- `value` may contain typed arrays, both using `ArrayBuffer`s and `SharedArrayBuffer`s.
- `value` may contain `WebAssembly.Module` instances.
- `value` may not contain native (C++-backed) objects other than:
  - `<CryptoKey>`s,
  - `<FileHandle>`s,
  - `<Histogram>`s,
  - `<KeyObject>`s,
  - `<MessagePort>`s,
  - `<net.BlockList>`s,
  - `<net.Server>`s (TCP only, when listed in `transferList`),
  - `<net.Socket>`s (TCP only, when listed in `transferList`),
  - `<net.SocketAddress>`es,
  - `<X509Certificate>`s.

    import { MessageChannel } from 'node:worker_threads';
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (message) => console.log(message));

    const circularData = {};
    circularData.foo = circularData;
    // Prints: { foo: [Circular] }
    port2.postMessage(circularData);
    const { MessageChannel } = require('node:worker_threads');
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (message) => console.log(message));

    const circularData = {};
    circularData.foo = circularData;
    // Prints: { foo: [Circular] }
    port2.postMessage(circularData);
    javascript

`transferList` may be a list of `<ArrayBuffer>`, `MessagePort`, `FileHandle`, `<net.Server>`, and `<net.Socket>` objects. After transferring, they are not usable on the sending side of the channel anymore (even if they are not contained in `value`).

Transferring a `<net.Server>` moves its listening socket — together with any pending connections in the accept queue — to the receiving thread's event loop. Transferring a `<net.Socket>` moves a single connection; the socket must be a freshly accepted or created TCP connection that has not yet started reading and has no buffered data, otherwise `postMessage()` throws `ERR_WORKER_HANDLE_NOT_TRANSFERABLE`. This makes it possible to accept connections on one thread and distribute them across a pool of worker threads. Only TCP handles are supported.

If `value` contains `<SharedArrayBuffer>` instances, those are accessible from either thread. They cannot be listed in `transferList`.

`value` may still contain `ArrayBuffer` instances that are not in `transferList`; in that case, the underlying memory is copied rather than moved.

    import { MessageChannel } from 'node:worker_threads';
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (message) => console.log(message));

    const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
    // This posts a copy of `uint8Array`:
    port2.postMessage(uint8Array);
    // This does not copy data, but renders `uint8Array` unusable:
    port2.postMessage(uint8Array, [ uint8Array.buffer ]);

    // The memory for the `sharedUint8Array` is accessible from both the
    // original and the copy received by `.on('message')`:
    const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
    port2.postMessage(sharedUint8Array);

    // This transfers a freshly created message port to the receiver.
    // This can be used, for example, to create communication channels between
    // multiple `Worker` threads that are children of the same parent thread.
    const otherChannel = new MessageChannel();
    port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);
    const { MessageChannel } = require('node:worker_threads');
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (message) => console.log(message));

    const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
    // This posts a copy of `uint8Array`:
    port2.postMessage(uint8Array);
    // This does not copy data, but renders `uint8Array` unusable:
    port2.postMessage(uint8Array, [ uint8Array.buffer ]);

    // The memory for the `sharedUint8Array` is accessible from both the
    // original and the copy received by `.on('message')`:
    const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
    port2.postMessage(sharedUint8Array);

    // This transfers a freshly created message port to the receiver.
    // This can be used, for example, to create communication channels between
    // multiple `Worker` threads that are children of the same parent thread.
    const otherChannel = new MessageChannel();
    port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);
    javascript

The message object is cloned immediately, and can be modified after posting without having side effects.

For more information on the serialization and deserialization mechanisms behind this API, see the serialization API of the `node:v8` module.

##### Considerations when transferring TypedArrays and Buffers#

All `<TypedArray>` | `<Buffer>` instances are views over an underlying `<ArrayBuffer>`. That is, it is the `ArrayBuffer` that actually stores the raw data while the `TypedArray` and `Buffer` objects provide a way of viewing and manipulating the data. It is possible and common for multiple views to be created over the same `ArrayBuffer` instance. Great care must be taken when using a transfer list to transfer an `ArrayBuffer` as doing so causes all `TypedArray` and `Buffer` instances that share that same `ArrayBuffer` to become unusable.

    const ab = new ArrayBuffer(10);

    const u1 = new Uint8Array(ab);
    const u2 = new Uint16Array(ab);

    console.log(u2.length);  // prints 5

    port.postMessage(u1, [u1.buffer]);

    console.log(u2.length);  // prints 0
    js

For `Buffer` instances, specifically, whether the underlying `ArrayBuffer` can be transferred or cloned depends entirely on how instances were created, which often cannot be reliably determined.

An `ArrayBuffer` can be marked with `markAsUntransferable()` to indicate that it should always be cloned and never transferred.

Depending on how a `Buffer` instance was created, it may or may not own its underlying `ArrayBuffer`. An `ArrayBuffer` must not be transferred unless it is known that the `Buffer` instance owns it. In particular, for `Buffer`s created from the internal `Buffer` pool (using, for instance `Buffer.from()` or `Buffer.allocUnsafe()`), transferring them is not possible and they are always cloned, which sends a copy of the entire `Buffer` pool. This behavior may come with unintended higher memory usage and possible security concerns.

See `Buffer.allocUnsafe()` for more details on `Buffer` pooling.

The `ArrayBuffer`s for `Buffer` instances created using `Buffer.alloc()` or `Buffer.allocUnsafeSlow()` can always be transferred but doing so renders all other existing views of those `ArrayBuffer`s unusable.

##### Considerations when cloning objects with prototypes, classes, and accessors#

Because object cloning uses the HTML structured clone algorithm, non-enumerable properties, property accessors, and object prototypes are not preserved. In particular, `<Buffer>` objects will be read as plain `<Uint8Array>`s on the receiving side, and instances of JavaScript classes will be cloned as plain JavaScript objects.

    const b = Symbol('b');

    class Foo {
      #a = 1;
      constructor() {
        this[b] = 2;
        this.c = 3;
      }

      get d() { return this.#a + 3; }
    }

    const { port1, port2 } = new MessageChannel();

    port1.onmessage = ({ data }) => console.log(data);

    port2.postMessage(new Foo());

    // Prints: { c: 3 }
    js

Some built-in objects cannot be cloned at all. For example, posting a `URL` object throws a `DataCloneError`:

    const { port1, port2 } = new MessageChannel();

    port2.postMessage(new URL('https://example.org'));
    // Throws DataCloneError: Cannot clone object of unsupported type.
    js

#### `port.hasRef()`#

Added in: v18.1.0, v16.17.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- Returns: `<boolean>`

If true, the `MessagePort` object will keep the Node.js event loop active.

#### `port.ref()`#

Added in: v10.5.0

Opposite of `unref()`. Calling `ref()` on a previously `unref()`ed port does not let the program exit if it's the only active handle left (the default behavior). If the port is `ref()`ed, calling `ref()` again has no effect.

If listeners are attached or removed using `.on('message')`, the port is `ref()`ed and `unref()`ed automatically depending on whether listeners for the event exist.

#### `port.start()`#

Added in: v10.5.0

Starts receiving messages on this `MessagePort`. When using this port as an event emitter, this is called automatically once `'message'` listeners are attached.

This method exists for parity with the Web `MessagePort` API. In Node.js, it is only useful for ignoring messages when no event listener is present. Node.js also diverges in its handling of `.onmessage`. Setting it automatically calls `.start()`, but unsetting it lets messages queue up until a new handler is set or the port is discarded.

#### `port.unref()`#

Added in: v10.5.0

Calling `unref()` on a port allows the thread to exit if this is the only active handle in the event system. If the port is already `unref()`ed calling `unref()` again has no effect.

If listeners are attached or removed using `.on('message')`, the port is `ref()`ed and `unref()`ed automatically depending on whether listeners for the event exist.

### Class: `Worker`#

Added in: v10.5.0

- Extends: `<EventEmitter>`

The `Worker` class represents an independent JavaScript execution thread. Most Node.js APIs are available inside of it.

Notable differences inside a Worker environment are:

- The `process.stdin`, `process.stdout`, and `process.stderr` streams may be redirected by the parent thread.
- The `require('node:worker_threads').isMainThread` property is set to `false`.
- The `require('node:worker_threads').parentPort` message port is available.
- `process.exit()` does not stop the whole program, just the single thread, and `process.abort()` is not available.
- `process.chdir()` and `process` methods that set group or user ids are not available.
- `process.env` is a copy of the parent thread's environment variables, unless otherwise specified. Changes to one copy are not visible in other threads, and are not visible to native add-ons (unless `worker.SHARE_ENV` is passed as the `env` option to the `Worker` constructor). On Windows, unlike the main thread, a copy of the environment variables operates in a case-sensitive manner.
- `process.title` cannot be modified.
- Signals are not delivered through `process.on('...')`.
- Execution may stop at any point as a result of `worker.terminate()` being invoked.
- IPC channels from parent processes are not accessible.
- The `trace_events` module is not supported.
- Native add-ons can only be loaded from multiple threads if they fulfill certain conditions.

Creating `Worker` instances inside of other `Worker`s is possible.

Like Web Workers and the `node:cluster` module, two-way communication can be achieved through inter-thread message passing. Internally, a `Worker` has a built-in pair of `MessagePort`s that are already associated with each other when the `Worker` is created. While the `MessagePort` object on the parent side is not directly exposed, its functionalities are exposed through `worker.postMessage()` and the `worker.on('message')` event on the `Worker` object for the parent thread.

To create custom messaging channels (which is encouraged over using the default global channel because it facilitates separation of concerns), users can create a `MessageChannel` object on either thread and pass one of the `MessagePort`s on that `MessageChannel` to the other thread through a pre-existing channel, such as the global one.

See `port.postMessage()` for more information on how messages are passed, and what kind of JavaScript values can be successfully transported through the thread barrier.

    import assert from 'node:assert';
    import {
      Worker, MessageChannel, MessagePort, isMainThread, parentPort,
    } from 'node:worker_threads';
    if (isMainThread) {
      const worker = new Worker(new URL(import.meta.url));
      const subChannel = new MessageChannel();
      worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
      subChannel.port2.on('message', (value) => {
        console.log('received:', value);
      });
    } else {
      parentPort.once('message', (value) => {
        assert(value.hereIsYourPort instanceof MessagePort);
        value.hereIsYourPort.postMessage('the worker is sending this');
        value.hereIsYourPort.close();
      });
    }
    const assert = require('node:assert');
    const {
      Worker, MessageChannel, MessagePort, isMainThread, parentPort,
    } = require('node:worker_threads');
    if (isMainThread) {
      const worker = new Worker(__filename);
      const subChannel = new MessageChannel();
      worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
      subChannel.port2.on('message', (value) => {
        console.log('received:', value);
      });
    } else {
      parentPort.once('message', (value) => {
        assert(value.hereIsYourPort instanceof MessagePort);
        value.hereIsYourPort.postMessage('the worker is sending this');
        value.hereIsYourPort.close();
      });
    }
    javascript

#### `new Worker(filename[, options])`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v19.8.0, v18.16.0 | Added support for a `name` option, which allows adding a name to worker title for debugging.
v14.9.0 | The `filename` parameter can be a WHATWG `URL` object using `data:` protocol.
v14.9.0 | The `trackUnmanagedFds` option was set to `true` by default.
v14.6.0, v12.19.0 | The `trackUnmanagedFds` option was introduced.
v13.13.0, v12.17.0 | The `transferList` option was introduced.
v13.12.0, v12.17.0 | The `filename` parameter can be a WHATWG `URL` object using `file:` protocol.
v13.4.0, v12.16.0 | The `argv` option was introduced.
v13.2.0, v12.16.0 | The `resourceLimits` option was introduced.

- `filename` `<string>` | `<URL>` The path to the Worker's main script or module. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with `./` or `../`, or a WHATWG `URL` object using `file:` or `data:` protocol. When using a `data:` URL, the data is interpreted based on MIME type using the ECMAScript module loader. If `options.eval` is `true`, this is a string containing JavaScript code rather than a path.
- `options` `<Object>`
  - `argv` `<any>`[] List of arguments which would be stringified and appended to `process.argv` in the worker. This is mostly similar to the `workerData` but the values are available on the global `process.argv` as if they were passed as CLI options to the script.
  - `env` `<Object>` If set, specifies the initial value of `process.env` inside the Worker thread. As a special value, `worker.SHARE_ENV` may be used to specify that the parent thread and the child thread should share their environment variables; in that case, changes to one thread's `process.env` object affect the other thread as well. Default: `process.env`.
  - `eval` `<boolean>` If `true` and the first argument is a `string`, interpret the first argument to the constructor as a script that is executed once the worker is online.
  - `execArgv` `<string>`[] List of node CLI options passed to the worker. V8 options (such as `--max-old-space-size`) and options that affect the process (such as `--title`) are not supported. If set, this is provided as `process.execArgv` inside the worker. By default, options are inherited from the parent thread.
  - `stdin` `<boolean>` If this is set to `true`, then `worker.stdin` provides a writable stream whose contents appear as `process.stdin` inside the Worker. By default, no data is provided.
  - `stdout` `<boolean>` If this is set to `true`, then `worker.stdout` is not automatically piped through to `process.stdout` in the parent.
  - `stderr` `<boolean>` If this is set to `true`, then `worker.stderr` is not automatically piped through to `process.stderr` in the parent.
  - `workerData` `<any>` Any JavaScript value that is cloned and made available as `require('node:worker_threads').workerData`. The cloning occurs as described in the HTML structured clone algorithm, and an error is thrown if the object cannot be cloned (e.g. because it contains `function`s).
  - `trackUnmanagedFds` `<boolean>` If this is set to `true`, then the Worker tracks raw file descriptors managed through `fs.open()` and `fs.close()`, and closes them when the Worker exits, similar to other resources like network sockets or file descriptors managed through the `FileHandle` API. This option is automatically inherited by all nested `Worker`s. Default: `true`.
  - `transferList` `<Object>`[] If one or more `MessagePort`-like objects are passed in `workerData`, a `transferList` is required for those items or `ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST` is thrown. See `port.postMessage()` for more information.
  - `resourceLimits` `<Object>` An optional set of resource limits for the new JS engine instance. Reaching these limits leads to termination of the `Worker` instance. These limits only affect the JS engine, and no external data, including no `ArrayBuffer`s. Even if these limits are set, the process may still abort if it encounters a global out-of-memory situation.
    - `maxOldGenerationSizeMb` `<number>` The maximum size of the main heap in MB. If the command-line argument `--max-old-space-size` is set, it overrides this setting.
    - `maxYoungGenerationSizeMb` `<number>` The maximum size of a heap space for recently created objects. If the command-line argument `--max-semi-space-size` is set, it overrides this setting.
    - `codeRangeSizeMb` `<number>` The size of a pre-allocated memory range used for generated code.
    - `stackSizeMb` `<number>` The default maximum stack size for the thread. Small values may lead to unusable Worker instances. Default: `4`.
  - `name` `<string>` An optional `name` to be replaced in the thread name and to the worker title for debugging/identification purposes, making the final title as `[worker ${id}] ${name}`. This parameter has a maximum allowed size, depending on the operating system. If the provided name exceeds the limit, it will be truncated
    - Maximum sizes:
      - Windows: 32,767 characters
      - macOS: 64 characters
      - Linux: 16 characters
      - NetBSD: limited to `PTHREAD_MAX_NAMELEN_NP`
      - FreeBSD and OpenBSD: limited to `MAXCOMLEN` Default: `'WorkerThread'`.

#### Event: `'error'`#

Added in: v10.5.0

- `err` `<any>`

The `'error'` event is emitted if the worker thread throws an uncaught exception. In that case, the worker is terminated.

#### Event: `'exit'`#

Added in: v10.5.0

- `exitCode` `<integer>`

The `'exit'` event is emitted once the worker has stopped. If the worker exited by calling `process.exit()`, the `exitCode` parameter is the passed exit code. If the worker was terminated, the `exitCode` parameter is `1`.

This is the final event emitted by any `Worker` instance.

#### Event: `'message'`#

Added in: v10.5.0

- `value` `<any>` The transmitted value

The `'message'` event is emitted when the worker thread has invoked `require('node:worker_threads').parentPort.postMessage()`. See the `port.on('message')` event for more details.

All messages sent from the worker thread are emitted before the `'exit'` event is emitted on the `Worker` object.

#### Event: `'messageerror'`#

Added in: v14.5.0, v12.19.0

- `error` `<Error>` An Error object

The `'messageerror'` event is emitted when deserializing a message failed.

#### Event: `'online'`#

Added in: v10.5.0

The `'online'` event is emitted when the worker thread has started executing JavaScript code.

#### `worker.cpuUsage([prev])`#

Added in: v24.6.0, v22.19.0

- Returns: `<Promise>`

This method returns a `Promise` that will resolve to an object identical to `process.threadCpuUsage()`, or reject with an `ERR_WORKER_NOT_RUNNING` error if the worker is no longer running. This methods allows the statistics to be observed from outside the actual thread.

#### `worker.getHeapSnapshot([options])`#

Added in: v13.9.0, v12.17.0
History

Version | Changes
--- | ---
v19.1.0 | Support options to configure the heap snapshot.

- `options` `<Object>`
  - `exposeInternals` `<boolean>` If true, expose internals in the heap snapshot. Default: `false`.
  - `exposeNumericValues` `<boolean>` If true, expose numeric values in artificial fields. Default: `false`.
- Returns: `<Promise>` A promise for a Readable Stream containing a V8 heap snapshot

Returns a readable stream for a V8 snapshot of the current state of the Worker. See `v8.getHeapSnapshot()` for more details.

If the Worker thread is no longer running, which may occur before the `'exit'` event is emitted, the returned `Promise` is rejected immediately with an `ERR_WORKER_NOT_RUNNING` error.

#### `worker.getHeapStatistics()`#

Added in: v24.0.0, v22.16.0

- Returns: `<Promise>`

This method returns a `Promise` that will resolve to an object identical to `v8.getHeapStatistics()`, or reject with an `ERR_WORKER_NOT_RUNNING` error if the worker is no longer running. This methods allows the statistics to be observed from outside the actual thread.

#### `worker.performance`#

Added in: v15.1.0, v14.17.0, v12.22.0

An object that can be used to query performance information from a worker instance.

##### `performance.eventLoopUtilization([utilization1[, utilization2]])`#

Added in: v15.1.0, v14.17.0, v12.22.0

- `utilization1` `<Object>` The result of a previous call to `eventLoopUtilization()`.
- `utilization2` `<Object>` The result of a previous call to `eventLoopUtilization()` prior to `utilization1`.
- Returns: `<Object>`
  - `idle` `<number>`
  - `active` `<number>`
  - `utilization` `<number>`

The same call as `perf_hooks` `eventLoopUtilization()`, except the values of the worker instance are returned.

One difference is that, unlike the main thread, bootstrapping within a worker is done within the event loop. So the event loop utilization is immediately available once the worker's script begins execution.

An `idle` time that does not increase does not indicate that the worker is stuck in bootstrap. The following examples shows how the worker's entire lifetime never accumulates any `idle` time, but is still be able to process messages.

    import { Worker, isMainThread, parentPort } from 'node:worker_threads';

    if (isMainThread) {
      const worker = new Worker(new URL(import.meta.url));
      setInterval(() => {
        worker.postMessage('hi');
        console.log(worker.performance.eventLoopUtilization());
      }, 100).unref();
    } else {
      parentPort.on('message', () => console.log('msg')).unref();
      (function r(n) {
        if (--n < 0) return;
        const t = Date.now();
        while (Date.now() - t < 300);
        setImmediate(r, n);
      })(10);
    }
    const { Worker, isMainThread, parentPort } = require('node:worker_threads');

    if (isMainThread) {
      const worker = new Worker(__filename);
      setInterval(() => {
        worker.postMessage('hi');
        console.log(worker.performance.eventLoopUtilization());
      }, 100).unref();
    } else {
      parentPort.on('message', () => console.log('msg')).unref();
      (function r(n) {
        if (--n < 0) return;
        const t = Date.now();
        while (Date.now() - t < 300);
        setImmediate(r, n);
      })(10);
    }
    javascript

The event loop utilization of a worker is available only after the `'online'` event emitted, and if called before this, or after the `'exit'` event, then all properties have the value of `0`.

#### `worker.postMessage(value[, transferList])`#

Added in: v10.5.0

- `value` `<any>`
- `transferList` `<Object>`[]

Send a message to the worker that is received via `require('node:worker_threads').parentPort.on('message')`. See `port.postMessage()` for more details.

#### `worker.ref()`#

Added in: v10.5.0

Opposite of `unref()`, calling `ref()` on a previously `unref()`ed worker does not let the program exit if it's the only active handle left (the default behavior). If the worker is `ref()`ed, calling `ref()` again has no effect.

#### `worker.resourceLimits`#

Added in: v13.2.0, v12.16.0

- Type: `<Object>`
  - `maxYoungGenerationSizeMb` `<number>`
  - `maxOldGenerationSizeMb` `<number>`
  - `codeRangeSizeMb` `<number>`
  - `stackSizeMb` `<number>`

Provides the set of JS engine resource constraints for this Worker thread. If the `resourceLimits` option was passed to the `Worker` constructor, this matches its values.

If the worker has stopped, the return value is an empty object.

#### `worker.startCpuProfile([options])`#

Added in: v24.8.0

- `options` `<Object>`
  - `sampleInterval` `<number>` Requested sampling interval in milliseconds. Default: `0`.
  - `maxBufferSize` `<integer>` Maximum number of samples to retain. Default: `4294967295`.
- Returns: `<Promise>`

Starting a CPU profile then return a Promise that fulfills with an error or an `CPUProfileHandle` object. This API supports `await using` syntax.

    const { Worker } = require('node:worker_threads');

    const worker = new Worker(`
      const { parentPort } = require('worker_threads');
      parentPort.on('message', () => {});
      `, { eval: true });

    worker.on('online', async () => {
      const handle = await worker.startCpuProfile({ sampleInterval: 1 });
      const profile = await handle.stop();
      console.log(profile);
      worker.terminate();
    });
    cjs

`await using` example.

    const { Worker } = require('node:worker_threads');

    const w = new Worker(`
      const { parentPort } = require('node:worker_threads');
      parentPort.on('message', () => {});
      `, { eval: true });

    w.on('online', async () => {
      // Stop profile automatically when return and profile will be discarded
      await using handle = await w.startCpuProfile();
    });
    cjs

#### `worker.startHeapProfile([options])`#

Added in: v24.9.0, v22.20.0

- `options` `<Object>`
  - `sampleInterval` `<number>` The average sampling interval in bytes. Default: `524288` (512 KiB).
  - `stackDepth` `<integer>` The maximum stack depth for samples. Default: `16`.
  - `forceGC` `<boolean>` Force garbage collection before taking the profile. Default: `false`.
  - `includeObjectsCollectedByMajorGC` `<boolean>` Include objects collected by major GC. Default: `false`.
  - `includeObjectsCollectedByMinorGC` `<boolean>` Include objects collected by minor GC. Default: `false`.
- Returns: `<Promise>`

Starting a Heap profile then return a Promise that fulfills with an error or an `HeapProfileHandle` object. This API supports `await using` syntax.

    const { Worker } = require('node:worker_threads');

    const worker = new Worker(`
      const { parentPort } = require('worker_threads');
      parentPort.on('message', () => {});
      `, { eval: true });

    worker.on('online', async () => {
      const handle = await worker.startHeapProfile();
      const profile = await handle.stop();
      console.log(profile);
      worker.terminate();
    });
    import { Worker } from 'node:worker_threads';

    const worker = new Worker(`
      const { parentPort } = require('node:worker_threads');
      parentPort.on('message', () => {});
      `, { eval: true });

    worker.on('online', async () => {
      const handle = await worker.startHeapProfile();
      const profile = await handle.stop();
      console.log(profile);
      worker.terminate();
    });
    javascript

`await using` example.

    const { Worker } = require('node:worker_threads');

    const w = new Worker(`
      const { parentPort } = require('node:worker_threads');
      parentPort.on('message', () => {});
      `, { eval: true });

    w.on('online', async () => {
      // Stop profile automatically when return and profile will be discarded
      await using handle = await w.startHeapProfile();
    });
    import { Worker } from 'node:worker_threads';

    const w = new Worker(`
      const { parentPort } = require('node:worker_threads');
      parentPort.on('message', () => {});
      `, { eval: true });

    w.on('online', async () => {
      // Stop profile automatically when return and profile will be discarded
      await using handle = await w.startHeapProfile();
    });
    javascript

#### `worker.stderr`#

Added in: v10.5.0

- Type: `<stream.Readable>`

This is a readable stream which contains data written to `process.stderr` inside the worker thread. If `stderr: true` was not passed to the `Worker` constructor, then data is piped to the parent thread's `process.stderr` stream.

#### `worker.stdin`#

Added in: v10.5.0

- Type: `<null>` | `<stream.Writable>`

If `stdin: true` was passed to the `Worker` constructor, this is a writable stream. The data written to this stream will be made available in the worker thread as `process.stdin`.

#### `worker.stdout`#

Added in: v10.5.0

- Type: `<stream.Readable>`

This is a readable stream which contains data written to `process.stdout` inside the worker thread. If `stdout: true` was not passed to the `Worker` constructor, then data is piped to the parent thread's `process.stdout` stream.

#### `worker.terminate()`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v12.5.0 | This function now returns a Promise. Passing a callback is deprecated, and was useless up to this version, as the Worker was actually terminated synchronously. Terminating is now a fully asynchronous operation.

- Returns: `<Promise>`

Stop all JavaScript execution in the worker thread as soon as possible. Returns a Promise for the exit code that is fulfilled when the `'exit'` event is emitted.

#### `worker.threadId`#

Added in: v10.5.0

- Type: `<integer>`

An integer identifier for the referenced thread. Inside the worker thread, it is available as `require('node:worker_threads').threadId`. This value is unique for each `Worker` instance inside a single process.

#### `worker.threadName`#

Added in: v24.6.0, v22.20.0

- `<string>` | `<null>`

A string identifier for the referenced thread or null if the thread is not running. Inside the worker thread, it is available as `require('node:worker_threads').threadName`.

#### `worker.unref()`#

Added in: v10.5.0

Calling `unref()` on a worker allows the thread to exit if this is the only active handle in the event system. If the worker is already `unref()`ed calling `unref()` again has no effect.

#### `worker[Symbol.asyncDispose]()`#

Added in: v24.2.0, v22.18.0

Calls `worker.terminate()` when the dispose scope is exited.

    async function example() {
      await using worker = new Worker('for (;;) {}', { eval: true });
      // Worker is automatically terminate when the scope is exited.
    }
    js

### Notes#

#### Synchronous blocking of stdio#

`Worker`s utilize message passing via `<MessagePort>` to implement interactions with `stdio`. This means that `stdio` output originating from a `Worker` can get blocked by synchronous code on the receiving end that is blocking the Node.js event loop.

    import {
      Worker,
      isMainThread,
    } from 'node:worker_threads';

    if (isMainThread) {
      new Worker(new URL(import.meta.url));
      for (let n = 0; n < 1e10; n++) {
        // Looping to simulate work.
      }
    } else {
      // This output will be blocked by the for loop in the main thread.
      console.log('foo');
    }
    const {
      Worker,
      isMainThread,
    } = require('node:worker_threads');

    if (isMainThread) {
      new Worker(__filename);
      for (let n = 0; n < 1e10; n++) {
        // Looping to simulate work.
      }
    } else {
      // This output will be blocked by the for loop in the main thread.
      console.log('foo');
    }
    javascript

#### Launching worker threads from preload scripts#

Take care when launching worker threads from preload scripts (scripts loaded and run using the `-r` command line flag). Unless the `execArgv` option is explicitly set, new Worker threads automatically inherit the command line flags from the running process and will preload the same preload scripts as the main thread. If the preload script unconditionally launches a worker thread, every thread spawned will spawn another until the application crashes.

## Zlib#

Source Code: lib/zlib.js

Stability: 2 - Stable

The `node:zlib` module provides compression functionality implemented using Gzip, Deflate/Inflate, Brotli, and Zstd.

To access it:

    import zlib from 'node:zlib';
    const zlib = require('node:zlib');
    javascript

Compression and decompression are built around the Node.js Streams API.

Compressing or decompressing a stream (such as a file) can be accomplished by piping the source stream through a `zlib` `Transform` stream into a destination stream:

    import {
      createReadStream,
      createWriteStream,
    } from 'node:fs';
    import process from 'node:process';
    import { createGzip } from 'node:zlib';
    import { pipeline } from 'node:stream';

    const gzip = createGzip();
    const source = createReadStream('input.txt');
    const destination = createWriteStream('input.txt.gz');

    pipeline(source, gzip, destination, (err) => {
      if (err) {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      }
    });
    const {
      createReadStream,
      createWriteStream,
    } = require('node:fs');
    const { createGzip } = require('node:zlib');
    const { pipeline } = require('node:stream');

    const gzip = createGzip();
    const source = createReadStream('input.txt');
    const destination = createWriteStream('input.txt.gz');

    pipeline(source, gzip, destination, (err) => {
      if (err) {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      }
    });
    javascript

Or, using the promise `pipeline` API:

    import {
      createReadStream,
      createWriteStream,
    } from 'node:fs';
    import { createGzip } from 'node:zlib';
    import { pipeline } from 'node:stream/promises';

    async function do_gzip(input, output) {
      const gzip = createGzip();
      const source = createReadStream(input);
      const destination = createWriteStream(output);
      await pipeline(source, gzip, destination);
    }

    await do_gzip('input.txt', 'input.txt.gz');
    const {
      createReadStream,
      createWriteStream,
    } = require('node:fs');
    const { createGzip } = require('node:zlib');
    const { pipeline } = require('node:stream/promises');

    async function do_gzip(input, output) {
      const gzip = createGzip();
      const source = createReadStream(input);
      const destination = createWriteStream(output);
      await pipeline(source, gzip, destination);
    }

    do_gzip('input.txt', 'input.txt.gz')
      .catch((err) => {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      });
    javascript

It is also possible to compress or decompress data in a single step:

    import process from 'node:process';
    import { Buffer } from 'node:buffer';
    import { deflate, unzip } from 'node:zlib';

    const input = '.................................';
    deflate(input, (err, buffer) => {
      if (err) {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      }
      console.log(buffer.toString('base64'));
    });

    const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
    unzip(buffer, (err, buffer) => {
      if (err) {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      }
      console.log(buffer.toString());
    });

    // Or, Promisified

    import { promisify } from 'node:util';
    const do_unzip = promisify(unzip);

    const unzippedBuffer = await do_unzip(buffer);
    console.log(unzippedBuffer.toString());
    const { deflate, unzip } = require('node:zlib');

    const input = '.................................';
    deflate(input, (err, buffer) => {
      if (err) {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      }
      console.log(buffer.toString('base64'));
    });

    const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
    unzip(buffer, (err, buffer) => {
      if (err) {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      }
      console.log(buffer.toString());
    });

    // Or, Promisified

    const { promisify } = require('node:util');
    const do_unzip = promisify(unzip);

    do_unzip(buffer)
      .then((buf) => console.log(buf.toString()))
      .catch((err) => {
        console.error('An error occurred:', err);
        process.exitCode = 1;
      });
    javascript

### Threadpool usage and performance considerations#

All `zlib` APIs, except those that are explicitly synchronous, use the Node.js internal threadpool. This can lead to surprising effects and performance limitations in some applications.

Creating and using a large number of zlib objects simultaneously can cause significant memory fragmentation.

    import zlib from 'node:zlib';
    import { Buffer } from 'node:buffer';

    const payload = Buffer.from('This is some data');

    // WARNING: DO NOT DO THIS!
    for (let i = 0; i < 30000; ++i) {
      zlib.deflate(payload, (err, buffer) => {});
    }
    const zlib = require('node:zlib');

    const payload = Buffer.from('This is some data');

    // WARNING: DO NOT DO THIS!
    for (let i = 0; i < 30000; ++i) {
      zlib.deflate(payload, (err, buffer) => {});
    }
    javascript

In the preceding example, 30,000 deflate instances are created concurrently. Because of how some operating systems handle memory allocation and deallocation, this may lead to significant memory fragmentation.

It is strongly recommended that the results of compression operations be cached to avoid duplication of effort.

### Compressing HTTP requests and responses#

The `node:zlib` module can be used to implement support for the `gzip`, `deflate`, `br`, and `zstd` content-encoding mechanisms defined by HTTP.

The HTTP `Accept-Encoding` header is used within an HTTP request to identify the compression encodings accepted by the client. The `Content-Encoding` header is used to identify the compression encodings actually applied to a message.

The examples given below are drastically simplified to show the basic concept. Using `zlib` encoding can be expensive, and the results ought to be cached. See Memory usage tuning for more information on the speed/memory/compression tradeoffs involved in `zlib` usage.

    // Client request example
    import fs from 'node:fs';
    import zlib from 'node:zlib';
    import http from 'node:http';
    import process from 'node:process';
    import { pipeline } from 'node:stream';

    const request = http.get({ host: 'example.com',
                               path: '/',
                               port: 80,
                               headers: { 'Accept-Encoding': 'br,gzip,deflate,zstd' } });
    request.on('response', (response) => {
      const output = fs.createWriteStream('example.com_index.html');

      const onError = (err) => {
        if (err) {
          console.error('An error occurred:', err);
          process.exitCode = 1;
        }
      };

      switch (response.headers['content-encoding']) {
        case 'br':
          pipeline(response, zlib.createBrotliDecompress(), output, onError);
          break;
        // Or, just use zlib.createUnzip() to handle both of the following cases:
        case 'gzip':
          pipeline(response, zlib.createGunzip(), output, onError);
          break;
        case 'deflate':
          pipeline(response, zlib.createInflate(), output, onError);
          break;
        case 'zstd':
          pipeline(response, zlib.createZstdDecompress(), output, onError);
          break;
        default:
          pipeline(response, output, onError);
          break;
      }
    });
    // Client request example
    const zlib = require('node:zlib');
    const http = require('node:http');
    const fs = require('node:fs');
    const { pipeline } = require('node:stream');

    const request = http.get({ host: 'example.com',
                               path: '/',
                               port: 80,
                               headers: { 'Accept-Encoding': 'br,gzip,deflate,zstd' } });
    request.on('response', (response) => {
      const output = fs.createWriteStream('example.com_index.html');

      const onError = (err) => {
        if (err) {
          console.error('An error occurred:', err);
          process.exitCode = 1;
        }
      };

      switch (response.headers['content-encoding']) {
        case 'br':
          pipeline(response, zlib.createBrotliDecompress(), output, onError);
          break;
        // Or, just use zlib.createUnzip() to handle both of the following cases:
        case 'gzip':
          pipeline(response, zlib.createGunzip(), output, onError);
          break;
        case 'deflate':
          pipeline(response, zlib.createInflate(), output, onError);
          break;
        case 'zstd':
          pipeline(response, zlib.createZstdDecompress(), output, onError);
          break;
        default:
          pipeline(response, output, onError);
          break;
      }
    });
    javascript

    // server example
    // Running a gzip operation on every request is quite expensive.
    // It would be much more efficient to cache the compressed buffer.
    import zlib from 'node:zlib';
    import http from 'node:http';
    import fs from 'node:fs';
    import { pipeline } from 'node:stream';

    http.createServer((request, response) => {
      const raw = fs.createReadStream('index.html');
      // Store both a compressed and an uncompressed version of the resource.
      response.setHeader('Vary', 'Accept-Encoding');
      const acceptEncoding = request.headers['accept-encoding'] || '';

      const onError = (err) => {
        if (err) {
          // If an error occurs, there's not much we can do because
          // the server has already sent the 200 response code and
          // some amount of data has already been sent to the client.
          // The best we can do is terminate the response immediately
          // and log the error.
          response.end();
          console.error('An error occurred:', err);
        }
      };

      // Note: This is not a conformant accept-encoding parser.
      // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
      if (/\bdeflate\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'deflate' });
        pipeline(raw, zlib.createDeflate(), response, onError);
      } else if (/\bgzip\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'gzip' });
        pipeline(raw, zlib.createGzip(), response, onError);
      } else if (/\bbr\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'br' });
        pipeline(raw, zlib.createBrotliCompress(), response, onError);
      } else if (/\bzstd\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'zstd' });
        pipeline(raw, zlib.createZstdCompress(), response, onError);
      } else {
        response.writeHead(200, {});
        pipeline(raw, response, onError);
      }
    }).listen(1337);
    // server example
    // Running a gzip operation on every request is quite expensive.
    // It would be much more efficient to cache the compressed buffer.
    const zlib = require('node:zlib');
    const http = require('node:http');
    const fs = require('node:fs');
    const { pipeline } = require('node:stream');

    http.createServer((request, response) => {
      const raw = fs.createReadStream('index.html');
      // Store both a compressed and an uncompressed version of the resource.
      response.setHeader('Vary', 'Accept-Encoding');
      const acceptEncoding = request.headers['accept-encoding'] || '';

      const onError = (err) => {
        if (err) {
          // If an error occurs, there's not much we can do because
          // the server has already sent the 200 response code and
          // some amount of data has already been sent to the client.
          // The best we can do is terminate the response immediately
          // and log the error.
          response.end();
          console.error('An error occurred:', err);
        }
      };

      // Note: This is not a conformant accept-encoding parser.
      // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
      if (/\bdeflate\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'deflate' });
        pipeline(raw, zlib.createDeflate(), response, onError);
      } else if (/\bgzip\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'gzip' });
        pipeline(raw, zlib.createGzip(), response, onError);
      } else if (/\bbr\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'br' });
        pipeline(raw, zlib.createBrotliCompress(), response, onError);
      } else if (/\bzstd\b/.test(acceptEncoding)) {
        response.writeHead(200, { 'Content-Encoding': 'zstd' });
        pipeline(raw, zlib.createZstdCompress(), response, onError);
      } else {
        response.writeHead(200, {});
        pipeline(raw, response, onError);
      }
    }).listen(1337);
    javascript

By default, the `zlib` methods will throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to decompress the last chunk of input data:

    // This is a truncated version of the buffer from the above examples
    const buffer = Buffer.from('eJzT0yMA', 'base64');

    zlib.unzip(
      buffer,
      // For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.
      // For Zstd, the equivalent is zlib.constants.ZSTD_e_flush.
      { finishFlush: zlib.constants.Z_SYNC_FLUSH },
      (err, buffer) => {
        if (err) {
          console.error('An error occurred:', err);
          process.exitCode = 1;
        }
        console.log(buffer.toString());
      });
    js

This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.

### Memory usage tuning#

#### For zlib-based streams#

From `zlib/zconf.h`, modified for Node.js usage:

The memory requirements for deflate are (in bytes):

    (1 << (windowBits + 2)) + (1 << (memLevel + 9));
    js

That is: 128K for `windowBits` = 15 + 128K for `memLevel` = 8 (default values) plus a few kilobytes for small objects.

For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:

    const options = { windowBits: 14, memLevel: 7 };
    js

This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes) `1 << windowBits`. That is, 32K for `windowBits` = 15 (default value) plus a few kilobytes for small objects.

This is in addition to a single internal output slab buffer of size `chunkSize`, which defaults to 16K.

The speed of `zlib` compression is affected most dramatically by the `level` setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster.

In general, greater memory usage options will mean that Node.js has to make fewer calls to `zlib` because it will be able to process more data on each `write` operation. So, this is another factor that affects the speed, at the cost of memory usage.

#### For Brotli-based streams#

There are equivalents to the zlib options for Brotli-based streams, although these options have different ranges than the zlib ones:

- zlib's `level` option matches Brotli's `BROTLI_PARAM_QUALITY` option.
- zlib's `windowBits` option matches Brotli's `BROTLI_PARAM_LGWIN` option.

See below for more details on Brotli-specific options.

#### For Zstd-based streams#

Stability: 1 - Experimental

There are equivalents to the zlib options for Zstd-based streams, although these options have different ranges than the zlib ones:

- zlib's `level` option matches Zstd's `ZSTD_c_compressionLevel` option.
- zlib's `windowBits` option matches Zstd's `ZSTD_c_windowLog` option.

See below for more details on Zstd-specific options.

### Flushing#

Calling `.flush()` on a compression stream will make `zlib` return as much output as currently possible. This may come at the cost of degraded compression quality, but can be useful when data needs to be available as soon as possible.

In the following example, `flush()` is used to write a compressed partial HTTP response to the client:

    import zlib from 'node:zlib';
    import http from 'node:http';
    import { pipeline } from 'node:stream';

    http.createServer((request, response) => {
      // For the sake of simplicity, the Accept-Encoding checks are omitted.
      response.writeHead(200, { 'content-encoding': 'gzip' });
      const output = zlib.createGzip();
      let i;

      pipeline(output, response, (err) => {
        if (err) {
          // If an error occurs, there's not much we can do because
          // the server has already sent the 200 response code and
          // some amount of data has already been sent to the client.
          // The best we can do is terminate the response immediately
          // and log the error.
          clearInterval(i);
          response.end();
          console.error('An error occurred:', err);
        }
      });

      i = setInterval(() => {
        output.write(`The current time is ${Date()}\n`, () => {
          // The data has been passed to zlib, but the compression algorithm may
          // have decided to buffer the data for more efficient compression.
          // Calling .flush() will make the data available as soon as the client
          // is ready to receive it.
          output.flush();
        });
      }, 1000);
    }).listen(1337);
    const zlib = require('node:zlib');
    const http = require('node:http');
    const { pipeline } = require('node:stream');

    http.createServer((request, response) => {
      // For the sake of simplicity, the Accept-Encoding checks are omitted.
      response.writeHead(200, { 'content-encoding': 'gzip' });
      const output = zlib.createGzip();
      let i;

      pipeline(output, response, (err) => {
        if (err) {
          // If an error occurs, there's not much we can do because
          // the server has already sent the 200 response code and
          // some amount of data has already been sent to the client.
          // The best we can do is terminate the response immediately
          // and log the error.
          clearInterval(i);
          response.end();
          console.error('An error occurred:', err);
        }
      });

      i = setInterval(() => {
        output.write(`The current time is ${Date()}\n`, () => {
          // The data has been passed to zlib, but the compression algorithm may
          // have decided to buffer the data for more efficient compression.
          // Calling .flush() will make the data available as soon as the client
          // is ready to receive it.
          output.flush();
        });
      }, 1000);
    }).listen(1337);
    javascript

### Constants#

Added in: v0.5.8

#### zlib constants#

All of the constants defined in `zlib.h` are also defined on `require('node:zlib').constants`. In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation.

Previously, the constants were available directly from `require('node:zlib')`, for instance `zlib.Z_NO_FLUSH`. Accessing the constants directly from the module is currently still possible but is deprecated.

Allowed flush values.

- `zlib.constants.Z_NO_FLUSH`
- `zlib.constants.Z_PARTIAL_FLUSH`
- `zlib.constants.Z_SYNC_FLUSH`
- `zlib.constants.Z_FULL_FLUSH`
- `zlib.constants.Z_FINISH`
- `zlib.constants.Z_BLOCK`

Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.

- `zlib.constants.Z_OK`
- `zlib.constants.Z_STREAM_END`
- `zlib.constants.Z_NEED_DICT`
- `zlib.constants.Z_ERRNO`
- `zlib.constants.Z_STREAM_ERROR`
- `zlib.constants.Z_DATA_ERROR`
- `zlib.constants.Z_MEM_ERROR`
- `zlib.constants.Z_BUF_ERROR`
- `zlib.constants.Z_VERSION_ERROR`

Compression levels.

- `zlib.constants.Z_NO_COMPRESSION`
- `zlib.constants.Z_BEST_SPEED`
- `zlib.constants.Z_BEST_COMPRESSION`
- `zlib.constants.Z_DEFAULT_COMPRESSION`

Compression strategy.

- `zlib.constants.Z_FILTERED`
- `zlib.constants.Z_HUFFMAN_ONLY`
- `zlib.constants.Z_RLE`
- `zlib.constants.Z_FIXED`
- `zlib.constants.Z_DEFAULT_STRATEGY`

#### Brotli constants#

Added in: v11.7.0, v10.16.0

There are several options and other constants available for Brotli-based streams:

##### Flush operations#

The following values are valid flush operations for Brotli-based streams:

- `zlib.constants.BROTLI_OPERATION_PROCESS` (default for all operations)
- `zlib.constants.BROTLI_OPERATION_FLUSH` (default when calling `.flush()`)
- `zlib.constants.BROTLI_OPERATION_FINISH` (default for the last chunk)
- `zlib.constants.BROTLI_OPERATION_EMIT_METADATA`
  - This particular operation may be hard to use in a Node.js context, as the streaming layer makes it hard to know which data will end up in this frame. Also, there is currently no way to consume this data through the Node.js API.

##### Compressor options#

There are several options that can be set on Brotli encoders, affecting compression efficiency and speed. Both the keys and the values can be accessed as properties of the `zlib.constants` object.

The most important options are:

- `BROTLI_PARAM_MODE`
  - `BROTLI_MODE_GENERIC` (default)
  - `BROTLI_MODE_TEXT`, adjusted for UTF-8 text
  - `BROTLI_MODE_FONT`, adjusted for WOFF 2.0 fonts
- `BROTLI_PARAM_QUALITY`
  - Ranges from `BROTLI_MIN_QUALITY` to `BROTLI_MAX_QUALITY`, with a default of `BROTLI_DEFAULT_QUALITY`.
- `BROTLI_PARAM_SIZE_HINT`
  - Integer value representing the expected input size; defaults to `0` for an unknown input size.

The following flags can be set for advanced control over the compression algorithm and memory usage tuning:

- `BROTLI_PARAM_LGWIN`
  - Ranges from `BROTLI_MIN_WINDOW_BITS` to `BROTLI_MAX_WINDOW_BITS`, with a default of `BROTLI_DEFAULT_WINDOW`, or up to `BROTLI_LARGE_MAX_WINDOW_BITS` if the `BROTLI_PARAM_LARGE_WINDOW` flag is set.
- `BROTLI_PARAM_LGBLOCK`
  - Ranges from `BROTLI_MIN_INPUT_BLOCK_BITS` to `BROTLI_MAX_INPUT_BLOCK_BITS`.
- `BROTLI_PARAM_DISABLE_LITERAL_CONTEXT_MODELING`
  - Boolean flag that decreases compression ratio in favour of decompression speed.
- `BROTLI_PARAM_LARGE_WINDOW`
  - Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).
- `BROTLI_PARAM_NPOSTFIX`
  - Ranges from `0` to `BROTLI_MAX_NPOSTFIX`.
- `BROTLI_PARAM_NDIRECT`
  - Ranges from `0` to `15 << NPOSTFIX` in steps of `1 << NPOSTFIX`.

##### Decompressor options#

These advanced options are available for controlling decompression:

- `BROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION`
  - Boolean flag that affects internal memory allocation patterns.
- `BROTLI_DECODER_PARAM_LARGE_WINDOW`
  - Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).

#### Zstd constants#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

There are several options and other constants available for Zstd-based streams:

##### Flush operations#

The following values are valid flush operations for Zstd-based streams:

- `zlib.constants.ZSTD_e_continue` (default for all operations)
- `zlib.constants.ZSTD_e_flush` (default when calling `.flush()`)
- `zlib.constants.ZSTD_e_end` (default for the last chunk)

##### Compressor options#

There are several options that can be set on Zstd encoders, affecting compression efficiency and speed. Both the keys and the values can be accessed as properties of the `zlib.constants` object.

The most important options are:

- `ZSTD_c_compressionLevel`
  - Set compression parameters according to pre-defined cLevel table. Default level is ZSTD_CLEVEL_DEFAULT==3.
- `ZSTD_c_strategy`
  - Select the compression strategy.
  - Possible values are listed in the strategy options section below.

##### Strategy options#

The following constants can be used as values for the `ZSTD_c_strategy` parameter:

- `zlib.constants.ZSTD_fast`
- `zlib.constants.ZSTD_dfast`
- `zlib.constants.ZSTD_greedy`
- `zlib.constants.ZSTD_lazy`
- `zlib.constants.ZSTD_lazy2`
- `zlib.constants.ZSTD_btlazy2`
- `zlib.constants.ZSTD_btopt`
- `zlib.constants.ZSTD_btultra`
- `zlib.constants.ZSTD_btultra2`

Example:

    const stream = zlib.createZstdCompress({
      params: {
        [zlib.constants.ZSTD_c_strategy]: zlib.constants.ZSTD_btultra,
      },
    });
    js

##### Pledged Source Size#

It's possible to specify the expected total size of the uncompressed input via `opts.pledgedSrcSize`, which must be a non-negative safe integer. If the size doesn't match at the end of the input, compression will fail with the code `ZSTD_error_srcSize_wrong`.

##### Decompressor options#

These advanced options are available for controlling decompression:

- `ZSTD_d_windowLogMax`
  - Select a size limit (in power of 2) beyond which the streaming API will refuse to allocate memory buffer in order to protect the host from unreasonable memory requirements.

### Class: `Options`#

Added in: v0.11.1
History

Version | Changes
--- | ---
v26.5.0 | The `rejectGarbageAfterEnd` option was added.
v14.5.0, v12.19.0 | The `maxOutputLength` option is supported now.
v9.4.0 | The `dictionary` option can be an `ArrayBuffer`.
v8.0.0 | The `dictionary` option can be an `Uint8Array` now.
v5.11.0 | The `finishFlush` option is supported now.

Each zlib-based class takes an `options` object. No options are required.

Some options are only relevant when compressing and are ignored by the decompression classes.

- `flush` `<integer>` Default: `zlib.constants.Z_NO_FLUSH`
- `finishFlush` `<integer>` Default: `zlib.constants.Z_FINISH`
- `chunkSize` `<integer>` Default: `16 * 1024`
- `windowBits` `<integer>`
- `level` `<integer>` (compression only)
- `memLevel` `<integer>` (compression only)
- `strategy` `<integer>` (compression only)
- `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` (deflate/inflate only, empty dictionary by default)
- `info` `<boolean>` (If `true`, returns an object with `buffer` and `engine`.)
- `maxOutputLength` `<integer>` Limits output size when using convenience methods. Default: `buffer.kMaxLength`
- `rejectGarbageAfterEnd` `<boolean>` If `true`, decompression fails when trailing input is detected after the end of the compressed stream. This includes unreadable bytes and, when decompressing gzip, additional gzip members following the first member. Default: `false`

See the `deflateInit2` and `inflateInit2` documentation for more information.

### Class: `BrotliOptions`#

Added in: v11.7.0
History

Version | Changes
--- | ---
v26.5.0 | The `rejectGarbageAfterEnd` option was added.
v14.5.0, v12.19.0 | The `maxOutputLength` option is supported now.

Each Brotli-based class takes an `options` object. All options are optional.

- `flush` `<integer>` Default: `zlib.constants.BROTLI_OPERATION_PROCESS`
- `finishFlush` `<integer>` Default: `zlib.constants.BROTLI_OPERATION_FINISH`
- `chunkSize` `<integer>` Default: `16 * 1024`
- `params` `<Object>` Key-value object containing indexed Brotli parameters.
- `maxOutputLength` `<integer>` Limits output size when using convenience methods. Default: `buffer.kMaxLength`
- `info` `<boolean>` If `true`, returns an object with `buffer` and `engine`. Default: `false`
- `rejectGarbageAfterEnd` `<boolean>` If `true`, decompression fails when input remains after the first complete compressed stream. Default: `false`

For example:

    const stream = zlib.createBrotliCompress({
      chunkSize: 32 * 1024,
      params: {
        [zlib.constants.BROTLI_PARAM_MODE]: zlib.constants.BROTLI_MODE_TEXT,
        [zlib.constants.BROTLI_PARAM_QUALITY]: 4,
        [zlib.constants.BROTLI_PARAM_SIZE_HINT]: fs.statSync(inputFile).size,
      },
    });
    js

### Class: `zlib.BrotliCompress`#

Added in: v11.7.0, v10.16.0

- Extends: `ZlibBase`

Compress data using the Brotli algorithm.

### Class: `zlib.BrotliDecompress`#

Added in: v11.7.0, v10.16.0

- Extends: `ZlibBase`

Decompress data using the Brotli algorithm.

### Class: `zlib.Deflate`#

Added in: v0.5.8

- Extends: `ZlibBase`

Compress data using deflate.

### Class: `zlib.DeflateRaw`#

Added in: v0.5.8

- Extends: `ZlibBase`

Compress data using deflate, and do not append a `zlib` header.

### Class: `zlib.Gunzip`#

Added in: v0.5.8
History

Version | Changes
--- | ---
v6.0.0 | Trailing garbage at the end of the input stream will now result in an `'error'` event.
v5.9.0 | Multiple concatenated gzip file members are supported now.
v5.0.0 | A truncated input stream will now result in an `'error'` event.

- Extends: `ZlibBase`

Decompress a gzip stream.

### Class: `zlib.Gzip`#

Added in: v0.5.8

- Extends: `ZlibBase`

Compress data using gzip.

### Class: `zlib.Inflate`#

Added in: v0.5.8
History

Version | Changes
--- | ---
v5.0.0 | A truncated input stream will now result in an `'error'` event.

- Extends: `ZlibBase`

Decompress a deflate stream.

### Class: `zlib.InflateRaw`#

Added in: v0.5.8
History

Version | Changes
--- | ---
v6.8.0 | Custom dictionaries are now supported by `InflateRaw`.
v5.0.0 | A truncated input stream will now result in an `'error'` event.

- Extends: `ZlibBase`

Decompress a raw deflate stream.

### Class: `zlib.Unzip`#

Added in: v0.5.8

- Extends: `ZlibBase`

Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.

### Class: `zlib.ZlibBase`#

Added in: v0.5.8
History

Version | Changes
--- | ---
v11.7.0, v10.16.0 | This class was renamed from `Zlib` to `ZlibBase`.

- Extends: `stream.Transform`

Not exported by the `node:zlib` module. It is documented here because it is the base class of the compressor/decompressor classes.

This class inherits from `stream.Transform`, allowing `node:zlib` objects to be used in pipes and similar stream operations.

#### `zlib.bytesWritten`#

Added in: v10.0.0

- Type: `<number>`

The `zlib.bytesWritten` property specifies the number of bytes written to the engine, before the bytes are processed (compressed or decompressed, as appropriate for the derived class).

#### `zlib.close([callback])`#

Added in: v0.9.4

- `callback` `<Function>`

Close the underlying handle.

#### `zlib.flush([kind, ]callback)`#

Added in: v0.5.8

- `kind` Default: `zlib.constants.Z_FULL_FLUSH` for zlib-based streams, `zlib.constants.BROTLI_OPERATION_FLUSH` for Brotli-based streams.
- `callback` `<Function>`

Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.

Calling this only flushes data from the internal `zlib` state, and does not perform flushing of any kind on the streams level. Rather, it behaves like a normal call to `.write()`, i.e. it will be queued up behind other pending writes and will only produce output when data is being read from the stream.

#### `zlib.params(level, strategy, callback)`#

Added in: v0.11.4

- `level` `<integer>`
- `strategy` `<integer>`
- `callback` `<Function>`

This function is only available for zlib-based streams, i.e. not Brotli.

Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.

#### `zlib.reset()`#

Added in: v0.7.0

Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.

### Class: `ZstdOptions`#

Added in: v23.8.0, v22.15.0
History

Version | Changes
--- | ---
v26.7.0 | The `dictionary` option can be a `TypedArray`, `DataView`, or `ArrayBuffer`.
v26.5.0 | The `rejectGarbageAfterEnd` option was added.

Stability: 1 - Experimental

Each Zstd-based class takes an `options` object. All options are optional.

- `flush` `<integer>` Default: `zlib.constants.ZSTD_e_continue`
- `finishFlush` `<integer>` Default: `zlib.constants.ZSTD_e_end`
- `chunkSize` `<integer>` Default: `16 * 1024`
- `params` `<Object>` Key-value object containing indexed Zstd parameters.
- `maxOutputLength` `<integer>` Limits output size when using convenience methods. Default: `buffer.kMaxLength`
- `info` `<boolean>` If `true`, returns an object with `buffer` and `engine`. Default: `false`
- `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` Optional dictionary used to improve compression efficiency when compressing or decompressing data that shares common patterns with the dictionary.
- `rejectGarbageAfterEnd` `<boolean>` If `true`, decompression fails when input remains after the first complete compressed stream. Default: `false`

For example:

    const stream = zlib.createZstdCompress({
      chunkSize: 32 * 1024,
      params: {
        [zlib.constants.ZSTD_c_compressionLevel]: 10,
        [zlib.constants.ZSTD_c_checksumFlag]: 1,
      },
    });
    js

### Class: `zlib.ZstdCompress`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

Compress data using the Zstd algorithm.

### Class: `zlib.ZstdDecompress`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

Decompress data using the Zstd algorithm.

### `zlib.constants`#

Added in: v7.0.0

Provides an object enumerating Zlib-related constants.

### `zlib.crc32(data[, value])`#

Added in: v22.2.0, v20.15.0

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` When `data` is a string, it will be encoded as UTF-8 before being used for computation.
- `value` `<integer>` An optional starting value. It must be a 32-bit unsigned integer. Default: `0`
- Returns: `<integer>` A 32-bit unsigned integer containing the checksum.

Computes a 32-bit Cyclic Redundancy Check checksum of `data`. If `value` is specified, it is used as the starting value of the checksum, otherwise, 0 is used as the starting value.

The CRC algorithm is designed to compute checksums and to detect error in data transmission. It's not suitable for cryptographic authentication.

To be consistent with other APIs, if the `data` is a string, it will be encoded with UTF-8 before being used for computation. If users only use Node.js to compute and match the checksums, this works well with other APIs that uses the UTF-8 encoding by default.

Some third-party JavaScript libraries compute the checksum on a string based on `str.charCodeAt()` so that it can be run in browsers. If users want to match the checksum computed with this kind of library in the browser, it's better to use the same library in Node.js if it also runs in Node.js. If users have to use `zlib.crc32()` to match the checksum produced by such a third-party library:

1. If the library accepts `Uint8Array` as input, use `TextEncoder` in the browser to encode the string into a `Uint8Array` with UTF-8 encoding, and compute the checksum based on the UTF-8 encoded string in the browser.
2. If the library only takes a string and compute the data based on `str.charCodeAt()`, on the Node.js side, convert the string into a buffer using `Buffer.from(str, 'utf16le')`.

    import zlib from 'node:zlib';
    import { Buffer } from 'node:buffer';

    let crc = zlib.crc32('hello');  // 907060870
    crc = zlib.crc32('world', crc);  // 4192936109

    crc = zlib.crc32(Buffer.from('hello', 'utf16le'));  // 1427272415
    crc = zlib.crc32(Buffer.from('world', 'utf16le'), crc);  // 4150509955
    const zlib = require('node:zlib');
    const { Buffer } = require('node:buffer');

    let crc = zlib.crc32('hello');  // 907060870
    crc = zlib.crc32('world', crc);  // 4192936109

    crc = zlib.crc32(Buffer.from('hello', 'utf16le'));  // 1427272415
    crc = zlib.crc32(Buffer.from('world', 'utf16le'), crc);  // 4150509955
    javascript

### `zlib.createBrotliCompress([options])`#

Added in: v11.7.0, v10.16.0

- `options` `<brotli options>`

Creates and returns a new `BrotliCompress` object.

### `zlib.createBrotliDecompress([options])`#

Added in: v11.7.0, v10.16.0

- `options` `<brotli options>`

Creates and returns a new `BrotliDecompress` object.

### `zlib.createDeflate([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `Deflate` object.

### `zlib.createDeflateRaw([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `DeflateRaw` object.

An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when `windowBits` is set to 8 for raw deflate streams. zlib would automatically set `windowBits` to 9 if was initially set to 8. Newer versions of zlib will throw an exception, so Node.js restored the original behavior of upgrading a value of 8 to 9, since passing `windowBits = 9` to zlib actually results in a compressed stream that effectively uses an 8-bit window only.

### `zlib.createGunzip([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `Gunzip` object.

### `zlib.createGzip([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `Gzip` object. See example.

### `zlib.createInflate([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `Inflate` object.

### `zlib.createInflateRaw([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `InflateRaw` object.

### `zlib.createUnzip([options])`#

Added in: v0.5.8

- `options` `<zlib options>`

Creates and returns a new `Unzip` object.

### `zlib.createZstdCompress([options])`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

- `options` `<zstd options>`

Creates and returns a new `ZstdCompress` object.

### `zlib.createZstdDecompress([options])`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

- `options` `<zstd options>`

Creates and returns a new `ZstdDecompress` object.

### Convenience methods#

All of these take a `<Buffer>`, `<TypedArray>`, `<DataView>`, `<ArrayBuffer>`, or string as the first argument, an optional second argument to supply options to the `zlib` classes and will call the supplied callback with `callback(error, result)`.

Every method has a `*Sync` counterpart, which accept the same arguments, but without a callback.

#### `zlib.brotliCompress(buffer[, options], callback)`#

Added in: v11.7.0, v10.16.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<brotli options>`
- `callback` `<Function>`

#### `zlib.brotliCompressSync(buffer[, options])`#

Added in: v11.7.0, v10.16.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<brotli options>`

Compress a chunk of data with `BrotliCompress`.

#### `zlib.brotliDecompress(buffer[, options], callback)`#

Added in: v11.7.0, v10.16.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<brotli options>`
- `callback` `<Function>`

#### `zlib.brotliDecompressSync(buffer[, options])`#

Added in: v11.7.0, v10.16.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<brotli options>`

Decompress a chunk of data with `BrotliDecompress`.

#### `zlib.deflate(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.deflateSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Compress a chunk of data with `Deflate`.

#### `zlib.deflateRaw(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.deflateRawSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Compress a chunk of data with `DeflateRaw`.

#### `zlib.gunzip(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.gunzipSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Decompress a chunk of data with `Gunzip`.

#### `zlib.gzip(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.gzipSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Compress a chunk of data with `Gzip`.

#### `zlib.inflate(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.inflateSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Decompress a chunk of data with `Inflate`.

#### `zlib.inflateRaw(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.inflateRawSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Decompress a chunk of data with `InflateRaw`.

#### `zlib.unzip(buffer[, options], callback)`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`
- `callback` `<Function>`

#### `zlib.unzipSync(buffer[, options])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v9.4.0 | The `buffer` parameter can be an `ArrayBuffer`.
v8.0.0 | The `buffer` parameter can be any `TypedArray` or `DataView`.
v8.0.0 | The `buffer` parameter can be an `Uint8Array` now.

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zlib options>`

Decompress a chunk of data with `Unzip`.

#### `zlib.zstdCompress(buffer[, options], callback)`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zstd options>`
- `callback` `<Function>`

#### `zlib.zstdCompressSync(buffer[, options])`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zstd options>`

Compress a chunk of data with `ZstdCompress`.

#### `zlib.zstdDecompress(buffer[, options], callback)`#

Added in: v23.8.0, v22.15.0

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zstd options>`
- `callback` `<Function>`

#### `zlib.zstdDecompressSync(buffer[, options])`#

Added in: v23.8.0, v22.15.0

Stability: 1 - Experimental

- `buffer` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` | `<string>`
- `options` `<zstd options>`

Decompress a chunk of data with `ZstdDecompress`.

### Iterable Compression#

Added in: v25.9.0

Stability: 1 - Experimental

The `node:zlib/iter` module provides compression and decompression transforms for use with the `node:stream/iter` iterable streams API.

This module is available only when the `--experimental-stream-iter` CLI flag is enabled.

Each algorithm has both an async variant (stateful async generator, for use with `pull()` and `pipeTo()`) and a sync variant (stateful sync generator, for use with `pullSync()` and `pipeToSync()`).

The async transforms run compression on the libuv threadpool, overlapping I/O with JavaScript execution. The sync transforms run compression directly on the main thread.

> Note: The defaults for these transforms are tuned for streaming throughput, and differ from the defaults in `node:zlib`. In particular, gzip/deflate default to level 4 (not 6) and memLevel 9 (not 8), and Brotli defaults to quality 6 (not 11). These choices match common HTTP server configurations and provide significantly faster compression with only a small reduction in compression ratio. All defaults can be overridden via options.

    import { from, pull, bytes, text } from 'node:stream/iter';
    import { compressGzip, decompressGzip } from 'node:zlib/iter';

    // Async round-trip
    const compressed = await bytes(pull(from('hello'), compressGzip()));
    const original = await text(pull(from(compressed), decompressGzip()));
    console.log(original); // 'hello'
    const { from, pull, bytes, text } = require('node:stream/iter');
    const { compressGzip, decompressGzip } = require('node:zlib/iter');

    async function run() {
      const compressed = await bytes(pull(from('hello'), compressGzip()));
      const original = await text(pull(from(compressed), decompressGzip()));
      console.log(original); // 'hello'
    }

    run().catch(console.error);
    javascript

    import { fromSync, pullSync, textSync } from 'node:stream/iter';
    import { compressGzipSync, decompressGzipSync } from 'node:zlib/iter';

    // Sync round-trip
    const compressed = pullSync(fromSync('hello'), compressGzipSync());
    const original = textSync(pullSync(compressed, decompressGzipSync()));
    console.log(original); // 'hello'
    const { fromSync, pullSync, textSync } = require('node:stream/iter');
    const { compressGzipSync, decompressGzipSync } = require('node:zlib/iter');

    const compressed = pullSync(fromSync('hello'), compressGzipSync());
    const original = textSync(pullSync(compressed, decompressGzipSync()));
    console.log(original); // 'hello'
    javascript

#### `compressBrotli([options])`#

#### `compressBrotliSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `params` `<Object>` Key-value object where keys and values are `zlib.constants` entries. The most important compressor parameters are:
    - `BROTLI_PARAM_MODE` -- `BROTLI_MODE_GENERIC` (default), `BROTLI_MODE_TEXT`, or `BROTLI_MODE_FONT`.
    - `BROTLI_PARAM_QUALITY` -- ranges from `BROTLI_MIN_QUALITY` to `BROTLI_MAX_QUALITY`. Default: `6` (not `BROTLI_DEFAULT_QUALITY` which is 11). Quality 6 is appropriate for streaming; quality 11 is intended for offline/build-time compression.
    - `BROTLI_PARAM_SIZE_HINT` -- expected input size. Default: `0` (unknown).
    - `BROTLI_PARAM_LGWIN` -- window size (log2). Default: `20` (1 MB). The Brotli library default is 22 (4 MB); the reduced default saves memory without significant compression impact for streaming workloads.
    - `BROTLI_PARAM_LGBLOCK` -- input block size (log2). See the Brotli compressor options in the zlib documentation for the full list.
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a Brotli compression transform. Output is compatible with `zlib.brotliDecompress()` and `decompressBrotli()`/`decompressBrotliSync()`.

#### `compressDeflate([options])`#

#### `compressDeflateSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `level` `<number>` Compression level (`0`-`9`). Default: `4`.
  - `windowBits` `<number>` Default: `Z_DEFAULT_WINDOWBITS` (15).
  - `memLevel` `<number>` Default: `9`.
  - `strategy` `<number>` Default: `Z_DEFAULT_STRATEGY`.
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a deflate compression transform. Output is compatible with `zlib.inflate()` and `decompressDeflate()`/`decompressDeflateSync()`.

#### `compressGzip([options])`#

#### `compressGzipSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `level` `<number>` Compression level (`0`-`9`). Default: `4`.
  - `windowBits` `<number>` Default: `Z_DEFAULT_WINDOWBITS` (15).
  - `memLevel` `<number>` Default: `9`.
  - `strategy` `<number>` Default: `Z_DEFAULT_STRATEGY`.
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a gzip compression transform. Output is compatible with `zlib.gunzip()` and `decompressGzip()`/`decompressGzipSync()`.

#### `compressZstd([options])`#

#### `compressZstdSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `params` `<Object>` Key-value object where keys and values are `zlib.constants` entries. The most important compressor parameters are:
    - `ZSTD_c_compressionLevel` -- Default: `ZSTD_CLEVEL_DEFAULT` (3).
    - `ZSTD_c_checksumFlag` -- generate a checksum. Default: `0`.
    - `ZSTD_c_strategy` -- compression strategy. Values include `ZSTD_fast`, `ZSTD_dfast`, `ZSTD_greedy`, `ZSTD_lazy`, `ZSTD_lazy2`, `ZSTD_btlazy2`, `ZSTD_btopt`, `ZSTD_btultra`, `ZSTD_btultra2`. See the Zstd compressor options in the zlib documentation for the full list.
  - `pledgedSrcSize` `<number>` Expected uncompressed size as a non-negative safe integer (optional hint).
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a Zstandard compression transform. Output is compatible with `zlib.zstdDecompress()` and `decompressZstd()`/`decompressZstdSync()`.

#### `decompressBrotli([options])`#

#### `decompressBrotliSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `params` `<Object>` Key-value object where keys and values are `zlib.constants` entries. Available decompressor parameters:
    - `BROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION` -- boolean flag affecting internal memory allocation.
    - `BROTLI_DECODER_PARAM_LARGE_WINDOW` -- boolean flag enabling "Large Window Brotli" mode (not compatible with RFC 7932). See the Brotli decompressor options in the zlib documentation for details.
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a Brotli decompression transform.

#### `decompressDeflate([options])`#

#### `decompressDeflateSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `windowBits` `<number>` Default: `Z_DEFAULT_WINDOWBITS` (15).
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a deflate decompression transform.

#### `decompressGzip([options])`#

#### `decompressGzipSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `windowBits` `<number>` Default: `Z_DEFAULT_WINDOWBITS` (15).
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a gzip decompression transform.

#### `decompressZstd([options])`#

#### `decompressZstdSync([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `chunkSize` `<number>` Output buffer size. Default: `65536` (64 KB).
  - `params` `<Object>` Key-value object where keys and values are `zlib.constants` entries. Available decompressor parameters:
    - `ZSTD_d_windowLogMax` -- maximum window size (log2) the decompressor will allocate. Limits memory usage against malicious input. See the Zstd decompressor options in the zlib documentation for details.
  - `dictionary` `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Object>` A stateful transform.

Create a Zstandard decompression transform.

## Performance measurement APIs#

Source Code: lib/perf_hooks.js

Stability: 2 - Stable

This module provides an implementation of a subset of the W3C Web Performance APIs as well as additional APIs for Node.js-specific performance measurements.

Node.js supports the following Web Performance APIs:

- High Resolution Time
- Performance Timeline
- User Timing
- Resource Timing

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((items) => {
      console.log(items.getEntries()[0].duration);
      performance.clearMarks();
    });
    obs.observe({ type: 'measure' });
    performance.measure('Start to Now');

    performance.mark('A');
    doSomeLongRunningProcess(() => {
      performance.measure('A to Now', 'A');

      performance.mark('B');
      performance.measure('A to B', 'A', 'B');
    });
    const { PerformanceObserver, performance } = require('node:perf_hooks');

    const obs = new PerformanceObserver((items) => {
      console.log(items.getEntries()[0].duration);
    });
    obs.observe({ type: 'measure' });
    performance.measure('Start to Now');

    performance.mark('A');
    (async function doSomeLongRunningProcess() {
      await new Promise((r) => setTimeout(r, 5000));
      performance.measure('A to Now', 'A');

      performance.mark('B');
      performance.measure('A to B', 'A', 'B');
    })();
    javascript

### `perf_hooks.performance`#

Added in: v8.5.0

An object that can be used to collect performance metrics from the current Node.js instance. It is similar to `window.performance` in browsers.

#### `performance.clearMarks([name])`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- `name` `<string>`

If `name` is not provided, removes all `PerformanceMark` objects from the Performance Timeline. If `name` is provided, removes only the named mark.

#### `performance.clearMeasures([name])`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- `name` `<string>`

If `name` is not provided, removes all `PerformanceMeasure` objects from the Performance Timeline. If `name` is provided, removes only the named measure.

#### `performance.clearResourceTimings([name])`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- `name` `<string>`

If `name` is not provided, removes all `PerformanceResourceTiming` objects from the Resource Timeline. If `name` is provided, removes only the named resource.

#### `performance.eventLoopUtilization([utilization1[, utilization2]])`#

Added in: v14.10.0, v12.19.0
History

Version | Changes
--- | ---
v25.2.0, v24.12.0 | Added `perf_hooks.eventLoopUtilization` alias.

- `utilization1` `<Object>` The result of a previous call to `eventLoopUtilization()`.
- `utilization2` `<Object>` The result of a previous call to `eventLoopUtilization()` prior to `utilization1`.
- Returns: `<Object>`
  - `idle` `<number>`
  - `active` `<number>`
  - `utilization` `<number>`

This is an alias of `perf_hooks.eventLoopUtilization()`.

This property is an extension by Node.js. It is not available in Web browsers.

#### `performance.getEntries()`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- Returns: `<PerformanceEntry>`[]

Returns a list of `PerformanceEntry` objects in chronological order with respect to `performanceEntry.startTime`. If you are only interested in performance entries of certain types or that have certain names, see `performance.getEntriesByType()` and `performance.getEntriesByName()`.

#### `performance.getEntriesByName(name[, type])`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- `name` `<string>`
- `type` `<string>`
- Returns: `<PerformanceEntry>`[]

Returns a list of `PerformanceEntry` objects in chronological order with respect to `performanceEntry.startTime` whose `performanceEntry.name` is equal to `name`, and optionally, whose `performanceEntry.entryType` is equal to `type`.

#### `performance.getEntriesByType(type)`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- `type` `<string>`
- Returns: `<PerformanceEntry>`[]

Returns a list of `PerformanceEntry` objects in chronological order with respect to `performanceEntry.startTime` whose `performanceEntry.entryType` is equal to `type`.

#### `performance.mark(name[, options])`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver. The name argument is no longer optional.
v16.0.0 | Updated to conform to the User Timing Level 3 specification.

- `name` `<string>`
- `options` `<Object>`
  - `detail` `<any>` Additional optional detail to include with the mark.
  - `startTime` `<number>` An optional timestamp to be used as the mark time. Default: `performance.now()`.

Creates a new `PerformanceMark` entry in the Performance Timeline. A `PerformanceMark` is a subclass of `PerformanceEntry` whose `performanceEntry.entryType` is always `'mark'`, and whose `performanceEntry.duration` is always `0`. Performance marks are used to mark specific significant moments in the Performance Timeline.

The created `PerformanceMark` entry is put in the global Performance Timeline and can be queried with `performance.getEntries`, `performance.getEntriesByName`, and `performance.getEntriesByType`. When the observation is performed, the entries should be cleared from the global Performance Timeline manually with `performance.clearMarks`.

#### `performance.markResourceTiming(timingInfo, requestedUrl, initiatorType, global, cacheMode, bodyInfo, responseStatus[, deliveryType])`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v22.2.0 | Added bodyInfo, responseStatus, and deliveryType arguments.

- `timingInfo` `<Object>` Fetch Timing Info
- `requestedUrl` `<string>` The resource url
- `initiatorType` `<string>` The initiator name, e.g: 'fetch'
- `global` `<Object>`
- `cacheMode` `<string>` The cache mode must be an empty string ('') or 'local'
- `bodyInfo` `<Object>` Fetch Response Body Info
- `responseStatus` `<number>` The response's status code
- `deliveryType` `<string>` The delivery type. Default: `''`.

This property is an extension by Node.js. It is not available in Web browsers.

Creates a new `PerformanceResourceTiming` entry in the Resource Timeline. A `PerformanceResourceTiming` is a subclass of `PerformanceEntry` whose `performanceEntry.entryType` is always `'resource'`. Performance resources are used to mark moments in the Resource Timeline.

The created `PerformanceMark` entry is put in the global Resource Timeline and can be queried with `performance.getEntries`, `performance.getEntriesByName`, and `performance.getEntriesByType`. When the observation is performed, the entries should be cleared from the global Performance Timeline manually with `performance.clearResourceTimings`.

#### `performance.measure(name[, startMarkOrOptions[, endMark]])`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.
v16.0.0 | Updated to conform to the User Timing Level 3 specification.
v13.13.0, v12.16.3 | Make `startMark` and `endMark` parameters optional.

- `name` `<string>`
- `startMarkOrOptions` `<string>` | `<Object>` Optional.
  - `detail` `<any>` Additional optional detail to include with the measure.
  - `duration` `<number>` Duration between start and end times.
  - `end` `<number>` | `<string>` Timestamp to be used as the end time, or a string identifying a previously recorded mark.
  - `start` `<number>` | `<string>` Timestamp to be used as the start time, or a string identifying a previously recorded mark.
- `endMark` `<string>` Optional. Must be omitted if `startMarkOrOptions` is an `<Object>`.

Creates a new `PerformanceMeasure` entry in the Performance Timeline. A `PerformanceMeasure` is a subclass of `PerformanceEntry` whose `performanceEntry.entryType` is always `'measure'`, and whose `performanceEntry.duration` measures the number of milliseconds elapsed since `startMark` and `endMark`.

The `startMark` argument may identify any existing `PerformanceMark` in the Performance Timeline, or may identify any of the timestamp properties provided by the `PerformanceNodeTiming` class. If the named `startMark` does not exist, an error is thrown.

The optional `endMark` argument must identify any existing `PerformanceMark` in the Performance Timeline or any of the timestamp properties provided by the `PerformanceNodeTiming` class. `endMark` will be `performance.now()` if no parameter is passed, otherwise if the named `endMark` does not exist, an error will be thrown.

The created `PerformanceMeasure` entry is put in the global Performance Timeline and can be queried with `performance.getEntries`, `performance.getEntriesByName`, and `performance.getEntriesByType`. When the observation is performed, the entries should be cleared from the global Performance Timeline manually with `performance.clearMeasures`.

#### `performance.nodeTiming`#

Added in: v8.5.0

- Type: `<PerformanceNodeTiming>`

This property is an extension by Node.js. It is not available in Web browsers.

An instance of the `PerformanceNodeTiming` class that provides performance metrics for specific Node.js operational milestones.

#### `performance.now()`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

- Returns: `<number>`

Returns the current high resolution millisecond timestamp, where 0 represents the start of the current `node` process.

#### `performance.setResourceTimingBufferSize(maxSize)`#

Added in: v18.8.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

Sets the global performance resource timing buffer size to the specified number of "resource" type performance entry objects.

By default the max buffer size is set to 250.

#### `performance.timeOrigin`#

Added in: v8.5.0

- Type: `<number>`

The `timeOrigin` specifies the high resolution millisecond timestamp at which the current `node` process began, measured in Unix time.

#### `performance.timerify(fn[, options])`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v25.2.0, v24.12.0 | Added `perf_hooks.timerify` alias.
v16.0.0 | Added the histogram option.
v16.0.0 | Re-implemented to use pure-JavaScript and the ability to time async functions.

- `fn` `<Function>`
- `options` `<Object>`
  - `histogram` `<RecordableHistogram>` A histogram object created using `perf_hooks.createHistogram()` that will record runtime durations in nanoseconds.

This is an alias of `perf_hooks.timerify()`.

This property is an extension by Node.js. It is not available in Web browsers.

#### `performance.toJSON()`#

Added in: v16.1.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `performance` object as the receiver.

An object which is JSON representation of the `performance` object. It is similar to `window.performance.toJSON` in browsers.

##### Event: `'resourcetimingbufferfull'`#

Added in: v18.8.0

The `'resourcetimingbufferfull'` event is fired when the global performance resource timing buffer is full. Adjust resource timing buffer size with `performance.setResourceTimingBufferSize()` or clear the buffer with `performance.clearResourceTimings()` in the event listener to allow more entries to be added to the performance timeline buffer.

### Class: `PerformanceEntry`#

Added in: v8.5.0

The constructor of this class is not exposed to users directly.

#### `performanceEntry.duration`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceEntry` object as the receiver.

- Type: `<number>`

The total number of milliseconds elapsed for this entry. This value will not be meaningful for all Performance Entry types.

#### `performanceEntry.entryType`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceEntry` object as the receiver.

- Type: `<string>`

The type of the performance entry. It may be one of:

- `'dns'` (Node.js only)
- `'function'` (Node.js only)
- `'gc'` (Node.js only)
- `'http2'` (Node.js only)
- `'http'` (Node.js only)
- `'mark'` (available on the Web)
- `'measure'` (available on the Web)
- `'net'` (Node.js only)
- `'node'` (Node.js only)
- `'resource'` (available on the Web)

#### `performanceEntry.name`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceEntry` object as the receiver.

- Type: `<string>`

The name of the performance entry.

#### `performanceEntry.startTime`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceEntry` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp marking the starting time of the Performance Entry.

### Class: `PerformanceMark`#

Added in: v18.2.0, v16.17.0

- Extends: `<PerformanceEntry>`

Exposes marks created via the `Performance.mark()` method.

#### `performanceMark.detail`#

Added in: v16.0.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceMark` object as the receiver.

- Type: `<any>`

Additional detail specified when creating with `Performance.mark()` method.

### Class: `PerformanceMeasure`#

Added in: v18.2.0, v16.17.0

- Extends: `<PerformanceEntry>`

Exposes measures created via the `Performance.measure()` method.

The constructor of this class is not exposed to users directly.

#### `performanceMeasure.detail`#

Added in: v16.0.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceMeasure` object as the receiver.

- Type: `<any>`

Additional detail specified when creating with `Performance.measure()` method.

### Class: `PerformanceNodeEntry`#

Added in: v19.0.0

- Extends: `<PerformanceEntry>`

This class is an extension by Node.js. It is not available in Web browsers.

Provides detailed Node.js timing data.

The constructor of this class is not exposed to users directly.

#### `performanceNodeEntry.detail`#

Added in: v16.0.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceNodeEntry` object as the receiver.

- Type: `<any>`

Additional detail specific to the `entryType`.

#### `performanceNodeEntry.flags`#

Added in: v13.9.0, v12.17.0
History

Version | Changes
--- | ---
v16.0.0 | Runtime deprecated. Now moved to the detail property when entryType is 'gc'.

Stability: 0 - Deprecated: Use `performanceNodeEntry.detail` instead.

- Type: `<number>`

When `performanceEntry.entryType` is equal to `'gc'`, the `performance.flags` property contains additional information about garbage collection operation. The value may be one of:

- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE`

#### `performanceNodeEntry.kind`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v16.0.0 | Runtime deprecated. Now moved to the detail property when entryType is 'gc'.

Stability: 0 - Deprecated: Use `performanceNodeEntry.detail` instead.

- Type: `<number>`

When `performanceEntry.entryType` is equal to `'gc'`, the `performance.kind` property identifies the type of garbage collection operation that occurred. The value may be one of:

- `perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR_MARK_SWEEP`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB`

#### Garbage Collection ('gc') Details#

When `performanceEntry.type` is equal to `'gc'`, the `performanceNodeEntry.detail` property will be an `<Object>` with two properties:

- `kind` `<number>` One of:
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR_MARK_SWEEP`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB`
- `flags` `<number>` One of:
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY`
  - `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE`

#### HTTP ('http') Details#

When `performanceEntry.type` is equal to `'http'`, the `performanceNodeEntry.detail` property will be an `<Object>` containing additional information.

If `performanceEntry.name` is equal to `HttpClient`, the `detail` will contain the following properties: `req`, `res`. And the `req` property will be an `<Object>` containing `method`, `url`, `headers`, the `res` property will be an `<Object>` containing `statusCode`, `statusMessage`, `headers`.

If `performanceEntry.name` is equal to `HttpRequest`, the `detail` will contain the following properties: `req`, `res`. And the `req` property will be an `<Object>` containing `method`, `url`, `headers`, the `res` property will be an `<Object>` containing `statusCode`, `statusMessage`, `headers`.

This could add additional memory overhead and should only be used for diagnostic purposes, not left turned on in production by default.

#### HTTP/2 ('http2') Details#

When `performanceEntry.type` is equal to `'http2'`, the `performanceNodeEntry.detail` property will be an `<Object>` containing additional performance information.

If `performanceEntry.name` is equal to `Http2Stream`, the `detail` will contain the following properties:

- `bytesRead` `<number>` The number of `DATA` frame bytes received for this `Http2Stream`.
- `bytesWritten` `<number>` The number of `DATA` frame bytes sent for this `Http2Stream`.
- `id` `<number>` The identifier of the associated `Http2Stream`
- `timeToFirstByte` `<number>` The number of milliseconds elapsed between the `PerformanceEntry` `startTime` and the reception of the first `DATA` frame.
- `timeToFirstByteSent` `<number>` The number of milliseconds elapsed between the `PerformanceEntry` `startTime` and sending of the first `DATA` frame.
- `timeToFirstHeader` `<number>` The number of milliseconds elapsed between the `PerformanceEntry` `startTime` and the reception of the first header.

If `performanceEntry.name` is equal to `Http2Session`, the `detail` will contain the following properties:

- `bytesRead` `<number>` The number of bytes received for this `Http2Session`.
- `bytesWritten` `<number>` The number of bytes sent for this `Http2Session`.
- `framesReceived` `<number>` The number of HTTP/2 frames received by the `Http2Session`.
- `framesSent` `<number>` The number of HTTP/2 frames sent by the `Http2Session`.
- `maxConcurrentStreams` `<number>` The maximum number of streams concurrently open during the lifetime of the `Http2Session`.
- `pingRTT` `<number>` The number of milliseconds elapsed since the transmission of a `PING` frame and the reception of its acknowledgment. Only present if a `PING` frame has been sent on the `Http2Session`.
- `streamAverageDuration` `<number>` The average duration (in milliseconds) for all `Http2Stream` instances.
- `streamCount` `<number>` The number of `Http2Stream` instances processed by the `Http2Session`.
- `type` `<string>` Either `'server'` or `'client'` to identify the type of `Http2Session`.

#### Timerify ('function') Details#

When `performanceEntry.type` is equal to `'function'`, the `performanceNodeEntry.detail` property will be an `<Array>` listing the input arguments to the timed function.

#### Net ('net') Details#

When `performanceEntry.type` is equal to `'net'`, the `performanceNodeEntry.detail` property will be an `<Object>` containing additional information.

If `performanceEntry.name` is equal to `connect`, the `detail` will contain the following properties: `host`, `port`.

#### DNS ('dns') Details#

When `performanceEntry.type` is equal to `'dns'`, the `performanceNodeEntry.detail` property will be an `<Object>` containing additional information.

If `performanceEntry.name` is equal to `lookup`, the `detail` will contain the following properties: `hostname`, `family`, `hints`, `verbatim`, `addresses`.

If `performanceEntry.name` is equal to `lookupService`, the `detail` will contain the following properties: `host`, `port`, `hostname`, `service`.

If `performanceEntry.name` is equal to `queryxxx` or `getHostByAddr`, the `detail` will contain the following properties: `host`, `ttl`, `result`. The value of `result` is same as the result of `queryxxx` or `getHostByAddr`.

### Class: `PerformanceNodeTiming`#

Added in: v8.5.0

- Extends: `<PerformanceEntry>`

This property is an extension by Node.js. It is not available in Web browsers.

Provides timing details for Node.js itself. The constructor of this class is not exposed to users.

#### `performanceNodeTiming.bootstrapComplete`#

Added in: v8.5.0

- Type: `<number>`

The high resolution millisecond timestamp at which the Node.js process completed bootstrapping. If bootstrapping has not yet finished, the property has the value of -1.

#### `performanceNodeTiming.environment`#

Added in: v8.5.0

- Type: `<number>`

The high resolution millisecond timestamp at which the Node.js environment was initialized.

#### `performanceNodeTiming.idleTime`#

Added in: v14.10.0, v12.19.0

- Type: `<number>`

The high resolution millisecond timestamp of the amount of time the event loop has been idle within the event loop's event provider (e.g. `epoll_wait`). This does not take CPU usage into consideration. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of 0.

#### `performanceNodeTiming.loopExit`#

Added in: v8.5.0

- Type: `<number>`

The high resolution millisecond timestamp at which the Node.js event loop exited. If the event loop has not yet exited, the property has the value of -1. It can only have a value of not -1 in a handler of the `'exit'` event.

#### `performanceNodeTiming.loopStart`#

Added in: v8.5.0

- Type: `<number>`

The high resolution millisecond timestamp at which the Node.js event loop started. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of -1.

#### `performanceNodeTiming.nodeStart`#

Added in: v8.5.0

- Type: `<number>`

The high resolution millisecond timestamp at which the Node.js process was initialized.

#### `performanceNodeTiming.uvMetricsInfo`#

Added in: v22.8.0, v20.18.0

- Returns: `<Object>`
  - `loopCount` `<number>` Number of event loop iterations.
  - `events` `<number>` Number of events that have been processed by the event handler.
  - `eventsWaiting` `<number>` Number of events that were waiting to be processed when the event provider was called.

This is a wrapper to the `uv_metrics_info` function. It returns the current set of event loop metrics.

It is recommended to use this property inside a function whose execution was scheduled using `setImmediate` to avoid collecting metrics before finishing all operations scheduled during the current loop iteration.

    const { performance } = require('node:perf_hooks');

    setImmediate(() => {
      console.log(performance.nodeTiming.uvMetricsInfo);
    });
    import { performance } from 'node:perf_hooks';

    setImmediate(() => {
      console.log(performance.nodeTiming.uvMetricsInfo);
    });
    javascript

#### `performanceNodeTiming.v8Start`#

Added in: v8.5.0

- Type: `<number>`

The high resolution millisecond timestamp at which the V8 platform was initialized.

### Class: `PerformanceResourceTiming`#

Added in: v18.2.0, v16.17.0

- Extends: `<PerformanceEntry>`

Provides detailed network timing data regarding the loading of an application's resources.

The constructor of this class is not exposed to users directly.

#### `performanceResourceTiming.workerStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp at immediately before dispatching the `fetch` request. If the resource is not intercepted by a worker the property will always return 0.

#### `performanceResourceTiming.redirectStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp that represents the start time of the fetch which initiates the redirect.

#### `performanceResourceTiming.redirectEnd`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp that will be created immediately after receiving the last byte of the response of the last redirect.

#### `performanceResourceTiming.fetchStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp immediately before the Node.js starts to fetch the resource.

#### `performanceResourceTiming.domainLookupStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp immediately before the Node.js starts the domain name lookup for the resource.

#### `performanceResourceTiming.domainLookupEnd`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp representing the time immediately after the Node.js finished the domain name lookup for the resource.

#### `performanceResourceTiming.connectStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp representing the time immediately before Node.js starts to establish the connection to the server to retrieve the resource.

#### `performanceResourceTiming.connectEnd`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp representing the time immediately after Node.js finishes establishing the connection to the server to retrieve the resource.

#### `performanceResourceTiming.secureConnectionStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp representing the time immediately before Node.js starts the handshake process to secure the current connection.

#### `performanceResourceTiming.requestStart`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp representing the time immediately before Node.js receives the first byte of the response from the server.

#### `performanceResourceTiming.responseEnd`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

The high resolution millisecond timestamp representing the time immediately after Node.js receives the last byte of the resource or immediately before the transport connection is closed, whichever comes first.

#### `performanceResourceTiming.transferSize`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

A number representing the size (in octets) of the fetched resource. The size includes the response header fields plus the response payload body.

#### `performanceResourceTiming.encodedBodySize`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

A number representing the size (in octets) received from the fetch (HTTP or cache), of the payload body, before removing any applied content-codings.

#### `performanceResourceTiming.decodedBodySize`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This property getter must be called with the `PerformanceResourceTiming` object as the receiver.

- Type: `<number>`

A number representing the size (in octets) received from the fetch (HTTP or cache), of the message body, after removing any applied content-codings.

#### `performanceResourceTiming.toJSON()`#

Added in: v18.2.0, v16.17.0
History

Version | Changes
--- | ---
v19.0.0 | This method must be called with the `PerformanceResourceTiming` object as the receiver.

Returns a `object` that is the JSON representation of the `PerformanceResourceTiming` object

### Class: `PerformanceObserver`#

Added in: v8.5.0

#### `PerformanceObserver.supportedEntryTypes`#

Added in: v16.0.0

- Type: `<string>`[]

Get supported types.

#### `new PerformanceObserver(callback)`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `callback` `<Function>`
  - `list` `<PerformanceObserverEntryList>`
  - `observer` `<PerformanceObserver>`

`PerformanceObserver` objects provide notifications when new `PerformanceEntry` instances have been added to the Performance Timeline.

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((list, observer) => {
      console.log(list.getEntries());

      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ entryTypes: ['mark'], buffered: true });

    performance.mark('test');
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    const obs = new PerformanceObserver((list, observer) => {
      console.log(list.getEntries());

      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ entryTypes: ['mark'], buffered: true });

    performance.mark('test');
    javascript

Because `PerformanceObserver` instances introduce their own additional performance overhead, instances should not be left subscribed to notifications indefinitely. Users should disconnect observers as soon as they are no longer needed.

The `callback` is invoked when a `PerformanceObserver` is notified about new `PerformanceEntry` instances. The callback receives a `PerformanceObserverEntryList` instance and a reference to the `PerformanceObserver`.

#### `performanceObserver.disconnect()`#

Added in: v8.5.0

Disconnects the `PerformanceObserver` instance from all notifications.

#### `performanceObserver.observe(options)`#

Added in: v8.5.0
History

Version | Changes
--- | ---
v16.7.0 | Updated to conform to Performance Timeline Level 2. The buffered option has been added back.
v16.0.0 | Updated to conform to User Timing Level 3. The buffered option has been removed.

- `options` `<Object>`
  - `type` `<string>` A single `<PerformanceEntry>` type. Must not be given if `entryTypes` is already specified.
  - `entryTypes` `<string>`[] An array of strings identifying the types of `<PerformanceEntry>` instances the observer is interested in. If not provided an error will be thrown.
  - `buffered` `<boolean>` If true, the observer callback is called with a list global `PerformanceEntry` buffered entries. If false, only `PerformanceEntry`s created after the time point are sent to the observer callback. Default: `false`.

Subscribes the `<PerformanceObserver>` instance to notifications of new `<PerformanceEntry>` instances identified either by `options.entryTypes` or `options.type`:

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((list, observer) => {
      // Called once asynchronously. `list` contains three items.
    });
    obs.observe({ type: 'mark' });

    for (let n = 0; n < 3; n++)
      performance.mark(`test${n}`);
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    const obs = new PerformanceObserver((list, observer) => {
      // Called once asynchronously. `list` contains three items.
    });
    obs.observe({ type: 'mark' });

    for (let n = 0; n < 3; n++)
      performance.mark(`test${n}`);
    javascript

#### `performanceObserver.takeRecords()`#

Added in: v16.0.0

- Returns: `<PerformanceEntry>`[] Current list of entries stored in the performance observer, emptying it out.

### Class: `PerformanceObserverEntryList`#

Added in: v8.5.0

The `PerformanceObserverEntryList` class is used to provide access to the `PerformanceEntry` instances passed to a `PerformanceObserver`. The constructor of this class is not exposed to users.

#### `performanceObserverEntryList.getEntries()`#

Added in: v8.5.0

- Returns: `<PerformanceEntry>`[]

Returns a list of `PerformanceEntry` objects in chronological order with respect to `performanceEntry.startTime`.

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((perfObserverList, observer) => {
      console.log(perfObserverList.getEntries());
      /**
       * [
       *   PerformanceEntry {
       *     name: 'test',
       *     entryType: 'mark',
       *     startTime: 81.465639,
       *     duration: 0,
       *     detail: null
       *   },
       *   PerformanceEntry {
       *     name: 'meow',
       *     entryType: 'mark',
       *     startTime: 81.860064,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */

      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ type: 'mark' });

    performance.mark('test');
    performance.mark('meow');
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    const obs = new PerformanceObserver((perfObserverList, observer) => {
      console.log(perfObserverList.getEntries());
      /**
       * [
       *   PerformanceEntry {
       *     name: 'test',
       *     entryType: 'mark',
       *     startTime: 81.465639,
       *     duration: 0,
       *     detail: null
       *   },
       *   PerformanceEntry {
       *     name: 'meow',
       *     entryType: 'mark',
       *     startTime: 81.860064,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */

      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ type: 'mark' });

    performance.mark('test');
    performance.mark('meow');
    javascript

#### `performanceObserverEntryList.getEntriesByName(name[, type])`#

Added in: v8.5.0

- `name` `<string>`
- `type` `<string>`
- Returns: `<PerformanceEntry>`[]

Returns a list of `PerformanceEntry` objects in chronological order with respect to `performanceEntry.startTime` whose `performanceEntry.name` is equal to `name`, and optionally, whose `performanceEntry.entryType` is equal to `type`.

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((perfObserverList, observer) => {
      console.log(perfObserverList.getEntriesByName('meow'));
      /**
       * [
       *   PerformanceEntry {
       *     name: 'meow',
       *     entryType: 'mark',
       *     startTime: 98.545991,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */
      console.log(perfObserverList.getEntriesByName('nope')); // []

      console.log(perfObserverList.getEntriesByName('test', 'mark'));
      /**
       * [
       *   PerformanceEntry {
       *     name: 'test',
       *     entryType: 'mark',
       *     startTime: 63.518931,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */
      console.log(perfObserverList.getEntriesByName('test', 'measure')); // []

      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ entryTypes: ['mark', 'measure'] });

    performance.mark('test');
    performance.mark('meow');
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    const obs = new PerformanceObserver((perfObserverList, observer) => {
      console.log(perfObserverList.getEntriesByName('meow'));
      /**
       * [
       *   PerformanceEntry {
       *     name: 'meow',
       *     entryType: 'mark',
       *     startTime: 98.545991,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */
      console.log(perfObserverList.getEntriesByName('nope')); // []

      console.log(perfObserverList.getEntriesByName('test', 'mark'));
      /**
       * [
       *   PerformanceEntry {
       *     name: 'test',
       *     entryType: 'mark',
       *     startTime: 63.518931,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */
      console.log(perfObserverList.getEntriesByName('test', 'measure')); // []

      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ entryTypes: ['mark', 'measure'] });

    performance.mark('test');
    performance.mark('meow');
    javascript

#### `performanceObserverEntryList.getEntriesByType(type)`#

Added in: v8.5.0

- `type` `<string>`
- Returns: `<PerformanceEntry>`[]

Returns a list of `PerformanceEntry` objects in chronological order with respect to `performanceEntry.startTime` whose `performanceEntry.entryType` is equal to `type`.

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const obs = new PerformanceObserver((perfObserverList, observer) => {
      console.log(perfObserverList.getEntriesByType('mark'));
      /**
       * [
       *   PerformanceEntry {
       *     name: 'test',
       *     entryType: 'mark',
       *     startTime: 55.897834,
       *     duration: 0,
       *     detail: null
       *   },
       *   PerformanceEntry {
       *     name: 'meow',
       *     entryType: 'mark',
       *     startTime: 56.350146,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */
      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ type: 'mark' });

    performance.mark('test');
    performance.mark('meow');
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    const obs = new PerformanceObserver((perfObserverList, observer) => {
      console.log(perfObserverList.getEntriesByType('mark'));
      /**
       * [
       *   PerformanceEntry {
       *     name: 'test',
       *     entryType: 'mark',
       *     startTime: 55.897834,
       *     duration: 0,
       *     detail: null
       *   },
       *   PerformanceEntry {
       *     name: 'meow',
       *     entryType: 'mark',
       *     startTime: 56.350146,
       *     duration: 0,
       *     detail: null
       *   }
       * ]
       */
      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ type: 'mark' });

    performance.mark('test');
    performance.mark('meow');
    javascript

### `perf_hooks.createHistogram([options])`#

Added in: v15.9.0, v14.18.0

- `options` `<Object>`
  - `lowest` `<number>` | `<bigint>` The lowest discernible value. Must be an integer value greater than 0. Default: `1`.
  - `highest` `<number>` | `<bigint>` The highest recordable value. Must be an integer value that is equal to or greater than two times `lowest`. Default: `Number.MAX_SAFE_INTEGER`.
  - `figures` `<number>` The number of accuracy digits. Must be a number between `1` and `5`. Default: `3`.
- Returns: `<RecordableHistogram>`

Returns a `<RecordableHistogram>`.

### `perf_hooks.eventLoopUtilization([utilization1[, utilization2]])`#

Added in: v25.2.0, v24.12.0

- `utilization1` `<Object>` The result of a previous call to `eventLoopUtilization()`.
- `utilization2` `<Object>` The result of a previous call to `eventLoopUtilization()` prior to `utilization1`.
- Returns: `<Object>`
  - `idle` `<number>`
  - `active` `<number>`
  - `utilization` `<number>`

The `eventLoopUtilization()` function returns an object that contains the cumulative duration of time the event loop has been both idle and active as a high resolution milliseconds timer. The `utilization` value is the calculated Event Loop Utilization (ELU).

If bootstrapping has not yet finished on the main thread the properties have the value of `0`. The ELU is immediately available on Worker threads since bootstrap happens within the event loop.

Both `utilization1` and `utilization2` are optional parameters.

If `utilization1` is passed, then the delta between the current call's `active` and `idle` times, as well as the corresponding `utilization` value are calculated and returned (similar to `process.hrtime()`).

If `utilization1` and `utilization2` are both passed, then the delta is calculated between the two arguments. This is a convenience option because, unlike `process.hrtime()`, calculating the ELU is more complex than a single subtraction.

ELU is similar to CPU utilization, except that it only measures event loop statistics and not CPU usage. It represents the percentage of time the event loop has spent outside the event loop's event provider (e.g. `epoll_wait`). No other CPU idle time is taken into consideration. The following is an example of how a mostly idle process will have a high ELU.

    import { eventLoopUtilization } from 'node:perf_hooks';
    import { spawnSync } from 'node:child_process';

    setImmediate(() => {
      const elu = eventLoopUtilization();
      spawnSync('sleep', ['5']);
      console.log(eventLoopUtilization(elu).utilization);
    });
    const { eventLoopUtilization } = require('node:perf_hooks');
    const { spawnSync } = require('node:child_process');

    setImmediate(() => {
      const elu = eventLoopUtilization();
      spawnSync('sleep', ['5']);
      console.log(eventLoopUtilization(elu).utilization);
    });
    javascript

Although the CPU is mostly idle while running this script, the value of `utilization` is `1`. This is because the call to `child_process.spawnSync()` blocks the event loop from proceeding.

Passing in a user-defined object instead of the result of a previous call to `eventLoopUtilization()` will lead to undefined behavior. The return values are not guaranteed to reflect any correct state of the event loop.

### `perf_hooks.monitorEventLoopDelay([options])`#

Added in: v11.10.0
History

Version | Changes
--- | ---
v26.5.0 | Added the `samplePerIteration` option.

- `options` `<Object>`
  - `samplePerIteration` `<boolean>` When `true`, samples are taken once per event loop iteration. Default: `false`.
  - `resolution` `<number>` The sampling rate in milliseconds for interval-based sampling. Must be greater than zero. This option is ignored when `samplePerIteration` is `true`. Default: `10`.
- Returns: `<ELDHistogram>`

This property is an extension by Node.js. It is not available in Web browsers.

Creates a histogram object that samples and reports the event loop delay over time. The delays will be reported in nanoseconds.

By default, the histogram is updated by a timer using the configured `resolution`. When `samplePerIteration` is `true`, samples are taken once per event loop iteration using `uv_prepare_t` and `uv_check_t` hooks. In that mode, the histogram does not keep the loop alive or force additional iterations when the application is idle. The two sampling modes produce significantly different results and should not be compared directly.

    import { monitorEventLoopDelay } from 'node:perf_hooks';

    const h = monitorEventLoopDelay({ resolution: 20 });
    h.enable();
    // Do something.
    h.disable();
    console.log(h.min);
    console.log(h.max);
    console.log(h.mean);
    console.log(h.stddev);
    console.log(h.percentiles);
    console.log(h.percentile(50));
    console.log(h.percentile(99));
    const { monitorEventLoopDelay } = require('node:perf_hooks');
    const h = monitorEventLoopDelay({ resolution: 20 });
    h.enable();
    // Do something.
    h.disable();
    console.log(h.min);
    console.log(h.max);
    console.log(h.mean);
    console.log(h.stddev);
    console.log(h.percentiles);
    console.log(h.percentile(50));
    console.log(h.percentile(99));
    javascript

### `perf_hooks.timerify(fn[, options])`#

Added in: v25.2.0, v24.12.0

- `fn` `<Function>`
- `options` `<Object>`
  - `histogram` `<RecordableHistogram>` A histogram object created using `perf_hooks.createHistogram()` that will record runtime durations in nanoseconds.

This property is an extension by Node.js. It is not available in Web browsers.

Wraps a function within a new function that measures the running time of the wrapped function. A `PerformanceObserver` must be subscribed to the `'function'` event type in order for the timing details to be accessed.

    import { timerify, performance, PerformanceObserver } from 'node:perf_hooks';

    function someFunction() {
      console.log('hello world');
    }

    const wrapped = timerify(someFunction);

    const obs = new PerformanceObserver((list) => {
      console.log(list.getEntries()[0].duration);

      performance.clearMarks();
      performance.clearMeasures();
      obs.disconnect();
    });
    obs.observe({ entryTypes: ['function'] });

    // A performance timeline entry will be created
    wrapped();
    const {
      timerify,
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    function someFunction() {
      console.log('hello world');
    }

    const wrapped = timerify(someFunction);

    const obs = new PerformanceObserver((list) => {
      console.log(list.getEntries()[0].duration);

      performance.clearMarks();
      performance.clearMeasures();
      obs.disconnect();
    });
    obs.observe({ entryTypes: ['function'] });

    // A performance timeline entry will be created
    wrapped();
    javascript

If the wrapped function returns a promise, a finally handler will be attached to the promise and the duration will be reported once the finally handler is invoked.

### Class: `Histogram`#

Added in: v11.10.0

#### `histogram.count`#

Added in: v17.4.0, v16.14.0

- Type: `<number>`

The number of samples recorded by the histogram.

#### `histogram.countBigInt`#

Added in: v17.4.0, v16.14.0

- Type: `<bigint>`

The number of samples recorded by the histogram.

#### `histogram.exceeds`#

Added in: v11.10.0

- Type: `<number>`

The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

#### `histogram.exceedsBigInt`#

Added in: v17.4.0, v16.14.0

- Type: `<bigint>`

The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

#### `histogram.max`#

Added in: v11.10.0

- Type: `<number>`

The maximum recorded event loop delay.

#### `histogram.maxBigInt`#

Added in: v17.4.0, v16.14.0

- Type: `<bigint>`

The maximum recorded event loop delay.

#### `histogram.mean`#

Added in: v11.10.0

- Type: `<number>`

The mean of the recorded event loop delays.

#### `histogram.min`#

Added in: v11.10.0

- Type: `<number>`

The minimum recorded event loop delay.

#### `histogram.minBigInt`#

Added in: v17.4.0, v16.14.0

- Type: `<bigint>`

The minimum recorded event loop delay.

#### `histogram.percentile(percentile)`#

Added in: v11.10.0

- `percentile` `<number>` A percentile value in the range (0, 100].
- Returns: `<number>`

Returns the value at the given percentile.

#### `histogram.percentileBigInt(percentile)`#

Added in: v17.4.0, v16.14.0

- `percentile` `<number>` A percentile value in the range (0, 100].
- Returns: `<bigint>`

Returns the value at the given percentile.

#### `histogram.percentiles`#

Added in: v11.10.0

- Type: `<Map>`

Returns a `Map` object detailing the accumulated percentile distribution.

#### `histogram.percentilesBigInt`#

Added in: v17.4.0, v16.14.0

- Type: `<Map>`

Returns a `Map` object detailing the accumulated percentile distribution.

#### `histogram.reset()`#

Added in: v11.10.0

Resets the collected histogram data.

#### `histogram.stddev`#

Added in: v11.10.0

- Type: `<number>`

The standard deviation of the recorded event loop delays.

### Class: `ELDHistogram extends Histogram`#

A `Histogram` that records event loop delay, returned by `perf_hooks.monitorEventLoopDelay()`.

#### `histogram.disable()`#

Added in: v11.10.0

- Returns: `<boolean>`

Disables event loop delay sampling. Returns `true` if sampling was stopped, `false` if it was already stopped.

#### `histogram.enable()`#

Added in: v11.10.0

- Returns: `<boolean>`

Enables event loop delay sampling. Returns `true` if sampling was started, `false` if it was already started.

#### `histogram[Symbol.dispose]()`#

Added in: v24.2.0

Disables event loop delay sampling when the histogram is disposed.

    const { monitorEventLoopDelay } = require('node:perf_hooks');
    {
      using hist = monitorEventLoopDelay({ resolution: 20 });
      hist.enable();
      // The histogram will be disabled when the block is exited.
    }
    js

#### Cloning an `ELDHistogram`#

`<ELDHistogram>` instances can be cloned via `<MessagePort>`. On the receiving end, the histogram is cloned as a plain `<Histogram>` object that does not implement the `enable()` and `disable()` methods.

### Class: `RecordableHistogram extends Histogram`#

Added in: v15.9.0, v14.18.0

#### `histogram.add(other)`#

Added in: v17.4.0, v16.14.0

- `other` `<RecordableHistogram>`

Adds the values from `other` to this histogram.

#### `histogram.record(val)`#

Added in: v15.9.0, v14.18.0

- `val` `<number>` | `<bigint>` The amount to record in the histogram.

#### `histogram.recordDelta()`#

Added in: v15.9.0, v14.18.0

Calculates the amount of time (in nanoseconds) that has passed since the previous call to `recordDelta()` and records that amount in the histogram.

### Examples#

#### Measuring the duration of async operations#

The following example uses the Async Hooks and Performance APIs to measure the actual duration of a Timeout operation (including the amount of time it took to execute the callback).

    import { createHook } from 'node:async_hooks';
    import { performance, PerformanceObserver } from 'node:perf_hooks';

    const set = new Set();
    const hook = createHook({
      init(id, type) {
        if (type === 'Timeout') {
          performance.mark(`Timeout-${id}-Init`);
          set.add(id);
        }
      },
      destroy(id) {
        if (set.has(id)) {
          set.delete(id);
          performance.mark(`Timeout-${id}-Destroy`);
          performance.measure(`Timeout-${id}`,
                              `Timeout-${id}-Init`,
                              `Timeout-${id}-Destroy`);
        }
      },
    });
    hook.enable();

    const obs = new PerformanceObserver((list, observer) => {
      console.log(list.getEntries()[0]);
      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ entryTypes: ['measure'], buffered: true });

    setTimeout(() => {}, 1000);
    const async_hooks = require('node:async_hooks');
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');

    const set = new Set();
    const hook = async_hooks.createHook({
      init(id, type) {
        if (type === 'Timeout') {
          performance.mark(`Timeout-${id}-Init`);
          set.add(id);
        }
      },
      destroy(id) {
        if (set.has(id)) {
          set.delete(id);
          performance.mark(`Timeout-${id}-Destroy`);
          performance.measure(`Timeout-${id}`,
                              `Timeout-${id}-Init`,
                              `Timeout-${id}-Destroy`);
        }
      },
    });
    hook.enable();

    const obs = new PerformanceObserver((list, observer) => {
      console.log(list.getEntries()[0]);
      performance.clearMarks();
      performance.clearMeasures();
      observer.disconnect();
    });
    obs.observe({ entryTypes: ['measure'] });

    setTimeout(() => {}, 1000);
    javascript

#### Measuring how long it takes to load dependencies#

The following example measures the duration of `require()` operations to load dependencies:

    import { performance, PerformanceObserver } from 'node:perf_hooks';

    // Activate the observer
    const obs = new PerformanceObserver((list) => {
      const entries = list.getEntries();
      entries.forEach((entry) => {
        console.log(`import('${entry[0]}')`, entry.duration);
      });
      performance.clearMarks();
      performance.clearMeasures();
      obs.disconnect();
    });
    obs.observe({ entryTypes: ['function'], buffered: true });

    const timedImport = performance.timerify(async (module) => {
      return await import(module);
    });

    await timedImport('some-module');
    const {
      performance,
      PerformanceObserver,
    } = require('node:perf_hooks');
    const mod = require('node:module');

    // Monkey patch the require function
    mod.Module.prototype.require =
      performance.timerify(mod.Module.prototype.require);
    require = performance.timerify(require);

    // Activate the observer
    const obs = new PerformanceObserver((list) => {
      const entries = list.getEntries();
      entries.forEach((entry) => {
        console.log(`require('${entry[0]}')`, entry.duration);
      });
      performance.clearMarks();
      performance.clearMeasures();
      obs.disconnect();
    });
    obs.observe({ entryTypes: ['function'] });

    require('some-module');
    javascript

#### Measuring how long one HTTP round-trip takes#

The following example is used to trace the time spent by HTTP client (`OutgoingMessage`) and HTTP request (`IncomingMessage`). For HTTP client, it means the time interval between starting the request and receiving the response, and for HTTP request, it means the time interval between receiving the request and sending the response:

    import { PerformanceObserver } from 'node:perf_hooks';
    import { createServer, get } from 'node:http';

    const obs = new PerformanceObserver((items) => {
      items.getEntries().forEach((item) => {
        console.log(item);
      });
    });

    obs.observe({ entryTypes: ['http'] });

    const PORT = 8080;

    createServer((req, res) => {
      res.end('ok');
    }).listen(PORT, () => {
      get(`http://127.0.0.1:${PORT}`);
    });
    const { PerformanceObserver } = require('node:perf_hooks');
    const http = require('node:http');

    const obs = new PerformanceObserver((items) => {
      items.getEntries().forEach((item) => {
        console.log(item);
      });
    });

    obs.observe({ entryTypes: ['http'] });

    const PORT = 8080;

    http.createServer((req, res) => {
      res.end('ok');
    }).listen(PORT, () => {
      http.get(`http://127.0.0.1:${PORT}`);
    });
    javascript

#### Measuring how long the `net.connect` (only for TCP) takes when the connection is successful#

    import { PerformanceObserver } from 'node:perf_hooks';
    import { connect, createServer } from 'node:net';

    const obs = new PerformanceObserver((items) => {
      items.getEntries().forEach((item) => {
        console.log(item);
      });
    });
    obs.observe({ entryTypes: ['net'] });
    const PORT = 8080;
    createServer((socket) => {
      socket.destroy();
    }).listen(PORT, () => {
      connect(PORT);
    });
    const { PerformanceObserver } = require('node:perf_hooks');
    const net = require('node:net');
    const obs = new PerformanceObserver((items) => {
      items.getEntries().forEach((item) => {
        console.log(item);
      });
    });
    obs.observe({ entryTypes: ['net'] });
    const PORT = 8080;
    net.createServer((socket) => {
      socket.destroy();
    }).listen(PORT, () => {
      net.connect(PORT);
    });
    javascript

#### Measuring how long the DNS takes when the request is successful#

    import { PerformanceObserver } from 'node:perf_hooks';
    import { lookup, promises } from 'node:dns';

    const obs = new PerformanceObserver((items) => {
      items.getEntries().forEach((item) => {
        console.log(item);
      });
    });
    obs.observe({ entryTypes: ['dns'] });
    lookup('localhost', () => {});
    promises.resolve('localhost');
    const { PerformanceObserver } = require('node:perf_hooks');
    const dns = require('node:dns');
    const obs = new PerformanceObserver((items) => {
      items.getEntries().forEach((item) => {
        console.log(item);
      });
    });
    obs.observe({ entryTypes: ['dns'] });
    dns.lookup('localhost', () => {});
    dns.promises.resolve('localhost');
    javascript

## Permissions#

Permissions can be used to control what system resources the Node.js process has access to or what actions the process can take with those resources.

- Process-based permissions control the Node.js process's access to resources. The resource can be entirely allowed or denied, or actions related to it can be controlled. For example, file system reads can be allowed while denying writes. This feature does not protect against malicious code. According to the Node.js Security Policy, Node.js trusts any code it is asked to run.

The permission model implements a "seat belt" approach, which prevents trusted code from unintentionally changing files or using resources that access has not explicitly been granted to. It does not provide security guarantees in the presence of malicious code. Malicious code can bypass the permission model and execute arbitrary code without the restrictions imposed by the permission model.

If you find a potential security vulnerability, please refer to our Security Policy.

### Process-based permissions#

#### Permission Model#

Added in: v20.0.0
History

Version | Changes
--- | ---
v23.5.0, v22.13.0 | This feature is no longer experimental.

Stability: 2 - Stable

The Node.js Permission Model is a mechanism for restricting access to specific resources during execution. The API exists behind a flag `--permission` which when enabled, will restrict access to all available permissions.

The available permissions are documented by the `--permission` flag.

The Permission Model has two operational modes:

- Enforce mode (default when using `--permission`): Access is denied and an `ERR_ACCESS_DENIED` error is thrown for any operation the process has not been granted permission to perform.
- Audit mode (when using `--permission-audit`): Permission checks are performed and violations are published through the diagnostics channel, but access is not denied. Execution continues normally. This mode is useful for discovering what permissions your application requires before deploying with enforce mode.

When starting Node.js with `--permission`, the ability to access the file system through the `fs` module, access the network, spawn processes, use `node:worker_threads`, use native addons, use WASI, use FFI, and enable the runtime inspector will be restricted (the listener for SIGUSR1 won't be created).

    $ node --permission index.js

    Error: Access to this API has been restricted
        at node:internal/main/run_main_module:23:47 {
      code: 'ERR_ACCESS_DENIED',
      permission: 'FileSystemRead',
      resource: '/home/user/index.js'
    }
    console

Allowing access to spawning a process and creating worker threads can be done using the `--allow-child-process` and `--allow-worker` respectively.

To allow network access, use `--allow-net` and for allowing native addons when using permission model, use the `--allow-addons` flag. For WASI, use the `--allow-wasi` flag. For FFI, use the `--allow-ffi` flag. The `node:ffi` module also requires the `--experimental-ffi` flag and is only available in builds with FFI support.

To allow use of OpenSSL STORE loaders, for example to load a private key from a `<URL>` passed to `crypto.createPrivateKey()`, use the `--allow-openssl-store` flag. This flag grants broad authority to configured OpenSSL STORE loaders, which may access files, devices, tokens, or the network. Access performed by a loader is not constrained by the `fs.read`, `fs.write`, or `net` permission scopes.

##### Runtime API#

When enabling the Permission Model through the `--permission` or `--permission-audit` flags, a new property `permission` is added to the `process` object. This property contains the following functions:

###### `permission.has(scope[, reference])`#

API call to check permissions at runtime (`permission.has()`)

    process.permission.has('fs.write'); // true
    process.permission.has('fs.write', '/home/rafaelgss/protected-folder'); // true

    process.permission.has('fs.read'); // true
    process.permission.has('fs.read', '/home/rafaelgss/protected-folder'); // false
    js

###### `permission.drop(scope[, reference])`#

API call to drop permissions at runtime. This operation is irreversible.

When called without a reference, the entire scope is dropped. When called with a reference, only the permission for that specific resource is revoked. Dropping a permission only affects future access checks. It does not close or revoke access to resources that are already open, such as file descriptors, network sockets, child processes, or worker threads. Applications are responsible for closing or terminating those resources when they are no longer needed.

You can only drop the exact resource that was explicitly granted. The reference passed to `drop()` must match the original grant. If a permission was granted using a wildcard (`*`), only the entire scope can be dropped (by calling `drop()` without a reference). If a directory was granted (e.g. `--allow-fs-read=/my/folder`), you cannot drop individual files inside it - you must drop the same directory that was originally granted.

    const fs = require('node:fs');

    // Read config at startup while we still have permission
    const config = fs.readFileSync('/etc/myapp/config.json', 'utf8');

    // Drop read access to /etc/myapp after initialization
    process.permission.drop('fs.read', '/etc/myapp');

    // This will now throw ERR_ACCESS_DENIED
    process.permission.has('fs.read', '/etc/myapp/config.json'); // false

    // Drop child process permission entirely
    process.permission.drop('child');
    js

##### Audit Mode#

The `--permission-audit` flag enables audit mode for the Permission Model. In audit mode, permission checks are performed but access is not denied — no `ERR_ACCESS_DENIED` error is thrown. Instead, each permission violation is published through the `node:diagnostics_channel` module, allowing the application to observe and log which operations would be denied under enforce mode. Execution continues normally.

Audit mode is useful for discovering what permissions your application requires before deploying with `--permission`. It can also be combined with the `--allow-fs-read`, `--allow-fs-write`, `--allow-net`, `--allow-child-process`, `--allow-worker`, `--allow-addons`, `--allow-wasi`, and `--allow-ffi` flags to audit a subset of permissions while granting others.

When a permission check fails in audit mode, a message is published to the diagnostics channel corresponding to the denied scope. The channel names are:

- `node:permission-model:fs` — File System (read and write)
- `node:permission-model:net` — Network
- `node:permission-model:child` — Child Process
- `node:permission-model:worker` — Worker Threads
- `node:permission-model:inspector` — Inspector
- `node:permission-model:wasi` — WASI
- `node:permission-model:addon` — Native Addons
- `node:permission-model:ffi` — FFI

Each message is an object with the following properties:

- `permission` `<string>` The name of the denied permission scope.
- `resource` `<string>` The resource that access was denied to (e.g. a file path or host).

    const diagnostics_channel = require('node:diagnostics_channel');

    diagnostics_channel.channel('node:permission-model:fs').subscribe((msg) => {
      console.log(`Permission denied: ${msg.permission} on ${msg.resource}`);
    });

    // Running with --permission-audit, this publishes a diagnostics channel
    // message but does not throw
    const fs = require('node:fs');
    fs.readFileSync('/etc/passwd');
    js

If both `--permission` and `--permission-audit` are specified, `--permission` takes precedence and the Permission Model runs in enforce mode.

##### File System Permissions#

The Permission Model, by default, restricts access to the file system through the `node:fs` module. It does not guarantee that users will not be able to access the file system through other means, such as through the `node:sqlite` module.

To allow access to the file system, use the `--allow-fs-read` and `--allow-fs-write` flags:

    $ node --permission --allow-fs-read=* --allow-fs-write=* index.js
    Hello world!
    console

By default the entrypoints of your application are included in the allowed file system read list. For example:

    $ node --permission index.js
    console

- `index.js` will be included in the allowed file system read list

    $ node -r /path/to/custom-require.js --permission index.js.
    console

- `/path/to/custom-require.js` will be included in the allowed file system read list.
- `index.js` will be included in the allowed file system read list.

The valid arguments for both flags are:

- `*` - To allow all `FileSystemRead` or `FileSystemWrite` operations, respectively.
- Relative paths to the current working directory.
- Absolute paths.

Example:

- `--allow-fs-read=*` - It will allow all `FileSystemRead` operations.
- `--allow-fs-write=*` - It will allow all `FileSystemWrite` operations.
- `--allow-fs-write=/tmp/` - It will allow `FileSystemWrite` access to the `/tmp/` folder.
- `--allow-fs-read=/tmp/ --allow-fs-read=/home/.gitignore` - It allows `FileSystemRead` access to the `/tmp/` folder and the `/home/.gitignore` path.

Wildcards are supported too:

- `--allow-fs-read=/home/test*` will allow read access to everything that matches the wildcard. e.g: `/home/test/file1` or `/home/test2`

After passing a wildcard character (`*`) all subsequent characters will be ignored. For example: `/home/*.js` will work similar to `/home/*`.

When the permission model is initialized, it will automatically add a wildcard (*) if the specified directory exists. For example, if `/home/test/files` exists, it will be treated as `/home/test/files/*`. However, if the directory does not exist, the wildcard will not be added, and access will be limited to `/home/test/files`. If you want to allow access to a folder that does not exist yet, make sure to explicitly include the wildcard: `/my-path/folder-do-not-exist/*`.

##### Configuration file support#

In addition to passing permission flags on the command line, they can also be declared in a Node.js configuration file when using the experimental [`--experimental-config-file`][] flag. Permission options must be placed inside the `permission` top-level object.

Example `node.config.json`:

    {
      "permission": {
        "allow-fs-read": ["./foo"],
        "allow-fs-write": ["./bar"],
        "allow-child-process": true,
        "allow-worker": true,
        "allow-net": true,
        "allow-addons": false,
        "allow-ffi": false,
        "allow-openssl-store": false
      }
    }
    json

When the `permission` namespace is present in the configuration file, Node.js automatically enables the `--permission` flag. Run with:

    $ node --experimental-default-config-file app.js
    console

##### Using the Permission Model with `npx`#

If you're using `npx` to execute a Node.js script, you can enable the Permission Model by passing the `--node-options` flag. For example:

    npx --node-options="--permission" package-name
    bash

This sets the `NODE_OPTIONS` environment variable for all Node.js processes spawned by `npx`, without affecting the `npx` process itself.

FileSystemRead Error with `npx`

The above command will likely throw a `FileSystemRead` invalid access error because Node.js requires file system read access to locate and execute the package. To avoid this:

1. Using a Globally Installed Package Grant read access to the global `node_modules` directory by running:

       npx --node-options="--permission --allow-fs-read=$(npm prefix -g)" package-name
       bash

2. Using the `npx` Cache If you are installing the package temporarily or relying on the `npx` cache, grant read access to the npm cache directory:

       npx --node-options="--permission --allow-fs-read=$(npm config get cache)" package-name
       bash

Any arguments you would normally pass to `node` (e.g., `--allow-*` flags) can also be passed through the `--node-options` flag. This flexibility makes it easy to configure permissions as needed when using `npx`.

##### Permission Model constraints#

There are constraints you need to know before using this system:

- The model does not inherit to a worker thread.
- When using the Permission Model the following features will be restricted:
  - Native modules
  - Network
  - Child process
  - Worker Threads
  - Inspector protocol
  - File system access
  - WASI
  - FFI
  - OpenSSL STORE loaders
- The Permission Model is initialized after the Node.js environment is set up. However, certain flags such as `--env-file` or `--openssl-config` are designed to read files before environment initialization. As a result, such flags are not subject to the rules of the Permission Model. The same applies for V8 flags that can be set via runtime through `v8.setFlagsFromString`.
- OpenSSL engines cannot be requested at runtime when the Permission Model is enabled, affecting the built-in crypto, https, and tls modules.
- Run-Time Loadable Extensions cannot be loaded when the Permission Model is enabled, affecting the sqlite module.
- Using existing file descriptors via the `node:fs` module bypasses the Permission Model.

##### process._debugProcess() and cross-process Inspector activation#

The `kInspector` permission scope restricts the current process from opening its own V8 Inspector. However, process._debugProcess(pid) — which sends an OS-level signal (SIGUSR1 on POSIX, a remote thread on Windows) to an external process — is not gated by the `kInspector` scope or any other Permission Model scope.

A sandboxed process running under --permission with no additional grants can call process._debugProcess(pid) to force another Node.js process to open its V8 Inspector. The target process does not need to be running under --permission for this to work — any Node.js process running on the same host under the same OS user can be signaled.

This is consistent with the Node.js threat model: Node.js trusts the OS environment in which it runs. Cross-process signaling is an operating-system-level capability; restricting it is the responsibility of the operator (for example, using OS-level process isolation, separate OS users per process, or seccomp/AppArmor profiles on Linux).

Developers relying on --permission to sandbox untrusted code should be aware that:

- process._debugProcess() is callable from any sandboxed process with no grants.
- If a target Node.js process is running on the same host under the same OS user, it can be forced to open its Inspector via this API.
- To prevent this, run sandboxed and target processes under different OS users, or use OS-level isolation mechanisms outside of Node.js.

##### Limitations and Known Issues#

- Symbolic links will be followed even to locations outside of the set of paths that access has been granted to. Relative symbolic links may allow access to arbitrary files and directories. When starting applications with the permission model enabled, you must ensure that no paths to which access has been granted contain relative symbolic links.

## Process#

Source Code: lib/process.js

The `process` object provides information about, and control over, the current Node.js process.

    import process from 'node:process';
    const process = require('node:process');
    javascript

### Process events#

The `process` object is an instance of `EventEmitter`.

#### Event: `'beforeExit'`#

Added in: v0.11.12

The `'beforeExit'` event is emitted when Node.js empties its event loop and has no additional work to schedule. Normally, the Node.js process will exit when there is no work scheduled, but a listener registered on the `'beforeExit'` event can make asynchronous calls, and thereby cause the Node.js process to continue.

The listener callback function is invoked with the value of `process.exitCode` passed as the only argument.

The `'beforeExit'` event is not emitted for conditions causing explicit termination, such as calling `process.exit()` or uncaught exceptions.

The `'beforeExit'` should not be used as an alternative to the `'exit'` event unless the intention is to schedule additional work.

    import process from 'node:process';

    process.on('beforeExit', (code) => {
      console.log('Process beforeExit event with code: ', code);
    });

    process.on('exit', (code) => {
      console.log('Process exit event with code: ', code);
    });

    console.log('This message is displayed first.');

    // Prints:
    // This message is displayed first.
    // Process beforeExit event with code: 0
    // Process exit event with code: 0
    process.on('beforeExit', (code) => {
      console.log('Process beforeExit event with code: ', code);
    });

    process.on('exit', (code) => {
      console.log('Process exit event with code: ', code);
    });

    console.log('This message is displayed first.');

    // Prints:
    // This message is displayed first.
    // Process beforeExit event with code: 0
    // Process exit event with code: 0
    javascript

#### Event: `'disconnect'`#

Added in: v0.7.7

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the `'disconnect'` event will be emitted when the IPC channel is closed.

#### Event: `'exit'`#

Added in: v0.1.7

- `code` `<integer>`

The `'exit'` event is emitted when the Node.js process is about to exit as a result of either:

- The `process.exit()` method being called explicitly;
- The Node.js event loop no longer having any additional work to perform.

There is no way to prevent the exiting of the event loop at this point, and once all `'exit'` listeners have finished running the Node.js process will terminate.

The listener callback function is invoked with the exit code specified either by the `process.exitCode` property, or the `exitCode` argument passed to the `process.exit()` method.

    import process from 'node:process';

    process.on('exit', (code) => {
      console.log(`About to exit with code: ${code}`);
    });
    process.on('exit', (code) => {
      console.log(`About to exit with code: ${code}`);
    });
    javascript

Listener functions must only perform synchronous operations. The Node.js process will exit immediately after calling the `'exit'` event listeners causing any additional work still queued in the event loop to be abandoned. In the following example, for instance, the timeout will never occur:

    import process from 'node:process';

    process.on('exit', (code) => {
      setTimeout(() => {
        console.log('This will not run');
      }, 0);
    });
    process.on('exit', (code) => {
      setTimeout(() => {
        console.log('This will not run');
      }, 0);
    });
    javascript

#### Event: `'message'`#

Added in: v0.5.10

- `message` `<Object>` | `<boolean>` | `<number>` | `<string>` | `<null>` a parsed JSON object or a serializable primitive value.
- `sendHandle` `<net.Server>` | `<net.Socket>` a `net.Server` or `net.Socket` object, or undefined.

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the `'message'` event is emitted whenever a message sent by a parent process using `childprocess.send()` is received by the child process.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the `serialization` option was set to `advanced` used when spawning the process, the `message` argument can contain data that JSON is not able to represent. See Advanced serialization for `child_process` for more details.

#### Event: `'rejectionHandled'`#

Added in: v1.4.1

- `promise` `<Promise>` The late handled promise.

The `'rejectionHandled'` event is emitted whenever a `Promise` has been rejected and an error handler was attached to it (using `promise.catch()`, for example) later than one turn of the Node.js event loop.

The `Promise` object would have previously been emitted in an `'unhandledRejection'` event, but during the course of processing gained a rejection handler.

There is no notion of a top level for a `Promise` chain at which rejections can always be handled. Being inherently asynchronous in nature, a `Promise` rejection can be handled at a future point in time, possibly much later than the event loop turn it takes for the `'unhandledRejection'` event to be emitted.

Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.

In synchronous code, the `'uncaughtException'` event is emitted when the list of unhandled exceptions grows.

In asynchronous code, the `'unhandledRejection'` event is emitted when the list of unhandled rejections grows, and the `'rejectionHandled'` event is emitted when the list of unhandled rejections shrinks.

    import process from 'node:process';

    const unhandledRejections = new Map();
    process.on('unhandledRejection', (reason, promise) => {
      unhandledRejections.set(promise, reason);
    });
    process.on('rejectionHandled', (promise) => {
      unhandledRejections.delete(promise);
    });
    const unhandledRejections = new Map();
    process.on('unhandledRejection', (reason, promise) => {
      unhandledRejections.set(promise, reason);
    });
    process.on('rejectionHandled', (promise) => {
      unhandledRejections.delete(promise);
    });
    javascript

In this example, the `unhandledRejections` `Map` will grow and shrink over time, reflecting rejections that start unhandled and then become handled. It is possible to record such errors in an error log, either periodically (which is likely best for long-running application) or upon process exit (which is likely most convenient for scripts).

#### Event: `'workerMessage'`#

Added in: v22.5.0, v20.19.0

- `value` `<any>` A value transmitted using `postMessageToThread()`.
- `source` `<number>` The transmitting worker thread ID or `0` for the main thread.

The `'workerMessage'` event is emitted for any incoming message send by the other party by using `postMessageToThread()`.

#### Event: `'uncaughtException'`#

Added in: v0.1.18
History

Version | Changes
--- | ---
v12.0.0, v10.17.0 | Added the `origin` argument.

- `err` `<Error>` The uncaught exception.
- `origin` `<string>` Indicates if the exception originates from an unhandled rejection or from a synchronous error. Can either be `'uncaughtException'` or `'unhandledRejection'`. The latter is used when an exception happens in a `Promise` based async context (or if a `Promise` is rejected) and `--unhandled-rejections` flag set to `strict` or `throw` (which is the default) and the rejection is not handled, or when a rejection happens during the command line entry point's ES module static loading phase.

The `'uncaughtException'` event is emitted when an uncaught JavaScript exception bubbles all the way back to the event loop. By default, Node.js handles such exceptions by printing the stack trace to `stderr` and exiting with code 1, overriding any previously set `process.exitCode`. Adding a handler for the `'uncaughtException'` event overrides this default behavior. Alternatively, change the `process.exitCode` in the `'uncaughtException'` handler which will result in the process exiting with the provided exit code. Otherwise, in the presence of such handler the process will exit with 0.

    import process from 'node:process';
    import fs from 'node:fs';

    process.on('uncaughtException', (err, origin) => {
      fs.writeSync(
        process.stderr.fd,
        `Caught exception: ${err}\n` +
        `Exception origin: ${origin}\n`,
      );
    });

    setTimeout(() => {
      console.log('This will still run.');
    }, 500);

    // Intentionally cause an exception, but don't catch it.
    nonexistentFunc();
    console.log('This will not run.');
    const fs = require('node:fs');

    process.on('uncaughtException', (err, origin) => {
      fs.writeSync(
        process.stderr.fd,
        `Caught exception: ${err}\n` +
        `Exception origin: ${origin}\n`,
      );
    });

    setTimeout(() => {
      console.log('This will still run.');
    }, 500);

    // Intentionally cause an exception, but don't catch it.
    nonexistentFunc();
    console.log('This will not run.');
    javascript

It is possible to monitor `'uncaughtException'` events without overriding the default behavior to exit the process by installing a `'uncaughtExceptionMonitor'` listener.

##### Warning: Using `'uncaughtException'` correctly#

`'uncaughtException'` is a crude mechanism for exception handling intended to be used only as a last resort. The event should not be used as an equivalent to `On Error Resume Next`. Unhandled exceptions inherently mean that an application is in an undefined state. Attempting to resume application code without properly recovering from the exception can cause additional unforeseen and unpredictable issues.

Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.

Attempting to resume normally after an uncaught exception can be similar to pulling out the power cord when upgrading a computer. Nine out of ten times, nothing happens. But the tenth time, the system becomes corrupted.

The correct use of `'uncaughtException'` is to perform synchronous cleanup of allocated resources (e.g. file descriptors, handles, etc) before shutting down the process. It is not safe to resume normal operation after `'uncaughtException'`.

To restart a crashed application in a more reliable way, whether `'uncaughtException'` is emitted or not, an external monitor should be employed in a separate process to detect application failures and recover or restart as needed.

#### Event: `'uncaughtExceptionMonitor'`#

Added in: v13.7.0, v12.17.0

- `err` `<Error>` The uncaught exception.
- `origin` `<string>` Indicates if the exception originates from an unhandled rejection or from synchronous errors. Can either be `'uncaughtException'` or `'unhandledRejection'`. The latter is used when an exception happens in a `Promise` based async context (or if a `Promise` is rejected) and `--unhandled-rejections` flag set to `strict` or `throw` (which is the default) and the rejection is not handled, or when a rejection happens during the command line entry point's ES module static loading phase.

The `'uncaughtExceptionMonitor'` event is emitted before an `'uncaughtException'` event is emitted or a hook installed via `process.setUncaughtExceptionCaptureCallback()` is called.

Installing an `'uncaughtExceptionMonitor'` listener does not change the behavior once an `'uncaughtException'` event is emitted. The process will still crash if no `'uncaughtException'` listener is installed.

    import process from 'node:process';

    process.on('uncaughtExceptionMonitor', (err, origin) => {
      MyMonitoringTool.logSync(err, origin);
    });

    // Intentionally cause an exception, but don't catch it.
    nonexistentFunc();
    // Still crashes Node.js
    process.on('uncaughtExceptionMonitor', (err, origin) => {
      MyMonitoringTool.logSync(err, origin);
    });

    // Intentionally cause an exception, but don't catch it.
    nonexistentFunc();
    // Still crashes Node.js
    javascript

#### Event: `'unhandledRejection'`#

Added in: v1.4.1
History

Version | Changes
--- | ---
v7.0.0 | Not handling `Promise` rejections is deprecated.
v6.6.0 | Unhandled `Promise` rejections will now emit a process warning.

- `reason` `<Error>` | `<any>` The object with which the promise was rejected (typically an `Error` object).
- `promise` `<Promise>` The rejected promise.

The `'unhandledRejection'` event is emitted whenever a `Promise` is rejected and no error handler is attached to the promise within a turn of the event loop. When programming with Promises, exceptions are encapsulated as "rejected promises". Rejections can be caught and handled using `promise.catch()` and are propagated through a `Promise` chain. The `'unhandledRejection'` event is useful for detecting and keeping track of promises that were rejected whose rejections have not yet been handled.

    import process from 'node:process';

    process.on('unhandledRejection', (reason, promise) => {
      console.log('Unhandled Rejection at:', promise, 'reason:', reason);
      // Application specific logging, throwing an error, or other logic here
    });

    somePromise.then((res) => {
      return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
    }); // No `.catch()` or `.then()`
    process.on('unhandledRejection', (reason, promise) => {
      console.log('Unhandled Rejection at:', promise, 'reason:', reason);
      // Application specific logging, throwing an error, or other logic here
    });

    somePromise.then((res) => {
      return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
    }); // No `.catch()` or `.then()`
    javascript

The following will also trigger the `'unhandledRejection'` event to be emitted:

    import process from 'node:process';

    function SomeResource() {
      // Initially set the loaded status to a rejected promise
      this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
    }

    const resource = new SomeResource();
    // no .catch or .then on resource.loaded for at least a turn
    function SomeResource() {
      // Initially set the loaded status to a rejected promise
      this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
    }

    const resource = new SomeResource();
    // no .catch or .then on resource.loaded for at least a turn
    javascript

In this example case, it is possible to track the rejection as a developer error as would typically be the case for other `'unhandledRejection'` events. To address such failures, a non-operational `.catch(() => { })` handler may be attached to `resource.loaded`, which would prevent the `'unhandledRejection'` event from being emitted.

If an `'unhandledRejection'` event is emitted but not handled it will be raised as an uncaught exception. This alongside other behaviors of `'unhandledRejection'` events can changed via the `--unhandled-rejections` flag.

#### Event: `'warning'`#

Added in: v6.0.0

- `warning` `<Error>` Key properties of the warning are:
  - `name` `<string>` The name of the warning. Default: `'Warning'`.
  - `message` `<string>` A system-provided description of the warning.
  - `stack` `<string>` A stack trace to the location in the code where the warning was issued.

The `'warning'` event is emitted whenever Node.js emits a process warning.

A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs, or security vulnerabilities.

    import process from 'node:process';

    process.on('warning', (warning) => {
      console.warn(warning.name);    // Print the warning name
      console.warn(warning.message); // Print the warning message
      console.warn(warning.stack);   // Print the stack trace
    });
    process.on('warning', (warning) => {
      console.warn(warning.name);    // Print the warning name
      console.warn(warning.message); // Print the warning message
      console.warn(warning.stack);   // Print the stack trace
    });
    javascript

By default, Node.js will print process warnings to `stderr`. The `--no-warnings` command-line option can be used to suppress the default console output but the `'warning'` event will still be emitted by the `process` object. Currently, it is not possible to suppress specific warning types other than deprecation warnings. To suppress deprecation warnings, check out the `--no-deprecation` flag.

The following example illustrates the warning that is printed to `stderr` when too many listeners have been added to an event:

    $ node
    > events.defaultMaxListeners = 1;
    > process.on('foo', () => {});
    > process.on('foo', () => {});
    > (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
    detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit
    console

In contrast, the following example turns off the default warning output and adds a custom handler to the `'warning'` event:

    $ node --no-warnings
    > const p = process.on('warning', (warning) => console.warn('Do not do that!'));
    > events.defaultMaxListeners = 1;
    > process.on('foo', () => {});
    > process.on('foo', () => {});
    > Do not do that!
    console

The `--trace-warnings` command-line option can be used to have the default console output for warnings include the full stack trace of the warning.

Launching Node.js using the `--throw-deprecation` command-line flag will cause custom deprecation warnings to be thrown as exceptions.

Using the `--trace-deprecation` command-line flag will cause the custom deprecation to be printed to `stderr` along with the stack trace.

Using the `--no-deprecation` command-line flag will suppress all reporting of the custom deprecation.

The `*-deprecation` command-line flags only affect warnings that use the name `'DeprecationWarning'`.

##### Emitting custom warnings#

See the `process.emitWarning()` method for issuing custom or application-specific warnings.

##### Node.js warning names#

There are no strict guidelines for warning types (as identified by the `name` property) emitted by Node.js. New types of warnings can be added at any time. A few of the warning types that are most common include:

- `'DeprecationWarning'` - Indicates use of a deprecated Node.js API or feature. Such warnings must include a `'code'` property identifying the deprecation code.
- `'ExperimentalWarning'` - Indicates use of an experimental Node.js API or feature. Such features must be used with caution as they may change at any time and are not subject to the same strict semantic-versioning and long-term support policies as supported features.
- `'MaxListenersExceededWarning'` - Indicates that too many listeners for a given event have been registered on either an `EventEmitter` or `EventTarget`. This is often an indication of a memory leak.
- `'TimeoutOverflowWarning'` - Indicates that a numeric value that cannot fit within a 32-bit signed integer has been provided to either the `setTimeout()` or `setInterval()` functions.
- `'TimeoutNegativeWarning'` - Indicates that a negative number has provided to either the `setTimeout()` or `setInterval()` functions.
- `'TimeoutNaNWarning'` - Indicates that a value which is not a number has provided to either the `setTimeout()` or `setInterval()` functions.
- `'UnsupportedWarning'` - Indicates use of an unsupported option or feature that will be ignored rather than treated as an error. One example is use of the HTTP response status message when using the HTTP/2 compatibility API.

#### Event: `'worker'`#

Added in: v16.2.0, v14.18.0

- `worker` `<Worker>` The `<Worker>` that was created.

The `'worker'` event is emitted after a new `<Worker>` thread has been created.

#### Signal events#

Signal events will be emitted when the Node.js process receives a signal. Please refer to `signal(7)` for a listing of standard POSIX signal names such as `'SIGINT'`, `'SIGHUP'`, etc.

Signals are not available on `Worker` threads.

The signal handler will receive the signal's name (`'SIGINT'`, `'SIGTERM'`, etc.) as the first argument.

The name of each event will be the uppercase common name for the signal (e.g. `'SIGINT'` for `SIGINT` signals).

    import process from 'node:process';

    // Begin reading from stdin so the process does not exit.
    process.stdin.resume();

    process.on('SIGINT', () => {
      console.log('Received SIGINT. Press Control-D to exit.');
    });

    // Using a single function to handle multiple signals
    function handle(signal) {
      console.log(`Received ${signal}`);
    }

    process.on('SIGINT', handle);
    process.on('SIGTERM', handle);
    // Begin reading from stdin so the process does not exit.
    process.stdin.resume();

    process.on('SIGINT', () => {
      console.log('Received SIGINT. Press Control-D to exit.');
    });

    // Using a single function to handle multiple signals
    function handle(signal) {
      console.log(`Received ${signal}`);
    }

    process.on('SIGINT', handle);
    process.on('SIGTERM', handle);
    javascript

- `'SIGUSR1'` is reserved by Node.js to start the debugger. It's possible to install a listener but doing so might interfere with the debugger.
- `'SIGTERM'` and `'SIGINT'` have default handlers on non-Windows platforms that reset the terminal mode before exiting with code `128 + signal number`. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).
- `'SIGPIPE'` is ignored by default. It can have a listener installed.
- `'SIGHUP'` is generated on Windows when the console window is closed, and on other platforms under various similar conditions. See `signal(7)`. It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior of `SIGHUP` is to terminate Node.js, but once a listener has been installed its default behavior will be removed.
- `'SIGTERM'` is not supported on Windows, it can be listened on.
- `'SIGINT'` from the terminal is supported on all platforms, and can usually be generated with `Ctrl`+`C` (though this may be configurable). It is not generated when terminal raw mode is enabled and `Ctrl`+`C` is used.
- `'SIGBREAK'` is delivered on Windows when `Ctrl`+`Break` is pressed. On non-Windows platforms, it can be listened on, but there is no way to send or generate it.
- `'SIGWINCH'` is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.
- `'SIGKILL'` cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.
- `'SIGSTOP'` cannot have a listener installed.
- `'SIGBUS'`, `'SIGFPE'`, `'SIGSEGV'`, and `'SIGILL'`, when not raised artificially using `kill(2)`, inherently leave the process in a state from which it is not safe to call JS listeners. Doing so might cause the process to stop responding.
- `0` can be sent to test for the existence of a process, it has no effect if the process exists, but will throw an error if the process does not exist.

Windows does not support signals so has no equivalent to termination by signal, but Node.js offers some emulation with `process.kill()`, and `subprocess.kill()`:

- Sending `SIGINT`, `SIGTERM`, and `SIGKILL` will cause the unconditional termination of the target process, and afterwards, subprocess will report that the process was terminated by signal.
- Sending signal `0` can be used as a platform independent way to test for the existence of a process.

### `process.abort()`#

Added in: v0.7.0

The `process.abort()` method causes the Node.js process to exit immediately and generate a core file.

This feature is not available in `Worker` threads.

### `process.addUncaughtExceptionCaptureCallback(fn)`#

Added in: v25.9.0

Stability: 1 - Experimental

- `fn` `<Function>`

The `process.addUncaughtExceptionCaptureCallback()` function adds a callback that will be invoked when an uncaught exception occurs, receiving the exception value as its first argument.

Unlike `process.setUncaughtExceptionCaptureCallback()`, this function allows multiple callbacks to be registered and does not conflict with the `domain` module. Callbacks are called in reverse order of registration (most recent first). If a callback returns `true`, subsequent callbacks and the default uncaught exception handling are skipped.

    import process from 'node:process';

    process.addUncaughtExceptionCaptureCallback((err) => {
      console.error('Caught exception:', err.message);
      return true; // Indicates exception was handled
    });
    process.addUncaughtExceptionCaptureCallback((err) => {
      console.error('Caught exception:', err.message);
      return true; // Indicates exception was handled
    });
    javascript

### `process.allowedNodeEnvironmentFlags`#

Added in: v10.10.0

- Type: `<Set>`

The `process.allowedNodeEnvironmentFlags` property is a special, read-only `Set` of flags allowable within the `NODE_OPTIONS` environment variable.

`process.allowedNodeEnvironmentFlags` extends `Set`, but overrides `Set.prototype.has` to recognize several different possible flag representations. `process.allowedNodeEnvironmentFlags.has()` will return `true` in the following cases:

- Flags may omit leading single (`-`) or double (`--`) dashes; e.g., `inspect-brk` for `--inspect-brk`, or `r` for `-r`.
- Flags passed through to V8 (as listed in `--v8-options`) may replace one or more non-leading dashes for an underscore, or vice-versa; e.g., `--perf_basic_prof`, `--perf-basic-prof`, `--perf_basic-prof`, etc.
- Flags may contain one or more equals (`=`) characters; all characters after and including the first equals will be ignored; e.g., `--stack-trace-limit=100`.
- Flags must be allowable within `NODE_OPTIONS`.

When iterating over `process.allowedNodeEnvironmentFlags`, flags will appear only once; each will begin with one or more dashes. Flags passed through to V8 will contain underscores instead of non-leading dashes:

    import { allowedNodeEnvironmentFlags } from 'node:process';

    allowedNodeEnvironmentFlags.forEach((flag) => {
      // -r
      // --inspect-brk
      // --abort_on_uncaught_exception
      // ...
    });
    const { allowedNodeEnvironmentFlags } = require('node:process');

    allowedNodeEnvironmentFlags.forEach((flag) => {
      // -r
      // --inspect-brk
      // --abort_on_uncaught_exception
      // ...
    });
    javascript

The methods `add()`, `clear()`, and `delete()` of `process.allowedNodeEnvironmentFlags` do nothing, and will fail silently.

If Node.js was compiled without `NODE_OPTIONS` support (shown in `process.config`), `process.allowedNodeEnvironmentFlags` will contain what would have been allowable.

### `process.arch`#

Added in: v0.5.0

- Type: `<string>`

The operating system CPU architecture for which the Node.js binary was compiled. Possible values are: `'arm'`, `'arm64'`, `'ia32'`, `'loong64'`, `'mips'`, `'mipsel'`, `'ppc64'`, `'riscv64'`, `'s390'`, `'s390x'`, and `'x64'`.

    import { arch } from 'node:process';

    console.log(`This processor architecture is ${arch}`);
    const { arch } = require('node:process');

    console.log(`This processor architecture is ${arch}`);
    javascript

### `process.argv`#

Added in: v0.1.27

- Type: `<string>`[]

The `process.argv` property returns an array containing the command-line arguments passed when the Node.js process was launched. The first element will be `process.execPath`. See `process.argv0` if access to the original value of `argv[0]` is needed. If a program entry point was provided, the second element will be the absolute path to it. The remaining elements are additional command-line arguments.

For example, assuming the following script for `process-args.js`:

    import { argv } from 'node:process';

    // print process.argv
    argv.forEach((val, index) => {
      console.log(`${index}: ${val}`);
    });
    const { argv } = require('node:process');

    // print process.argv
    argv.forEach((val, index) => {
      console.log(`${index}: ${val}`);
    });
    javascript

Launching the Node.js process as:

    node process-args.js one two=three four
    bash

Would generate the output:

    0: /usr/local/bin/node
    1: /Users/mjr/work/node/process-args.js
    2: one
    3: two=three
    4: four
    text

### `process.argv0`#

Added in: v6.4.0

- Type: `<string>`

The `process.argv0` property stores a read-only copy of the original value of `argv[0]` passed when Node.js starts.

    $ bash -c 'exec -a customArgv0 ./node'
    > process.argv[0]
    '/Volumes/code/external/node/out/Release/node'
    > process.argv0
    'customArgv0'
    console

### `process.availableMemory()`#

Added in: v22.0.0, v20.13.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Change stability index for this feature from Experimental to Stable.

- Returns: `<number>`

Gets the amount of free memory that is still available to the process (in bytes).

See `uv_get_available_memory` for more information.

### `process.channel`#

Added in: v7.1.0
History

Version | Changes
--- | ---
v14.0.0 | The object no longer accidentally exposes native C++ bindings.

- Type: `<Object>`

If the Node.js process was spawned with an IPC channel (see the Child Process documentation), the `process.channel` property is a reference to the IPC channel. If no IPC channel exists, this property is `undefined`.

#### `process.channel.ref()`#

Added in: v7.1.0

This method makes the IPC channel keep the event loop of the process running if `.unref()` has been called before.

Typically, this is managed through the number of `'disconnect'` and `'message'` listeners on the `process` object. However, this method can be used to explicitly request a specific behavior.

#### `process.channel.unref()`#

Added in: v7.1.0

This method makes the IPC channel not keep the event loop of the process running, and lets it finish even while the channel is open.

Typically, this is managed through the number of `'disconnect'` and `'message'` listeners on the `process` object. However, this method can be used to explicitly request a specific behavior.

### `process.chdir(directory)`#

Added in: v0.1.17

- `directory` `<string>`

The `process.chdir()` method changes the current working directory of the Node.js process or throws an exception if doing so fails (for instance, if the specified `directory` does not exist).

    import { chdir, cwd } from 'node:process';

    console.log(`Starting directory: ${cwd()}`);
    try {
      chdir('/tmp');
      console.log(`New directory: ${cwd()}`);
    } catch (err) {
      console.error(`chdir: ${err}`);
    }
    const { chdir, cwd } = require('node:process');

    console.log(`Starting directory: ${cwd()}`);
    try {
      chdir('/tmp');
      console.log(`New directory: ${cwd()}`);
    } catch (err) {
      console.error(`chdir: ${err}`);
    }
    javascript

This feature is not available in `Worker` threads.

### `process.config`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v19.0.0 | The `process.config` object is now frozen.
v16.0.0 | Modifying process.config has been deprecated.

- Type: `<Object>`

The `process.config` property returns a frozen `Object` containing the JavaScript representation of the configure options used to compile the current Node.js executable. This is the same as the `config.gypi` file that was produced when running the `./configure` script.

An example of the possible output looks like:

    {
      "target_defaults":
       { "cflags": [],
         "default_configuration": "Release",
         "defines": [],
         "include_dirs": [],
         "libraries": [] },
      "variables":
       {
         "host_arch": "x64",
         "napi_build_version": 5,
         "node_install_npm": "true",
         "node_prefix": "",
         "node_shared_cares": "false",
         "node_shared_http_parser": "false",
         "node_shared_libuv": "false",
         "node_shared_zlib": "false",
         "node_use_openssl": "true",
         "node_shared_openssl": "false",
         "target_arch": "x64",
         "v8_use_snapshot": 1
       }
    }
    json

### `process.connected`#

Added in: v0.7.2

- Type: `<boolean>`

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the `process.connected` property will return `true` so long as the IPC channel is connected and will return `false` after `process.disconnect()` is called.

Once `process.connected` is `false`, it is no longer possible to send messages over the IPC channel using `process.send()`.

### `process.constrainedMemory()`#

Added in: v19.6.0, v18.15.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Change stability index for this feature from Experimental to Stable.
v22.0.0, v20.13.0 | Aligned return value with `uv_get_constrained_memory`.

- Returns: `<number>`

Gets the amount of memory available to the process (in bytes) based on limits imposed by the OS. If there is no such constraint, or the constraint is unknown, `0` is returned.

See `uv_get_constrained_memory` for more information.

### `process.cpuUsage([previousValue])`#

Added in: v6.1.0

- `previousValue` `<Object>` A previous return value from calling `process.cpuUsage()`
- Returns: `<Object>`
  - `user` `<integer>`
  - `system` `<integer>`

The `process.cpuUsage()` method returns the user and system CPU time usage of the current process, in an object with properties `user` and `system`, whose values are microsecond values (millionth of a second). These values measure time spent in user and system code respectively, and may end up being greater than actual elapsed time if multiple CPU cores are performing work for this process.

The result of a previous call to `process.cpuUsage()` can be passed as the argument to the function, to get a diff reading.

    import { cpuUsage } from 'node:process';

    const startUsage = cpuUsage();
    // { user: 38579, system: 6986 }

    // spin the CPU for 500 milliseconds
    const now = Date.now();
    while (Date.now() - now < 500);

    console.log(cpuUsage(startUsage));
    // { user: 514883, system: 11226 }
    const { cpuUsage } = require('node:process');

    const startUsage = cpuUsage();
    // { user: 38579, system: 6986 }

    // spin the CPU for 500 milliseconds
    const now = Date.now();
    while (Date.now() - now < 500);

    console.log(cpuUsage(startUsage));
    // { user: 514883, system: 11226 }
    javascript

### `process.cwd()`#

Added in: v0.1.8

- Returns: `<string>`

The `process.cwd()` method returns the current working directory of the Node.js process.

    import { cwd } from 'node:process';

    console.log(`Current directory: ${cwd()}`);
    const { cwd } = require('node:process');

    console.log(`Current directory: ${cwd()}`);
    javascript

### `process.debugPort`#

Added in: v0.7.2

- Type: `<number>`

The port used by the Node.js debugger when enabled.

    import process from 'node:process';

    process.debugPort = 5858;
    process.debugPort = 5858;
    javascript

### `process.disconnect()`#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the `process.disconnect()` method will close the IPC channel to the parent process, allowing the child process to exit gracefully once there are no other connections keeping it alive.

The effect of calling `process.disconnect()` is the same as calling `ChildProcess.disconnect()` from the parent process.

If the Node.js process was not spawned with an IPC channel, `process.disconnect()` will be `undefined`.

### `process.dlopen(module, filename[, flags])`#

Added in: v0.1.16
History

Version | Changes
--- | ---
v9.0.0 | Added support for the `flags` argument.

- `module` `<Object>`
- `filename` `<string>`
- `flags` `<os.constants.dlopen>` Default: `os.constants.dlopen.RTLD_LAZY`

The `process.dlopen()` method allows dynamically loading shared objects. It is primarily used by `require()` to load C++ Addons, and should not be used directly, except in special cases. In other words, `require()` should be preferred over `process.dlopen()` unless there are specific reasons such as custom dlopen flags or loading from ES modules.

The `flags` argument is an integer that allows to specify dlopen behavior. See the `os.constants.dlopen` documentation for details.

An important requirement when calling `process.dlopen()` is that the `module` instance must be passed. Functions exported by the C++ Addon are then accessible via `module.exports`.

The example below shows how to load a C++ Addon, named `local.node`, that exports a `foo` function. All the symbols are loaded before the call returns, by passing the `RTLD_NOW` constant. In this example the constant is assumed to be available.

    import { dlopen } from 'node:process';
    import { constants } from 'node:os';
    import { fileURLToPath } from 'node:url';

    const module = { exports: {} };
    dlopen(module, fileURLToPath(new URL('local.node', import.meta.url)),
           constants.dlopen.RTLD_NOW);
    module.exports.foo();
    const { dlopen } = require('node:process');
    const { constants } = require('node:os');
    const { join } = require('node:path');

    const module = { exports: {} };
    dlopen(module, join(__dirname, 'local.node'), constants.dlopen.RTLD_NOW);
    module.exports.foo();
    javascript

### `process.emitWarning(warning[, options])`#

Added in: v8.0.0

- `warning` `<string>` | `<Error>` The warning to emit.
- `options` `<Object>`
  - `type` `<string>` When `warning` is a `String`, `type` is the name to use for the type of warning being emitted. Default: `'Warning'`.
  - `code` `<string>` A unique identifier for the warning instance being emitted.
  - `ctor` `<Function>` When `warning` is a `String`, `ctor` is an optional function used to limit the generated stack trace. Default: `process.emitWarning`.
  - `detail` `<string>` Additional text to include with the error.

The `process.emitWarning()` method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the `'warning'` event.

    import { emitWarning } from 'node:process';

    // Emit a warning with a code and additional detail.
    emitWarning('Something happened!', {
      code: 'MY_WARNING',
      detail: 'This is some additional information',
    });
    // Emits:
    // (node:56338) [MY_WARNING] Warning: Something happened!
    // This is some additional information
    const { emitWarning } = require('node:process');

    // Emit a warning with a code and additional detail.
    emitWarning('Something happened!', {
      code: 'MY_WARNING',
      detail: 'This is some additional information',
    });
    // Emits:
    // (node:56338) [MY_WARNING] Warning: Something happened!
    // This is some additional information
    javascript

In this example, an `Error` object is generated internally by `process.emitWarning()` and passed through to the `'warning'` handler.

    import process from 'node:process';

    process.on('warning', (warning) => {
      console.warn(warning.name);    // 'Warning'
      console.warn(warning.message); // 'Something happened!'
      console.warn(warning.code);    // 'MY_WARNING'
      console.warn(warning.stack);   // Stack trace
      console.warn(warning.detail);  // 'This is some additional information'
    });
    process.on('warning', (warning) => {
      console.warn(warning.name);    // 'Warning'
      console.warn(warning.message); // 'Something happened!'
      console.warn(warning.code);    // 'MY_WARNING'
      console.warn(warning.stack);   // Stack trace
      console.warn(warning.detail);  // 'This is some additional information'
    });
    javascript

If `warning` is passed as an `Error` object, the `options` argument is ignored.

### `process.emitWarning(warning[, type[, code]][, ctor])`#

Added in: v6.0.0

- `warning` `<string>` | `<Error>` The warning to emit.
- `type` `<string>` When `warning` is a `String`, `type` is the name to use for the type of warning being emitted. Default: `'Warning'`.
- `code` `<string>` A unique identifier for the warning instance being emitted.
- `ctor` `<Function>` When `warning` is a `String`, `ctor` is an optional function used to limit the generated stack trace. Default: `process.emitWarning`.

The `process.emitWarning()` method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the `'warning'` event.

    import { emitWarning } from 'node:process';

    // Emit a warning using a string.
    emitWarning('Something happened!');
    // Emits: (node: 56338) Warning: Something happened!
    const { emitWarning } = require('node:process');

    // Emit a warning using a string.
    emitWarning('Something happened!');
    // Emits: (node: 56338) Warning: Something happened!
    javascript

    import { emitWarning } from 'node:process';

    // Emit a warning using a string and a type.
    emitWarning('Something Happened!', 'CustomWarning');
    // Emits: (node:56338) CustomWarning: Something Happened!
    const { emitWarning } = require('node:process');

    // Emit a warning using a string and a type.
    emitWarning('Something Happened!', 'CustomWarning');
    // Emits: (node:56338) CustomWarning: Something Happened!
    javascript

    import { emitWarning } from 'node:process';

    emitWarning('Something happened!', 'CustomWarning', 'WARN001');
    // Emits: (node:56338) [WARN001] CustomWarning: Something happened!
    const { emitWarning } = require('node:process');

    process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
    // Emits: (node:56338) [WARN001] CustomWarning: Something happened!
    javascript

In each of the previous examples, an `Error` object is generated internally by `process.emitWarning()` and passed through to the `'warning'` handler.

    import process from 'node:process';

    process.on('warning', (warning) => {
      console.warn(warning.name);
      console.warn(warning.message);
      console.warn(warning.code);
      console.warn(warning.stack);
    });
    process.on('warning', (warning) => {
      console.warn(warning.name);
      console.warn(warning.message);
      console.warn(warning.code);
      console.warn(warning.stack);
    });
    javascript

If `warning` is passed as an `Error` object, it will be passed through to the `'warning'` event handler unmodified (and the optional `type`, `code` and `ctor` arguments will be ignored):

    import { emitWarning } from 'node:process';

    // Emit a warning using an Error object.
    const myWarning = new Error('Something happened!');
    // Use the Error name property to specify the type name
    myWarning.name = 'CustomWarning';
    myWarning.code = 'WARN001';

    emitWarning(myWarning);
    // Emits: (node:56338) [WARN001] CustomWarning: Something happened!
    const { emitWarning } = require('node:process');

    // Emit a warning using an Error object.
    const myWarning = new Error('Something happened!');
    // Use the Error name property to specify the type name
    myWarning.name = 'CustomWarning';
    myWarning.code = 'WARN001';

    emitWarning(myWarning);
    // Emits: (node:56338) [WARN001] CustomWarning: Something happened!
    javascript

A `TypeError` is thrown if `warning` is anything other than a string or `Error` object.

While process warnings use `Error` objects, the process warning mechanism is not a replacement for normal error handling mechanisms.

The following additional handling is implemented if the warning `type` is `'DeprecationWarning'`:

- If the `--throw-deprecation` command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event.
- If the `--no-deprecation` command-line flag is used, the deprecation warning is suppressed.
- If the `--trace-deprecation` command-line flag is used, the deprecation warning is printed to `stderr` along with the full stack trace.

#### Avoiding duplicate warnings#

As a best practice, warnings should be emitted only once per process. To do so, place the `emitWarning()` behind a boolean.

    import { emitWarning } from 'node:process';

    function emitMyWarning() {
      if (!emitMyWarning.warned) {
        emitMyWarning.warned = true;
        emitWarning('Only warn once!');
      }
    }
    emitMyWarning();
    // Emits: (node: 56339) Warning: Only warn once!
    emitMyWarning();
    // Emits nothing
    const { emitWarning } = require('node:process');

    function emitMyWarning() {
      if (!emitMyWarning.warned) {
        emitMyWarning.warned = true;
        emitWarning('Only warn once!');
      }
    }
    emitMyWarning();
    // Emits: (node: 56339) Warning: Only warn once!
    emitMyWarning();
    // Emits nothing
    javascript

### `process.env`#

Added in: v0.1.27
History

Version | Changes
--- | ---
v11.14.0 | Worker threads will now use a copy of the parent thread's `process.env` by default, configurable through the `env` option of the `Worker` constructor.
v10.0.0 | Implicit conversion of variable value to string is deprecated.

- Type: `<Object>`

The `process.env` property returns an object containing the user environment. See `environ(7)`.

An example of this object looks like:

    {
      "TERM": "xterm-256color",
      "SHELL": "/usr/local/bin/bash",
      "USER": "maciej",
      "PATH": "~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin",
      "PWD": "/Users/maciej",
      "EDITOR": "vim",
      "SHLVL": "1",
      "HOME": "/Users/maciej",
      "LOGNAME": "maciej",
      "_": "/usr/local/bin/node"
    }
    json

It is possible to modify this object, but such modifications will not be reflected outside the Node.js process, or (unless explicitly requested) to other `Worker` threads. In other words, the following example would not work:

    node -e 'process.env.foo = "bar"' && echo $foo
    bash

While the following will:

    import { env } from 'node:process';

    env.foo = 'bar';
    console.log(env.foo);
    const { env } = require('node:process');

    env.foo = 'bar';
    console.log(env.foo);
    javascript

Assigning a property on `process.env` will implicitly convert the value to a string. This behavior is deprecated. Future versions of Node.js may throw an error when the value is not a string, number, or boolean.

    import { env } from 'node:process';

    env.test = null;
    console.log(env.test);
    // => 'null'
    env.test = undefined;
    console.log(env.test);
    // => 'undefined'
    const { env } = require('node:process');

    env.test = null;
    console.log(env.test);
    // => 'null'
    env.test = undefined;
    console.log(env.test);
    // => 'undefined'
    javascript

Use `delete` to delete a property from `process.env`.

    import { env } from 'node:process';

    env.TEST = 1;
    delete env.TEST;
    console.log(env.TEST);
    // => undefined
    const { env } = require('node:process');

    env.TEST = 1;
    delete env.TEST;
    console.log(env.TEST);
    // => undefined
    javascript

On Windows operating systems, environment variables are case-insensitive.

    import { env } from 'node:process';

    env.TEST = 1;
    console.log(env.test);
    // => 1
    const { env } = require('node:process');

    env.TEST = 1;
    console.log(env.test);
    // => 1
    javascript

Unless explicitly specified when creating a `Worker` instance, each `Worker` thread has its own copy of `process.env`, based on its parent thread's `process.env`, or whatever was specified as the `env` option to the `Worker` constructor. Changes to `process.env` will not be visible across `Worker` threads, and only the main thread can make changes that are visible to the operating system or to native add-ons. On Windows, a copy of `process.env` on a `Worker` instance operates in a case-sensitive manner unlike the main thread.

### `process.execArgv`#

Added in: v0.7.7

- Type: `<string>`[]

The `process.execArgv` property returns the set of Node.js-specific command-line options passed when the Node.js process was launched. These options do not appear in the array returned by the `process.argv` property, and do not include the Node.js executable, the name of the script, or any options following the script name. These options are useful in order to spawn child processes with the same execution environment as the parent.

    node --icu-data-dir=./foo --require ./bar.js script.js --version
    bash

Results in `process.execArgv`:

    ["--icu-data-dir=./foo", "--require", "./bar.js"]
    json

And `process.argv`:

    ["/usr/local/bin/node", "script.js", "--version"]
    json

Refer to `Worker` constructor for the detailed behavior of worker threads with this property.

### `process.execPath`#

Added in: v0.1.100

- Type: `<string>`

The `process.execPath` property returns the absolute pathname of the executable that started the Node.js process. Symbolic links, if any, are resolved.

    "/usr/local/bin/node"
    json

### `process.execve(file[, args[, env]])`#

Added in: v23.11.0, v22.15.0
History

Version | Changes
--- | ---
v26.1.0 | A failed `execve(2)` system call now throws an exception instead of aborting the process. Native `AtExit` callbacks registered via the embedder API are no longer invoked before the `execve(2)` call.

Stability: 1 - Experimental

- `file` `<string>` The name or path of the executable file to run.
- `args` `<string>`[] List of string arguments. No argument can contain a null-byte (`\u0000`).
- `env` `<Object>` Environment key-value pairs. No key or value can contain a null-byte (`\u0000`). Default: `process.env`.

Replaces the current process with a new process.

This is achieved by using the `execve` POSIX function and therefore no memory or other resources from the current process are preserved, except for the standard input, standard output and standard error file descriptor.

On success, all other resources are discarded by the system when the processes are swapped, without triggering any exit or close events, without running any JavaScript cleanup handler (for example `process.on('exit')`), and without invoking native `AtExit` callbacks registered through the embedder API. Callers that need to run cleanup logic should do so before calling `process.execve()`.

This function does not return on success. If the underlying `execve(2)` system call fails, an `Error` is thrown whose `code` property is set to the corresponding `errno` string (for example, `'ENOENT'` when `file` does not exist), with `syscall` set to `'execve'` and `path` set to `file`. When `execve(2)` fails the current process continues to run with its state unchanged, so a caller may handle the error and take another action.

This function is not available on Windows or IBM i.

### `process.exit([code])`#

Added in: v0.1.13
History

Version | Changes
--- | ---
v20.0.0 | Only accepts a code of type number, or of type string if it represents an integer.

- `code` `<integer>` | `<string>` | `<null>` | `<undefined>` The exit code. For string type, only integer strings (e.g.,'1') are allowed. Default: `0`.

The `process.exit()` method instructs Node.js to terminate the process synchronously with an exit status of `code`. If `code` is omitted, exit uses either the 'success' code `0` or the value of `process.exitCode` if it has been set. Node.js will not terminate until all the `'exit'` event listeners are called.

To exit with a 'failure' code:

    import { exit } from 'node:process';

    exit(1);
    const { exit } = require('node:process');

    exit(1);
    javascript

The shell that executed Node.js should see the exit code as `1`.

Calling `process.exit()` will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to `process.stdout` and `process.stderr`.

In most situations, it is not actually necessary to call `process.exit()` explicitly. The Node.js process will exit on its own if there is no additional work pending in the event loop. The `process.exitCode` property can be set to tell the process which exit code to use when the process exits gracefully.

For instance, the following example illustrates a misuse of the `process.exit()` method that could lead to data printed to stdout being truncated and lost:

    import { exit } from 'node:process';

    // This is an example of what *not* to do:
    if (someConditionNotMet()) {
      printUsageToStdout();
      exit(1);
    }
    const { exit } = require('node:process');

    // This is an example of what *not* to do:
    if (someConditionNotMet()) {
      printUsageToStdout();
      exit(1);
    }
    javascript

The reason this is problematic is because writes to `process.stdout` in Node.js are sometimes asynchronous and may occur over multiple ticks of the Node.js event loop. Calling `process.exit()`, however, forces the process to exit before those additional writes to `stdout` can be performed.

Rather than calling `process.exit()` directly, the code should set the `process.exitCode` and allow the process to exit naturally by avoiding scheduling any additional work for the event loop:

    import process from 'node:process';

    // How to properly set the exit code while letting
    // the process exit gracefully.
    if (someConditionNotMet()) {
      printUsageToStdout();
      process.exitCode = 1;
    }
    // How to properly set the exit code while letting
    // the process exit gracefully.
    if (someConditionNotMet()) {
      printUsageToStdout();
      process.exitCode = 1;
    }
    javascript

If it is necessary to terminate the Node.js process due to an error condition, throwing an uncaught error and allowing the process to terminate accordingly is safer than calling `process.exit()`.

In `Worker` threads, this function stops the current thread rather than the current process.

### `process.exitCode`#

Added in: v0.11.8
History

Version | Changes
--- | ---
v20.0.0 | Only accepts a code of type number, or of type string if it represents an integer.

- Type: `<integer>` | `<string>` | `<null>` | `<undefined>` The exit code. For string type, only integer strings (e.g.,'1') are allowed. Default: `undefined`.

A number which will be the process exit code, when the process either exits gracefully, or is exited via `process.exit()` without specifying a code.

The value of `process.exitCode` can be updated by either assigning a value to `process.exitCode` or by passing an argument to `process.exit()`:

    $ node -e 'process.exitCode = 9'; echo $?
    9
    $ node -e 'process.exit(42)'; echo $?
    42
    $ node -e 'process.exitCode = 9; process.exit(42)'; echo $?
    42
    console

The value can also be set implicitly by Node.js when unrecoverable errors occur (e.g. such as the encountering of an unsettled top-level await). However explicit manipulations of the exit code always take precedence over implicit ones:

    $ node --input-type=module -e 'await new Promise(() => {})'; echo $?
    13
    $ node --input-type=module -e 'process.exitCode = 9; await new Promise(() => {})'; echo $?
    9
    console

### `process.features.cached_builtins`#

Added in: v12.0.0

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build is caching builtin modules.

### `process.features.debug`#

Added in: v0.5.5

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build is a debug build.

### `process.features.inspector`#

Added in: v11.10.0

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes the inspector.

### `process.features.ipv6`#

Added in: v0.5.3Deprecated in: v23.4.0, v22.13.0

Stability: 0 - Deprecated. This property is always true, and any checks based on it are redundant.

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes support for IPv6.

Since all Node.js builds have IPv6 support, this value is always `true`.

### `process.features.require_module`#

Added in: v23.0.0, v22.10.0, v20.19.0

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build supports loading ECMAScript modules using `require()`.

### `process.features.tls`#

Added in: v0.5.3

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes support for TLS.

### `process.features.tls_alpn`#

Added in: v4.8.0Deprecated in: v23.4.0, v22.13.0

Stability: 0 - Deprecated. Use `process.features.tls` instead.

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes support for ALPN in TLS.

In Node.js 11.0.0 and later versions, the OpenSSL dependencies feature unconditional ALPN support. This value is therefore identical to that of `process.features.tls`.

### `process.features.tls_ocsp`#

Added in: v0.11.13Deprecated in: v23.4.0, v22.13.0

Stability: 0 - Deprecated. Use `process.features.tls` instead.

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes support for OCSP in TLS.

In Node.js 11.0.0 and later versions, the OpenSSL dependencies feature unconditional OCSP support. This value is therefore identical to that of `process.features.tls`.

### `process.features.tls_sni`#

Added in: v0.5.3Deprecated in: v23.4.0, v22.13.0

Stability: 0 - Deprecated. Use `process.features.tls` instead.

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes support for SNI in TLS.

In Node.js 11.0.0 and later versions, the OpenSSL dependencies feature unconditional SNI support. This value is therefore identical to that of `process.features.tls`.

### `process.features.typescript`#

Added in: v23.0.0, v22.10.0
History

Version | Changes
--- | ---
v26.0.0 | Removed `transform` value.
v25.2.0, v24.12.0 | Type stripping is now stable.

Stability: 1.2 - Release candidate

- Type: `<boolean>` | `<string>`

A value that is `"strip"` by default, and `false` if Node.js is run with `--no-strip-types`.

### `process.features.uv`#

Added in: v0.5.3Deprecated in: v23.4.0, v22.13.0

Stability: 0 - Deprecated. This property is always true, and any checks based on it are redundant.

- Type: `<boolean>`

A boolean value that is `true` if the current Node.js build includes support for libuv.

Since it's not possible to build Node.js without libuv, this value is always `true`.

### `process.finalization.register(ref, callback)`#

Added in: v22.5.0

Stability: 1.1 - Active Development

- `ref` `<Object>` | `<Function>` The reference to the resource that is being tracked.
- `callback` `<Function>` The callback function to be called when the resource is finalized.
  - `ref` `<Object>` | `<Function>` The reference to the resource that is being tracked.
  - `event` `<string>` The event that triggered the finalization. Defaults to 'exit'.

This function registers a callback to be called when the process emits the `exit` event if the `ref` object was not garbage collected. If the object `ref` was garbage collected before the `exit` event is emitted, the callback will be removed from the finalization registry, and it will not be called on process exit.

Inside the callback you can release the resources allocated by the `ref` object. Be aware that all limitations applied to the `beforeExit` event are also applied to the `callback` function, this means that there is a possibility that the callback will not be called under special circumstances.

The idea of ​​this function is to help you free up resources when the starts process exiting, but also let the object be garbage collected if it is no longer being used.

Eg: you can register an object that contains a buffer, you want to make sure that buffer is released when the process exit, but if the object is garbage collected before the process exit, we no longer need to release the buffer, so in this case we just remove the callback from the finalization registry.

    const { finalization } = require('node:process');

    // Please make sure that the function passed to finalization.register()
    // does not create a closure around unnecessary objects.
    function onFinalize(obj, event) {
      // You can do whatever you want with the object
      obj.dispose();
    }

    function setup() {
      // This object can be safely garbage collected,
      // and the resulting shutdown function will not be called.
      // There are no leaks.
      const myDisposableObject = {
        dispose() {
          // Free your resources synchronously
        },
      };

      finalization.register(myDisposableObject, onFinalize);
    }

    setup();
    import { finalization } from 'node:process';

    // Please make sure that the function passed to finalization.register()
    // does not create a closure around unnecessary objects.
    function onFinalize(obj, event) {
      // You can do whatever you want with the object
      obj.dispose();
    }

    function setup() {
      // This object can be safely garbage collected,
      // and the resulting shutdown function will not be called.
      // There are no leaks.
      const myDisposableObject = {
        dispose() {
          // Free your resources synchronously
        },
      };

      finalization.register(myDisposableObject, onFinalize);
    }

    setup();
    javascript

The code above relies on the following assumptions:

- arrow functions are avoided
- regular functions are recommended to be within the global context (root)

Regular functions could reference the context where the `obj` lives, making the `obj` not garbage collectible.

Arrow functions will hold the previous context. Consider, for example:

    class Test {
      constructor() {
        finalization.register(this, (ref) => ref.dispose());

        // Even something like this is highly discouraged
        // finalization.register(this, () => this.dispose());
      }
      dispose() {}
    }
    js

It is very unlikely (not impossible) that this object will be garbage collected, but if it is not, `dispose` will be called when `process.exit` is called.

Be careful and avoid relying on this feature for the disposal of critical resources, as it is not guaranteed that the callback will be called under all circumstances.

### `process.finalization.registerBeforeExit(ref, callback)`#

Added in: v22.5.0

Stability: 1.1 - Active Development

- `ref` `<Object>` | `<Function>` The reference to the resource that is being tracked.
- `callback` `<Function>` The callback function to be called when the resource is finalized.
  - `ref` `<Object>` | `<Function>` The reference to the resource that is being tracked.
  - `event` `<string>` The event that triggered the finalization. Defaults to 'beforeExit'.

This function behaves exactly like the `register`, except that the callback will be called when the process emits the `beforeExit` event if `ref` object was not garbage collected.

Be aware that all limitations applied to the `beforeExit` event are also applied to the `callback` function, this means that there is a possibility that the callback will not be called under special circumstances.

### `process.finalization.unregister(ref)`#

Added in: v22.5.0

Stability: 1.1 - Active Development

- `ref` `<Object>` | `<Function>` The reference to the resource that was registered previously.

This function remove the register of the object from the finalization registry, so the callback will not be called anymore.

    const { finalization } = require('node:process');

    // Please make sure that the function passed to finalization.register()
    // does not create a closure around unnecessary objects.
    function onFinalize(obj, event) {
      // You can do whatever you want with the object
      obj.dispose();
    }

    function setup() {
      // This object can be safely garbage collected,
      // and the resulting shutdown function will not be called.
      // There are no leaks.
      const myDisposableObject = {
        dispose() {
          // Free your resources synchronously
        },
      };

      finalization.register(myDisposableObject, onFinalize);

      // Do something

      myDisposableObject.dispose();
      finalization.unregister(myDisposableObject);
    }

    setup();
    import { finalization } from 'node:process';

    // Please make sure that the function passed to finalization.register()
    // does not create a closure around unnecessary objects.
    function onFinalize(obj, event) {
      // You can do whatever you want with the object
      obj.dispose();
    }

    function setup() {
      // This object can be safely garbage collected,
      // and the resulting shutdown function will not be called.
      // There are no leaks.
      const myDisposableObject = {
        dispose() {
          // Free your resources synchronously
        },
      };

      // Please make sure that the function passed to finalization.register()
      // does not create a closure around unnecessary objects.
      function onFinalize(obj, event) {
        // You can do whatever you want with the object
        obj.dispose();
      }

      finalization.register(myDisposableObject, onFinalize);

      // Do something

      myDisposableObject.dispose();
      finalization.unregister(myDisposableObject);
    }

    setup();
    javascript

### `process.getActiveResourcesInfo()`#

Added in: v17.3.0, v16.14.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Change stability index for this feature from Experimental to Stable.

- Returns: `<string>`[]

The `process.getActiveResourcesInfo()` method returns an array of strings containing the types of the active resources that are currently keeping the event loop alive.

    import { getActiveResourcesInfo } from 'node:process';
    import { setTimeout } from 'node:timers';

    console.log('Before:', getActiveResourcesInfo());
    setTimeout(() => {}, 1000);
    console.log('After:', getActiveResourcesInfo());
    // Prints:
    //   Before: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
    //   After: [ 'CloseReq', 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]
    const { getActiveResourcesInfo } = require('node:process');
    const { setTimeout } = require('node:timers');

    console.log('Before:', getActiveResourcesInfo());
    setTimeout(() => {}, 1000);
    console.log('After:', getActiveResourcesInfo());
    // Prints:
    //   Before: [ 'TTYWrap', 'TTYWrap', 'TTYWrap' ]
    //   After: [ 'TTYWrap', 'TTYWrap', 'TTYWrap', 'Timeout' ]
    javascript

### `process.getBuiltinModule(id)`#

Added in: v22.3.0, v20.16.0

- `id` `<string>` ID of the built-in module being requested.
- Returns: `<Object>` | `<undefined>`

`process.getBuiltinModule(id)` provides a way to load built-in modules in a globally available function. ES Modules that need to support other environments can use it to conditionally load a Node.js built-in when it is run in Node.js, without having to deal with the resolution error that can be thrown by `import` in a non-Node.js environment or having to use dynamic `import()` which either turns the module into an asynchronous module, or turns a synchronous API into an asynchronous one.

    if (globalThis.process?.getBuiltinModule) {
      // Run in Node.js, use the Node.js fs module.
      const fs = globalThis.process.getBuiltinModule('fs');
      // If `require()` is needed to load user-modules, use createRequire()
      const module = globalThis.process.getBuiltinModule('module');
      const require = module.createRequire(import.meta.url);
      const foo = require('foo');
    }
    mjs

If `id` specifies a built-in module available in the current Node.js process, `process.getBuiltinModule(id)` method returns the corresponding built-in module. If `id` does not correspond to any built-in module, `undefined` is returned.

`process.getBuiltinModule(id)` accepts built-in module IDs that are recognized by `module.isBuiltin(id)`. Some built-in modules must be loaded with the `node:` prefix, see built-in modules with mandatory `node:` prefix. The references returned by `process.getBuiltinModule(id)` always point to the built-in module corresponding to `id` even if users modify `require.cache` so that `require(id)` returns something else.

### `process.getegid()`#

Added in: v2.0.0

The `process.getegid()` method returns the numerical effective group identity of the Node.js process. (See `getegid(2)`.)

    import process from 'node:process';

    if (process.getegid) {
      console.log(`Current gid: ${process.getegid()}`);
    }
    if (process.getegid) {
      console.log(`Current gid: ${process.getegid()}`);
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android).

### `process.geteuid()`#

Added in: v2.0.0

- Returns: `<Object>`

The `process.geteuid()` method returns the numerical effective user identity of the process. (See `geteuid(2)`.)

    import process from 'node:process';

    if (process.geteuid) {
      console.log(`Current uid: ${process.geteuid()}`);
    }
    if (process.geteuid) {
      console.log(`Current uid: ${process.geteuid()}`);
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android).

### `process.getgid()`#

Added in: v0.1.31

- Returns: `<Object>`

The `process.getgid()` method returns the numerical group identity of the process. (See `getgid(2)`.)

    import process from 'node:process';

    if (process.getgid) {
      console.log(`Current gid: ${process.getgid()}`);
    }
    if (process.getgid) {
      console.log(`Current gid: ${process.getgid()}`);
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android).

### `process.getgroups()`#

Added in: v0.9.4

- Returns: `<integer>`[]

The `process.getgroups()` method returns an array with the supplementary group IDs. POSIX leaves it unspecified if the effective group ID is included but Node.js ensures it always is.

    import process from 'node:process';

    if (process.getgroups) {
      console.log(process.getgroups()); // [ 16, 21, 297 ]
    }
    if (process.getgroups) {
      console.log(process.getgroups()); // [ 16, 21, 297 ]
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android).

### `process.getuid()`#

Added in: v0.1.28

- Returns: `<integer>`

The `process.getuid()` method returns the numeric user identity of the process. (See `getuid(2)`.)

    import process from 'node:process';

    if (process.getuid) {
      console.log(`Current uid: ${process.getuid()}`);
    }
    if (process.getuid) {
      console.log(`Current uid: ${process.getuid()}`);
    }
    javascript

This function not available on Windows.

### `process.hasUncaughtExceptionCaptureCallback()`#

Added in: v9.3.0

- Returns: `<boolean>`

Indicates whether a callback has been set using `process.setUncaughtExceptionCaptureCallback()`.

### `process.hrtime([time])`#

Added in: v0.7.6

Stability: 3 - Legacy. Use `process.hrtime.bigint()` instead.

- `time` `<integer>`[] The result of a previous call to `process.hrtime()`
- Returns: `<integer>`[]

This is the legacy version of `process.hrtime.bigint()` before `bigint` was introduced in JavaScript.

The `process.hrtime()` method returns the current high-resolution real time in a `[seconds, nanoseconds]` tuple `Array`, where `nanoseconds` is the remaining part of the real time that can't be represented in second precision.

`time` is an optional parameter that must be the result of a previous `process.hrtime()` call to diff with the current time. If the parameter passed in is not a tuple `Array`, a `TypeError` will be thrown. Passing in a user-defined array instead of the result of a previous call to `process.hrtime()` will lead to undefined behavior.

These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals:

    import { hrtime } from 'node:process';

    const NS_PER_SEC = 1e9;
    const time = hrtime();
    // [ 1800216, 25 ]

    setTimeout(() => {
      const diff = hrtime(time);
      // [ 1, 552 ]

      console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
      // Benchmark took 1000000552 nanoseconds
    }, 1000);
    const { hrtime } = require('node:process');

    const NS_PER_SEC = 1e9;
    const time = hrtime();
    // [ 1800216, 25 ]

    setTimeout(() => {
      const diff = hrtime(time);
      // [ 1, 552 ]

      console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
      // Benchmark took 1000000552 nanoseconds
    }, 1000);
    javascript

### `process.hrtime.bigint()`#

Added in: v10.7.0

- Returns: `<bigint>`

The `bigint` version of the `process.hrtime()` method returning the current high-resolution real time in nanoseconds as a `bigint`.

Unlike `process.hrtime()`, it does not support an additional `time` argument since the difference can just be computed directly by subtraction of the two `bigint`s.

    import { hrtime } from 'node:process';

    const start = hrtime.bigint();
    // 191051479007711n

    setTimeout(() => {
      const end = hrtime.bigint();
      // 191052633396993n

      console.log(`Benchmark took ${end - start} nanoseconds`);
      // Benchmark took 1154389282 nanoseconds
    }, 1000);
    const { hrtime } = require('node:process');

    const start = hrtime.bigint();
    // 191051479007711n

    setTimeout(() => {
      const end = hrtime.bigint();
      // 191052633396993n

      console.log(`Benchmark took ${end - start} nanoseconds`);
      // Benchmark took 1154389282 nanoseconds
    }, 1000);
    javascript

### `process.initgroups(user, extraGroup)`#

Added in: v0.9.4

- `user` `<string>` | `<number>` The user name or numeric identifier.
- `extraGroup` `<string>` | `<number>` A group name or numeric identifier.

The `process.initgroups()` method reads the `/etc/group` file and initializes the group access list, using all groups of which the user is a member. This is a privileged operation that requires that the Node.js process either have `root` access or the `CAP_SETGID` capability.

Use care when dropping privileges:

    import { getgroups, initgroups, setgid } from 'node:process';

    console.log(getgroups());         // [ 0 ]
    initgroups('nodeuser', 1000);     // switch user
    console.log(getgroups());         // [ 27, 30, 46, 1000, 0 ]
    setgid(1000);                     // drop root gid
    console.log(getgroups());         // [ 27, 30, 46, 1000 ]
    const { getgroups, initgroups, setgid } = require('node:process');

    console.log(getgroups());         // [ 0 ]
    initgroups('nodeuser', 1000);     // switch user
    console.log(getgroups());         // [ 27, 30, 46, 1000, 0 ]
    setgid(1000);                     // drop root gid
    console.log(getgroups());         // [ 27, 30, 46, 1000 ]
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in `Worker` threads.

### `process.kill(pid[, signal])`#

Added in: v0.0.6

- `pid` `<number>` A process ID
- `signal` `<string>` | `<number>` The signal to send, either as a string or number. Default: `'SIGTERM'`.

The `process.kill()` method sends the `signal` to the process identified by `pid`.

Signal names are strings such as `'SIGINT'` or `'SIGHUP'`. See Signal Events and `kill(2)` for more information.

This method will throw an error if the target `pid` does not exist. As a special case, a signal of `0` can be used to test for the existence of a process. Windows platforms will throw an error if the `pid` is used to kill a process group.

Even though the name of this function is `process.kill()`, it is really just a signal sender, like the `kill` system call. The signal sent may do something other than kill the target process.

    import process, { kill } from 'node:process';

    process.on('SIGHUP', () => {
      console.log('Got SIGHUP signal.');
    });

    setTimeout(() => {
      console.log('Exiting.');
      process.exit(0);
    }, 100);

    kill(process.pid, 'SIGHUP');
    process.on('SIGHUP', () => {
      console.log('Got SIGHUP signal.');
    });

    setTimeout(() => {
      console.log('Exiting.');
      process.exit(0);
    }, 100);

    process.kill(process.pid, 'SIGHUP');
    javascript

When `SIGUSR1` is received by a Node.js process, Node.js will start the debugger. See Signal Events.

### `process.loadEnvFile(path)`#

Added in: v21.7.0, v20.12.0
History

Version | Changes
--- | ---
v24.10.0, v22.21.0 | This API is no longer experimental.

- `path` `<string>` | `<URL>` | `<Buffer>` | `<undefined>`. Default: `'./.env'`

Loads the `.env` file into `process.env`. Usage of `NODE_OPTIONS` in the `.env` file will not have any effect on Node.js.

    const { loadEnvFile } = require('node:process');
    loadEnvFile();
    import { loadEnvFile } from 'node:process';
    loadEnvFile();
    javascript

### `process.mainModule`#

Added in: v0.1.17Deprecated in: v14.0.0

Stability: 0 - Deprecated: Use `require.main` instead.

- Type: `<Object>`

The `process.mainModule` property provides an alternative way of retrieving `require.main`. The difference is that if the main module changes at runtime, `require.main` may still refer to the original main module in modules that were required before the change occurred. Generally, it's safe to assume that the two refer to the same module.

As with `require.main`, `process.mainModule` will be `undefined` if there is no entry script.

### `process.memoryUsage()`#

Added in: v0.1.16
History

Version | Changes
--- | ---
v13.9.0, v12.17.0 | Added `arrayBuffers` to the returned object.
v7.2.0 | Added `external` to the returned object.

- Returns: `<Object>`
  - `rss` `<integer>`
  - `heapTotal` `<integer>`
  - `heapUsed` `<integer>`
  - `external` `<integer>`
  - `arrayBuffers` `<integer>`

Returns an object describing the memory usage of the Node.js process measured in bytes.

    import { memoryUsage } from 'node:process';

    console.log(memoryUsage());
    // Prints:
    // {
    //  rss: 4935680,
    //  heapTotal: 1826816,
    //  heapUsed: 650472,
    //  external: 49879,
    //  arrayBuffers: 9386
    // }
    const { memoryUsage } = require('node:process');

    console.log(memoryUsage());
    // Prints:
    // {
    //  rss: 4935680,
    //  heapTotal: 1826816,
    //  heapUsed: 650472,
    //  external: 49879,
    //  arrayBuffers: 9386
    // }
    javascript

- `heapTotal` and `heapUsed` refer to V8's memory usage.
- `external` refers to the memory usage of C++ objects bound to JavaScript objects managed by V8.
- `rss`, Resident Set Size, is the amount of space occupied in the main memory device (that is a subset of the total allocated memory) for the process, including all C++ and JavaScript objects and code.
- `arrayBuffers` refers to memory allocated for `ArrayBuffer`s and `SharedArrayBuffer`s, including all Node.js `Buffer`s. This is also included in the `external` value. When Node.js is used as an embedded library, this value may be `0` because allocations for `ArrayBuffer`s may not be tracked in that case.

When using `Worker` threads, `rss` will be a value that is valid for the entire process, while the other fields will only refer to the current thread.

The `process.memoryUsage()` method iterates over each page to gather information about memory usage which might be slow depending on the program memory allocations.

#### A note on process memoryUsage#

On Linux or other systems where glibc is commonly used, an application may have sustained `rss` growth despite stable `heapTotal` due to fragmentation caused by the glibc `malloc` implementation. See nodejs/node#21973 on how to switch to an alternative `malloc` implementation to address the performance issue.

### `process.memoryUsage.rss()`#

Added in: v15.6.0, v14.18.0

- Returns: `<integer>`

The `process.memoryUsage.rss()` method returns an integer representing the Resident Set Size (RSS) in bytes.

The Resident Set Size, is the amount of space occupied in the main memory device (that is a subset of the total allocated memory) for the process, including all C++ and JavaScript objects and code.

This is the same value as the `rss` property provided by `process.memoryUsage()` but `process.memoryUsage.rss()` is faster.

    import { memoryUsage } from 'node:process';

    console.log(memoryUsage.rss());
    // 35655680
    const { memoryUsage } = require('node:process');

    console.log(memoryUsage.rss());
    // 35655680
    javascript

### `process.nextTick(callback[, ...args])`#

Added in: v0.1.26
History

Version | Changes
--- | ---
v22.7.0, v20.18.0 | Changed stability to Legacy.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v1.8.1 | Additional arguments after `callback` are now supported.

Stability: 3 - Legacy: Use `queueMicrotask()` instead.

- `callback` `<Function>`
- `...args` `<any>` Additional arguments to pass when invoking the `callback`

`process.nextTick()` adds `callback` to the "next tick queue". This queue is fully drained after the current operation on the JavaScript stack runs to completion and before the event loop is allowed to continue. It's possible to create an infinite loop if one were to recursively call `process.nextTick()`. See the Event Loop guide for more background.

    import { nextTick } from 'node:process';

    console.log('start');
    nextTick(() => {
      console.log('nextTick callback');
    });
    console.log('scheduled');
    // Output:
    // start
    // scheduled
    // nextTick callback
    const { nextTick } = require('node:process');

    console.log('start');
    nextTick(() => {
      console.log('nextTick callback');
    });
    console.log('scheduled');
    // Output:
    // start
    // scheduled
    // nextTick callback
    javascript

This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:

    import { nextTick } from 'node:process';

    function MyThing(options) {
      this.setupOptions(options);

      nextTick(() => {
        this.startDoingStuff();
      });
    }

    const thing = new MyThing();
    thing.getReadyForStuff();

    // thing.startDoingStuff() gets called now, not before.
    const { nextTick } = require('node:process');

    function MyThing(options) {
      this.setupOptions(options);

      nextTick(() => {
        this.startDoingStuff();
      });
    }

    const thing = new MyThing();
    thing.getReadyForStuff();

    // thing.startDoingStuff() gets called now, not before.
    javascript

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

    // WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
    function maybeSync(arg, cb) {
      if (arg) {
        cb();
        return;
      }

      fs.stat('file', cb);
    }
    js

This API is hazardous because in the following case:

    const maybeTrue = Math.random() > 0.5;

    maybeSync(maybeTrue, () => {
      foo();
    });

    bar();
    js

It is not clear whether `foo()` or `bar()` will be called first.

The following approach is much better:

    import { nextTick } from 'node:process';

    function definitelyAsync(arg, cb) {
      if (arg) {
        nextTick(cb);
        return;
      }

      fs.stat('file', cb);
    }
    const { nextTick } = require('node:process');

    function definitelyAsync(arg, cb) {
      if (arg) {
        nextTick(cb);
        return;
      }

      fs.stat('file', cb);
    }
    javascript

#### When to use `queueMicrotask()` vs. `process.nextTick()`#

The `queueMicrotask()` API is an alternative to `process.nextTick()` that instead of using the "next tick queue" defers execution of a function using the same microtask queue used to execute the then, catch, and finally handlers of resolved promises.

Within Node.js, every time the "next tick queue" is drained, the microtask queue is drained immediately after.

So in CJS modules `process.nextTick()` callbacks are always run before `queueMicrotask()` ones. However since ESM modules are processed already as part of the microtask queue, there `queueMicrotask()` callbacks are always executed before `process.nextTick()` ones since Node.js is already in the process of draining the microtask queue.

    import { nextTick } from 'node:process';

    Promise.resolve().then(() => console.log('resolve'));
    queueMicrotask(() => console.log('microtask'));
    nextTick(() => console.log('nextTick'));
    // Output:
    // resolve
    // microtask
    // nextTick
    const { nextTick } = require('node:process');

    Promise.resolve().then(() => console.log('resolve'));
    queueMicrotask(() => console.log('microtask'));
    nextTick(() => console.log('nextTick'));
    // Output:
    // nextTick
    // resolve
    // microtask
    javascript

For most userland use cases, the `queueMicrotask()` API provides a portable and reliable mechanism for deferring execution that works across multiple JavaScript platform environments and should be favored over `process.nextTick()`. In simple scenarios, `queueMicrotask()` can be a drop-in replacement for `process.nextTick()`.

    console.log('start');
    queueMicrotask(() => {
      console.log('microtask callback');
    });
    console.log('scheduled');
    // Output:
    // start
    // scheduled
    // microtask callback
    js

One note-worthy difference between the two APIs is that `process.nextTick()` allows specifying additional values that will be passed as arguments to the deferred function when it is called. Achieving the same result with `queueMicrotask()` requires using either a closure or a bound function:

    function deferred(a, b) {
      console.log('microtask', a + b);
    }

    console.log('start');
    queueMicrotask(deferred.bind(undefined, 1, 2));
    console.log('scheduled');
    // Output:
    // start
    // scheduled
    // microtask 3
    js

There are minor differences in the way errors raised from within the next tick queue and microtask queue are handled. Errors thrown within a queued microtask callback should be handled within the queued callback when possible. If they are not, the `process.on('uncaughtException')` event handler can be used to capture and handle the errors.

When in doubt, unless the specific capabilities of `process.nextTick()` are needed, use `queueMicrotask()`.

### `process.noDeprecation`#

Added in: v0.8.0

- Type: `<boolean>`

The `process.noDeprecation` property indicates whether the `--no-deprecation` flag is set on the current Node.js process. See the documentation for the `'warning'` event and the `emitWarning()` method for more information about this flag's behavior.

### `process.permission`#

Added in: v20.0.0

- Type: `<Object>`

This API is available through the `--permission` or `--permission-audit` flags.

`process.permission` is an object whose methods are used to manage permissions for the current process. Additional documentation is available in the Permission Model.

#### `process.permission.has(scope[, reference])`#

Added in: v20.0.0

- `scope` `<string>`
- `reference` `<string>`
- Returns: `<boolean>`

Verifies that the process is able to access the given scope and reference. If no reference is provided, a global scope is assumed, for instance, `process.permission.has('fs.read')` will check if the process has ALL file system read permissions.

In audit mode (`--permission-audit`), this method still returns the actual permission status, but denied operations will not throw `ERR_ACCESS_DENIED`.

The reference has a meaning based on the provided scope. For example, the reference when the scope is File System means files and folders.

The available scopes are:

- `fs` - All File System
- `fs.read` - File System read operations
- `fs.write` - File System write operations
- `child` - Child process spawning operations
- `openssl.store` - Loading keys through OpenSSL STORE loaders
- `worker` - Worker thread spawning operation
- `ffi` - Foreign function interface operations

    // Check if the process has permission to read the README file
    process.permission.has('fs.read', './README.md');
    // Check if the process has read permission operations
    process.permission.has('fs.read');
    js

#### `process.permission.drop(scope[, reference])`#

Added in: v26.3.0

Stability: 1.1 - Active Development

- `scope` `<string>`
- `reference` `<string>`

Drops the specified permission from the current process. This operation is irreversible — once a permission is dropped, it cannot be restored through any Node.js API.

In audit mode (`--permission-audit`), dropping a permission takes effect, but since denied operations do not throw, the impact is limited to changing the return value of `permission.has()`.

If no reference is provided, the entire scope is dropped. For example, `process.permission.drop('fs.read')` will revoke ALL file system read permissions.

When a reference is provided, only the permission for that specific resource is dropped. For example, `process.permission.drop('fs.read', '/etc/myapp')` will revoke read access to that directory while keeping other read permissions intact.

Important: You can only drop the exact resource that was explicitly granted. The reference passed to `drop()` must match the original grant:

- If a permission was granted using a wildcard (`*`), such as `--allow-fs-read=*`, individual paths cannot be dropped - only the entire scope can be dropped (by calling `drop()` without a reference).
- If a directory was granted (e.g. `--allow-fs-read=/my/folder`), you cannot drop access to individual files inside it. You must drop the same directory that was granted. Any remaining grants continue to apply.

The available scopes are the same as `process.permission.has()`:

- `fs` - All File System (drops both read and write)
- `fs.read` - File System read operations
- `fs.write` - File System write operations
- `child` - Child process spawning operations
- `openssl.store` - Loading keys through OpenSSL STORE loaders
- `worker` - Worker thread spawning operation
- `net` - Network operations
- `inspector` - Inspector operations
- `wasi` - WASI operations
- `addon` - Native addon operations

    const fs = require('node:fs');

    // Read configuration during startup
    const config = fs.readFileSync('/etc/myapp/config.json', 'utf8');

    // Drop read access to the config directory after initialization
    process.permission.drop('fs.read', '/etc/myapp');

    // This will now throw ERR_ACCESS_DENIED
    fs.readFileSync('/etc/myapp/config.json');
    js

### `process.pid`#

Added in: v0.1.15

- Type: `<integer>`

The `process.pid` property returns the PID of the process.

    import { pid } from 'node:process';

    console.log(`This process is pid ${pid}`);
    const { pid } = require('node:process');

    console.log(`This process is pid ${pid}`);
    javascript

### `process.platform`#

Added in: v0.1.16

- Type: `<string>`

The `process.platform` property returns a string identifying the operating system platform for which the Node.js binary was compiled.

Currently possible values are:

- `'aix'`
- `'darwin'`
- `'freebsd'`
- `'linux'`
- `'openbsd'`
- `'sunos'`
- `'win32'`

    import { platform } from 'node:process';

    console.log(`This platform is ${platform}`);
    const { platform } = require('node:process');

    console.log(`This platform is ${platform}`);
    javascript

The value `'android'` may also be returned if Node.js is built on the Android operating system. However, Android support in Node.js is experimental.

### `process.ppid`#

Added in: v9.2.0, v8.10.0, v6.13.0

- Type: `<integer>`

The `process.ppid` property returns the PID of the parent of the current process.

    import { ppid } from 'node:process';

    console.log(`The parent process is pid ${ppid}`);
    const { ppid } = require('node:process');

    console.log(`The parent process is pid ${ppid}`);
    javascript

### `process.ref(maybeRefable)`#

Added in: v23.6.0, v22.14.0

Stability: 1 - Experimental

- `maybeRefable` `<any>` An object that may be "refable".

An object is "refable" if it implements the Node.js "Refable protocol". Specifically, this means that the object implements the `Symbol.for('nodejs.ref')` and `Symbol.for('nodejs.unref')` methods. "Ref'd" objects will keep the Node.js event loop alive, while "unref'd" objects will not. Historically, this was implemented by using `ref()` and `unref()` methods directly on the objects. This pattern, however, is being deprecated in favor of the "Refable protocol" in order to better support Web Platform API types whose APIs cannot be modified to add `ref()` and `unref()` methods but still need to support that behavior.

### `process.release`#

Added in: v3.0.0
History

Version | Changes
--- | ---
v4.2.0 | The `lts` property is now supported.

- Type: `<Object>`

The `process.release` property returns an `Object` containing metadata related to the current release, including URLs for the source tarball and headers-only tarball.

`process.release` contains the following properties:

- `name` `<string>` A value that will always be `'node'`.
- `sourceUrl` `<string>` an absolute URL pointing to a `.tar.gz` file containing the source code of the current release.
- `headersUrl``<string>` an absolute URL pointing to a `.tar.gz` file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.
- `libUrl` `<string>` | `<undefined>` an absolute URL pointing to a `node.lib` file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.
- `lts` `<string>` | `<undefined>` a string label identifying the LTS label for this release. This property only exists for LTS releases and is `undefined` for all other release types, including Current releases. Valid values include the LTS Release code names (including those that are no longer supported).
  - `'Fermium'` for the 14.x LTS line beginning with 14.15.0.
  - `'Gallium'` for the 16.x LTS line beginning with 16.13.0.
  - `'Hydrogen'` for the 18.x LTS line beginning with 18.12.0. For other LTS Release code names, see Node.js Changelog Archive

    {
      "name": "node",
      "lts": "Hydrogen",
      "sourceUrl": "https://nodejs.org/download/release/v18.12.0/node-v18.12.0.tar.gz",
      "headersUrl": "https://nodejs.org/download/release/v18.12.0/node-v18.12.0-headers.tar.gz",
      "libUrl": "https://nodejs.org/download/release/v18.12.0/win-x64/node.lib"
    }
    json

In custom builds from non-release versions of the source tree, only the `name` property may be present. The additional properties should not be relied upon to exist.

### `process.report`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- Type: `<Object>`

`process.report` is an object whose methods are used to generate diagnostic reports for the current process. Additional documentation is available in the report documentation.

#### `process.report.compact`#

Added in: v13.12.0, v12.17.0

- Type: `<boolean>`

Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

    import { report } from 'node:process';

    console.log(`Reports are compact? ${report.compact}`);
    const { report } = require('node:process');

    console.log(`Reports are compact? ${report.compact}`);
    javascript

#### `process.report.directory`#

Added in: v11.12.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- Type: `<string>`

Directory where the report is written. The default value is the empty string, indicating that reports are written to the current working directory of the Node.js process.

    import { report } from 'node:process';

    console.log(`Report directory is ${report.directory}`);
    const { report } = require('node:process');

    console.log(`Report directory is ${report.directory}`);
    javascript

#### `process.report.filename`#

Added in: v11.12.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- Type: `<string>`

Filename where the report is written. If set to the empty string, the output filename will be comprised of a timestamp, PID, and sequence number. The default value is the empty string.

If the value of `process.report.filename` is set to `'stdout'` or `'stderr'`, the report is written to the stdout or stderr of the process respectively.

    import { report } from 'node:process';

    console.log(`Report filename is ${report.filename}`);
    const { report } = require('node:process');

    console.log(`Report filename is ${report.filename}`);
    javascript

#### `process.report.getReport([err])`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- `err` `<Error>` A custom error used for reporting the JavaScript stack.
- Returns: `<Object>`

Returns a JavaScript Object representation of a diagnostic report for the running process. The report's JavaScript stack trace is taken from `err`, if present.

    import { report } from 'node:process';
    import util from 'node:util';

    const data = report.getReport();
    console.log(data.header.nodejsVersion);

    // Similar to process.report.writeReport()
    import fs from 'node:fs';
    fs.writeFileSync('my-report.log', util.inspect(data), 'utf8');
    const { report } = require('node:process');
    const util = require('node:util');

    const data = report.getReport();
    console.log(data.header.nodejsVersion);

    // Similar to process.report.writeReport()
    const fs = require('node:fs');
    fs.writeFileSync('my-report.log', util.inspect(data), 'utf8');
    javascript

Additional documentation is available in the report documentation.

#### `process.report.reportOnFatalError`#

Added in: v11.12.0
History

Version | Changes
--- | ---
v15.0.0, v14.17.0 | This API is no longer experimental.

- Type: `<boolean>`

If `true`, a diagnostic report is generated on fatal errors, such as out of memory errors or failed C++ assertions.

    import { report } from 'node:process';

    console.log(`Report on fatal error: ${report.reportOnFatalError}`);
    const { report } = require('node:process');

    console.log(`Report on fatal error: ${report.reportOnFatalError}`);
    javascript

#### `process.report.reportOnSignal`#

Added in: v11.12.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- Type: `<boolean>`

If `true`, a diagnostic report is generated when the process receives the signal specified by `process.report.signal`.

    import { report } from 'node:process';

    console.log(`Report on signal: ${report.reportOnSignal}`);
    const { report } = require('node:process');

    console.log(`Report on signal: ${report.reportOnSignal}`);
    javascript

#### `process.report.reportOnUncaughtException`#

Added in: v11.12.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- Type: `<boolean>`

If `true`, a diagnostic report is generated on uncaught exception.

    import { report } from 'node:process';

    console.log(`Report on exception: ${report.reportOnUncaughtException}`);
    const { report } = require('node:process');

    console.log(`Report on exception: ${report.reportOnUncaughtException}`);
    javascript

#### `process.report.excludeEnv`#

Added in: v23.3.0, v22.13.0

- Type: `<boolean>`

If `true`, a diagnostic report is generated without the environment variables.

#### `process.report.signal`#

Added in: v11.12.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- Type: `<string>`

The signal used to trigger the creation of a diagnostic report. Defaults to `'SIGUSR2'`.

    import { report } from 'node:process';

    console.log(`Report signal: ${report.signal}`);
    const { report } = require('node:process');

    console.log(`Report signal: ${report.signal}`);
    javascript

#### `process.report.writeReport([filename][, err])`#

Added in: v11.8.0
History

Version | Changes
--- | ---
v13.12.0, v12.17.0 | This API is no longer experimental.

- `filename` `<string>` Name of the file where the report is written. This should be a relative path, that will be appended to the directory specified in `process.report.directory`, or the current working directory of the Node.js process, if unspecified.

- `err` `<Error>` A custom error used for reporting the JavaScript stack.

- Returns: `<string>` Returns the filename of the generated report.

Writes a diagnostic report to a file. If `filename` is not provided, the default filename includes the date, time, PID, and a sequence number. The report's JavaScript stack trace is taken from `err`, if present.

If the value of `filename` is set to `'stdout'` or `'stderr'`, the report is written to the stdout or stderr of the process respectively.

    import { report } from 'node:process';

    report.writeReport();
    const { report } = require('node:process');

    report.writeReport();
    javascript

Additional documentation is available in the report documentation.

### `process.resourceUsage()`#

Added in: v12.6.0

- Returns: `<Object>` the resource usage for the current process. All of these values come from the `uv_getrusage` call which returns a `uv_rusage_t` struct.
  - `userCPUTime` `<integer>` maps to `ru_utime` computed in microseconds. It is the same value as `process.cpuUsage().user`.
  - `systemCPUTime` `<integer>` maps to `ru_stime` computed in microseconds. It is the same value as `process.cpuUsage().system`.
  - `maxRSS` `<integer>` maps to `ru_maxrss` which is the maximum resident set size used in kibibytes (1024 bytes).
  - `sharedMemorySize` `<integer>` maps to `ru_ixrss` but is not supported by any platform.
  - `unsharedDataSize` `<integer>` maps to `ru_idrss` but is not supported by any platform.
  - `unsharedStackSize` `<integer>` maps to `ru_isrss` but is not supported by any platform.
  - `minorPageFault` `<integer>` maps to `ru_minflt` which is the number of minor page faults for the process, see this article for more details.
  - `majorPageFault` `<integer>` maps to `ru_majflt` which is the number of major page faults for the process, see this article for more details. This field is not supported on Windows.
  - `swappedOut` `<integer>` maps to `ru_nswap` but is not supported by any platform.
  - `fsRead` `<integer>` maps to `ru_inblock` which is the number of times the file system had to perform input.
  - `fsWrite` `<integer>` maps to `ru_oublock` which is the number of times the file system had to perform output.
  - `ipcSent` `<integer>` maps to `ru_msgsnd` but is not supported by any platform.
  - `ipcReceived` `<integer>` maps to `ru_msgrcv` but is not supported by any platform.
  - `signalsCount` `<integer>` maps to `ru_nsignals` but is not supported by any platform.
  - `voluntaryContextSwitches` `<integer>` maps to `ru_nvcsw` which is the number of times a CPU context switch resulted due to a process voluntarily giving up the processor before its time slice was completed (usually to await availability of a resource). This field is not supported on Windows.
  - `involuntaryContextSwitches` `<integer>` maps to `ru_nivcsw` which is the number of times a CPU context switch resulted due to a higher priority process becoming runnable or because the current process exceeded its time slice. This field is not supported on Windows.

    import { resourceUsage } from 'node:process';

    console.log(resourceUsage());
    /*
      Will output:
      {
        userCPUTime: 82872,
        systemCPUTime: 4143,
        maxRSS: 33164,
        sharedMemorySize: 0,
        unsharedDataSize: 0,
        unsharedStackSize: 0,
        minorPageFault: 2469,
        majorPageFault: 0,
        swappedOut: 0,
        fsRead: 0,
        fsWrite: 8,
        ipcSent: 0,
        ipcReceived: 0,
        signalsCount: 0,
        voluntaryContextSwitches: 79,
        involuntaryContextSwitches: 1
      }
    */
    const { resourceUsage } = require('node:process');

    console.log(resourceUsage());
    /*
      Will output:
      {
        userCPUTime: 82872,
        systemCPUTime: 4143,
        maxRSS: 33164,
        sharedMemorySize: 0,
        unsharedDataSize: 0,
        unsharedStackSize: 0,
        minorPageFault: 2469,
        majorPageFault: 0,
        swappedOut: 0,
        fsRead: 0,
        fsWrite: 8,
        ipcSent: 0,
        ipcReceived: 0,
        signalsCount: 0,
        voluntaryContextSwitches: 79,
        involuntaryContextSwitches: 1
      }
    */
    javascript

### `process.send(message[, sendHandle[, options]][, callback])`#

Added in: v0.5.9

- `message` `<Object>`
- `sendHandle` `<net.Server>` | `<net.Socket>`
- `options` `<Object>` used to parameterize the sending of certain types of handles.`options` supports the following properties:
  - `keepOpen` `<boolean>` A value that can be used when passing instances of `net.Socket`. When `true`, the socket is kept open in the sending process. Default: `false`.
- `callback` `<Function>`
- Returns: `<boolean>`

If Node.js is spawned with an IPC channel, the `process.send()` method can be used to send messages to the parent process. Messages will be received as a `'message'` event on the parent's `ChildProcess` object.

If Node.js was not spawned with an IPC channel, `process.send` will be `undefined`.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

### `process.setegid(id)`#

Added in: v2.0.0

- `id` `<string>` | `<number>` A group name or ID

The `process.setegid()` method sets the effective group identity of the process. (See `setegid(2)`.) The `id` can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated a numeric ID.

    import process from 'node:process';

    if (process.getegid && process.setegid) {
      console.log(`Current gid: ${process.getegid()}`);
      try {
        process.setegid(501);
        console.log(`New gid: ${process.getegid()}`);
      } catch (err) {
        console.error(`Failed to set gid: ${err}`);
      }
    }
    if (process.getegid && process.setegid) {
      console.log(`Current gid: ${process.getegid()}`);
      try {
        process.setegid(501);
        console.log(`New gid: ${process.getegid()}`);
      } catch (err) {
        console.error(`Failed to set gid: ${err}`);
      }
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in `Worker` threads.

### `process.seteuid(id)`#

Added in: v2.0.0

- `id` `<string>` | `<number>` A user name or ID

The `process.seteuid()` method sets the effective user identity of the process. (See `seteuid(2)`.) The `id` can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

    import process from 'node:process';

    if (process.geteuid && process.seteuid) {
      console.log(`Current uid: ${process.geteuid()}`);
      try {
        process.seteuid(501);
        console.log(`New uid: ${process.geteuid()}`);
      } catch (err) {
        console.error(`Failed to set uid: ${err}`);
      }
    }
    if (process.geteuid && process.seteuid) {
      console.log(`Current uid: ${process.geteuid()}`);
      try {
        process.seteuid(501);
        console.log(`New uid: ${process.geteuid()}`);
      } catch (err) {
        console.error(`Failed to set uid: ${err}`);
      }
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in `Worker` threads.

### `process.setgid(id)`#

Added in: v0.1.31

- `id` `<string>` | `<number>` The group name or ID

The `process.setgid()` method sets the group identity of the process. (See `setgid(2)`.) The `id` can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated numeric ID.

    import process from 'node:process';

    if (process.getgid && process.setgid) {
      console.log(`Current gid: ${process.getgid()}`);
      try {
        process.setgid(501);
        console.log(`New gid: ${process.getgid()}`);
      } catch (err) {
        console.error(`Failed to set gid: ${err}`);
      }
    }
    if (process.getgid && process.setgid) {
      console.log(`Current gid: ${process.getgid()}`);
      try {
        process.setgid(501);
        console.log(`New gid: ${process.getgid()}`);
      } catch (err) {
        console.error(`Failed to set gid: ${err}`);
      }
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in `Worker` threads.

### `process.setgroups(groups)`#

Added in: v0.9.4

- `groups` `<integer>`[]

The `process.setgroups()` method sets the supplementary group IDs for the Node.js process. This is a privileged operation that requires the Node.js process to have `root` or the `CAP_SETGID` capability.

The `groups` array can contain numeric group IDs, group names, or both.

    import process from 'node:process';

    if (process.getgroups && process.setgroups) {
      try {
        process.setgroups([501]);
        console.log(process.getgroups()); // new groups
      } catch (err) {
        console.error(`Failed to set groups: ${err}`);
      }
    }
    if (process.getgroups && process.setgroups) {
      try {
        process.setgroups([501]);
        console.log(process.getgroups()); // new groups
      } catch (err) {
        console.error(`Failed to set groups: ${err}`);
      }
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in `Worker` threads.

### `process.setuid(id)`#

Added in: v0.1.28

- `id` `<integer>` | `<string>`

The `process.setuid(id)` method sets the user identity of the process. (See `setuid(2)`.) The `id` can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

    import process from 'node:process';

    if (process.getuid && process.setuid) {
      console.log(`Current uid: ${process.getuid()}`);
      try {
        process.setuid(501);
        console.log(`New uid: ${process.getuid()}`);
      } catch (err) {
        console.error(`Failed to set uid: ${err}`);
      }
    }
    if (process.getuid && process.setuid) {
      console.log(`Current uid: ${process.getuid()}`);
      try {
        process.setuid(501);
        console.log(`New uid: ${process.getuid()}`);
      } catch (err) {
        console.error(`Failed to set uid: ${err}`);
      }
    }
    javascript

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in `Worker` threads.

### `process.setSourceMapsEnabled(val)`#

Added in: v16.6.0, v14.18.0

Stability: 1 - Experimental: Use `module.setSourceMapsSupport()` instead.

- `val` `<boolean>`

This function enables or disables the Source Map support for stack traces.

It provides same features as launching Node.js process with commandline options `--enable-source-maps`.

Only source maps in JavaScript files that are loaded after source maps has been enabled will be parsed and loaded.

This implies calling `module.setSourceMapsSupport()` with an option `{ nodeModules: true, generatedCode: true }`.

### `process.setUncaughtExceptionCaptureCallback(fn)`#

Added in: v9.3.0
History

Version | Changes
--- | ---
v25.9.0 | Use `process.addUncaughtExceptionCaptureCallback()` to register multiple callbacks.

- `fn` `<Function>` | `<null>`

The `process.setUncaughtExceptionCaptureCallback()` function sets a function that will be invoked when an uncaught exception occurs, which will receive the exception value itself as its first argument.

If such a function is set, the `'uncaughtException'` event will not be emitted. If `--abort-on-uncaught-exception` was passed from the command line or set through `v8.setFlagsFromString()`, the process will not abort. Actions configured to take place on exceptions such as report generations will be affected too

To unset the capture function, `process.setUncaughtExceptionCaptureCallback(null)` may be used. Calling this method with a non-`null` argument while another capture function is set will throw an error.

To register multiple callbacks that can coexist, use `process.addUncaughtExceptionCaptureCallback()` instead.

### `process.sourceMapsEnabled`#

Added in: v20.7.0, v18.19.0

Stability: 1 - Experimental: Use `module.getSourceMapsSupport()` instead.

- Type: `<boolean>`

The `process.sourceMapsEnabled` property returns whether the Source Map support for stack traces is enabled.

### `process.stderr`#

- Type: `<Stream>`

The `process.stderr` property returns a stream connected to `stderr` (fd `2`). It is a `net.Socket` (which is a Duplex stream) unless fd `2` refers to a file, in which case it is a Writable stream.

`process.stderr` differs from other Node.js streams in important ways. See note on process I/O for more information.

#### `process.stderr.fd`#

- Type: `<number>`

This property refers to the value of underlying file descriptor of `process.stderr`. The value is fixed at `2`. In `Worker` threads, this field does not exist.

### `process.stdin`#

- Type: `<Stream>`

The `process.stdin` property returns a stream connected to `stdin` (fd `0`). It is a `net.Socket` (which is a Duplex stream) unless fd `0` refers to a file, in which case it is a Readable stream.

For details of how to read from `stdin` see `readable.read()`.

As a Duplex stream, `process.stdin` can also be used in "old" mode that is compatible with scripts written for Node.js prior to v0.10. For more information see Stream compatibility.

In "old" streams mode the `stdin` stream is paused by default, so one must call `process.stdin.resume()` to read from it. Note also that calling `process.stdin.resume()` itself would switch stream to "old" mode.

#### `process.stdin.fd`#

- Type: `<number>`

This property refers to the value of underlying file descriptor of `process.stdin`. The value is fixed at `0`. In `Worker` threads, this field does not exist.

### `process.stdout`#

- Type: `<Stream>`

The `process.stdout` property returns a stream connected to `stdout` (fd `1`). It is a `net.Socket` (which is a Duplex stream) unless fd `1` refers to a file, in which case it is a Writable stream.

For example, to copy `process.stdin` to `process.stdout`:

    import { stdin, stdout } from 'node:process';

    stdin.pipe(stdout);
    const { stdin, stdout } = require('node:process');

    stdin.pipe(stdout);
    javascript

`process.stdout` differs from other Node.js streams in important ways. See note on process I/O for more information.

#### `process.stdout.fd`#

- Type: `<number>`

This property refers to the value of underlying file descriptor of `process.stdout`. The value is fixed at `1`. In `Worker` threads, this field does not exist.

#### A note on process I/O#

`process.stdout` and `process.stderr` differ from other Node.js streams in important ways:

1. They are used internally by `console.log()` and `console.error()`, respectively.
2. Writes may be synchronous depending on what the stream is connected to and whether the system is Windows or POSIX:
   - Files: synchronous on Windows and POSIX
   - TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
   - Pipes (and sockets): synchronous on Windows, asynchronous on POSIX

These behaviors are partly for historical reasons, as changing them would create backward incompatibility, but they are also expected by some users.

Synchronous writes avoid problems such as output written with `console.log()` or `console.error()` being unexpectedly interleaved, or not written at all if `process.exit()` is called before an asynchronous write completes. See `process.exit()` for more information.

Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, it's possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.

To check if a stream is connected to a TTY context, check the `isTTY` property.

For instance:

    $ node -p "Boolean(process.stdin.isTTY)"
    true
    $ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
    false
    $ node -p "Boolean(process.stdout.isTTY)"
    true
    $ node -p "Boolean(process.stdout.isTTY)" | cat
    false
    console

See the TTY documentation for more information.

### `process.throwDeprecation`#

Added in: v0.9.12

- Type: `<boolean>`

The initial value of `process.throwDeprecation` indicates whether the `--throw-deprecation` flag is set on the current Node.js process. `process.throwDeprecation` is mutable, so whether or not deprecation warnings result in errors may be altered at runtime. See the documentation for the `'warning'` event and the `emitWarning()` method for more information.

    $ node --throw-deprecation -p "process.throwDeprecation"
    true
    $ node -p "process.throwDeprecation"
    undefined
    $ node
    > process.emitWarning('test', 'DeprecationWarning');
    undefined
    > (node:26598) DeprecationWarning: test
    > process.throwDeprecation = true;
    true
    > process.emitWarning('test', 'DeprecationWarning');
    Thrown:
    [DeprecationWarning: test] { name: 'DeprecationWarning' }
    console

### `process.threadCpuUsage([previousValue])`#

Added in: v23.9.0, v22.19.0

- `previousValue` `<Object>` A previous return value from calling `process.threadCpuUsage()`
- Returns: `<Object>`
  - `user` `<integer>`
  - `system` `<integer>`

The `process.threadCpuUsage()` method returns the user and system CPU time usage of the current worker thread, in an object with properties `user` and `system`, whose values are microsecond values (millionth of a second).

The result of a previous call to `process.threadCpuUsage()` can be passed as the argument to the function, to get a diff reading.

### `process.title`#

Added in: v0.1.104

- Type: `<string>`

The `process.title` property returns the current process title (i.e. returns the current value of `ps`). Assigning a new value to `process.title` modifies the current value of `ps`.

When a new value is assigned, different platforms will impose different maximum length restrictions on the title. Usually such restrictions are quite limited. For instance, on Linux and macOS, `process.title` is limited to the size of the binary name plus the length of the command-line arguments because setting the `process.title` overwrites the `argv` memory of the process. Node.js 0.8 allowed for longer process title strings by also overwriting the `environ` memory but that was potentially insecure and confusing in some (rather obscure) cases.

Assigning a value to `process.title` might not result in an accurate label within process manager applications such as macOS Activity Monitor or Windows Services Manager.

### `process.traceDeprecation`#

Added in: v0.8.0

- Type: `<boolean>`

The `process.traceDeprecation` property indicates whether the `--trace-deprecation` flag is set on the current Node.js process. See the documentation for the `'warning'` event and the `emitWarning()` method for more information about this flag's behavior.

### `process.traceProcessWarnings`#

Added in: v6.10.0

- `<boolean>`

The `process.traceProcessWarnings` property indicates whether the `--trace-warnings` flag is set on the current Node.js process. This property allows programmatic control over the tracing of warnings, enabling or disabling stack traces for warnings at runtime.

    // Enable trace warnings
    process.traceProcessWarnings = true;

    // Emit a warning with a stack trace
    process.emitWarning('Warning with stack trace');

    // Disable trace warnings
    process.traceProcessWarnings = false;
    js

### `process.umask()`#

Added in: v0.1.19
History

Version | Changes
--- | ---
v14.0.0, v12.19.0 | Calling `process.umask()` with no arguments is deprecated.

Stability: 0 - Deprecated. Calling `process.umask()` with no argument causes the process-wide umask to be written twice. This introduces a race condition between threads, and is a potential security vulnerability. There is no safe, cross-platform alternative API.

`process.umask()` returns the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process.

### `process.umask(mask)`#

Added in: v0.1.19

- `mask` `<string>` | `<integer>`

`process.umask(mask)` sets the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process. Returns the previous mask.

    import { umask } from 'node:process';

    const newmask = 0o022;
    const oldmask = umask(newmask);
    console.log(
      `Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`,
    );
    const { umask } = require('node:process');

    const newmask = 0o022;
    const oldmask = umask(newmask);
    console.log(
      `Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`,
    );
    javascript

In `Worker` threads, `process.umask(mask)` will throw an exception.

### `process.unref(maybeRefable)`#

Added in: v23.6.0, v22.14.0

Stability: 1 - Experimental

- `maybeRefable` `<any>` An object that may be "unref'd".

An object is "unrefable" if it implements the Node.js "Refable protocol". Specifically, this means that the object implements the `Symbol.for('nodejs.ref')` and `Symbol.for('nodejs.unref')` methods. "Ref'd" objects will keep the Node.js event loop alive, while "unref'd" objects will not. Historically, this was implemented by using `ref()` and `unref()` methods directly on the objects. This pattern, however, is being deprecated in favor of the "Refable protocol" in order to better support Web Platform API types whose APIs cannot be modified to add `ref()` and `unref()` methods but still need to support that behavior.

### `process.uptime()`#

Added in: v0.5.0

- Returns: `<number>`

The `process.uptime()` method returns the number of seconds the current Node.js process has been running.

The return value includes fractions of a second. Use `Math.floor()` to get whole seconds.

### `process.version`#

Added in: v0.1.3

- Type: `<string>`

The `process.version` property contains the Node.js version string.

    import { version } from 'node:process';

    console.log(`Version: ${version}`);
    // Version: v14.8.0
    const { version } = require('node:process');

    console.log(`Version: ${version}`);
    // Version: v14.8.0
    javascript

To get the version string without the prepended v, use `process.versions.node`.

### `process.versions`#

Added in: v0.2.0
History

Version | Changes
--- | ---
v9.0.0 | The `v8` property now includes a Node.js specific suffix.
v4.2.0 | The `icu` property is now supported.

- Type: `<Object>`

The `process.versions` property returns an object listing the version strings of Node.js and its dependencies. `process.versions.modules` indicates the current ABI version, which is increased whenever a C++ API changes. Node.js will refuse to load modules that were compiled against a different module ABI version.

    import { versions } from 'node:process';

    console.log(versions);
    const { versions } = require('node:process');

    console.log(versions);
    javascript

Will generate an object similar to:

    { node: '26.0.0-pre',
      acorn: '8.15.0',
      ada: '3.4.1',
      amaro: '1.1.5',
      ares: '1.34.6',
      brotli: '1.2.0',
      merve: '1.0.0',
      cldr: '48.0',
      icu: '78.2',
      llhttp: '9.3.0',
      modules: '144',
      napi: '10',
      nbytes: '0.1.1',
      ncrypto: '0.0.1',
      nghttp2: '1.68.0',
      nghttp3: '',
      ngtcp2: '',
      openssl: '3.5.4',
      simdjson: '4.2.4',
      simdutf: '7.3.3',
      sqlite: '3.51.2',
      tz: '2025c',
      undici: '7.18.2',
      unicode: '17.0',
      uv: '1.51.0',
      uvwasi: '0.0.23',
      v8: '14.3.127.18-node.10',
      zlib: '1.3.1-e00f703',
      zstd: '1.5.7' }
    console

### Exit codes#

Node.js will normally exit with a `0` status code when no more async operations are pending. The following status codes are used in other cases:

- `1` Uncaught Fatal Exception: There was an uncaught exception, and it was not handled by a domain or an `'uncaughtException'` event handler.
- `2`: Unused (reserved by Bash for builtin misuse)
- `3` Internal JavaScript Parse Error: The JavaScript source code internal in the Node.js bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.
- `4` Internal JavaScript Evaluation Failure: The JavaScript source code internal in the Node.js bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.
- `5` Fatal Error: There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix `FATAL ERROR`.
- `6` Non-function Internal Exception Handler: There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
- `7` Internal Exception Handler Run-Time Failure: There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if an `'uncaughtException'` or `domain.on('error')` handler throws an error.
- `8`: Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.
- `9` Invalid Argument: Either an unknown option was specified, or an option requiring a value was provided without a value.
- `10` Internal JavaScript Run-Time Failure: The JavaScript source code internal in the Node.js bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.
- `12` Invalid Debug Argument: The `--inspect` and/or `--inspect-brk` options were set, but the port number chosen was invalid or unavailable.
- `13` Unsettled Top-Level Await: `await` was used outside of a function in the top-level code, but the passed `Promise` never settled.
- `14` Snapshot Failure: Node.js was started to build a V8 startup snapshot and it failed because certain requirements of the state of the application were not met.
- `>128` Signal Exits: If Node.js receives a fatal signal such as `SIGKILL` or `SIGHUP`, then its exit code will be `128` plus the value of the signal code. This is a standard POSIX practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code. For example, signal `SIGABRT` has value `6`, so the expected exit code will be `128` + `6`, or `134`.

## Punycode#

Source Code: lib/punycode.js Deprecated in: v7.0.0

Stability: 0 - Deprecated

The version of the punycode module bundled in Node.js is being deprecated. In a future major version of Node.js this module will be removed. Users currently depending on the `punycode` module should switch to using the userland-provided Punycode.js module instead. For punycode-based URL encoding, see `url.domainToASCII` or, more generally, the WHATWG URL API.

The `punycode` module is a bundled version of the Punycode.js module. It can be accessed using:

    const punycode = require('node:punycode');
    js

Punycode is a character encoding scheme defined by RFC 3492 that is primarily intended for use in Internationalized Domain Names. Because host names in URLs are limited to ASCII characters only, Domain Names that contain non-ASCII characters must be converted into ASCII using the Punycode scheme. For instance, the Japanese character that translates into the English word, `'example'` is `'例'`. The Internationalized Domain Name, `'例.com'` (equivalent to `'example.com'`) is represented by Punycode as the ASCII string `'xn--fsq.com'`.

The `punycode` module provides a simple implementation of the Punycode standard.

The `punycode` module is a third-party dependency used by Node.js and made available to developers as a convenience. Fixes or other modifications to the module must be directed to the Punycode.js project.

### `punycode.decode(string)`#

Added in: v0.5.1

- `string` `<string>`

The `punycode.decode()` method converts a Punycode string of ASCII-only characters to the equivalent string of Unicode codepoints.

    punycode.decode('maana-pta'); // 'mañana'
    punycode.decode('--dqo34k'); // '☃-⌘'
    js

### `punycode.encode(string)`#

Added in: v0.5.1

- `string` `<string>`

The `punycode.encode()` method converts a string of Unicode codepoints to a Punycode string of ASCII-only characters.

    punycode.encode('mañana'); // 'maana-pta'
    punycode.encode('☃-⌘'); // '--dqo34k'
    js

### `punycode.toASCII(domain)`#

Added in: v0.6.1

- `domain` `<string>`

The `punycode.toASCII()` method converts a Unicode string representing an Internationalized Domain Name to Punycode. Only the non-ASCII parts of the domain name will be converted. Calling `punycode.toASCII()` on a string that already only contains ASCII characters will have no effect.

    // encode domain names
    punycode.toASCII('mañana.com');  // 'xn--maana-pta.com'
    punycode.toASCII('☃-⌘.com');   // 'xn----dqo34k.com'
    punycode.toASCII('example.com'); // 'example.com'
    js

### `punycode.toUnicode(domain)`#

Added in: v0.6.1

- `domain` `<string>`

The `punycode.toUnicode()` method converts a string representing a domain name containing Punycode encoded characters into Unicode. Only the Punycode encoded parts of the domain name are converted.

    // decode domain names
    punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
    punycode.toUnicode('xn----dqo34k.com');  // '☃-⌘.com'
    punycode.toUnicode('example.com');       // 'example.com'
    js

### `punycode.ucs2`#

Added in: v0.7.0

#### `punycode.ucs2.decode(string)`#

Added in: v0.7.0

- `string` `<string>`

The `punycode.ucs2.decode()` method returns an array containing the numeric codepoint values of each Unicode symbol in the string.

    punycode.ucs2.decode('abc'); // [0x61, 0x62, 0x63]
    // surrogate pair for U+1D306 tetragram for centre:
    punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]
    js

#### `punycode.ucs2.encode(codePoints)`#

Added in: v0.7.0

- `codePoints` `<integer>`[]

The `punycode.ucs2.encode()` method returns a string based on an array of numeric code point values.

    punycode.ucs2.encode([0x61, 0x62, 0x63]); // 'abc'
    punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'
    js

### `punycode.version`#

Added in: v0.6.1

- Type: `<string>`

Returns a string identifying the current Punycode.js version number.

## Query string#

Source Code: lib/querystring.js

Stability: 2 - Stable

The `node:querystring` module provides utilities for parsing and formatting URL query strings. It can be accessed using:

    const querystring = require('node:querystring');
    js

`querystring` is more performant than `<URLSearchParams>` but is not a standardized API. Use `<URLSearchParams>` when performance is not critical or when compatibility with browser code is desirable.

### `querystring.decode()`#

Added in: v0.1.99

The `querystring.decode()` function is an alias for `querystring.parse()`.

### `querystring.encode()`#

Added in: v0.1.99

The `querystring.encode()` function is an alias for `querystring.stringify()`.

### `querystring.escape(str)`#

Added in: v0.1.25

- `str` `<string>`

The `querystring.escape()` method performs URL percent-encoding on the given `str` in a manner that is optimized for the specific requirements of URL query strings.

The `querystring.escape()` method is used by `querystring.stringify()` and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement percent-encoding implementation if necessary by assigning `querystring.escape` to an alternative function.

### `querystring.parse(str[, sep[, eq[, options]]])`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v8.0.0 | Multiple empty entries are now parsed correctly (e.g. `&=&=`).
v6.0.0 | The returned object no longer inherits from `Object.prototype`.
v6.0.0, v4.2.4 | The `eq` parameter may now have a length of more than `1`.

- `str` `<string>` The URL query string to parse
- `sep` `<string>` The substring used to delimit key and value pairs in the query string. Default: `'&'`.
- `eq` `<string>`. The substring used to delimit keys and values in the query string. Default: `'='`.
- `options` `<Object>`
  - `decodeURIComponent` `<Function>` The function to use when decoding percent-encoded characters in the query string. Default: `querystring.unescape()`.
  - `maxKeys` `<number>` Specifies the maximum number of keys to parse. Specify `0` to remove key counting limitations. Default: `1000`.

The `querystring.parse()` method parses a URL query string (`str`) into a collection of key and value pairs.

For example, the query string `'foo=bar&abc=xyz&abc=123'` is parsed into:

    {
      "foo": "bar",
      "abc": ["xyz", "123"]
    }
    json

The object returned by the `querystring.parse()` method does not prototypically inherit from the JavaScript `Object`. This means that typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others are not defined and will not work.

By default, percent-encoded characters within the query string will be assumed to use UTF-8 encoding. If an alternative character encoding is used, then an alternative `decodeURIComponent` option will need to be specified:

    // Assuming gbkDecodeURIComponent function already exists...

    querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
                      { decodeURIComponent: gbkDecodeURIComponent });
    js

### `querystring.stringify(obj[, sep[, eq[, options]]])`#

Added in: v0.1.25

- `obj` `<Object>` The object to serialize into a URL query string
- `sep` `<string>` The substring used to delimit key and value pairs in the query string. Default: `'&'`.
- `eq` `<string>`. The substring used to delimit keys and values in the query string. Default: `'='`.
- `options`
  - `encodeURIComponent` `<Function>` The function to use when converting URL-unsafe characters to percent-encoding in the query string. Default: `querystring.escape()`.

The `querystring.stringify()` method produces a URL query string from a given `obj` by iterating through the object's "own properties".

It serializes the following types of values passed in `obj`: `<string>` | `<number>` | `<bigint>` | `<boolean>` | `<string>`[] | `<number>`[] | `<bigint>`[] | `<boolean>`[] The numeric values must be finite. Any other input values will be coerced to empty strings.

    querystring.stringify({ foo: 'bar', baz: ['qux', 'quux'], corge: '' });
    // Returns 'foo=bar&baz=qux&baz=quux&corge='

    querystring.stringify({ foo: 'bar', baz: 'qux' }, ';', ':');
    // Returns 'foo:bar;baz:qux'
    js

By default, characters requiring percent-encoding within the query string will be encoded as UTF-8. If an alternative encoding is required, then an alternative `encodeURIComponent` option will need to be specified:

    // Assuming gbkEncodeURIComponent function already exists,

    querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
                          { encodeURIComponent: gbkEncodeURIComponent });
    js

### `querystring.unescape(str)`#

Added in: v0.1.25

- `str` `<string>`

The `querystring.unescape()` method performs decoding of URL percent-encoded characters on the given `str`.

The `querystring.unescape()` method is used by `querystring.parse()` and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement decoding implementation if necessary by assigning `querystring.unescape` to an alternative function.

By default, the `querystring.unescape()` method will attempt to use the JavaScript built-in `decodeURIComponent()` method to decode. If that fails, a safer equivalent that does not throw on malformed URLs will be used.

## Readline#

Source Code: lib/readline.js

Stability: 2 - Stable

The `node:readline` module provides an interface for reading data from a Readable stream (such as `process.stdin`) one line at a time.

To use the promise-based APIs:

    import * as readline from 'node:readline/promises';
    const readline = require('node:readline/promises');
    javascript

To use the callback and sync APIs:

    import * as readline from 'node:readline';
    const readline = require('node:readline');
    javascript

The following simple example illustrates the basic use of the `node:readline` module.

    import * as readline from 'node:readline/promises';
    import { stdin as input, stdout as output } from 'node:process';

    const rl = readline.createInterface({ input, output });

    const answer = await rl.question('What do you think of Node.js? ');

    console.log(`Thank you for your valuable feedback: ${answer}`);

    rl.close();
    const readline = require('node:readline');
    const { stdin: input, stdout: output } = require('node:process');

    const rl = readline.createInterface({ input, output });

    rl.question('What do you think of Node.js? ', (answer) => {
      // TODO: Log the answer in a database
      console.log(`Thank you for your valuable feedback: ${answer}`);

      rl.close();
    });
    javascript

Once this code is invoked, the Node.js application will not terminate until the `readline.Interface` is closed because the interface waits for data to be received on the `input` stream.

### Class: `InterfaceConstructor`#

Added in: v0.1.104

- Extends: `<EventEmitter>`

Instances of the `InterfaceConstructor` class are constructed using the `readlinePromises.createInterface()` or `readline.createInterface()` method. Every instance is associated with a single `input` Readable stream and a single `output` Writable stream. The `output` stream is used to print prompts for user input that arrives on, and is read from, the `input` stream.

#### Event: `'close'`#

Added in: v0.1.98

The `'close'` event is emitted when one of the following occur:

- The `rl.close()` method is called and the `InterfaceConstructor` instance has relinquished control over the `input` and `output` streams;
- The `input` stream receives its `'end'` event;
- The `input` stream receives `Ctrl`+`D` to signal end-of-transmission (EOT);
- The `input` stream receives `Ctrl`+`C` to signal `SIGINT` and there is no `'SIGINT'` event listener registered on the `InterfaceConstructor` instance.

The listener function is called without passing any arguments.

The `InterfaceConstructor` instance is finished once the `'close'` event is emitted.

#### Event: `'error'`#

Added in: v16.0.0

The `'error'` event is emitted when an error occurs on the `input` stream associated with the `node:readline` `Interface`.

The listener function is called with an `Error` object passed as the single argument.

#### Event: `'line'`#

Added in: v0.1.98

The `'line'` event is emitted whenever the `input` stream receives an end-of-line input (`\n`, `\r`, or `\r\n`). This usually occurs when the user presses `Enter` or `Return`.

The `'line'` event is also emitted if new data has been read from a stream and that stream ends without a final end-of-line marker.

The listener function is called with a string containing the single line of received input.

    rl.on('line', (input) => {
      console.log(`Received: ${input}`);
    });
    js

#### Event: `'history'`#

Added in: v15.8.0, v14.18.0

The `'history'` event is emitted whenever the history array has changed.

The listener function is called with an array containing the history array. It will reflect all changes, added lines and removed lines due to `historySize` and `removeHistoryDuplicates`.

The primary purpose is to allow a listener to persist the history. It is also possible for the listener to change the history object. This could be useful to prevent certain lines to be added to the history, like a password.

    rl.on('history', (history) => {
      console.log(`Received: ${history}`);
    });
    js

#### Event: `'pause'`#

Added in: v0.7.5

The `'pause'` event is emitted when one of the following occur:

- The `input` stream is paused.
- The `input` stream is not paused and receives the `'SIGCONT'` event. (See events `'SIGTSTP'` and `'SIGCONT'`.)

The listener function is called without passing any arguments.

    rl.on('pause', () => {
      console.log('Readline paused.');
    });
    js

#### Event: `'resume'`#

Added in: v0.7.5

The `'resume'` event is emitted whenever the `input` stream is resumed.

The listener function is called without passing any arguments.

    rl.on('resume', () => {
      console.log('Readline resumed.');
    });
    js

#### Event: `'SIGCONT'`#

Added in: v0.7.5

The `'SIGCONT'` event is emitted when a Node.js process previously moved into the background using `Ctrl`+`Z` (i.e. `SIGTSTP`) is then brought back to the foreground using `fg(1p)`.

If the `input` stream was paused before the `SIGTSTP` request, this event will not be emitted.

The listener function is invoked without passing any arguments.

    rl.on('SIGCONT', () => {
      // `prompt` will automatically resume the stream
      rl.prompt();
    });
    js

The `'SIGCONT'` event is not supported on Windows.

#### Event: `'SIGINT'`#

Added in: v0.3.0

The `'SIGINT'` event is emitted whenever the `input` stream receives a `Ctrl+C` input, known typically as `SIGINT`. If there are no `'SIGINT'` event listeners registered when the `input` stream receives a `SIGINT`, the `'pause'` event will be emitted.

The listener function is invoked without passing any arguments.

    rl.on('SIGINT', () => {
      rl.question('Are you sure you want to exit? ', (answer) => {
        if (answer.match(/^y(es)?$/i)) rl.pause();
      });
    });
    js

#### Event: `'SIGTSTP'`#

Added in: v0.7.5

The `'SIGTSTP'` event is emitted when the `input` stream receives a `Ctrl`+`Z` input, typically known as `SIGTSTP`. If there are no `'SIGTSTP'` event listeners registered when the `input` stream receives a `SIGTSTP`, the Node.js process will be sent to the background.

When the program is resumed using `fg(1p)`, the `'pause'` and `'SIGCONT'` events will be emitted. These can be used to resume the `input` stream.

The `'pause'` and `'SIGCONT'` events will not be emitted if the `input` was paused before the process was sent to the background.

The listener function is invoked without passing any arguments.

    rl.on('SIGTSTP', () => {
      // This will override SIGTSTP and prevent the program from going to the
      // background.
      console.log('Caught SIGTSTP.');
    });
    js

The `'SIGTSTP'` event is not supported on Windows.

#### `rl.close()`#

Added in: v0.1.98

The `rl.close()` method closes the `InterfaceConstructor` instance and relinquishes control over the `input` and `output` streams. When called, the `'close'` event will be emitted.

Calling `rl.close()` does not immediately stop other events (including `'line'`) from being emitted by the `InterfaceConstructor` instance.

#### `rl[Symbol.dispose]()`#

Added in: v23.10.0, v22.15.0

Alias for `rl.close()`.

#### `rl.pause()`#

Added in: v0.3.4

The `rl.pause()` method pauses the `input` stream, allowing it to be resumed later if necessary.

Calling `rl.pause()` does not immediately pause other events (including `'line'`) from being emitted by the `InterfaceConstructor` instance.

#### `rl.prompt([preserveCursor])`#

Added in: v0.1.98

- `preserveCursor` `<boolean>` If `true`, prevents the cursor placement from being reset to `0`.

The `rl.prompt()` method writes the `InterfaceConstructor` instances configured `prompt` to a new line in `output` in order to provide a user with a new location at which to provide input.

When called, `rl.prompt()` will resume the `input` stream if it has been paused.

If the `InterfaceConstructor` was created with `output` set to `null` or `undefined` the prompt is not written.

#### `rl.resume()`#

Added in: v0.3.4

The `rl.resume()` method resumes the `input` stream if it has been paused.

#### `rl.setPrompt(prompt)`#

Added in: v0.1.98

- `prompt` `<string>`

The `rl.setPrompt()` method sets the prompt that will be written to `output` whenever `rl.prompt()` is called.

#### `rl.getPrompt()`#

Added in: v15.3.0, v14.17.0

- Returns: `<string>` the current prompt string

The `rl.getPrompt()` method returns the current prompt used by `rl.prompt()`.

#### `rl.write(data[, key])`#

Added in: v0.1.98

- `data` `<string>`
- `key` `<Object>`
  - `ctrl` `<boolean>` `true` to indicate the `Ctrl` key.
  - `meta` `<boolean>` `true` to indicate the `Meta` key.
  - `shift` `<boolean>` `true` to indicate the `Shift` key.
  - `name` `<string>` The name of the a key.

The `rl.write()` method will write either `data` or a key sequence identified by `key` to the `output`. The `key` argument is supported only if `output` is a TTY text terminal. See TTY keybindings for a list of key combinations.

If `key` is specified, `data` is ignored.

When called, `rl.write()` will resume the `input` stream if it has been paused.

If the `InterfaceConstructor` was created with `output` set to `null` or `undefined` the `data` and `key` are not written.

    rl.write('Delete this!');
    // Simulate Ctrl+U to delete the line written previously
    rl.write(null, { ctrl: true, name: 'u' });
    js

The `rl.write()` method will write the data to the `readline` `Interface`'s `input` as if it were provided by the user.

#### `rl[Symbol.asyncIterator]()`#

Added in: v11.4.0, v10.16.0
History

Version | Changes
--- | ---
v11.14.0, v10.17.0 | Symbol.asyncIterator support is no longer experimental.

- Returns: `<AsyncIterator>`

Create an `AsyncIterator` object that iterates through each line in the input stream as a string. This method allows asynchronous iteration of `InterfaceConstructor` objects through `for await...of` loops.

Errors in the input stream are not forwarded.

If the loop is terminated with `break`, `throw`, or `return`, `rl.close()` will be called. In other words, iterating over a `InterfaceConstructor` will always consume the input stream fully.

Performance is not on par with the traditional `'line'` event API. Use `'line'` instead for performance-sensitive applications.

    async function processLineByLine() {
      const rl = readline.createInterface({
        // ...
      });

      for await (const line of rl) {
        // Each line in the readline input will be successively available here as
        // `line`.
      }
    }
    js

`readline.createInterface()` will start to consume the input stream once invoked. Having asynchronous operations between interface creation and asynchronous iteration may result in missed lines.

#### `rl.line`#

Added in: v0.1.98
History

Version | Changes
--- | ---
v15.8.0, v14.18.0 | Value will always be a string, never undefined.

- Type: `<string>`

The current input data being processed by node.

This can be used when collecting input from a TTY stream to retrieve the current value that has been processed thus far, prior to the `line` event being emitted. Once the `line` event has been emitted, this property will be an empty string.

Be aware that modifying the value during the instance runtime may have unintended consequences if `rl.cursor` is not also controlled.

If not using a TTY stream for input, use the `'line'` event.

One possible use case would be as follows:

    const values = ['lorem ipsum', 'dolor sit amet'];
    const rl = readline.createInterface(process.stdin);
    const showResults = debounce(() => {
      console.log(
        '\n',
        values.filter((val) => val.startsWith(rl.line)).join(' '),
      );
    }, 300);
    process.stdin.on('keypress', (c, k) => {
      showResults();
    });
    js

#### `rl.cursor`#

Added in: v0.1.98

- Type: `<number>` | `<undefined>`

The cursor position relative to `rl.line`.

This will track where the current cursor lands in the input string, when reading input from a TTY stream. The position of cursor determines the portion of the input string that will be modified as input is processed, as well as the column where the terminal caret will be rendered.

#### `rl.getCursorPos()`#

Added in: v13.5.0, v12.16.0

- Returns: `<Object>`
  - `rows` `<number>` the row of the prompt the cursor currently lands on
  - `cols` `<number>` the screen column the cursor currently lands on

Returns the real position of the cursor in relation to the input prompt + string. Long input (wrapping) strings, as well as multiple line prompts are included in the calculations.

### Promises API#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

#### Class: `readlinePromises.Interface`#

Added in: v17.0.0

- Extends: `<readline.InterfaceConstructor>`

Instances of the `readlinePromises.Interface` class are constructed using the `readlinePromises.createInterface()` method. Every instance is associated with a single `input` Readable stream and a single `output` Writable stream. The `output` stream is used to print prompts for user input that arrives on, and is read from, the `input` stream.

##### `rl.question(query[, options])`#

Added in: v17.0.0

- `query` `<string>` A statement or query to write to `output`, prepended to the prompt.
- `options` `<Object>`
  - `signal` `<AbortSignal>` Optionally allows the `question()` to be canceled using an `AbortSignal`.
- Returns: `<Promise>` A promise that is fulfilled with the user's input in response to the `query`.

The `rl.question()` method displays the `query` by writing it to the `output`, waits for user input to be provided on `input`, then invokes the `callback` function passing the provided input as the first argument.

When called, `rl.question()` will resume the `input` stream if it has been paused.

If the `readlinePromises.Interface` was created with `output` set to `null` or `undefined` the `query` is not written.

If the question is called after `rl.close()`, it returns a rejected promise.

Example usage:

    const answer = await rl.question('What is your favorite food? ');
    console.log(`Oh, so your favorite food is ${answer}`);
    mjs

Using an `AbortSignal` to cancel a question.

    const signal = AbortSignal.timeout(10_000);

    signal.addEventListener('abort', () => {
      console.log('The food question timed out');
    }, { once: true });

    const answer = await rl.question('What is your favorite food? ', { signal });
    console.log(`Oh, so your favorite food is ${answer}`);
    mjs

#### Class: `readlinePromises.Readline`#

Added in: v17.0.0

##### `new readlinePromises.Readline(stream[, options])`#

Added in: v17.0.0

- `stream` `<stream.Writable>` A TTY stream.
- `options` `<Object>`
  - `autoCommit` `<boolean>` If `true`, no need to call `rl.commit()`.

##### `rl.clearLine(dir)`#

Added in: v17.0.0

- `dir` `<integer>`
  - `-1`: to the left from cursor
  - `1`: to the right from cursor
  - `0`: the entire line
- Returns: this

The `rl.clearLine()` method adds to the internal list of pending action an action that clears current line of the associated `stream` in a specified direction identified by `dir`. Call `rl.commit()` to see the effect of this method, unless `autoCommit: true` was passed to the constructor.

##### `rl.clearScreenDown()`#

Added in: v17.0.0

- Returns: this

The `rl.clearScreenDown()` method adds to the internal list of pending action an action that clears the associated stream from the current position of the cursor down. Call `rl.commit()` to see the effect of this method, unless `autoCommit: true` was passed to the constructor.

##### `rl.commit()`#

Added in: v17.0.0

- Returns: `<Promise>`

The `rl.commit()` method sends all the pending actions to the associated `stream` and clears the internal list of pending actions.

##### `rl.cursorTo(x[, y])`#

Added in: v17.0.0

- `x` `<integer>`
- `y` `<integer>`
- Returns: this

The `rl.cursorTo()` method adds to the internal list of pending action an action that moves cursor to the specified position in the associated `stream`. Call `rl.commit()` to see the effect of this method, unless `autoCommit: true` was passed to the constructor.

##### `rl.moveCursor(dx, dy)`#

Added in: v17.0.0

- `dx` `<integer>`
- `dy` `<integer>`
- Returns: this

The `rl.moveCursor()` method adds to the internal list of pending action an action that moves the cursor relative to its current position in the associated `stream`. Call `rl.commit()` to see the effect of this method, unless `autoCommit: true` was passed to the constructor.

##### `rl.rollback()`#

Added in: v17.0.0

- Returns: this

The `rl.rollback` methods clears the internal list of pending actions without sending it to the associated `stream`.

#### `readlinePromises.createInterface(options)`#

Added in: v17.0.0

- `options` `<Object>`
  - `input` `<stream.Readable>` The Readable stream to listen to. This option is required.
  - `output` `<stream.Writable>` The Writable stream to write readline data to.
  - `completer` `<Function>` An optional function used for Tab autocompletion.
  - `terminal` `<boolean>` `true` if the `input` and `output` streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checking `isTTY` on the `output` stream upon instantiation.
  - `history` `<string>`[] Initial list of history lines. This option makes sense only if `terminal` is set to `true` by the user or by an internal `output` check, otherwise the history caching mechanism is not initialized at all. Default: `[]`.
  - `historySize` `<number>` Maximum number of history lines retained. To disable the history set this value to `0`. This option makes sense only if `terminal` is set to `true` by the user or by an internal `output` check, otherwise the history caching mechanism is not initialized at all. Default: `30`.
  - `removeHistoryDuplicates` `<boolean>` If `true`, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default: `false`.
  - `prompt` `<string>` The prompt string to use. Default: `'> '`.
  - `crlfDelay` `<number>` If the delay between `\r` and `\n` exceeds `crlfDelay` milliseconds, both `\r` and `\n` will be treated as separate end-of-line input. `crlfDelay` will be coerced to a number no less than `100`. It can be set to `Infinity`, in which case `\r` followed by `\n` will always be considered a single newline (which may be reasonable for reading files with `\r\n` line delimiter). Default: `100`.
  - `escapeCodeTimeout` `<number>` The duration `readlinePromises` will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default: `500`.
  - `tabSize` `<integer>` The number of spaces a tab is equal to (minimum 1). Default: `8`.
  - `signal` `<AbortSignal>` Allows closing the interface using an AbortSignal.
- Returns: `<readlinePromises.Interface>`

The `readlinePromises.createInterface()` method creates a new `readlinePromises.Interface` instance.

    import { createInterface } from 'node:readline/promises';
    import { stdin, stdout } from 'node:process';
    const rl = createInterface({
      input: stdin,
      output: stdout,
    });
    const { createInterface } = require('node:readline/promises');
    const rl = createInterface({
      input: process.stdin,
      output: process.stdout,
    });
    javascript

Once the `readlinePromises.Interface` instance is created, the most common case is to listen for the `'line'` event:

    rl.on('line', (line) => {
      console.log(`Received: ${line}`);
    });
    js

If `terminal` is `true` for this instance then the `output` stream will get the best compatibility if it defines an `output.columns` property and emits a `'resize'` event on the `output` if or when the columns ever change (`process.stdout` does this automatically when it is a TTY).

##### Use of the `completer` function#

The `completer` function takes the current line entered by the user as an argument, and returns an `Array` with 2 entries:

- An `Array` with matching entries for the completion.
- The substring that was used for the matching.

For instance: `[[substr1, substr2, ...], originalsubstring]`.

    function completer(line) {
      const completions = '.help .error .exit .quit .q'.split(' ');
      const hits = completions.filter((c) => c.startsWith(line));
      // Show all completions if none found
      return [hits.length ? hits : completions, line];
    }
    js

The `completer` function can also return a `<Promise>`, or be asynchronous:

    async function completer(linePartial) {
      await someAsyncWork();
      return [['123'], linePartial];
    }
    js

### Callback API#

Added in: v0.1.104

#### Class: `readline.Interface`#

Added in: v0.1.104
History

Version | Changes
--- | ---
v17.0.0 | The class `readline.Interface` now inherits from `Interface`.

- Extends: `<readline.InterfaceConstructor>`

Instances of the `readline.Interface` class are constructed using the `readline.createInterface()` method. Every instance is associated with a single `input` Readable stream and a single `output` Writable stream. The `output` stream is used to print prompts for user input that arrives on, and is read from, the `input` stream.

##### `rl.question(query[, options], callback)`#

Added in: v0.3.3

- `query` `<string>` A statement or query to write to `output`, prepended to the prompt.
- `options` `<Object>`
  - `signal` `<AbortSignal>` Optionally allows the `question()` to be canceled using an `AbortController`.
- `callback` `<Function>` A callback function that is invoked with the user's input in response to the `query`.

The `rl.question()` method displays the `query` by writing it to the `output`, waits for user input to be provided on `input`, then invokes the `callback` function passing the provided input as the first argument.

When called, `rl.question()` will resume the `input` stream if it has been paused.

If the `readline.Interface` was created with `output` set to `null` or `undefined` the `query` is not written.

The `callback` function passed to `rl.question()` does not follow the typical pattern of accepting an `Error` object or `null` as the first argument. The `callback` is called with the provided answer as the only argument.

An error will be thrown if calling `rl.question()` after `rl.close()`.

Example usage:

    rl.question('What is your favorite food? ', (answer) => {
      console.log(`Oh, so your favorite food is ${answer}`);
    });
    js

Using an `AbortController` to cancel a question.

    const ac = new AbortController();
    const signal = ac.signal;

    rl.question('What is your favorite food? ', { signal }, (answer) => {
      console.log(`Oh, so your favorite food is ${answer}`);
    });

    signal.addEventListener('abort', () => {
      console.log('The food question timed out');
    }, { once: true });

    setTimeout(() => ac.abort(), 10000);
    js

#### `readline.clearLine(stream, dir[, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v12.7.0 | The stream's write() callback and return value are exposed.

- `stream` `<stream.Writable>`
- `dir` `<number>`
  - `-1`: to the left from cursor
  - `1`: to the right from cursor
  - `0`: the entire line
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if `stream` wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

The `readline.clearLine()` method clears current line of given TTY stream in a specified direction identified by `dir`.

#### `readline.clearScreenDown(stream[, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v12.7.0 | The stream's write() callback and return value are exposed.

- `stream` `<stream.Writable>`
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if `stream` wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

The `readline.clearScreenDown()` method clears the given TTY stream from the current position of the cursor down.

#### `readline.createInterface(options)`#

Added in: v0.1.98
History

Version | Changes
--- | ---
v15.14.0, v14.18.0 | The `signal` option is supported now.
v15.8.0, v14.18.0 | The `history` option is supported now.
v13.9.0 | The `tabSize` option is supported now.
v8.3.0, v6.11.4 | Remove max limit of `crlfDelay` option.
v6.6.0 | The `crlfDelay` option is supported now.
v6.3.0 | The `prompt` option is supported now.
v6.0.0 | The `historySize` option can be `0` now.

- `options` `<Object>`
  - `input` `<stream.Readable>` The Readable stream to listen to. This option is required.
  - `output` `<stream.Writable>` The Writable stream to write readline data to.
  - `completer` `<Function>` An optional function used for Tab autocompletion.
  - `terminal` `<boolean>` `true` if the `input` and `output` streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checking `isTTY` on the `output` stream upon instantiation.
  - `history` `<string>`[] Initial list of history lines. This option makes sense only if `terminal` is set to `true` by the user or by an internal `output` check, otherwise the history caching mechanism is not initialized at all. Default: `[]`.
  - `historySize` `<number>` Maximum number of history lines retained. To disable the history set this value to `0`. This option makes sense only if `terminal` is set to `true` by the user or by an internal `output` check, otherwise the history caching mechanism is not initialized at all. Default: `30`.
  - `removeHistoryDuplicates` `<boolean>` If `true`, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default: `false`.
  - `prompt` `<string>` The prompt string to use. Default: `'> '`.
  - `crlfDelay` `<number>` If the delay between `\r` and `\n` exceeds `crlfDelay` milliseconds, both `\r` and `\n` will be treated as separate end-of-line input. `crlfDelay` will be coerced to a number no less than `100`. It can be set to `Infinity`, in which case `\r` followed by `\n` will always be considered a single newline (which may be reasonable for reading files with `\r\n` line delimiter). Default: `100`.
  - `escapeCodeTimeout` `<number>` The duration `readline` will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default: `500`.
  - `tabSize` `<integer>` The number of spaces a tab is equal to (minimum 1). Default: `8`.
  - `signal` `<AbortSignal>` Allows closing the interface using an AbortSignal. Aborting the signal will internally call `close` on the interface.
- Returns: `<readline.Interface>`

The `readline.createInterface()` method creates a new `readline.Interface` instance.

    import { createInterface } from 'node:readline';
    import { stdin, stdout } from 'node:process';
    const rl = createInterface({
      input: stdin,
      output: stdout,
    });
    const { createInterface } = require('node:readline');
    const rl = createInterface({
      input: process.stdin,
      output: process.stdout,
    });
    javascript

Once the `readline.Interface` instance is created, the most common case is to listen for the `'line'` event:

    rl.on('line', (line) => {
      console.log(`Received: ${line}`);
    });
    js

If `terminal` is `true` for this instance then the `output` stream will get the best compatibility if it defines an `output.columns` property and emits a `'resize'` event on the `output` if or when the columns ever change (`process.stdout` does this automatically when it is a TTY).

When creating a `readline.Interface` using `stdin` as input, the program will not terminate until it receives an EOF character. To exit without waiting for user input, call `process.stdin.unref()`.

##### Use of the `completer` function#

The `completer` function takes the current line entered by the user as an argument, and returns an `Array` with 2 entries:

- An `Array` with matching entries for the completion.
- The substring that was used for the matching.

For instance: `[[substr1, substr2, ...], originalsubstring]`.

    function completer(line) {
      const completions = '.help .error .exit .quit .q'.split(' ');
      const hits = completions.filter((c) => c.startsWith(line));
      // Show all completions if none found
      return [hits.length ? hits : completions, line];
    }
    js

The `completer` function can be called asynchronously if it accepts two arguments:

    function completer(linePartial, callback) {
      callback(null, [['123'], linePartial]);
    }
    js

#### `readline.cursorTo(stream, x[, y][, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v12.7.0 | The stream's write() callback and return value are exposed.

- `stream` `<stream.Writable>`
- `x` `<number>`
- `y` `<number>`
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if `stream` wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

The `readline.cursorTo()` method moves cursor to the specified position in a given TTY `stream`.

#### `readline.moveCursor(stream, dx, dy[, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v12.7.0 | The stream's write() callback and return value are exposed.

- `stream` `<stream.Writable>`
- `dx` `<number>`
- `dy` `<number>`
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if `stream` wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

The `readline.moveCursor()` method moves the cursor relative to its current position in a given TTY `stream`.

### `readline.emitKeypressEvents(stream[, interface])`#

Added in: v0.7.7

- `stream` `<stream.Readable>`
- `interface` `<readline.InterfaceConstructor>`

The `readline.emitKeypressEvents()` method causes the given Readable stream to begin emitting `'keypress'` events corresponding to received input.

Optionally, `interface` specifies a `readline.Interface` instance for which autocompletion is disabled when copy-pasted input is detected.

If the `stream` is a TTY, then it must be in raw mode.

This is automatically called by any readline instance on its `input` if the `input` is a terminal. Closing the `readline` instance does not stop the `input` from emitting `'keypress'` events.

    readline.emitKeypressEvents(process.stdin);
    if (process.stdin.isTTY)
      process.stdin.setRawMode(true);
    js

### Example: Tiny CLI#

The following example illustrates the use of `readline.Interface` class to implement a small command-line interface:

    import { createInterface } from 'node:readline';
    import { exit, stdin, stdout } from 'node:process';
    const rl = createInterface({
      input: stdin,
      output: stdout,
      prompt: 'OHAI> ',
    });

    rl.prompt();

    rl.on('line', (line) => {
      switch (line.trim()) {
        case 'hello':
          console.log('world!');
          break;
        default:
          console.log(`Say what? I might have heard '${line.trim()}'`);
          break;
      }
      rl.prompt();
    }).on('close', () => {
      console.log('Have a great day!');
      exit(0);
    });
    const { createInterface } = require('node:readline');
    const rl = createInterface({
      input: process.stdin,
      output: process.stdout,
      prompt: 'OHAI> ',
    });

    rl.prompt();

    rl.on('line', (line) => {
      switch (line.trim()) {
        case 'hello':
          console.log('world!');
          break;
        default:
          console.log(`Say what? I might have heard '${line.trim()}'`);
          break;
      }
      rl.prompt();
    }).on('close', () => {
      console.log('Have a great day!');
      process.exit(0);
    });
    javascript

### Example: Read file stream line-by-Line#

A common use case for `readline` is to consume an input file one line at a time. The easiest way to do so is leveraging the `fs.ReadStream` API as well as a `for await...of` loop:

    import { createReadStream } from 'node:fs';
    import { createInterface } from 'node:readline';

    async function processLineByLine() {
      const fileStream = createReadStream('input.txt');

      const rl = createInterface({
        input: fileStream,
        crlfDelay: Infinity,
      });
      // Note: we use the crlfDelay option to recognize all instances of CR LF
      // ('\r\n') in input.txt as a single line break.

      for await (const line of rl) {
        // Each line in input.txt will be successively available here as `line`.
        console.log(`Line from file: ${line}`);
      }
    }

    processLineByLine();
    const { createReadStream } = require('node:fs');
    const { createInterface } = require('node:readline');

    async function processLineByLine() {
      const fileStream = createReadStream('input.txt');

      const rl = createInterface({
        input: fileStream,
        crlfDelay: Infinity,
      });
      // Note: we use the crlfDelay option to recognize all instances of CR LF
      // ('\r\n') in input.txt as a single line break.

      for await (const line of rl) {
        // Each line in input.txt will be successively available here as `line`.
        console.log(`Line from file: ${line}`);
      }
    }

    processLineByLine();
    javascript

Alternatively, one could use the `'line'` event:

    import { createReadStream } from 'node:fs';
    import { createInterface } from 'node:readline';

    const rl = createInterface({
      input: createReadStream('sample.txt'),
      crlfDelay: Infinity,
    });

    rl.on('line', (line) => {
      console.log(`Line from file: ${line}`);
    });
    const { createReadStream } = require('node:fs');
    const { createInterface } = require('node:readline');

    const rl = createInterface({
      input: createReadStream('sample.txt'),
      crlfDelay: Infinity,
    });

    rl.on('line', (line) => {
      console.log(`Line from file: ${line}`);
    });
    javascript

Currently, `for await...of` loop can be a bit slower. If `async` / `await` flow and speed are both essential, a mixed approach can be applied:

    import { once } from 'node:events';
    import { createReadStream } from 'node:fs';
    import { createInterface } from 'node:readline';

    (async function processLineByLine() {
      try {
        const rl = createInterface({
          input: createReadStream('big-file.txt'),
          crlfDelay: Infinity,
        });

        rl.on('line', (line) => {
          // Process the line.
        });

        await once(rl, 'close');

        console.log('File processed.');
      } catch (err) {
        console.error(err);
      }
    })();
    const { once } = require('node:events');
    const { createReadStream } = require('node:fs');
    const { createInterface } = require('node:readline');

    (async function processLineByLine() {
      try {
        const rl = createInterface({
          input: createReadStream('big-file.txt'),
          crlfDelay: Infinity,
        });

        rl.on('line', (line) => {
          // Process the line.
        });

        await once(rl, 'close');

        console.log('File processed.');
      } catch (err) {
        console.error(err);
      }
    })();
    javascript

### TTY keybindings#

Keybindings | Description | Notes
--- | --- | ---
`Ctrl`+`Shift`+`Backspace` | Delete line left | Doesn't work on Linux, Mac and Windows
`Ctrl`+`Shift`+`Delete` | Delete line right | Doesn't work on Mac
`Ctrl`+`C` | Emit `SIGINT` or close the readline instance |
`Ctrl`+`H` | Delete left |
`Ctrl`+`D` | Delete right or close the readline instance in case the current line is empty / EOF | Doesn't work on Windows
`Ctrl`+`U` | Delete from the current position to the line start |
`Ctrl`+`K` | Delete from the current position to the end of line |
`Ctrl`+`Y` | Yank (Recall) the previously deleted text | Only works with text deleted by `Ctrl`+`U` or `Ctrl`+`K`
`Meta`+`Y` | Cycle among previously deleted texts | Only available when the last keystroke is `Ctrl`+`Y` or `Meta`+`Y`
`Ctrl`+`A` | Go to start of line |
`Ctrl`+`E` | Go to end of line |
`Ctrl`+`B` | Back one character |
`Ctrl`+`F` | Forward one character |
`Ctrl`+`L` | Clear screen |
`Ctrl`+`N` | Next history item |
`Ctrl`+`P` | Previous history item |
`Ctrl`+`-` | Undo previous change | Any keystroke that emits key code `0x1F` will do this action. In many terminals, for example `xterm`, this is bound to `Ctrl`+`-`.
`Ctrl`+`6` | Redo previous change | Many terminals don't have a default redo keystroke. We choose key code `0x1E` to perform redo. In `xterm`, it is bound to `Ctrl`+`6` by default.
`Ctrl`+`Z` | Moves running process into background. Type `fg` and press `Enter` to return. | Doesn't work on Windows
`Ctrl`+`W` or `Ctrl` +`Backspace` | Delete backward to a word boundary | `Ctrl`+`Backspace` Doesn't work on Linux, Mac and Windows
`Ctrl`+`Delete` | Delete forward to a word boundary | Doesn't work on Mac
`Ctrl`+`Left arrow` or `Meta`+`B` | Word left | `Ctrl`+`Left arrow` Doesn't work on Mac
`Ctrl`+`Right arrow` or `Meta`+`F` | Word right | `Ctrl`+`Right arrow` Doesn't work on Mac
`Meta`+`D` or `Meta` +`Delete` | Delete word right | `Meta`+`Delete` Doesn't work on windows
`Meta`+`Backspace` | Delete word left | Doesn't work on Mac

## REPL#

Source Code: lib/repl.js

Stability: 2 - Stable

The `node:repl` module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:

    import repl from 'node:repl';
    const repl = require('node:repl');
    javascript

### Design and features#

The `node:repl` module exports the `repl.REPLServer` class. While running, instances of `repl.REPLServer` will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from `stdin` and `stdout`, respectively, or may be connected to any Node.js stream.

Instances of `repl.REPLServer` support automatic completion of inputs, completion preview, simplistic Emacs-style line editing, multi-line inputs, ZSH-like reverse-i-search, ZSH-like substring-based history search, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions. Terminals that do not support ANSI styles and Emacs-style line editing automatically fall back to a limited feature set.

#### Commands and special keys#

The following special commands are supported by all REPL instances:

- `.break`: When in the process of inputting a multi-line expression, enter the `.break` command (or press `Ctrl`+`C`) to abort further input or processing of that expression.
- `.clear`: Resets the REPL `context` to an empty object and clears any multi-line expression being input.
- `.exit`: Close the I/O stream, causing the REPL to exit.
- `.help`: Show this list of special commands.
- `.save`: Save the current REPL session to a file: `> .save ./file/to/save.js`
- `.load`: Load a file into the current REPL session. `> .load ./file/to/load.js`
- `.editor`: Enter editor mode (`Ctrl`+`D` to finish, `Ctrl`+`C` to cancel).

    > .editor
    // Entering editor mode (^D to finish, ^C to cancel)
    function welcome(name) {
      return `Hello ${name}!`;
    }

    welcome('Node.js User');

    // ^D
    'Hello Node.js User!'
    >
    console

The following key combinations in the REPL have these special effects:

- `Ctrl`+`C`: When pressed once, has the same effect as the `.break` command. When pressed twice on a blank line, has the same effect as the `.exit` command.
- `Ctrl`+`D`: Has the same effect as the `.exit` command.
- `Tab`: When pressed on a blank line, displays global and local (scope) variables. When pressed while entering other input, displays relevant autocompletion options.

For key bindings related to the reverse-i-search, see `reverse-i-search`. For all other key bindings, see TTY keybindings.

#### Default evaluation#

By default, all instances of `repl.REPLServer` use an evaluation function that evaluates JavaScript expressions and provides access to Node.js built-in modules. This default behavior can be overridden by passing in an alternative evaluation function when the `repl.REPLServer` instance is created.

##### JavaScript expressions#

The default evaluator supports direct evaluation of JavaScript expressions:

    > 1 + 1
    2
    > const m = 2
    undefined
    > m + 1
    3
    console

Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the `const`, `let`, or `var` keywords are declared at the global scope.

##### Global and local scope#

The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the `context` object associated with each `REPLServer`:

    import repl from 'node:repl';
    const msg = 'message';

    repl.start('> ').context.m = msg;
    const repl = require('node:repl');
    const msg = 'message';

    repl.start('> ').context.m = msg;
    javascript

Properties in the `context` object appear as local within the REPL:

    $ node repl_test.js
    > m
    'message'
    console

Context properties are not read-only by default. To specify read-only globals, context properties must be defined using `Object.defineProperty()`:

    import repl from 'node:repl';
    const msg = 'message';

    const r = repl.start('> ');
    Object.defineProperty(r.context, 'm', {
      configurable: false,
      enumerable: true,
      value: msg,
    });
    const repl = require('node:repl');
    const msg = 'message';

    const r = repl.start('> ');
    Object.defineProperty(r.context, 'm', {
      configurable: false,
      enumerable: true,
      value: msg,
    });
    javascript

##### Accessing core Node.js modules#

The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input `fs` will be evaluated on-demand as `global.fs = require('node:fs')`.

    > fs.createReadStream('./some/file');
    console

##### Global uncaught exceptions#

History

Version | Changes
--- | ---
v12.3.0 | The `'uncaughtException'` event is from now on triggered if the repl is used as standalone program.

The REPL uses the `domain` module to catch all uncaught exceptions for that REPL session.

This use of the `domain` module in the REPL has these side effects:

- Uncaught exceptions only emit the `'uncaughtException'` event in the standalone REPL. Adding a listener for this event in a REPL within another Node.js program results in `ERR_INVALID_REPL_INPUT`.

      const r = repl.start();

      r.write('process.on("uncaughtException", () => console.log("Foobar"));\n');
      // Output stream includes:
      //   TypeError [ERR_INVALID_REPL_INPUT]: Listeners for `uncaughtException`
      //   cannot be used in the REPL

      r.close();
      js

- Trying to use `process.setUncaughtExceptionCaptureCallback()` throws an `ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE` error.

##### Assignment of the `_` (underscore) variable#

History

Version | Changes
--- | ---
v9.8.0 | Added `_error` support.

The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable `_` (underscore). Explicitly setting `_` to a value will disable this behavior.

    > [ 'a', 'b', 'c' ]
    [ 'a', 'b', 'c' ]
    > _.length
    3
    > _ += 1
    Expression assignment to _ now disabled.
    4
    > 1 + 1
    2
    > _
    4
    console

Similarly, `_error` will refer to the last seen error, if there was any. Explicitly setting `_error` to a value will disable this behavior.

    > throw new Error('foo');
    Uncaught Error: foo
    > _error.message
    'foo'
    console

##### `await` keyword#

Support for the `await` keyword is enabled at the top level.

    > await Promise.resolve(123)
    123
    > await Promise.reject(new Error('REPL await'))
    Uncaught Error: REPL await
        at REPL2:1:54
    > const timeout = util.promisify(setTimeout);
    undefined
    > const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
    1002
    undefined
    console

One known limitation of using the `await` keyword in the REPL is that it will invalidate the lexical scoping of the `const` keywords.

For example:

    > const m = await Promise.resolve(123)
    undefined
    > m
    123
    > m = await Promise.resolve(234)
    234
    // redeclaring the constant does error
    > const m = await Promise.resolve(345)
    Uncaught SyntaxError: Identifier 'm' has already been declared
    console

`--no-experimental-repl-await` shall disable top-level await in REPL.

#### Reverse-i-search#

Added in: v13.6.0, v12.17.0

The REPL supports bi-directional reverse-i-search similar to ZSH. It is triggered with `Ctrl`+`R` to search backward and `Ctrl`+`S` to search forwards.

Duplicated history entries will be skipped.

Entries are accepted as soon as any key is pressed that doesn't correspond with the reverse search. Cancelling is possible by pressing `Esc` or `Ctrl`+`C`.

Changing the direction immediately searches for the next entry in the expected direction from the current position on.

#### Custom evaluation functions#

When a new `repl.REPLServer` is created, a custom evaluation function may be provided. This can be used, for instance, to implement fully customized REPL applications.

An evaluation function accepts the following four arguments:

- `code` `<string>` The code to be executed (e.g. `1 + 1`).
- `context` `<Object>` The context in which the code is executed. This can either be the JavaScript `global` context or a context specific to the REPL instance, depending on the `useGlobal` option.
- `replResourceName` `<string>` An identifier for the REPL resource associated with the current code evaluation. This can be useful for debugging purposes.
- `callback` `<Function>` A function to invoke once the code evaluation is complete. The callback takes two parameters:
  - An error object to provide if an error occurred during evaluation, or `null`/`undefined` if no error occurred.
  - The result of the code evaluation (this is not relevant if an error is provided).

The following illustrates an example of a REPL that squares a given number, an error is instead printed if the provided input is not actually a number:

    import repl from 'node:repl';

    function byThePowerOfTwo(number) {
      return number * number;
    }

    function myEval(code, context, replResourceName, callback) {
      if (isNaN(code)) {
        callback(new Error(`${code.trim()} is not a number`));
      } else {
        callback(null, byThePowerOfTwo(code));
      }
    }

    repl.start({ prompt: 'Enter a number: ', eval: myEval });
    const repl = require('node:repl');

    function byThePowerOfTwo(number) {
      return number * number;
    }

    function myEval(code, context, replResourceName, callback) {
      if (isNaN(code)) {
        callback(new Error(`${code.trim()} is not a number`));
      } else {
        callback(null, byThePowerOfTwo(code));
      }
    }

    repl.start({ prompt: 'Enter a number: ', eval: myEval });
    javascript

##### Recoverable errors#

At the REPL prompt, pressing `Enter` sends the current line of input to the `eval` function. In order to support multi-line input, the `eval` function can return an instance of `repl.Recoverable` to the provided callback function:

    function myEval(cmd, context, filename, callback) {
      let result;
      try {
        result = vm.runInThisContext(cmd);
      } catch (e) {
        if (isRecoverableError(e)) {
          return callback(new repl.Recoverable(e));
        }
      }
      callback(null, result);
    }

    function isRecoverableError(error) {
      if (error.name === 'SyntaxError') {
        return /^(Unexpected end of input|Unexpected token)/.test(error.message);
      }
      return false;
    }
    js

#### Customizing REPL output#

By default, `repl.REPLServer` instances format output using the `util.inspect()` method before writing the output to the provided `Writable` stream (`process.stdout` by default). The `showProxy` inspection option is set to true by default and the `colors` option is set to true depending on the REPL's `useColors` option.

The `useColors` boolean option can be specified at construction to instruct the default writer to use ANSI style codes to colorize the output from the `util.inspect()` method.

If the REPL is run as standalone program, it is also possible to change the REPL's inspection defaults from inside the REPL by using the `inspect.replDefaults` property which mirrors the `defaultOptions` from `util.inspect()`.

    > util.inspect.replDefaults.compact = false;
    false
    > [1]
    [
      1
    ]
    >
    console

To fully customize the output of a `repl.REPLServer` instance pass in a new function for the `writer` option on construction. The following example, for instance, simply converts any input text to upper case:

    import repl from 'node:repl';

    const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });

    function myEval(cmd, context, filename, callback) {
      callback(null, cmd);
    }

    function myWriter(output) {
      return output.toUpperCase();
    }
    const repl = require('node:repl');

    const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });

    function myEval(cmd, context, filename, callback) {
      callback(null, cmd);
    }

    function myWriter(output) {
      return output.toUpperCase();
    }
    javascript

### Class: `REPLServer`#

Added in: v0.1.91

- `options` `<Object>` | `<string>` See `repl.start()`
- Extends: `<readline.Interface>`

Instances of `repl.REPLServer` are created using the `repl.start()` method or directly using the JavaScript `new` keyword.

    import repl from 'node:repl';

    const options = { useColors: true };

    const firstInstance = repl.start(options);
    const secondInstance = new repl.REPLServer(options);
    const repl = require('node:repl');

    const options = { useColors: true };

    const firstInstance = repl.start(options);
    const secondInstance = new repl.REPLServer(options);
    javascript

#### Event: `'exit'`#

Added in: v0.7.7

The `'exit'` event is emitted when the REPL is exited either by receiving the `.exit` command as input, the user pressing `Ctrl`+`C` twice to signal `SIGINT`, or by pressing `Ctrl`+`D` to signal `'end'` on the input stream. The listener callback is invoked without any arguments.

    replServer.on('exit', () => {
      console.log('Received "exit" event from repl!');
      process.exit();
    });
    js

#### Event: `'reset'`#

Added in: v0.11.0

The `'reset'` event is emitted when the REPL's context is reset. This occurs whenever the `.clear` command is received as input unless the REPL is using the default evaluator and the `repl.REPLServer` instance was created with the `useGlobal` option set to `true`. The listener callback will be called with a reference to the `context` object as the only argument.

This can be used primarily to re-initialize REPL context to some pre-defined state:

    import repl from 'node:repl';

    function initializeContext(context) {
      context.m = 'test';
    }

    const r = repl.start({ prompt: '> ' });
    initializeContext(r.context);

    r.on('reset', initializeContext);
    const repl = require('node:repl');

    function initializeContext(context) {
      context.m = 'test';
    }

    const r = repl.start({ prompt: '> ' });
    initializeContext(r.context);

    r.on('reset', initializeContext);
    javascript

When this code is executed, the global `'m'` variable can be modified but then reset to its initial value using the `.clear` command:

    $ ./node example.js
    > m
    'test'
    > m = 1
    1
    > m
    1
    > .clear
    Clearing context...
    > m
    'test'
    >
    console

#### `replServer.defineCommand(keyword, cmd)`#

Added in: v0.3.0

- `keyword` `<string>` The command keyword (without a leading `.` character).
- `cmd` `<Object>` | `<Function>` The function to invoke when the command is processed.

The `replServer.defineCommand()` method is used to add new `.`-prefixed commands to the REPL instance. Such commands are invoked by typing a `.` followed by the `keyword`. The `cmd` is either a `Function` or an `Object` with the following properties:

- `help` `<string>` Help text to be displayed when `.help` is entered (Optional).
- `action` `<Function>` The function to execute, optionally accepting a single string argument.

The following example shows two new commands added to the REPL instance:

    import repl from 'node:repl';

    const replServer = repl.start({ prompt: '> ' });
    replServer.defineCommand('sayhello', {
      help: 'Say hello',
      action(name) {
        this.clearBufferedCommand();
        console.log(`Hello, ${name}!`);
        this.displayPrompt();
      },
    });
    replServer.defineCommand('saybye', function saybye() {
      console.log('Goodbye!');
      this.close();
    });
    const repl = require('node:repl');

    const replServer = repl.start({ prompt: '> ' });
    replServer.defineCommand('sayhello', {
      help: 'Say hello',
      action(name) {
        this.clearBufferedCommand();
        console.log(`Hello, ${name}!`);
        this.displayPrompt();
      },
    });
    replServer.defineCommand('saybye', function saybye() {
      console.log('Goodbye!');
      this.close();
    });
    javascript

The new commands can then be used from within the REPL instance:

    > .sayhello Node.js User
    Hello, Node.js User!
    > .saybye
    Goodbye!
    console

#### `replServer.displayPrompt([preserveCursor])`#

Added in: v0.1.91

- `preserveCursor` `<boolean>`

The `replServer.displayPrompt()` method readies the REPL instance for input from the user, printing the configured `prompt` to a new line in the `output` and resuming the `input` to accept new input.

When multi-line input is being entered, a pipe `'|'` is printed rather than the 'prompt'.

When `preserveCursor` is `true`, the cursor placement will not be reset to `0`.

The `replServer.displayPrompt` method is primarily intended to be called from within the action function for commands registered using the `replServer.defineCommand()` method.

#### `replServer.clearBufferedCommand()`#

Added in: v9.0.0

The `replServer.clearBufferedCommand()` method clears any command that has been buffered but not yet executed. This method is primarily intended to be called from within the action function for commands registered using the `replServer.defineCommand()` method.

#### `replServer.setupHistory(historyConfig, callback)`#

Added in: v11.10.0
History

Version | Changes
--- | ---
v24.2.0 | Updated the `historyConfig` parameter to accept an object with `filePath`, `size`, `removeHistoryDuplicates` and `onHistoryFileLoaded` properties.

- `historyConfig` `<Object>` | `<string>` the path to the history file If it is a string, it is the path to the history file. If it is an object, it can have the following properties:
  - `filePath` `<string>` the path to the history file
  - `size` `<number>` Maximum number of history lines retained. To disable the history set this value to `0`. This option makes sense only if `terminal` is set to `true` by the user or by an internal `output` check, otherwise the history caching mechanism is not initialized at all. Default: `30`.
  - `removeHistoryDuplicates` `<boolean>` If `true`, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default: `false`.
  - `onHistoryFileLoaded` `<Function>` called when history writes are ready or upon error
    - `err` `<Error>`
    - `repl` `<repl.REPLServer>`
- `callback` `<Function>` called when history writes are ready or upon error (Optional if provided as `onHistoryFileLoaded` in `historyConfig`)
  - `err` `<Error>`
  - `repl` `<repl.REPLServer>`

Initializes a history log file for the REPL instance. When executing the Node.js binary and using the command-line REPL, a history file is initialized by default. However, this is not the case when creating a REPL programmatically. Use this method to initialize a history log file when working with REPL instances programmatically.

### `repl.builtinModules`#

Added in: v14.5.0Deprecated in: v24.0.0, v22.16.0

Stability: 0 - Deprecated. Use `module.builtinModules` instead.

- Type: `<string>`[]

A list of the names of some Node.js modules, e.g., `'http'`.

An automated migration is available (source):

    npx codemod@latest @nodejs/repl-builtin-modules
    bash

### `repl.start([options])`#

Added in: v0.1.91
History

Version | Changes
--- | ---
v25.9.0 | The `handleError` parameter has been added.
v24.1.0 | Added the possibility to add/edit/remove multilines while adding a multiline command.
v24.0.0 | The multi-line indicator is now "|" instead of "...". Added support for multi-line history. It is now possible to "fix" multi-line commands with syntax errors by visiting the history and editing the command. When visiting the multiline history from an old node version, the multiline structure is not preserved.
v13.4.0, v12.17.0 | The `preview` option is now available.
v12.0.0 | The `terminal` option now follows the default description in all cases and `useColors` checks `hasColors()` if available.
v10.0.0 | The `REPL_MAGIC_MODE` `replMode` was removed.
v6.3.0 | The `breakEvalOnSigint` option is supported now.
v5.8.0 | The `options` parameter is optional now.

- `options` `<Object>` | `<string>`
  - `prompt` `<string>` The input prompt to display. Default: `'> '` (with a trailing space).
  - `input` `<stream.Readable>` The `Readable` stream from which REPL input will be read. Default: `process.stdin`.
  - `output` `<stream.Writable>` The `Writable` stream to which REPL output will be written. Default: `process.stdout`.
  - `terminal` `<boolean>` If `true`, specifies that the `output` should be treated as a TTY terminal. Default: checking the value of the `isTTY` property on the `output` stream upon instantiation.
  - `eval` `<Function>` The function to be used when evaluating each given line of input. Default: an async wrapper for the JavaScript `eval()` function. An `eval` function can error with `repl.Recoverable` to indicate the input was incomplete and prompt for additional lines. See the custom evaluation functions section for more details.
  - `useColors` `<boolean>` If `true`, specifies that the default `writer` function should include ANSI color styling to REPL output. If a custom `writer` function is provided then this has no effect. Default: checking color support on the `output` stream if the REPL instance's `terminal` value is `true`.
  - `useGlobal` `<boolean>` If `true`, specifies that the default evaluation function will use the JavaScript `global` as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value to `true`. Default: `false`.
  - `ignoreUndefined` `<boolean>` If `true`, specifies that the default writer will not output the return value of a command if it evaluates to `undefined`. Default: `false`.
  - `writer` `<Function>` The function to invoke to format the output of each command before writing to `output`. Default: `util.inspect()`.
  - `completer` `<Function>` An optional function used for custom Tab auto completion. See `readline.InterfaceCompleter` for an example.
  - `replMode` `<symbol>` A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:
    - `repl.REPL_MODE_SLOPPY` to evaluate expressions in sloppy mode.
    - `repl.REPL_MODE_STRICT` to evaluate expressions in strict mode. This is equivalent to prefacing every repl statement with `'use strict'`.
  - `breakEvalOnSigint` `<boolean>` Stop evaluating the current piece of code when `SIGINT` is received, such as when `Ctrl`+`C` is pressed. This cannot be used together with a custom `eval` function. Default: `false`.
  - `preview` `<boolean>` Defines if the repl prints autocomplete and output previews or not. Default: `true` with the default eval function and `false` in case a custom eval function is used. If `terminal` is falsy, then there are no previews and the value of `preview` has no effect.
  - `handleError` `<Function>` This function customizes error handling in the REPL. It receives the thrown exception as its first argument and must return one of the following values synchronously:
    - `'print'` to print the error to the output stream (default behavior).
    - `'ignore'` to skip all remaining error handling.
    - `'unhandled'` to treat the exception as fully unhandled. In this case, the error will be passed to process-wide exception handlers, such as the `'uncaughtException'` event. The `'unhandled'` value may or may not be desirable in situations where the `REPLServer` instance has been closed, depending on the particular use case.
- Returns: `<repl.REPLServer>`

The `repl.start()` method creates and starts a `repl.REPLServer` instance.

If `options` is a string, then it specifies the input prompt:

    import repl from 'node:repl';

    // a Unix style prompt
    repl.start('$ ');
    const repl = require('node:repl');

    // a Unix style prompt
    repl.start('$ ');
    javascript

### The Node.js REPL#

Node.js itself uses the `node:repl` module to provide its own interactive interface for executing JavaScript. This can be used by executing the Node.js binary without passing any arguments (or by passing the `-i` argument):

    $ node
    > const a = [1, 2, 3];
    undefined
    > a
    [ 1, 2, 3 ]
    > a.forEach((v) => {
    ...   console.log(v);
    ...   });
    1
    2
    3
    console

#### Environment variable options#

Various behaviors of the Node.js REPL can be customized using the following environment variables:

- `NODE_REPL_HISTORY`: When a valid path is given, persistent REPL history will be saved to the specified file rather than `.node_repl_history` in the user's home directory. Setting this value to `''` (an empty string) will disable persistent REPL history. Whitespace will be trimmed from the value. On Windows platforms environment variables with empty values are invalid so set this variable to one or more spaces to disable persistent REPL history.
- `NODE_REPL_HISTORY_SIZE`: Controls how many lines of history will be persisted if history is available. Must be a positive number. Default: `1000`.
- `NODE_REPL_MODE`: May be either `'sloppy'` or `'strict'`. Default: `'sloppy'`, which will allow non-strict mode code to be run.

#### Persistent history#

By default, the Node.js REPL will persist history between `node` REPL sessions by saving inputs to a `.node_repl_history` file located in the user's home directory. This can be disabled by setting the environment variable `NODE_REPL_HISTORY=''`.

#### Using the Node.js REPL with advanced line-editors#

For advanced line-editors, start Node.js with the environment variable `NODE_NO_READLINE=1`. This will start the main and debugger REPL in canonical terminal settings, which will allow use with `rlwrap`.

For example, the following can be added to a `.bashrc` file:

    alias node="env NODE_NO_READLINE=1 rlwrap node"
    bash

#### Starting multiple REPL instances in the same process#

It is possible to create and run multiple REPL instances against a single running instance of Node.js that share a single `global` object (by setting the `useGlobal` option to `true`) but have separate I/O interfaces.

The following example, for instance, provides separate REPLs on `stdin`, a Unix socket, and a TCP socket, all sharing the same `global` object:

    import net from 'node:net';
    import repl from 'node:repl';
    import process from 'node:process';
    import fs from 'node:fs';

    let connections = 0;

    repl.start({
      prompt: 'Node.js via stdin> ',
      useGlobal: true,
      input: process.stdin,
      output: process.stdout,
    });

    const unixSocketPath = '/tmp/node-repl-sock';

    // If the socket file already exists let's remove it
    fs.rmSync(unixSocketPath, { force: true });

    net.createServer((socket) => {
      connections += 1;
      repl.start({
        prompt: 'Node.js via Unix socket> ',
        useGlobal: true,
        input: socket,
        output: socket,
      }).on('exit', () => {
        socket.end();
      });
    }).listen(unixSocketPath);

    net.createServer((socket) => {
      connections += 1;
      repl.start({
        prompt: 'Node.js via TCP socket> ',
        useGlobal: true,
        input: socket,
        output: socket,
      }).on('exit', () => {
        socket.end();
      });
    }).listen(5001);
    const net = require('node:net');
    const repl = require('node:repl');
    const fs = require('node:fs');

    let connections = 0;

    repl.start({
      prompt: 'Node.js via stdin> ',
      useGlobal: true,
      input: process.stdin,
      output: process.stdout,
    });

    const unixSocketPath = '/tmp/node-repl-sock';

    // If the socket file already exists let's remove it
    fs.rmSync(unixSocketPath, { force: true });

    net.createServer((socket) => {
      connections += 1;
      repl.start({
        prompt: 'Node.js via Unix socket> ',
        useGlobal: true,
        input: socket,
        output: socket,
      }).on('exit', () => {
        socket.end();
      });
    }).listen(unixSocketPath);

    net.createServer((socket) => {
      connections += 1;
      repl.start({
        prompt: 'Node.js via TCP socket> ',
        useGlobal: true,
        input: socket,
        output: socket,
      }).on('exit', () => {
        socket.end();
      });
    }).listen(5001);
    javascript

Running this application from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. `telnet`, for instance, is useful for connecting to TCP sockets, while `socat` can be used to connect to both Unix and TCP sockets.

By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.

#### Examples#

##### Full-featured "terminal" REPL over `net.Server` and `net.Socket`#

This is an example on how to run a "full-featured" (terminal) REPL using `net.Server` and `net.Socket`

The following script starts an HTTP server on port `1337` that allows clients to establish socket connections to its REPL instance.

    // repl-server.js
    import repl from 'node:repl';
    import net from 'node:net';

    net
      .createServer((socket) => {
        const r = repl.start({
          prompt: `socket ${socket.remoteAddress}:${socket.remotePort}> `,
          input: socket,
          output: socket,
          terminal: true,
          useGlobal: false,
        });
        r.on('exit', () => {
          socket.end();
        });
        r.context.socket = socket;
      })
      .listen(1337);
    // repl-server.js
    const repl = require('node:repl');
    const net = require('node:net');

    net
      .createServer((socket) => {
        const r = repl.start({
          prompt: `socket ${socket.remoteAddress}:${socket.remotePort}> `,
          input: socket,
          output: socket,
          terminal: true,
          useGlobal: false,
        });
        r.on('exit', () => {
          socket.end();
        });
        r.context.socket = socket;
      })
      .listen(1337);
    javascript

While the following implements a client that can create a socket connection with the above defined server over port `1337`.

    // repl-client.js
    import net from 'node:net';
    import process from 'node:process';

    const sock = net.connect(1337);

    process.stdin.pipe(sock);
    sock.pipe(process.stdout);

    sock.on('connect', () => {
      process.stdin.resume();
      process.stdin.setRawMode(true);
    });

    sock.on('close', () => {
      process.stdin.setRawMode(false);
      process.stdin.pause();
      sock.removeListener('close', done);
    });

    process.stdin.on('end', () => {
      sock.destroy();
      console.log();
    });

    process.stdin.on('data', (b) => {
      if (b.length === 1 && b[0] === 4) {
        process.stdin.emit('end');
      }
    });
    // repl-client.js
    const net = require('node:net');

    const sock = net.connect(1337);

    process.stdin.pipe(sock);
    sock.pipe(process.stdout);

    sock.on('connect', () => {
      process.stdin.resume();
      process.stdin.setRawMode(true);
    });

    sock.on('close', () => {
      process.stdin.setRawMode(false);
      process.stdin.pause();
      sock.removeListener('close', done);
    });

    process.stdin.on('end', () => {
      sock.destroy();
      console.log();
    });

    process.stdin.on('data', (b) => {
      if (b.length === 1 && b[0] === 4) {
        process.stdin.emit('end');
      }
    });
    javascript

To run the example open two different terminals on your machine, start the server with `node repl-server.js` in one terminal and `node repl-client.js` on the other.

Original code from https://gist.github.com/TooTallNate/2209310.

##### REPL over `curl`#

This is an example on how to run a REPL instance over `curl()`

The following script starts an HTTP server on port `8000` that can accept a connection established via `curl()`.

    import http from 'node:http';
    import repl from 'node:repl';

    const server = http.createServer((req, res) => {
      res.setHeader('content-type', 'multipart/octet-stream');

      repl.start({
        prompt: 'curl repl> ',
        input: req,
        output: res,
        terminal: false,
        useColors: true,
        useGlobal: false,
      });
    });

    server.listen(8000);
    const http = require('node:http');
    const repl = require('node:repl');

    const server = http.createServer((req, res) => {
      res.setHeader('content-type', 'multipart/octet-stream');

      repl.start({
        prompt: 'curl repl> ',
        input: req,
        output: res,
        terminal: false,
        useColors: true,
        useGlobal: false,
      });
    });

    server.listen(8000);
    javascript

When the above script is running you can then use `curl()` to connect to the server and connect to its REPL instance by running `curl --no-progress-meter -sSNT. localhost:8000`.

Warning This example is intended purely for educational purposes to demonstrate how Node.js REPLs can be started using different I/O streams. It should not be used in production environments or any context where security is a concern without additional protective measures. If you need to implement REPLs in a real-world application, consider alternative approaches that mitigate these risks, such as using secure input mechanisms and avoiding open network interfaces.

Original code from https://gist.github.com/TooTallNate/2053342.

## Single executable applications#

Source Code: src/node_sea.cc Added in: v19.7.0, v18.16.0
History

Version | Changes
--- | ---
v25.5.0 | Added built-in single executable application generation via the CLI flag `--build-sea`.
v20.6.0 | Added support for "useSnapshot".
v20.6.0 | Added support for "useCodeCache".

Stability: 1.1 - Active development

This feature allows the distribution of a Node.js application conveniently to a system that does not have Node.js installed.

Node.js supports the creation of single executable applications by allowing the injection of a blob prepared by Node.js, which can contain a bundled script, into the `node` binary. During start up, the program checks if anything has been injected. If the blob is found, it executes the script in the blob. Otherwise Node.js operates as it normally does.

The single executable application feature supports running a single embedded script using the CommonJS or the ECMAScript Modules module system.

Users can create a single executable application from their bundled script with the `node` binary itself and any tool which can inject resources into the binary.

1. Create a JavaScript file:

       echo 'console.log(`Hello, ${process.argv[2]}!`);' > hello.js
       bash

2. Create a configuration file building a blob that can be injected into the single executable application (see Generating single executable preparation blobs for details):

   - On systems other than Windows:

       echo '{ "main": "hello.js", "output": "sea" }' > sea-config.json
       bash

   - On Windows:

       echo '{ "main": "hello.js", "output": "sea.exe" }' > sea-config.json
       bash

   The `.exe` extension is necessary.

3. Generate the target executable:

       node --build-sea sea-config.json
       bash

4. Sign the binary (macOS and Windows only):

   - On macOS:

       codesign --sign - sea
       bash

   - On Windows (optional):

   A certificate needs to be present for this to work. However, the unsigned binary would still be runnable.

       signtool sign /fd SHA256 sea.exe
       powershell

5. Run the binary:

   - On systems other than Windows

       $ ./sea world
       Hello, world!
       console

   - On Windows

       $ .\sea.exe world
       Hello, world!
       console

### Generating single executable applications with `--build-sea`#

To generate a single executable application directly, the `--build-sea` flag can be used. It takes a path to a configuration file in JSON format. If the path passed to it isn't absolute, Node.js will use the path relative to the current working directory.

The configuration currently reads the following top-level fields:

    {
      "main": "/path/to/bundled/script.js",
      "mainFormat": "commonjs", // Default: "commonjs", options: "commonjs", "module"
      "executable": "/path/to/node/binary", // Optional, if not specified, uses the current Node.js binary
      "output": "/path/to/write/the/generated/executable",
      "disableExperimentalSEAWarning": true, // Default: false
      "useSnapshot": false,  // Default: false
      "useCodeCache": true, // Default: false
      "execArgv": ["--no-warnings", "--max-old-space-size=4096"], // Optional
      "execArgvExtension": "env", // Default: "env", options: "none", "env", "cli"
      "assets": {  // Optional
        "a.dat": "/path/to/a.dat",
        "b.txt": "/path/to/b.txt"
      }
    }
    json

If the paths are not absolute, Node.js will use the path relative to the current working directory. The version of the Node.js binary used to produce the blob must be the same as the one to which the blob will be injected.

Note: When generating cross-platform SEAs (e.g., generating a SEA for `linux-x64` on `darwin-arm64`), `useCodeCache` and `useSnapshot` must be set to false to avoid generating incompatible executables. Since code cache and snapshots can only be loaded on the same platform where they are compiled, the generated executable might crash on startup when trying to load code cache or snapshots built on a different platform.

#### Assets#

Users can include assets by adding a key-path dictionary to the configuration as the `assets` field. At build time, Node.js would read the assets from the specified paths and bundle them into the preparation blob. In the generated executable, users can retrieve the assets using the `sea.getAsset()` and `sea.getAssetAsBlob()` APIs.

    {
      "main": "/path/to/bundled/script.js",
      "output": "/path/to/write/the/generated/executable",
      "assets": {
        "a.jpg": "/path/to/a.jpg",
        "b.txt": "/path/to/b.txt"
      }
    }
    json

The single-executable application can access the assets as follows:

    const { getAsset, getAssetAsBlob, getRawAsset, getAssetKeys } = require('node:sea');
    // Get all asset keys.
    const keys = getAssetKeys();
    console.log(keys); // ['a.jpg', 'b.txt']
    // Returns a copy of the data in an ArrayBuffer.
    const image = getAsset('a.jpg');
    // Returns a string decoded from the asset as UTF8.
    const text = getAsset('b.txt', 'utf8');
    // Returns a Blob containing the asset.
    const blob = getAssetAsBlob('a.jpg');
    // Returns an ArrayBuffer containing the raw asset without copying.
    const raw = getRawAsset('a.jpg');
    cjs

See documentation of the `sea.getAsset()`, `sea.getAssetAsBlob()`, `sea.getRawAsset()` and `sea.getAssetKeys()` APIs for more information.

#### Startup snapshot support#

The `useSnapshot` field can be used to enable startup snapshot support. In this case, the `main` script would not be executed when the final executable is launched. Instead, it would be run when the single executable application preparation blob is generated on the building machine. The generated preparation blob would then include a snapshot capturing the states initialized by the `main` script. The final executable, with the preparation blob injected, would deserialize the snapshot at run time.

When `useSnapshot` is true, the main script must invoke the `v8.startupSnapshot.setDeserializeMainFunction()` API to configure code that needs to be run when the final executable is launched by the users.

The typical pattern for an application to use snapshot in a single executable application is:

1. At build time, on the building machine, the main script is run to initialize the heap to a state that's ready to take user input. The script should also configure a main function with `v8.startupSnapshot.setDeserializeMainFunction()`. This function will be compiled and serialized into the snapshot, but not invoked at build time.
2. At run time, the main function will be run on top of the deserialized heap on the user machine to process user input and generate output.

The general constraints of the startup snapshot scripts also apply to the main script when it's used to build snapshot for the single executable application, and the main script can use the `v8.startupSnapshot` API to adapt to these constraints. See documentation about startup snapshot support in Node.js.

#### V8 code cache support#

When `useCodeCache` is set to `true` in the configuration, during the generation of the single executable preparation blob, Node.js will compile the `main` script to generate the V8 code cache. The generated code cache would be part of the preparation blob and get injected into the final executable. When the single executable application is launched, instead of compiling the `main` script from scratch, Node.js would use the code cache to speed up the compilation, then execute the script, which would improve the startup performance.

Note: `import()` does not work when `useCodeCache` is `true`.

#### Execution arguments#

The `execArgv` field can be used to specify Node.js-specific arguments that will be automatically applied when the single executable application starts. This allows application developers to configure Node.js runtime options without requiring end users to be aware of these flags.

For example, the following configuration:

    {
      "main": "/path/to/bundled/script.js",
      "output": "/path/to/write/the/generated/executable",
      "execArgv": ["--no-warnings", "--max-old-space-size=2048"]
    }
    json

will instruct the SEA to be launched with the `--no-warnings` and `--max-old-space-size=2048` flags. In the scripts embedded in the executable, these flags can be accessed using the `process.execArgv` property:

    // If the executable is launched with `sea user-arg1 user-arg2`
    console.log(process.execArgv);
    // Prints: ['--no-warnings', '--max-old-space-size=2048']
    console.log(process.argv);
    // Prints ['/path/to/sea', 'path/to/sea', 'user-arg1', 'user-arg2']
    js

The user-provided arguments are in the `process.argv` array starting from index 2, similar to what would happen if the application is started with:

    node --no-warnings --max-old-space-size=2048 /path/to/bundled/script.js user-arg1 user-arg2
    console

#### Execution argument extension#

The `execArgvExtension` field controls how additional execution arguments can be provided beyond those specified in the `execArgv` field. It accepts one of three string values:

- `"none"`: No extension is allowed. Only the arguments specified in `execArgv` will be used, and the `NODE_OPTIONS` environment variable will be ignored.
- `"env"`: (Default) The `NODE_OPTIONS` environment variable can extend the execution arguments. This is the default behavior to maintain backward compatibility.
- `"cli"`: The executable can be launched with `--node-options="--flag1 --flag2"`, and those flags will be parsed as execution arguments for Node.js instead of being passed to the user script. This allows using arguments that are not supported by the `NODE_OPTIONS` environment variable.

For example, with `"execArgvExtension": "cli"`:

    {
      "main": "/path/to/bundled/script.js",
      "output": "/path/to/write/the/generated/executable",
      "execArgv": ["--no-warnings"],
      "execArgvExtension": "cli"
    }
    json

The executable can be launched as:

    ./my-sea --node-options="--trace-exit" user-arg1 user-arg2
    console

This would be equivalent to running:

    node --no-warnings --trace-exit /path/to/bundled/script.js user-arg1 user-arg2
    console

### Single-executable application API#

The `node:sea` builtin allows interaction with the single-executable application from the JavaScript main script embedded into the executable.

#### `sea.isSea()`#

Added in: v21.7.0, v20.12.0

- Returns: `<boolean>` Whether this script is running inside a single-executable application.

#### `sea.getAsset(key[, encoding])`#

Added in: v21.7.0, v20.12.0

This method can be used to retrieve the assets configured to be bundled into the single-executable application at build time. An error is thrown when no matching asset can be found.

- `key` `<string>` the key for the asset in the dictionary specified by the `assets` field in the single-executable application configuration.
- `encoding` `<string>` If specified, the asset will be decoded as a string. Any encoding supported by the `TextDecoder` is accepted. If unspecified, an `ArrayBuffer` containing a copy of the asset would be returned instead.
- Returns: `<string>` | `<ArrayBuffer>`

#### `sea.getAssetAsBlob(key[, options])`#

Added in: v21.7.0, v20.12.0

Similar to `sea.getAsset()`, but returns the result in a `<Blob>`. An error is thrown when no matching asset can be found.

- `key` `<string>` the key for the asset in the dictionary specified by the `assets` field in the single-executable application configuration.
- `options` `<Object>`
  - `type` `<string>` An optional mime type for the blob.
- Returns: `<Blob>`

#### `sea.getRawAsset(key)`#

Added in: v21.7.0, v20.12.0

This method can be used to retrieve the assets configured to be bundled into the single-executable application at build time. An error is thrown when no matching asset can be found.

Unlike `sea.getAsset()` or `sea.getAssetAsBlob()`, this method does not return a copy. Instead, it returns the raw asset bundled inside the executable.

For now, users should avoid writing to the returned array buffer. If the injected section is not marked as writable or not aligned properly, writes to the returned array buffer is likely to result in a crash.

- `key` `<string>` the key for the asset in the dictionary specified by the `assets` field in the single-executable application configuration.
- Returns: `<ArrayBuffer>`

#### `sea.getAssetKeys()`#

Added in: v24.8.0, v22.20.0

- Returns `<string>`[] An array containing all the keys of the assets embedded in the executable. If no assets are embedded, returns an empty array.

This method can be used to retrieve an array of all the keys of assets embedded into the single-executable application. An error is thrown when not running inside a single-executable application.

### In the injected main script#

#### Module format of the injected main script#

To specify how Node.js should interpret the injected main script, use the `mainFormat` field in the single-executable application configuration. The accepted values are:

- `"commonjs"`: The injected main script is treated as a CommonJS module.
- `"module"`: The injected main script is treated as an ECMAScript module.

If the `mainFormat` field is not specified, it defaults to `"commonjs"`.

Currently, `"mainFormat": "module"` cannot be used together with `"useSnapshot"`.

#### Module loading in the injected main script#

In the injected main script, module loading does not read from the file system. By default, both `require()` and `import` statements would only be able to load the built-in modules. Attempting to load a module that can only be found in the file system will throw an error.

Users can bundle their application into a standalone JavaScript file to inject into the executable. This also ensures a more deterministic dependency graph.

To load modules from the file system in the injected main script, users can create a `require` function that can load from the file system using `module.createRequire()`. For example, in a CommonJS entry point:

    const { createRequire } = require('node:module');
    require = createRequire(__filename);
    js

#### `require()` in the injected main script#

`require()` in the injected main script is not the same as the `require()` available to modules that are not injected. Currently, it does not have any of the properties that non-injected `require()` has except `require.main`.

#### `__filename` and `module.filename` in the injected main script#

The values of `__filename` and `module.filename` in the injected main script are equal to `process.execPath`.

#### `__dirname` in the injected main script#

The value of `__dirname` in the injected main script is equal to the directory name of `process.execPath`.

#### `import.meta` in the injected main script#

When using `"mainFormat": "module"`, `import.meta` is available in the injected main script with the following properties:

- `import.meta.url`: A `file:` URL corresponding to `process.execPath`.
- `import.meta.filename`: Equal to `process.execPath`.
- `import.meta.dirname`: The directory name of `process.execPath`.
- `import.meta.main`: `true`.

`import.meta.resolve` is currently not supported.

#### `import()` in the injected main script#

When using `"mainFormat": "module"`, `import()` can be used to dynamically load built-in modules. Attempting to use `import()` to load modules from the file system will throw an error.

#### Using native addons in the injected main script#

Native addons can be bundled as assets into the single-executable application by specifying them in the `assets` field of the configuration file used to generate the single-executable application preparation blob. The addon can then be loaded in the injected main script by writing the asset to a temporary file and loading it with `process.dlopen()`.

    {
      "main": "/path/to/bundled/script.js",
      "output": "/path/to/write/the/generated/executable",
      "assets": {
        "myaddon.node": "/path/to/myaddon/build/Release/myaddon.node"
      }
    }
    json

    // script.js
    const fs = require('node:fs');
    const os = require('node:os');
    const path = require('node:path');
    const { getRawAsset } = require('node:sea');
    const addonPath = path.join(os.tmpdir(), 'myaddon.node');
    fs.writeFileSync(addonPath, new Uint8Array(getRawAsset('myaddon.node')));
    const myaddon = { exports: {} };
    process.dlopen(myaddon, addonPath);
    console.log(myaddon.exports);
    fs.rmSync(addonPath);
    js

Known caveat: if the single-executable application is produced by postject running on a Linux arm64 docker container, the produced ELF binary does not have the correct hash table to load the addons and will crash on `process.dlopen()`. Build the single-executable application on other platforms, or at least on a non-container Linux arm64 environment to work around this issue.

### Notes#

#### Single executable application creation process#

The process documented here is subject to change.

##### 1. Generating single executable preparation blobs#

To build a single executable application, Node.js would first generate a blob that contains all the necessary information to run the bundled script. When using `--build-sea`, this step is done internally along with the injection.

###### Dumping the preparation blob to disk#

Before `--build-sea` was introduced, an older workflow was introduced to write the preparation blob to disk for injection by external tools. This can still be used for verification purposes.

To dump the preparation blob to disk for verification, use `--experimental-sea-config`. This writes a file that can be injected into a Node.js binary using tools like postject.

The configuration is similar to that of `--build-sea`, except that the `output` field specifies the path to write the generated blob file instead of the final executable.

    {
      "main": "/path/to/bundled/script.js",
      // Instead of the final executable, this is the path to write the blob.
      "output": "/path/to/write/the/generated/blob.blob"
    }
    json

##### 2. Injecting the preparation blob into the `node` binary#

To complete the creation of a single executable application, the generated blob needs to be injected into a copy of the `node` binary, as documented below.

When using `--build-sea`, this step is done internally along with the blob generation.

- If the `node` binary is a PE file, the blob should be injected as a resource named `NODE_SEA_BLOB`.
- If the `node` binary is a Mach-O file, the blob should be injected as a section named `NODE_SEA_BLOB` in the `NODE_SEA` segment.
- If the `node` binary is an ELF file, the blob should be injected as a note named `NODE_SEA_BLOB`.

Then, the SEA building process searches the binary for the `NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2:0` fuse string and flip the last character to `1` to indicate that a resource has been injected.

###### Injecting the preparation blob manually#

Before `--build-sea` was introduced, an older workflow was introduced to allow external tools to inject the generated blob into a copy of the `node` binary.

For example, with postject:

1. Create a copy of the `node` executable and name it according to your needs:

   - On systems other than Windows:

       cp $(command -v node) hello
       bash

   - On Windows:

       node -e "require('fs').copyFileSync(process.execPath, 'hello.exe')"
       text

   The `.exe` extension is necessary.

2. Remove the signature of the binary (macOS and Windows only):

   - On macOS:

       codesign --remove-signature hello
       bash

   - On Windows (optional):

   signtool can be used from the installed Windows SDK. If this step is skipped, ignore any signature-related warning from postject.

       signtool remove /s hello.exe
       powershell

3. Inject the blob into the copied binary by running `postject` with the following options:

   - `hello` / `hello.exe` - The name of the copy of the `node` executable created in step 4.
   - `NODE_SEA_BLOB` - The name of the resource / note / section in the binary where the contents of the blob will be stored.
   - `sea-prep.blob` - The name of the blob created in step 1.
   - `--sentinel-fuse NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2` - The fuse used by the Node.js project to detect if a file has been injected.
   - `--macho-segment-name NODE_SEA` (only needed on macOS) - The name of the segment in the binary where the contents of the blob will be stored.

   To summarize, here is the required command for each platform:

   - On Linux:

         npx postject hello NODE_SEA_BLOB sea-prep.blob \
             --sentinel-fuse NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2
         bash

   - On Windows - PowerShell:

         npx postject hello.exe NODE_SEA_BLOB sea-prep.blob `
             --sentinel-fuse NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2
         powershell

   - On Windows - Command Prompt:

         npx postject hello.exe NODE_SEA_BLOB sea-prep.blob ^
             --sentinel-fuse NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2
         text

   - On macOS:

         npx postject hello NODE_SEA_BLOB sea-prep.blob \
             --sentinel-fuse NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2 \
             --macho-segment-name NODE_SEA
         bash

#### Platform support#

Single-executable support is tested regularly on CI only on the following platforms:

- Windows
- macOS (arm64 only; x64 is not currently supported and is skipped in the tests)
- Linux (all distributions supported by Node.js except Alpine and all architectures supported by Node.js except s390x)

This is due to a lack of better tools to generate single-executables that can be used to test this feature on other platforms.

Suggestions for other resource injection tools/workflows are welcomed. Please start a discussion at https://github.com/nodejs/single-executable/discussions to help us document them.

## SQLite#

Source Code: lib/sqlite.js Added in: v22.5.0
History

Version | Changes
--- | ---
v25.7.0 | SQLite is now a release candidate.
v23.4.0, v22.13.0 | SQLite is no longer behind `--experimental-sqlite` but still experimental.

Stability: 1.2 - Release candidate.

The `node:sqlite` module facilitates working with SQLite databases. To access it:

    import sqlite from 'node:sqlite';
    const sqlite = require('node:sqlite');
    javascript

This module is only available under the `node:` scheme.

The following example shows the basic usage of the `node:sqlite` module to open an in-memory database, write data to the database, and then read the data back.

    import { DatabaseSync } from 'node:sqlite';
    const database = new DatabaseSync(':memory:');

    // Execute SQL statements from strings.
    database.exec(`
      CREATE TABLE data(
        key INTEGER PRIMARY KEY,
        value TEXT
      ) STRICT
    `);
    // Create a prepared statement to insert data into the database.
    const insert = database.prepare('INSERT INTO data (key, value) VALUES (?, ?)');
    // Execute the prepared statement with bound values.
    insert.run(1, 'hello');
    insert.run(2, 'world');
    // Create a prepared statement to read data from the database.
    const query = database.prepare('SELECT * FROM data ORDER BY key');
    // Execute the prepared statement and log the result set.
    console.log(query.all());
    // Prints: [ { key: 1, value: 'hello' }, { key: 2, value: 'world' } ]
    const { DatabaseSync } = require('node:sqlite');
    const database = new DatabaseSync(':memory:');

    // Execute SQL statements from strings.
    database.exec(`
      CREATE TABLE data(
        key INTEGER PRIMARY KEY,
        value TEXT
      ) STRICT
    `);
    // Create a prepared statement to insert data into the database.
    const insert = database.prepare('INSERT INTO data (key, value) VALUES (?, ?)');
    // Execute the prepared statement with bound values.
    insert.run(1, 'hello');
    insert.run(2, 'world');
    // Create a prepared statement to read data from the database.
    const query = database.prepare('SELECT * FROM data ORDER BY key');
    // Execute the prepared statement and log the result set.
    console.log(query.all());
    // Prints: [ { key: 1, value: 'hello' }, { key: 2, value: 'world' } ]
    javascript

### Type conversion between JavaScript and SQLite#

When Node.js writes to or reads from SQLite, it is necessary to convert between JavaScript data types and SQLite's data types. Because JavaScript supports more data types than SQLite, only a subset of JavaScript types are supported. Attempting to write an unsupported data type to SQLite will result in an exception.

Storage class | JavaScript to SQLite | SQLite to JavaScript
--- | --- | ---
`NULL` | `<null>` | `<null>`
`INTEGER` | `<number>` or `<bigint>` | `<number>` or `<bigint>` (configurable)
`REAL` | `<number>` | `<number>`
`TEXT` | `<string>` | `<string>`
`BLOB` | `<TypedArray>` or `<DataView>` | `<Uint8Array>`

APIs that read values from SQLite have a configuration option that determines whether `INTEGER` values are converted to `number` or `bigint` in JavaScript, such as the `readBigInts` option for statements and the `useBigIntArguments` option for user-defined functions. If Node.js reads an `INTEGER` value from SQLite that is outside the JavaScript safe integer range, and the option to read BigInts is not enabled, then an `ERR_OUT_OF_RANGE` error will be thrown.

### Class: `DatabaseSync`#

Added in: v22.5.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Add `timeout` option.
v23.10.0, v22.15.0 | The `path` argument now supports Buffer and URL objects.

This class represents a single connection to a SQLite database. All APIs exposed by this class execute synchronously.

#### `new DatabaseSync(path[, options])`#

Added in: v22.5.0
History

Version | Changes
--- | ---
v25.5.0, v24.14.0 | Enable `defensive` by default.
v25.1.0, v24.12.0 | Add `defensive` option.
v24.4.0, v22.18.0 | Add new SQLite database options.

- `path` `<string>` | `<Buffer>` | `<URL>` The path of the database. A SQLite database can be stored in a file or completely in memory. To use a file-backed database, the path should be a file path. To use an in-memory database, the path should be the special name `':memory:'`.
- `options` `<Object>` Configuration options for the database connection. The following options are supported:
  - `open` `<boolean>` If `true`, the database is opened by the constructor. When this value is `false`, the database must be opened via the `open()` method. Default: `true`.
  - `readOnly` `<boolean>` If `true`, the database is opened in read-only mode. If the database does not exist, opening it will fail. Default: `false`.
  - `enableForeignKeyConstraints` `<boolean>` If `true`, foreign key constraints are enabled. This is recommended but can be disabled for compatibility with legacy database schemas. The enforcement of foreign key constraints can be enabled and disabled after opening the database using `PRAGMA foreign_keys`. Default: `true`.
  - `enableDoubleQuotedStringLiterals` `<boolean>` If `true`, SQLite will accept double-quoted string literals. This is not recommended but can be enabled for compatibility with legacy database schemas. Default: `false`.
  - `allowExtension` `<boolean>` If `true`, the `loadExtension` SQL function and the `loadExtension()` method are enabled. You can call `enableLoadExtension(false)` later to disable this feature. Default: `false`.
  - `timeout` `<number>` The busy timeout in milliseconds. This is the maximum amount of time that SQLite will wait for a database lock to be released before returning an error. Default: `0`.
  - `readBigInts` `<boolean>` If `true`, integer fields are read as JavaScript `BigInt` values. If `false`, integer fields are read as JavaScript numbers. Default: `false`.
  - `returnArrays` `<boolean>` If `true`, query results are returned as arrays instead of objects. Default: `false`.
  - `allowBareNamedParameters` `<boolean>` If `true`, allows binding named parameters without the prefix character (e.g., `foo` instead of `:foo`). Default: `true`.
  - `allowUnknownNamedParameters` `<boolean>` If `true`, unknown named parameters are ignored when binding. If `false`, an exception is thrown for unknown named parameters. Default: `false`.
  - `defensive` `<boolean>` If `true`, enables the defensive flag. When the defensive flag is enabled, language features that allow ordinary SQL to deliberately corrupt the database file are disabled. The defensive flag can also be set using `enableDefensive()`. Default: `true`.
  - `limits` `<Object>` Configuration for various SQLite limits. These limits can be used to prevent excessive resource consumption when handling potentially malicious input. See Run-Time Limits and Limit Constants in the SQLite documentation for details. Default values are determined by SQLite's compile-time defaults and may vary depending on how SQLite was built. The following properties are supported:
    - `length` `<number>` Maximum length of a string or BLOB.
    - `sqlLength` `<number>` Maximum length of an SQL statement.
    - `column` `<number>` Maximum number of columns.
    - `exprDepth` `<number>` Maximum depth of an expression tree.
    - `compoundSelect` `<number>` Maximum number of terms in a compound SELECT.
    - `vdbeOp` `<number>` Maximum number of VDBE instructions.
    - `functionArg` `<number>` Maximum number of function arguments.
    - `attach` `<number>` Maximum number of attached databases.
    - `likePatternLength` `<number>` Maximum length of a LIKE pattern.
    - `variableNumber` `<number>` Maximum number of SQL variables.
    - `triggerDepth` `<number>` Maximum trigger recursion depth.

Constructs a new `DatabaseSync` instance.

#### `database.aggregate(name, options)`#

Added in: v24.0.0, v22.16.0

Registers a new aggregate function with the SQLite database. This method is a wrapper around `sqlite3_create_window_function()`.

- `name` `<string>` The name of the SQLite function to create.
- `options` `<Object>` Function configuration settings.
  - `deterministic` `<boolean>` If `true`, the `SQLITE_DETERMINISTIC` flag is set on the created function. Default: `false`.
  - `directOnly` `<boolean>` If `true`, the `SQLITE_DIRECTONLY` flag is set on the created function. Default: `false`.
  - `useBigIntArguments` `<boolean>` If `true`, integer arguments to `options.step` and `options.inverse` are converted to `BigInt`s. If `false`, integer arguments are passed as JavaScript numbers. Default: `false`.
  - `varargs` `<boolean>` If `true`, `options.step` and `options.inverse` may be invoked with any number of arguments (between zero and `SQLITE_MAX_FUNCTION_ARG`). If `false`, `inverse` and `step` must be invoked with exactly `length` arguments. Default: `false`.
  - `start` `<number>` | `<string>` | `<null>` | `<Array>` | `<Object>` | `<Function>` The identity value for the aggregation function. This value is used when the aggregation function is initialized. When a `<Function>` is passed the identity will be its return value.
  - `step` `<Function>` The function to call for each row in the aggregation. The function receives the current state and the row value. The return value of this function should be the new state.
  - `result` `<Function>` The function to call to get the result of the aggregation. The function receives the final state and should return the result of the aggregation.
  - `inverse` `<Function>` When this function is provided, the `aggregate` method will work as a window function. The function receives the current state and the dropped row value. The return value of this function should be the new state.

When used as a window function, the `result` function will be called multiple times.

    const { DatabaseSync } = require('node:sqlite');

    const db = new DatabaseSync(':memory:');
    db.exec(`
      CREATE TABLE t3(x, y);
      INSERT INTO t3 VALUES ('a', 4),
                            ('b', 5),
                            ('c', 3),
                            ('d', 8),
                            ('e', 1);
    `);

    db.aggregate('sumint', {
      start: 0,
      step: (acc, value) => acc + value,
    });

    db.prepare('SELECT sumint(y) as total FROM t3').get(); // { total: 21 }
    import { DatabaseSync } from 'node:sqlite';

    const db = new DatabaseSync(':memory:');
    db.exec(`
      CREATE TABLE t3(x, y);
      INSERT INTO t3 VALUES ('a', 4),
                            ('b', 5),
                            ('c', 3),
                            ('d', 8),
                            ('e', 1);
    `);

    db.aggregate('sumint', {
      start: 0,
      step: (acc, value) => acc + value,
    });

    db.prepare('SELECT sumint(y) as total FROM t3').get(); // { total: 21 }
    javascript

#### `database.close()`#

Added in: v22.5.0

Closes the database connection. An exception is thrown if the database is not open. This method is a wrapper around `sqlite3_close_v2()`.

#### `database.loadExtension(path[, entryPoint])`#

Added in: v23.5.0, v22.13.0

- `path` `<string>` The path to the shared library to load.
- `entryPoint` `<string>` The name of the extension's entry-point function. When omitted, SQLite derives the entry point from the shared library's filename; pass this argument explicitly when the derived name does not match.

Loads a shared library into the database connection. This method is a wrapper around `sqlite3_load_extension()`. It is required to enable the `allowExtension` option when constructing the `DatabaseSync` instance.

    import { DatabaseSync } from 'node:sqlite';
    const database = new DatabaseSync(':memory:', { allowExtension: true });

    // Load using the entry point derived from the filename.
    database.loadExtension('./decimal.dylib');

    // Override the entry point when the derived name does not match.
    database.loadExtension('./base64.dylib', 'sqlite3_base64_init');
    const { DatabaseSync } = require('node:sqlite');
    const database = new DatabaseSync(':memory:', { allowExtension: true });

    // Load using the entry point derived from the filename.
    database.loadExtension('./decimal.dylib');

    // Override the entry point when the derived name does not match.
    database.loadExtension('./base64.dylib', 'sqlite3_base64_init');
    javascript

#### `database.enableLoadExtension(allow)`#

Added in: v23.5.0, v22.13.0

- `allow` `<boolean>` Whether to allow loading extensions.

Enables or disables the `loadExtension` SQL function, and the `loadExtension()` method. When `allowExtension` is `false` when constructing, you cannot enable loading extensions for security reasons.

#### `database.enableDefensive(active)`#

Added in: v25.1.0, v24.12.0

- `active` `<boolean>` Whether to set the defensive flag.

Enables or disables the defensive flag. When the defensive flag is active, language features that allow ordinary SQL to deliberately corrupt the database file are disabled. See `SQLITE_DBCONFIG_DEFENSIVE` in the SQLite documentation for details.

#### `database.location([dbName])`#

Added in: v24.0.0, v22.16.0

- `dbName` `<string>` Name of the database. This can be `'main'` (the default primary database) or any other database that has been added with `ATTACH DATABASE` Default: `'main'`.
- Returns: `<string>` | `<null>` The location of the database file. When using an in-memory database, this method returns null.

This method is a wrapper around `sqlite3_db_filename()`

#### `database.exec(sql)`#

Added in: v22.5.0

- `sql` `<string>` A SQL string to execute.

This method allows one or more SQL statements to be executed without returning any results. This method is useful when executing SQL statements read from a file. This method is a wrapper around `sqlite3_exec()`.

#### `database.function(name[, options], fn)`#

Added in: v23.5.0, v22.13.0

- `name` `<string>` The name of the SQLite function to create.
- `options` `<Object>` Optional configuration settings for the function. The following properties are supported:
  - `deterministic` `<boolean>` If `true`, the `SQLITE_DETERMINISTIC` flag is set on the created function. Default: `false`.
  - `directOnly` `<boolean>` If `true`, the `SQLITE_DIRECTONLY` flag is set on the created function. Default: `false`.
  - `useBigIntArguments` `<boolean>` If `true`, integer arguments to `function` are converted to `BigInt`s. If `false`, integer arguments are passed as JavaScript numbers. Default: `false`.
  - `varargs` `<boolean>` If `true`, `function` may be invoked with any number of arguments (between zero and `SQLITE_MAX_FUNCTION_ARG`). If `false`, `function` must be invoked with exactly `function.length` arguments. Default: `false`.
- `fn` `<Function>` The JavaScript function to call when the SQLite function is invoked. The return value of this function should be a valid SQLite data type: see Type conversion between JavaScript and SQLite. The result defaults to `NULL` if the return value is `undefined`.

This method is used to create SQLite user-defined functions. This method is a wrapper around `sqlite3_create_function_v2()`.

#### `database.setAuthorizer(callback)`#

Added in: v24.10.0

- `callback` `<Function>` | `<null>` The authorizer function to set, or `null` to clear the current authorizer.

Sets an authorizer callback that SQLite will invoke whenever it attempts to access data or modify the database schema through prepared statements. This can be used to implement security policies, audit access, or restrict certain operations. This method is a wrapper around `sqlite3_set_authorizer()`.

When invoked, the callback receives five arguments:

- `actionCode` `<number>` The type of operation being performed (e.g., `SQLITE_INSERT`, `SQLITE_UPDATE`, `SQLITE_SELECT`).
- `arg1` `<string>` | `<null>` The first argument (context-dependent, often a table name).
- `arg2` `<string>` | `<null>` The second argument (context-dependent, often a column name).
- `dbName` `<string>` | `<null>` The name of the database.
- `triggerOrView` `<string>` | `<null>` The name of the trigger or view causing the access.

The callback must return one of the following constants:

- `SQLITE_OK` - Allow the operation.
- `SQLITE_DENY` - Deny the operation (causes an error).
- `SQLITE_IGNORE` - Ignore the operation (silently skip).

    const { DatabaseSync, constants } = require('node:sqlite');
    const db = new DatabaseSync(':memory:');

    // Set up an authorizer that denies all table creation
    db.setAuthorizer((actionCode) => {
      if (actionCode === constants.SQLITE_CREATE_TABLE) {
        return constants.SQLITE_DENY;
      }
      return constants.SQLITE_OK;
    });

    // This will work
    db.prepare('SELECT 1').get();

    // This will throw an error due to authorization denial
    try {
      db.exec('CREATE TABLE blocked (id INTEGER)');
    } catch (err) {
      console.log('Operation blocked:', err.message);
    }
    import { DatabaseSync, constants } from 'node:sqlite';
    const db = new DatabaseSync(':memory:');

    // Set up an authorizer that denies all table creation
    db.setAuthorizer((actionCode) => {
      if (actionCode === constants.SQLITE_CREATE_TABLE) {
        return constants.SQLITE_DENY;
      }
      return constants.SQLITE_OK;
    });

    // This will work
    db.prepare('SELECT 1').get();

    // This will throw an error due to authorization denial
    try {
      db.exec('CREATE TABLE blocked (id INTEGER)');
    } catch (err) {
      console.log('Operation blocked:', err.message);
    }
    javascript

#### `database.isOpen`#

Added in: v23.11.0, v22.15.0

- Type: `<boolean>` Whether the database is currently open or not.

#### `database.isTransaction`#

Added in: v24.0.0, v22.16.0

- Type: `<boolean>` Whether the database is currently within a transaction. This method is a wrapper around `sqlite3_get_autocommit()`.

#### `database.limits`#

Added in: v25.8.0

- Type: `<Object>`

An object for getting and setting SQLite database limits at runtime. Each property corresponds to an SQLite limit and can be read or written.

    const db = new DatabaseSync(':memory:');

    // Read current limit
    console.log(db.limits.length);

    // Set a new limit
    db.limits.sqlLength = 100000;

    // Reset a limit to its compile-time maximum
    db.limits.sqlLength = Infinity;
    js

Available properties: `length`, `sqlLength`, `column`, `exprDepth`, `compoundSelect`, `vdbeOp`, `functionArg`, `attach`, `likePatternLength`, `variableNumber`, `triggerDepth`.

Setting a property to `Infinity` resets the limit to its compile-time maximum value.

#### `database.open()`#

Added in: v22.5.0

Opens the database specified in the `path` argument of the `DatabaseSync` constructor. This method should only be used when the database is not opened via the constructor. An exception is thrown if the database is already open.

#### `database.serialize([dbName])`#

Added in: v26.1.0

- `dbName` `<string>` Name of the database to serialize. This can be `'main'` (the default primary database) or any other database that has been added with `ATTACH DATABASE`. Default: `'main'`.
- Returns: `<Uint8Array>` A binary representation of the database.

Serializes the database into a binary representation, returned as a `Uint8Array`. This is useful for saving, cloning, or transferring an in-memory database. This method is a wrapper around `sqlite3_serialize()`.

    import { DatabaseSync } from 'node:sqlite';

    const db = new DatabaseSync(':memory:');
    db.exec('CREATE TABLE t(key INTEGER PRIMARY KEY, value TEXT)');
    db.exec("INSERT INTO t VALUES (1, 'hello')");
    const buffer = db.serialize();
    console.log(buffer.length); // Prints the byte length of the database
    const { DatabaseSync } = require('node:sqlite');

    const db = new DatabaseSync(':memory:');
    db.exec('CREATE TABLE t(key INTEGER PRIMARY KEY, value TEXT)');
    db.exec("INSERT INTO t VALUES (1, 'hello')");
    const buffer = db.serialize();
    console.log(buffer.length); // Prints the byte length of the database
    javascript

#### `database.deserialize(buffer[, options])`#

Added in: v26.1.0

- `buffer` `<Uint8Array>` A binary representation of a database, such as the output of `database.serialize()`.
- `options` `<Object>` Optional configuration for the deserialization.
  - `dbName` `<string>` Name of the database to deserialize into. Default: `'main'`.

Loads a serialized database into this connection, replacing the current database. The deserialized database is writable. Existing prepared statements are finalized before deserialization is attempted, even if the operation subsequently fails. This method is a wrapper around `sqlite3_deserialize()`.

    import { DatabaseSync } from 'node:sqlite';

    const original = new DatabaseSync(':memory:');
    original.exec('CREATE TABLE t(key INTEGER PRIMARY KEY, value TEXT)');
    original.exec("INSERT INTO t VALUES (1, 'hello')");
    const buffer = original.serialize();
    original.close();

    const clone = new DatabaseSync(':memory:');
    clone.deserialize(buffer);
    console.log(clone.prepare('SELECT value FROM t').get());
    // Prints: { value: 'hello' }
    const { DatabaseSync } = require('node:sqlite');

    const original = new DatabaseSync(':memory:');
    original.exec('CREATE TABLE t(key INTEGER PRIMARY KEY, value TEXT)');
    original.exec("INSERT INTO t VALUES (1, 'hello')");
    const buffer = original.serialize();
    original.close();

    const clone = new DatabaseSync(':memory:');
    clone.deserialize(buffer);
    console.log(clone.prepare('SELECT value FROM t').get());
    // Prints: { value: 'hello' }
    javascript

#### `database.prepare(sql[, options])`#

Added in: v22.5.0

- `sql` `<string>` A SQL string to compile to a prepared statement.
- `options` `<Object>` Optional configuration for the prepared statement.
  - `readBigInts` `<boolean>` If `true`, integer fields are read as `BigInt`s. Default: inherited from database options or `false`.
  - `returnArrays` `<boolean>` If `true`, results are returned as arrays. Default: inherited from database options or `false`.
  - `allowBareNamedParameters` `<boolean>` If `true`, allows binding named parameters without the prefix character. Default: inherited from database options or `true`.
  - `allowUnknownNamedParameters` `<boolean>` If `true`, unknown named parameters are ignored. Default: inherited from database options or `false`.
- Returns: `<StatementSync>` The prepared statement.

Compiles a SQL statement into a prepared statement. This method is a wrapper around `sqlite3_prepare_v2()`.

#### `database.createTagStore([maxSize])`#

Added in: v24.9.0

- `maxSize` `<integer>` The maximum number of prepared statements to cache. Default: `1000`.
- Returns: `<SQLTagStore>` A new SQL tag store for caching prepared statements.

Creates a new `SQLTagStore`, which is a Least Recently Used (LRU) cache for storing prepared statements. This allows for the efficient reuse of prepared statements by tagging them with a unique identifier.

When a tagged SQL literal is executed, the `SQLTagStore` checks if a prepared statement for the corresponding SQL query string already exists in the cache. If it does, the cached statement is used. If not, a new prepared statement is created, executed, and then stored in the cache for future use. This mechanism helps to avoid the overhead of repeatedly parsing and preparing the same SQL statements.

Tagged statements bind the placeholder values from the template literal as parameters to the underlying prepared statement. For example:

    sqlTagStore.get`SELECT ${value}`;
    js

is equivalent to:

    db.prepare('SELECT ?').get(value);
    js

However, in the first example, the tag store will cache the underlying prepared statement for future use.

> Note: The `${value}` syntax in tagged statements binds a parameter to the prepared statement. This differs from its behavior in untagged template literals, where it performs string interpolation.

>     // This a safe example of binding a parameter to a tagged statement.
>     sqlTagStore.run`INSERT INTO t1 (id) VALUES (${id})`;

>     // This is an *unsafe* example of an untagged template string.
>     // `id` is interpolated into the query text as a string.
>     // This can lead to SQL injection and data corruption.
>     db.run(`INSERT INTO t1 (id) VALUES (${id})`);
>     js

The tag store will match a statement from the cache if the query strings (including the positions of any bound placeholders) are identical.

    // The following statements will match in the cache:
    sqlTagStore.get`SELECT * FROM t1 WHERE id = ${id} AND active = 1`;
    sqlTagStore.get`SELECT * FROM t1 WHERE id = ${12345} AND active = 1`;

    // The following statements will not match, as the query strings
    // and bound placeholders differ:
    sqlTagStore.get`SELECT * FROM t1 WHERE id = ${id} AND active = 1`;
    sqlTagStore.get`SELECT * FROM t1 WHERE id = 12345 AND active = 1`;

    // The following statements will not match, as matches are case-sensitive:
    sqlTagStore.get`SELECT * FROM t1 WHERE id = ${id} AND active = 1`;
    sqlTagStore.get`select * from t1 where id = ${id} and active = 1`;
    js

The only way of binding parameters in tagged statements is with the `${value}` syntax. Do not add parameter binding placeholders (`?` etc.) to the SQL query string itself.

    import { DatabaseSync } from 'node:sqlite';

    const db = new DatabaseSync(':memory:');
    const sql = db.createTagStore();

    db.exec('CREATE TABLE users (id INT, name TEXT)');

    // Using the 'run' method to insert data.
    // The tagged literal is used to identify the prepared statement.
    sql.run`INSERT INTO users VALUES (1, 'Alice')`;
    sql.run`INSERT INTO users VALUES (2, 'Bob')`;

    // Using the 'get' method to retrieve a single row.
    const name = 'Alice';
    const user = sql.get`SELECT * FROM users WHERE name = ${name}`;
    console.log(user); // { id: 1, name: 'Alice' }

    // Using the 'all' method to retrieve all rows.
    const allUsers = sql.all`SELECT * FROM users ORDER BY id`;
    console.log(allUsers);
    // [
    //   { id: 1, name: 'Alice' },
    //   { id: 2, name: 'Bob' }
    // ]
    const { DatabaseSync } = require('node:sqlite');

    const db = new DatabaseSync(':memory:');
    const sql = db.createTagStore();

    db.exec('CREATE TABLE users (id INT, name TEXT)');

    // Using the 'run' method to insert data.
    // The tagged literal is used to identify the prepared statement.
    sql.run`INSERT INTO users VALUES (1, 'Alice')`;
    sql.run`INSERT INTO users VALUES (2, 'Bob')`;

    // Using the 'get' method to retrieve a single row.
    const name = 'Alice';
    const user = sql.get`SELECT * FROM users WHERE name = ${name}`;
    console.log(user); // { id: 1, name: 'Alice' }

    // Using the 'all' method to retrieve all rows.
    const allUsers = sql.all`SELECT * FROM users ORDER BY id`;
    console.log(allUsers);
    // [
    //   { id: 1, name: 'Alice' },
    //   { id: 2, name: 'Bob' }
    // ]
    javascript

#### `database.createSession([options])`#

Added in: v23.3.0, v22.12.0

- `options` `<Object>` The configuration options for the session.
  - `table` `<string>` A specific table to track changes for. By default, changes to all tables are tracked.
  - `db` `<string>` Name of the database to track. This is useful when multiple databases have been added using `ATTACH DATABASE`. Default: `'main'`.
- Returns: `<Session>` A session handle.

Creates and attaches a session to the database. This method is a wrapper around `sqlite3session_create()` and `sqlite3session_attach()`.

#### `database.applyChangeset(changeset[, options])`#

Added in: v23.3.0, v22.12.0

- `changeset` `<Uint8Array>` A binary changeset or patchset.
- `options` `<Object>` The configuration options for how the changes will be applied.

  - `filter` `<Function>` for each table affected by at least one change in the changeset, the `filter` callback is invoked with the table name as the first argument. If the return value is falsy, then no attempt is made to apply any changes to the table. Otherwise, if the return value is truthy or no `filter` callback is provided, all changes related to the table are attempted.

  - `onConflict` `<Function>` A function that determines how to handle conflicts. The function receives one argument, which can be one of the following values:

    - `SQLITE_CHANGESET_DATA`: A `DELETE` or `UPDATE` change does not contain the expected "before" values.
    - `SQLITE_CHANGESET_NOTFOUND`: A row matching the primary key of the `DELETE` or `UPDATE` change does not exist.
    - `SQLITE_CHANGESET_CONFLICT`: An `INSERT` change results in a duplicate primary key.
    - `SQLITE_CHANGESET_FOREIGN_KEY`: Applying a change would result in a foreign key violation.
    - `SQLITE_CHANGESET_CONSTRAINT`: Applying a change results in a `UNIQUE`, `CHECK`, or `NOT NULL` constraint violation.

    The function should return one of the following values:

    - `SQLITE_CHANGESET_OMIT`: Omit conflicting changes.
    - `SQLITE_CHANGESET_REPLACE`: Replace existing values with conflicting changes (only valid with `SQLITE_CHANGESET_DATA` or `SQLITE_CHANGESET_CONFLICT` conflicts).
    - `SQLITE_CHANGESET_ABORT`: Abort on conflict and roll back the database.

    When an error is thrown in the conflict handler or when any other value is returned from the handler, applying the changeset is aborted and the database is rolled back.

    Default: A function that returns `SQLITE_CHANGESET_ABORT`.

- Returns: `<boolean>` Whether the changeset was applied successfully without being aborted.

An exception is thrown if the database is not open. This method is a wrapper around `sqlite3changeset_apply()`.

    import { DatabaseSync } from 'node:sqlite';

    const sourceDb = new DatabaseSync(':memory:');
    const targetDb = new DatabaseSync(':memory:');

    sourceDb.exec('CREATE TABLE data(key INTEGER PRIMARY KEY, value TEXT)');
    targetDb.exec('CREATE TABLE data(key INTEGER PRIMARY KEY, value TEXT)');

    const session = sourceDb.createSession();

    const insert = sourceDb.prepare('INSERT INTO data (key, value) VALUES (?, ?)');
    insert.run(1, 'hello');
    insert.run(2, 'world');

    const changeset = session.changeset();
    targetDb.applyChangeset(changeset);
    // Now that the changeset has been applied, targetDb contains the same data as sourceDb.
    const { DatabaseSync } = require('node:sqlite');

    const sourceDb = new DatabaseSync(':memory:');
    const targetDb = new DatabaseSync(':memory:');

    sourceDb.exec('CREATE TABLE data(key INTEGER PRIMARY KEY, value TEXT)');
    targetDb.exec('CREATE TABLE data(key INTEGER PRIMARY KEY, value TEXT)');

    const session = sourceDb.createSession();

    const insert = sourceDb.prepare('INSERT INTO data (key, value) VALUES (?, ?)');
    insert.run(1, 'hello');
    insert.run(2, 'world');

    const changeset = session.changeset();
    targetDb.applyChangeset(changeset);
    // Now that the changeset has been applied, targetDb contains the same data as sourceDb.
    javascript

#### `database[Symbol.dispose]()`#

Added in: v23.11.0, v22.15.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Closes the database connection. If the database connection is already closed then this is a no-op.

### Class: `Session`#

Added in: v23.3.0, v22.12.0

#### `session.changeset()`#

Added in: v23.3.0, v22.12.0

- Returns: `<Uint8Array>` Binary changeset that can be applied to other databases.

Retrieves a changeset containing all changes since the changeset was created. Can be called multiple times. An exception is thrown if the database or the session is not open. This method is a wrapper around `sqlite3session_changeset()`.

#### `session.patchset()`#

Added in: v23.3.0, v22.12.0

- Returns: `<Uint8Array>` Binary patchset that can be applied to other databases.

Similar to the method above, but generates a more compact patchset. See Changesets and Patchsets in the documentation of SQLite. An exception is thrown if the database or the session is not open. This method is a wrapper around `sqlite3session_patchset()`.

#### `session.close()`#

Closes the session. An exception is thrown if the database or the session is not open. This method is a wrapper around `sqlite3session_delete()`.

#### `session[Symbol.dispose]()`#

Added in: v24.9.0

Closes the session. If the session is already closed, does nothing.

### Class: `StatementSync`#

Added in: v22.5.0

This class represents a single prepared statement. This class cannot be instantiated via its constructor. Instead, instances are created via the `database.prepare()` method. All APIs exposed by this class execute synchronously.

A prepared statement is an efficient binary representation of the SQL used to create it. Prepared statements are parameterizable, and can be invoked multiple times with different bound values. Parameters also offer protection against SQL injection attacks. For these reasons, prepared statements are preferred over hand-crafted SQL strings when handling user input.

#### `statement.all([namedParameters][, ...anonymousParameters])`#

Added in: v22.5.0
History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Add support for `DataView` and typed array objects for `anonymousParameters`.

- `namedParameters` `<Object>` An optional object used to bind named parameters. The keys of this object are used to configure the mapping.
- `...anonymousParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Zero or more values to bind to anonymous parameters.
- Returns: `<Array>` An array of objects. Each object corresponds to a row returned by executing the prepared statement. The keys and values of each object correspond to the column names and values of the row.

This method executes a prepared statement and returns all results as an array of objects. If the prepared statement does not return any results, this method returns an empty array. The prepared statement parameters are bound using the values in `namedParameters` and `anonymousParameters`.

#### `statement.columns()`#

Added in: v23.11.0, v22.16.0

- Returns: `<Array>` An array of objects. Each object corresponds to a column in the prepared statement, and contains the following properties:

  - `column` `<string>` | `<null>` The unaliased name of the column in the origin table, or `null` if the column is the result of an expression or subquery. This property is the result of `sqlite3_column_origin_name()`.
  - `database` `<string>` | `<null>` The unaliased name of the origin database, or `null` if the column is the result of an expression or subquery. This property is the result of `sqlite3_column_database_name()`.
  - `name` `<string>` The name assigned to the column in the result set of a `SELECT` statement. This property is the result of `sqlite3_column_name()`.
  - `table` `<string>` | `<null>` The unaliased name of the origin table, or `null` if the column is the result of an expression or subquery. This property is the result of `sqlite3_column_table_name()`.
  - `type` `<string>` | `<null>` The declared data type of the column, or `null` if the column is the result of an expression or subquery. This property is the result of `sqlite3_column_decltype()`.

This method is used to retrieve information about the columns returned by the prepared statement.

#### `statement.expandedSQL`#

Added in: v22.5.0

- Type: `<string>` The source SQL expanded to include parameter values.

The source SQL text of the prepared statement with parameter placeholders replaced by the values that were used during the most recent execution of this prepared statement. This property is a wrapper around `sqlite3_expanded_sql()`.

#### `statement.get([namedParameters][, ...anonymousParameters])`#

Added in: v22.5.0
History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Add support for `DataView` and typed array objects for `anonymousParameters`.

- `namedParameters` `<Object>` An optional object used to bind named parameters. The keys of this object are used to configure the mapping.
- `...anonymousParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Zero or more values to bind to anonymous parameters.
- Returns: `<Object>` | `<undefined>` An object corresponding to the first row returned by executing the prepared statement. The keys and values of the object correspond to the column names and values of the row. If no rows were returned from the database then this method returns `undefined`.

This method executes a prepared statement and returns the first result as an object. If the prepared statement does not return any results, this method returns `undefined`. The prepared statement parameters are bound using the values in `namedParameters` and `anonymousParameters`.

#### `statement.iterate([namedParameters][, ...anonymousParameters])`#

Added in: v23.4.0, v22.13.0
History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Add support for `DataView` and typed array objects for `anonymousParameters`.

- `namedParameters` `<Object>` An optional object used to bind named parameters. The keys of this object are used to configure the mapping.
- `...anonymousParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Zero or more values to bind to anonymous parameters.
- Returns: `<Iterator>` An iterable iterator of objects. Each object corresponds to a row returned by executing the prepared statement. The keys and values of each object correspond to the column names and values of the row.

This method executes a prepared statement and returns an iterator of objects. If the prepared statement does not return any results, this method returns an empty iterator. The prepared statement parameters are bound using the values in `namedParameters` and `anonymousParameters`.

#### `statement.run([namedParameters][, ...anonymousParameters])`#

Added in: v22.5.0
History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Add support for `DataView` and typed array objects for `anonymousParameters`.

- `namedParameters` `<Object>` An optional object used to bind named parameters. The keys of this object are used to configure the mapping.
- `...anonymousParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Zero or more values to bind to anonymous parameters.
- Returns: `<Object>`
  - `changes` `<number>` | `<bigint>` The number of rows modified, inserted, or deleted by the most recently completed `INSERT`, `UPDATE`, or `DELETE` statement. This field is either a number or a `BigInt` depending on the prepared statement's configuration. This property is the result of `sqlite3_changes64()`.
  - `lastInsertRowid` `<number>` | `<bigint>` The most recently inserted rowid. This field is either a number or a `BigInt` depending on the prepared statement's configuration. This property is the result of `sqlite3_last_insert_rowid()`.

This method executes a prepared statement and returns an object summarizing the resulting changes. The prepared statement parameters are bound using the values in `namedParameters` and `anonymousParameters`.

#### `statement.setAllowBareNamedParameters(enabled)`#

Added in: v22.5.0

- `enabled` `<boolean>` Enables or disables support for binding named parameters without the prefix character.

The names of SQLite parameters begin with a prefix character. By default, `node:sqlite` requires that this prefix character is present when binding parameters. However, with the exception of dollar sign character, these prefix characters also require extra quoting when used in object keys.

To improve ergonomics, this method can be used to also allow bare named parameters, which do not require the prefix character in JavaScript code. There are several caveats to be aware of when enabling bare named parameters:

- The prefix character is still required in SQL.
- The prefix character is still allowed in JavaScript. In fact, prefixed names will have slightly better binding performance.
- Using ambiguous named parameters, such as `$k` and `@k`, in the same prepared statement will result in an exception as it cannot be determined how to bind a bare name.

#### `statement.setAllowUnknownNamedParameters(enabled)`#

Added in: v23.11.0, v22.15.0

- `enabled` `<boolean>` Enables or disables support for unknown named parameters.

By default, if an unknown name is encountered while binding parameters, an exception is thrown. This method allows unknown named parameters to be ignored.

#### `statement.setReturnArrays(enabled)`#

Added in: v24.0.0, v22.16.0

- `enabled` `<boolean>` Enables or disables the return of query results as arrays.

When enabled, query results returned by the `all()`, `get()`, and `iterate()` methods will be returned as arrays instead of objects.

#### `statement.setReadBigInts(enabled)`#

Added in: v22.5.0

- `enabled` `<boolean>` Enables or disables the use of `BigInt`s when reading `INTEGER` fields from the database.

When reading from the database, SQLite `INTEGER`s are mapped to JavaScript numbers by default. However, SQLite `INTEGER`s can store values larger than JavaScript numbers are capable of representing. In such cases, this method can be used to read `INTEGER` data using JavaScript `BigInt`s. This method has no impact on database write operations where numbers and `BigInt`s are both supported at all times.

#### `statement.sourceSQL`#

Added in: v22.5.0

- Type: `<string>` The source SQL used to create this prepared statement.

The source SQL text of the prepared statement. This property is a wrapper around `sqlite3_sql()`.

### Class: `SQLTagStore`#

Added in: v24.9.0

This class represents a single LRU (Least Recently Used) cache for storing prepared statements.

Instances of this class are created via the `database.createTagStore()` method, not by using a constructor. The store caches prepared statements based on the provided SQL query string. When the same query is seen again, the store retrieves the cached statement and safely applies the new values through parameter binding, thereby preventing attacks like SQL injection.

The cache has a maxSize that defaults to 1000 statements, but a custom size can be provided (e.g., `database.createTagStore(100)`). All APIs exposed by this class execute synchronously.

#### `sqlTagStore.all(stringElements[, ...boundParameters])`#

Added in: v24.9.0

- `stringElements` `<string>`[] Template literal elements containing the SQL query.
- `...boundParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Parameter values to be bound to placeholders in the template string.
- Returns: `<Array>` An array of objects representing the rows returned by the query.

Executes the given SQL query and returns all resulting rows as an array of objects.

This function is intended to be used as a template literal tag, not to be called directly.

#### `sqlTagStore.get(stringElements[, ...boundParameters])`#

Added in: v24.9.0

- `stringElements` `<string>`[] Template literal elements containing the SQL query.
- `...boundParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Parameter values to be bound to placeholders in the template string.
- Returns: `<Object>` | `<undefined>` An object representing the first row returned by the query, or `undefined` if no rows are returned.

Executes the given SQL query and returns the first resulting row as an object.

This function is intended to be used as a template literal tag, not to be called directly.

#### `sqlTagStore.iterate(stringElements[, ...boundParameters])`#

Added in: v24.9.0

- `stringElements` `<string>`[] Template literal elements containing the SQL query.
- `...boundParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Parameter values to be bound to placeholders in the template string.
- Returns: `<Iterator>` An iterator that yields objects representing the rows returned by the query.

Executes the given SQL query and returns an iterator over the resulting rows.

This function is intended to be used as a template literal tag, not to be called directly.

#### `sqlTagStore.run(stringElements[, ...boundParameters])`#

Added in: v24.9.0

- `stringElements` `<string>`[] Template literal elements containing the SQL query.
- `...boundParameters` `<null>` | `<number>` | `<bigint>` | `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Parameter values to be bound to placeholders in the template string.
- Returns: `<Object>` An object containing information about the execution, including `changes` and `lastInsertRowid`.

Executes the given SQL query, which is expected to not return any rows (e.g., INSERT, UPDATE, DELETE).

This function is intended to be used as a template literal tag, not to be called directly.

#### `sqlTagStore.size`#

Added in: v24.9.0
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | Changed from a method to a getter.

- Type: `<integer>`

A read-only property that returns the number of prepared statements currently in the cache.

#### `sqlTagStore.capacity`#

Added in: v24.9.0

- Type: `<integer>`

A read-only property that returns the maximum number of prepared statements the cache can hold.

#### `sqlTagStore.db`#

Added in: v24.9.0

- Type: `<DatabaseSync>`

A read-only property that returns the `DatabaseSync` object associated with this `SQLTagStore`.

#### `sqlTagStore.clear()`#

Added in: v24.9.0

Resets the LRU cache, clearing all stored prepared statements.

### `sqlite.backup(sourceDb, path[, options])`#

Added in: v23.8.0, v22.16.0
History

Version | Changes
--- | ---
v23.10.0 | The `path` argument now supports Buffer and URL objects.

- `sourceDb` `<DatabaseSync>` The database to backup. The source database must be open.
- `path` `<string>` | `<Buffer>` | `<URL>` The path where the backup will be created. If the file already exists, the contents will be overwritten.
- `options` `<Object>` Optional configuration for the backup. The following properties are supported:
  - `source` `<string>` Name of the source database. This can be `'main'` (the default primary database) or any other database that have been added with `ATTACH DATABASE` Default: `'main'`.
  - `target` `<string>` Name of the target database. This can be `'main'` (the default primary database) or any other database that have been added with `ATTACH DATABASE` Default: `'main'`.
  - `rate` `<number>` Number of pages to be transmitted in each batch of the backup. Default: `100`.
  - `progress` `<Function>` An optional callback function that will be called after each backup step. The argument passed to this callback is an `<Object>` with `remainingPages` and `totalPages` properties, describing the current progress of the backup operation.
- Returns: `<Promise>` A promise that fulfills with the total number of backed-up pages upon completion, or rejects if an error occurs.

This method makes a database backup. This method abstracts the `sqlite3_backup_init()`, `sqlite3_backup_step()` and `sqlite3_backup_finish()` functions.

The backed-up database can be used normally during the backup process. Mutations coming from the same connection - same `<DatabaseSync>` - object will be reflected in the backup right away. However, mutations from other connections will cause the backup process to restart.

    const { backup, DatabaseSync } = require('node:sqlite');

    (async () => {
      const sourceDb = new DatabaseSync('source.db');
      const totalPagesTransferred = await backup(sourceDb, 'backup.db', {
        rate: 1, // Copy one page at a time.
        progress: ({ totalPages, remainingPages }) => {
          console.log('Backup in progress', { totalPages, remainingPages });
        },
      });

      console.log('Backup completed', totalPagesTransferred);
    })();
    import { backup, DatabaseSync } from 'node:sqlite';

    const sourceDb = new DatabaseSync('source.db');
    const totalPagesTransferred = await backup(sourceDb, 'backup.db', {
      rate: 1, // Copy one page at a time.
      progress: ({ totalPages, remainingPages }) => {
        console.log('Backup in progress', { totalPages, remainingPages });
      },
    });

    console.log('Backup completed', totalPagesTransferred);
    javascript

### `sqlite.constants`#

Added in: v23.5.0, v22.13.0

- Type: `<Object>`

An object containing commonly used constants for SQLite operations.

#### SQLite constants#

The following constants are exported by the `sqlite.constants` object.

##### Conflict resolution constants#

One of the following constants is available as an argument to the `onConflict` conflict resolution handler passed to `database.applyChangeset()`. See also Constants Passed To The Conflict Handler in the SQLite documentation.

Constant | Description
--- | ---
`SQLITE_CHANGESET_DATA` | The conflict handler is invoked with this constant when processing a DELETE or UPDATE change if a row with the required PRIMARY KEY fields is present in the database, but one or more other (non primary-key) fields modified by the update do not contain the expected "before" values.
`SQLITE_CHANGESET_NOTFOUND` | The conflict handler is invoked with this constant when processing a DELETE or UPDATE change if a row with the required PRIMARY KEY fields is not present in the database.
`SQLITE_CHANGESET_CONFLICT` | This constant is passed to the conflict handler while processing an INSERT change if the operation would result in duplicate primary key values.
`SQLITE_CHANGESET_CONSTRAINT` | If foreign key handling is enabled, and applying a changeset leaves the database in a state containing foreign key violations, the conflict handler is invoked with this constant exactly once before the changeset is committed. If the conflict handler returns `SQLITE_CHANGESET_OMIT`, the changes, including those that caused the foreign key constraint violation, are committed. Or, if it returns `SQLITE_CHANGESET_ABORT`, the changeset is rolled back.
`SQLITE_CHANGESET_FOREIGN_KEY` | If any other constraint violation occurs while applying a change (i.e. a UNIQUE, CHECK or NOT NULL constraint), the conflict handler is invoked with this constant.

One of the following constants must be returned from the `onConflict` conflict resolution handler passed to `database.applyChangeset()`. See also Constants Returned From The Conflict Handler in the SQLite documentation.

Constant | Description
--- | ---
`SQLITE_CHANGESET_OMIT` | Conflicting changes are omitted.
`SQLITE_CHANGESET_REPLACE` | Conflicting changes replace existing values. Note that this value can only be returned when the type of conflict is either `SQLITE_CHANGESET_DATA` or `SQLITE_CHANGESET_CONFLICT`.
`SQLITE_CHANGESET_ABORT` | Abort when a change encounters a conflict and roll back database.

##### Authorization constants#

The following constants are used with the `database.setAuthorizer()` method.

###### Authorization result codes#

One of the following constants must be returned from the authorizer callback function passed to `database.setAuthorizer()`.

Constant | Description
--- | ---
`SQLITE_OK` | Allow the operation to proceed normally.
`SQLITE_DENY` | Deny the operation and cause an error to be returned.
`SQLITE_IGNORE` | Ignore the operation and continue as if it had never been requested.

###### Authorization action codes#

The following constants are passed as the first argument to the authorizer callback function to indicate what type of operation is being authorized.

Constant | Description
--- | ---
`SQLITE_CREATE_INDEX` | Create an index
`SQLITE_CREATE_TABLE` | Create a table
`SQLITE_CREATE_TEMP_INDEX` | Create a temporary index
`SQLITE_CREATE_TEMP_TABLE` | Create a temporary table
`SQLITE_CREATE_TEMP_TRIGGER` | Create a temporary trigger
`SQLITE_CREATE_TEMP_VIEW` | Create a temporary view
`SQLITE_CREATE_TRIGGER` | Create a trigger
`SQLITE_CREATE_VIEW` | Create a view
`SQLITE_DELETE` | Delete from a table
`SQLITE_DROP_INDEX` | Drop an index
`SQLITE_DROP_TABLE` | Drop a table
`SQLITE_DROP_TEMP_INDEX` | Drop a temporary index
`SQLITE_DROP_TEMP_TABLE` | Drop a temporary table
`SQLITE_DROP_TEMP_TRIGGER` | Drop a temporary trigger
`SQLITE_DROP_TEMP_VIEW` | Drop a temporary view
`SQLITE_DROP_TRIGGER` | Drop a trigger
`SQLITE_DROP_VIEW` | Drop a view
`SQLITE_INSERT` | Insert into a table
`SQLITE_PRAGMA` | Execute a PRAGMA statement
`SQLITE_READ` | Read from a table
`SQLITE_SELECT` | Execute a SELECT statement
`SQLITE_TRANSACTION` | Begin, commit, or rollback a transaction
`SQLITE_UPDATE` | Update a table
`SQLITE_ATTACH` | Attach a database
`SQLITE_DETACH` | Detach a database
`SQLITE_ALTER_TABLE` | Alter a table
`SQLITE_REINDEX` | Reindex
`SQLITE_ANALYZE` | Analyze the database
`SQLITE_CREATE_VTABLE` | Create a virtual table
`SQLITE_DROP_VTABLE` | Drop a virtual table
`SQLITE_FUNCTION` | Use a function
`SQLITE_SAVEPOINT` | Create, release, or rollback a savepoint
`SQLITE_COPY` | Copy data (legacy)
`SQLITE_RECURSIVE` | Recursive query

## Stream#

Source Code: lib/stream.js

Stability: 2 - Stable

A stream is an abstract interface for working with streaming data in Node.js. The `node:stream` module provides an API for implementing the stream interface.

There are many stream objects provided by Node.js. For instance, a request to an HTTP server and `process.stdout` are both stream instances.

Streams can be readable, writable, or both. All streams are instances of `EventEmitter`.

To access the `node:stream` module:

    const stream = require('node:stream');
    js

The `node:stream` module is useful for creating new types of stream instances. It is usually not necessary to use the `node:stream` module to consume streams.

### Organization of this document#

This document contains two primary sections and a third section for notes. The first section explains how to use existing streams within an application. The second section explains how to create new types of streams.

### Types of streams#

There are four fundamental stream types within Node.js:

- `Writable`: streams to which data can be written (for example, `fs.createWriteStream()`).
- `Readable`: streams from which data can be read (for example, `fs.createReadStream()`).
- `Duplex`: streams that are both `Readable` and `Writable` (for example, `net.Socket`).
- `Transform`: `Duplex` streams that can modify or transform the data as it is written and read (for example, `zlib.createDeflate()`).

Additionally, this module includes the utility functions `stream.duplexPair()`, `stream.pipeline()`, `stream.finished()` `stream.Readable.from()`, and `stream.addAbortSignal()`.

#### Streams Promises API#

Added in: v15.0.0

The `stream/promises` API provides an alternative set of asynchronous utility functions for streams that return `Promise` objects rather than using callbacks. The API is accessible via `require('node:stream/promises')` or `require('node:stream').promises`.

#### `stream.pipeline(streams[, options])`#

#### `stream.pipeline(source[, ...transforms], destination[, options])`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v19.7.0, v18.16.0 | Added support for webstreams.
v18.0.0, v17.2.0, v16.14.0 | Add the `end` option, which can be set to `false` to prevent automatically closing the destination stream when the source ends.

- `streams` {Stream[]|Iterable[]|AsyncIterable[]|Function[]| ReadableStream[]|WritableStream[]|TransformStream[]}
- `source` `<Stream>` | `<Iterable>` | `<AsyncIterable>` | `<Function>` | `<ReadableStream>`
  - Returns: `<Promise>` | `<AsyncIterable>`
- `...transforms` `<Stream>` | `<Function>` | `<TransformStream>`
  - `source` `<AsyncIterable>`
  - Returns: `<Promise>` | `<AsyncIterable>`
- `destination` `<Stream>` | `<Function>` | `<WritableStream>`
  - `source` `<AsyncIterable>`
  - Returns: `<Promise>` | `<AsyncIterable>`
- `options` `<Object>` Pipeline options
  - `signal` `<AbortSignal>`
  - `end` `<boolean>` End the destination stream when the source stream ends. Transform streams are always ended, even if this value is `false`. Default: `true`.
- Returns: `<Promise>` Fulfills when the pipeline is complete.

    const { pipeline } = require('node:stream/promises');
    const fs = require('node:fs');
    const zlib = require('node:zlib');

    async function run() {
      await pipeline(
        fs.createReadStream('archive.tar'),
        zlib.createGzip(),
        fs.createWriteStream('archive.tar.gz'),
      );
      console.log('Pipeline succeeded.');
    }

    run().catch(console.error);
    import { pipeline } from 'node:stream/promises';
    import { createReadStream, createWriteStream } from 'node:fs';
    import { createGzip } from 'node:zlib';

    await pipeline(
      createReadStream('archive.tar'),
      createGzip(),
      createWriteStream('archive.tar.gz'),
    );
    console.log('Pipeline succeeded.');
    javascript

To use an `AbortSignal`, pass it inside an options object, as the last argument. When the signal is aborted, `destroy` will be called on the underlying pipeline, with an `AbortError`.

    const { pipeline } = require('node:stream/promises');
    const fs = require('node:fs');
    const zlib = require('node:zlib');

    async function run() {
      const ac = new AbortController();
      const signal = ac.signal;

      setImmediate(() => ac.abort());
      await pipeline(
        fs.createReadStream('archive.tar'),
        zlib.createGzip(),
        fs.createWriteStream('archive.tar.gz'),
        { signal },
      );
    }

    run().catch(console.error); // AbortError
    import { pipeline } from 'node:stream/promises';
    import { createReadStream, createWriteStream } from 'node:fs';
    import { createGzip } from 'node:zlib';

    const ac = new AbortController();
    const { signal } = ac;
    setImmediate(() => ac.abort());
    try {
      await pipeline(
        createReadStream('archive.tar'),
        createGzip(),
        createWriteStream('archive.tar.gz'),
        { signal },
      );
    } catch (err) {
      console.error(err); // AbortError
    }
    javascript

The `pipeline` API also supports async generators:

    const { pipeline } = require('node:stream/promises');
    const fs = require('node:fs');

    async function run() {
      await pipeline(
        fs.createReadStream('lowercase.txt'),
        async function* (source, { signal }) {
          source.setEncoding('utf8');  // Work with strings rather than `Buffer`s.
          for await (const chunk of source) {
            yield await processChunk(chunk, { signal });
          }
        },
        fs.createWriteStream('uppercase.txt'),
      );
      console.log('Pipeline succeeded.');
    }

    run().catch(console.error);
    import { pipeline } from 'node:stream/promises';
    import { createReadStream, createWriteStream } from 'node:fs';

    await pipeline(
      createReadStream('lowercase.txt'),
      async function* (source, { signal }) {
        source.setEncoding('utf8');  // Work with strings rather than `Buffer`s.
        for await (const chunk of source) {
          yield await processChunk(chunk, { signal });
        }
      },
      createWriteStream('uppercase.txt'),
    );
    console.log('Pipeline succeeded.');
    javascript

Remember to handle the `signal` argument passed into the async generator. Especially in the case where the async generator is the source for the pipeline (i.e. first argument) or the pipeline will never complete.

    const { pipeline } = require('node:stream/promises');
    const fs = require('node:fs');

    async function run() {
      await pipeline(
        async function* ({ signal }) {
          await someLongRunningfn({ signal });
          yield 'asd';
        },
        fs.createWriteStream('uppercase.txt'),
      );
      console.log('Pipeline succeeded.');
    }

    run().catch(console.error);
    import { pipeline } from 'node:stream/promises';
    import fs from 'node:fs';
    await pipeline(
      async function* ({ signal }) {
        await someLongRunningfn({ signal });
        yield 'asd';
      },
      fs.createWriteStream('uppercase.txt'),
    );
    console.log('Pipeline succeeded.');
    javascript

The `pipeline` API provides callback version:

#### `stream.finished(stream[, options])`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v19.5.0, v18.14.0 | Added support for `ReadableStream` and `WritableStream`.
v19.1.0, v18.13.0 | The `cleanup` option was added.

- `stream` `<Stream>` | `<ReadableStream>` | `<WritableStream>` A readable and/or writable stream/webstream.
- `options` `<Object>`
  - `error` `<boolean>` | `<undefined>`
  - `readable` `<boolean>` | `<undefined>`
  - `writable` `<boolean>` | `<undefined>`
  - `signal` `<AbortSignal>` | `<undefined>`
  - `cleanup` `<boolean>` | `<undefined>` If `true`, removes the listeners registered by this function before the promise is fulfilled. Default: `false`.
- Returns: `<Promise>` Fulfills when the stream is no longer readable or writable.

    const { finished } = require('node:stream/promises');
    const fs = require('node:fs');

    const rs = fs.createReadStream('archive.tar');

    async function run() {
      await finished(rs);
      console.log('Stream is done reading.');
    }

    run().catch(console.error);
    rs.resume(); // Drain the stream.
    import { finished } from 'node:stream/promises';
    import { createReadStream } from 'node:fs';

    const rs = createReadStream('archive.tar');

    async function run() {
      await finished(rs);
      console.log('Stream is done reading.');
    }

    run().catch(console.error);
    rs.resume(); // Drain the stream.
    javascript

The `finished` API also provides a callback version.

`stream.finished()` leaves dangling event listeners (in particular `'error'`, `'end'`, `'finish'` and `'close'`) after the returned promise is resolved or rejected. The reason for this is so that unexpected `'error'` events (due to incorrect stream implementations) do not cause unexpected crashes. If this is unwanted behavior then `options.cleanup` should be set to `true`:

    await finished(rs, { cleanup: true });
    mjs

#### Object mode#

All streams created by Node.js APIs operate exclusively on strings, `<Buffer>`, `<TypedArray>` and `<DataView>` objects:

- `Strings` and `Buffers` are the most common types used with streams.
- `TypedArray` and `DataView` lets you handle binary data with types like `Int32Array` or `Uint8Array`. When you write a TypedArray or DataView to a stream, Node.js processes the raw bytes.

It is possible, however, for stream implementations to work with other types of JavaScript values (with the exception of `null`, which serves a special purpose within streams). Such streams are considered to operate in "object mode".

Stream instances are switched into object mode using the `objectMode` option when the stream is created. Attempting to switch an existing stream into object mode is not safe.

#### Buffering#

Both `Writable` and `Readable` streams will store data in an internal buffer.

The amount of data potentially buffered depends on the `highWaterMark` option passed into the stream's constructor. For normal streams, the `highWaterMark` option specifies a total number of bytes. For streams operating in object mode, the `highWaterMark` specifies a total number of objects. For streams operating on (but not decoding) strings, the `highWaterMark` specifies a total number of UTF-16 code units.

Data is buffered in `Readable` streams when the implementation calls `stream.push(chunk)`. If the consumer of the Stream does not call `stream.read()`, the data will sit in the internal queue until it is consumed.

Once the total size of the internal read buffer reaches the threshold specified by `highWaterMark`, the stream will temporarily stop reading data from the underlying resource until the data currently buffered can be consumed (that is, the stream will stop calling the internal `readable._read()` method that is used to fill the read buffer).

Data is buffered in `Writable` streams when the `writable.write(chunk)` method is called repeatedly. While the total size of the internal write buffer is below the threshold set by `highWaterMark`, calls to `writable.write()` will return `true`. Once the size of the internal buffer reaches or exceeds the `highWaterMark`, `false` will be returned.

A key goal of the `stream` API, particularly the `stream.pipe()` method, is to limit the buffering of data to acceptable levels such that sources and destinations of differing speeds will not overwhelm the available memory.

The `highWaterMark` option is a threshold, not a limit: it dictates the amount of data that a stream buffers before it stops asking for more data. It does not enforce a strict memory limitation in general. Specific stream implementations may choose to enforce stricter limits but doing so is optional.

Because `Duplex` and `Transform` streams are both `Readable` and `Writable`, each maintains two separate internal buffers used for reading and writing, allowing each side to operate independently of the other while maintaining an appropriate and efficient flow of data. For example, `net.Socket` instances are `Duplex` streams whose `Readable` side allows consumption of data received from the socket and whose `Writable` side allows writing data to the socket. Because data may be written to the socket at a faster or slower rate than data is received, each side should operate (and buffer) independently of the other.

The mechanics of the internal buffering are an internal implementation detail and may be changed at any time. However, for certain advanced implementations, the internal buffers can be retrieved using `writable.writableBuffer` or `readable.readableBuffer`. Use of these undocumented properties is discouraged.

### API for stream consumers#

Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:

    const http = require('node:http');

    const server = http.createServer((req, res) => {
      // `req` is an http.IncomingMessage, which is a readable stream.
      // `res` is an http.ServerResponse, which is a writable stream.

      let body = '';
      // Get the data as utf8 strings.
      // If an encoding is not set, Buffer objects will be received.
      req.setEncoding('utf8');

      // Readable streams emit 'data' events once a listener is added.
      req.on('data', (chunk) => {
        body += chunk;
      });

      // The 'end' event indicates that the entire body has been received.
      req.on('end', () => {
        try {
          const data = JSON.parse(body);
          // Write back something interesting to the user:
          res.write(typeof data);
          res.end();
        } catch (er) {
          // uh oh! bad json!
          res.statusCode = 400;
          return res.end(`error: ${er.message}`);
        }
      });
    });

    server.listen(1337);

    // $ curl localhost:1337 -d "{}"
    // object
    // $ curl localhost:1337 -d "\"foo\""
    // string
    // $ curl localhost:1337 -d "not json"
    // error: Unexpected token 'o', "not json" is not valid JSON
    js

`Writable` streams (such as `res` in the example) expose methods such as `write()` and `end()` that are used to write data onto the stream.

`Readable` streams use the `EventEmitter` API for notifying application code when data is available to be read off the stream. That available data can be read from the stream in multiple ways.

Both `Writable` and `Readable` streams use the `EventEmitter` API in various ways to communicate the current state of the stream.

`Duplex` and `Transform` streams are both `Writable` and `Readable`.

Applications that are either writing data to or consuming data from a stream are not required to implement the stream interfaces directly and will generally have no reason to call `require('node:stream')`.

Developers wishing to implement new types of streams should refer to the section API for stream implementers.

#### Writable streams#

Writable streams are an abstraction for a destination to which data is written.

Examples of `Writable` streams include:

- HTTP requests, on the client
- HTTP responses, on the server
- fs write streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdin
- `process.stdout`, `process.stderr`

Some of these examples are actually `Duplex` streams that implement the `Writable` interface.

All `Writable` streams implement the interface defined by the `stream.Writable` class.

While specific instances of `Writable` streams may differ in various ways, all `Writable` streams follow the same fundamental usage pattern as illustrated in the example below:

    const myStream = getWritableStreamSomehow();
    myStream.write('some data');
    myStream.write('some more data');
    myStream.end('done writing data');
    js

##### Class: `stream.Writable`#

Added in: v0.9.4

###### Event: `'close'`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v10.0.0 | Add `emitClose` option to specify if `'close'` is emitted on destroy.

The `'close'` event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

A `Writable` stream will always emit the `'close'` event if it is created with the `emitClose` option.

###### Event: `'drain'`#

Added in: v0.9.4

If a call to `stream.write(chunk)` returns `false`, the `'drain'` event will be emitted when it is appropriate to resume writing data to the stream.

    // Write the data to the supplied writable stream one million times.
    // Be attentive to back-pressure.
    function writeOneMillionTimes(writer, data, encoding, callback) {
      let i = 1000000;
      write();
      function write() {
        let ok = true;
        do {
          i--;
          if (i === 0) {
            // Last time!
            writer.write(data, encoding, callback);
          } else {
            // See if we should continue, or wait.
            // Don't pass the callback, because we're not done yet.
            ok = writer.write(data, encoding);
          }
        } while (i > 0 && ok);
        if (i > 0) {
          // Had to stop early!
          // Write some more once it drains.
          writer.once('drain', write);
        }
      }
    }
    js

###### Event: `'error'`#

Added in: v0.9.4

- Type: `<Error>`

The `'error'` event is emitted if an error occurred while writing or piping data. The listener callback is passed a single `Error` argument when called.

The stream is closed when the `'error'` event is emitted unless the `autoDestroy` option was set to `false` when creating the stream.

After `'error'`, no further events other than `'close'` should be emitted (including `'error'` events).

###### Event: `'finish'`#

Added in: v0.9.4

The `'finish'` event is emitted after the `stream.end()` method has been called, and all data has been flushed to the underlying system.

    const writer = getWritableStreamSomehow();
    for (let i = 0; i < 100; i++) {
      writer.write(`hello, #${i}!\n`);
    }
    writer.on('finish', () => {
      console.log('All writes are now complete.');
    });
    writer.end('This is the end\n');
    js

###### Event: `'pipe'`#

Added in: v0.9.4

- `src` `<stream.Readable>` source stream that is piping to this writable

The `'pipe'` event is emitted when the `stream.pipe()` method is called on a readable stream, adding this writable to its set of destinations.

    const writer = getWritableStreamSomehow();
    const reader = getReadableStreamSomehow();
    writer.on('pipe', (src) => {
      console.log('Something is piping into the writer.');
      assert.equal(src, reader);
    });
    reader.pipe(writer);
    js

###### Event: `'unpipe'`#

Added in: v0.9.4

- `src` `<stream.Readable>` The source stream that unpiped this writable

The `'unpipe'` event is emitted when the `stream.unpipe()` method is called on a `Readable` stream, removing this `Writable` from its set of destinations.

This is also emitted in case this `Writable` stream emits an error when a `Readable` stream pipes into it.

    const writer = getWritableStreamSomehow();
    const reader = getReadableStreamSomehow();
    writer.on('unpipe', (src) => {
      console.log('Something has stopped piping into the writer.');
      assert.equal(src, reader);
    });
    reader.pipe(writer);
    reader.unpipe(writer);
    js

###### `writable.cork()`#

Added in: v0.11.2

The `writable.cork()` method forces all written data to be buffered in memory. The buffered data will be flushed when either the `stream.uncork()` or `stream.end()` methods are called.

The primary intent of `writable.cork()` is to accommodate a situation in which several small chunks are written to the stream in rapid succession. Instead of immediately forwarding them to the underlying destination, `writable.cork()` buffers all the chunks until `writable.uncork()` is called, which will pass them all to `writable._writev()`, if present. This prevents a head-of-line blocking situation where data is being buffered while waiting for the first small chunk to be processed. However, use of `writable.cork()` without implementing `writable._writev()` may have an adverse effect on throughput.

See also: `writable.uncork()`, `writable._writev()`.

###### `writable.destroy([error])`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v14.0.0 | Work as a no-op on a stream that has already been destroyed.

- `error` `<Error>` Optional, an error to emit with `'error'` event.
- Returns: `<this>`

Destroy the stream. Optionally emit an `'error'` event, and emit a `'close'` event (unless `emitClose` is set to `false`). After this call, the writable stream has ended and subsequent calls to `write()` or `end()` will result in an `ERR_STREAM_DESTROYED` error. This is a destructive and immediate way to destroy a stream. Previous calls to `write()` may not have drained, and may trigger an `ERR_STREAM_DESTROYED` error. Use `end()` instead of destroy if data should flush before close, or wait for the `'drain'` event before destroying the stream.

    const { Writable } = require('node:stream');

    const myStream = new Writable();

    const fooErr = new Error('foo error');
    myStream.destroy(fooErr);
    myStream.on('error', (fooErr) => console.error(fooErr.message)); // foo error
    cjs

    const { Writable } = require('node:stream');

    const myStream = new Writable();

    myStream.destroy();
    myStream.on('error', function wontHappen() {});
    cjs

    const { Writable } = require('node:stream');

    const myStream = new Writable();
    myStream.destroy();

    myStream.write('foo', (error) => console.error(error.code));
    // ERR_STREAM_DESTROYED
    cjs

Once `destroy()` has been called any further calls will be a no-op and no further errors except from `_destroy()` may be emitted as `'error'`.

Implementors should not override this method, but instead implement `writable._destroy()`.

###### `writable.closed`#

Added in: v18.0.0

- Type: `<boolean>`

Is `true` after `'close'` has been emitted.

###### `writable.destroyed`#

Added in: v8.0.0

- Type: `<boolean>`

Is `true` after `writable.destroy()` has been called.

    const { Writable } = require('node:stream');

    const myStream = new Writable();

    console.log(myStream.destroyed); // false
    myStream.destroy();
    console.log(myStream.destroyed); // true
    cjs

###### `writable.end([chunk[, encoding]][, callback])`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | The `chunk` argument can now be a `TypedArray` or `DataView` instance.
v15.0.0 | The `callback` is invoked before 'finish' or on error.
v14.0.0 | The `callback` is invoked if 'finish' or 'error' is emitted.
v10.0.0 | This method now returns a reference to `writable`.
v8.0.0 | The `chunk` argument can now be a `Uint8Array` instance.

- `chunk` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<any>` Optional data to write. For streams not operating in object mode, `chunk` must be a `<string>`, `<Buffer>`, `<TypedArray>` or `<DataView>`. For object mode streams, `chunk` may be any JavaScript value other than `null`.
- `encoding` `<string>` The encoding if `chunk` is a string
- `callback` `<Function>` Callback for when the stream is finished.
- Returns: `<this>`

Calling the `writable.end()` method signals that no more data will be written to the `Writable`. The optional `chunk` and `encoding` arguments allow one final additional chunk of data to be written immediately before closing the stream.

Calling the `stream.write()` method after calling `stream.end()` will raise an error.

    // Write 'hello, ' and then end with 'world!'.
    const fs = require('node:fs');
    const file = fs.createWriteStream('example.txt');
    file.write('hello, ');
    file.end('world!');
    // Writing more now is not allowed!
    js

###### `writable.setDefaultEncoding(encoding)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v6.1.0 | This method now returns a reference to `writable`.

- `encoding` `<string>` The new default encoding
- Returns: `<this>`

The `writable.setDefaultEncoding()` method sets the default `encoding` for a `Writable` stream.

###### `writable.uncork()`#

Added in: v0.11.2

The `writable.uncork()` method flushes all data buffered since `stream.cork()` was called.

When using `writable.cork()` and `writable.uncork()` to manage the buffering of writes to a stream, defer calls to `writable.uncork()` using `process.nextTick()`. Doing so allows batching of all `writable.write()` calls that occur within a given Node.js event loop phase.

    stream.cork();
    stream.write('some ');
    stream.write('data ');
    process.nextTick(() => stream.uncork());
    js

If the `writable.cork()` method is called multiple times on a stream, the same number of calls to `writable.uncork()` must be called to flush the buffered data.

    stream.cork();
    stream.write('some ');
    stream.cork();
    stream.write('data ');
    process.nextTick(() => {
      stream.uncork();
      // The data will not be flushed until uncork() is called a second time.
      stream.uncork();
    });
    js

See also: `writable.cork()`.

###### `writable.writable`#

Added in: v11.4.0

- Type: `<boolean>`

Is `true` if it is safe to call `writable.write()`, which means the stream has not been destroyed, errored, or ended.

###### `writable.writableAborted`#

Added in: v18.0.0, v16.17.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- Type: `<boolean>`

Returns whether the stream was destroyed or errored before emitting `'finish'`.

###### `writable.writableEnded`#

Added in: v12.9.0

- Type: `<boolean>`

Is `true` after `writable.end()` has been called. This property does not indicate whether the data has been flushed, for this use `writable.writableFinished` instead.

###### `writable.writableCorked`#

Added in: v13.2.0, v12.16.0

- Type: `<integer>`

Number of times `writable.uncork()` needs to be called in order to fully uncork the stream.

###### `writable.errored`#

Added in: v18.0.0

- Type: `<Error>`

Returns error if the stream has been destroyed with an error.

###### `writable.writableFinished`#

Added in: v12.6.0

- Type: `<boolean>`

Is set to `true` immediately before the `'finish'` event is emitted.

###### `writable.writableHighWaterMark`#

Added in: v9.3.0

- Type: `<number>`

Return the value of `highWaterMark` passed when creating this `Writable`.

###### `writable.writableLength`#

Added in: v9.4.0

- Type: `<number>`

This property contains the number of bytes (or objects) in the queue ready to be written. The value provides introspection data regarding the status of the `highWaterMark`.

###### `writable.writableNeedDrain`#

Added in: v15.2.0, v14.17.0

- Type: `<boolean>`

Is `true` if the stream's buffer has been full and stream will emit `'drain'`.

###### `writable.writableObjectMode`#

Added in: v12.3.0

- Type: `<boolean>`

Getter for the property `objectMode` of a given `Writable` stream.

###### `writable[Symbol.asyncDispose]()`#

Added in: v22.4.0, v20.16.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `writable.destroy()` with an `AbortError` and returns a promise that fulfills when the stream is finished.

###### `writable.write(chunk[, encoding][, callback])`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | The `chunk` argument can now be a `TypedArray` or `DataView` instance.
v8.0.0 | The `chunk` argument can now be a `Uint8Array` instance.
v6.0.0 | Passing `null` as the `chunk` parameter will always be considered invalid now, even in object mode.

- `chunk` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<any>` Optional data to write. For streams not operating in object mode, `chunk` must be a `<string>`, `<Buffer>`, `<TypedArray>` or `<DataView>`. For object mode streams, `chunk` may be any JavaScript value other than `null`.
- `encoding` `<string>` | `<null>` The encoding, if `chunk` is a string. Default: `'utf8'`
- `callback` `<Function>` Callback for when this chunk of data is flushed.
- Returns: `<boolean>` `false` if the stream wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

The `writable.write()` method writes some data to the stream, and calls the supplied `callback` once the data has been fully handled. If an error occurs, the `callback` will be called with the error as its first argument. The `callback` is called asynchronously and before `'error'` is emitted.

The return value is `true` if the internal buffer is less than the `highWaterMark` configured when the stream was created after admitting `chunk`. If `false` is returned, further attempts to write data to the stream should stop until the `'drain'` event is emitted.

While a stream is not draining, calls to `write()` will buffer `chunk`, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the `'drain'` event will be emitted. Once `write()` returns false, do not write more chunks until the `'drain'` event is emitted. While calling `write()` on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a `Transform`, because the `Transform` streams are paused by default until they are piped or a `'data'` or `'readable'` event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a `Readable` and use `stream.pipe()`. However, if calling `write()` is preferred, it is possible to respect backpressure and avoid memory issues using the `'drain'` event:

    function write(data, cb) {
      if (!stream.write(data)) {
        stream.once('drain', cb);
      } else {
        process.nextTick(cb);
      }
    }

    // Wait for cb to be called before doing any other write.
    write('hello', () => {
      console.log('Write completed, do more writes now.');
    });
    js

A `Writable` stream in object mode will always ignore the `encoding` argument.

#### Readable streams#

Readable streams are an abstraction for a source from which data is consumed.

Examples of `Readable` streams include:

- HTTP responses, on the client
- HTTP requests, on the server
- fs read streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdout and stderr
- `process.stdin`

All `Readable` streams implement the interface defined by the `stream.Readable` class.

##### Two reading modes#

`Readable` streams effectively operate in one of two modes: flowing and paused. These modes are separate from object mode. A `Readable` stream can be in object mode or not, regardless of whether it is in flowing mode or paused mode.

- In flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the `EventEmitter` interface.

- In paused mode, the `stream.read()` method must be called explicitly to read chunks of data from the stream.

All `Readable` streams begin in paused mode but can be switched to flowing mode in one of the following ways:

- Adding a `'data'` event handler.
- Calling the `stream.resume()` method.
- Calling the `stream.pipe()` method to send the data to a `Writable`.

The `Readable` can switch back to paused mode using one of the following:

- If there are no pipe destinations, by calling the `stream.pause()` method.
- If there are pipe destinations, by removing all pipe destinations. Multiple pipe destinations may be removed by calling the `stream.unpipe()` method.

The important concept to remember is that a `Readable` will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the `Readable` will attempt to stop generating the data.

For backward compatibility reasons, removing `'data'` event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling `stream.pause()` will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

If a `Readable` is switched into flowing mode and there are no consumers available to handle the data, that data will be lost. This can occur, for instance, when the `readable.resume()` method is called without a listener attached to the `'data'` event, or when a `'data'` event handler is removed from the stream.

Adding a `'readable'` event handler automatically makes the stream stop flowing, and the data has to be consumed via `readable.read()`. If the `'readable'` event handler is removed, then the stream will start flowing again if there is a `'data'` event handler.

##### Three states#

The "two modes" of operation for a `Readable` stream are a simplified abstraction for the more complicated internal state management that is happening within the `Readable` stream implementation.

Specifically, at any given point in time, every `Readable` is in one of three possible states:

- `readable.readableFlowing === null`
- `readable.readableFlowing === false`
- `readable.readableFlowing === true`

When `readable.readableFlowing` is `null`, no mechanism for consuming the stream's data is provided. Therefore, the stream will not generate data. While in this state, attaching a listener for the `'data'` event, calling the `readable.pipe()` method, or calling the `readable.resume()` method will switch `readable.readableFlowing` to `true`, causing the `Readable` to begin actively emitting events as data is generated.

Calling `readable.pause()`, `readable.unpipe()`, or receiving backpressure will cause the `readable.readableFlowing` to be set as `false`, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the `'data'` event will not switch `readable.readableFlowing` to `true`.

    const { PassThrough, Writable } = require('node:stream');
    const pass = new PassThrough();
    const writable = new Writable();

    pass.pipe(writable);
    pass.unpipe(writable);
    // readableFlowing is now false.

    pass.on('data', (chunk) => { console.log(chunk.toString()); });
    // readableFlowing is still false.
    pass.write('ok');  // Will not emit 'data'.
    pass.resume();     // Must be called to make stream emit 'data'.
    // readableFlowing is now true.
    js

While `readable.readableFlowing` is `false`, data may be accumulating within the stream's internal buffer.

##### Choose one API style#

The `Readable` stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream. Specifically, using a combination of `on('data')`, `on('readable')`, `pipe()`, or async iterators could lead to unintuitive behavior.

##### Class: `stream.Readable`#

Added in: v0.9.4

###### Event: `'close'`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v10.0.0 | Add `emitClose` option to specify if `'close'` is emitted on destroy.

The `'close'` event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

A `Readable` stream will always emit the `'close'` event if it is created with the `emitClose` option.

###### Event: `'data'`#

Added in: v0.9.4

- `chunk` `<Buffer>` | `<string>` | `<any>` The chunk of data. For streams that are not operating in object mode, the chunk will be either a string or `Buffer`. For streams that are in object mode, the chunk can be any JavaScript value other than `null`.

The `'data'` event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling `readable.pipe()`, `readable.resume()`, or by attaching a listener callback to the `'data'` event. The `'data'` event will also be emitted whenever the `readable.read()` method is called and a chunk of data is available to be returned.

Attaching a `'data'` event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the `readable.setEncoding()` method; otherwise the data will be passed as a `Buffer`.

    const readable = getReadableStreamSomehow();
    readable.on('data', (chunk) => {
      console.log(`Received ${chunk.length} bytes of data.`);
    });
    js

###### Event: `'end'`#

Added in: v0.9.4

The `'end'` event is emitted when there is no more data to be consumed from the stream.

The `'end'` event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling `stream.read()` repeatedly until all data has been consumed.

    const readable = getReadableStreamSomehow();
    readable.on('data', (chunk) => {
      console.log(`Received ${chunk.length} bytes of data.`);
    });
    readable.on('end', () => {
      console.log('There will be no more data.');
    });
    js

###### Event: `'error'`#

Added in: v0.9.4

- Type: `<Error>`

The `'error'` event may be emitted by a `Readable` implementation at any time. Typically, this may occur if the underlying stream is unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single `Error` object.

###### Event: `'pause'`#

Added in: v0.9.4

The `'pause'` event is emitted when `stream.pause()` is called and `readableFlowing` is not `false`.

###### Event: `'readable'`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v10.0.0 | The `'readable'` is always emitted in the next tick after `.push()` is called.
v10.0.0 | Using `'readable'` requires calling `.read()`.

The `'readable'` event is emitted when there is data available to be read from the stream, up to the configured high water mark (`state.highWaterMark`). Effectively, it indicates that the stream has new information within the buffer. If data is available within this buffer, `stream.read()` can be called to retrieve that data. Additionally, the `'readable'` event may also be emitted when the end of the stream has been reached.

    const readable = getReadableStreamSomehow();
    readable.on('readable', function() {
      // There is some data to read now.
      let data;

      while ((data = this.read()) !== null) {
        console.log(data);
      }
    });
    js

If the end of the stream has been reached, calling `stream.read()` will return `null` and trigger the `'end'` event. This is also true if there never was any data to be read. For instance, in the following example, `foo.txt` is an empty file:

    const fs = require('node:fs');
    const rr = fs.createReadStream('foo.txt');
    rr.on('readable', () => {
      console.log(`readable: ${rr.read()}`);
    });
    rr.on('end', () => {
      console.log('end');
    });
    js

The output of running this script is:

    $ node test.js
    readable: null
    end
    console

In some cases, attaching a listener for the `'readable'` event will cause some amount of data to be read into an internal buffer.

In general, the `readable.pipe()` and `'data'` event mechanisms are easier to understand than the `'readable'` event. However, handling `'readable'` might result in increased throughput.

If both `'readable'` and `'data'` are used at the same time, `'readable'` takes precedence in controlling the flow, i.e. `'data'` will be emitted only when `stream.read()` is called. The `readableFlowing` property would become `false`. If there are `'data'` listeners when `'readable'` is removed, the stream will start flowing, i.e. `'data'` events will be emitted without calling `.resume()`.

###### Event: `'resume'`#

Added in: v0.9.4

The `'resume'` event is emitted when `stream.resume()` is called and `readableFlowing` is not `true`.

###### `readable.destroy([error])`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v14.0.0 | Work as a no-op on a stream that has already been destroyed.

- `error` `<Error>` Error which will be passed as payload in `'error'` event
- Returns: `<this>`

Destroy the stream. Optionally emit an `'error'` event, and emit a `'close'` event (unless `emitClose` is set to `false`). After this call, the readable stream will release any internal resources and subsequent calls to `push()` will be ignored.

Once `destroy()` has been called any further calls will be a no-op and no further errors except from `_destroy()` may be emitted as `'error'`.

Implementors should not override this method, but instead implement `readable._destroy()`.

###### `readable.closed`#

Added in: v18.0.0

- Type: `<boolean>`

Is `true` after `'close'` has been emitted.

###### `readable.destroyed`#

Added in: v8.0.0

- Type: `<boolean>`

Is `true` after `readable.destroy()` has been called.

###### `readable.isPaused()`#

Added in: v0.11.14

- Returns: `<boolean>`

The `readable.isPaused()` method returns the current operating state of the `Readable`. This is used primarily by the mechanism that underlies the `readable.pipe()` method. In most typical cases, there will be no reason to use this method directly.

    const readable = new stream.Readable();

    readable.isPaused(); // === false
    readable.pause();
    readable.isPaused(); // === true
    readable.resume();
    readable.isPaused(); // === false
    js

###### `readable.pause()`#

Added in: v0.9.4

- Returns: `<this>`

The `readable.pause()` method will cause a stream in flowing mode to stop emitting `'data'` events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

    const readable = getReadableStreamSomehow();
    readable.on('data', (chunk) => {
      console.log(`Received ${chunk.length} bytes of data.`);
      readable.pause();
      console.log('There will be no additional data for 1 second.');
      setTimeout(() => {
        console.log('Now data will start flowing again.');
        readable.resume();
      }, 1000);
    });
    js

The `readable.pause()` method has no effect if there is a `'readable'` event listener.

###### `readable.pipe(destination[, options])`#

Added in: v0.9.4

- `destination` `<stream.Writable>` The destination for writing data
- `options` `<Object>` Pipe options
  - `end` `<boolean>` End the writer when the reader ends. Default: `true`.
- Returns: `<stream.Writable>` The destination, allowing for a chain of pipes if it is a `Duplex` or a `Transform` stream

The `readable.pipe()` method attaches a `Writable` stream to the `readable`, causing it to switch automatically into flowing mode and push all of its data to the attached `Writable`. The flow of data will be automatically managed so that the destination `Writable` stream is not overwhelmed by a faster `Readable` stream.

The following example pipes all of the data from the `readable` into a file named `file.txt`:

    const fs = require('node:fs');
    const readable = getReadableStreamSomehow();
    const writable = fs.createWriteStream('file.txt');
    // All the data from readable goes into 'file.txt'.
    readable.pipe(writable);
    js

It is possible to attach multiple `Writable` streams to a single `Readable` stream.

The `readable.pipe()` method returns a reference to the destination stream making it possible to set up chains of piped streams:

    const fs = require('node:fs');
    const zlib = require('node:zlib');
    const r = fs.createReadStream('file.txt');
    const z = zlib.createGzip();
    const w = fs.createWriteStream('file.txt.gz');
    r.pipe(z).pipe(w);
    js

By default, `stream.end()` is called on the destination `Writable` stream when the source `Readable` stream emits `'end'`, so that the destination is no longer writable. To disable this default behavior, the `end` option can be passed as `false`, causing the destination stream to remain open:

    reader.pipe(writer, { end: false });
    reader.on('end', () => {
      writer.end('Goodbye\n');
    });
    js

One important caveat is that if the `Readable` stream emits an error during processing, the `Writable` destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.

The `process.stderr` and `process.stdout` `Writable` streams are never closed until the Node.js process exits, regardless of the specified options.

###### `readable.read([size])`#

Added in: v0.9.4

- `size` `<number>` Optional argument to specify how much data to read.
- Returns: `<string>` | `<Buffer>` | `<null>` | `<any>`

The `readable.read()` method reads data out of the internal buffer and returns it. If no data is available to be read, `null` is returned. By default, the data is returned as a `Buffer` object unless an encoding has been specified using the `readable.setEncoding()` method or the stream is operating in object mode.

The optional `size` argument specifies a specific number of bytes to read. If `size` bytes are not available to be read, `null` will be returned unless the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

If the `size` argument is not specified, all of the data contained in the internal buffer will be returned.

The `size` argument must be less than or equal to 1 GiB.

The `readable.read()` method should only be called on `Readable` streams operating in paused mode. In flowing mode, `readable.read()` is called automatically until the internal buffer is fully drained.

    const readable = getReadableStreamSomehow();

    // 'readable' may be triggered multiple times as data is buffered in
    readable.on('readable', () => {
      let chunk;
      console.log('Stream is readable (new data received in buffer)');
      // Use a loop to make sure we read all currently available data
      while (null !== (chunk = readable.read())) {
        console.log(`Read ${chunk.length} bytes of data...`);
      }
    });

    // 'end' will be triggered once when there is no more data available
    readable.on('end', () => {
      console.log('Reached end of stream.');
    });
    js

Each call to `readable.read()` returns a chunk of data or `null`, signifying that there's no more data to read at that moment. These chunks aren't automatically concatenated. Because a single `read()` call does not return all the data, using a while loop may be necessary to continuously read chunks until all data is retrieved. When reading a large file, `.read()` might return `null` temporarily, indicating that it has consumed all buffered content but there may be more data yet to be buffered. In such cases, a new `'readable'` event is emitted once there's more data in the buffer, and the `'end'` event signifies the end of data transmission.

Therefore to read a file's whole contents from a `readable`, it is necessary to collect chunks across multiple `'readable'` events:

    const chunks = [];

    readable.on('readable', () => {
      let chunk;
      while (null !== (chunk = readable.read())) {
        chunks.push(chunk);
      }
    });

    readable.on('end', () => {
      const content = chunks.join('');
    });
    js

A `Readable` stream in object mode will always return a single item from a call to `readable.read(size)`, regardless of the value of the `size` argument.

If the `readable.read()` method returns a chunk of data, a `'data'` event will also be emitted.

Calling `stream.read([size])` after the `'end'` event has been emitted will return `null`. No runtime error will be raised.

###### `readable.readable`#

Added in: v11.4.0

- Type: `<boolean>`

Is `true` if it is safe to call `readable.read()`, which means the stream has not been destroyed or emitted `'error'` or `'end'`.

###### `readable.readableAborted`#

Added in: v16.8.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- Type: `<boolean>`

Returns whether the stream was destroyed or errored before emitting `'end'`.

###### `readable.readableDidRead`#

Added in: v16.7.0, v14.18.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- Type: `<boolean>`

Returns whether `'data'` has been emitted.

###### `readable.readableEncoding`#

Added in: v12.7.0

- Type: `<null>` | `<string>`

Getter for the property `encoding` of a given `Readable` stream. The `encoding` property can be set using the `readable.setEncoding()` method.

###### `readable.readableEnded`#

Added in: v12.9.0

- Type: `<boolean>`

Becomes `true` when `'end'` event is emitted.

###### `readable.errored`#

Added in: v18.0.0

- Type: `<Error>`

Returns error if the stream has been destroyed with an error.

###### `readable.readableFlowing`#

Added in: v9.4.0

- Type: `<boolean>`

This property reflects the current state of a `Readable` stream as described in the Three states section.

###### `readable.readableHighWaterMark`#

Added in: v9.3.0

- Type: `<number>`

Returns the value of `highWaterMark` passed when creating this `Readable`.

###### `readable.readableLength`#

Added in: v9.4.0

- Type: `<number>`

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the `highWaterMark`.

###### `readable.readableObjectMode`#

Added in: v12.3.0

- Type: `<boolean>`

Getter for the property `objectMode` of a given `Readable` stream.

###### `readable.resume()`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v10.0.0 | The `resume()` has no effect if there is a `'readable'` event listening.

- Returns: `<this>`

The `readable.resume()` method causes an explicitly paused `Readable` stream to resume emitting `'data'` events, switching the stream into flowing mode.

The `readable.resume()` method can be used to fully consume the data from a stream without actually processing any of that data:

    getReadableStreamSomehow()
      .resume()
      .on('end', () => {
        console.log('Reached the end, but did not read anything.');
      });
    js

The `readable.resume()` method has no effect if there is a `'readable'` event listener.

###### `readable.setEncoding(encoding)`#

Added in: v0.9.4

- `encoding` `<string>` The encoding to use.
- Returns: `<this>`

The `readable.setEncoding()` method sets the character encoding for data read from the `Readable` stream.

By default, no encoding is assigned and stream data will be returned as `Buffer` objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as `Buffer` objects. For instance, calling `readable.setEncoding('utf8')` will cause the output data to be interpreted as UTF-8 data, and passed as strings. Calling `readable.setEncoding('hex')` will cause the data to be encoded in hexadecimal string format.

The `Readable` stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as `Buffer` objects.

    const readable = getReadableStreamSomehow();
    readable.setEncoding('utf8');
    readable.on('data', (chunk) => {
      assert.equal(typeof chunk, 'string');
      console.log('Got %d characters of string data:', chunk.length);
    });
    js

###### `readable.unpipe([destination])`#

Added in: v0.9.4

- `destination` `<stream.Writable>` Optional specific stream to unpipe
- Returns: `<this>`

The `readable.unpipe()` method detaches a `Writable` stream previously attached using the `stream.pipe()` method.

If the `destination` is not specified, then all pipes are detached.

If the `destination` is specified, but no pipe is set up for it, then the method does nothing.

    const fs = require('node:fs');
    const readable = getReadableStreamSomehow();
    const writable = fs.createWriteStream('file.txt');
    // All the data from readable goes into 'file.txt',
    // but only for the first second.
    readable.pipe(writable);
    setTimeout(() => {
      console.log('Stop writing to file.txt.');
      readable.unpipe(writable);
      console.log('Manually close the file stream.');
      writable.end();
    }, 1000);
    js

###### `readable.unshift(chunk[, encoding])`#

Added in: v0.9.11
History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | The `chunk` argument can now be a `TypedArray` or `DataView` instance.
v8.0.0 | The `chunk` argument can now be a `Uint8Array` instance.

- `chunk` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<string>` | `<null>` | `<any>` Chunk of data to unshift onto the read queue. For streams not operating in object mode, `chunk` must be a `<string>`, `<Buffer>`, `<TypedArray>`, `<DataView>` or `null`. For object mode streams, `chunk` may be any JavaScript value.
- `encoding` `<string>` Encoding of string chunks. Must be a valid `Buffer` encoding, such as `'utf8'` or `'ascii'`.

Passing `chunk` as `null` signals the end of the stream (EOF) and behaves the same as `readable.push(null)`, after which no more data can be written. The EOF signal is put at the end of the buffer and any buffered data will still be flushed.

The `readable.unshift()` method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

The `stream.unshift(chunk)` method cannot be called after the `'end'` event has been emitted or a runtime error will be thrown.

Developers using `stream.unshift()` often should consider switching to use of a `Transform` stream instead. See the API for stream implementers section for more information.

    // Pull off a header delimited by \n\n.
    // Use unshift() if we get too much.
    // Call the callback with (error, header, stream).
    const { StringDecoder } = require('node:string_decoder');
    function parseHeader(stream, callback) {
      stream.on('error', callback);
      stream.on('readable', onReadable);
      const decoder = new StringDecoder('utf8');
      let header = '';
      function onReadable() {
        let chunk;
        while (null !== (chunk = stream.read())) {
          const str = decoder.write(chunk);
          if (str.includes('\n\n')) {
            // Found the header boundary.
            const split = str.split(/\n\n/);
            header += split.shift();
            const remaining = split.join('\n\n');
            const buf = Buffer.from(remaining, 'utf8');
            stream.removeListener('error', callback);
            // Remove the 'readable' listener before unshifting.
            stream.removeListener('readable', onReadable);
            if (buf.length)
              stream.unshift(buf);
            // Now the body of the message can be read from the stream.
            callback(null, header, stream);
            return;
          }
          // Still reading the header.
          header += str;
        }
      }
    }
    js

Unlike `stream.push(chunk)`, `stream.unshift(chunk)` will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if `readable.unshift()` is called during a read (i.e. from within a `stream._read()` implementation on a custom stream). Following the call to `readable.unshift()` with an immediate `stream.push('')` will reset the reading state appropriately, however it is best to simply avoid calling `readable.unshift()` while in the process of performing a read.

###### `readable.wrap(stream)`#

Added in: v0.9.4

- `stream` `<Stream>` An "old style" readable stream
- Returns: `<this>`

Prior to Node.js 0.10, streams did not implement the entire `node:stream` module API as it is currently defined. (See Compatibility for more information.)

When using an older Node.js library that emits `'data'` events and has a `stream.pause()` method that is advisory only, the `readable.wrap()` method can be used to create a `Readable` stream that uses the old stream as its data source.

It will rarely be necessary to use `readable.wrap()` but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

    const { OldReader } = require('./old-api-module.js');
    const { Readable } = require('node:stream');
    const oreader = new OldReader();
    const myReader = new Readable().wrap(oreader);

    myReader.on('readable', () => {
      myReader.read(); // etc.
    });
    js

###### `readable[Symbol.asyncIterator]()`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v11.14.0 | Symbol.asyncIterator support is no longer experimental.

- Returns: `<AsyncIterator>` to fully consume the stream.

    const fs = require('node:fs');

    async function print(readable) {
      readable.setEncoding('utf8');
      let data = '';
      for await (const chunk of readable) {
        data += chunk;
      }
      console.log(data);
    }

    print(fs.createReadStream('file')).catch(console.error);
    js

If the loop terminates with a `break`, `return`, or a `throw`, the stream will be destroyed. In other terms, iterating over a stream will consume the stream fully. The stream will be read in chunks of size equal to the `highWaterMark` option. In the code example above, data will be in a single chunk if the file has less than 64 KiB of data because no `highWaterMark` option is provided to `fs.createReadStream()`.

###### `readable[Symbol.for('Stream.toAsyncStreamable')]()`#

Added in: v26.1.0

Stability: 1 - Experimental

- Returns: `<AsyncIterable>` An `AsyncIterable<Uint8Array[]>` that yields batched chunks from the stream.

When the `--experimental-stream-iter` flag is enabled, `Readable` streams implement the `Stream.toAsyncStreamable` protocol, enabling efficient consumption by the `stream/iter` API.

This provides a batched async iterator that drains the stream's internal buffer into `Uint8Array[]` batches, amortizing the per-chunk Promise overhead of the standard `Symbol.asyncIterator` path. For byte-mode streams, chunks are yielded directly as `Buffer` instances (which are `Uint8Array` subclasses). For object-mode or encoded streams, each chunk is normalized to `Uint8Array` before batching.

The returned iterator is tagged as a validated source, so `from()` passes it through without additional normalization.

    import { Readable } from 'node:stream';
    import { text, from } from 'node:stream/iter';

    const readable = new Readable({
      read() { this.push('hello'); this.push(null); },
    });

    // Readable is automatically consumed via toAsyncStreamable
    console.log(await text(from(readable))); // 'hello'
    const { Readable } = require('node:stream');
    const { text, from } = require('node:stream/iter');

    async function run() {
      const readable = new Readable({
        read() { this.push('hello'); this.push(null); },
      });

      console.log(await text(from(readable))); // 'hello'
    }

    run().catch(console.error);
    javascript

Without the `--experimental-stream-iter` flag, calling this method throws `ERR_STREAM_ITER_MISSING_FLAG`.

###### `readable[Symbol.asyncDispose]()`#

Added in: v20.4.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `readable.destroy()` with an `AbortError` and returns a promise that fulfills when the stream is finished.

###### `readable.compose(stream[, options])`#

Added in: v19.1.0, v18.13.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `stream` `<Writable>` | `<Duplex>` | `<WritableStream>` | `<TransformStream>` | `<Function>`
- `options` `<Object>`
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Duplex>` a stream composed with the stream `stream`.

    import { Readable } from 'node:stream';

    async function* splitToWords(source) {
      for await (const chunk of source) {
        const words = String(chunk).split(' ');

        for (const word of words) {
          yield word;
        }
      }
    }

    const wordsStream = Readable.from(['text passed through', 'composed stream']).compose(splitToWords);
    const words = await wordsStream.toArray();

    console.log(words); // prints ['text', 'passed', 'through', 'composed', 'stream']
    mjs

`readable.compose(s)` is equivalent to `stream.compose(readable, s)`.

This method also allows for an `<AbortSignal>` to be provided, which will destroy the composed stream when aborted.

See `stream.compose(...streams)` for more information.

###### `readable.iterator([options])`#

Added in: v16.3.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `options` `<Object>`
  - `destroyOnReturn` `<boolean>` When set to `false`, calling `return` on the async iterator, or exiting a `for await...of` iteration using a `break`, `return`, or `throw` will not destroy the stream. Default: `true`.
- Returns: `<AsyncIterator>` to consume the stream.

The iterator created by this method gives users the option to cancel the destruction of the stream if the `for await...of` loop is exited by `return`, `break`, or `throw`, or if the iterator should destroy the stream if the stream emitted an error during iteration.

    const { Readable } = require('node:stream');

    async function printIterator(readable) {
      for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
        console.log(chunk); // 1
        break;
      }

      console.log(readable.destroyed); // false

      for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
        console.log(chunk); // Will print 2 and then 3
      }

      console.log(readable.destroyed); // True, stream was totally consumed
    }

    async function printSymbolAsyncIterator(readable) {
      for await (const chunk of readable) {
        console.log(chunk); // 1
        break;
      }

      console.log(readable.destroyed); // true
    }

    async function showBoth() {
      await printIterator(Readable.from([1, 2, 3]));
      await printSymbolAsyncIterator(Readable.from([1, 2, 3]));
    }

    showBoth();
    js

###### `readable.map(fn[, options])`#

Added in: v17.4.0, v16.14.0
History

Version | Changes
--- | ---
v20.7.0, v18.19.0 | added `highWaterMark` in options.

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a function to map over every chunk in the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `highWaterMark` `<number>` how many items to buffer while waiting for user consumption of the mapped items. Default: `concurrency * 2 - 1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Readable>` a stream mapped with the function `fn`.

This method allows mapping over the stream. The `fn` function will be called for every chunk in the stream. If the `fn` function returns a promise - that promise will be `await`ed before being passed to the result stream.

    import { Readable } from 'node:stream';
    import { Resolver } from 'node:dns/promises';

    // With a synchronous mapper.
    for await (const chunk of Readable.from([1, 2, 3, 4]).map((x) => x * 2)) {
      console.log(chunk); // 2, 4, 6, 8
    }
    // With an asynchronous mapper, making at most 2 queries at a time.
    const resolver = new Resolver();
    const dnsResults = Readable.from([
      'nodejs.org',
      'openjsf.org',
      'www.linuxfoundation.org',
    ]).map((domain) => resolver.resolve4(domain), { concurrency: 2 });
    for await (const result of dnsResults) {
      console.log(result); // Logs the DNS result of resolver.resolve4.
    }
    mjs

###### `readable.filter(fn[, options])`#

Added in: v17.4.0, v16.14.0
History

Version | Changes
--- | ---
v20.7.0, v18.19.0 | added `highWaterMark` in options.

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a function to filter chunks from the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `highWaterMark` `<number>` how many items to buffer while waiting for user consumption of the filtered items. Default: `concurrency * 2 - 1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Readable>` a stream filtered with the predicate `fn`.

This method allows filtering the stream. For each chunk in the stream the `fn` function will be called and if it returns a truthy value, the chunk will be passed to the result stream. If the `fn` function returns a promise - that promise will be `await`ed.

    import { Readable } from 'node:stream';
    import { Resolver } from 'node:dns/promises';

    // With a synchronous predicate.
    for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
      console.log(chunk); // 3, 4
    }
    // With an asynchronous predicate, making at most 2 queries at a time.
    const resolver = new Resolver();
    const dnsResults = Readable.from([
      'nodejs.org',
      'openjsf.org',
      'www.linuxfoundation.org',
    ]).filter(async (domain) => {
      const { address } = await resolver.resolve4(domain, { ttl: true });
      return address.ttl > 60;
    }, { concurrency: 2 });
    for await (const result of dnsResults) {
      // Logs domains with more than 60 seconds on the resolved dns record.
      console.log(result);
    }
    mjs

###### `readable.forEach(fn[, options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a function to call on each chunk of the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Promise>` a promise for when the stream has finished.

This method allows iterating a stream. For each chunk in the stream the `fn` function will be called. If the `fn` function returns a promise - that promise will be `await`ed.

This method is different from `for await...of` loops in that it can optionally process chunks concurrently. In addition, a `forEach` iteration can only be stopped by having passed a `signal` option and aborting the related `AbortController` while `for await...of` can be stopped with `break` or `return`. In either case the stream will be destroyed.

This method is different from listening to the `'data'` event in that it uses the `readable` event in the underlying machinery and can limit the number of concurrent `fn` calls.

    import { Readable } from 'node:stream';
    import { Resolver } from 'node:dns/promises';

    // With a synchronous predicate.
    for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
      console.log(chunk); // 3, 4
    }
    // With an asynchronous predicate, making at most 2 queries at a time.
    const resolver = new Resolver();
    const dnsResults = Readable.from([
      'nodejs.org',
      'openjsf.org',
      'www.linuxfoundation.org',
    ]).map(async (domain) => {
      const { address } = await resolver.resolve4(domain, { ttl: true });
      return address;
    }, { concurrency: 2 });
    await dnsResults.forEach((result) => {
      // Logs result, similar to `for await (const result of dnsResults)`
      console.log(result);
    });
    console.log('done'); // Stream has finished
    mjs

###### `readable.toArray([options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `options` `<Object>`
  - `signal` `<AbortSignal>` allows cancelling the toArray operation if the signal is aborted.
- Returns: `<Promise>` a promise containing an array with the contents of the stream.

This method allows easily obtaining the contents of a stream.

As this method reads the entire stream into memory, it negates the benefits of streams. It's intended for interoperability and convenience, not as the primary way to consume streams.

    import { Readable } from 'node:stream';
    import { Resolver } from 'node:dns/promises';

    await Readable.from([1, 2, 3, 4]).toArray(); // [1, 2, 3, 4]

    const resolver = new Resolver();

    // Make dns queries concurrently using .map and collect
    // the results into an array using toArray
    const dnsResults = await Readable.from([
      'nodejs.org',
      'openjsf.org',
      'www.linuxfoundation.org',
    ]).map(async (domain) => {
      const { address } = await resolver.resolve4(domain, { ttl: true });
      return address;
    }, { concurrency: 2 }).toArray();
    mjs

###### `readable.some(fn[, options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a function to call on each chunk of the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Promise>` a promise evaluating to `true` if `fn` returned a truthy value for at least one of the chunks.

This method is similar to `Array.prototype.some` and calls `fn` on each chunk in the stream until the awaited return value is `true` (or any truthy value). Once an `fn` call on a chunk awaited return value is truthy, the stream is destroyed and the promise is fulfilled with `true`. If none of the `fn` calls on the chunks return a truthy value, the promise is fulfilled with `false`.

    import { Readable } from 'node:stream';
    import { stat } from 'node:fs/promises';

    // With a synchronous predicate.
    await Readable.from([1, 2, 3, 4]).some((x) => x > 2); // true
    await Readable.from([1, 2, 3, 4]).some((x) => x < 0); // false

    // With an asynchronous predicate, making at most 2 file checks at a time.
    const anyBigFile = await Readable.from([
      'file1',
      'file2',
      'file3',
    ]).some(async (fileName) => {
      const stats = await stat(fileName);
      return stats.size > 1024 * 1024;
    }, { concurrency: 2 });
    console.log(anyBigFile); // `true` if any file in the list is bigger than 1MB
    console.log('done'); // Stream has finished
    mjs

###### `readable.find(fn[, options])`#

Added in: v17.5.0, v16.17.0

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a function to call on each chunk of the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Promise>` a promise evaluating to the first chunk for which `fn` evaluated with a truthy value, or `undefined` if no element was found.

This method is similar to `Array.prototype.find` and calls `fn` on each chunk in the stream to find a chunk with a truthy value for `fn`. Once an `fn` call's awaited return value is truthy, the stream is destroyed and the promise is fulfilled with value for which `fn` returned a truthy value. If all of the `fn` calls on the chunks return a falsy value, the promise is fulfilled with `undefined`.

    import { Readable } from 'node:stream';
    import { stat } from 'node:fs/promises';

    // With a synchronous predicate.
    await Readable.from([1, 2, 3, 4]).find((x) => x > 2); // 3
    await Readable.from([1, 2, 3, 4]).find((x) => x > 0); // 1
    await Readable.from([1, 2, 3, 4]).find((x) => x > 10); // undefined

    // With an asynchronous predicate, making at most 2 file checks at a time.
    const foundBigFile = await Readable.from([
      'file1',
      'file2',
      'file3',
    ]).find(async (fileName) => {
      const stats = await stat(fileName);
      return stats.size > 1024 * 1024;
    }, { concurrency: 2 });
    console.log(foundBigFile); // File name of large file, if any file in the list is bigger than 1MB
    console.log('done'); // Stream has finished
    mjs

###### `readable.every(fn[, options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a function to call on each chunk of the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Promise>` a promise evaluating to `true` if `fn` returned a truthy value for all of the chunks.

This method is similar to `Array.prototype.every` and calls `fn` on each chunk in the stream to check if all awaited return values are truthy value for `fn`. Once an `fn` call on a chunk awaited return value is falsy, the stream is destroyed and the promise is fulfilled with `false`. If all of the `fn` calls on the chunks return a truthy value, the promise is fulfilled with `true`.

    import { Readable } from 'node:stream';
    import { stat } from 'node:fs/promises';

    // With a synchronous predicate.
    await Readable.from([1, 2, 3, 4]).every((x) => x > 2); // false
    await Readable.from([1, 2, 3, 4]).every((x) => x > 0); // true

    // With an asynchronous predicate, making at most 2 file checks at a time.
    const allBigFiles = await Readable.from([
      'file1',
      'file2',
      'file3',
    ]).every(async (fileName) => {
      const stats = await stat(fileName);
      return stats.size > 1024 * 1024;
    }, { concurrency: 2 });
    // `true` if all files in the list are bigger than 1MiB
    console.log(allBigFiles);
    console.log('done'); // Stream has finished
    mjs

###### `readable.flatMap(fn[, options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncGeneratorFunction>` | `<AsyncFunction>` a function to map over every chunk in the stream.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `options` `<Object>`
  - `concurrency` `<number>` the maximum concurrent invocation of `fn` to call on the stream at once. Default: `1`.
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Readable>` a stream flat-mapped with the function `fn`.

This method returns a new stream by applying the given callback to each chunk of the stream and then flattening the result.

It is possible to return a stream or another iterable or async iterable from `fn` and the result streams will be merged (flattened) into the returned stream.

    import { Readable } from 'node:stream';
    import { createReadStream } from 'node:fs';

    // With a synchronous mapper.
    for await (const chunk of Readable.from([1, 2, 3, 4]).flatMap((x) => [x, x])) {
      console.log(chunk); // 1, 1, 2, 2, 3, 3, 4, 4
    }
    // With an asynchronous mapper, combine the contents of 4 files
    const concatResult = Readable.from([
      './1.mjs',
      './2.mjs',
      './3.mjs',
      './4.mjs',
    ]).flatMap((fileName) => createReadStream(fileName));
    for await (const result of concatResult) {
      // This will contain the contents (all chunks) of all 4 files
      console.log(result);
    }
    mjs

###### `readable.drop(limit[, options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `limit` `<number>` the number of chunks to drop from the readable.
- `options` `<Object>`
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Readable>` a stream with `limit` chunks dropped.

This method returns a new stream with the first `limit` chunks dropped.

    import { Readable } from 'node:stream';

    await Readable.from([1, 2, 3, 4]).drop(2).toArray(); // [3, 4]
    mjs

###### `readable.take(limit[, options])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `limit` `<number>` the number of chunks to take from the readable.
- `options` `<Object>`
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Readable>` a stream with `limit` chunks taken.

This method returns a new stream with the first `limit` chunks.

    import { Readable } from 'node:stream';

    await Readable.from([1, 2, 3, 4]).take(2).toArray(); // [1, 2]
    mjs

###### `readable.reduce(fn[, initial[, options]])`#

Added in: v17.5.0, v16.15.0

Stability: 1 - Experimental

- `fn` `<Function>` | `<AsyncFunction>` a reducer function to call over every chunk in the stream.
  - `previous` `<any>` the value obtained from the last call to `fn` or the `initial` value if specified or the first chunk of the stream otherwise.
  - `data` `<any>` a chunk of data from the stream.
  - `options` `<Object>`
    - `signal` `<AbortSignal>` aborted if the stream is destroyed allowing to abort the `fn` call early.
- `initial` `<any>` the initial value to use in the reduction.
- `options` `<Object>`
  - `signal` `<AbortSignal>` allows destroying the stream if the signal is aborted.
- Returns: `<Promise>` a promise for the final value of the reduction.

This method calls `fn` on each chunk of the stream in order, passing it the result from the calculation on the previous element. It returns a promise for the final value of the reduction.

If no `initial` value is supplied the first chunk of the stream is used as the initial value. If the stream is empty, the promise is rejected with a `TypeError` with the `ERR_INVALID_ARGS` code property.

    import { Readable } from 'node:stream';
    import { readdir, stat } from 'node:fs/promises';
    import { join } from 'node:path';

    const directoryPath = './src';
    const filesInDir = await readdir(directoryPath);

    const folderSize = await Readable.from(filesInDir)
      .reduce(async (totalSize, file) => {
        const { size } = await stat(join(directoryPath, file));
        return totalSize + size;
      }, 0);

    console.log(folderSize);
    mjs

The reducer function iterates the stream element-by-element which means that there is no `concurrency` parameter or parallelism. To perform a `reduce` concurrently, you can extract the async function to `readable.map` method.

    import { Readable } from 'node:stream';
    import { readdir, stat } from 'node:fs/promises';
    import { join } from 'node:path';

    const directoryPath = './src';
    const filesInDir = await readdir(directoryPath);

    const folderSize = await Readable.from(filesInDir)
      .map((file) => stat(join(directoryPath, file)), { concurrency: 2 })
      .reduce((totalSize, { size }) => totalSize + size, 0);

    console.log(folderSize);
    mjs

#### Duplex and transform streams#

##### Class: `stream.Duplex`#

Added in: v0.9.4
History

Version | Changes
--- | ---
v6.8.0 | Instances of `Duplex` now return `true` when checking `instanceof stream.Writable`.

Duplex streams are streams that implement both the `Readable` and `Writable` interfaces.

Examples of `Duplex` streams include:

- TCP sockets
- zlib streams
- crypto streams

###### `duplex.allowHalfOpen`#

Added in: v0.9.4

- Type: `<boolean>`

If `false` then the stream will automatically end the writable side when the readable side ends. Set initially by the `allowHalfOpen` constructor option, which defaults to `true`.

This can be changed manually to change the half-open behavior of an existing `Duplex` stream instance, but must be changed before the `'end'` event is emitted.

##### Class: `stream.Transform`#

Added in: v0.9.4

Transform streams are `Duplex` streams where the output is in some way related to the input. Like all `Duplex` streams, `Transform` streams implement both the `Readable` and `Writable` interfaces.

Examples of `Transform` streams include:

- zlib streams
- crypto streams

###### `transform.destroy([error])`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v14.0.0 | Work as a no-op on a stream that has already been destroyed.

- `error` `<Error>`
- Returns: `<this>`

Destroy the stream, and optionally emit an `'error'` event. After this call, the transform stream would release any internal resources. Implementors should not override this method, but instead implement `readable._destroy()`. The default implementation of `_destroy()` for `Transform` also emit `'close'` unless `emitClose` is set in false.

Once `destroy()` has been called, any further calls will be a no-op and no further errors except from `_destroy()` may be emitted as `'error'`.

##### `stream.duplexPair([options])`#

Added in: v22.6.0, v20.17.0

- `options` `<Object>` A value to pass to both `Duplex` constructors, to set options such as buffering.
- Returns: `<Array>` of two `Duplex` instances.

The utility function `duplexPair` returns an Array with two items, each being a `Duplex` stream connected to the other side:

    const [ sideA, sideB ] = duplexPair();
    js

Whatever is written to one stream is made readable on the other. It provides behavior analogous to a network connection, where the data written by the client becomes readable by the server, and vice-versa.

The Duplex streams are symmetrical; one or the other may be used without any difference in behavior.

#### `stream.finished(stream[, options], callback)`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v19.5.0 | Added support for `ReadableStream` and `WritableStream`.
v15.11.0 | The `signal` option was added.
v14.0.0 | The `finished(stream, cb)` will wait for the `'close'` event before invoking the callback. The implementation tries to detect legacy streams and only apply this behavior to streams which are expected to emit `'close'`.
v14.0.0 | Emitting `'close'` before `'end'` on a `Readable` stream will cause an `ERR_STREAM_PREMATURE_CLOSE` error.
v14.0.0 | Callback will be invoked on streams which have already finished before the call to `finished(stream, cb)`.

- `stream` `<Stream>` | `<ReadableStream>` | `<WritableStream>` A readable and/or writable stream/webstream.
- `options` `<Object>`
  - `error` `<boolean>` If set to `false`, then a call to `emit('error', err)` is not treated as finished. Default: `true`.
  - `readable` `<boolean>` When set to `false`, the callback will be called when the stream ends even though the stream might still be readable. Default: `true`.
  - `writable` `<boolean>` When set to `false`, the callback will be called when the stream ends even though the stream might still be writable. Default: `true`.
  - `signal` `<AbortSignal>` allows aborting the wait for the stream finish. The underlying stream will not be aborted if the signal is aborted. The callback will get called with an `AbortError`. All registered listeners added by this function will also be removed.
- `callback` `<Function>` A callback function that takes an optional error argument.
- Returns: `<Function>` A cleanup function which removes all registered listeners.

A function to get notified when a stream is no longer readable, writable or has experienced an error or a premature close event.

    const { finished } = require('node:stream');
    const fs = require('node:fs');

    const rs = fs.createReadStream('archive.tar');

    finished(rs, (err) => {
      if (err) {
        console.error('Stream failed.', err);
      } else {
        console.log('Stream is done reading.');
      }
    });

    rs.resume(); // Drain the stream.
    js

Especially useful in error handling scenarios where a stream is destroyed prematurely (like an aborted HTTP request), and will not emit `'end'` or `'finish'`.

The `finished` API provides promise version.

`stream.finished()` leaves dangling event listeners (in particular `'error'`, `'end'`, `'finish'` and `'close'`) after `callback` has been invoked. The reason for this is so that unexpected `'error'` events (due to incorrect stream implementations) do not cause unexpected crashes. If this is unwanted behavior then the returned cleanup function needs to be invoked in the callback:

    const cleanup = finished(rs, (err) => {
      cleanup();
      // ...
    });
    js

#### `stream.pipeline(source[, ...transforms], destination, callback)`#

#### `stream.pipeline(streams, callback)`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v19.7.0, v18.16.0 | Added support for webstreams.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v14.0.0 | The `pipeline(..., cb)` will wait for the `'close'` event before invoking the callback. The implementation tries to detect legacy streams and only apply this behavior to streams which are expected to emit `'close'`.
v13.10.0 | Add support for async generators.

- `streams` {Stream[]|Iterable[]|AsyncIterable[]|Function[]| ReadableStream[]|WritableStream[]|TransformStream[]}
- `source` `<Stream>` | `<Iterable>` | `<AsyncIterable>` | `<Function>` | `<ReadableStream>`
  - Returns: `<Iterable>` | `<AsyncIterable>`
- `...transforms` `<Stream>` | `<Function>` | `<TransformStream>`
  - `source` `<AsyncIterable>`
  - Returns: `<AsyncIterable>`
- `destination` `<Stream>` | `<Function>` | `<WritableStream>`
  - `source` `<AsyncIterable>`
  - Returns: `<AsyncIterable>` | `<Promise>`
- `callback` `<Function>` Called when the pipeline is fully done.
  - `err` `<Error>`
  - `val` Resolved value of `Promise` returned by `destination`.
- Returns: `<Stream>`

A module method to pipe between streams and generators forwarding errors and properly cleaning up and provide a callback when the pipeline is complete.

    const { pipeline } = require('node:stream');
    const fs = require('node:fs');
    const zlib = require('node:zlib');

    // Use the pipeline API to easily pipe a series of streams
    // together and get notified when the pipeline is fully done.

    // A pipeline to gzip a potentially huge tar file efficiently:

    pipeline(
      fs.createReadStream('archive.tar'),
      zlib.createGzip(),
      fs.createWriteStream('archive.tar.gz'),
      (err) => {
        if (err) {
          console.error('Pipeline failed.', err);
        } else {
          console.log('Pipeline succeeded.');
        }
      },
    );
    js

The `pipeline` API provides a promise version.

`stream.pipeline()` will call `stream.destroy(err)` on all streams except:

- `Readable` streams which have emitted `'end'` or `'close'`.
- `Writable` streams which have emitted `'finish'` or `'close'`.

`stream.pipeline()` leaves dangling event listeners on the streams after the `callback` has been invoked. In the case of reuse of streams after failure, this can cause event listener leaks and swallowed errors. If the last stream is readable, dangling event listeners will be removed so that the last stream can be consumed later.

`stream.pipeline()` closes all the streams when an error is raised. The `IncomingRequest` usage with `pipeline` could lead to an unexpected behavior once it would destroy the socket without sending the expected response. See the example below:

    const fs = require('node:fs');
    const http = require('node:http');
    const { pipeline } = require('node:stream');

    const server = http.createServer((req, res) => {
      const fileStream = fs.createReadStream('./fileNotExist.txt');
      pipeline(fileStream, res, (err) => {
        if (err) {
          console.log(err); // No such file
          // this message can't be sent once `pipeline` already destroyed the socket
          return res.end('error!!!');
        }
      });
    });
    js

#### `stream.compose(...streams)`#

Added in: v16.9.0
History

Version | Changes
--- | ---
v26.2.0 | Marking the API stable.
v21.1.0, v20.10.0 | Added support for stream class.
v19.8.0, v18.16.0 | Added support for webstreams.

- `streams` {Stream[]|Iterable[]|AsyncIterable[]|Function[]| ReadableStream[]|WritableStream[]|TransformStream[]|Duplex[]|Function}
- Returns: `<stream.Duplex>`

Combines two or more streams into a `Duplex` stream that writes to the first stream and reads from the last. Each provided stream is piped into the next, using `stream.pipeline`. If any of the streams error then all are destroyed, including the outer `Duplex` stream.

Because `stream.compose` returns a new stream that in turn can (and should) be piped into other streams, it enables composition. In contrast, when passing streams to `stream.pipeline`, typically the first stream is a readable stream and the last a writable stream, forming a closed circuit.

If passed a `Function` it must be a factory method taking a `source` `Iterable`.

    import { compose, Transform } from 'node:stream';

    const removeSpaces = new Transform({
      transform(chunk, encoding, callback) {
        callback(null, String(chunk).replace(' ', ''));
      },
    });

    async function* toUpper(source) {
      for await (const chunk of source) {
        yield String(chunk).toUpperCase();
      }
    }

    let res = '';
    for await (const buf of compose(removeSpaces, toUpper).end('hello world')) {
      res += buf;
    }

    console.log(res); // prints 'HELLOWORLD'
    mjs

`stream.compose` can be used to convert async iterables, generators and functions into streams.

- `AsyncIterable` converts into a readable `Duplex`. Cannot yield `null`.
- `AsyncGeneratorFunction` converts into a readable/writable transform `Duplex`. Must take a source `AsyncIterable` as first parameter. Cannot yield `null`.
- `AsyncFunction` converts into a writable `Duplex`. Must return either `null` or `undefined`.

    import { compose } from 'node:stream';
    import { finished } from 'node:stream/promises';

    // Convert AsyncIterable into readable Duplex.
    const s1 = compose(async function*() {
      yield 'Hello';
      yield 'World';
    }());

    // Convert AsyncGenerator into transform Duplex.
    const s2 = compose(async function*(source) {
      for await (const chunk of source) {
        yield String(chunk).toUpperCase();
      }
    });

    let res = '';

    // Convert AsyncFunction into writable Duplex.
    const s3 = compose(async function(source) {
      for await (const chunk of source) {
        res += chunk;
      }
    });

    await finished(compose(s1, s2, s3));

    console.log(res); // prints 'HELLOWORLD'
    mjs

For convenience, the `readable.compose(stream)` method is available on `<Readable>` and `<Duplex>` streams as a wrapper for this function.

#### `stream.isDestroyed(stream)`#

Added in: v19.9.0, v18.17.0

- `stream` `<Readable>` | `<Writable>` | `<Duplex>`
- Returns: `<boolean>` | `<null>` - Only returns `null` if `stream` is not a valid `Readable`, `Writable` or `Duplex`.

Returns whether the stream has been destroyed.

#### `stream.isErrored(stream)`#

Added in: v17.3.0, v16.14.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `stream` `<Readable>` | `<Writable>` | `<Duplex>` | `<WritableStream>` | `<ReadableStream>`
- Returns: `<boolean>`

Returns whether the stream has encountered an error.

#### `stream.isReadable(stream)`#

Added in: v17.4.0, v16.14.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `stream` `<Readable>` | `<Duplex>` | `<ReadableStream>`
- Returns: `<boolean>` | `<null>` - Only returns `null` if `stream` is not a valid `Readable`, `Duplex` or `ReadableStream`.

Returns whether the stream is readable.

#### `stream.isWritable(stream)`#

- `stream` `<Writable>` | `<Duplex>` | `<WritableStream>`
- Returns: `<boolean>` | `<null>` - Only returns `null` if `stream` is not a valid `Writable`, `Duplex` or `WritableStream`.

Returns whether the stream is writable.

#### `stream.Readable.from(iterable[, options])`#

Added in: v12.3.0, v10.17.0

- `iterable` `<Iterable>` Object implementing the `Symbol.asyncIterator` or `Symbol.iterator` iterable protocol. Emits an 'error' event if a null value is passed.
- `options` `<Object>` Options provided to `new stream.Readable([options])`. By default, `Readable.from()` will set `options.objectMode` to `true`, unless this is explicitly opted out by setting `options.objectMode` to `false`.
- Returns: `<stream.Readable>`

A utility method for creating readable streams out of iterators.

    const { Readable } = require('node:stream');

    async function * generate() {
      yield 'hello';
      yield 'streams';
    }

    const readable = Readable.from(generate());

    readable.on('data', (chunk) => {
      console.log(chunk);
    });
    js

Calling `Readable.from(string)` or `Readable.from(buffer)` will not have the strings or buffers be iterated to match the other streams semantics for performance reasons.

If an `Iterable` object containing promises is passed as an argument, it might result in unhandled rejection.

    const { Readable } = require('node:stream');

    Readable.from([
      new Promise((resolve) => setTimeout(resolve('1'), 1500)),
      new Promise((_, reject) => setTimeout(reject(new Error('2')), 1000)), // Unhandled rejection
    ]);
    js

#### `stream.Readable.fromWeb(readableStream[, options])`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `readableStream` `<ReadableStream>`
- `options` `<Object>`
  - `encoding` `<string>`
  - `highWaterMark` `<number>`
  - `objectMode` `<boolean>`
  - `signal` `<AbortSignal>`
- Returns: `<stream.Readable>`

#### `stream.Readable.isDisturbed(stream)`#

Added in: v16.8.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `stream` `<stream.Readable>` | `<ReadableStream>`
- Returns: `boolean`

Returns whether the stream has been read from or cancelled.

#### `stream.Readable.toWeb(streamReadable[, options])`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v25.4.0, v24.14.0 | Add 'type' option to specify 'bytes'.
v24.0.0, v22.17.0 | Marking the API stable.
v18.7.0 | include strategy options on Readable.

- `streamReadable` `<stream.Readable>`
- `options` `<Object>`
  - `strategy` `<Object>`
    - `highWaterMark` `<number>` The maximum internal queue size (of the created `ReadableStream`) before backpressure is applied in reading from the given `stream.Readable`. If no value is provided, it will be taken from the given `stream.Readable`.
    - `size` `<Function>` A function that size of the given chunk of data. If no value is provided, the size will be `1` for all the chunks.
      - `chunk` `<any>`
      - Returns: `<number>`
  - `type` `<string>` Specifies the type of the created `ReadableStream`. Must be `'bytes'` or undefined.
- Returns: `<ReadableStream>`

#### `stream.Writable.fromWeb(writableStream[, options])`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `writableStream` `<WritableStream>`
- `options` `<Object>`
  - `decodeStrings` `<boolean>`
  - `highWaterMark` `<number>`
  - `objectMode` `<boolean>`
  - `signal` `<AbortSignal>`
- Returns: `<stream.Writable>`

#### `stream.Writable.toWeb(streamWritable)`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `streamWritable` `<stream.Writable>`
- Returns: `<WritableStream>`

#### `stream.Duplex.from(src)`#

Added in: v16.8.0
History

Version | Changes
--- | ---
v19.5.0, v18.17.0 | The `src` argument can now be a `ReadableStream` or `WritableStream`.

- `src` {Stream|Blob|ArrayBuffer|string|Iterable|AsyncIterable| AsyncGeneratorFunction|AsyncFunction|Promise|Object| ReadableStream|WritableStream}

A utility method for creating duplex streams.

- `Stream` converts writable stream into writable `Duplex` and readable stream to `Duplex`.
- `Blob` converts into readable `Duplex`.
- `string` converts into readable `Duplex`.
- `ArrayBuffer` converts into readable `Duplex`.
- `AsyncIterable` converts into a readable `Duplex`. Cannot yield `null`.
- `AsyncGeneratorFunction` converts into a readable/writable transform `Duplex`. Must take a source `AsyncIterable` as first parameter. Cannot yield `null`.
- `AsyncFunction` converts into a writable `Duplex`. Must return either `null` or `undefined`
- `Object ({ writable, readable })` converts `readable` and `writable` into `Stream` and then combines them into `Duplex` where the `Duplex` will write to the `writable` and read from the `readable`.
- `Promise` converts into readable `Duplex`. Value `null` is ignored.
- `ReadableStream` converts into readable `Duplex`.
- `WritableStream` converts into writable `Duplex`.
- Returns: `<stream.Duplex>`

If an `Iterable` object containing promises is passed as an argument, it might result in unhandled rejection.

    const { Duplex } = require('node:stream');

    Duplex.from([
      new Promise((resolve) => setTimeout(resolve('1'), 1500)),
      new Promise((_, reject) => setTimeout(reject(new Error('2')), 1000)), // Unhandled rejection
    ]);
    js

#### `stream.Duplex.fromWeb(pair[, options])`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `pair` `<Object>`
  - `readable` `<ReadableStream>`
  - `writable` `<WritableStream>`
- `options` `<Object>`
  - `allowHalfOpen` `<boolean>`
  - `decodeStrings` `<boolean>`
  - `encoding` `<string>`
  - `highWaterMark` `<number>`
  - `objectMode` `<boolean>`
  - `signal` `<AbortSignal>`
- Returns: `<stream.Duplex>`

    import { Duplex } from 'node:stream';
    import {
      ReadableStream,
      WritableStream,
    } from 'node:stream/web';

    const readable = new ReadableStream({
      start(controller) {
        controller.enqueue('world');
      },
    });

    const writable = new WritableStream({
      write(chunk) {
        console.log('writable', chunk);
      },
    });

    const pair = {
      readable,
      writable,
    };
    const duplex = Duplex.fromWeb(pair, { encoding: 'utf8', objectMode: true });

    duplex.write('hello');

    for await (const chunk of duplex) {
      console.log('readable', chunk);
    }
    const { Duplex } = require('node:stream');
    const {
      ReadableStream,
      WritableStream,
    } = require('node:stream/web');

    const readable = new ReadableStream({
      start(controller) {
        controller.enqueue('world');
      },
    });

    const writable = new WritableStream({
      write(chunk) {
        console.log('writable', chunk);
      },
    });

    const pair = {
      readable,
      writable,
    };
    const duplex = Duplex.fromWeb(pair, { encoding: 'utf8', objectMode: true });

    duplex.write('hello');
    duplex.once('readable', () => console.log('readable', duplex.read()));
    javascript

#### `stream.Duplex.toWeb(streamDuplex[, options])`#

Added in: v17.0.0
History

Version | Changes
--- | ---
v25.7.0 | Added the 'readableType' option to specify the ReadableStream type. The 'type' option is deprecated.
v25.4.0, v24.14.0 | Added the 'type' option to specify the ReadableStream type.
v24.0.0, v22.17.0 | Marking the API stable.

- `streamDuplex` `<stream.Duplex>`
- `options` `<Object>`
  - `readableType` `<string>` Specifies the type of the `ReadableStream` half of the created readable-writable pair. Must be `'bytes'` or undefined. (`options.type` is a deprecated alias for this option.)
- Returns: `<Object>`
  - `readable` `<ReadableStream>`
  - `writable` `<WritableStream>`

    import { Duplex } from 'node:stream';

    const duplex = Duplex({
      objectMode: true,
      read() {
        this.push('world');
        this.push(null);
      },
      write(chunk, encoding, callback) {
        console.log('writable', chunk);
        callback();
      },
    });

    const { readable, writable } = Duplex.toWeb(duplex);
    writable.getWriter().write('hello');

    const { value } = await readable.getReader().read();
    console.log('readable', value);
    const { Duplex } = require('node:stream');

    const duplex = Duplex({
      objectMode: true,
      read() {
        this.push('world');
        this.push(null);
      },
      write(chunk, encoding, callback) {
        console.log('writable', chunk);
        callback();
      },
    });

    const { readable, writable } = Duplex.toWeb(duplex);
    writable.getWriter().write('hello');

    readable.getReader().read().then((result) => {
      console.log('readable', result.value);
    });
    javascript

#### `stream.addAbortSignal(signal, stream)`#

Added in: v15.4.0
History

Version | Changes
--- | ---
v19.7.0, v18.16.0 | Added support for `ReadableStream` and `WritableStream`.

- `signal` `<AbortSignal>` A signal representing possible cancellation
- `stream` `<Stream>` | `<ReadableStream>` | `<WritableStream>` A stream to attach a signal to.

Attaches an AbortSignal to a readable or writable stream. This lets code control stream destruction using an `AbortController`.

Calling `abort` on the `AbortController` corresponding to the passed `AbortSignal` will behave the same way as calling `.destroy(new AbortError())` on the stream, and `controller.error(new AbortError())` for webstreams.

    const fs = require('node:fs');

    const controller = new AbortController();
    const read = addAbortSignal(
      controller.signal,
      fs.createReadStream(('object.json')),
    );
    // Later, abort the operation closing the stream
    controller.abort();
    js

Or using an `AbortSignal` with a readable stream as an async iterable:

    const controller = new AbortController();
    setTimeout(() => controller.abort(), 10_000); // set a timeout
    const stream = addAbortSignal(
      controller.signal,
      fs.createReadStream(('object.json')),
    );
    (async () => {
      try {
        for await (const chunk of stream) {
          await process(chunk);
        }
      } catch (e) {
        if (e.name === 'AbortError') {
          // The operation was cancelled
        } else {
          throw e;
        }
      }
    })();
    js

Or using an `AbortSignal` with a ReadableStream:

    const controller = new AbortController();
    const rs = new ReadableStream({
      start(controller) {
        controller.enqueue('hello');
        controller.enqueue('world');
        controller.close();
      },
    });

    addAbortSignal(controller.signal, rs);

    finished(rs, (err) => {
      if (err) {
        if (err.name === 'AbortError') {
          // The operation was cancelled
        }
      }
    });

    const reader = rs.getReader();

    reader.read().then(({ value, done }) => {
      console.log(value); // hello
      console.log(done); // false
      controller.abort();
    });
    js

#### `stream.getDefaultHighWaterMark(objectMode)`#

Added in: v19.9.0, v18.17.0

- `objectMode` `<boolean>`
- Returns: `<integer>`

Returns the default highWaterMark used by streams. Defaults to `65536` (64 KiB), or `16` for `objectMode`.

#### `stream.setDefaultHighWaterMark(objectMode, value)`#

Added in: v19.9.0, v18.17.0

- `objectMode` `<boolean>`
- `value` `<integer>` highWaterMark value

Sets the default highWaterMark used by streams.

### API for stream implementers#

The `node:stream` module API has been designed to make it possible to easily implement streams using JavaScript's prototypal inheritance model.

First, a stream developer would declare a new JavaScript class that extends one of the four basic stream classes (`stream.Writable`, `stream.Readable`, `stream.Duplex`, or `stream.Transform`), making sure they call the appropriate parent class constructor:

    const { Writable } = require('node:stream');

    class MyWritable extends Writable {
      constructor({ highWaterMark, ...options }) {
        super({ highWaterMark });
        // ...
      }
    }
    js

When extending streams, keep in mind what options the user can and should provide before forwarding these to the base constructor. For example, if the implementation makes assumptions in regard to the `autoDestroy` and `emitClose` options, do not allow the user to override these. Be explicit about what options are forwarded instead of implicitly forwarding all options.

The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:

Use-case | Class | Method(s) to implement
--- | --- | ---
Reading only | `Readable` | `_read()`
Writing only | `Writable` | `_write()`, `_writev()`, `_final()`
Reading and writing | `Duplex` | `_read()`, `_write()`, `_writev()`, `_final()`
Operate on written data, then read the result | `Transform` | `_transform()`, `_flush()`, `_final()`

The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for stream consumers section). Doing so may lead to adverse side effects in application code consuming the stream.

Avoid overriding public methods such as `write()`, `end()`, `cork()`, `uncork()`, `read()` and `destroy()`, or emitting internal events such as `'error'`, `'data'`, `'end'`, `'finish'` and `'close'` through `.emit()`. Doing so can break current and future stream invariants leading to behavior and/or compatibility issues with other streams, stream utilities, and user expectations.

#### Simplified construction#

Added in: v1.2.0

For many simple cases, it is possible to create a stream without relying on inheritance. This can be accomplished by directly creating instances of the `stream.Writable`, `stream.Readable`, `stream.Duplex`, or `stream.Transform` objects and passing appropriate methods as constructor options.

    const { Writable } = require('node:stream');

    const myWritable = new Writable({
      construct(callback) {
        // Initialize state and load resources...
      },
      write(chunk, encoding, callback) {
        // ...
      },
      destroy() {
        // Free resources...
      },
    });
    js

#### Implementing a writable stream#

The `stream.Writable` class is extended to implement a `Writable` stream.

Custom `Writable` streams must call the `new stream.Writable([options])` constructor and implement the `writable._write()` and/or `writable._writev()` method.

##### `new stream.Writable([options])`#

History

Version | Changes
--- | ---
v22.0.0 | bump default highWaterMark.
v15.5.0 | support passing in an AbortSignal.
v14.0.0 | Change `autoDestroy` option default to `true`.
v11.2.0, v10.16.0 | Add `autoDestroy` option to automatically `destroy()` the stream when it emits `'finish'` or errors.
v10.0.0 | Add `emitClose` option to specify if `'close'` is emitted on destroy.

- `options` `<Object>`
  - `highWaterMark` `<number>` Buffer level when `stream.write()` starts returning `false`. Default: `65536` (64 KiB), or `16` for `objectMode` streams.
  - `decodeStrings` `<boolean>` Whether to encode `string`s passed to `stream.write()` to `Buffer`s (with the encoding specified in the `stream.write()` call) before passing them to `stream._write()`. Other types of data are not converted (i.e. `Buffer`s are not decoded into `string`s). Setting to false will prevent `string`s from being converted. Default: `true`.
  - `defaultEncoding` `<string>` The default encoding that is used when no encoding is specified as an argument to `stream.write()`. Default: `'utf8'`.
  - `objectMode` `<boolean>` Whether or not the `stream.write(anyObj)` is a valid operation. When set, it becomes possible to write JavaScript values other than string, `<Buffer>`, `<TypedArray>` or `<DataView>` if supported by the stream implementation. Default: `false`.
  - `emitClose` `<boolean>` Whether or not the stream should emit `'close'` after it has been destroyed. Default: `true`.
  - `write` `<Function>` Implementation for the `stream._write()` method.
  - `writev` `<Function>` Implementation for the `stream._writev()` method.
  - `destroy` `<Function>` Implementation for the `stream._destroy()` method.
  - `final` `<Function>` Implementation for the `stream._final()` method.
  - `construct` `<Function>` Implementation for the `stream._construct()` method.
  - `autoDestroy` `<boolean>` Whether this stream should automatically call `.destroy()` on itself after ending. Default: `true`.
  - `signal` `<AbortSignal>` A signal representing possible cancellation.

    const { Writable } = require('node:stream');

    class MyWritable extends Writable {
      constructor(options) {
        // Calls the stream.Writable() constructor.
        super(options);
        // ...
      }
    }
    js

Or, when using pre-ES6 style constructors:

    const { Writable } = require('node:stream');
    const util = require('node:util');

    function MyWritable(options) {
      if (!(this instanceof MyWritable))
        return new MyWritable(options);
      Writable.call(this, options);
    }
    util.inherits(MyWritable, Writable);
    js

Or, using the simplified constructor approach:

    const { Writable } = require('node:stream');

    const myWritable = new Writable({
      write(chunk, encoding, callback) {
        // ...
      },
      writev(chunks, callback) {
        // ...
      },
    });
    js

Calling `abort` on the `AbortController` corresponding to the passed `AbortSignal` will behave the same way as calling `.destroy(new AbortError())` on the writable stream.

    const { Writable } = require('node:stream');

    const controller = new AbortController();
    const myWritable = new Writable({
      write(chunk, encoding, callback) {
        // ...
      },
      writev(chunks, callback) {
        // ...
      },
      signal: controller.signal,
    });
    // Later, abort the operation closing the stream
    controller.abort();
    js

##### `writable._construct(callback)`#

Added in: v15.0.0

- `callback` `<Function>` Call this function (optionally with an error argument) when the stream has finished initializing.

The `_construct()` method MUST NOT be called directly. It may be implemented by child classes, and if so, will be called by the internal `Writable` class methods only.

This optional function will be called in a tick after the stream constructor has returned, delaying any `_write()`, `_final()` and `_destroy()` calls until `callback` is called. This is useful to initialize state or asynchronously initialize resources before the stream can be used.

    const { Writable } = require('node:stream');
    const fs = require('node:fs');

    class WriteStream extends Writable {
      constructor(filename) {
        super();
        this.filename = filename;
        this.fd = null;
      }
      _construct(callback) {
        fs.open(this.filename, 'w', (err, fd) => {
          if (err) {
            callback(err);
          } else {
            this.fd = fd;
            callback();
          }
        });
      }
      _write(chunk, encoding, callback) {
        fs.write(this.fd, chunk, callback);
      }
      _destroy(err, callback) {
        if (this.fd) {
          fs.close(this.fd, (er) => callback(er || err));
        } else {
          callback(err);
        }
      }
    }
    js

##### `writable._write(chunk, encoding, callback)`#

History

Version | Changes
--- | ---
v12.11.0 | _write() is optional when providing _writev().

- `chunk` `<Buffer>` | `<string>` | `<any>` The `Buffer` to be written, converted from the `string` passed to `stream.write()`. If the stream's `decodeStrings` option is `false` or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed to `stream.write()`.
- `encoding` `<string>` If the chunk is a string, then `encoding` is the character encoding of that string. If chunk is a `Buffer`, or if the stream is operating in object mode, `encoding` may be ignored.
- `callback` `<Function>` Call this function (optionally with an error argument) when processing is complete for the supplied chunk.

All `Writable` stream implementations must provide a `writable._write()` and/or `writable._writev()` method to send data to the underlying resource.

`Transform` streams provide their own implementation of the `writable._write()`.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal `Writable` class methods only.

The `callback` function must be called synchronously inside of `writable._write()` or asynchronously (i.e. different tick) to signal either that the write completed successfully or failed with an error. The first argument passed to the `callback` must be the `Error` object if the call failed or `null` if the write succeeded.

All calls to `writable.write()` that occur between the time `writable._write()` is called and the `callback` is called will cause the written data to be buffered. When the `callback` is invoked, the stream might emit a `'drain'` event. If a stream implementation is capable of processing multiple chunks of data at once, the `writable._writev()` method should be implemented.

If the `decodeStrings` property is explicitly set to `false` in the constructor options, then `chunk` will remain the same object that is passed to `.write()`, and may be a string rather than a `Buffer`. This is to support implementations that have an optimized handling for certain string data encodings. In that case, the `encoding` argument will indicate the character encoding of the string. Otherwise, the `encoding` argument can be safely ignored.

The `writable._write()` method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

##### `writable._writev(chunks, callback)`#

- `chunks` `<Object>`[] The data to be written. The value is an array of `<Object>` that each represent a discrete chunk of data to write. The properties of these objects are:
  - `chunk` `<Buffer>` | `<string>` A buffer instance or string containing the data to be written. The `chunk` will be a string if the `Writable` was created with the `decodeStrings` option set to `false` and a string was passed to `write()`.
  - `encoding` `<string>` The character encoding of the `chunk`. If `chunk` is a `Buffer`, the `encoding` will be `'buffer'`.
- `callback` `<Function>` A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal `Writable` class methods only.

The `writable._writev()` method may be implemented in addition or alternatively to `writable._write()` in stream implementations that are capable of processing multiple chunks of data at once. If implemented and if there is buffered data from previous writes, `_writev()` will be called instead of `_write()`.

The `writable._writev()` method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

##### `writable._destroy(err, callback)`#

Added in: v8.0.0

- `err` `<Error>` A possible error.
- `callback` `<Function>` A callback function that takes an optional error argument.

The `_destroy()` method is called by `writable.destroy()`. It can be overridden by child classes but it must not be called directly.

##### `writable._final(callback)`#

Added in: v8.0.0

- `callback` `<Function>` Call this function (optionally with an error argument) when finished writing any remaining data.

The `_final()` method must not be called directly. It may be implemented by child classes, and if so, will be called by the internal `Writable` class methods only.

This optional function will be called before the stream closes, delaying the `'finish'` event until `callback` is called. This is useful to close resources or write buffered data before a stream ends.

##### Errors while writing#

Errors occurring during the processing of the `writable._write()`, `writable._writev()` and `writable._final()` methods must be propagated by invoking the callback and passing the error as the first argument. Throwing an `Error` from within these methods or manually emitting an `'error'` event results in undefined behavior.

If a `Readable` stream pipes into a `Writable` stream when `Writable` emits an error, the `Readable` stream will be unpiped.

    const { Writable } = require('node:stream');

    const myWritable = new Writable({
      write(chunk, encoding, callback) {
        if (chunk.toString().indexOf('a') >= 0) {
          callback(new Error('chunk is invalid'));
        } else {
          callback();
        }
      },
    });
    js

##### An example writable stream#

The following illustrates a rather simplistic (and somewhat pointless) custom `Writable` stream implementation. While this specific `Writable` stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom `Writable` stream instance:

    const { Writable } = require('node:stream');

    class MyWritable extends Writable {
      _write(chunk, encoding, callback) {
        if (chunk.toString().indexOf('a') >= 0) {
          callback(new Error('chunk is invalid'));
        } else {
          callback();
        }
      }
    }
    js

##### Decoding buffers in a writable stream#

Decoding buffers is a common task, for instance, when using transformers whose input is a string. This is not a trivial process when using multi-byte characters encoding, such as UTF-8. The following example shows how to decode multi-byte strings using `StringDecoder` and `Writable`.

    const { Writable } = require('node:stream');
    const { StringDecoder } = require('node:string_decoder');

    class StringWritable extends Writable {
      constructor(options) {
        super(options);
        this._decoder = new StringDecoder(options?.defaultEncoding);
        this.data = '';
      }
      _write(chunk, encoding, callback) {
        if (encoding === 'buffer') {
          chunk = this._decoder.write(chunk);
        }
        this.data += chunk;
        callback();
      }
      _final(callback) {
        this.data += this._decoder.end();
        callback();
      }
    }

    const euro = [[0xE2, 0x82], [0xAC]].map(Buffer.from);
    const w = new StringWritable();

    w.write('currency: ');
    w.write(euro[0]);
    w.end(euro[1]);

    console.log(w.data); // currency: €
    js

#### Implementing a readable stream#

The `stream.Readable` class is extended to implement a `Readable` stream.

Custom `Readable` streams must call the `new stream.Readable([options])` constructor and implement the `readable._read()` method.

##### `new stream.Readable([options])`#

History

Version | Changes
--- | ---
v22.0.0 | bump default highWaterMark.
v15.5.0 | support passing in an AbortSignal.
v14.0.0 | Change `autoDestroy` option default to `true`.
v11.2.0, v10.16.0 | Add `autoDestroy` option to automatically `destroy()` the stream when it emits `'end'` or errors.

- `options` `<Object>`
  - `highWaterMark` `<number>` The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Default: `65536` (64 KiB), or `16` for `objectMode` streams.
  - `encoding` `<string>` If specified, then buffers will be decoded to strings using the specified encoding. Default: `null`.
  - `objectMode` `<boolean>` Whether this stream should behave as a stream of objects. Meaning that `stream.read(n)` returns a single value instead of a `Buffer` of size `n`. Default: `false`.
  - `emitClose` `<boolean>` Whether or not the stream should emit `'close'` after it has been destroyed. Default: `true`.
  - `read` `<Function>` Implementation for the `stream._read()` method.
  - `destroy` `<Function>` Implementation for the `stream._destroy()` method.
  - `construct` `<Function>` Implementation for the `stream._construct()` method.
  - `autoDestroy` `<boolean>` Whether this stream should automatically call `.destroy()` on itself after ending. Default: `true`.
  - `signal` `<AbortSignal>` A signal representing possible cancellation.

    const { Readable } = require('node:stream');

    class MyReadable extends Readable {
      constructor(options) {
        // Calls the stream.Readable(options) constructor.
        super(options);
        // ...
      }
    }
    js

Or, when using pre-ES6 style constructors:

    const { Readable } = require('node:stream');
    const util = require('node:util');

    function MyReadable(options) {
      if (!(this instanceof MyReadable))
        return new MyReadable(options);
      Readable.call(this, options);
    }
    util.inherits(MyReadable, Readable);
    js

Or, using the simplified constructor approach:

    const { Readable } = require('node:stream');

    const myReadable = new Readable({
      read(size) {
        // ...
      },
    });
    js

Calling `abort` on the `AbortController` corresponding to the passed `AbortSignal` will behave the same way as calling `.destroy(new AbortError())` on the readable created.

    const { Readable } = require('node:stream');
    const controller = new AbortController();
    const read = new Readable({
      read(size) {
        // ...
      },
      signal: controller.signal,
    });
    // Later, abort the operation closing the stream
    controller.abort();
    js

##### `readable._construct(callback)`#

Added in: v15.0.0

- `callback` `<Function>` Call this function (optionally with an error argument) when the stream has finished initializing.

The `_construct()` method MUST NOT be called directly. It may be implemented by child classes, and if so, will be called by the internal `Readable` class methods only.

This optional function will be scheduled in the next tick by the stream constructor, delaying any `_read()` and `_destroy()` calls until `callback` is called. This is useful to initialize state or asynchronously initialize resources before the stream can be used.

    const { Readable } = require('node:stream');
    const fs = require('node:fs');

    class ReadStream extends Readable {
      constructor(filename) {
        super();
        this.filename = filename;
        this.fd = null;
      }
      _construct(callback) {
        fs.open(this.filename, (err, fd) => {
          if (err) {
            callback(err);
          } else {
            this.fd = fd;
            callback();
          }
        });
      }
      _read(n) {
        const buf = Buffer.alloc(n);
        fs.read(this.fd, buf, 0, n, null, (err, bytesRead) => {
          if (err) {
            this.destroy(err);
          } else {
            this.push(bytesRead > 0 ? buf.slice(0, bytesRead) : null);
          }
        });
      }
      _destroy(err, callback) {
        if (this.fd) {
          fs.close(this.fd, (er) => callback(er || err));
        } else {
          callback(err);
        }
      }
    }
    js

##### `readable._read(size)`#

Added in: v0.9.4

- `size` `<number>` Number of bytes to read asynchronously

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal `Readable` class methods only.

All `Readable` stream implementations must provide an implementation of the `readable._read()` method to fetch data from the underlying resource.

When `readable._read()` is called, if data is available from the resource, the implementation should begin pushing that data into the read queue using the `this.push(dataChunk)` method. `_read()` will be called again after each call to `this.push(dataChunk)` once the stream is ready to accept more data. `_read()` may continue reading from the resource and pushing data until `readable.push()` returns `false`. Only when `_read()` is called again after it has stopped should it resume pushing additional data into the queue.

Once the `readable._read()` method has been called, it will not be called again until more data is pushed through the `readable.push()` method. Empty data such as empty buffers and strings will not cause `readable._read()` to be called.

The `size` argument is advisory. For implementations where a "read" is a single operation that returns data can use the `size` argument to determine how much data to fetch. Other implementations may ignore this argument and simply provide data whenever it becomes available. There is no need to "wait" until `size` bytes are available before calling `stream.push(chunk)`.

The `readable._read()` method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

##### `readable._destroy(err, callback)`#

Added in: v8.0.0

- `err` `<Error>` A possible error.
- `callback` `<Function>` A callback function that takes an optional error argument.

The `_destroy()` method is called by `readable.destroy()`. It can be overridden by child classes but it must not be called directly.

##### `readable.push(chunk[, encoding])`#

History

Version | Changes
--- | ---
v22.0.0, v20.13.0 | The `chunk` argument can now be a `TypedArray` or `DataView` instance.
v8.0.0 | The `chunk` argument can now be a `Uint8Array` instance.

- `chunk` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<string>` | `<null>` | `<any>` Chunk of data to push into the read queue. For streams not operating in object mode, `chunk` must be a `<string>`, `<Buffer>`, `<TypedArray>` or `<DataView>`. For object mode streams, `chunk` may be any JavaScript value.
- `encoding` `<string>` Encoding of string chunks. Must be a valid `Buffer` encoding, such as `'utf8'` or `'ascii'`.
- Returns: `<boolean>` `true` if additional chunks of data may continue to be pushed; `false` otherwise.

When `chunk` is a `<Buffer>`, `<TypedArray>`, `<DataView>` or `<string>`, the `chunk` of data will be added to the internal queue for users of the stream to consume. Passing `chunk` as `null` signals the end of the stream (EOF), after which no more data can be written.

When the `Readable` is operating in paused mode, the data added with `readable.push()` can be read out by calling the `readable.read()` method when the `'readable'` event is emitted.

When the `Readable` is operating in flowing mode, the data added with `readable.push()` will be delivered by emitting a `'data'` event.

The `readable.push()` method is designed to be as flexible as possible. For example, when wrapping a lower-level source that provides some form of pause/resume mechanism, and a data callback, the low-level source can be wrapped by the custom `Readable` instance:

    // `_source` is an object with readStop() and readStart() methods,
    // and an `ondata` member that gets called when it has data, and
    // an `onend` member that gets called when the data is over.

    class SourceWrapper extends Readable {
      constructor(options) {
        super(options);

        this._source = getLowLevelSourceObject();

        // Every time there's data, push it into the internal buffer.
        this._source.ondata = (chunk) => {
          // If push() returns false, then stop reading from source.
          if (!this.push(chunk))
            this._source.readStop();
        };

        // When the source ends, push the EOF-signaling `null` chunk.
        this._source.onend = () => {
          this.push(null);
        };
      }
      // _read() will be called when the stream wants to pull more data in.
      // The advisory size argument is ignored in this case.
      _read(size) {
        this._source.readStart();
      }
    }
    js

The `readable.push()` method is used to push the content into the internal buffer. It can be driven by the `readable._read()` method.

For streams not operating in object mode, if the `chunk` parameter of `readable.push()` is `undefined`, it will be treated as empty string or buffer. See `readable.push('')` for more information.

##### Errors while reading#

Errors occurring during processing of the `readable._read()` must be propagated through the `readable.destroy(err)` method. Throwing an `Error` from within `readable._read()` or manually emitting an `'error'` event results in undefined behavior.

    const { Readable } = require('node:stream');

    const myReadable = new Readable({
      read(size) {
        const err = checkSomeErrorCondition();
        if (err) {
          this.destroy(err);
        } else {
          // Do some work.
        }
      },
    });
    js

##### An example counting stream#

The following is a basic example of a `Readable` stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.

    const { Readable } = require('node:stream');

    class Counter extends Readable {
      constructor(opt) {
        super(opt);
        this._max = 1000000;
        this._index = 1;
      }

      _read() {
        const i = this._index++;
        if (i > this._max)
          this.push(null);
        else {
          const str = String(i);
          const buf = Buffer.from(str, 'ascii');
          this.push(buf);
        }
      }
    }
    js

#### Implementing a duplex stream#

A `Duplex` stream is one that implements both `Readable` and `Writable`, such as a TCP socket connection.

Because JavaScript does not have support for multiple inheritance, the `stream.Duplex` class is extended to implement a `Duplex` stream (as opposed to extending the `stream.Readable` and `stream.Writable` classes).

The `stream.Duplex` class prototypically inherits from `stream.Readable` and parasitically from `stream.Writable`, but `instanceof` will work properly for both base classes due to overriding `Symbol.hasInstance` on `stream.Writable`.

Custom `Duplex` streams must call the `new stream.Duplex([options])` constructor and implement both the `readable._read()` and `writable._write()` methods.

##### `new stream.Duplex(options)`#

History

Version | Changes
--- | ---
v8.4.0 | The `readableHighWaterMark` and `writableHighWaterMark` options are supported now.

- `options` `<Object>` Passed to both `Writable` and `Readable` constructors. Also has the following fields:
  - `allowHalfOpen` `<boolean>` If set to `false`, then the stream will automatically end the writable side when the readable side ends. Default: `true`.
  - `readable` `<boolean>` Sets whether the `Duplex` should be readable. Default: `true`.
  - `writable` `<boolean>` Sets whether the `Duplex` should be writable. Default: `true`.
  - `readableObjectMode` `<boolean>` Sets `objectMode` for readable side of the stream. Has no effect if `objectMode` is `true`. Default: `false`.
  - `writableObjectMode` `<boolean>` Sets `objectMode` for writable side of the stream. Has no effect if `objectMode` is `true`. Default: `false`.
  - `readableHighWaterMark` `<number>` Sets `highWaterMark` for the readable side of the stream. Has no effect if `highWaterMark` is provided.
  - `writableHighWaterMark` `<number>` Sets `highWaterMark` for the writable side of the stream. Has no effect if `highWaterMark` is provided.

    const { Duplex } = require('node:stream');

    class MyDuplex extends Duplex {
      constructor(options) {
        super(options);
        // ...
      }
    }
    js

Or, when using pre-ES6 style constructors:

    const { Duplex } = require('node:stream');
    const util = require('node:util');

    function MyDuplex(options) {
      if (!(this instanceof MyDuplex))
        return new MyDuplex(options);
      Duplex.call(this, options);
    }
    util.inherits(MyDuplex, Duplex);
    js

Or, using the simplified constructor approach:

    const { Duplex } = require('node:stream');

    const myDuplex = new Duplex({
      read(size) {
        // ...
      },
      write(chunk, encoding, callback) {
        // ...
      },
    });
    js

When using pipeline:

    const { Transform, pipeline } = require('node:stream');
    const fs = require('node:fs');

    pipeline(
      fs.createReadStream('object.json')
        .setEncoding('utf8'),
      new Transform({
        decodeStrings: false, // Accept string input rather than Buffers
        construct(callback) {
          this.data = '';
          callback();
        },
        transform(chunk, encoding, callback) {
          this.data += chunk;
          callback();
        },
        flush(callback) {
          try {
            // Make sure is valid json.
            JSON.parse(this.data);
            this.push(this.data);
            callback();
          } catch (err) {
            callback(err);
          }
        },
      }),
      fs.createWriteStream('valid-object.json'),
      (err) => {
        if (err) {
          console.error('failed', err);
        } else {
          console.log('completed');
        }
      },
    );
    js

##### An example duplex stream#

The following illustrates a simple example of a `Duplex` stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read, albeit using an API that is not compatible with Node.js streams. The following illustrates a simple example of a `Duplex` stream that buffers incoming written data via the `Writable` interface that is read back out via the `Readable` interface.

    const { Duplex } = require('node:stream');
    const kSource = Symbol('source');

    class MyDuplex extends Duplex {
      constructor(source, options) {
        super(options);
        this[kSource] = source;
      }

      _write(chunk, encoding, callback) {
        // The underlying source only deals with strings.
        if (Buffer.isBuffer(chunk))
          chunk = chunk.toString();
        this[kSource].writeSomeData(chunk);
        callback();
      }

      _read(size) {
        this[kSource].fetchSomeData(size, (data, encoding) => {
          this.push(Buffer.from(data, encoding));
        });
      }
    }
    js

The most important aspect of a `Duplex` stream is that the `Readable` and `Writable` sides operate independently of one another despite co-existing within a single object instance.

##### Object mode duplex streams#

For `Duplex` streams, `objectMode` can be set exclusively for either the `Readable` or `Writable` side using the `readableObjectMode` and `writableObjectMode` options respectively.

In the following example, for instance, a new `Transform` stream (which is a type of `Duplex` stream) is created that has an object mode `Writable` side that accepts JavaScript numbers that are converted to hexadecimal strings on the `Readable` side.

    const { Transform } = require('node:stream');

    // All Transform streams are also Duplex Streams.
    const myTransform = new Transform({
      writableObjectMode: true,

      transform(chunk, encoding, callback) {
        // Coerce the chunk to a number if necessary.
        chunk |= 0;

        // Transform the chunk into something else.
        const data = chunk.toString(16);

        // Push the data onto the readable queue.
        callback(null, '0'.repeat(data.length % 2) + data);
      },
    });

    myTransform.setEncoding('ascii');
    myTransform.on('data', (chunk) => console.log(chunk));

    myTransform.write(1);
    // Prints: 01
    myTransform.write(10);
    // Prints: 0a
    myTransform.write(100);
    // Prints: 64
    js

#### Implementing a transform stream#

A `Transform` stream is a `Duplex` stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.

There is no requirement that the output be the same size as the input, the same number of chunks, or arrive at the same time. For example, a `Hash` stream will only ever have a single chunk of output which is provided when the input is ended. A `zlib` stream will produce output that is either much smaller or much larger than its input.

The `stream.Transform` class is extended to implement a `Transform` stream.

The `stream.Transform` class prototypically inherits from `stream.Duplex` and implements its own versions of the `writable._write()` and `readable._read()` methods. Custom `Transform` implementations must implement the `transform._transform()` method and may also implement the `transform._flush()` method.

Care must be taken when using `Transform` streams in that data written to the stream can cause the `Writable` side of the stream to become paused if the output on the `Readable` side is not consumed.

##### `new stream.Transform([options])`#

- `options` `<Object>` Passed to both `Writable` and `Readable` constructors. Also has the following fields:
  - `transform` `<Function>` Implementation for the `stream._transform()` method.
  - `flush` `<Function>` Implementation for the `stream._flush()` method.

    const { Transform } = require('node:stream');

    class MyTransform extends Transform {
      constructor(options) {
        super(options);
        // ...
      }
    }
    js

Or, when using pre-ES6 style constructors:

    const { Transform } = require('node:stream');
    const util = require('node:util');

    function MyTransform(options) {
      if (!(this instanceof MyTransform))
        return new MyTransform(options);
      Transform.call(this, options);
    }
    util.inherits(MyTransform, Transform);
    js

Or, using the simplified constructor approach:

    const { Transform } = require('node:stream');

    const myTransform = new Transform({
      transform(chunk, encoding, callback) {
        // ...
      },
    });
    js

##### Event: `'end'`#

The `'end'` event is from the `stream.Readable` class. The `'end'` event is emitted after all data has been output, which occurs after the callback in `transform._flush()` has been called. In the case of an error, `'end'` should not be emitted.

##### Event: `'finish'`#

The `'finish'` event is from the `stream.Writable` class. The `'finish'` event is emitted after `stream.end()` is called and all chunks have been processed by `stream._transform()`. In the case of an error, `'finish'` should not be emitted.

##### `transform._flush(callback)`#

- `callback` `<Function>` A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal `Readable` class methods only.

In some cases, a transform operation may need to emit an additional bit of data at the end of the stream. For example, a `zlib` compression stream will store an amount of internal state used to optimally compress the output. When the stream ends, however, that additional data needs to be flushed so that the compressed data will be complete.

Custom `Transform` implementations may implement the `transform._flush()` method. This will be called when there is no more written data to be consumed, but before the `'end'` event is emitted signaling the end of the `Readable` stream.

Within the `transform._flush()` implementation, the `transform.push()` method may be called zero or more times, as appropriate. The `callback` function must be called when the flush operation is complete.

The `transform._flush()` method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

##### `transform._transform(chunk, encoding, callback)`#

- `chunk` `<Buffer>` | `<string>` | `<any>` The `Buffer` to be transformed, converted from the `string` passed to `stream.write()`. If the stream's `decodeStrings` option is `false` or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed to `stream.write()`.
- `encoding` `<string>` If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value `'buffer'`. Ignore it in that case.
- `callback` `<Function>` A callback function (optionally with an error argument and data) to be called after the supplied `chunk` has been processed.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal `Readable` class methods only.

All `Transform` stream implementations must provide a `_transform()` method to accept input and produce output. The `transform._transform()` implementation handles the bytes being written, computes an output, then passes that output off to the readable portion using the `transform.push()` method.

The `transform.push()` method may be called zero or more times to generate output from a single input chunk, depending on how much is to be output as a result of the chunk.

It is possible that no output is generated from any given chunk of input data.

The `callback` function must be called only when the current chunk is completely consumed. The first argument passed to the `callback` must be an `Error` object if an error occurred while processing the input or `null` otherwise. If a second argument is passed to the `callback`, it will be forwarded on to the `transform.push()` method, but only if the first argument is falsy. In other words, the following are equivalent:

    transform.prototype._transform = function(data, encoding, callback) {
      this.push(data);
      callback();
    };

    transform.prototype._transform = function(data, encoding, callback) {
      callback(null, data);
    };
    js

The `transform._transform()` method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

`transform._transform()` is never called in parallel; streams implement a queue mechanism, and to receive the next chunk, `callback` must be called, either synchronously or asynchronously.

##### Class: `stream.PassThrough`#

The `stream.PassThrough` class is a trivial implementation of a `Transform` stream that simply passes the input bytes across to the output. Its purpose is primarily for examples and testing, but there are some use cases where `stream.PassThrough` is useful as a building block for novel sorts of streams.

### Additional notes#

#### Streams compatibility with async generators and async iterators#

With the support of async generators and iterators in JavaScript, async generators are effectively a first-class language-level stream construct at this point.

Some common interop cases of using Node.js streams with async generators and async iterators are provided below.

##### Consuming readable streams with async iterators#

    (async function() {
      for await (const chunk of readable) {
        console.log(chunk);
      }
    })();
    js

Async iterators register a permanent error handler on the stream to prevent any unhandled post-destroy errors.

##### Creating readable streams with async generators#

A Node.js readable stream can be created from an asynchronous generator using the `Readable.from()` utility method:

    const { Readable } = require('node:stream');

    const ac = new AbortController();
    const signal = ac.signal;

    async function * generate() {
      yield 'a';
      await someLongRunningFn({ signal });
      yield 'b';
      yield 'c';
    }

    const readable = Readable.from(generate());
    readable.on('close', () => {
      ac.abort();
    });

    readable.on('data', (chunk) => {
      console.log(chunk);
    });
    js

##### Piping to writable streams from async iterators#

When writing to a writable stream from an async iterator, ensure correct handling of backpressure and errors. `stream.pipeline()` abstracts away the handling of backpressure and backpressure-related errors:

    const fs = require('node:fs');
    const { pipeline } = require('node:stream');
    const { pipeline: pipelinePromise } = require('node:stream/promises');

    const writable = fs.createWriteStream('./file');

    const ac = new AbortController();
    const signal = ac.signal;

    const iterator = createIterator({ signal });

    // Callback Pattern
    pipeline(iterator, writable, (err, value) => {
      if (err) {
        console.error(err);
      } else {
        console.log(value, 'value returned');
      }
    }).on('close', () => {
      ac.abort();
    });

    // Promise Pattern
    pipelinePromise(iterator, writable)
      .then((value) => {
        console.log(value, 'value returned');
      })
      .catch((err) => {
        console.error(err);
        ac.abort();
      });
    js

#### Compatibility with older Node.js versions#

Prior to Node.js 0.10, the `Readable` stream interface was simpler, but also less powerful and less useful.

- Rather than waiting for calls to the `stream.read()` method, `'data'` events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost.
- The `stream.pause()` method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive `'data'` events even when the stream was in a paused state.

In Node.js 0.10, the `Readable` class was added. For backward compatibility with older Node.js programs, `Readable` streams switch into "flowing mode" when a `'data'` event handler is added, or when the `stream.resume()` method is called. The effect is that, even when not using the new `stream.read()` method and `'readable'` event, it is no longer necessary to worry about losing `'data'` chunks.

While most applications will continue to function normally, this introduces an edge case in the following conditions:

- No `'data'` event listener is added.
- The `stream.resume()` method is never called.
- The stream is not piped to any writable destination.

For example, consider the following code:

    // WARNING!  BROKEN!
    net.createServer((socket) => {

      // We add an 'end' listener, but never consume the data.
      socket.on('end', () => {
        // It will never get here.
        socket.end('The message was received but was not processed.\n');
      });

    }).listen(1337);
    js

Prior to Node.js 0.10, the incoming message data would be simply discarded. However, in Node.js 0.10 and beyond, the socket remains paused forever.

The workaround in this situation is to call the `stream.resume()` method to begin the flow of data:

    // Workaround.
    net.createServer((socket) => {
      socket.on('end', () => {
        socket.end('The message was received but was not processed.\n');
      });

      // Start the flow of data, discarding it.
      socket.resume();
    }).listen(1337);
    js

In addition to new `Readable` streams switching into flowing mode, pre-0.10 style streams can be wrapped in a `Readable` class using the `readable.wrap()` method.

#### `readable.read(0)`#

There are some cases where it is necessary to trigger a refresh of the underlying readable stream mechanisms, without actually consuming any data. In such cases, it is possible to call `readable.read(0)`, which will always return `null`.

If the internal read buffer is below the `highWaterMark`, and the stream is not currently reading, then calling `stream.read(0)` will trigger a low-level `stream._read()` call.

While most applications will almost never need to do this, there are situations within Node.js where this is done, particularly in the `Readable` stream class internals.

#### `readable.push('')`#

Use of `readable.push('')` is not recommended.

Pushing a zero-byte `<string>`, `<Buffer>`, `<TypedArray>` or `<DataView>` to a stream that is not in object mode has an interesting side effect. Because it is a call to `readable.push()`, the call will end the reading process. However, because the argument is an empty string, no data is added to the readable buffer so there is nothing for a user to consume.

#### `highWaterMark` discrepancy after calling `readable.setEncoding()`#

The use of `readable.setEncoding()` will change the behavior of how the `highWaterMark` operates in non-object mode.

Typically, the size of the current buffer is measured against the `highWaterMark` in bytes. However, after `setEncoding()` is called, the comparison function will begin to measure the buffer's size in characters.

This is not a problem in common cases with `latin1` or `ascii`. But it is advised to be mindful about this behavior when working with strings that could contain multi-byte characters.

## Iterable Streams#

Source Code: lib/stream/iter.js

Stability: 1 - Experimental – Enable this API with the `--experimental-stream-iter` CLI flag.

The `node:stream/iter` module provides a streaming API built on iterables rather than the event-driven `Readable`/`Writable`/`Transform` class hierarchy, or the Web Streams `ReadableStream`/`WritableStream`/`TransformStream` interfaces.

Streams are represented as `<AsyncIterable>` (async) or `<Iterable>` (sync). There are no base classes to extend -- any object implementing the iterable protocol can participate. Transforms are plain functions or objects with a `transform` method.

Data flows in batches (`<Uint8Array>`[] per iteration) to amortize the cost of async operations.

    import { from, pull, text } from 'node:stream/iter';
    import { compressGzip, decompressGzip } from 'node:zlib/iter';

    // Compress and decompress a string
    const compressed = pull(from('Hello, world!'), compressGzip());
    const result = await text(pull(compressed, decompressGzip()));
    console.log(result); // 'Hello, world!'
    const { from, pull, text } = require('node:stream/iter');
    const { compressGzip, decompressGzip } = require('node:zlib/iter');

    async function run() {
      // Compress and decompress a string
      const compressed = pull(from('Hello, world!'), compressGzip());
      const result = await text(pull(compressed, decompressGzip()));
      console.log(result); // 'Hello, world!'
    }

    run().catch(console.error);
    javascript

    import { open } from 'node:fs/promises';
    import { text, pipeTo } from 'node:stream/iter';
    import { compressGzip, decompressGzip } from 'node:zlib/iter';

    // Read a file, compress, write to another file
    const src = await open('input.txt', 'r');
    const dst = await open('output.gz', 'w');
    await pipeTo(src.pull(), compressGzip(), dst.writer({ autoClose: true }));
    await src.close();

    // Read it back
    const gz = await open('output.gz', 'r');
    console.log(await text(gz.pull(decompressGzip(), { autoClose: true })));
    const { open } = require('node:fs/promises');
    const { text, pipeTo } = require('node:stream/iter');
    const { compressGzip, decompressGzip } = require('node:zlib/iter');

    async function run() {
      // Read a file, compress, write to another file
      const src = await open('input.txt', 'r');
      const dst = await open('output.gz', 'w');
      await pipeTo(src.pull(), compressGzip(), dst.writer({ autoClose: true }));
      await src.close();

      // Read it back
      const gz = await open('output.gz', 'r');
      console.log(await text(gz.pull(decompressGzip(), { autoClose: true })));
    }

    run().catch(console.error);
    javascript

### Concepts#

#### Byte streams#

All data in this API is represented as `<Uint8Array>` bytes. Strings are automatically UTF-8 encoded when passed to `from()`, `push()`, or `pipeTo()`. This removes ambiguity around encodings and enables zero-copy transfers between streams and native code.

#### Batching#

Each iteration yields a batch -- an `<Array>` of `<Uint8Array>` chunks (`<Uint8Array>`[]). Batching amortizes the cost of `await` and `<Promise>` creation across multiple chunks. A consumer that processes one chunk at a time can simply iterate the inner array:

    for await (const batch of source) {
      for (const chunk of batch) {
        handle(chunk);
      }
    }
    async function run() {
      for await (const batch of source) {
        for (const chunk of batch) {
          handle(chunk);
        }
      }
    }
    javascript

#### Transforms#

Transforms come in two forms:

- Stateless -- a function `(chunks, options) => result` called once per batch. Receives `Uint8Array[]` (or `null` as the flush signal) and an `options` object. Returns `<Uint8Array>`[] | `<null>` | `<Iterable>`.

- Stateful -- an object `{ transform(source, options) }` where `transform` is a generator (sync or async) that receives the entire upstream iterable and an `options` object, and yields output. This form is used for compression, encryption, and any transform that needs to buffer across batches.

Both forms receive an `options` parameter with the following property:

- `options.signal` `<AbortSignal>` An AbortSignal that fires when the pipeline is cancelled, encounters an error, or the consumer stops reading. Transforms can check `signal.aborted` or listen for the `'abort'` event to perform early cleanup.

The flush signal (`null`) is sent after the source ends, giving transforms a chance to emit trailing data (e.g., compression footers).

    // Stateless: uppercase transform
    const upper = (chunks) => {
      if (chunks === null) return null; // flush
      return chunks.map((c) => new TextEncoder().encode(
        new TextDecoder().decode(c).toUpperCase(),
      ));
    };

    // Stateful: line splitter
    const lines = {
      transform: async function*(source) {
        let partial = '';
        for await (const chunks of source) {
          if (chunks === null) {
            if (partial) yield [new TextEncoder().encode(partial)];
            continue;
          }
          for (const chunk of chunks) {
            const str = partial + new TextDecoder().decode(chunk);
            const parts = str.split('\n');
            partial = parts.pop();
            for (const line of parts) {
              yield [new TextEncoder().encode(`${line}\n`)];
            }
          }
        }
      },
    };
    js

#### Pull vs. push#

The API supports two models:

- Pull -- data flows on demand. `pull()` and `pullSync()` create lazy pipelines that only read from the source when the consumer iterates.

- Push -- data is written explicitly. `push()` creates a writer/readable pair with backpressure. The writer pushes data in; the readable is consumed as an async iterable.

#### Backpressure#

Pull streams have natural backpressure -- the consumer drives the pace, so the source is never read faster than the consumer can process. Push streams need explicit backpressure because the producer and consumer run independently. The `budget` and `backpressure` options on `push()`, `broadcast()`, and `share()` control how this works.

##### The two-buffer model#

Push streams use a two-part buffering system. Think of it like a bucket (buffer) being filled through a hose (pending writes), with a float valve that closes when the bucket is full:

                              budget (e.g., 16384)
                                     |
        Producer                     v
           |                    +---------+
           v                    |         |
      [ write() ] ----+    +--->| buffer  |---> Consumer pulls
      [ write() ]     |    |    | (bucket)|     for await (...)
      [ write() ]     v    |    +---------+
                  +--------+         ^
                  | pending|         |
                  | writes |    float valve
                  | (hose) |    (backpressure)
                  +--------+
                       ^
                       |
              'strict' mode limits this too!
    text

- Buffer (the bucket) -- data ready for the consumer, capped at `budget` bytes. When the consumer pulls, it drains all buffered data at once into a single batch.

- Pending writes (the hose) -- writes waiting for buffer space. After the consumer drains, pending writes are promoted into the now-empty buffer and their promises settle.

How each policy uses these buffers:

Policy | Buffer limit | Pending writes limit
--- | --- | ---
`'strict'` | `budget` | 1
`'unbounded'` | `budget` | Unbounded
`'drop-oldest'` | `budget` | N/A (never waits)
`'drop-newest'` | `budget` | N/A (never waits)

##### Strict (default)#

Strict mode catches "fire-and-forget" patterns where the producer calls `write()` without awaiting, which would cause unbounded memory growth. It limits the buffer to `budget` bytes and the pending writes queue to a single entry.

If you properly await each write, you can only ever have one pending write at a time (yours), so you never hit the pending writes limit. Unawaited writes accumulate in the pending queue and throw once it overflows:

    import { push, text } from 'node:stream/iter';

    const { writer, readable } = push({ budget: 16384 });

    // Consumer must run concurrently -- without it, the first write
    // that fills the buffer blocks the producer forever.
    const consuming = text(readable);

    // GOOD: awaited writes. The producer waits for the consumer to
    // make room when the buffer is full.
    for (const item of dataset) {
      await writer.write(item);
    }
    await writer.end();
    console.log(await consuming);
    const { push, text } = require('node:stream/iter');

    async function run() {
      const { writer, readable } = push({ budget: 16384 });

      // Consumer must run concurrently -- without it, the first write
      // that fills the buffer blocks the producer forever.
      const consuming = text(readable);

      // GOOD: awaited writes. The producer waits for the consumer to
      // make room when the buffer is full.
      for (const item of dataset) {
        await writer.write(item);
      }
      await writer.end();
      console.log(await consuming);
    }

    run().catch(console.error);
    javascript

Forgetting to `await` will eventually throw:

    // BAD: fire-and-forget. Strict mode throws once both buffers fill.
    for (const item of dataset) {
      writer.write(item); // Not awaited -- queues without bound
    }
    // --> throws "Backpressure violation: too many pending writes"
    js

##### Unbounded#

Unbounded mode caps buffered bytes at `budget` but places no limit on the pending writes queue. Awaited writes block until the consumer makes room, just like strict mode. The difference is that unawaited writes silently queue forever instead of throwing -- a potential memory leak if the producer forgets to `await`.

This is the mode that existing Node.js classic streams and Web Streams default to. Use it when you control the producer and know it awaits properly, or when migrating code from those APIs.

    import { push, text } from 'node:stream/iter';

    const { writer, readable } = push({
      budget: 16384,
      backpressure: 'unbounded',
    });

    const consuming = text(readable);

    // Safe -- awaited writes block until the consumer reads.
    for (const item of dataset) {
      await writer.write(item);
    }
    await writer.end();
    console.log(await consuming);
    const { push, text } = require('node:stream/iter');

    async function run() {
      const { writer, readable } = push({
        budget: 16384,
        backpressure: 'unbounded',
      });

      const consuming = text(readable);

      // Safe -- awaited writes block until the consumer reads.
      for (const item of dataset) {
        await writer.write(item);
      }
      await writer.end();
      console.log(await consuming);
    }

    run().catch(console.error);
    javascript

##### Drop-oldest#

Writes never wait. When the slots buffer is full, the oldest buffered chunk is evicted to make room for the incoming write. The consumer always sees the most recent data. Useful for live feeds, telemetry, or any scenario where stale data is less valuable than current data.

    import { push } from 'node:stream/iter';

    // Keep only the most recent ~16 KB of readings
    const { writer, readable } = push({
      budget: 16384,
      backpressure: 'drop-oldest',
    });
    const { push } = require('node:stream/iter');

    // Keep only the most recent ~16 KB of readings
    const { writer, readable } = push({
      budget: 16384,
      backpressure: 'drop-oldest',
    });
    javascript

##### Drop-newest#

Writes never wait. When the slots buffer is full, the incoming write is silently discarded. The consumer processes what is already buffered without being overwhelmed by new data. Useful for rate-limiting or shedding load under pressure.

    import { push } from 'node:stream/iter';

    // Accept up to 16 KB of buffered data; discard anything beyond that
    const { writer, readable } = push({
      budget: 16384,
      backpressure: 'drop-newest',
    });
    const { push } = require('node:stream/iter');

    // Accept up to 16 KB of buffered data; discard anything beyond that
    const { writer, readable } = push({
      budget: 16384,
      backpressure: 'drop-newest',
    });
    javascript

#### Writer interface#

A writer is any object conforming to the Writer interface. Only `write()` is required; all other methods are optional.

Each async method has a synchronous `*Sync` counterpart designed for a try-fallback pattern: attempt the fast synchronous path first, and fall back to the async version only when the synchronous call indicates it could not complete:

    if (!writer.writeSync(chunk)) await writer.write(chunk);
    if (!writer.writevSync(chunks)) await writer.writev(chunks);
    if (writer.endSync() < 0) await writer.end();
    writer.fail(err);  // Always synchronous, no fallback needed
    mjs

##### `writer.canWrite`#

- `<boolean>` | `<null>`

Returns `true` if the next write is likely to be accepted (buffered data is below capacity), `false` if backpressure is active, or `null` if the writer is closed or the consumer has disconnected.

This is a hint, not a guarantee: the state can change between the check and the write. Use `ondrain()` to wait for capacity rather than polling.

##### `writer.end([options])`#

- `options` `<Object>`
  - `signal` `<AbortSignal>` Cancel just this operation. The signal cancels only the pending `end()` call; it does not fail the writer itself.
- Returns: `<Promise>` Fulfills with the total number of bytes written.

Signal that no more data will be written.

##### `writer.endSync()`#

- Returns: `<number>` Total bytes written, or `-1` if the writer is not open.

Synchronous variant of `writer.end()`. Returns `-1` if the writer is already closed or errored. Can be used as a try-fallback pattern:

    const result = writer.endSync();
    if (result < 0) {
      writer.end();
    }
    cjs

##### `writer.fail(reason)`#

- `reason` `<any>`

Put the writer into a terminal error state. If the writer is already closed or errored, this is a no-op. Unlike `write()` and `end()`, `fail()` is unconditionally synchronous because failing a writer is a pure state transition with no async work to perform.

##### `writer.write(chunk[, options])`#

- `chunk` `<Uint8Array>` | `<string>`
- `options` `<Object>`
  - `signal` `<AbortSignal>` Cancel just this write operation. The signal cancels only the pending `write()` call; it does not fail the writer itself.
- Returns: `<Promise>` Fulfills with `undefined` when buffer space is available.

Write a chunk.

##### `writer.writeSync(chunk)`#

- `chunk` `<Uint8Array>` | `<string>`
- Returns: `<boolean>` `true` if the write was accepted, `false` if the buffer is full.

Synchronous write. Does not block; returns `false` if backpressure is active.

##### `writer.writev(chunks[, options])`#

- `chunks` `<Uint8Array>`[] | `<string>`[]
- `options` `<Object>`
  - `signal` `<AbortSignal>` Cancel just this write operation. The signal cancels only the pending `writev()` call; it does not fail the writer itself.
- Returns: `<Promise>`

Write multiple chunks as a single batch.

##### `writer.writevSync(chunks)`#

- `chunks` `<Uint8Array>`[] | `<string>`[]
- Returns: `<boolean>` `true` if the write was accepted, `false` if the buffer is full.

Synchronous batch write.

### The `stream/iter` module#

All functions are available both as named exports and as properties of the `Stream` namespace object:

    // Named exports
    import { from, pull, bytes, Stream } from 'node:stream/iter';

    // Namespace access
    Stream.from('hello');
    // Named exports
    const { from, pull, bytes, Stream } = require('node:stream/iter');

    // Namespace access
    Stream.from('hello');
    javascript

Including the `node:` prefix on the module specifier is optional.

### Sources#

#### `from(input)`#

Added in: v25.9.0

- `input` `<string>` | `<ArrayBuffer>` | `<ArrayBufferView>` | `<Iterable>` | `<AsyncIterable>` | `<Object>` Must not be `null` or `undefined`.
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[].

Create an async byte stream from the given input. Strings are UTF-8 encoded. `ArrayBuffer` and `ArrayBufferView` values are wrapped as `Uint8Array`. Arrays and iterables in `input` are recursively flattened and normalized.

Objects implementing `Symbol.for('Stream.toAsyncStreamable')` or `Symbol.for('Stream.toStreamable')` are converted via those protocols. The `toAsyncStreamable` protocol takes precedence over `toStreamable`, which takes precedence over the iteration protocols (`Symbol.asyncIterator`, `Symbol.iterator`).

    import { Buffer } from 'node:buffer';
    import { from, text } from 'node:stream/iter';

    console.log(await text(from('hello')));       // 'hello'
    console.log(await text(from(Buffer.from('hello')))); // 'hello'
    const { Buffer } = require('node:buffer');
    const { from, text } = require('node:stream/iter');

    async function run() {
      console.log(await text(from('hello')));       // 'hello'
      console.log(await text(from(Buffer.from('hello')))); // 'hello'
    }

    run().catch(console.error);
    javascript

#### `fromSync(input)`#

Added in: v25.9.0

- `input` `<string>` | `<ArrayBuffer>` | `<ArrayBufferView>` | `<Iterable>` | `<Object>` Must not be `null` or `undefined`.
- Returns: `<Iterable>` whose chunks return `<Uint8Array>`[]

Synchronous version of `from()`. Returns a sync iterable. Cannot accept async iterables or promises. Objects implementing `Symbol.for('Stream.toStreamable')` are converted via that protocol (takes precedence over `Symbol.iterator`). The `toAsyncStreamable` protocol is ignored entirely.

    import { fromSync, textSync } from 'node:stream/iter';

    console.log(textSync(fromSync('hello'))); // 'hello'
    const { fromSync, textSync } = require('node:stream/iter');

    console.log(textSync(fromSync('hello'))); // 'hello'
    javascript

### Pipelines#

#### `pipeTo(source[, ...transforms], writer[, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` | `<Iterable>` The data source.
- `...transforms` `<Function>` | `<Object>` Zero or more transforms to apply.
- `writer` `<Object>` Destination with `write(chunk)` method.
- `options` `<Object>`
  - `signal` `<AbortSignal>` Abort the pipeline.
  - `preventClose` `<boolean>` If `true`, do not call `writer.end()` when the source ends. Default: `false`.
  - `preventFail` `<boolean>` If `true`, do not call `writer.fail()` on error. Default: `false`.
- Returns: `<Promise>` Fulfills with the total number of bytes written.

Pipe a source through transforms into a writer. If the writer has a `writev(chunks)` method, entire batches are passed in a single call (enabling scatter/gather I/O).

If the writer implements the optional `*Sync` methods (`writeSync`, `writevSync`, `endSync`), `pipeTo()` will attempt to use the synchronous methods first as a fast path, and fall back to the async versions only when the sync methods indicate they cannot complete (e.g., backpressure or waiting for the next tick). `fail()` is always called synchronously.

    import { from, pipeTo } from 'node:stream/iter';
    import { compressGzip } from 'node:zlib/iter';
    import { open } from 'node:fs/promises';

    const fh = await open('output.gz', 'w');
    const totalBytes = await pipeTo(
      from('Hello, world!'),
      compressGzip(),
      fh.writer({ autoClose: true }),
    );
    const { from, pipeTo } = require('node:stream/iter');
    const { compressGzip } = require('node:zlib/iter');
    const { open } = require('node:fs/promises');

    async function run() {
      const fh = await open('output.gz', 'w');
      const totalBytes = await pipeTo(
        from('Hello, world!'),
        compressGzip(),
        fh.writer({ autoClose: true }),
      );
    }

    run().catch(console.error);
    javascript

#### `pipeToSync(source[, ...transforms], writer[, options])`#

Added in: v25.9.0

- `source` `<Iterable>` The sync data source.
- `...transforms` `<Function>` | `<Object>` Zero or more sync transforms.
- `writer` `<Object>` Destination with `write(chunk)` method.
- `options` `<Object>`
  - `preventClose` `<boolean>` Default: `false`.
  - `preventFail` `<boolean>` Default: `false`.
- Returns: `<number>` Total bytes written.

Synchronous version of `pipeTo()`. The `source`, all transforms, and the `writer` must be synchronous. Cannot accept async iterables or promises.

The `writer` must have the `*Sync` methods (`writeSync`, `writevSync`, `endSync`) and `fail()` for this to work.

#### `pull(source[, ...transforms][, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` | `<Iterable>` The data source.
- `...transforms` `<Function>` | `<Object>` Zero or more transforms to apply.
- `options` `<Object>`
  - `signal` `<AbortSignal>` Abort the pipeline.
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Create a lazy async pipeline. Data is not read from `source` until the returned iterable is consumed. Transforms are applied in order.

    import { from, pull, text } from 'node:stream/iter';

    const asciiUpper = (chunks) => {
      if (chunks === null) return null;
      return chunks.map((c) => {
        for (let i = 0; i < c.length; i++) {
          c[i] -= (c[i] >= 97 && c[i] <= 122) * 32;
        }
        return c;
      });
    };

    const result = pull(from('hello'), asciiUpper);
    console.log(await text(result)); // 'HELLO'
    const { from, pull, text } = require('node:stream/iter');

    const asciiUpper = (chunks) => {
      if (chunks === null) return null;
      return chunks.map((c) => {
        for (let i = 0; i < c.length; i++) {
          c[i] -= (c[i] >= 97 && c[i] <= 122) * 32;
        }
        return c;
      });
    };

    async function run() {
      const result = pull(from('hello'), asciiUpper);
      console.log(await text(result)); // 'HELLO'
    }

    run().catch(console.error);
    javascript

Using an `AbortSignal`:

    import { pull } from 'node:stream/iter';

    const ac = new AbortController();
    const result = pull(source, transform, { signal: ac.signal });
    ac.abort(); // Pipeline throws AbortError on next iteration
    const { pull } = require('node:stream/iter');

    const ac = new AbortController();
    const result = pull(source, transform, { signal: ac.signal });
    ac.abort(); // Pipeline throws AbortError on next iteration
    javascript

#### `pullSync(source[, ...transforms])`#

Added in: v25.9.0

- `source` `<Iterable>` The sync data source.
- `...transforms` `<Function>` | `<Object>` Zero or more sync transforms.
- Returns: `<Iterable>` whose chunks return `<Uint8Array>`[]

Synchronous version of `pull()`. All transforms must be synchronous.

### Push streams#

#### `push([...transforms][, options])`#

Added in: v25.9.0

- `...transforms` `<Function>` | `<Object>` Optional transforms applied to the readable side.
- `options` `<Object>`
  - `budget` `<number>` Maximum number of buffered bytes before backpressure is applied. Must be >= 16384. Default: `16384`.
  - `backpressure` `<string>` Backpressure policy: `'strict'`, `'unbounded'`, `'drop-oldest'`, or `'drop-newest'`. Default: `'strict'`.
  - `signal` `<AbortSignal>` Abort the stream.
- Returns: `<Object>`
  - `writer` `<Writable>` The writer side.
  - `readable` `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Create a push stream with backpressure. The writer pushes data in; the readable side is consumed as an async iterable.

    import { push, text } from 'node:stream/iter';

    const { writer, readable } = push();

    // Producer and consumer must run concurrently. With strict backpressure
    // (the default), awaited writes block until the consumer reads.
    const producing = (async () => {
      await writer.write('hello');
      await writer.write(' world');
      await writer.end();
    })();

    console.log(await text(readable)); // 'hello world'
    await producing;
    const { push, text } = require('node:stream/iter');

    async function run() {
      const { writer, readable } = push();

      // Producer and consumer must run concurrently. With strict backpressure
      // (the default), awaited writes block until the consumer reads.
      const producing = (async () => {
        await writer.write('hello');
        await writer.write(' world');
        await writer.end();
      })();

      console.log(await text(readable)); // 'hello world'
      await producing;
    }

    run().catch(console.error);
    javascript

The writer returned by `push()` conforms to the [Writer interface][].

### Duplex channels#

#### `duplex([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `budget` `<number>` Buffer size in bytes for both directions. Default: `16384`.
  - `backpressure` `<string>` Policy for both directions. Default: `'strict'`.
  - `signal` `<AbortSignal>` Cancellation signal for both channels.
  - `a` `<Object>` Options specific to the A-to-B direction. Overrides shared options.
    - `budget` `<number>`
    - `backpressure` `<string>`
  - `b` `<Object>` Options specific to the B-to-A direction. Overrides shared options.
    - `budget` `<number>`
    - `backpressure` `<string>`
- Returns: `<Array>` A pair `[channelA, channelB]` of duplex channels.

Create a pair of connected duplex channels for bidirectional communication, similar to `socketpair()`. Data written to one channel's writer appears in the other channel's readable.

Each channel has:

- `writer` — a [Writer interface][] object for sending data to the peer.
- `readable` — an `<AsyncIterable>` for reading data from the peer.
- `close()` — close this end of the channel (idempotent).
- `[Symbol.asyncDispose]()` — async dispose support for `await using`.

    import { duplex, text } from 'node:stream/iter';

    const [client, server] = duplex();

    // Server echoes back
    const serving = (async () => {
      for await (const chunks of server.readable) {
        await server.writer.writev(chunks);
      }
    })();

    await client.writer.write('hello');
    await client.writer.end();

    console.log(await text(server.readable)); // handled by echo
    await serving;
    const { duplex, text } = require('node:stream/iter');

    async function run() {
      const [client, server] = duplex();

      // Server echoes back
      const serving = (async () => {
        for await (const chunks of server.readable) {
          await server.writer.writev(chunks);
        }
      })();

      await client.writer.write('hello');
      await client.writer.end();

      console.log(await text(server.readable)); // handled by echo
      await serving;
    }

    run().catch(console.error);
    javascript

### Consumers#

#### `array(source[, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` | `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `signal` `<AbortSignal>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<Promise>` Fulfills with an array of `Uint8Array` objects.

Collect all chunks as an array of `Uint8Array` values (without concatenating).

#### `arrayBuffer(source[, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` | `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `signal` `<AbortSignal>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<Promise>` Fulfills with an `ArrayBuffer` object.

Collect all bytes into an `ArrayBuffer`.

#### `arrayBufferSync(source[, options])`#

Added in: v25.9.0

- `source` `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<ArrayBuffer>`

Synchronous version of `arrayBuffer()`.

#### `arraySync(source[, options])`#

Added in: v25.9.0

- `source` `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<Uint8Array>`[]

Synchronous version of `array()`.

#### `bytes(source[, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` | `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `signal` `<AbortSignal>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<Promise>` Fulfills with an `Uint8Array` object.

Collect all bytes from a stream into a single `Uint8Array`.

    import { from, bytes } from 'node:stream/iter';

    const data = await bytes(from('hello'));
    console.log(data); // Uint8Array(5) [ 104, 101, 108, 108, 111 ]
    const { from, bytes } = require('node:stream/iter');

    async function run() {
      const data = await bytes(from('hello'));
      console.log(data); // Uint8Array(5) [ 104, 101, 108, 108, 111 ]
    }

    run().catch(console.error);
    javascript

#### `bytesSync(source[, options])`#

Added in: v25.9.0

- `source` `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<Uint8Array>`

Synchronous version of `bytes()`.

#### `text(source[, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` | `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `encoding` `<string>` Text encoding. Default: `'utf-8'`.
  - `signal` `<AbortSignal>`
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<Promise>` Fulfills with a `string`.

Collect all bytes and decode as text.

    import { from, text } from 'node:stream/iter';

    console.log(await text(from('hello'))); // 'hello'
    const { from, text } = require('node:stream/iter');

    async function run() {
      console.log(await text(from('hello'))); // 'hello'
    }

    run().catch(console.error);
    javascript

#### `textSync(source[, options])`#

Added in: v25.9.0

- `source` `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `encoding` `<string>` Default: `'utf-8'`.
  - `limit` `<number>` Maximum number of bytes to consume. If the total bytes collected exceeds limit, an `ERR_OUT_OF_RANGE` error is thrown
- Returns: `<string>`

Synchronous version of `text()`.

### Utilities#

#### `ondrain(drainable)`#

Added in: v25.9.0

- `drainable` `<Object>` An object implementing the drainable protocol.
- Returns: `<Promise>` | `<null>`

Wait for a drainable writer's backpressure to clear. Returns `null` if the object does not implement the drainable protocol, or a promise that fulfills with `true` when the writer can accept more data.

    import { push, ondrain, text } from 'node:stream/iter';

    const { writer, readable } = push({ budget: 16384 });
    const chunk = new Uint8Array(8192);  // 8 KB
    writer.writeSync(chunk);
    writer.writeSync(chunk);  // 16 KB total -- buffer full

    // Start consuming so the buffer can actually drain
    const consuming = text(readable);

    // Buffer is full -- wait for drain
    const canWrite = await ondrain(writer);
    if (canWrite) {
      await writer.write('c');
    }
    await writer.end();
    await consuming;
    const { push, ondrain, text } = require('node:stream/iter');

    async function run() {
      const { writer, readable } = push({ budget: 16384 });
      const chunk = new Uint8Array(8192);  // 8 KB
      writer.writeSync(chunk);
      writer.writeSync(chunk);  // 16 KB total -- buffer full

      // Start consuming so the buffer can actually drain
      const consuming = text(readable);

      // Buffer is full -- wait for drain
      const canWrite = await ondrain(writer);
      if (canWrite) {
        await writer.write('c');
      }
      await writer.end();
      await consuming;
    }

    run().catch(console.error);
    javascript

#### `merge(...sources[, options])`#

Added in: v25.9.0

- `...sources` `<AsyncIterable>` | `<Iterable>` whose chunks must be `<Uint8Array>`[]
- `options` `<Object>`
  - `signal` `<AbortSignal>`
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Merge multiple async iterables by yielding batches in temporal order (whichever source produces data first). All sources are consumed concurrently.

    import { from, merge, text } from 'node:stream/iter';

    const merged = merge(from('hello '), from('world'));
    console.log(await text(merged)); // Order depends on timing
    const { from, merge, text } = require('node:stream/iter');

    async function run() {
      const merged = merge(from('hello '), from('world'));
      console.log(await text(merged)); // Order depends on timing
    }

    run().catch(console.error);
    javascript

#### `tap(callback)`#

Added in: v25.9.0

- `callback` `<Function>` `(chunks) => void` Called with each batch.
- Returns: `<Function>` A stateless transform.

Create a pass-through transform that observes batches without modifying them. Useful for logging, metrics, or debugging.

    import { from, pull, text, tap } from 'node:stream/iter';

    const result = pull(
      from('hello'),
      tap((chunks) => console.log('Batch size:', chunks.length)),
    );
    console.log(await text(result));
    const { from, pull, text, tap } = require('node:stream/iter');

    async function run() {
      const result = pull(
        from('hello'),
        tap((chunks) => console.log('Batch size:', chunks.length)),
      );
      console.log(await text(result));
    }

    run().catch(console.error);
    javascript

`tap()` intentionally does not prevent in-place modification of the chunks by the tapping callback; but return values are ignored.

#### `tapSync(callback)`#

Added in: v25.9.0

- `callback` `<Function>`
- Returns: `<Function>`

Synchronous version of `tap()`.

### Multi-consumer#

#### `broadcast([options])`#

Added in: v25.9.0

- `options` `<Object>`
  - `budget` `<number>` Buffer size in bytes. Must be >= 16384. Default: `65536`.
  - `backpressure` `<string>` `'strict'`, `'unbounded'`, `'drop-oldest'`, or `'drop-newest'`. Default: `'strict'`.
  - `signal` `<AbortSignal>`
- Returns: `<Object>`
  - `writer` `<Writable>`
  - `broadcast` `<BroadcastChannel>`

Create a push-model multi-consumer broadcast channel. A single writer pushes data to multiple consumers. Each consumer has an independent cursor into a shared buffer.

    import { broadcast, text } from 'node:stream/iter';

    const { writer, broadcast: bc } = broadcast();

    // Create consumers before writing
    const c1 = bc.push();  // Consumer 1
    const c2 = bc.push();  // Consumer 2

    // Producer and consumers must run concurrently. Awaited writes
    // block when the buffer fills until consumers read.
    const producing = (async () => {
      await writer.write('hello');
      await writer.end();
    })();

    const [r1, r2] = await Promise.all([text(c1), text(c2)]);
    console.log(r1); // 'hello'
    console.log(r2); // 'hello'
    await producing;
    const { broadcast, text } = require('node:stream/iter');

    async function run() {
      const { writer, broadcast: bc } = broadcast();

      // Create consumers before writing
      const c1 = bc.push();  // Consumer 1
      const c2 = bc.push();  // Consumer 2

      // Producer and consumers must run concurrently. Awaited writes
      // block when the buffer fills until consumers read.
      const producing = (async () => {
        await writer.write('hello');
        await writer.end();
      })();

      const [r1, r2] = await Promise.all([text(c1), text(c2)]);
      console.log(r1); // 'hello'
      console.log(r2); // 'hello'
      await producing;
    }

    run().catch(console.error);
    javascript

##### `broadcast.cancel([reason])`#

- `reason` `<Error>`

Cancel the broadcast. All consumers receive an error.

##### `broadcast.consumerCount`#

- `<number>`

The number of active consumers.

##### `broadcast.push([...transforms][, options])`#

- `...transforms` `<Function>` | `<Object>`
- `options` `<Object>`
  - `signal` `<AbortSignal>`
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Create a new consumer. Each consumer receives all data written to the broadcast from the point of subscription onward. Optional transforms are applied to this consumer's view of the data.

##### `broadcast[Symbol.dispose]()`#

Alias for `broadcast.cancel()`.

#### `Broadcast.from(input[, options])`#

Added in: v25.9.0

- `input` `<AsyncIterable>` | `<Iterable>` | `<BroadcastChannel>`
- `options` `<Object>` Same as `broadcast()`.
- Returns: `<Object>` `{ writer, broadcast }`

Create a `<BroadcastChannel>` from an existing source. The source is consumed automatically and pushed to all subscribers.

#### `share(source[, options])`#

Added in: v25.9.0

- `source` `<AsyncIterable>` The source to share.
- `options` `<Object>`
  - `budget` `<number>` Buffer size in bytes. Must be >= 16384. Default: `65536`.
  - `backpressure` `<string>` `'strict'`, `'unbounded'`, `'drop-oldest'`, or `'drop-newest'`. Default: `'strict'`.
- Returns: `<Share>`

Create a pull-model multi-consumer shared stream. Unlike `broadcast()`, the source is only read when a consumer pulls. Multiple consumers share a single buffer.

    import { from, share, text } from 'node:stream/iter';

    const shared = share(from('hello'));

    const c1 = shared.pull();
    const c2 = shared.pull();

    // Consume concurrently to avoid deadlock with small buffers.
    const [r1, r2] = await Promise.all([text(c1), text(c2)]);
    console.log(r1); // 'hello'
    console.log(r2); // 'hello'
    const { from, share, text } = require('node:stream/iter');

    async function run() {
      const shared = share(from('hello'));

      const c1 = shared.pull();
      const c2 = shared.pull();

      // Consume concurrently to avoid deadlock with small buffers.
      const [r1, r2] = await Promise.all([text(c1), text(c2)]);
      console.log(r1); // 'hello'
      console.log(r2); // 'hello'
    }

    run().catch(console.error);
    javascript

#### Class: `Share`#

##### Static method: `Share.from(input[, options])`#

Added in: v25.9.0

- `input` `<AsyncIterable>` | `<Shareable>`
- `options` `<Object>` Same as `share()`.
- Returns: `<Share>`

Create a `<Share>` from an existing source.

##### `share.cancel([reason])`#

- `reason` `<Error>`

Cancel the share. All consumers receive an error.

##### `share.consumerCount`#

- `<number>`

The number of active consumers.

##### `share.pull([...transforms][, options])`#

- `...transforms` `<Function>` | `<Object>`
- `options` `<Object>`
  - `signal` `<AbortSignal>`
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Create a new consumer of the shared source.

##### `share[Symbol.dispose]()`#

Alias for `share.cancel()`.

#### Interface: `Shareable`#

##### `sharable[Symbol.for('Stream.shareProtocol')]`#

- `<Function>` that returns a `<Share>`.

#### Interface: `SyncShareable`#

##### `sharable[Symbol.for('Stream.shareSyncProtocol')]`#

- `<Function>` that returns a `<SyncShare>`.

#### `shareSync(source[, options])`#

Added in: v25.9.0

- `source` `<Iterable>` The sync source to share.
- `options` `<Object>`
  - `budget` `<number>` Must be >= 16384. Default: `65536`.
  - `backpressure` `<string>` Default: `'strict'`.
- Returns: `<SyncShare>`

Synchronous version of `share()`.

#### Class: `SyncShare`#

##### Static method: `SyncShare.fromSync(input[, options])`#

Added in: v25.9.0

- `input` `<Iterable>` | `<SyncShareable>`
- `options` `<Object>`
- Returns: `<SyncShare>`

##### `share.bufferSize`#

- `<number>`

The number of chunks currently buffered.

##### `share.cancel([reason])`#

- `reason` `<Error>`

Cancel the share. All consumers receive an error.

##### `share.consumerCount`#

- `<number>`

The number of active consumers.

##### `share.pull([...transforms][, options])`#

- `...transforms` `<Function>` | `<Object>`
- `options` `<Object>`
  - `signal` `<AbortSignal>`
- Returns: `<Iterable>` whose chunks return `<Uint8Array>`[]

Create a new consumer of the shared source.

##### `share[Symbol.dispose]()`#

Alias for `share.cancel()`.

### Compression and decompression transforms#

Compression and decompression transforms for use with `pull()`, `pullSync()`, `pipeTo()`, and `pipeToSync()` are available via the `node:zlib/iter` module. See the `node:zlib/iter` documentation for details.

### Classic stream interop#

These utility functions bridge between classic `stream.Readable`/`stream.Writable` streams and the `stream/iter` API.

Both `fromReadable()` and `fromWritable()` accept duck-typed objects -- they do not require the input to extend `stream.Readable` or `stream.Writable` directly. The minimum contract is described below for each function.

#### `fromReadable(readable)`#

Added in: v26.1.0

Stability: 1 - Experimental

- `readable` `<stream.Readable>` | `<Object>` A classic Readable stream or any object with `read()`, `on()`, and `off()` methods.
- Returns: `<AsyncIterable>` whose chunks fulfill with `<Uint8Array>`[]

Converts a classic Readable stream (or duck-typed equivalent) into a stream/iter async iterable source that can be passed to `from()`, `pull()`, `text()`, etc.

If the object implements the `toAsyncStreamable` protocol (as `stream.Readable` does), that protocol is used. Otherwise, the function duck-types on `read()`, `on()`, and `off()` (EventEmitter) and wraps the stream with a batched async iterator.

The result is cached per instance -- calling `fromReadable()` twice with the same stream returns the same iterable.

For object-mode or encoded Readable streams, chunks are automatically normalized to `Uint8Array`.

    import { Readable } from 'node:stream';
    import { fromReadable, text } from 'node:stream/iter';

    const readable = new Readable({
      read() { this.push('hello world'); this.push(null); },
    });

    const result = await text(fromReadable(readable));
    console.log(result); // 'hello world'
    const { Readable } = require('node:stream');
    const { fromReadable, text } = require('node:stream/iter');

    const readable = new Readable({
      read() { this.push('hello world'); this.push(null); },
    });

    async function run() {
      const result = await text(fromReadable(readable));
      console.log(result); // 'hello world'
    }
    run();
    javascript

#### `fromWritable(writable[, options])`#

Added in: v26.1.0

Stability: 1 - Experimental

- `writable` `<stream.Writable>` | `<Object>` A classic Writable stream or any object with `write()` and `on()` methods.
- `options` `<Object>`
  - `backpressure` `<string>` Backpressure policy. Default: `'strict'`.
    - `'strict'` -- writes are rejected when the buffer is full. Catches callers that ignore backpressure.
    - `'unbounded'` -- writes wait for drain when the buffer is full. Recommended for use with `pipeTo()`.
    - `'drop-newest'` -- writes are silently discarded when the buffer is full.
    - `'drop-oldest'` -- not supported. Throws `ERR_INVALID_ARG_VALUE`.
- Returns: `<Object>` A stream/iter Writer adapter.

Creates a stream/iter Writer adapter from a classic Writable stream (or duck-typed equivalent). The adapter can be passed to `pipeTo()` as a destination.

Since all writes on a classic Writable are fundamentally asynchronous, the synchronous Writer methods (`writeSync`, `writevSync`, `endSync`) always return `false` or `-1`, deferring to the async path. The per-write `options.signal` parameter from the Writer interface is also ignored.

The result is cached per instance and backpressure policy -- calling `fromWritable()` twice with the same stream and `backpressure` option returns the same Writer.

For duck-typed streams that do not expose `writableHighWaterMark`, `writableLength`, or similar properties, sensible defaults are used. Object-mode writables (if detectable) are rejected since the Writer interface is bytes-only.

    import { Writable } from 'node:stream';
    import { from, fromWritable, pipeTo } from 'node:stream/iter';

    const writable = new Writable({
      write(chunk, encoding, cb) { console.log(chunk.toString()); cb(); },
    });

    await pipeTo(from('hello world'),
                 fromWritable(writable, { backpressure: 'unbounded' }));
    const { Writable } = require('node:stream');
    const { from, fromWritable, pipeTo } = require('node:stream/iter');

    async function run() {
      const writable = new Writable({
        write(chunk, encoding, cb) { console.log(chunk.toString()); cb(); },
      });

      await pipeTo(from('hello world'),
                   fromWritable(writable, { backpressure: 'unbounded' }));
    }
    run();
    javascript

#### `toReadable(source[, options])`#

Added in: v26.1.0

Stability: 1 - Experimental

- `source` `<AsyncIterable>` whose chunks must fulfill with `<Uint8Array>`[] the return value of `pull()` or `from()`.
- `options` `<Object>`
  - `highWaterMark` `<number>` The internal buffer size in bytes before backpressure is applied. Default: `65536` (64 KB).
  - `signal` `<AbortSignal>` An optional signal to abort the readable.
- Returns: `<stream.Readable>`

Creates a byte-mode `stream.Readable` from the `source` (the native batch format used by the stream/iter API). Each `Uint8Array` in a yielded batch is pushed as a separate chunk into the Readable.

    import { createWriteStream } from 'node:fs';
    import { from, pull, toReadable } from 'node:stream/iter';
    import { compressGzip } from 'node:zlib/iter';

    const source = pull(from('hello world'), compressGzip());
    const readable = toReadable(source);

    readable.pipe(createWriteStream('output.gz'));
    const { createWriteStream } = require('node:fs');
    const { from, pull, toReadable } = require('node:stream/iter');
    const { compressGzip } = require('node:zlib/iter');

    const source = pull(from('hello world'), compressGzip());
    const readable = toReadable(source);

    readable.pipe(createWriteStream('output.gz'));
    javascript

#### `toReadableSync(source[, options])`#

Added in: v26.1.0

Stability: 1 - Experimental

- `source` `<Iterable>` whose chunks must return `<Uint8Array>`[], such as the return value of `pullSync()` or `fromSync()`.
- `options` `<Object>`
  - `highWaterMark` `<number>` The internal buffer size in bytes before backpressure is applied. Default: `65536` (64 KB).
- Returns: `<stream.Readable>`

Creates a byte-mode `stream.Readable` from the `source`. The `_read()` method pulls from the iterator synchronously, so data is available immediately via `readable.read()`.

    import { fromSync, toReadableSync } from 'node:stream/iter';

    const source = fromSync('hello world');
    const readable = toReadableSync(source);

    console.log(readable.read().toString()); // 'hello world'
    const { fromSync, toReadableSync } = require('node:stream/iter');

    const source = fromSync('hello world');
    const readable = toReadableSync(source);

    console.log(readable.read().toString()); // 'hello world'
    javascript

#### `toWritable(writer)`#

Added in: v26.1.0

Stability: 1 - Experimental

- `writer` `<Object>` A stream/iter Writer. Only the `write()` method is required; `end()`, `fail()`, `writeSync()`, `writevSync()`, `endSync()`, and `writev()` are optional.
- Returns: `<stream.Writable>`

Creates a classic `stream.Writable` backed by a stream/iter Writer.

Each `_write()` / `_writev()` call attempts the Writer's synchronous method first (`writeSync` / `writevSync`), falling back to the async method if the sync path returns `false`. Similarly, `_final()` tries `endSync()` before `end()`. When the sync path succeeds, the callback is deferred via `queueMicrotask` to preserve the async resolution contract.

The Writable's `highWaterMark` is set to `Number.MAX_SAFE_INTEGER` to effectively disable its internal buffering, allowing the underlying Writer to manage backpressure directly.

    import { push, toWritable } from 'node:stream/iter';

    const { writer, readable } = push();
    const writable = toWritable(writer);

    writable.write('hello');
    writable.end();
    const { push, toWritable } = require('node:stream/iter');

    const { writer, readable } = push();
    const writable = toWritable(writer);

    writable.write('hello');
    writable.end();
    javascript

### Protocol symbols#

These well-known symbols allow third-party objects to participate in the streaming protocol without importing from `node:stream/iter` directly.

#### `Stream.broadcastProtocol`#

- Value: `Symbol.for('Stream.broadcastProtocol')`

The value must be a function. When called by `Broadcast.from()`, it receives the options passed to `Broadcast.from()` and must return an object conforming to the `<BroadcastChannel>` interface. The implementation is fully custom -- it can manage consumers, buffering, and backpressure however it wants.

    import { Broadcast, text } from 'node:stream/iter';

    // This example defers to the built-in Broadcast, but a custom
    // implementation could use any mechanism.
    class MessageBus {
      #broadcast;
      #writer;

      constructor() {
        const { writer, broadcast } = Broadcast();
        this.#writer = writer;
        this.#broadcast = broadcast;
      }

      [Symbol.for('Stream.broadcastProtocol')](options) {
        return this.#broadcast;
      }

      send(data) {
        this.#writer.write(new TextEncoder().encode(data));
      }

      close() {
        this.#writer.end();
      }
    }

    const bus = new MessageBus();
    const { broadcast } = Broadcast.from(bus);
    const consumer = broadcast.push();
    bus.send('hello');
    bus.close();
    console.log(await text(consumer)); // 'hello'
    const { Broadcast, text } = require('node:stream/iter');

    // This example defers to the built-in Broadcast, but a custom
    // implementation could use any mechanism.
    class MessageBus {
      #broadcast;
      #writer;

      constructor() {
        const { writer, broadcast } = Broadcast();
        this.#writer = writer;
        this.#broadcast = broadcast;
      }

      [Symbol.for('Stream.broadcastProtocol')](options) {
        return this.#broadcast;
      }

      send(data) {
        this.#writer.write(new TextEncoder().encode(data));
      }

      close() {
        this.#writer.end();
      }
    }

    const bus = new MessageBus();
    const { broadcast } = Broadcast.from(bus);
    const consumer = broadcast.push();
    bus.send('hello');
    bus.close();
    text(consumer).then(console.log); // 'hello'
    javascript

#### `Stream.drainableProtocol`#

- Value: `Symbol.for('Stream.drainableProtocol')`

Implement to make a writer compatible with `ondrain()`. The method should return `null` if no backpressure, or a promise that fulfills with a truthy value when backpressure clears.

    import { ondrain } from 'node:stream/iter';

    class CustomWriter {
      #queue = [];
      #drain = null;
      #closed = false;
      [Symbol.for('Stream.drainableProtocol')]() {
        if (this.#closed) return null;
        if (this.#queue.length < 3) return Promise.resolve(true);
        this.#drain ??= Promise.withResolvers();
        return this.#drain.promise;
      }
      write(chunk) {
        this.#queue.push(chunk);
      }
      flush() {
        this.#queue.length = 0;
        this.#drain?.resolve(true);
        this.#drain = null;
      }
      close() {
        this.#closed = true;
      }
    }
    const writer = new CustomWriter();
    const ready = ondrain(writer);
    console.log(ready); // Promise { true } -- no backpressure
    const { ondrain } = require('node:stream/iter');

    class CustomWriter {
      #queue = [];
      #drain = null;
      #closed = false;

      [Symbol.for('Stream.drainableProtocol')]() {
        if (this.#closed) return null;
        if (this.#queue.length < 3) return Promise.resolve(true);
        this.#drain ??= Promise.withResolvers();
        return this.#drain.promise;
      }

      write(chunk) {
        this.#queue.push(chunk);
      }

      flush() {
        this.#queue.length = 0;
        this.#drain?.resolve(true);
        this.#drain = null;
      }

      close() {
        this.#closed = true;
      }
    }

    const writer = new CustomWriter();
    const ready = ondrain(writer);
    console.log(ready); // Promise { true } -- no backpressure
    javascript

#### `Stream.shareProtocol`#

- Value: `Symbol.for('Stream.shareProtocol')`

The value must be a function. When called by `Share.from()`, it receives the options passed to `Share.from()` and must return an object conforming to the `<Share>` interface. The implementation is fully custom -- it can manage the shared source, consumers, buffering, and backpressure however it wants.

    import { share, Share, text } from 'node:stream/iter';

    // This example defers to the built-in share(), but a custom
    // implementation could use any mechanism.
    class DataPool {
      #share;

      constructor(source) {
        this.#share = share(source);
      }

      [Symbol.for('Stream.shareProtocol')](options) {
        return this.#share;
      }
    }

    const pool = new DataPool(
      (async function* () {
        yield 'hello';
      })(),
    );

    const shared = Share.from(pool);
    const consumer = shared.pull();
    console.log(await text(consumer)); // 'hello'
    const { share, Share, text } = require('node:stream/iter');

    // This example defers to the built-in share(), but a custom
    // implementation could use any mechanism.
    class DataPool {
      #share;

      constructor(source) {
        this.#share = share(source);
      }

      [Symbol.for('Stream.shareProtocol')](options) {
        return this.#share;
      }
    }

    const pool = new DataPool(
      (async function* () {
        yield 'hello';
      })(),
    );

    const shared = Share.from(pool);
    const consumer = shared.pull();
    text(consumer).then(console.log); // 'hello'
    javascript

#### `Stream.shareSyncProtocol`#

- Value: `Symbol.for('Stream.shareSyncProtocol')`

The value must be a function. When called by `SyncShare.fromSync()`, it receives the options passed to `SyncShare.fromSync()` and must return an object conforming to the `<SyncShare>` interface. The implementation is fully custom -- it can manage the shared source, consumers, and buffering however it wants.

    import { shareSync, SyncShare, textSync } from 'node:stream/iter';

    // This example defers to the built-in shareSync(), but a custom
    // implementation could use any mechanism.
    class SyncDataPool {
      #share;

      constructor(source) {
        this.#share = shareSync(source);
      }

      [Symbol.for('Stream.shareSyncProtocol')](options) {
        return this.#share;
      }
    }

    const encoder = new TextEncoder();
    const pool = new SyncDataPool(
      function* () {
        yield [encoder.encode('hello')];
      }(),
    );

    const shared = SyncShare.fromSync(pool);
    const consumer = shared.pull();
    console.log(textSync(consumer)); // 'hello'
    const { shareSync, SyncShare, textSync } = require('node:stream/iter');

    // This example defers to the built-in shareSync(), but a custom
    // implementation could use any mechanism.
    class SyncDataPool {
      #share;

      constructor(source) {
        this.#share = shareSync(source);
      }

      [Symbol.for('Stream.shareSyncProtocol')](options) {
        return this.#share;
      }
    }

    const encoder = new TextEncoder();
    const pool = new SyncDataPool(
      function* () {
        yield [encoder.encode('hello')];
      }(),
    );

    const shared = SyncShare.fromSync(pool);
    const consumer = shared.pull();
    console.log(textSync(consumer)); // 'hello'
    javascript

#### `Stream.toAsyncStreamable`#

- Value: `Symbol.for('Stream.toAsyncStreamable')`

The value must be a function that converts the object into a streamable value. When the object is encountered anywhere in the streaming pipeline (as a source passed to `from()`, or as a value returned from a transform), this method is called to produce the actual data. It may return any value that resolves to: a string, `Uint8Array`, `AsyncIterable`, `Iterable`, or another streamable object.

    import { from, text } from 'node:stream/iter';

    class Greeting {
      #name;

      constructor(name) {
        this.#name = name;
      }

      [Symbol.for('Stream.toAsyncStreamable')]() {
        return `hello ${this.#name}`;
      }
    }

    const stream = from(new Greeting('world'));
    console.log(await text(stream)); // 'hello world'
    const { from, text } = require('node:stream/iter');

    class Greeting {
      #name;

      constructor(name) {
        this.#name = name;
      }

      [Symbol.for('Stream.toAsyncStreamable')]() {
        return `hello ${this.#name}`;
      }
    }

    const stream = from(new Greeting('world'));
    text(stream).then(console.log); // 'hello world'
    javascript

#### `Stream.toStreamable`#

- Value: `Symbol.for('Stream.toStreamable')`

The value must be a function that synchronously converts the object into a streamable value. When the object is encountered anywhere in the streaming pipeline (as a source passed to `fromSync()`, or as a value returned from a sync transform), this method is called to produce the actual data. It must synchronously return a streamable value: a string, `Uint8Array`, or `Iterable`.

    import { fromSync, textSync } from 'node:stream/iter';

    class Greeting {
      #name;

      constructor(name) {
        this.#name = name;
      }

      [Symbol.for('Stream.toStreamable')]() {
        return `hello ${this.#name}`;
      }
    }

    const stream = fromSync(new Greeting('world'));
    console.log(textSync(stream)); // 'hello world'
    const { fromSync, textSync } = require('node:stream/iter');

    class Greeting {
      #name;

      constructor(name) {
        this.#name = name;
      }

      [Symbol.for('Stream.toStreamable')]() {
        return `hello ${this.#name}`;
      }
    }

    const stream = fromSync(new Greeting('world'));
    console.log(textSync(stream)); // 'hello world'
    javascript

## Modules: `node:module` API#

Added in: v0.3.7

### The `Module` object#

- Type: `<Object>`

Provides general utility methods when interacting with instances of `Module`, the `module` variable often seen in CommonJS modules. Accessed via `import 'node:module'` or `require('node:module')`.

#### `module.builtinModules`#

Added in: v9.3.0, v8.10.0, v6.13.0
History

Version | Changes
--- | ---
v23.5.0 | The list now also contains prefix-only modules.

- Type: `<string>`[]

A list of the names of all modules provided by Node.js. Can be used to verify if a module is maintained by a third party or not.

`module` in this context isn't the same object that's provided by the module wrapper. To access it, require the `Module` module:

    // module.mjs
    // In an ECMAScript module
    import { builtinModules as builtin } from 'node:module';
    // module.cjs
    // In a CommonJS module
    const builtin = require('node:module').builtinModules;
    javascript

#### `module.createRequire(filename)`#

Added in: v12.2.0

- `filename` `<string>` | `<URL>` Filename to be used to construct the require function. Must be a file URL object, file URL string, or absolute path string.
- Returns: `<require>` Require function

    import { createRequire } from 'node:module';
    const require = createRequire(import.meta.url);

    // sibling-module.js is a CommonJS module.
    const siblingModule = require('./sibling-module');
    mjs

#### `module.findPackageJSON(specifier[, base])`#

Added in: v23.2.0, v22.14.0

Stability: 1.1 - Active Development

- `specifier` `<string>` | `<URL>` The specifier for the module whose `package.json` to retrieve. When passing a bare specifier, the `package.json` at the root of the package is returned. When passing a relative specifier or an absolute specifier, the closest parent `package.json` is returned.
- `base` `<string>` | `<URL>` The absolute location (`file:` URL string or FS path) of the containing module. For CJS, use `__filename` (not `__dirname`!); for ESM, use `import.meta.url`. You do not need to pass it if `specifier` is an `absolute specifier`.
- Returns: `<string>` | `<undefined>` A path if the `package.json` is found. When `specifier` is a package, the package's root `package.json`; when a relative or unresolved, the closest `package.json` to the `specifier`.

> Caveat: Do not use this to try to determine module format. There are many things affecting that determination; the `type` field of package.json is the least definitive (ex file extension supersedes it, and a loader hook supersedes that).

> Caveat: This currently leverages only the built-in default resolver; if `resolve` customization hooks are registered, they will not affect the resolution. This may change in the future.

    // /path/to/project/packages/bar/bar.js
    import { findPackageJSON } from 'node:module';

    findPackageJSON('..', import.meta.url);
    // '/path/to/project/package.json'
    // Same result when passing an absolute specifier instead:
    findPackageJSON(new URL('../', import.meta.url));
    findPackageJSON(import.meta.resolve('../'));

    findPackageJSON('some-package', import.meta.url);
    // '/path/to/project/packages/bar/node_modules/some-package/package.json'
    // When passing an absolute specifier, you might get a different result if the
    // resolved module is inside a subfolder that has nested `package.json`.
    findPackageJSON(import.meta.resolve('some-package'));
    // '/path/to/project/packages/bar/node_modules/some-package/some-subfolder/package.json'

    findPackageJSON('@foo/qux', import.meta.url);
    // '/path/to/project/packages/qux/package.json'
    // /path/to/project/packages/bar/bar.js
    const { findPackageJSON } = require('node:module');
    const { pathToFileURL } = require('node:url');
    const path = require('node:path');

    findPackageJSON('..', __filename);
    // '/path/to/project/package.json'
    // Same result when passing an absolute specifier instead:
    findPackageJSON(pathToFileURL(path.join(__dirname, '..')));

    findPackageJSON('some-package', __filename);
    // '/path/to/project/packages/bar/node_modules/some-package/package.json'
    // When passing an absolute specifier, you might get a different result if the
    // resolved module is inside a subfolder that has nested `package.json`.
    findPackageJSON(pathToFileURL(require.resolve('some-package')));
    // '/path/to/project/packages/bar/node_modules/some-package/some-subfolder/package.json'

    findPackageJSON('@foo/qux', __filename);
    // '/path/to/project/packages/qux/package.json'
    javascript

#### `module.isBuiltin(moduleName)`#

Added in: v18.6.0, v16.17.0

- `moduleName` `<string>` name of the module
- Returns: `<boolean>` returns true if the module is builtin else returns false

    import { isBuiltin } from 'node:module';
    isBuiltin('node:fs'); // true
    isBuiltin('fs'); // true
    isBuiltin('wss'); // false
    mjs

#### `module.register(specifier[, parentURL][, options])`#

Added in: v20.6.0, v18.19.0Deprecated in: v25.9.0
History

Version | Changes
--- | ---
v26.0.0 | Runtime deprecation (DEP0205).
v23.6.1, v22.13.1, v20.18.2 | Using this feature with the permission model enabled requires passing `--allow-worker`.
v20.8.0, v18.19.0 | Add support for WHATWG URL instances.

Stability: 0 - Deprecated: Use `module.registerHooks()` instead.

- `specifier` `<string>` | `<URL>` Customization hooks to be registered; this should be the same string that would be passed to `import()`, except that if it is relative, it is resolved relative to `parentURL`.
- `parentURL` `<string>` | `<URL>` If you want to resolve `specifier` relative to a base URL, such as `import.meta.url`, you can pass that URL here. Default: `'data:'`
- `options` `<Object>`
  - `parentURL` `<string>` | `<URL>` If you want to resolve `specifier` relative to a base URL, such as `import.meta.url`, you can pass that URL here. This property is ignored if the `parentURL` is supplied as the second argument. Default: `'data:'`
  - `data` `<any>` Any arbitrary, cloneable JavaScript value to pass into the `initialize` hook.
  - `transferList` `<Object>`[] transferable objects to be passed into the `initialize` hook.

Register a module that exports hooks that customize Node.js module resolution and loading behavior. See Customization hooks.

This feature requires `--allow-worker` if used with the Permission Model.

#### `module.registerHooks(options)`#

Added in: v23.5.0, v22.15.0
History

Version | Changes
--- | ---
v25.4.0, v24.13.1 | Synchronous and in-thread hooks are now release candidate.

Stability: 1.2 - Release candidate

- `options` `<Object>`
  - `load` `<Function>` | `<undefined>` See load hook. Default: `undefined`.
  - `resolve` `<Function>` | `<undefined>` See resolve hook. Default: `undefined`.
- Returns: `<Object>` An object with the following property:
  - `deregister()` `<Function>` Remove the registered hooks so that they are no longer called. Hooks are otherwise retained for the lifetime of the running process.
  - `[Symbol.dispose]` `<Function>` The same as `deregister`.

Register hooks that customize Node.js module resolution and loading behavior. See Customization hooks. The returned object can be used to deregister the hooks.

#### `module.stripTypeScriptTypes(code[, options])`#

Added in: v23.2.0, v22.13.0
History

Version | Changes
--- | ---
v26.0.0 | Removed `transform` and `sourceMap` options.

Stability: 1.2 - Release candidate

- `code` `<string>` The code to strip type annotations from.
- `options` `<Object>`
  - `mode` `<string>` Default: `'strip'`. Possible values are:
    - `'strip'` Only strip type annotations without performing the transformation of TypeScript features.
  - `sourceUrl` `<string>` Specifies the source url used in the source map.
- Returns: `<string>` The code with type annotations stripped.

`module.stripTypeScriptTypes()` removes type annotations from TypeScript code. It can be used to strip type annotations from TypeScript code before running it with `vm.runInContext()` or `vm.compileFunction()`.

By default, it will throw an error if the code contains TypeScript features that require transformation, such as `enum`s. See type-stripping for more information.

WARNING: The output of this function should not be considered stable across Node.js versions, due to changes in the TypeScript parser.

    import { stripTypeScriptTypes } from 'node:module';
    const code = 'const a: number = 1;';
    const strippedCode = stripTypeScriptTypes(code);
    console.log(strippedCode);
    // Prints: const a         = 1;
    const { stripTypeScriptTypes } = require('node:module');
    const code = 'const a: number = 1;';
    const strippedCode = stripTypeScriptTypes(code);
    console.log(strippedCode);
    // Prints: const a         = 1;
    javascript

If `sourceUrl` is provided, it will be used appended as a comment at the end of the output:

    import { stripTypeScriptTypes } from 'node:module';
    const code = 'const a: number = 1;';
    const strippedCode = stripTypeScriptTypes(code, { mode: 'strip', sourceUrl: 'source.ts' });
    console.log(strippedCode);
    // Prints: const a         = 1\n\n//# sourceURL=source.ts;
    const { stripTypeScriptTypes } = require('node:module');
    const code = 'const a: number = 1;';
    const strippedCode = stripTypeScriptTypes(code, { mode: 'strip', sourceUrl: 'source.ts' });
    console.log(strippedCode);
    // Prints: const a         = 1\n\n//# sourceURL=source.ts;
    javascript

#### `module.syncBuiltinESMExports()`#

Added in: v12.12.0

The `module.syncBuiltinESMExports()` method updates all the live bindings for builtin ES Modules to match the properties of the CommonJS exports. It does not add or remove exported names from the ES Modules.

    const fs = require('node:fs');
    const assert = require('node:assert');
    const { syncBuiltinESMExports } = require('node:module');

    fs.readFile = newAPI;

    delete fs.readFileSync;

    function newAPI() {
      // ...
    }

    fs.newAPI = newAPI;

    syncBuiltinESMExports();

    import('node:fs').then((esmFS) => {
      // It syncs the existing readFile property with the new value
      assert.strictEqual(esmFS.readFile, newAPI);
      // readFileSync has been deleted from the required fs
      assert.strictEqual('readFileSync' in fs, false);
      // syncBuiltinESMExports() does not remove readFileSync from esmFS
      assert.strictEqual('readFileSync' in esmFS, true);
      // syncBuiltinESMExports() does not add names
      assert.strictEqual(esmFS.newAPI, undefined);
    });
    js

### Module compile cache#

Added in: v22.1.0
History

Version | Changes
--- | ---
v22.8.0 | add initial JavaScript APIs for runtime access.

The module compile cache can be enabled either using the `module.enableCompileCache()` method or the `NODE_COMPILE_CACHE=dir` environment variable. After it is enabled, whenever Node.js compiles a CommonJS, an ECMAScript Module, or a TypeScript module, it will use on-disk V8 code cache persisted in the specified directory to speed up the compilation. This may slow down the first load of a module graph, but subsequent loads of the same module graph may get a significant speedup if the contents of the modules do not change.

To clean up the generated compile cache on disk, simply remove the cache directory. The cache directory will be recreated the next time the same directory is used for compile cache storage. To avoid filling up the disk with stale cache, it is recommended to use a directory under the `os.tmpdir()`. If the compile cache is enabled by a call to `module.enableCompileCache()` without specifying the `directory`, Node.js will use the `NODE_COMPILE_CACHE=dir` environment variable if it's set, or defaults to `path.join(os.tmpdir(), 'node-compile-cache')` otherwise. To locate the compile cache directory used by a running Node.js instance, use `module.getCompileCacheDir()`.

The enabled module compile cache can be disabled by the `NODE_DISABLE_COMPILE_CACHE=1` environment variable. This can be useful when the compile cache leads to unexpected or undesired behaviors (e.g. less precise test coverage).

At the moment, when the compile cache is enabled and a module is loaded afresh, the code cache is generated from the compiled code immediately, but will only be written to disk when the Node.js instance is about to exit. This is subject to change. The `module.flushCompileCache()` method can be used to ensure the accumulated code cache is flushed to disk in case the application wants to spawn other Node.js instances and let them share the cache long before the parent exits.

The compile cache layout on disk is an implementation detail and should not be relied upon. The compile cache generated is typically only reusable in the same version of Node.js, and should be not assumed to be compatible across different versions of Node.js.

#### Portability of the compile cache#

By default, caches are invalidated when the absolute paths of the modules being cached are changed. To keep the cache working after moving the project directory, enable portable compile cache. This allows previously compiled modules to be reused across different directory locations as long as the layout relative to the cache directory remains the same. This would be done on a best-effort basis. If Node.js cannot compute the location of a module relative to the cache directory, the module will not be cached.

There are two ways to enable the portable mode:

1. Using the portable option in `module.enableCompileCache()`:

       // Non-portable cache (default): cache breaks if project is moved
       module.enableCompileCache({ directory: '/path/to/cache/storage/dir' });

       // Portable cache: cache works after the project is moved
       module.enableCompileCache({ directory: '/path/to/cache/storage/dir', portable: true });
       js

2. Setting the environment variable: `NODE_COMPILE_CACHE_PORTABLE=1`

#### Limitations of the compile cache#

Currently when using the compile cache with V8 JavaScript code coverage, the coverage being collected by V8 may be less precise in functions that are deserialized from the code cache. It's recommended to turn this off when running tests to generate precise coverage.

Compilation cache generated by one version of Node.js can not be reused by a different version of Node.js. Cache generated by different versions of Node.js will be stored separately if the same base directory is used to persist the cache, so they can co-exist.

#### `module.constants.compileCacheStatus`#

Added in: v22.8.0
History

Version | Changes
--- | ---
v25.4.0 | This feature is no longer experimental.

The following constants are returned as the `status` field in the object returned by `module.enableCompileCache()` to indicate the result of the attempt to enable the module compile cache.

Constant | Description
--- | ---
`ENABLED` | Node.js has enabled the compile cache successfully. The directory used to store the compile cache will be returned in the `directory` field in the returned object.
`ALREADY_ENABLED` | The compile cache has already been enabled before, either by a previous call to `module.enableCompileCache()`, or by the `NODE_COMPILE_CACHE=dir` environment variable. The directory used to store the compile cache will be returned in the `directory` field in the returned object.
`FAILED` | Node.js fails to enable the compile cache. This can be caused by the lack of permission to use the specified directory, or various kinds of file system errors. The detail of the failure will be returned in the `message` field in the returned object.
`DISABLED` | Node.js cannot enable the compile cache because the environment variable `NODE_DISABLE_COMPILE_CACHE=1` has been set.

#### `module.enableCompileCache([options])`#

Added in: v22.8.0
History

Version | Changes
--- | ---
v25.4.0 | This feature is no longer experimental.
v25.0.0, v24.12.0 | Add `portable` option to enable portable compile cache.
v25.0.0, v24.12.0 | Rename the unreleased `path` option to `directory` to maintain consistency.

- `options` `<string>` | `<Object>` Optional. If a string is passed, it is considered to be `options.directory`.
  - `directory` `<string>` Optional. Directory to store the compile cache. If not specified, the directory specified by the `NODE_COMPILE_CACHE=dir` environment variable will be used if it's set, or `path.join(os.tmpdir(), 'node-compile-cache')` otherwise.
  - `portable` `<boolean>` Optional. If `true`, enables portable compile cache so that the cache can be reused even if the project directory is moved. This is a best-effort feature. If not specified, it will depend on whether the environment variable `NODE_COMPILE_CACHE_PORTABLE=1` is set.
- Returns: `<Object>`
  - `status` `<integer>` One of the `module.constants.compileCacheStatus`
  - `message` `<string>` | `<undefined>` If Node.js cannot enable the compile cache, this contains the error message. Only set if `status` is `module.constants.compileCacheStatus.FAILED`.
  - `directory` `<string>` | `<undefined>` If the compile cache is enabled, this contains the directory where the compile cache is stored. Only set if `status` is `module.constants.compileCacheStatus.ENABLED` or `module.constants.compileCacheStatus.ALREADY_ENABLED`.

Enable module compile cache in the current Node.js instance.

For general use cases, it's recommended to call `module.enableCompileCache()` without specifying the `options.directory`, so that the directory can be overridden by the `NODE_COMPILE_CACHE` environment variable when necessary.

Since compile cache is supposed to be an optimization that is not mission critical, this method is designed to not throw any exception when the compile cache cannot be enabled. Instead, it will return an object containing an error message in the `message` field to aid debugging. If compile cache is enabled successfully, the `directory` field in the returned object contains the path to the directory where the compile cache is stored. The `status` field in the returned object would be one of the `module.constants.compileCacheStatus` values to indicate the result of the attempt to enable the module compile cache.

This method only affects the current Node.js instance. To enable it in child worker threads, either call this method in child worker threads too, or set the `process.env.NODE_COMPILE_CACHE` value to compile cache directory so the behavior can be inherited into the child workers. The directory can be obtained either from the `directory` field returned by this method, or with `module.getCompileCacheDir()`.

#### `module.flushCompileCache()`#

Added in: v23.0.0, v22.10.0
History

Version | Changes
--- | ---
v25.4.0 | This feature is no longer experimental.

Flush the module compile cache accumulated from modules already loaded in the current Node.js instance to disk. This returns after all the flushing file system operations come to an end, no matter they succeed or not. If there are any errors, this will fail silently, since compile cache misses should not interfere with the actual operation of the application.

#### `module.getCompileCacheDir()`#

Added in: v22.8.0
History

Version | Changes
--- | ---
v25.4.0 | This feature is no longer experimental.

- Returns: `<string>` | `<undefined>` Path to the module compile cache directory if it is enabled, or `undefined` otherwise.

### Customization Hooks#

Added in: v8.8.0
History

Version | Changes
--- | ---
v25.4.0, v24.13.1 | Synchronous and in-thread hooks are now release candidate.
v23.5.0, v22.15.0 | Add support for synchronous and in-thread hooks.
v20.6.0, v18.19.0 | Added `initialize` hook to replace `globalPreload`.
v18.6.0, v16.17.0 | Add support for chaining loaders.
v16.12.0 | Removed `getFormat`, `getSource`, `transformSource`, and `globalPreload`; added `load` hook and `getGlobalPreload` hook.

Node.js currently supports two types of module customization hooks:

1. `module.registerHooks(options)`: takes synchronous hook functions that are run directly on the thread where the modules are loaded.
2. `module.register(specifier[, parentURL][, options])`: takes specifier to a module that exports asynchronous hook functions. The functions are run on a separate loader thread.

The asynchronous hooks incur extra overhead from inter-thread communication, and have several caveats especially when customizing CommonJS modules in the module graph. In most cases, it's recommended to use synchronous hooks via `module.registerHooks()` for simplicity.

#### Synchronous customization hooks#

Stability: 1.2 - Release candidate

##### Registration of synchronous customization hooks#

To register synchronous customization hooks, use `module.registerHooks()`, which takes synchronous hook functions directly in-line.

    // register-hooks.js
    import { registerHooks } from 'node:module';
    registerHooks({
      resolve(specifier, context, nextResolve) { /* implementation */ },
      load(url, context, nextLoad) { /* implementation */ },
    });
    // register-hooks.js
    const { registerHooks } = require('node:module');
    registerHooks({
      resolve(specifier, context, nextResolve) { /* implementation */ },
      load(url, context, nextLoad) { /* implementation */ },
    });
    javascript

###### Registering hooks before application code runs with flags#

The hooks can be registered before the application code is run by using the `--import` or `--require` flag:

    node --import ./register-hooks.js ./my-app.js
    node --require ./register-hooks.js ./my-app.js
    bash

The specifier passed to `--import` or `--require` can also come from a package:

    node --import some-package/register ./my-app.js
    node --require some-package/register ./my-app.js
    bash

Where `some-package` has an `"exports"` field defining the `/register` export to map to a file that calls `registerHooks()`, like the `register-hooks.js` examples above.

Using `--import` or `--require` ensures that the hooks are registered before any application code is loaded, including the entry point of the application and for any worker threads by default as well.

###### Registering hooks before application code runs programmatically#

Alternatively, `registerHooks()` can be called from the entry point.

If the entry point needs to load other modules and the loading process needs to be customized, load them using either `require()` or dynamic `import()` after the hooks are registered. Do not use static `import` statements to load modules that need to be customized in the same module that registers the hooks, because static `import` statements are evaluated before any code in the importer module is run, including the call to `registerHooks()`, regardless of where the static `import` statements appear in the importer module.

    import { registerHooks } from 'node:module';

    registerHooks({ /* implementation of synchronous hooks */ });

    // If loaded using static import, the hooks would not be applied when loading
    // my-app.mjs, because statically imported modules are all executed before its
    // importer regardless of where the static import appears.
    // import './my-app.mjs';

    // my-app.mjs must be loaded dynamically to ensure the hooks are applied.
    await import('./my-app.mjs');
    const { registerHooks } = require('node:module');

    registerHooks({ /* implementation of synchronous hooks */ });

    import('./my-app.mjs');
    // Or, if my-app.mjs does not have top-level await or it's a CommonJS module,
    // require() can also be used:
    // require('./my-app.mjs');
    javascript

###### Registering hooks before application code runs with a `data:` URL#

Alternatively, inline JavaScript code can be embedded in `data:` URLs to register the hooks before the application code runs. For example,

    node --import 'data:text/javascript,import {registerHooks} from "node:module"; registerHooks(/* hooks code */);' ./my-app.js
    bash

##### Convention of hooks and chaining#

Hooks are part of a chain, even if that chain consists of only one custom (user-provided) hook and the default hook, which is always present.

Hook functions nest: each one must always return a plain object, and chaining happens as a result of each function calling `next<hookName>()`, which is a reference to the subsequent loader's hook (in LIFO order).

It's possible to call `registerHooks()` more than once:

    // entrypoint.mjs
    import { registerHooks } from 'node:module';

    const hook1 = { /* implementation of hooks */ };
    const hook2 = { /* implementation of hooks */ };
    // hook2 runs before hook1.
    registerHooks(hook1);
    registerHooks(hook2);
    // entrypoint.cjs
    const { registerHooks } = require('node:module');

    const hook1 = { /* implementation of hooks */ };
    const hook2 = { /* implementation of hooks */ };
    // hook2 runs before hook1.
    registerHooks(hook1);
    registerHooks(hook2);
    javascript

In this example, the registered hooks will form chains. These chains run last-in, first-out (LIFO). If both `hook1` and `hook2` define a `resolve` hook, they will be called like so (note the right-to-left, starting with `hook2.resolve`, then `hook1.resolve`, then the Node.js default):

Node.js default `resolve` ← `hook1.resolve` ← `hook2.resolve`

The same applies to all the other hooks.

A hook that returns a value lacking a required property triggers an exception. A hook that returns without calling `next<hookName>()` and without returning `shortCircuit: true` also triggers an exception. These errors are to help prevent unintentional breaks in the chain. Return `shortCircuit: true` from a hook to signal that the chain is intentionally ending at your hook.

If a hook should be applied when loading other hook modules, the other hook modules should be loaded after the hook is registered.

##### Deregistration of synchronous customization hooks#

The object returned by `registerHooks()` has a `deregister()` method that can be used to remove the hooks from the chain. Once `deregister()` is called, the hooks will no longer be invoked during module resolution or loading.

This is currently only available for synchronous hooks registered via `registerHooks()`, not for asynchronous hooks registered via `module.register()`.

    import { registerHooks } from 'node:module';

    const hooks = registerHooks({
      resolve(specifier, context, nextResolve) {
        console.log('resolve hook called for', specifier);
        return nextResolve(specifier, context);
      },
      load(url, context, nextLoad) {
        return nextLoad(url, context);
      },
    });

    // At this point, the hooks are active and will be called for
    // any subsequent import() or require() calls.
    await import('./my-module.mjs');

    // Later, remove the hooks from the chain.
    hooks.deregister();

    // Subsequent loads will no longer trigger the hooks.
    await import('./another-module.mjs');
    const { registerHooks } = require('node:module');

    const hooks = registerHooks({
      resolve(specifier, context, nextResolve) {
        console.log('resolve hook called for', specifier);
        return nextResolve(specifier, context);
      },
      load(url, context, nextLoad) {
        return nextLoad(url, context);
      },
    });

    // At this point, the hooks are active and will be called for
    // any subsequent require() calls.
    require('./my-module.cjs');

    // Later, remove the hooks from the chain.
    hooks.deregister();

    // Subsequent loads will no longer trigger the hooks.
    require('./another-module.cjs');
    javascript

##### Hook functions accepted by `module.registerHooks()`#

Added in: v23.5.0, v22.15.0

The `module.registerHooks()` method accepts the following synchronous hook functions.

    function resolve(specifier, context, nextResolve) {
      // Take an `import` or `require` specifier and resolve it to a URL.
    }

    function load(url, context, nextLoad) {
      // Take a resolved URL and return the source code to be evaluated.
    }
    mjs

Synchronous hooks are run in the same thread and the same realm where the modules are loaded, the code in the hook function can pass values to the modules being referenced directly via global variables or other shared states.

Unlike the asynchronous hooks, the synchronous hooks are not inherited into child worker threads by default, though if the hooks are registered using a file preloaded by `--import` or `--require`, child worker threads can inherit the preloaded scripts via `process.execArgv` inheritance. See the documentation of `Worker` for details.

##### Synchronous `resolve(specifier, context, nextResolve)`#

History

Version | Changes
--- | ---
v23.5.0, v22.15.0 | Add support for synchronous and in-thread hooks.

- `specifier` `<string>`
- `context` `<Object>`
  - `conditions` `<string>`[] Export conditions of the relevant `package.json`
  - `importAttributes` `<Object>` An object whose key-value pairs represent the attributes for the module to import
  - `parentURL` `<string>` | `<undefined>` The module importing this one, or undefined if this is the Node.js entry point
- `nextResolve` `<Function>` The subsequent `resolve` hook in the chain, or the Node.js default `resolve` hook after the last user-supplied `resolve` hook
  - `specifier` `<string>`
  - `context` `<Object>` | `<undefined>` When omitted, the defaults are provided. When provided, defaults are merged in with preference to the provided properties.
- Returns: `<Object>`
  - `format` `<string>` | `<null>` | `<undefined>` A hint to the `load` hook (it might be ignored). It can be a module format (such as `'commonjs'` or `'module'`) or an arbitrary value like `'css'` or `'yaml'`.
  - `importAttributes` `<Object>` | `<undefined>` The import attributes to use when caching the module (optional; if excluded the input will be used)
  - `shortCircuit` `<undefined>` | `<boolean>` A signal that this hook intends to terminate the chain of `resolve` hooks. Default: `false`
  - `url` `<string>` The absolute URL to which this input resolves

The `resolve` hook chain is responsible for telling Node.js where to find and how to cache a given `import` statement or expression, or `require` call. It can optionally return a format (such as `'module'`) as a hint to the `load` hook. If a format is specified, the `load` hook is ultimately responsible for providing the final `format` value (and it is free to ignore the hint provided by `resolve`); if `resolve` provides a `format`, a custom `load` hook is required even if only to pass the value to the Node.js default `load` hook.

Import type attributes are part of the cache key for saving loaded modules into the internal module cache. The `resolve` hook is responsible for returning an `importAttributes` object if the module should be cached with different attributes than were present in the source code.

The `conditions` property in `context` is an array of conditions that will be used to match package exports conditions for this resolution request. They can be used for looking up conditional mappings elsewhere or to modify the list when calling the default resolution logic.

The current package exports conditions are always in the `context.conditions` array passed into the hook. To guarantee default Node.js module specifier resolution behavior when calling `defaultResolve`, the `context.conditions` array passed to it must include all elements of the `context.conditions` array originally passed into the `resolve` hook.

    import { registerHooks } from 'node:module';

    function resolve(specifier, context, nextResolve) {
      // When calling `defaultResolve`, the arguments can be modified. For example,
      // to change the specifier or to add applicable export conditions.
      if (specifier.includes('foo')) {
        specifier = specifier.replace('foo', 'bar');
        return nextResolve(specifier, {
          ...context,
          conditions: [...context.conditions, 'another-condition'],
        });
      }

      // The hook can also skip default resolution and provide a custom URL.
      if (specifier === 'special-module') {
        return {
          url: 'file:///path/to/special-module.mjs',
          format: 'module',
          shortCircuit: true,  // This is mandatory if nextResolve() is not called.
        };
      }

      // If no customization is needed, defer to the next hook in the chain which would be the
      // Node.js default resolve if this is the last user-specified loader.
      return nextResolve(specifier);
    }

    registerHooks({ resolve });
    mjs

##### Synchronous `load(url, context, nextLoad)`#

History

Version | Changes
--- | ---
v23.5.0, v22.15.0 | Add support for synchronous and in-thread version.

- `url` `<string>` The URL returned by the `resolve` chain
- `context` `<Object>`
  - `conditions` `<string>`[] Export conditions of the relevant `package.json`
  - `format` `<string>` | `<null>` | `<undefined>` The format optionally supplied by the `resolve` hook chain. This can be any string value as an input; input values do not need to conform to the list of acceptable return values described below.
  - `importAttributes` `<Object>`
- `nextLoad` `<Function>` The subsequent `load` hook in the chain, or the Node.js default `load` hook after the last user-supplied `load` hook
  - `url` `<string>`
  - `context` `<Object>` | `<undefined>` When omitted, defaults are provided. When provided, defaults are merged in with preference to the provided properties. In the default `nextLoad`, if the module pointed to by `url` does not have explicit module type information, `context.format` is mandatory.
- Returns: `<Object>`
  - `format` `<string>` One of the acceptable module formats listed below.
  - `shortCircuit` `<undefined>` | `<boolean>` A signal that this hook intends to terminate the chain of `load` hooks. Default: `false`
  - `source` `<string>` | `<ArrayBuffer>` | `<TypedArray>` The source for Node.js to evaluate

The `load` hook provides a way to define a custom method for retrieving the source code of a resolved URL. This would allow a loader to potentially avoid reading files from disk. It could also be used to map an unrecognized format to a supported one, for example `yaml` to `module`.

    import { registerHooks } from 'node:module';
    import { Buffer } from 'node:buffer';

    function load(url, context, nextLoad) {
      // The hook can skip default loading and provide a custom source code.
      if (url === 'special-module') {
        return {
          source: 'export const special = 42;',
          format: 'module',
          shortCircuit: true,  // This is mandatory if nextLoad() is not called.
        };
      }

      // It's possible to modify the source code loaded by the next - possibly default - step,
      // for example, replacing 'foo' with 'bar' in the source code of the module.
      const result = nextLoad(url, context);
      const source = typeof result.source === 'string' ?
        result.source : Buffer.from(result.source).toString('utf8');
      return {
        source: source.replace(/foo/g, 'bar'),
        ...result,
      };
    }

    registerHooks({ load });
    mjs

In a more advanced scenario, this can also be used to transform an unsupported source to a supported one (see Examples below).

###### Accepted final formats returned by `load`#

The final value of `format` must be one of the following:

`format` | Description | Acceptable types for `source` returned by `load`
--- | --- | ---
`'addon'` | Load a Node.js addon | `<null>`
`'builtin'` | Load a Node.js builtin module | `<null>`
`'commonjs-typescript'` | Load a Node.js CommonJS module with TypeScript syntax | `<string>` | `<ArrayBuffer>` | `<TypedArray>` | `<null>` | `<undefined>`
`'commonjs'` | Load a Node.js CommonJS module | `<string>` | `<ArrayBuffer>` | `<TypedArray>` | `<null>` | `<undefined>`
`'json'` | Load a JSON file | `<string>` | `<ArrayBuffer>` | `<TypedArray>`
`'module-typescript'` | Load an ES module with TypeScript syntax | `<string>` | `<ArrayBuffer>` | `<TypedArray>`
`'module'` | Load an ES module | `<string>` | `<ArrayBuffer>` | `<TypedArray>`
`'wasm'` | Load a WebAssembly module | `<ArrayBuffer>` | `<TypedArray>`

The value of `source` is ignored for format `'builtin'` because currently it is not possible to replace the value of a Node.js builtin (core) module.

> These types all correspond to classes defined in ECMAScript.

- The specific `<ArrayBuffer>` object is a `<SharedArrayBuffer>`.
- The specific `<TypedArray>` object is a `<Uint8Array>`.

If the source value of a text-based format (i.e., `'json'`, `'module'`) is not a string, it is converted to a string using `util.TextDecoder`.

#### Asynchronous customization hooks#

Stability: 1.1 - Active Development

##### Caveats of asynchronous customization hooks#

The asynchronous customization hooks have many caveats and it is uncertain if their issues can be resolved. Users are encouraged to use the synchronous customization hooks via `module.registerHooks()` instead to avoid these caveats.

- Asynchronous hooks run on a separate thread, so the hook functions cannot directly mutate the global state of the modules being customized. It's typical to use message channels and atomics to pass data between the two or to affect control flows. See Communication with asynchronous module customization hooks.
- Asynchronous hooks do not affect all `require()` calls in the module graph.
  - Custom `require` functions created using `module.createRequire()` are not affected.
  - If the asynchronous `load` hook does not override the `source` for CommonJS modules that go through it, the child modules loaded by those CommonJS modules via built-in `require()` would not be affected by the asynchronous hooks either.
- There are several caveats that the asynchronous hooks need to handle when customizing CommonJS modules. See asynchronous `resolve` hook and asynchronous `load` hook for details.
- When `require()` calls inside CommonJS modules are customized by asynchronous hooks, Node.js may need to load the source code of the CommonJS module multiple times to maintain compatibility with existing CommonJS monkey-patching. If the module code changes between loads, this may lead to unexpected behaviors.
  - As a side effect, if both asynchronous hooks and synchronous hooks are registered and the asynchronous hooks choose to customize the CommonJS module, the synchronous hooks may be invoked multiple times for the `require()` calls in that CommonJS module.

##### Registration of asynchronous customization hooks#

Asynchronous customization hooks are registered using `module.register()` which takes a path or URL to another module that exports the asynchronous hook functions.

Similar to `registerHooks()`, `register()` can be called in a module preloaded by `--import` or `--require`, or called directly within the entry point.

    // Use module.register() to register asynchronous hooks in a dedicated thread.
    import { register } from 'node:module';
    register('./hooks.mjs', import.meta.url);

    // If my-app.mjs is loaded statically here as `import './my-app.mjs'`, since ESM
    // dependencies are evaluated before the module that imports them,
    // it's loaded _before_ the hooks are registered above and won't be affected.
    // To ensure the hooks are applied, dynamic import() must be used to load ESM
    // after the hooks are registered.
    import('./my-app.mjs');
    const { register } = require('node:module');
    const { pathToFileURL } = require('node:url');
    // Use module.register() to register asynchronous hooks in a dedicated thread.
    register('./hooks.mjs', pathToFileURL(__filename));

    import('./my-app.mjs');
    javascript

In `hooks.mjs`:

    // hooks.mjs
    export async function resolve(specifier, context, nextResolve) {
      /* implementation */
    }
    export async function load(url, context, nextLoad) {
      /* implementation */
    }
    mjs

Unlike synchronous hooks, the asynchronous hooks would not run for these modules loaded in the file that calls `register()`:

    // register-hooks.js
    import { register, createRequire } from 'node:module';
    register('./hooks.mjs', import.meta.url);

    // Asynchronous hooks does not affect modules loaded via custom require()
    // functions created by module.createRequire().
    const userRequire = createRequire(import.meta.filename);
    userRequire('./my-app-2.cjs');  // Hooks won't affect this
    // register-hooks.js
    const { register, createRequire } = require('node:module');
    const { pathToFileURL } = require('node:url');
    register('./hooks.mjs', pathToFileURL(__filename));

    // Asynchronous hooks does not affect modules loaded via built-in require()
    // in the module calling `register()`
    require('./my-app-2.cjs');  // Hooks won't affect this
    // .. or custom require() functions created by module.createRequire().
    const userRequire = createRequire(__filename);
    userRequire('./my-app-3.cjs');  // Hooks won't affect this
    javascript

Asynchronous hooks can also be registered using a `data:` URL with the `--import` flag:

    node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register("my-instrumentation", pathToFileURL("./"));' ./my-app.js
    bash

##### Chaining of asynchronous customization hooks#

Chaining of `register()` work similarly to `registerHooks()`. If synchronous and asynchronous hooks are mixed, the synchronous hooks are always run first before the asynchronous hooks start running, that is, in the last synchronous hook being run, its next hook includes invocation of the asynchronous hooks.

    // entrypoint.mjs
    import { register } from 'node:module';

    register('./foo.mjs', import.meta.url);
    register('./bar.mjs', import.meta.url);
    await import('./my-app.mjs');
    // entrypoint.cjs
    const { register } = require('node:module');
    const { pathToFileURL } = require('node:url');

    const parentURL = pathToFileURL(__filename);
    register('./foo.mjs', parentURL);
    register('./bar.mjs', parentURL);
    import('./my-app.mjs');
    javascript

If `foo.mjs` and `bar.mjs` define a `resolve` hook, they will be called like so (note the right-to-left, starting with `./bar.mjs`, then `./foo.mjs`, then the Node.js default):

Node.js default ← `./foo.mjs` ← `./bar.mjs`

When using the asynchronous hooks, the registered hooks also affect subsequent `register` calls, which takes care of loading hook modules. In the example above, `bar.mjs` will be resolved and loaded via the hooks registered by `foo.mjs` (because `foo`'s hooks will have already been added to the chain). This allows for things like writing hooks in non-JavaScript languages, so long as earlier registered hooks transpile into JavaScript.

The `register()` method cannot be called from the thread running the hook module that exports the asynchronous hooks or its dependencies.

##### Communication with asynchronous module customization hooks#

Asynchronous hooks run on a dedicated thread, separate from the main thread that runs application code. This means mutating global variables won't affect the other thread(s), and message channels must be used to communicate between the threads.

The `register` method can be used to pass data to an `initialize` hook. The data passed to the hook may include transferable objects like ports.

    import { register } from 'node:module';
    import { MessageChannel } from 'node:worker_threads';

    // This example demonstrates how a message channel can be used to
    // communicate with the hooks, by sending `port2` to the hooks.
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (msg) => {
      console.log(msg);
    });
    port1.unref();

    register('./my-hooks.mjs', {
      parentURL: import.meta.url,
      data: { number: 1, port: port2 },
      transferList: [port2],
    });
    const { register } = require('node:module');
    const { pathToFileURL } = require('node:url');
    const { MessageChannel } = require('node:worker_threads');

    // This example showcases how a message channel can be used to
    // communicate with the hooks, by sending `port2` to the hooks.
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (msg) => {
      console.log(msg);
    });
    port1.unref();

    register('./my-hooks.mjs', {
      parentURL: pathToFileURL(__filename),
      data: { number: 1, port: port2 },
      transferList: [port2],
    });
    javascript

##### Asynchronous hooks accepted by `module.register()`#

Added in: v8.8.0
History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | Added `initialize` hook to replace `globalPreload`.
v18.6.0, v16.17.0 | Add support for chaining loaders.
v16.12.0 | Removed `getFormat`, `getSource`, `transformSource`, and `globalPreload`; added `load` hook and `getGlobalPreload` hook.

The `register` method can be used to register a module that exports a set of hooks. The hooks are functions that are called by Node.js to customize the module resolution and loading process. The exported functions must have specific names and signatures, and they must be exported as named exports.

    export async function initialize({ number, port }) {
      // Receives data from `register`.
    }

    export async function resolve(specifier, context, nextResolve) {
      // Take an `import` or `require` specifier and resolve it to a URL.
    }

    export async function load(url, context, nextLoad) {
      // Take a resolved URL and return the source code to be evaluated.
    }
    mjs

Asynchronous hooks are run in a separate thread, isolated from the main thread where application code runs. That means it is a different realm. The hooks thread may be terminated by the main thread at any time, so do not depend on asynchronous operations (like `console.log`) to complete. They are inherited into child workers by default.

##### `initialize()`#

Added in: v20.6.0, v18.19.0

- `data` `<any>` The data from `register(loader, import.meta.url, { data })`.

The `initialize` hook is only accepted by `register`. `registerHooks()` does not support nor need it since initialization done for synchronous hooks can be run directly before the call to `registerHooks()`.

The `initialize` hook provides a way to define a custom function that runs in the hooks thread when the hooks module is initialized. Initialization happens when the hooks module is registered via `register`.

This hook can receive data from a `register` invocation, including ports and other transferable objects. The return value of `initialize` can be a `<Promise>`, in which case it will be awaited before the main application thread execution resumes.

Module customization code:

    // path-to-my-hooks.js

    export async function initialize({ number, port }) {
      port.postMessage(`increment: ${number + 1}`);
    }
    mjs

Caller code:

    import assert from 'node:assert';
    import { register } from 'node:module';
    import { MessageChannel } from 'node:worker_threads';

    // This example showcases how a message channel can be used to communicate
    // between the main (application) thread and the hooks running on the hooks
    // thread, by sending `port2` to the `initialize` hook.
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (msg) => {
      assert.strictEqual(msg, 'increment: 2');
    });
    port1.unref();

    register('./path-to-my-hooks.js', {
      parentURL: import.meta.url,
      data: { number: 1, port: port2 },
      transferList: [port2],
    });
    const assert = require('node:assert');
    const { register } = require('node:module');
    const { pathToFileURL } = require('node:url');
    const { MessageChannel } = require('node:worker_threads');

    // This example showcases how a message channel can be used to communicate
    // between the main (application) thread and the hooks running on the hooks
    // thread, by sending `port2` to the `initialize` hook.
    const { port1, port2 } = new MessageChannel();

    port1.on('message', (msg) => {
      assert.strictEqual(msg, 'increment: 2');
    });
    port1.unref();

    register('./path-to-my-hooks.js', {
      parentURL: pathToFileURL(__filename),
      data: { number: 1, port: port2 },
      transferList: [port2],
    });
    javascript

##### Asynchronous `resolve(specifier, context, nextResolve)`#

History

Version | Changes
--- | ---
v21.0.0, v20.10.0, v18.19.0 | The property `context.importAssertions` is replaced with `context.importAttributes`. Using the old name is still supported and will emit an experimental warning.
v18.6.0, v16.17.0 | Add support for chaining resolve hooks. Each hook must either call `nextResolve()` or include a `shortCircuit` property set to `true` in its return.
v17.1.0, v16.14.0 | Add support for import assertions.

- `specifier` `<string>`
- `context` `<Object>`
  - `conditions` `<string>`[] Export conditions of the relevant `package.json`
  - `importAttributes` `<Object>` An object whose key-value pairs represent the attributes for the module to import
  - `parentURL` `<string>` | `<undefined>` The module importing this one, or undefined if this is the Node.js entry point
- `nextResolve` `<Function>` The subsequent `resolve` hook in the chain, or the Node.js default `resolve` hook after the last user-supplied `resolve` hook
  - `specifier` `<string>`
  - `context` `<Object>` | `<undefined>` When omitted, the defaults are provided. When provided, defaults are merged in with preference to the provided properties.
- Returns: `<Object>` | `<Promise>` The asynchronous version takes either an object containing the following properties, or a `Promise` that will resolve to such an object.
  - `format` `<string>` | `<null>` | `<undefined>` A hint to the `load` hook (it might be ignored). It can be a module format (such as `'commonjs'` or `'module'`) or an arbitrary value like `'css'` or `'yaml'`.
  - `importAttributes` `<Object>` | `<undefined>` The import attributes to use when caching the module (optional; if excluded the input will be used)
  - `shortCircuit` `<undefined>` | `<boolean>` A signal that this hook intends to terminate the chain of `resolve` hooks. Default: `false`
  - `url` `<string>` The absolute URL to which this input resolves

The asynchronous version works similarly to the synchronous version, only that the `nextResolve` function returns a `Promise`, and the `resolve` hook itself can return a `Promise`.

> Warning In the case of the asynchronous version, despite support for returning promises and async functions, calls to `resolve` may still block the main thread which can impact performance.

> Warning The `resolve` hook invoked for `require()` calls inside CommonJS modules customized by asynchronous hooks does not receive the original specifier passed to `require()`. Instead, it receives a URL already fully resolved using the default CommonJS resolution.

> Warning In the CommonJS modules that are customized by the asynchronous customization hooks, `require.resolve()` and `require()` will use `"import"` export condition instead of `"require"`, which may cause unexpected behaviors when loading dual packages.

    export async function resolve(specifier, context, nextResolve) {
      // When calling `defaultResolve`, the arguments can be modified. For example,
      // to change the specifier or add conditions.
      if (specifier.includes('foo')) {
        specifier = specifier.replace('foo', 'bar');
        return nextResolve(specifier, {
          ...context,
          conditions: [...context.conditions, 'another-condition'],
        });
      }

      // The hook can also skips default resolution and provide a custom URL.
      if (specifier === 'special-module') {
        return {
          url: 'file:///path/to/special-module.mjs',
          format: 'module',
          shortCircuit: true,  // This is mandatory if not calling nextResolve().
        };
      }

      // If no customization is needed, defer to the next hook in the chain which would be the
      // Node.js default resolve if this is the last user-specified loader.
      return nextResolve(specifier);
    }
    mjs

##### Asynchronous `load(url, context, nextLoad)`#

History

Version | Changes
--- | ---
v22.6.0 | Add support for `source` with format `commonjs-typescript` and `module-typescript`.
v20.6.0 | Add support for `source` with format `commonjs`.
v18.6.0, v16.17.0 | Add support for chaining load hooks. Each hook must either call `nextLoad()` or include a `shortCircuit` property set to `true` in its return.

- `url` `<string>` The URL returned by the `resolve` chain
- `context` `<Object>`
  - `conditions` `<string>`[] Export conditions of the relevant `package.json`
  - `format` `<string>` | `<null>` | `<undefined>` The format optionally supplied by the `resolve` hook chain. This can be any string value as an input; input values do not need to conform to the list of acceptable return values described below.
  - `importAttributes` `<Object>`
- `nextLoad` `<Function>` The subsequent `load` hook in the chain, or the Node.js default `load` hook after the last user-supplied `load` hook
  - `url` `<string>`
  - `context` `<Object>` | `<undefined>` When omitted, defaults are provided. When provided, defaults are merged in with preference to the provided properties. In the default `nextLoad`, if the module pointed to by `url` does not have explicit module type information, `context.format` is mandatory.
- Returns: `<Promise>` The asynchronous version takes either an object containing the following properties, or a `Promise` that will resolve to such an object.
  - `format` `<string>`
  - `shortCircuit` `<undefined>` | `<boolean>` A signal that this hook intends to terminate the chain of `load` hooks. Default: `false`
  - `source` `<string>` | `<ArrayBuffer>` | `<TypedArray>` The source for Node.js to evaluate

> Warning: The asynchronous `load` hook and namespaced exports from CommonJS modules are incompatible. Attempting to use them together will result in an empty object from the import. This may be addressed in the future. This does not apply to the synchronous `load` hook, in which case exports can be used as usual.

The asynchronous version works similarly to the synchronous version, though when using the asynchronous `load` hook, omitting vs providing a `source` for `'commonjs'` has very different effects:

- When a `source` is provided, all `require` calls from this module will be processed by the ESM loader with registered `resolve` and `load` hooks; all `require.resolve` calls from this module will be processed by the ESM loader with registered `resolve` hooks; only a subset of the CommonJS API will be available (e.g. no `require.extensions`, no `require.cache`, no `require.resolve.paths`) and monkey-patching on the CommonJS module loader will not apply.
- If `source` is undefined or `null`, it will be handled by the CommonJS module loader and `require`/`require.resolve` calls will not go through the registered hooks. This behavior for nullish `source` is temporary — in the future, nullish `source` will not be supported.

These caveats do not apply to the synchronous `load` hook, in which case the complete set of CommonJS APIs available to the customized CommonJS modules, and `require`/`require.resolve` always go through the registered hooks.

The Node.js internal asynchronous `load` implementation, which is the value of `next` for the last hook in the `load` chain, returns `null` for `source` when `format` is `'commonjs'` for backward compatibility. Here is an example hook that would opt-in to using the non-default behavior:

    import { readFile } from 'node:fs/promises';

    // Asynchronous version accepted by module.register(). This fix is not needed
    // for the synchronous version accepted by module.registerHooks().
    export async function load(url, context, nextLoad) {
      const result = await nextLoad(url, context);
      if (result.format === 'commonjs') {
        result.source ??= await readFile(new URL(result.responseURL ?? url));
      }
      return result;
    }
    mjs

This doesn't apply to the synchronous `load` hook either, in which case the `source` returned contains source code loaded by the next hook, regardless of module format.

#### Examples#

The various module customization hooks can be used together to accomplish wide-ranging customizations of the Node.js code loading and evaluation behaviors.

##### Import from HTTPS#

The hook below registers hooks to enable rudimentary support for such specifiers. While this may seem like a significant improvement to Node.js core functionality, there are substantial downsides to actually using these hooks: performance is much slower than loading files from disk, there is no caching, and there is no security.

    // https-hooks.mjs
    import { get } from 'node:https';

    export function load(url, context, nextLoad) {
      // For JavaScript to be loaded over the network, we need to fetch and
      // return it.
      if (url.startsWith('https://')) {
        return new Promise((resolve, reject) => {
          get(url, (res) => {
            let data = '';
            res.setEncoding('utf8');
            res.on('data', (chunk) => data += chunk);
            res.on('end', () => resolve({
              // This example assumes all network-provided JavaScript is ES module
              // code.
              format: 'module',
              shortCircuit: true,
              source: data,
            }));
          }).on('error', (err) => reject(err));
        });
      }

      // Let Node.js handle all other URLs.
      return nextLoad(url);
    }
    mjs

    // main.mjs
    import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';

    console.log(VERSION);
    mjs

With the preceding hooks module, running `node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./https-hooks.mjs"));' ./main.mjs` prints the current version of CoffeeScript per the module at the URL in `main.mjs`.

##### Transpilation#

Sources that are in formats Node.js doesn't understand can be converted into JavaScript using the `load` hook.

This is less performant than transpiling source files before running Node.js; transpiler hooks should only be used for development and testing purposes.

###### Asynchronous version#

    // coffeescript-hooks.mjs
    import { readFile } from 'node:fs/promises';
    import { findPackageJSON } from 'node:module';
    import coffeescript from 'coffeescript';

    const extensionsRegex = /\.(coffee|litcoffee|coffee\.md)$/;

    export async function load(url, context, nextLoad) {
      if (extensionsRegex.test(url)) {
        // CoffeeScript files can be either CommonJS or ES modules. Use a custom format
        // to tell Node.js not to detect its module type.
        const { source: rawSource } = await nextLoad(url, { ...context, format: 'coffee' });
        // This hook converts CoffeeScript source code into JavaScript source code
        // for all imported CoffeeScript files.
        const transformedSource = coffeescript.compile(rawSource.toString(), url);

        // To determine how Node.js would interpret the transpilation result,
        // search up the file system for the nearest parent package.json file
        // and read its "type" field.
        return {
          format: await getPackageType(url),
          shortCircuit: true,
          source: transformedSource,
        };
      }

      // Let Node.js handle all other URLs.
      return nextLoad(url, context);
    }

    async function getPackageType(url) {
      // `url` is only a file path during the first iteration when passed the
      // resolved url from the load() hook
      // an actual file path from load() will contain a file extension as it's
      // required by the spec
      // this simple truthy check for whether `url` contains a file extension will
      // work for most projects but does not cover some edge-cases (such as
      // extensionless files or a url ending in a trailing space)
      const pJson = findPackageJSON(url);

      return readFile(pJson, 'utf8')
        .then(JSON.parse)
        .then((json) => json?.type)
        .catch(() => undefined);
    }
    mjs

###### Synchronous version#

    // coffeescript-sync-hooks.mjs
    import { readFileSync } from 'node:fs';
    import { registerHooks, findPackageJSON } from 'node:module';
    import coffeescript from 'coffeescript';

    const extensionsRegex = /\.(coffee|litcoffee|coffee\.md)$/;

    function load(url, context, nextLoad) {
      if (extensionsRegex.test(url)) {
        const { source: rawSource } = nextLoad(url, { ...context, format: 'coffee' });
        const transformedSource = coffeescript.compile(rawSource.toString(), url);

        return {
          format: getPackageType(url),
          shortCircuit: true,
          source: transformedSource,
        };
      }

      return nextLoad(url, context);
    }

    function getPackageType(url) {
      const pJson = findPackageJSON(url);
      if (!pJson) {
        return undefined;
      }
      try {
        const file = readFileSync(pJson, 'utf-8');
        return JSON.parse(file)?.type;
      } catch {
        return undefined;
      }
    }

    registerHooks({ load });
    mjs

##### Running hooks#

    # main.coffee
    import { scream } from './scream.coffee'
    console.log scream 'hello, world'

    import { version } from 'node:process'
    console.log "Brought to you by Node.js version #{version}"
    coffee

    # scream.coffee
    export scream = (str) -> str.toUpperCase()
    coffee

For the sake of running the example, add a `package.json` file containing the module type of the CoffeeScript files.

    {
      "type": "module"
    }
    json

This is only for running the example. In real world loaders, `getPackageType()` must be able to return an `format` known to Node.js even in the absence of an explicit type in a `package.json`, or otherwise the `nextLoad` call would throw `ERR_UNKNOWN_FILE_EXTENSION` (if undefined) or `ERR_UNKNOWN_MODULE_FORMAT` (if it's not a known format listed in the load hook documentation).

With the preceding hooks modules, running `node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./coffeescript-hooks.mjs"));' ./main.coffee` or `node --import ./coffeescript-sync-hooks.mjs ./main.coffee` causes `main.coffee` to be turned into JavaScript after its source code is loaded from disk but before Node.js executes it; and so on for any `.coffee`, `.litcoffee` or `.coffee.md` files referenced via `import` statements of any loaded file.

##### Import maps#

The previous two examples defined `load` hooks. This is an example of a `resolve` hook. This hooks module reads an `import-map.json` file that defines which specifiers to override to other URLs (this is a very simplistic implementation of a small subset of the "import maps" specification).

###### Asynchronous version#

    // import-map-hooks.js
    import fs from 'node:fs/promises';

    const { imports } = JSON.parse(await fs.readFile('import-map.json'));

    export async function resolve(specifier, context, nextResolve) {
      if (Object.hasOwn(imports, specifier)) {
        return nextResolve(imports[specifier], context);
      }

      return nextResolve(specifier, context);
    }
    mjs

###### Synchronous version#

    // import-map-sync-hooks.js
    import fs from 'node:fs/promises';
    import module from 'node:module';

    const { imports } = JSON.parse(fs.readFileSync('import-map.json', 'utf-8'));

    function resolve(specifier, context, nextResolve) {
      if (Object.hasOwn(imports, specifier)) {
        return nextResolve(imports[specifier], context);
      }

      return nextResolve(specifier, context);
    }

    module.registerHooks({ resolve });
    mjs

###### Using the hooks#

With these files:

    // main.js
    import 'a-module';
    mjs

    // import-map.json
    {
      "imports": {
        "a-module": "./some-module.js"
      }
    }
    json

    // some-module.js
    console.log('some module!');
    mjs

Running `node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./import-map-hooks.js"));' main.js` or `node --import ./import-map-sync-hooks.js main.js` should print `some module!`.

### Source Map Support#

Added in: v13.7.0, v12.17.0

Stability: 1 - Experimental

Node.js supports TC39 ECMA-426 Source Map format (it was called Source map revision 3 format).

The APIs in this section are helpers for interacting with the source map cache. This cache is populated when source map parsing is enabled and source map include directives are found in a modules' footer.

To enable source map parsing, Node.js must be run with the flag `--enable-source-maps`, or with code coverage enabled by setting `NODE_V8_COVERAGE=dir`, or be enabled programmatically via `module.setSourceMapsSupport()`.

    // module.mjs
    // In an ECMAScript module
    import { findSourceMap, SourceMap } from 'node:module';
    // module.cjs
    // In a CommonJS module
    const { findSourceMap, SourceMap } = require('node:module');
    javascript

#### `module.getSourceMapsSupport()`#

Added in: v23.7.0, v22.14.0

- Returns: `<Object>`
  - `enabled` `<boolean>` If the source maps support is enabled
  - `nodeModules` `<boolean>` If the support is enabled for files in `node_modules`.
  - `generatedCode` `<boolean>` If the support is enabled for generated code from `eval` or `new Function`.

This method returns whether the Source Map v3 support for stack traces is enabled.

#### `module.findSourceMap(path)`#

Added in: v13.7.0, v12.17.0

- `path` `<string>`
- Returns: `<module.SourceMap>` | `<undefined>` Returns `module.SourceMap` if a source map is found, `undefined` otherwise.

`path` is the resolved path for the file for which a corresponding source map should be fetched.

#### `module.setSourceMapsSupport(enabled[, options])`#

Added in: v23.7.0, v22.14.0

- `enabled` `<boolean>` Enable the source map support.
- `options` `<Object>` Optional
  - `nodeModules` `<boolean>` If enabling the support for files in `node_modules`. Default: `false`.
  - `generatedCode` `<boolean>` If enabling the support for generated code from `eval` or `new Function`. Default: `false`.

This function enables or disables the Source Map v3 support for stack traces.

It provides same features as launching Node.js process with commandline options `--enable-source-maps`, with additional options to alter the support for files in `node_modules` or generated codes.

Only source maps in JavaScript files that are loaded after source maps has been enabled will be parsed and loaded. Preferably, use the commandline options `--enable-source-maps` to avoid losing track of source maps of modules loaded before this API call.

#### Class: `module.SourceMap`#

Added in: v13.7.0, v12.17.0

##### `new SourceMap(payload[, { lineLengths }])`#

History

Version | Changes
--- | ---
v20.5.0 | Add support for `lineLengths`.

- `payload` `<Object>`
- `lineLengths` `<number>`[]

Creates a new `sourceMap` instance.

`payload` is an object with keys matching the Source map format:

- `file` `<string>`
- `version` `<number>`
- `sources` `<string>`[]
- `sourcesContent` `<string>`[]
- `names` `<string>`[]
- `mappings` `<string>`
- `sourceRoot` `<string>`

`lineLengths` is an optional array of the length of each line in the generated code.

##### `sourceMap.payload`#

- Returns: `<Object>`

Getter for the payload used to construct the `SourceMap` instance.

##### `sourceMap.findEntry(lineOffset, columnOffset)`#

- `lineOffset` `<number>` The zero-indexed line number offset in the generated source
- `columnOffset` `<number>` The zero-indexed column number offset in the generated source
- Returns: `<Object>`

Given a line offset and column offset in the generated source file, returns an object representing the SourceMap range in the original file if found, or an empty object if not.

The object returned contains the following keys:

- `generatedLine` `<number>` The line offset of the start of the range in the generated source
- `generatedColumn` `<number>` The column offset of start of the range in the generated source
- `originalSource` `<string>` The file name of the original source, as reported in the SourceMap
- `originalLine` `<number>` The line offset of the start of the range in the original source
- `originalColumn` `<number>` The column offset of start of the range in the original source
- `name` `<string>`

The returned value represents the raw range as it appears in the SourceMap, based on zero-indexed offsets, not 1-indexed line and column numbers as they appear in Error messages and CallSite objects.

To get the corresponding 1-indexed line and column numbers from a lineNumber and columnNumber as they are reported by Error stacks and CallSite objects, use `sourceMap.findOrigin(lineNumber, columnNumber)`

##### `sourceMap.findOrigin(lineNumber, columnNumber)`#

Added in: v20.4.0, v18.18.0

- `lineNumber` `<number>` The 1-indexed line number of the call site in the generated source
- `columnNumber` `<number>` The 1-indexed column number of the call site in the generated source
- Returns: `<Object>`

Given a 1-indexed `lineNumber` and `columnNumber` from a call site in the generated source, find the corresponding call site location in the original source.

If the `lineNumber` and `columnNumber` provided are not found in any source map, then an empty object is returned. Otherwise, the returned object contains the following keys:

- `name` `<string>` | `<undefined>` The name of the range in the source map, if one was provided
- `fileName` `<string>` The file name of the original source, as reported in the SourceMap
- `lineNumber` `<number>` The 1-indexed lineNumber of the corresponding call site in the original source
- `columnNumber` `<number>` The 1-indexed columnNumber of the corresponding call site in the original source

## Modules: CommonJS modules#

Stability: 2 - Stable

CommonJS modules are the original way to package JavaScript code for Node.js. Node.js also supports the ECMAScript modules standard used by browsers and other JavaScript runtimes.

In Node.js, each file is treated as a separate module. For example, consider a file named `foo.js`:

    const circle = require('./circle.js');
    console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);
    js

On the first line, `foo.js` loads the module `circle.js` that is in the same directory as `foo.js`.

Here are the contents of `circle.js`:

    const { PI } = Math;

    exports.area = (r) => PI * r ** 2;

    exports.circumference = (r) => 2 * PI * r;
    js

The module `circle.js` has exported the functions `area()` and `circumference()`. Functions and objects are added to the root of a module by specifying additional properties on the special `exports` object.

Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable `PI` is private to `circle.js`.

The `module.exports` property can be assigned a new value (such as a function or object).

In the following code, `bar.js` makes use of the `square` module, which exports a Square class:

    const Square = require('./square.js');
    const mySquare = new Square(2);
    console.log(`The area of mySquare is ${mySquare.area()}`);
    js

The `square` module is defined in `square.js`:

    // Assigning to exports will not modify module, must use module.exports
    module.exports = class Square {
      constructor(width) {
        this.width = width;
      }

      area() {
        return this.width ** 2;
      }
    };
    js

The CommonJS module system is implemented in the `module` core module.

### Enabling#

Node.js has two module systems: CommonJS modules and ECMAScript modules.

By default, Node.js will treat the following as CommonJS modules:

- Files with a `.cjs` extension.

- Files with a `.js` extension or without an extension, when the nearest parent `package.json` file contains a top-level field `"type"` with a value of `"commonjs"`.

- Files with a `.js` extension or without an extension, when the nearest parent `package.json` file doesn't contain a top-level field `"type"` or there is no `package.json` in any parent folder; unless the file contains syntax that errors unless it is evaluated as an ES module. Package authors should include the `"type"` field, even in packages where all sources are CommonJS. Being explicit about the `type` of the package will make things easier for build tools and loaders to determine how the files in the package should be interpreted.

- Files with an extension that is not `.mjs`, `.cjs`, `.json`, `.node`, or `.js`, when the nearest parent `package.json` file contains a top-level field `"type"` with a value of `"module"`.

See Determining module system for more details.

Calling `require()` always use the CommonJS module loader. Calling `import()` always use the ECMAScript module loader.

### Accessing the main module#

When a file is run directly from Node.js, `require.main` is set to its `module`. That means that it is possible to determine whether a file has been run directly by testing `require.main === module`.

For a file `foo.js`, this will be `true` if run via `node foo.js`, but `false` if run by `require('./foo')`.

When the entry point is not a CommonJS module, `require.main` is `undefined`, and the main module is out of reach.

### Package manager tips#

The semantics of the Node.js `require()` function were designed to be general enough to support reasonable directory structures. Package manager programs such as `dpkg`, `rpm`, and `npm` will hopefully find it possible to build native packages from Node.js modules without modification.

In the following, we give a suggested directory structure that could work:

Let's say that we wanted to have the folder at `/usr/lib/node/<some-package>/<some-version>` hold the contents of a specific version of a package.

Packages can depend on one another. In order to install package `foo`, it may be necessary to install a specific version of package `bar`. The `bar` package may itself have dependencies, and in some cases, these may even collide or form cyclic dependencies.

Because Node.js looks up the `realpath` of any modules it loads (that is, it resolves symlinks) and then looks for their dependencies in `node_modules` folders, this situation can be resolved with the following architecture:

- `/usr/lib/node/foo/1.2.3/`: Contents of the `foo` package, version 1.2.3.
- `/usr/lib/node/bar/4.3.2/`: Contents of the `bar` package that `foo` depends on.
- `/usr/lib/node/foo/1.2.3/node_modules/bar`: Symbolic link to `/usr/lib/node/bar/4.3.2/`.
- `/usr/lib/node/bar/4.3.2/node_modules/*`: Symbolic links to the packages that `bar` depends on.

Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.

When the code in the `foo` package does `require('bar')`, it will get the version that is symlinked into `/usr/lib/node/foo/1.2.3/node_modules/bar`. Then, when the code in the `bar` package calls `require('quux')`, it'll get the version that is symlinked into `/usr/lib/node/bar/4.3.2/node_modules/quux`.

Furthermore, to make the module lookup process even more optimal, rather than putting packages directly in `/usr/lib/node`, we could put them in `/usr/lib/node_modules/<name>/<version>`. Then Node.js will not bother looking for missing dependencies in `/usr/node_modules` or `/node_modules`.

In order to make modules available to the Node.js REPL, it might be useful to also add the `/usr/lib/node_modules` folder to the `$NODE_PATH` environment variable. Since the module lookups using `node_modules` folders are all relative, and based on the real path of the files making the calls to `require()`, the packages themselves can be anywhere.

### Loading ECMAScript modules using `require()`#

Added in: v22.0.0, v20.17.0
History

Version | Changes
--- | ---
v25.4.0 | This feature is no longer experimental.
v23.5.0, v22.13.0, v20.19.0 | This feature no longer emits an experimental warning by default, though the warning can still be emitted by --trace-require-module.
v23.0.0, v22.12.0, v20.19.0 | This feature is no longer behind the `--experimental-require-module` CLI flag.
v23.0.0, v22.12.0 | Support `'module.exports'` interop export in `require(esm)`.

The `.mjs` extension is reserved for ECMAScript Modules. See Determining module system section for more info regarding which files are parsed as ECMAScript modules.

`require()` only supports loading ECMAScript modules that meet the following requirements:

- The module is fully synchronous (contains no top-level `await`); and
- One of these conditions are met:
  1. The file has a `.mjs` extension.
  2. The file has a `.js` extension, and the closest `package.json` contains `"type": "module"`
  3. The file has a `.js` extension, the closest `package.json` does not contain `"type": "commonjs"`, and the module contains ES module syntax.

If the ES Module being loaded meets the requirements, `require()` can load it and return the module namespace object. In this case it is similar to dynamic `import()` but is run synchronously and returns the name space object directly.

With the following ES Modules:

    // distance.mjs
    export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }
    mjs

    // point.mjs
    export default class Point {
      constructor(x, y) { this.x = x; this.y = y; }
    }
    mjs

A CommonJS module can load them with `require()`:

    const distance = require('./distance.mjs');
    console.log(distance);
    // [Module: null prototype] {
    //   distance: [Function: distance]
    // }

    const point = require('./point.mjs');
    console.log(point);
    // [Module: null prototype] {
    //   default: [class Point],
    //   __esModule: true,
    // }
    cjs

For interoperability with existing tools that convert ES Modules into CommonJS, which could then load real ES Modules through `require()`, the returned namespace would contain a `__esModule: true` property if it has a `default` export so that consuming code generated by tools can recognize the default exports in real ES Modules. If the namespace already defines `__esModule`, this would not be added. This property is experimental and can change in the future. It should only be used by tools converting ES modules into CommonJS modules, following existing ecosystem conventions. Code authored directly in CommonJS should avoid depending on it.

The result returned by `require()` is the module namespace object, which places the default export in the `.default` property, similar to the results returned by `import()`. To customize what should be returned by `require(esm)` directly, the ES Module can export the desired value using the string name `"module.exports"`.

    // point.mjs
    export default class Point {
      constructor(x, y) { this.x = x; this.y = y; }
    }

    // `distance` is lost to CommonJS consumers of this module, unless it's
    // added to `Point` as a static property.
    export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }
    export { Point as 'module.exports' };
    const Point = require('./point.mjs');
    console.log(Point); // [class Point]

    // Named exports are lost when 'module.exports' is used
    const { distance } = require('./point.mjs');
    console.log(distance); // undefined
    javascript

Notice in the example above, when the `module.exports` export name is used, named exports will be lost to CommonJS consumers. To allow CommonJS consumers to continue accessing named exports, the module can make sure that the default export is an object with the named exports attached to it as properties. For example with the example above, `distance` can be attached to the default export, the `Point` class, as a static method.

    export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }

    export default class Point {
      constructor(x, y) { this.x = x; this.y = y; }
      static distance = distance;
    }

    export { Point as 'module.exports' };
    const Point = require('./point.mjs');
    console.log(Point); // [class Point]

    const { distance } = require('./point.mjs');
    console.log(distance); // [Function: distance]
    javascript

If the module being `require()`'d contains top-level `await`, or the module graph it `import`s contains top-level `await`, `ERR_REQUIRE_ASYNC_MODULE` will be thrown. In this case, users should load the asynchronous module using `import()`.

If `--experimental-print-required-tla` is enabled and the error is uncaught, Node.js will try to locate the top-level `await`s in the `require()`'d module graph and print the locations in the stderr.

If support for loading ES modules using `require()` results in unexpected breakage, it can be disabled using `--no-require-module`. To print where this feature is used, use `--trace-require-module`.

This feature can be detected by checking if `process.features.require_module` is `true`.

### All together#

To get the exact filename that will be loaded when `require()` is called, use the `require.resolve()` function.

Putting together all of the above, here is the high-level algorithm in pseudocode of what `require()` does:

    require(X) from module at path Y
    1. If X is a core module,
       a. return the core module
       b. STOP
    2. If X begins with '/'
       a. set Y to the file system root
    3. If X is equal to '.', or X begins with './', '/' or '../'
       a. LOAD_AS_FILE(Y + X)
       b. LOAD_AS_DIRECTORY(Y + X)
       c. THROW "not found"
    4. If X begins with '#'
       a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
    5. LOAD_PACKAGE_SELF(X, dirname(Y))
    6. If a package map PACKAGE_MAP exists,
       a. Find the package ID for the package owning Y
            1. Let PARENT_PACKAGE_ID be FIND_PACKAGE_ID(dirname(Y), PACKAGE_MAP)
       b. LOAD_PACKAGE_MAP(X, PARENT_PACKAGE_ID, PACKAGE_MAP)
    7. LOAD_NODE_MODULES(X, dirname(Y))
    8. THROW "not found"

    MAYBE_DETECT_AND_LOAD(X)
    1. If X parses as a CommonJS module, load X as a CommonJS module. STOP.
    2. Else, if the source code of X can be parsed as ECMAScript module using
      DETECT_MODULE_SYNTAX defined in the ESM resolver,
      a. Load X as an ECMAScript module. STOP.
    3. THROW the SyntaxError from attempting to parse X as CommonJS in 1. STOP.

    LOAD_AS_FILE(X)
    1. If X is a file, load X as its file extension format. STOP
    2. If X.js is a file,
        a. Find the closest package scope SCOPE to X.
        b. If no scope was found
          1. MAYBE_DETECT_AND_LOAD(X.js)
        c. If the SCOPE/package.json contains "type" field,
          1. If the "type" field is "module", load X.js as an ECMAScript module. STOP.
          2. If the "type" field is "commonjs", load X.js as a CommonJS module. STOP.
        d. MAYBE_DETECT_AND_LOAD(X.js)
    3. If X.json is a file, load X.json to a JavaScript Object. STOP
    4. If X.node is a file, load X.node as binary addon. STOP

    LOAD_INDEX(X)
    1. If X/index.js is a file
        a. Find the closest package scope SCOPE to X.
        b. If no scope was found, load X/index.js as a CommonJS module. STOP.
        c. If the SCOPE/package.json contains "type" field,
          1. If the "type" field is "module", load X/index.js as an ECMAScript module. STOP.
          2. Else, load X/index.js as a CommonJS module. STOP.
    2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
    3. If X/index.node is a file, load X/index.node as binary addon. STOP

    LOAD_AS_DIRECTORY(X)
    1. If X/package.json is a file,
       a. Parse X/package.json, and look for "main" field.
       b. If "main" is a falsy value, GOTO 2.
       c. let M = X + (json main field)
       d. LOAD_AS_FILE(M)
       e. LOAD_INDEX(M)
       f. LOAD_INDEX(X) DEPRECATED
       g. THROW "not found"
    2. LOAD_INDEX(X)

    LOAD_NODE_MODULES(X, START)
    1. Try to interpret X as a combination of NAME and SUBPATH where the name
       may have a @scope/ prefix and the subpath begins with a slash (`/`).
    2. let DIRS = NODE_MODULES_PATHS(START)
    3. for each DIR in DIRS:
       a. LOAD_PACKAGE_EXPORTS(SUBPATH, DIR/NAME)
       b. LOAD_AS_FILE(DIR/X)
       c. LOAD_AS_DIRECTORY(DIR/X)

    NODE_MODULES_PATHS(START)
    1. let PARTS = path split(START)
    2. let I = count of PARTS - 1
    3. let DIRS = []
    4. while I >= 0,
       a. if PARTS[I] = "node_modules", GOTO d.
       b. DIR = path join(PARTS[0 .. I] + "node_modules")
       c. DIRS = DIRS + DIR
       d. let I = I - 1
    5. return DIRS + GLOBAL_FOLDERS

    FIND_PACKAGE_ID(PATH, PACKAGE_MAP)
    1. Find the PACKAGE_ID for the entry whose "path" is a parent directory of PATH
    2. If multiple entries are found, THROW "ambiguous resolution"
    3. If no entry was found, THROW "external file".
    4. return PACKAGE_ID

    LOAD_PACKAGE_MAP(X, PARENT_PACKAGE_ID, PACKAGE_MAP)
    1. Try to interpret X as a combination of NAME and SUBPATH where the name
       may have a @scope/ prefix and the subpath begins with a slash (`/`).
    2. Find the package map entry for key PARENT_PACKAGE_ID
    3. Look up NAME in the entry's "dependencies" map.
    4. If NAME is not found, THROW "not found".
    5. Let TARGET be PACKAGE_MAP.packages[dependencies[name]]
    6. Let PACKAGE_PATH be the resolved path of TARGET.
    7. LOAD_PACKAGE_EXPORTS(SUBPATH, PACKAGE_PATH)
    8. LOAD_AS_FILE(PACKAGE_PATH/SUBPATH)
    9. LOAD_AS_DIRECTORY(PACKAGE_PATH/SUBPATH)
    10. THROW "not found"

    LOAD_PACKAGE_IMPORTS(X, DIR)
    1. Find the closest package scope SCOPE to DIR.
    2. If no scope was found, return.
    3. If the SCOPE/package.json "imports" is null or undefined, return.
    4. If `--no-require-module` is not enabled
      a. let CONDITIONS = ["node", "require", "module-sync"]
      b. Else, let CONDITIONS = ["node", "require"]
    5. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
      CONDITIONS) defined in the ESM resolver.
    6. RESOLVE_ESM_MATCH(MATCH).

    LOAD_PACKAGE_EXPORTS(SUBPATH, PACKAGE_DIR)
    1. Parse PACKAGE_DIR/package.json, and look for "exports" field.
    2. If "exports" is null or undefined, return.
    3. If `--no-require-module` is not enabled
      a. let CONDITIONS = ["node", "require", "module-sync"]
      b. Else, let CONDITIONS = ["node", "require"]
    4. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(PACKAGE_DIR), "." + SUBPATH,
       `package.json` "exports", CONDITIONS) defined in the ESM resolver.
    5. RESOLVE_ESM_MATCH(MATCH)

    LOAD_PACKAGE_SELF(X, DIR)
    1. Find the closest package scope SCOPE to DIR.
    2. If no scope was found, return.
    3. If the SCOPE/package.json "exports" is null or undefined, return.
    4. If the SCOPE/package.json "name" is not the first segment of X, return.
    5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(SCOPE),
       "." + X.slice("name".length), `package.json` "exports", ["node", "require"])
       defined in the ESM resolver.
    6. RESOLVE_ESM_MATCH(MATCH)

    RESOLVE_ESM_MATCH(MATCH)
    1. let RESOLVED_PATH = fileURLToPath(MATCH)
    2. If the file at RESOLVED_PATH exists, load RESOLVED_PATH as its extension
       format. STOP
    3. THROW "not found"
    text

The "ESM resolver" is defined in the ESM documentation.

### Caching#

Modules are cached after the first time they are loaded. This means (among other things) that every call to `require('foo')` will get exactly the same object returned, if it would resolve to the same file.

Provided `require.cache` is not modified, multiple calls to `require('foo')` will not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles.

To have a module execute code multiple times, export a function, and call that function.

#### Module caching caveats#

Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from `node_modules` folders), it is not a guarantee that `require('foo')` will always return the exact same object, if it would resolve to different files.

Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, `require('./foo')` and `require('./FOO')` return two different objects, irrespective of whether or not `./foo` and `./FOO` are the same file.

### Built-in modules#

History

Version | Changes
--- | ---
v16.0.0, v14.18.0 | Added `node:` import support to `require(...)`.

Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.

The built-in modules are defined within the Node.js source and are located in the `lib/` folder.

Built-in modules can be identified using the `node:` prefix, in which case it bypasses the `require` cache. For instance, `require('node:http')` will always return the built in HTTP module, even if there is `require.cache` entry by that name.

Some built-in modules are always preferentially loaded if their identifier is passed to `require()`. For instance, `require('http')` will always return the built-in HTTP module, even if there is a file by that name.

The list of all the built-in modules can be retrieved from `module.builtinModules`. The modules being all listed without the `node:` prefix, except those that mandate such prefix (as explained in the next section).

#### Built-in modules with mandatory `node:` prefix#

When being loaded by `require()`, some built-in modules must be requested with the `node:` prefix. This requirement exists to prevent newly introduced built-in modules from having a conflict with user land packages that already have taken the name. Currently the built-in modules that requires the `node:` prefix are:

- `node:ffi`
- `node:sea`
- `node:sqlite`
- `node:test`
- `node:test/reporters`

The list of these modules is exposed in `module.builtinModules`, including the prefix.

### Cycles#

When there are circular `require()` calls, a module might not have finished executing when it is returned.

Consider this situation:

`a.js`:

    console.log('a starting');
    exports.done = false;
    const b = require('./b.js');
    console.log('in a, b.done = %j', b.done);
    exports.done = true;
    console.log('a done');
    js

`b.js`:

    console.log('b starting');
    exports.done = false;
    const a = require('./a.js');
    console.log('in b, a.done = %j', a.done);
    exports.done = true;
    console.log('b done');
    js

`main.js`:

    console.log('main starting');
    const a = require('./a.js');
    const b = require('./b.js');
    console.log('in main, a.done = %j, b.done = %j', a.done, b.done);
    js

When `main.js` loads `a.js`, then `a.js` in turn loads `b.js`. At that point, `b.js` tries to load `a.js`. In order to prevent an infinite loop, an unfinished copy of the `a.js` exports object is returned to the `b.js` module. `b.js` then finishes loading, and its `exports` object is provided to the `a.js` module.

By the time `main.js` has loaded both modules, they're both finished. The output of this program would thus be:

    $ node main.js
    main starting
    a starting
    b starting
    in b, a.done = false
    b done
    in a, b.done = true
    a done
    in main, a.done = true, b.done = true
    console

Careful planning is required to allow cyclic module dependencies to work correctly within an application.

### File modules#

If the exact filename is not found, then Node.js will attempt to load the required filename with the added extensions: `.js`, `.json`, and finally `.node`. When loading a file that has a different extension (e.g. `.cjs`), its full name must be passed to `require()`, including its file extension (e.g. `require('./file.cjs')`).

`.json` files are parsed as JSON text files, `.node` files are interpreted as compiled addon modules loaded with `process.dlopen()`. Files using any other extension (or no extension at all) are parsed as JavaScript text files. Refer to the Determining module system section to understand what parse goal will be used.

A required module prefixed with `'/'` is an absolute path to the file. For example, `require('/home/marco/foo.js')` will load the file at `/home/marco/foo.js`.

A required module prefixed with `'./'` is relative to the file calling `require()`. That is, `circle.js` must be in the same directory as `foo.js` for `require('./circle')` to find it.

Without a leading `'/'`, `'./'`, or `'../'` to indicate a file, the module must either be a core module or is loaded from a `node_modules` folder.

If the given path does not exist, `require()` will throw a `MODULE_NOT_FOUND` error.

### Folders as modules#

Stability: 3 - Legacy: Use subpath exports or subpath imports instead.

There are three ways in which a folder may be passed to `require()` as an argument.

The first is to create a `package.json` file in the root of the folder, which specifies a `main` module. An example `package.json` file might look like this:

    { "name" : "some-library",
      "main" : "./lib/some-library.js" }
    json

If this was in a folder at `./some-library`, then `require('./some-library')` would attempt to load `./some-library/lib/some-library.js`.

If there is no `package.json` file present in the directory, or if the `"main"` entry is missing or cannot be resolved, then Node.js will attempt to load an `index.js` or `index.node` file out of that directory. For example, if there was no `package.json` file in the previous example, then `require('./some-library')` would attempt to load:

- `./some-library/index.js`
- `./some-library/index.node`

If these attempts fail, then Node.js will report the entire module as missing with the default error:

    Error: Cannot find module 'some-library'
    console

In all three above cases, an `import('./some-library')` call would result in a `ERR_UNSUPPORTED_DIR_IMPORT` error. Using package subpath exports or subpath imports can provide the same containment organization benefits as folders as modules, and work for both `require` and `import`.

### Loading from `node_modules` folders#

If the module identifier passed to `require()` is not a built-in module, and does not begin with `'/'`, `'../'`, or `'./'`, then Node.js starts at the directory of the current module, and adds `/node_modules`, and attempts to load the module from that location. Node.js will not append `node_modules` to a path already ending in `node_modules`.

If it is not found there, then it moves to the parent directory, and so on, until the root of the file system is reached.

For example, if the file at `'/home/ry/projects/foo.js'` called `require('bar.js')`, then Node.js would look in the following locations, in this order:

- `/home/ry/projects/node_modules/bar.js`
- `/home/ry/node_modules/bar.js`
- `/home/node_modules/bar.js`
- `/node_modules/bar.js`

This allows programs to localize their dependencies, so that they do not clash.

It is possible to require specific files or sub modules distributed with a module by including a path suffix after the module name. For instance `require('example-module/path/to/file')` would resolve `path/to/file` relative to where `example-module` is located. The suffixed path follows the same module resolution semantics.

### Loading from the global folders#

If the `NODE_PATH` environment variable is set to a colon-delimited list of absolute paths, then Node.js will search those paths for modules if they are not found elsewhere.

On Windows, `NODE_PATH` is delimited by semicolons (`;`) instead of colons.

`NODE_PATH` was originally created to support loading modules from varying paths before the current module resolution algorithm was defined.

`NODE_PATH` is still supported, but is less necessary now that the Node.js ecosystem has settled on a convention for locating dependent modules. Sometimes deployments that rely on `NODE_PATH` show surprising behavior when people are unaware that `NODE_PATH` must be set. Sometimes a module's dependencies change, causing a different version (or even a different module) to be loaded as the `NODE_PATH` is searched.

Additionally, Node.js will search in the following list of GLOBAL_FOLDERS:

- 1: `$HOME/.node_modules`
- 2: `$HOME/.node_libraries`
- 3: `$PREFIX/lib/node`

Where `$HOME` is the user's home directory, and `$PREFIX` is the Node.js configured `node_prefix`.

These are mostly for historic reasons.

It is strongly encouraged to place dependencies in the local `node_modules` folder. These will be loaded faster, and more reliably.

### The module wrapper#

Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:

    (function(exports, require, module, __filename, __dirname) {
    // Module code actually lives in here
    });
    js

By doing this, Node.js achieves a few things:

- It keeps top-level variables (defined with `var`, `const`, or `let`) scoped to the module rather than the global object.
- It helps to provide some global-looking variables that are actually specific to the module, such as:
  - The `module` and `exports` objects that the implementor can use to export values from the module.
  - The convenience variables `__filename` and `__dirname`, containing the module's absolute filename and directory path.

### The module scope#

#### `__dirname`#

Added in: v0.1.27

- Type: `<string>`

The directory name of the current module. This is the same as the `path.dirname()` of the `__filename`.

Example: running `node example.js` from `/Users/mjr`

    console.log(__dirname);
    // Prints: /Users/mjr
    console.log(path.dirname(__filename));
    // Prints: /Users/mjr
    js

#### `__filename`#

Added in: v0.0.1

- Type: `<string>`

The file name of the current module. This is the current module file's absolute path with symlinks resolved.

For a main program this is not necessarily the same as the file name used in the command line.

See `__dirname` for the directory name of the current module.

Examples:

Running `node example.js` from `/Users/mjr`

    console.log(__filename);
    // Prints: /Users/mjr/example.js
    console.log(__dirname);
    // Prints: /Users/mjr
    js

Given two modules: `a` and `b`, where `b` is a dependency of `a` and there is a directory structure of:

- `/Users/mjr/app/a.js`
- `/Users/mjr/app/node_modules/b/b.js`

References to `__filename` within `b.js` will return `/Users/mjr/app/node_modules/b/b.js` while references to `__filename` within `a.js` will return `/Users/mjr/app/a.js`.

#### `exports`#

Added in: v0.1.12

- Type: `<Object>`

A reference to the `module.exports` that is shorter to type. See the section about the exports shortcut for details on when to use `exports` and when to use `module.exports`.

#### `module`#

Added in: v0.1.16

- Type: `<module>`

A reference to the current module, see the section about the `module` object. In particular, `module.exports` is used for defining what a module exports and makes available through `require()`.

#### `require(id)`#

Added in: v0.1.13

- `id` `<string>` module name or path
- Returns: `<any>` exported module content

Used to import modules, `JSON`, and local files. Modules can be imported from `node_modules`. Local modules and JSON files can be imported using a relative path (e.g. `./`, `./foo`, `./bar/baz`, `../foo`) that will be resolved against the directory named by `__dirname` (if defined) or the current working directory. The relative paths of POSIX style are resolved in an OS independent fashion, meaning that the examples above will work on Windows in the same way they would on Unix systems.

    // Importing a local module with a path relative to the `__dirname` or current
    // working directory. (On Windows, this would resolve to .\path\myLocalModule.)
    const myLocalModule = require('./path/myLocalModule');

    // Importing a JSON file:
    const jsonData = require('./path/filename.json');

    // Importing a module from node_modules or Node.js built-in module:
    const crypto = require('node:crypto');
    js

##### `require.cache`#

Added in: v0.3.0

- Type: `<Object>`

Modules are cached in this object when they are required. By deleting a key value from this object, the next `require` will reload the module. This does not apply to native addons, for which reloading will result in an error.

Adding or replacing entries is also possible. This cache is checked before built-in modules and if a name matching a built-in module is added to the cache, only `node:`-prefixed require calls are going to receive the built-in module. Use with care!

    const assert = require('node:assert');
    const realFs = require('node:fs');

    const fakeFs = {};
    require.cache.fs = { exports: fakeFs };

    assert.strictEqual(require('fs'), fakeFs);
    assert.strictEqual(require('node:fs'), realFs);
    js

##### `require.extensions`#

Added in: v0.3.0Deprecated in: v0.10.6

Stability: 0 - Deprecated

- Type: `<Object>`

Instruct `require` on how to handle certain file extensions.

Process files with the extension `.sjs` as `.js`:

    require.extensions['.sjs'] = require.extensions['.js'];
    js

Deprecated. In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.

Avoid using `require.extensions`. Use could cause subtle bugs and resolving the extensions gets slower with each registered extension.

##### `require.main`#

Added in: v0.1.17

- Type: `<module>` | `<undefined>`

The `Module` object representing the entry script loaded when the Node.js process launched, or `undefined` if the entry point of the program is not a CommonJS module. See "Accessing the main module".

In `entry.js` script:

    console.log(require.main);
    js

    node entry.js
    bash

    Module {
      id: '.',
      path: '/absolute/path/to',
      exports: {},
      filename: '/absolute/path/to/entry.js',
      loaded: false,
      children: [],
      paths:
       [ '/absolute/path/to/node_modules',
         '/absolute/path/node_modules',
         '/absolute/node_modules',
         '/node_modules' ] }
    js

##### `require.resolve(request[, options])`#

Added in: v0.3.0
History

Version | Changes
--- | ---
v8.9.0 | The `paths` option is now supported.

- `request` `<string>` The module path to resolve.
- `options` `<Object>`
  - `paths` `<string>`[] Paths to resolve module location from. If present, these paths are used instead of the default resolution paths, with the exception of GLOBAL_FOLDERS like `$HOME/.node_modules`, which are always included. Each of these paths is used as a starting point for the module resolution algorithm, meaning that the `node_modules` hierarchy is checked from this location.
- Returns: `<string>`

Use the internal `require()` machinery to look up the location of a module, but rather than loading the module, just return the resolved filename.

If the module can not be found, a `MODULE_NOT_FOUND` error is thrown.

###### `require.resolve.paths(request)`#

Added in: v8.9.0

- `request` `<string>` The module path whose lookup paths are being retrieved.
- Returns: `<string>`[] | `<null>`

Returns an array containing the paths searched during resolution of `request` or `null` if the `request` string references a core module, for example `http` or `fs`.

### The `module` object#

Added in: v0.1.16

- Type: `<Object>`

In each module, the `module` free variable is a reference to the object representing the current module. For convenience, `module.exports` is also accessible via the `exports` module-global. `module` is not actually a global but rather local to each module.

#### `module.children`#

Added in: v0.1.16

- Type: `<module>`[]

The module objects required for the first time by this one.

#### `module.exports`#

Added in: v0.1.16

- Type: `<Object>`

The `module.exports` object is created by the `Module` system. Sometimes this is not acceptable; many want their module to be an instance of some class. To do this, assign the desired export object to `module.exports`. Assigning the desired object to `exports` will simply rebind the local `exports` variable, which is probably not what is desired.

For example, suppose we were making a module called `a.js`:

    const EventEmitter = require('node:events');

    module.exports = new EventEmitter();

    // Do some work, and after some time emit
    // the 'ready' event from the module itself.
    setTimeout(() => {
      module.exports.emit('ready');
    }, 1000);
    js

Then in another file we could do:

    const a = require('./a');
    a.on('ready', () => {
      console.log('module "a" is ready');
    });
    js

Assignment to `module.exports` must be done immediately. It cannot be done in any callbacks. This does not work:

`x.js`:

    setTimeout(() => {
      module.exports = { a: 'hello' };
    }, 0);
    js

`y.js`:

    const x = require('./x');
    console.log(x.a);
    js

##### `exports` shortcut#

Added in: v0.1.16

The `exports` variable is available within a module's file-level scope, and is assigned the value of `module.exports` before the module is evaluated.

It allows a shortcut, so that `module.exports.f = ...` can be written more succinctly as `exports.f = ...`. However, be aware that like any variable, if a new value is assigned to `exports`, it is no longer bound to `module.exports`:

    module.exports.hello = true; // Exported from require of module
    exports = { hello: false };  // Not exported, only available in the module
    js

When the `module.exports` property is being completely replaced by a new object, it is common to also reassign `exports`:

    module.exports = exports = function Constructor() {
      // ... etc.
    };
    js

To illustrate the behavior, imagine this hypothetical implementation of `require()`, which is quite similar to what is actually done by `require()`:

    function require(/* ... */) {
      const module = { exports: {} };
      ((module, exports) => {
        // Module code here. In this example, define a function.
        function someFunc() {}
        exports = someFunc;
        // At this point, exports is no longer a shortcut to module.exports, and
        // this module will still export an empty default object.
        module.exports = someFunc;
        // At this point, the module will now export someFunc, instead of the
        // default object.
      })(module, module.exports);
      return module.exports;
    }
    js

#### `module.filename`#

Added in: v0.1.16

- Type: `<string>`

The fully resolved filename of the module.

#### `module.id`#

Added in: v0.1.16

- Type: `<string>`

The identifier for the module. Typically this is the fully resolved filename.

#### `module.isPreloading`#

Added in: v15.4.0, v14.17.0

- Type: `<boolean>` `true` if the module is running during the Node.js preload phase.

#### `module.loaded`#

Added in: v0.1.16

- Type: `<boolean>`

Whether or not the module is done loading, or is in the process of loading.

#### `module.parent`#

Added in: v0.1.16Deprecated in: v14.6.0, v12.19.0

Stability: 0 - Deprecated: Please use `require.main` and `module.children` instead.

- Type: `<module>` | `<null>` | `<undefined>`

The module that first required this one, or `null` if the current module is the entry point of the current process, or `undefined` if the module was loaded by something that is not a CommonJS module (E.G.: REPL or `import`).

#### `module.path`#

Added in: v11.14.0

- Type: `<string>`

The directory name of the module. This is usually the same as the `path.dirname()` of the `module.id`.

#### `module.paths`#

Added in: v0.4.0

- Type: `<string>`[]

The search paths for the module.

#### `module.require(id)`#

Added in: v0.5.1

- `id` `<string>`
- Returns: `<any>` exported module content

The `module.require()` method provides a way to load a module as if `require()` was called from the original module.

In order to do this, it is necessary to get a reference to the `module` object. Since `require()` returns the `module.exports`, and the `module` is typically only available within a specific module's code, it must be explicitly exported in order to be used.

### The `Module` object#

This section was moved to Modules: `module` core module.

- `module.builtinModules`
- `module.createRequire(filename)`
- `module.syncBuiltinESMExports()`

### Source map v3 support#

This section was moved to Modules: `module` core module.

- `module.findSourceMap(path)`
- Class: `module.SourceMap`
  - `new SourceMap(payload)`
  - `sourceMap.payload`
  - `sourceMap.findEntry(lineNumber, columnNumber)`

## Modules: ECMAScript modules#

Added in: v8.5.0
History

Version | Changes
--- | ---
v23.1.0, v22.12.0, v20.18.3, v18.20.5 | Import attributes are no longer experimental.
v22.0.0 | Drop support for import assertions.
v21.0.0, v20.10.0, v18.20.0 | Add experimental support for import attributes.
v20.0.0, v18.19.0 | Module customization hooks are executed off the main thread.
v18.6.0, v16.17.0 | Add support for chaining module customization hooks.
v17.1.0, v16.14.0 | Add experimental support for import assertions.
v17.0.0, v16.12.0 | Consolidate customization hooks, removed `getFormat`, `getSource`, `transformSource`, and `getGlobalPreloadCode` hooks added `load` and `globalPreload` hooks allowed returning `format` from either `resolve` or `load` hooks.
v15.3.0, v14.17.0, v12.22.0 | Stabilize modules implementation.
v14.13.0, v12.20.0 | Support for detection of CommonJS named exports.
v14.8.0 | Unflag Top-Level Await.
v14.0.0, v13.14.0, v12.20.0 | Remove experimental modules warning.
v13.2.0, v12.17.0 | Loading ECMAScript modules no longer requires a command-line flag.
v12.0.0 | Add support for ES modules using `.js` file extension via `package.json` `"type"` field.

Stability: 2 - Stable

### Introduction#

ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of `import` and `export` statements.

The following example of an ES module exports a function:

    // addTwo.mjs
    function addTwo(num) {
      return num + 2;
    }

    export { addTwo };
    js

The following example of an ES module imports the function from `addTwo.mjs`:

    // app.mjs
    import { addTwo } from './addTwo.mjs';

    // Prints: 6
    console.log(addTwo(4));
    js

Node.js fully supports ECMAScript modules as they are currently specified and provides interoperability between them and its original module format, CommonJS.

### Enabling#

Node.js has two module systems: CommonJS modules and ECMAScript modules.

Authors can tell Node.js to interpret JavaScript as an ES module via the `.mjs` file extension, the `package.json` `"type"` field with a value `"module"`, or the `--input-type` flag with a value of `"module"`. These are explicit markers of code being intended to run as an ES module.

Inversely, authors can explicitly tell Node.js to interpret JavaScript as CommonJS via the `.cjs` file extension, the `package.json` `"type"` field with a value `"commonjs"`, or the `--input-type` flag with a value of `"commonjs"`.

When code lacks explicit markers for either module system, Node.js will inspect the source code of a module to look for ES module syntax. If such syntax is found, Node.js will run the code as an ES module; otherwise it will run the module as CommonJS. See Determining module system for more details.

### Packages#

This section was moved to Modules: Packages.

### `import` Specifiers#

#### Terminology#

The specifier of an `import` statement is the string after the `from` keyword, e.g. `'node:path'` in `import { sep } from 'node:path'`. Specifiers are also used in `export from` statements, and as the argument to an `import()` expression.

There are three types of specifiers:

- Relative specifiers like `'./startup.js'` or `'../config.mjs'`. They refer to a path relative to the location of the importing file. The file extension is always necessary for these.

- Bare specifiers like `'some-package'` or `'some-package/shuffle'`. They can refer to the main entry point of a package by the package name, or a specific feature module within a package prefixed by the package name as per the examples respectively. Including the file extension is only necessary for packages without an `"exports"` field.

- Absolute specifiers like `'file:///opt/nodejs/config.js'`. They refer directly and explicitly to a full path.

Bare specifier resolutions are handled by the Node.js module resolution and loading algorithm. All other specifier resolutions are always only resolved with the standard relative URL resolution semantics.

Like in CommonJS, module files within packages can be accessed by appending a path to the package name unless the package's `package.json` contains an `"exports"` field, in which case files within packages can only be accessed via the paths defined in `"exports"`.

For details on these package resolution rules that apply to bare specifiers in the Node.js module resolution, see the packages documentation.

#### Mandatory file extensions#

A file extension must be provided when using the `import` keyword to resolve relative or absolute specifiers. Directory indexes (e.g. `'./startup/index.js'`) must also be fully specified.

This behavior matches how `import` behaves in browser environments, assuming a typically configured server.

#### URLs#

ES modules are resolved and cached as URLs. This means that special characters must be percent-encoded, such as `#` with `%23` and `?` with `%3F`.

`file:`, `node:`, and `data:` URL schemes are supported. A specifier like `'https://example.com/app.js'` is not supported natively in Node.js unless using a custom HTTPS loader.

##### `file:` URLs#

Modules are loaded multiple times if the `import` specifier used to resolve them has a different query or fragment.

    import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
    import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"
    js

The volume root may be referenced via `/`, `//`, or `file:///`. Given the differences between URL and path resolution (such as percent encoding details), it is recommended to use url.pathToFileURL when importing a path.

##### `data:` imports#

Added in: v12.10.0

`data:` URLs are supported for importing with the following MIME types:

- `text/javascript` for ES modules
- `application/json` for JSON
- `application/wasm` for Wasm

    import 'data:text/javascript,console.log("hello!");';
    import _ from 'data:application/json,"world!"' with { type: 'json' };
    js

`data:` URLs only resolve bare specifiers for builtin modules and absolute specifiers. Resolving relative specifiers does not work because `data:` is not a special scheme. For example, attempting to load `./foo` from `data:text/javascript,import "./foo";` fails to resolve because there is no concept of relative resolution for `data:` URLs.

##### `node:` imports#

Added in: v14.13.1, v12.20.0
History

Version | Changes
--- | ---
v16.0.0, v14.18.0 | Added `node:` import support to `require(...)`.

`node:` URLs are supported as an alternative means to load Node.js builtin modules. This URL scheme allows for builtin modules to be referenced by valid absolute URL strings.

    import fs from 'node:fs/promises';
    js

### Import attributes#

Added in: v17.1.0, v16.14.0
History

Version | Changes
--- | ---
v21.0.0, v20.10.0, v18.20.0 | Switch from Import Assertions to Import Attributes.

Import attributes are an inline syntax for module import statements to pass on more information alongside the module specifier.

    import fooData from './foo.json' with { type: 'json' };

    const { default: barData } =
      await import('./bar.json', { with: { type: 'json' } });
    js

Node.js only supports the `type` attribute, for which it supports the following values:

Attribute `type` | Needed for
--- | ---
`'json'` | JSON modules
`'text'` | Text modules

The `type: 'json'` attribute is mandatory when importing JSON modules. The `type: 'text'` attribute is mandatory when importing text modules.

### Built-in modules#

Built-in modules provide named exports of their public API. A default export is also provided which is the value of the CommonJS exports. The default export can be used for, among other things, modifying the named exports. Named exports of built-in modules are updated only by calling `module.syncBuiltinESMExports()`.

    import EventEmitter from 'node:events';
    const e = new EventEmitter();
    js

    import { readFile } from 'node:fs';
    readFile('./foo.txt', (err, source) => {
      if (err) {
        console.error(err);
      } else {
        console.log(source);
      }
    });
    js

    import fs, { readFileSync } from 'node:fs';
    import { syncBuiltinESMExports } from 'node:module';
    import { Buffer } from 'node:buffer';

    fs.readFileSync = () => Buffer.from('Hello, ESM');
    syncBuiltinESMExports();

    fs.readFileSync === readFileSync;
    js

> When importing built-in modules, all the named exports (i.e. properties of the module exports object) are populated even if they are not individually accessed. This can make initial imports of built-in modules slightly slower compared to loading them with `require()` or `process.getBuiltinModule()`, where the module exports object is evaluated immediately, but some of its properties may only be initialized when first accessed individually.

### `import()` expressions#

Dynamic `import()` provides an asynchronous way to import modules. It is supported in both CommonJS and ES modules, and can be used to load both CommonJS and ES modules.

### `import.meta`#

- Type: `<Object>`

The `import.meta` meta property is an `Object` that contains the following properties. It is only supported in ES modules.

#### `import.meta.dirname`#

Added in: v21.2.0, v20.11.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | This property is no longer experimental.

- Type: `<string>` The directory name of the current module.

This is the same as the `path.dirname()` of the `import.meta.filename`.

> Caveat: only present on `file:` modules.

#### `import.meta.filename`#

Added in: v21.2.0, v20.11.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | This property is no longer experimental.

- Type: `<string>` The full absolute path and filename of the current module, with symlinks resolved.

This is the same as the `url.fileURLToPath()` of the `import.meta.url`.

> Caveat only local modules support this property. Modules not using the `file:` protocol will not provide it.

#### `import.meta.url`#

- Type: `<string>` The absolute `file:` URL of the module.

This is defined exactly the same as it is in browsers providing the URL of the current module file.

This enables useful patterns such as relative file loading:

    import { readFileSync } from 'node:fs';
    const buffer = readFileSync(new URL('./data.proto', import.meta.url));
    js

#### `import.meta.main`#

Added in: v24.2.0, v22.18.0

Stability: 1.0 - Early development

- Type: `<boolean>` `true` when the current module is the entry point of the current process; `false` otherwise.

Equivalent to `require.main === module` in CommonJS.

Analogous to Python's `__name__ == "__main__"`.

    export function foo() {
      return 'Hello, world';
    }

    function main() {
      const message = foo();
      console.log(message);
    }

    if (import.meta.main) main();
    // `foo` can be imported from another module without possible side-effects from `main`
    js

#### `import.meta.resolve(specifier)`#

Added in: v13.9.0, v12.16.2
History

Version | Changes
--- | ---
v20.6.0, v18.19.0 | No longer behind `--experimental-import-meta-resolve` CLI flag, except for the non-standard `parentURL` parameter.
v20.6.0, v18.19.0 | This API no longer throws when targeting `file:` URLs that do not map to an existing file on the local FS.
v20.0.0, v18.19.0 | This API now returns a string synchronously instead of a Promise.
v16.2.0, v14.18.0 | Add support for WHATWG `URL` object to `parentURL` parameter.

Stability: 1.2 - Release candidate

- `specifier` `<string>` The module specifier to resolve relative to the current module.
- Returns: `<string>` The absolute URL string that the specifier would resolve to.

`import.meta.resolve` is a module-relative resolution function scoped to each module, returning the URL string.

    const dependencyAsset = import.meta.resolve('component-lib/asset.css');
    // file:///app/node_modules/component-lib/asset.css
    import.meta.resolve('./dep.js');
    // file:///app/dep.js
    js

All features of the Node.js module resolution are supported. Dependency resolutions are subject to the permitted exports resolutions within the package.

Caveats:

- This can result in synchronous file-system operations, which can impact performance similarly to `require.resolve`.
- This feature is not available within custom loaders (it would create a deadlock).

Non-standard API:

When using the `--experimental-import-meta-resolve` flag, that function accepts a second argument:

- `parent` `<string>` | `<URL>` An optional absolute parent module URL to resolve from. Default: `import.meta.url`

### Interoperability with CommonJS#

#### `import` statements#

An `import` statement can reference an ES module or a CommonJS module. `import` statements are permitted only in ES modules, but dynamic `import()` expressions are supported in CommonJS for loading ES modules.

When importing CommonJS modules, the `module.exports` object is provided as the default export. Named exports may be available, provided by static analysis as a convenience for better ecosystem compatibility.

#### `require`#

The CommonJS module `require` currently only supports loading synchronous ES modules (that is, ES modules that do not use top-level `await`).

See Loading ECMAScript modules using `require()` for details.

#### CommonJS Namespaces#

Added in: v14.13.0
History

Version | Changes
--- | ---
v23.0.0 | Added `'module.exports'` export marker to CJS namespaces.

CommonJS modules consist of a `module.exports` object which can be of any type.

To support this, when importing CommonJS from an ECMAScript module, a namespace wrapper for the CommonJS module is constructed, which always provides a `default` export key pointing to the CommonJS `module.exports` value.

In addition, a heuristic static analysis is performed against the source text of the CommonJS module to get a best-effort static list of exports to provide on the namespace from values on `module.exports`. This is necessary since these namespaces must be constructed prior to the evaluation of the CJS module.

These CommonJS namespace objects also provide the `default` export as a `'module.exports'` named export, in order to unambiguously indicate that their representation in CommonJS uses this value, and not the namespace value. This mirrors the semantics of the handling of the `'module.exports'` export name in `require(esm)` interop support.

When importing a CommonJS module, it can be reliably imported using the ES module default import or its corresponding sugar syntax:

    import { default as cjs } from 'cjs';
    // Identical to the above
    import cjsSugar from 'cjs';

    console.log(cjs);
    console.log(cjs === cjsSugar);
    // Prints:
    //   <module.exports>
    //   true
    js

This Module Namespace Exotic Object can be directly observed either when using `import * as m from 'cjs'` or a dynamic import:

    import * as m from 'cjs';
    console.log(m);
    console.log(m === await import('cjs'));
    // Prints:
    //   [Module] { default: <module.exports>, 'module.exports': <module.exports> }
    //   true
    js

For better compatibility with existing usage in the JS ecosystem, Node.js in addition attempts to determine the CommonJS named exports of every imported CommonJS module to provide them as separate ES module exports using a static analysis process.

For example, consider a CommonJS module written:

    // cjs.cjs
    exports.name = 'exported';
    cjs

The preceding module supports named imports in ES modules:

    import { name } from './cjs.cjs';
    console.log(name);
    // Prints: 'exported'

    import cjs from './cjs.cjs';
    console.log(cjs);
    // Prints: { name: 'exported' }

    import * as m from './cjs.cjs';
    console.log(m);
    // Prints:
    //   [Module] {
    //     default: { name: 'exported' },
    //     'module.exports': { name: 'exported' },
    //     name: 'exported'
    //   }
    js

As can be seen from the last example of the Module Namespace Exotic Object being logged, the `name` export is copied off of the `module.exports` object and set directly on the ES module namespace when the module is imported.

Live binding updates or new exports added to `module.exports` are not detected for these named exports.

The detection of named exports is based on common syntax patterns but does not always correctly detect named exports. In these cases, using the default import form described above can be a better option.

Named exports detection covers many common export patterns, reexport patterns and build tool and transpiler outputs. See merve for the exact semantics implemented.

#### Differences between ES modules and CommonJS#

##### No `require`, `exports`, or `module.exports`#

In most cases, the ES module `import` can be used to load CommonJS modules.

If needed, a `require` function can be constructed within an ES module using `module.createRequire()`.

##### No `__filename` or `__dirname`#

These CommonJS variables are not available in ES modules.

`__filename` and `__dirname` use cases can be replicated via `import.meta.filename` and `import.meta.dirname`.

##### No Addon Loading#

Addons are not currently supported with ES module imports.

They can instead be loaded with `module.createRequire()` or `process.dlopen`.

##### No `require.main`#

To replace `require.main === module`, there is the `import.meta.main` API.

##### No `require.resolve`#

Relative resolution can be handled via `new URL('./local', import.meta.url)`.

For a complete `require.resolve` replacement, there is the import.meta.resolve API.

Alternatively `module.createRequire()` can be used.

##### No `NODE_PATH`#

`NODE_PATH` is not part of resolving `import` specifiers. Please use symlinks if this behavior is desired.

##### No `require.extensions`#

`require.extensions` is not used by `import`. Module customization hooks can provide a replacement.

##### No `require.cache`#

`require.cache` is not used by `import` as the ES module loader has its own separate cache.

### JSON modules#

History

Version | Changes
--- | ---
v23.1.0, v22.12.0, v20.18.3, v18.20.5 | JSON modules are no longer experimental.

JSON files can be referenced by `import`:

    import packageConfig from './package.json' with { type: 'json' };
    js

The `with { type: 'json' }` syntax is mandatory; see Import Attributes.

The imported JSON only exposes a `default` export. There is no support for named exports. A cache entry is created in the CommonJS cache to avoid duplication. The same object is returned in CommonJS if the JSON module has already been imported from the same path.

### Text modules#

Stability: 1.0 - Early development

Text modules are available behind the `--experimental-import-text` flag.

Text files can be referenced by `import`:

    import message from './message.txt' with { type: 'text' };
    js

The `with { type: 'text' }` syntax is mandatory; see Import Attributes.

The imported text only exposes a `default` export whose value is the module source as a string.

### Wasm modules#

History

Version | Changes
--- | ---
v24.5.0, v22.19.0 | Wasm modules no longer require the `--experimental-wasm-modules` flag.

Importing both WebAssembly module instances and WebAssembly source phase imports is supported.

Both of these integrations are in line with the ES Module Integration Proposal for WebAssembly.

#### Wasm Source Phase Imports#

Added in: v24.0.0

Stability: 1.2 - Release candidate

The Source Phase Imports proposal allows the `import source` keyword combination to import a `WebAssembly.Module` object directly, instead of getting a module instance already instantiated with its dependencies.

This is useful when needing custom instantiations for Wasm, while still resolving and loading it through the ES module integration.

For example, to create multiple instances of a module, or to pass custom imports into a new instance of `library.wasm`:

    import source libraryModule from './library.wasm';

    const instance1 = await WebAssembly.instantiate(libraryModule, importObject1);

    const instance2 = await WebAssembly.instantiate(libraryModule, importObject2);
    js

In addition to the static source phase, there is also a dynamic variant of the source phase via the `import.source` dynamic phase import syntax:

    const dynamicLibrary = await import.source('./library.wasm');

    const instance = await WebAssembly.instantiate(dynamicLibrary, importObject);
    js

#### JavaScript String Builtins#

Added in: v24.5.0, v22.19.0

Stability: 1.2 - Release candidate

When importing WebAssembly modules, the WebAssembly JS String Builtins Proposal is automatically enabled through the ESM Integration. This allows WebAssembly modules to directly use efficient compile-time string builtins from the `wasm:js-string` namespace.

For example, the following Wasm module exports a string `getLength` function using the `wasm:js-string` `length` builtin:

    (module
      ;; Compile-time import of the string length builtin.
      (import "wasm:js-string" "length" (func $string_length (param externref) (result i32)))

      ;; Define getLength, taking a JS value parameter assumed to be a string,
      ;; calling string length on it and returning the result.
      (func $getLength (param $str externref) (result i32)
        local.get $str
        call $string_length
      )

      ;; Export the getLength function.
      (export "getLength" (func $get_length))
    )
    text

    import { getLength } from './string-len.wasm';
    getLength('foo'); // Returns 3.
    js

Wasm builtins are compile-time imports that are linked during module compilation rather than during instantiation. They do not behave like normal module graph imports and they cannot be inspected via `WebAssembly.Module.imports(mod)` or virtualized unless recompiling the module using the direct `WebAssembly.compile` API with string builtins disabled.

String constants may also be imported from the `wasm:js/string-constants` builtin import URL, allowing static JS string globals to be defined:

    (module
      (import "wasm:js/string-constants" "hello" (global $hello externref))
    )
    text

Importing a module in the source phase before it has been instantiated will also use the compile-time builtins automatically:

    import source mod from './string-len.wasm';
    const { exports: { getLength } } = await WebAssembly.instantiate(mod, {});
    getLength('foo'); // Also returns 3.
    js

#### Wasm Instance Phase Imports#

Stability: 1.1 - Active development

Instance imports allow any `.wasm` files to be imported as normal modules, supporting their module imports in turn.

For example, an `index.js` containing:

    import * as M from './library.wasm';
    console.log(M);
    js

executed under:

    node index.mjs
    bash

would provide the exports interface for the instantiation of `library.wasm`.

#### Reserved Wasm Namespaces#

Added in: v24.5.0, v22.19.0

When importing WebAssembly module instances, they cannot use import module names or import/export names that start with reserved prefixes:

- `wasm-js:` - reserved in all module import names, module names and export names.
- `wasm:` - reserved in module import names and export names (imported module names are allowed in order to support future builtin polyfills).

Importing a module using the above reserved names will throw a `WebAssembly.LinkError`.

### Top-level `await`#

Added in: v14.8.0

The `await` keyword may be used in the top level body of an ECMAScript module.

Assuming an `a.mjs` with

    export const five = await Promise.resolve(5);
    js

And a `b.mjs` with

    import { five } from './a.mjs';

    console.log(five); // Logs `5`
    js

    node b.mjs # works
    bash

If a top level `await` expression never resolves, the `node` process will exit with a `13` status code.

    import { spawn } from 'node:child_process';
    import { execPath } from 'node:process';

    spawn(execPath, [
      '--input-type=module',
      '--eval',
      // Never-resolving Promise:
      'await new Promise(() => {})',
    ]).once('exit', (code) => {
      console.log(code); // Logs `13`
    });
    js

### Loaders#

The former Loaders documentation is now at Modules: Customization hooks.

### Resolution and loading algorithm#

#### Features#

The default resolver has the following properties:

- FileURL-based resolution as is used by ES modules
- Relative and absolute URL resolution
- No default extensions
- No folder mains
- Bare specifier package resolution lookup through node_modules
- Does not fail on unknown extensions or protocols
- Can optionally provide a hint of the format to the loading phase

The default loader has the following properties

- Support for builtin module loading via `node:` URLs
- Support for "inline" module loading via `data:` URLs
- Support for `file:` module loading
- Fails on any other URL protocol
- Fails on unknown extensions for `file:` loading (supports only `.cjs`, `.js`, and `.mjs`)

When the `--experimental-package-map` flag is enabled, bare specifier resolution first consults the package map configuration. If the importing module is within a mapped package and the specifier matches a declared dependency, the package map resolution takes precedence. See Package maps for details.

#### Resolution algorithm#

The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL.

The resolution algorithm determines the full resolved URL for a module load, along with its suggested module format. The resolution algorithm does not determine whether the resolved URL protocol can be loaded, or whether the file extensions are permitted, instead these validations are applied by Node.js during the load phase (for example, if it was asked to load a URL that has a protocol that is not `file:`, `data:` or `node:`.

The algorithm also tries to determine the format of the file based on the extension (see `ESM_FILE_FORMAT` algorithm below). If it does not recognize the file extension (eg if it is not `.mjs`, `.cjs`, or `.json`), then a format of `undefined` is returned, which will throw during the load phase.

The algorithm to determine the module format of a resolved URL is provided by ESM_FILE_FORMAT, which returns the unique module format for any file. The "module" format is returned for an ECMAScript Module, while the "commonjs" format is used to indicate loading through the legacy CommonJS loader. Additional formats such as "addon" can be extended in future updates.

In the following algorithms, all subroutine errors are propagated as errors of these top-level routines unless stated otherwise.

defaultConditions is the conditional environment name array, `["node", "import"]`.

The resolver can throw the following errors:

- Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
- Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
- Invalid Package Target: Package exports or imports define a target module for the package that is an invalid type or string target.
- Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
- Package Import Not Defined: Package imports do not define the specifier.
- Module Not Found: The package or module requested does not exist.
- Unsupported Directory Import: The resolved path corresponds to a directory, which is not a supported target for module imports.

#### Resolution Algorithm Specification#

ESM_RESOLVE(specifier, parentURL)

> 1. Let resolved be undefined.
> 2. If specifier is a valid URL, then
>    1. Set resolved to the result of parsing and reserializing specifier as a URL.
> 3. Otherwise, if specifier starts with "/", "./", or "../", then
>    1. Set resolved to the URL resolution of specifier relative to parentURL.
> 4. Otherwise, if specifier starts with "#", then
>    1. Set resolved to the result of PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, defaultConditions).
> 5. Otherwise,
>    1. Note: specifier is now a bare specifier.
>    2. Set resolved the result of PACKAGE_RESOLVE(specifier, parentURL).
> 6. Let format be undefined.
> 7. If resolved is a "file:" URL, then
>    1. If resolved contains any percent encodings of "/" or "\" ("%2F" and "%5C" respectively), then
>       1. Throw an Invalid Module Specifier error.
>    2. If the file at resolved is a directory, then
>       1. Throw an Unsupported Directory Import error.
>    3. If the file at resolved does not exist, then
>       1. Throw a Module Not Found error.
>    4. Set resolved to the real path of resolved, maintaining the same URL querystring and fragment components.
>    5. Set format to the result of ESM_FILE_FORMAT(resolved).
> 8. Otherwise,
>    1. Set format the module format of the content type associated with the URL resolved.
> 9. Return format and resolved to the loading phase

PACKAGE_RESOLVE(packageSpecifier, parentURL)

> 1. Let packageName be undefined.
> 2. If packageSpecifier is an empty string, then
>    1. Throw an Invalid Module Specifier error.
> 3. If packageSpecifier is a Node.js builtin module name, then
>    1. Return the string "node:" concatenated with packageSpecifier.
> 4. If packageSpecifier does not start with "@", then
>    1. Set packageName to the substring of packageSpecifier until the first "/" separator or the end of the string.
> 5. Otherwise,
>    1. If packageSpecifier does not contain a "/" separator, then
>       1. Throw an Invalid Module Specifier error.
>    2. Set packageName to the substring of packageSpecifier until the second "/" separator or the end of the string.
> 6. If packageName starts with "." or contains "\" or "%", then
>    1. Throw an Invalid Module Specifier error.
> 7. Let packageSubpath be "." concatenated with the substring of packageSpecifier from the position at the length of packageName.
> 8. Let selfUrl be the result of PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL).
> 9. If selfUrl is not undefined, return selfUrl.
> 10. While parentURL is not the file system root,
>     1. Let packageURL be the URL resolution of "node_modules/" concatenated with packageName, relative to parentURL.
>     2. Set parentURL to the parent folder URL of parentURL.
>     3. If the folder at packageURL does not exist, then
>        1. Continue the next loop iteration.
>     4. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
>     5. If pjson is not null and pjson.exports is not null or undefined, then
>        1. Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
>     6. Otherwise, if packageSubpath is equal to ".", then
>        1. If pjson.main is a string, then
>           1. Return the URL resolution of main in packageURL.
>     7. Otherwise,
>        1. Return the URL resolution of packageSubpath in packageURL.
> 11. Throw a Module Not Found error.

PACKAGE_SELF_RESOLVE(packageName, packageSubpath, parentURL)

> 1. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
> 2. If packageURL is null, then
>    1. Return undefined.
> 3. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
> 4. If pjson is null or if pjson.exports is null or undefined, then
>    1. Return undefined.
> 5. If pjson.name is equal to packageName, then
>    1. Return the result of PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports, defaultConditions).
> 6. Otherwise, return undefined.

PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, exports, conditions)

Note: This function is directly invoked by the CommonJS resolution algorithm.

> 1. If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.
> 2. If subpath is equal to ".", then
>    1. Let mainExport be undefined.
>    2. If exports is a String or Array, or an Object containing no keys starting with ".", then
>       1. Set mainExport to exports.
>    3. Otherwise if exports is an Object containing a "." property, then
>       1. Set mainExport to exports["."].
>    4. If mainExport is not undefined, then
>       1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, mainExport, null, false, conditions).
>       2. If resolved is not null or undefined, return resolved.
> 3. Otherwise, if exports is an Object and all keys of exports start with ".", then
>    1. Assert: subpath begins with "./".
>    2. Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( subpath, exports, packageURL, false, conditions).
>    3. If resolved is not null or undefined, return resolved.
> 4. Throw a Package Path Not Exported error.

PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, conditions)

Note: This function is directly invoked by the CommonJS resolution algorithm.

> 1. Assert: specifier begins with "#".
> 2. If specifier is exactly equal to "#", then
>    1. Throw an Invalid Module Specifier error.
> 3. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(parentURL).
> 4. If packageURL is not null, then
>    1. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
>    2. If pjson.imports is a non-null Object, then
>       1. Let resolved be the result of PACKAGE_IMPORTS_EXPORTS_RESOLVE( specifier, pjson.imports, packageURL, true, conditions).
>       2. If resolved is not null or undefined, return resolved.
> 5. Throw a Package Import Not Defined error.

PACKAGE_IMPORTS_EXPORTS_RESOLVE(matchKey, matchObj, packageURL, isImports, conditions)

> 1. If matchKey ends in "/", then
>    1. Throw an Invalid Module Specifier error.
> 2. If matchKey is a key of matchObj and does not contain "*", then
>    1. Let target be the value of matchObj[matchKey].
>    2. Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, null, isImports, conditions).
> 3. Let expansionKeys be the list of keys of matchObj containing only a single "*", sorted by the sorting function PATTERN_KEY_COMPARE which orders in descending order of specificity.
> 4. For each key expansionKey in expansionKeys, do
>    1. Let patternBase be the substring of expansionKey up to but excluding the first "*" character.
>    2. If matchKey starts with but is not equal to patternBase, then
>       1. Let patternTrailer be the substring of expansionKey from the index after the first "*" character.
>       2. If patternTrailer has zero length, or if matchKey ends with patternTrailer and the length of matchKey is greater than or equal to the length of expansionKey, then
>          1. Let target be the value of matchObj[expansionKey].
>          2. Let patternMatch be the substring of matchKey starting at the index of the length of patternBase up to the length of matchKey minus the length of patternTrailer.
>          3. Return the result of PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions).
> 5. Return null.

PATTERN_KEY_COMPARE(keyA, keyB)

> 1. Assert: keyA contains only a single "*".
> 2. Assert: keyB contains only a single "*".
> 3. Let baseLengthA be the index of "*" in keyA.
> 4. Let baseLengthB be the index of "*" in keyB.
> 5. If baseLengthA is greater than baseLengthB, return -1.
> 6. If baseLengthB is greater than baseLengthA, return 1.
> 7. If the length of keyA is greater than the length of keyB, return -1.
> 8. If the length of keyB is greater than the length of keyA, return 1.
> 9. Return 0.

PACKAGE_TARGET_RESOLVE(packageURL, target, patternMatch, isImports, conditions)

> 1. If target is a String, then
>    1. If target does not start with "./", then
>       1. If isImports is false, or if target starts with "../" or "/", or if target is a valid URL, then
>          1. Throw an Invalid Package Target error.
>       2. If patternMatch is a String, then
>          1. Return PACKAGE_RESOLVE(target with every instance of "*" replaced by patternMatch, packageURL + "/").
>       3. Return PACKAGE_RESOLVE(target, packageURL + "/").
>    2. If target split on "/" or "\" contains any "", ".", "..", or "node_modules" segments after the first "." segment, case insensitive and including percent encoded variants, throw an Invalid Package Target error.
>    3. Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
>    4. Assert: packageURL is contained in resolvedTarget.
>    5. If patternMatch is null, then
>       1. Return resolvedTarget.
>    6. If patternMatch split on "/" or "\" contains any "", ".", "..", or "node_modules" segments, case insensitive and including percent encoded variants, throw an Invalid Module Specifier error.
>    7. Return the URL resolution of resolvedTarget with every instance of "*" replaced with patternMatch.
> 2. Otherwise, if target is a non-null Object, then
>    1. If target contains any index property keys, as defined in ECMA-262 6.1.7 Array Index, throw an Invalid Package Configuration error.
>    2. For each property p of target, in object insertion order as,
>       1. If p equals "default" or conditions contains an entry for p, then
>          1. Let targetValue be the value of the p property in target.
>          2. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions).
>          3. If resolved is equal to undefined, continue the loop.
>          4. Return resolved.
>    3. Return undefined.
> 3. Otherwise, if target is an Array, then
>    1. If _target.length is zero, return null.
>    2. For each item targetValue in target, do
>       1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions), continuing the loop on any Invalid Package Target error.
>       2. If resolved is undefined, continue the loop.
>       3. Return resolved.
>    3. Return or throw the last fallback resolution null return or error.
> 4. Otherwise, if target is null, return null.
> 5. Otherwise throw an Invalid Package Target error.

ESM_FILE_FORMAT(url)

> 1. Assert: url corresponds to an existing file.
> 2. If url ends in ".mjs", then
>    1. Return "module".
> 3. If url ends in ".cjs", then
>    1. Return "commonjs".
> 4. If url ends in ".json", then
>    1. Return "json".
> 5. If url ends in ".wasm", then
>    1. Return "wasm".
> 6. If `--experimental-addon-modules` is enabled and url ends in ".node", then
>    1. Return "addon".
> 7. Let packageURL be the result of LOOKUP_PACKAGE_SCOPE(url).
> 8. Let pjson be the result of READ_PACKAGE_JSON(packageURL).
> 9. Let packageType be null.
> 10. If pjson?.type is "module" or "commonjs", then
>     1. Set packageType to pjson.type.
> 11. If url ends in ".js", then
>     1. If packageType is not null, then
>        1. Return packageType.
>     2. If the result of DETECT_MODULE_SYNTAX(source) is true, then
>        1. Return "module".
>     3. Return "commonjs".
> 12. If url does not have any extension, then
>     1. If packageType is "module" and the file at url contains the "application/wasm" content type header for a WebAssembly module, then
>        1. Return "wasm".
>     2. If packageType is not null, then
>        1. Return packageType.
>     3. If the result of DETECT_MODULE_SYNTAX(source) is true, then
>        1. Return "module".
>     4. Return "commonjs".
> 13. Return undefined (will throw during load phase).

LOOKUP_PACKAGE_SCOPE(url)

> 1. Let scopeURL be url.
> 2. While scopeURL is not the file system root,
>    1. Set scopeURL to the parent URL of scopeURL.
>    2. If scopeURL ends in a "node_modules" path segment, return null.
>    3. Let pjsonURL be the resolution of "package.json" within scopeURL.
>    4. if the file at pjsonURL exists, then
>       1. Return scopeURL.
> 3. Return null.

READ_PACKAGE_JSON(packageURL)

> 1. Let pjsonURL be the resolution of "package.json" within packageURL.
> 2. If the file at pjsonURL does not exist, then
>    1. Return null.
> 3. If the file at packageURL does not parse as valid JSON, then
>    1. Throw an Invalid Package Configuration error.
> 4. Return the parsed JSON source of the file at pjsonURL.

DETECT_MODULE_SYNTAX(source)

> 1. Parse source as an ECMAScript module.
> 2. If the parse is successful, then
>    1. If source contains top-level `await`, static `import` or `export` statements, or `import.meta`, return true.
>    2. If source contains a top-level lexical declaration (`const`, `let`, or `class`) of any of the CommonJS wrapper variables (`require`, `exports`, `module`, `__filename`, or `__dirname`) then return true.
> 3. Return false.

#### Customizing ESM specifier resolution algorithm#

Module customization hooks provide a mechanism for customizing the ESM specifier resolution algorithm. An example that provides CommonJS-style resolution for ESM specifiers is commonjs-extension-resolution-loader.

## Modules: Packages#

History

Version | Changes
--- | ---
v14.13.0, v12.20.0 | Add support for `"exports"` patterns.
v14.6.0, v12.19.0 | Add package `"imports"` field.
v13.7.0, v12.17.0 | Unflag conditional exports.
v13.7.0, v12.16.0 | Remove the `--experimental-conditional-exports` option. In 12.16.0, conditional exports are still behind `--experimental-modules`.
v13.6.0, v12.16.0 | Unflag self-referencing a package using its name.
v12.7.0 | Introduce `"exports"` `package.json` field as a more powerful alternative to the classic `"main"` field.
v12.0.0 | Add support for ES modules using `.js` file extension via `package.json` `"type"` field.

### Introduction#

A package is a folder tree described by a `package.json` file. The package consists of the folder containing the `package.json` file and all subfolders until the next folder containing another `package.json` file, or a folder named `node_modules`.

This page provides guidance for package authors writing `package.json` files along with a reference for the `package.json` fields defined by Node.js.

### Determining module system#

#### Introduction#

Node.js will treat the following as ES modules when passed to `node` as the initial input, or when referenced by `import` statements or `import()` expressions:

- Files with an `.mjs` extension.

- Files with a `.js` extension when the nearest parent `package.json` file contains a top-level `"type"` field with a value of `"module"`.

- Strings passed in as an argument to `--eval`, or piped to `node` via `STDIN`, with the flag `--input-type=module`.

- Code containing syntax only successfully parsed as ES modules, such as `import` or `export` statements or `import.meta`, with no explicit marker of how it should be interpreted. Explicit markers are `.mjs` or `.cjs` extensions, `package.json` `"type"` fields with either `"module"` or `"commonjs"` values, or the `--input-type` flag. Dynamic `import()` expressions are supported in either CommonJS or ES modules and would not force a file to be treated as an ES module. See Syntax detection.

Node.js will treat the following as CommonJS when passed to `node` as the initial input, or when referenced by `import` statements or `import()` expressions:

- Files with a `.cjs` extension.

- Files with a `.js` extension when the nearest parent `package.json` file contains a top-level field `"type"` with a value of `"commonjs"`.

- Strings passed in as an argument to `--eval` or `--print`, or piped to `node` via `STDIN`, with the flag `--input-type=commonjs`.

- Files with a `.js` extension with no parent `package.json` file or where the nearest parent `package.json` file lacks a `type` field, and where the code can evaluate successfully as CommonJS. In other words, Node.js tries to run such "ambiguous" files as CommonJS first, and will retry evaluating them as ES modules if the evaluation as CommonJS fails because the parser found ES module syntax.

Writing ES module syntax in "ambiguous" files incurs a performance cost, and therefore it is encouraged that authors be explicit wherever possible. In particular, package authors should always include the `"type"` field in their `package.json` files, even in packages where all sources are CommonJS. Being explicit about the `type` of the package will future-proof the package in case the default type of Node.js ever changes, and it will also make things easier for build tools and loaders to determine how the files in the package should be interpreted.

#### Syntax detection#

Added in: v21.1.0, v20.10.0
History

Version | Changes
--- | ---
v22.7.0, v20.19.0 | Syntax detection is enabled by default.

Stability: 1.2 - Release candidate

Node.js will inspect the source code of ambiguous input to determine whether it contains ES module syntax; if such syntax is detected, the input will be treated as an ES module.

Ambiguous input is defined as:

- Files with a `.js` extension or no extension; and either no controlling `package.json` file or one that lacks a `type` field.
- String input (`--eval` or `STDIN`) when `--input-type`is not specified.

ES module syntax is defined as syntax that would throw when evaluated as CommonJS. This includes the following:

- `import` statements (but not `import()` expressions, which are valid in CommonJS).
- `export` statements.
- `import.meta` references.
- `await` at the top level of a module.
- Lexical redeclarations of the CommonJS wrapper variables (`require`, `module`, `exports`, `__dirname`, `__filename`).

#### Module resolution and loading#

Node.js has two types of module resolution and loading, chosen based on how the module is requested.

When a module is requested via `require()` (available by default in CommonJS modules, and can be dynamically generated using `createRequire()` in both CommonJS and ES Modules):

- Resolution:
  - The resolution initiated by `require()` supports folders as modules.
  - When resolving a specifier, if no exact match is found, `require()` will try to add extensions (`.js`, `.json`, and finally `.node`) and then attempt to resolve folders as modules.
  - It does not support URLs as specifiers by default.
- Loading:
  - `.json` files are treated as JSON text files.
  - `.node` files are interpreted as compiled addon modules loaded with `process.dlopen()`.
  - `.ts`, `.mts` and `.cts` files are treated as TypeScript text files.
  - Files with any other extension, or without extensions, are treated as JavaScript text files.
  - `require()` can only be used to load ECMAScript modules from CommonJS modules if the ECMAScript module and its dependencies are synchronous (i.e. they do not contain top-level `await`).

When a module is requested via static `import` statements (only available in ES Modules) or `import()` expressions (available in both CommonJS and ES Modules):

- Resolution:
  - The resolution of `import`/`import()` does not support folders as modules, directory indexes (e.g. `'./startup/index.js'`) must be fully specified.
  - It does not perform extension searching. A file extension must be provided when the specifier is a relative or absolute file URL.
  - It supports `file://` and `data:` URLs as specifiers by default.
- Loading:
  - `.json` files are treated as JSON text files. When importing JSON modules, an import type attribute is required (e.g. `import json from './data.json' with { type: 'json' }`).
  - `.node` files are interpreted as compiled addon modules loaded with `process.dlopen()`, if `--experimental-addon-modules` is enabled.
  - `.ts`, `.mts` and `.cts` files are treated as TypeScript text files.
  - It accepts only `.js`, `.mjs`, and `.cjs` extensions for JavaScript text files.
  - `.wasm` files are treated as WebAssembly modules.
  - Any other file extensions will result in a `ERR_UNKNOWN_FILE_EXTENSION` error. Additional file extensions can be facilitated via customization hooks.
  - `import`/`import()` can be used to load JavaScript CommonJS modules. Such modules are passed through merve to try to identify named exports, which are available if they can be determined through static analysis.

Regardless of how a module is requested, the resolution and loading process can be customized using customization hooks.

#### `package.json` and file extensions#

Within a package, the `package.json` `"type"` field defines how Node.js should interpret `.js` files. If a `package.json` file does not have a `"type"` field, `.js` files are treated as CommonJS.

A `package.json` `"type"` value of `"module"` tells Node.js to interpret `.js` files within that package as using ES module syntax.

The `"type"` field applies not only to initial entry points (`node my-app.js`) but also to files referenced by `import` statements and `import()` expressions.

    // my-app.js, treated as an ES module because there is a package.json
    // file in the same folder with "type": "module".

    import './startup/init.js';
    // Loaded as ES module since ./startup contains no package.json file,
    // and therefore inherits the "type" value from one level up.

    import 'commonjs-package';
    // Loaded as CommonJS since ./node_modules/commonjs-package/package.json
    // lacks a "type" field or contains "type": "commonjs".

    import './node_modules/commonjs-package/index.js';
    // Loaded as CommonJS since ./node_modules/commonjs-package/package.json
    // lacks a "type" field or contains "type": "commonjs".
    js

Files ending with `.mjs` are always loaded as ES modules regardless of the nearest parent `package.json`.

Files ending with `.cjs` are always loaded as CommonJS regardless of the nearest parent `package.json`.

    import './legacy-file.cjs';
    // Loaded as CommonJS since .cjs is always loaded as CommonJS.

    import 'commonjs-package/src/index.mjs';
    // Loaded as ES module since .mjs is always loaded as ES module.
    js

The `.mjs` and `.cjs` extensions can be used to mix types within the same package:

- Within a `"type": "module"` package, Node.js can be instructed to interpret a particular file as CommonJS by naming it with a `.cjs` extension (since both `.js` and `.mjs` files are treated as ES modules within a `"module"` package).

- Within a `"type": "commonjs"` package, Node.js can be instructed to interpret a particular file as an ES module by naming it with an `.mjs` extension (since both `.js` and `.cjs` files are treated as CommonJS within a `"commonjs"` package).

#### `--input-type` flag#

Added in: v12.0.0

Strings passed in as an argument to `--eval` (or `-e`), or piped to `node` via `STDIN`, are treated as ES modules when the `--input-type=module` flag is set.

    node --input-type=module --eval "import { sep } from 'node:path'; console.log(sep);"

    echo "import { sep } from 'node:path'; console.log(sep);" | node --input-type=module
    bash

For completeness there is also `--input-type=commonjs`, for explicitly running string input as CommonJS. This is the default behavior if `--input-type` is unspecified.

### Package entry points#

In a package's `package.json` file, two fields can define entry points for a package: `"main"` and `"exports"`. Both fields apply to both ES module and CommonJS module entry points.

The `"main"` field is supported in all versions of Node.js, but its capabilities are limited: it only defines the main entry point of the package.

The `"exports"` provides a modern alternative to `"main"` allowing multiple entry points to be defined, conditional entry resolution support between environments, and preventing any other entry points besides those defined in `"exports"`. This encapsulation allows module authors to clearly define the public interface for their package.

For new packages targeting the currently supported versions of Node.js, the `"exports"` field is recommended. For packages supporting Node.js 10 and below, the `"main"` field is required. If both `"exports"` and `"main"` are defined, the `"exports"` field takes precedence over `"main"` in supported versions of Node.js.

Conditional exports can be used within `"exports"` to define different package entry points per environment, including whether the package is referenced via `require` or via `import`. For more information about supporting both CommonJS and ES modules in a single package please consult the dual CommonJS/ES module packages section.

Existing packages introducing the `"exports"` field will prevent consumers of the package from using any entry points that are not defined, including the `package.json` (e.g. `require('your-package/package.json')`). This will likely be a breaking change.

To make the introduction of `"exports"` non-breaking, ensure that every previously supported entry point is exported. It is best to explicitly specify entry points so that the package's public API is well-defined. For example, a project that previously exported `main`, `lib`, `feature`, and the `package.json` could use the following `package.exports`:

    {
      "name": "my-package",
      "exports": {
        ".": "./lib/index.js",
        "./lib": "./lib/index.js",
        "./lib/index": "./lib/index.js",
        "./lib/index.js": "./lib/index.js",
        "./feature": "./feature/index.js",
        "./feature/index": "./feature/index.js",
        "./feature/index.js": "./feature/index.js",
        "./package.json": "./package.json"
      }
    }
    json

Alternatively a project could choose to export entire folders both with and without extensioned subpaths using export patterns:

    {
      "name": "my-package",
      "exports": {
        ".": "./lib/index.js",
        "./lib": "./lib/index.js",
        "./lib/*": "./lib/*.js",
        "./lib/*.js": "./lib/*.js",
        "./feature": "./feature/index.js",
        "./feature/*": "./feature/*.js",
        "./feature/*.js": "./feature/*.js",
        "./package.json": "./package.json"
      }
    }
    json

With the above providing backwards-compatibility for any minor package versions, a future major change for the package can then properly restrict the exports to only the specific feature exports exposed:

    {
      "name": "my-package",
      "exports": {
        ".": "./lib/index.js",
        "./feature/*.js": "./feature/*.js",
        "./feature/internal/*": null
      }
    }
    json

#### Main entry point export#

When writing a new package, it is recommended to use the `"exports"` field:

    {
      "exports": "./index.js"
    }
    json

When the `"exports"` field is defined, all subpaths of the package are encapsulated and no longer available to importers. For example, `require('pkg/subpath.js')` throws an `ERR_PACKAGE_PATH_NOT_EXPORTED` error.

This encapsulation of exports provides more reliable guarantees about package interfaces for tools and when handling semver upgrades for a package. It is not a strong encapsulation since a direct require of any absolute subpath of the package such as `require('/path/to/node_modules/pkg/subpath.js')` will still load `subpath.js`.

All currently supported versions of Node.js and modern build tools support the `"exports"` field. For projects using an older version of Node.js or a related build tool, compatibility can be achieved by including the `"main"` field alongside `"exports"` pointing to the same module:

    {
      "main": "./index.js",
      "exports": "./index.js"
    }
    json

#### Subpath exports#

Added in: v12.7.0

When using the `"exports"` field, custom subpaths can be defined along with the main entry point by treating the main entry point as the `"."` subpath:

    {
      "exports": {
        ".": "./index.js",
        "./submodule.js": "./src/submodule.js"
      }
    }
    json

Now only the defined subpath in `"exports"` can be imported by a consumer:

    import submodule from 'es-module-package/submodule.js';
    // Loads ./node_modules/es-module-package/src/submodule.js
    js

While other subpaths will error:

    import submodule from 'es-module-package/private-module.js';
    // Throws ERR_PACKAGE_PATH_NOT_EXPORTED
    js

##### Extensions in subpaths#

Package authors should provide either extensioned (`import 'pkg/subpath.js'`) or extensionless (`import 'pkg/subpath'`) subpaths in their exports. This ensures that there is only one subpath for each exported module so that all dependents import the same consistent specifier, keeping the package contract clear for consumers and simplifying package subpath completions.

Traditionally, packages tended to use the extensionless style, which has the benefits of readability and of masking the true path of the file within the package.

With import maps now providing a standard for package resolution in browsers and other JavaScript runtimes, using the extensionless style can result in bloated import map definitions. Explicit file extensions can avoid this issue by enabling the import map to utilize a packages folder mapping to map multiple subpaths where possible instead of a separate map entry per package subpath export. This also mirrors the requirement of using the full specifier path in relative and absolute import specifiers.

##### Path Rules and Validation for Export Targets#

When defining paths as targets in the `"exports"` field, Node.js enforces several rules to ensure security, predictability, and proper encapsulation. Understanding these rules is crucial for authors publishing packages.

###### Targets must be relative URLs#

All target paths in the `"exports"` map (the values associated with export keys) must be relative URL strings starting with `./`.

    // package.json
    {
      "name": "my-package",
      "exports": {
        ".": "./dist/main.js",          // Correct
        "./feature": "./lib/feature.js", // Correct
        // "./origin-relative": "/dist/main.js", // Incorrect: Must start with ./
        // "./absolute": "file:///dev/null", // Incorrect: Must start with ./
        // "./outside": "../common/util.js" // Incorrect: Must start with ./
      }
    }
    json

Reasons for this behavior include:

- Security: Prevents exporting arbitrary files from outside the package's own directory.
- Encapsulation: Ensures all exported paths are resolved relative to the package root, making the package self-contained.

###### No path traversal or invalid segments#

Export targets must not resolve to a location outside the package's root directory. Additionally, path segments like `.` (single dot), `..` (double dot), or `node_modules` (and their URL-encoded equivalents) are generally disallowed within the `target` string after the initial `./` and in any `subpath` part substituted into a target pattern.

    // package.json
    {
      "name": "my-package",
      "exports": {
        // ".": "./dist/../../elsewhere/file.js", // Invalid: path traversal
        // ".": "././dist/main.js",             // Invalid: contains "." segment
        // ".": "./dist/../dist/main.js",       // Invalid: contains ".." segment
        // "./utils/./helper.js": "./utils/helper.js" // Key has invalid segment
      }
    }
    json

#### Exports sugar#

Added in: v12.11.0

If the `"."` export is the only export, the `"exports"` field provides sugar for this case being the direct `"exports"` field value.

    {
      "exports": {
        ".": "./index.js"
      }
    }
    json

can be written:

    {
      "exports": "./index.js"
    }
    json

#### Subpath imports#

Added in: v14.6.0, v12.19.0
History

Version | Changes
--- | ---
v25.4.0, v24.14.0 | Allow subpath imports that start with `#/`.

In addition to the `"exports"` field, there is a package `"imports"` field to create private mappings that only apply to import specifiers from within the package itself.

Entries in the `"imports"` field must always start with `#` to ensure they are disambiguated from external package specifiers.

For example, the imports field can be used to gain the benefits of conditional exports for internal modules:

    // package.json
    {
      "imports": {
        "#dep": {
          "node": "dep-node-native",
          "default": "./dep-polyfill.js"
        }
      },
      "dependencies": {
        "dep-node-native": "^1.0.0"
      }
    }
    json

where `import '#dep'` does not get the resolution of the external package `dep-node-native` (including its exports in turn), and instead gets the local file `./dep-polyfill.js` relative to the package in other environments.

Unlike the `"exports"` field, the `"imports"` field permits mapping to external packages.

The resolution rules for the imports field are otherwise analogous to the exports field.

#### Subpath patterns#

Added in: v14.13.0, v12.20.0
History

Version | Changes
--- | ---
v16.10.0, v14.19.0 | Support pattern trailers in "imports" field.
v16.9.0, v14.19.0 | Support pattern trailers.

For packages with a small number of exports or imports, we recommend explicitly listing each exports subpath entry. But for packages that have large numbers of subpaths, this might cause `package.json` bloat and maintenance issues.

For these use cases, subpath export patterns can be used instead:

    // ./node_modules/es-module-package/package.json
    {
      "exports": {
        "./features/*.js": "./src/features/*.js"
      },
      "imports": {
        "#internal/*.js": "./src/internal/*.js"
      }
    }
    json

`*` maps expose nested subpaths as it is a string replacement syntax only.

All instances of `*` on the right hand side will then be replaced with this value, including if it contains any `/` separators.

    import featureX from 'es-module-package/features/x.js';
    // Loads ./node_modules/es-module-package/src/features/x.js

    import featureY from 'es-module-package/features/y/y.js';
    // Loads ./node_modules/es-module-package/src/features/y/y.js

    import internalZ from '#internal/z.js';
    // Loads ./src/internal/z.js
    js

This is a direct static matching and replacement without any special handling for file extensions. Including the `"*.js"` on both sides of the mapping restricts the exposed package exports to only JS files.

The property of exports being statically enumerable is maintained with exports patterns since the individual exports for a package can be determined by treating the right hand side target pattern as a `**` glob against the list of files within the package. Because `node_modules` paths are forbidden in exports targets, this expansion is dependent on only the files of the package itself.

To exclude private subfolders from patterns, `null` targets can be used:

    // ./node_modules/es-module-package/package.json
    {
      "exports": {
        "./features/*.js": "./src/features/*.js",
        "./features/private-internal/*": null
      }
    }
    json

    import featureInternal from 'es-module-package/features/private-internal/m.js';
    // Throws: ERR_PACKAGE_PATH_NOT_EXPORTED

    import featureX from 'es-module-package/features/x.js';
    // Loads ./node_modules/es-module-package/src/features/x.js
    js

#### Conditional exports#

Added in: v13.2.0, v12.16.0
History

Version | Changes
--- | ---
v13.7.0, v12.16.0 | Unflag conditional exports.

Conditional exports provide a way to map to different paths depending on certain conditions. They are supported for both CommonJS and ES module imports.

For example, a package that wants to provide different ES module exports for `require()` and `import` can be written:

    // package.json
    {
      "exports": {
        "import": "./index-module.js",
        "require": "./index-require.cjs"
      },
      "type": "module"
    }
    json

Node.js implements the following conditions, listed in order from most specific to least specific as conditions should be defined:

- `"node-addons"` - similar to `"node"` and matches for any Node.js environment. This condition can be used to provide an entry point which uses native C++ addons as opposed to an entry point which is more universal and doesn't rely on native addons. This condition can be disabled via the `--no-addons` flag.
- `"node"` - matches for any Node.js environment. Can be a CommonJS or ES module file. In most cases explicitly calling out the Node.js platform is not necessary.
- `"import"` - matches when the package is loaded via `import` or `import()`, or via any top-level import or resolve operation by the ECMAScript module loader. Applies regardless of the module format of the target file. Always mutually exclusive with `"require"`.
- `"require"` - matches when the package is loaded via `require()`. The referenced file should be loadable with `require()` although the condition matches regardless of the module format of the target file. Expected formats include CommonJS, JSON, native addons, and ES modules. Always mutually exclusive with `"import"`.
- `"module-sync"` - matches no matter the package is loaded via `import`, `import()` or `require()`. The format is expected to be ES modules that does not contain top-level await in its module graph - if it does, `ERR_REQUIRE_ASYNC_MODULE` will be thrown when the module is `require()`-ed.
- `"default"` - the generic fallback that always matches. Can be a CommonJS or ES module file. This condition should always come last.

Within the `"exports"` object, key order is significant. During condition matching, earlier entries have higher priority and take precedence over later entries. The general rule is that conditions should be from most specific to least specific in object order.

Using the `"import"` and `"require"` conditions can lead to some hazards, which are further explained in the dual CommonJS/ES module packages section.

The `"node-addons"` condition can be used to provide an entry point which uses native C++ addons. However, this condition can be disabled via the `--no-addons` flag. When using `"node-addons"`, it's recommended to treat `"default"` as an enhancement that provides a more universal entry point, e.g. using WebAssembly instead of a native addon.

Conditional exports can also be extended to exports subpaths, for example:

    {
      "exports": {
        ".": "./index.js",
        "./feature.js": {
          "node": "./feature-node.js",
          "default": "./feature.js"
        }
      }
    }
    json

Defines a package where `require('pkg/feature.js')` and `import 'pkg/feature.js'` could provide different implementations between Node.js and other JS environments.

When using environment branches, always include a `"default"` condition where possible. Providing a `"default"` condition ensures that any unknown JS environments are able to use this universal implementation, which helps avoid these JS environments from having to pretend to be existing environments in order to support packages with conditional exports. For this reason, using `"node"` and `"default"` condition branches is usually preferable to using `"node"` and `"browser"` condition branches.

#### Nested conditions#

In addition to direct mappings, Node.js also supports nested condition objects.

For example, to define a package that only has dual mode entry points for use in Node.js but not the browser:

    {
      "exports": {
        "node": {
          "import": "./feature-node.mjs",
          "require": "./feature-node.cjs"
        },
        "default": "./feature.mjs"
      }
    }
    json

Conditions continue to be matched in order as with flat conditions. If a nested condition does not have any mapping it will continue checking the remaining conditions of the parent condition. In this way nested conditions behave analogously to nested JavaScript `if` statements.

#### Resolving user conditions#

Added in: v14.9.0, v12.19.0

When running Node.js, custom user conditions can be added with the `--conditions` flag:

    node --conditions=development index.js
    bash

which would then resolve the `"development"` condition in package imports and exports, while resolving the existing `"node"`, `"node-addons"`, `"default"`, `"import"`, and `"require"` conditions as appropriate.

Any number of custom conditions can be set with repeat flags.

Typical conditions should only contain alphanumerical characters, using ":", "-", or "=" as separators if necessary. Anything else may run into compatibility issues outside of node.

In node, conditions have very few restrictions, but specifically these include:

1. They must contain at least one character.
2. They cannot start with "." since they may appear in places that also allow relative paths.
3. They cannot contain "," since they may be parsed as a comma-separated list by some CLI tools.
4. They cannot be integer property keys like "10" since that can have unexpected effects on property key ordering for JS objects.

#### Community Conditions Definitions#

Condition strings other than the `"import"`, `"require"`, `"node"`, `"module-sync"`, `"node-addons"` and `"default"` conditions implemented in Node.js core are ignored by default.

Other platforms may implement other conditions and user conditions can be enabled in Node.js via the `--conditions` / `-C` flag.

Since custom package conditions require clear definitions to ensure correct usage, a list of common known package conditions and their strict definitions is provided below to assist with ecosystem coordination.

- `"types"` - can be used by typing systems to resolve the typing file for the given export. This condition should always be included first.
- `"browser"` - any web browser environment.
- `"development"` - can be used to define a development-only environment entry point, for example to provide additional debugging context such as better error messages when running in a development mode. Must always be mutually exclusive with `"production"`.
- `"production"` - can be used to define a production environment entry point. Must always be mutually exclusive with `"development"`.

For other runtimes, platform-specific condition key definitions are maintained by the WinterCG in the Runtime Keys proposal specification.

New conditions definitions may be added to this list by creating a pull request to the Node.js documentation for this section. The requirements for listing a new condition definition here are that:

- The definition should be clear and unambiguous for all implementers.
- The use case for why the condition is needed should be clearly justified.
- There should exist sufficient existing implementation usage.
- The condition name should not conflict with another condition definition or condition in wide usage.
- The listing of the condition definition should provide a coordination benefit to the ecosystem that wouldn't otherwise be possible. For example, this would not necessarily be the case for company-specific or application-specific conditions.
- The condition should be such that a Node.js user would expect it to be in Node.js core documentation. The `"types"` condition is a good example: It doesn't really belong in the Runtime Keys proposal but is a good fit here in the Node.js docs.

The above definitions may be moved to a dedicated conditions registry in due course.

#### Self-referencing a package using its name#

Added in: v13.1.0, v12.16.0
History

Version | Changes
--- | ---
v13.6.0, v12.16.0 | Unflag self-referencing a package using its name.

Within a package, the values defined in the package's `package.json` `"exports"` field can be referenced via the package's name. For example, assuming the `package.json` is:

    // package.json
    {
      "name": "a-package",
      "exports": {
        ".": "./index.mjs",
        "./foo.js": "./foo.js"
      }
    }
    json

Then any module in that package can reference an export in the package itself:

    // ./a-module.mjs
    import { something } from 'a-package'; // Imports "something" from ./index.mjs.
    js

Self-referencing is available only if `package.json` has `"exports"`, and will allow importing only what that `"exports"` (in the `package.json`) allows. So the code below, given the previous package, will generate a runtime error:

    // ./another-module.mjs

    // Imports "another" from ./m.mjs. Fails because
    // the "package.json" "exports" field
    // does not provide an export named "./m.mjs".
    import { another } from 'a-package/m.mjs';
    js

Self-referencing is also available when using `require`, both in an ES module, and in a CommonJS one. For example, this code will also work:

    // ./a-module.js
    const { something } = require('a-package/foo.js'); // Loads from ./foo.js.
    cjs

Finally, self-referencing also works with scoped packages. For example, this code will also work:

    // package.json
    {
      "name": "@my/package",
      "exports": "./index.js"
    }
    json

    // ./index.js
    module.exports = 42;
    cjs

    // ./other.js
    console.log(require('@my/package'));
    cjs

    $ node other.js
    42
    console

### Dual CommonJS/ES module packages#

See the package examples repository for details.

### Package maps#

Added in: v26.4.0

Stability: 1 - Experimental. Enable this API with `--experimental-package-map`.

Package maps provide a mechanism to control package resolution without relying on the `node_modules` folder structure. When enabled via the `--experimental-package-map` flag, Node.js uses a JSON configuration file to determine how bare specifiers are resolved.

This feature is useful for:

- Monorepos: Define explicit dependency relationships between workspace packages without symlinks or hoisting complexities.
- Dependency isolation: Prevent packages from accessing undeclared dependencies (phantom dependencies).
- Low file system coupling: The package resolution algorithm runs without inspecting the file system, relying instead on static data tables.

#### Configuration file format#

The package map configuration file is a JSON file with a `packages` object. Each key in `packages` is called a package ID and is a unique identifier for a package entry:

    {
      "packages": {
        "app": {
          "url": "./packages/app",
          "dependencies": {
            "@myorg/utils": "utils",
            "@myorg/ui-lib": "ui-lib"
          }
        },
        "utils": {
          "url": "./packages/utils"
        },
        "ui-lib": {
          "url": "./packages/ui-lib",
          "dependencies": {
            "@myorg/utils": "utils"
          }
        }
      }
    }
    json

Each package entry has the following fields:

- `url` `<string>` Required. An absolute or relative URL. This is parsed using the WHATWG `URL` API, using the configuration file URL as base. Only `file:` protocol is supported. Multiple packages are allowed to share the same URL; consumers must key module instances by both module url and package IDs to differentiate them.
- `dependencies` `<Object>` An object mapping bare specifiers to package keys. Each key is the import name used in source code, and each value is the corresponding package key in the `packages` object. Defaults to an empty object.

#### Resolution algorithm#

When a bare specifier is encountered:

1. Node.js determines which package performs the resolution request.
   - If possible the package ID for the importer file should be provided to the resolution algorithm.
   - Failing that, the resolution will check if the file path is within any package location decoded from its `url`.
2. If no package ID is provided and the importing file is not within any mapped package, an `ERR_PACKAGE_MAP_EXTERNAL_FILE` error is thrown.
3. Node.js looks up the specifier's package name in the importing package's `dependencies` object to find the corresponding package key.
4. If found, the resolution algorithm locates the target package location from the package's `url` field in the package map.
5. If the specifier is not in `dependencies`, a `MODULE_NOT_FOUND` error is thrown.
6. The package location is forwarded to the regular Node.js resolution algorithm to finish the resolution (`index.js`, exports field, etc).

More details can be found in the resolution algorithm pseudo-code.

#### Multiple package versions#

Different packages can depend on different versions of the same package. Because `dependencies` maps bare specifiers to package keys, two packages can map the same specifier to different targets:

    {
      "packages": {
        "app": {
          "url": "./app",
          "dependencies": {
            "component": "component-v2"
          }
        },
        "legacy": {
          "url": "./legacy",
          "dependencies": {
            "component": "component-v1"
          }
        },
        "component-v1": {
          "url": "./vendor/component-1.0.0"
        },
        "component-v2": {
          "url": "./vendor/component-2.0.0"
        }
      }
    }
    json

Both `app` and `legacy` can `import 'component'`, but they resolve to different paths based on their declared dependencies.

#### Multiple packages for the same URL#

To address complex hoisting situations, multiple packages may share the same URL, which introduces ambiguity when determining which package an import originates from:

    {
      "packages": {
        "app-old": {
          "url": "./app-old",
          "dependencies": {
            "lib": "lib-old"
          }
        },
        "app-new": {
          "url": "./app-new",
          "dependencies": {
            "lib": "lib-new"
          }
        },
        "lib-old": {
          "url": "./lib",
          "dependencies": {
            "react": "react-15"
          }
        },
        "lib-new": {
          "url": "./lib",
          "dependencies": {
            "react": "react-18"
          }
        }
      }
    }
    json

In the example above both `lib-old` and `lib-new` use the same `./lib` folder to store their sources, the only difference being in which version of `react` they'll access when performing `require` calls or using `import`.

Because multiple package entries share the same URL, resolving a bare specifier from a file within that URL is ambiguous unless the originating package ID is known. If the package ID cannot be determined (for example, because the caller did not propagate it from a previous resolution), Node.js will throw an error rather than guess.

To support this pattern, implementers must key module instances by package ID and propagate it from each resolution result to subsequent resolution requests. This ensures that when `lib` requires `react`, the runtime knows whether the request comes from `lib-old` or `lib-new` and can select the correct dependency.

#### Interaction with other resolution#

Package maps only apply to bare specifiers that are not Node.js builtin modules. The following cases are not affected by package maps and continue to use standard resolution:

- Relative paths or URLs (`./` or `../`).
- Absolute paths or URLs.
- Node.js builtin modules (`node:fs`, etc.).

#### Limitations#

- Package maps must be a single static file; dynamic configuration is not supported.
- Circular dependency detection is not performed by the package map resolver.
- The package map file is loaded synchronously at startup.

### Node.js `package.json` field definitions#

This section describes the fields used by the Node.js runtime. Other tools (such as npm) use additional fields which are ignored by Node.js and not documented here.

The following fields in `package.json` files are used in Node.js:

- `"name"` - Relevant when using named imports within a package. Also used by package managers as the name of the package.
- `"main"` - The default module when loading the package, if exports is not specified, and in versions of Node.js prior to the introduction of exports.
- `"type"` - The package type determining whether to load `.js` files as CommonJS or ES modules.
- `"exports"` - Package exports and conditional exports. When present, limits which submodules can be loaded from within the package.
- `"imports"` - Package imports, for use by modules within the package itself.

#### `"name"`#

Added in: v13.1.0, v12.16.0
History

Version | Changes
--- | ---
v13.6.0, v12.16.0 | Remove the `--experimental-resolve-self` option.

- Type: `<string>`

    {
      "name": "package-name"
    }
    json

The `"name"` field defines your package's name. Publishing to the npm registry requires a name that satisfies certain requirements.

The `"name"` field can be used in addition to the `"exports"` field to self-reference a package using its name.

#### `"main"`#

Added in: v0.4.0

- Type: `<string>`

    {
      "main": "./index.js"
    }
    json

The `"main"` field defines the entry point of a package when imported by name via a `node_modules` lookup. Its value is a path.

The `"exports"` field, if it exists, takes precedence over the `"main"` field when importing the package by name.

It also defines the script that is used when the package directory is loaded via `require()`.

    // This resolves to ./path/to/directory/index.js.
    require('./path/to/directory');
    cjs

#### `"type"`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v13.2.0, v12.17.0 | Unflag `--experimental-modules`.

- Type: `<string>`

The `"type"` field defines the module format that Node.js uses for all `.js` files that have that `package.json` file as their nearest parent.

Files ending with `.js` are loaded as ES modules when the nearest parent `package.json` file contains a top-level field `"type"` with a value of `"module"`.

The nearest parent `package.json` is defined as the first `package.json` found when searching in the current folder, that folder's parent, and so on up until a node_modules folder or the volume root is reached.

    // package.json
    {
      "type": "module"
    }
    json

    # In same folder as preceding package.json
    node my-app.js # Runs as ES module
    bash

If the nearest parent `package.json` lacks a `"type"` field, or contains `"type": "commonjs"`, `.js` files are treated as CommonJS. If the volume root is reached and no `package.json` is found, `.js` files are treated as CommonJS.

`import` statements of `.js` files are treated as ES modules if the nearest parent `package.json` contains `"type": "module"`.

    // my-app.js, part of the same example as above
    import './startup.js'; // Loaded as ES module because of package.json
    js

Regardless of the value of the `"type"` field, `.mjs` files are always treated as ES modules and `.cjs` files are always treated as CommonJS.

#### `"exports"`#

Added in: v12.7.0
History

Version | Changes
--- | ---
v14.13.0, v12.20.0 | Add support for `"exports"` patterns.
v13.7.0, v12.17.0 | Unflag conditional exports.
v13.7.0, v12.16.0 | Implement logical conditional exports ordering.
v13.7.0, v12.16.0 | Remove the `--experimental-conditional-exports` option. In 12.16.0, conditional exports are still behind `--experimental-modules`.
v13.2.0, v12.16.0 | Implement conditional exports.

- Type: `<Object>` | `<string>` | `<string>`[]

    {
      "exports": "./index.js"
    }
    json

The `"exports"` field allows defining the entry points of a package when imported by name loaded either via a `node_modules` lookup or a self-reference to its own name. It is supported in Node.js 12+ as an alternative to the `"main"` that can support defining subpath exports and conditional exports while encapsulating internal unexported modules.

Conditional Exports can also be used within `"exports"` to define different package entry points per environment, including whether the package is referenced via `require` or via `import`.

All paths defined in the `"exports"` must be relative file URLs starting with `./`.

#### `"imports"`#

Added in: v14.6.0, v12.19.0

- Type: `<Object>`

    // package.json
    {
      "imports": {
        "#dep": {
          "node": "dep-node-native",
          "default": "./dep-polyfill.js"
        }
      },
      "dependencies": {
        "dep-node-native": "^1.0.0"
      }
    }
    json

Entries in the imports field must be strings starting with `#`.

Package imports permit mapping to external packages.

This field defines subpath imports for the current package.

## Modules: TypeScript#

History

Version | Changes
--- | ---
v26.0.0 | Removed `--experimental-transform-types` flag.
v25.2.0, v24.12.0 | Type stripping is now stable.
v24.3.0, v22.18.0 | Type stripping no longer emits an experimental warning.
v23.6.0, v22.18.0 | Type stripping is enabled by default.
v22.7.0 | Added `--experimental-transform-types` flag.

Stability: 2 - Stable

### Enabling#

There are two ways to enable runtime TypeScript support in Node.js:

1. For full support of all of TypeScript's syntax and features, including using any version of TypeScript, use a third-party package.

2. For lightweight support, you can use the built-in support for type stripping.

### Full TypeScript support#

To use TypeScript with full support for all TypeScript features, including `tsconfig.json`, you can use a third-party package. These instructions use `tsx` as an example but there are many other similar libraries available.

1. Install the package as a development dependency using whatever package manager you're using for your project. For example, with `npm`:

       npm install --save-dev tsx
       bash

2. Then you can run your TypeScript code via:

       npx tsx your-file.ts
       bash

   Or alternatively, you can run with `node` via:

       node --import=tsx your-file.ts
       bash

### Type stripping#

Added in: v22.6.0
History

Version | Changes
--- | ---
v25.2.0, v24.12.0 | Type stripping is now stable.

By default Node.js will execute TypeScript files that contains only erasable TypeScript syntax. Node.js will replace TypeScript syntax with whitespace, and no type checking is performed. To disable this feature, use the flag `--no-strip-types`.

Node.js ignores `tsconfig.json` files and therefore features that depend on settings within `tsconfig.json`, such as paths or converting newer JavaScript syntax to older standards, are intentionally unsupported. To get full TypeScript support, see Full TypeScript support.

The type stripping feature is designed to be lightweight. By intentionally not supporting syntaxes that require JavaScript code generation, and by replacing inline types with whitespace, Node.js can run TypeScript code without the need for source maps.

Type stripping is compatible with most versions of TypeScript but we recommend version 5.8 or newer with the following `tsconfig.json` settings:

    {
      "compilerOptions": {
         "noEmit": true, // Optional - see note below
         "target": "esnext",
         "module": "nodenext",
         "rewriteRelativeImportExtensions": true,
         "erasableSyntaxOnly": true,
         "verbatimModuleSyntax": true
      }
    }
    json

Use the `noEmit` option if you intend to only execute `*.ts` files, for example a build script. You won't need this flag if you intend to distribute `*.js` files.

#### Determining module system#

Node.js supports both CommonJS and ES Modules syntax in TypeScript files. Node.js will not convert from one module system to another; if you want your code to run as an ES module, you must use `import` and `export` syntax, and if you want your code to run as CommonJS you must use `require` and `module.exports`.

- `.ts` files will have their module system determined the same way as `.js` files. To use `import` and `export` syntax, add `"type": "module"` to the nearest parent `package.json`.
- `.mts` files will always be run as ES modules, similar to `.mjs` files.
- `.cts` files will always be run as CommonJS modules, similar to `.cjs` files.
- `.tsx` files are unsupported.

As in JavaScript files, file extensions are mandatory in `import` statements and `import()` expressions: `import './file.ts'`, not `import './file'`. Because of backward compatibility, file extensions are also mandatory in `require()` calls: `require('./file.ts')`, not `require('./file')`, similar to how the `.cjs` extension is mandatory in `require` calls in CommonJS files.

The `tsconfig.json` option `allowImportingTsExtensions` will allow the TypeScript compiler `tsc` to type-check files with `import` specifiers that include the `.ts` extension.

#### TypeScript features#

Since Node.js is only removing inline types, any TypeScript features that involve replacing TypeScript syntax with new JavaScript syntax will error.

The most prominent features that require transformation are:

- `Enum` declarations
- `namespace` with runtime code
- parameter properties
- import aliases

`namespace`s that do not contain runtime code are supported. This example will work correctly:

    // This namespace is exporting a type
    namespace TypeOnly {
       export type A = string;
    }
    ts

This will result in `ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX` error:

    // This namespace is exporting a value
    namespace A {
       export let x = 1
    }
    ts

Since Decorators are currently a TC39 Stage 3 proposal, they are not transformed and will result in a parser error. Node.js does not provide polyfills and thus will not support decorators until they are supported natively in JavaScript.

In addition, Node.js does not read `tsconfig.json` files and does not support features that depend on settings within `tsconfig.json`, such as paths or converting newer JavaScript syntax into older standards.

#### Importing types without `type` keyword#

Due to the nature of type stripping, the `type` keyword is necessary to correctly strip type imports. Without the `type` keyword, Node.js will treat the import as a value import, which will result in a runtime error. The tsconfig option `verbatimModuleSyntax` can be used to match this behavior.

This example will work correctly:

    import type { Type1, Type2 } from './module.ts';
    import { fn, type FnParams } from './fn.ts';
    ts

This will result in a runtime error:

    import { Type1, Type2 } from './module.ts';
    import { fn, FnParams } from './fn.ts';
    ts

#### Non-file forms of input#

Type stripping can be enabled for `--eval` and STDIN. The module system will be determined by `--input-type`, as it is for JavaScript.

TypeScript syntax is unsupported in the REPL, `--check`, and `inspect`.

#### Source maps#

Since inline types are replaced by whitespace, source maps are unnecessary for correct line numbers in stack traces; and Node.js does not generate them.

#### Type stripping in dependencies#

To discourage package authors from publishing packages written in TypeScript, Node.js refuses to handle TypeScript files inside folders under a `node_modules` path.

#### Paths aliases#

`tsconfig` "paths" won't be transformed and therefore produce an error. The closest feature available is subpath imports with the limitation that they need to start with `#`.

## Net#

Source Code: lib/net.js

Stability: 2 - Stable

The `node:net` module provides an asynchronous network API for creating stream-based TCP or IPC servers (`net.createServer()`) and clients (`net.createConnection()`).

It can be accessed using:

    import net from 'node:net';
    const net = require('node:net');
    javascript

### IPC support#

History

Version | Changes
--- | ---
v20.8.0 | Support binding to abstract Unix domain socket path like `\0abstract`. We can bind '\0' for Node.js `< v20.4.0`.

The `node:net` module supports IPC with named pipes on Windows, and Unix domain sockets on other operating systems.

#### Identifying paths for IPC connections#

`net.connect()`, `net.createConnection()`, `server.listen()`, and `socket.connect()` take a `path` parameter to identify IPC endpoints.

On Unix, the local domain is also known as the Unix domain. The path is a file system pathname. It will throw an error when the length of pathname is greater than the length of `sizeof(sockaddr_un.sun_path)`. Typical values are 107 bytes on Linux and 103 bytes on macOS. If a Node.js API abstraction creates the Unix domain socket, it will unlink the Unix domain socket as well. For example, `net.createServer()` may create a Unix domain socket and `server.close()` will unlink it. But if a user creates the Unix domain socket outside of these abstractions, the user will need to remove it. The same applies when a Node.js API creates a Unix domain socket but the program then crashes. In short, a Unix domain socket will be visible in the file system and will persist until unlinked. On Linux, You can use Unix abstract socket by adding `\0` to the beginning of the path, such as `\0abstract`. The path to the Unix abstract socket is not visible in the file system and it will disappear automatically when all open references to the socket are closed.

On Windows, the local domain is implemented using a named pipe. The path must refer to an entry in `\\?\pipe\` or `\\.\pipe\`. Any characters are permitted, but the latter may do some processing of pipe names, such as resolving `..` sequences. Despite how it might look, the pipe namespace is flat. Pipes will not persist. They are removed when the last reference to them is closed. Unlike Unix domain sockets, Windows will close and remove the pipe when the owning process exits.

JavaScript string escaping requires paths to be specified with extra backslash escaping such as:

    net.createServer().listen(
      path.join('\\\\?\\pipe', process.cwd(), 'myctl'));
    js

### Class: `net.BlockList`#

Added in: v15.0.0, v14.18.0

The `BlockList` object can be used with some network APIs to specify rules for disabling inbound or outbound access to specific IP addresses, IP ranges, or IP subnets.

#### `blockList.addAddress(address[, type])`#

Added in: v15.0.0, v14.18.0

- `address` `<string>` | `<net.SocketAddress>` An IPv4 or IPv6 address.
- `type` `<string>` Either `'ipv4'` or `'ipv6'`. Default: `'ipv4'`.

Adds a rule to block the given IP address.

#### `blockList.addRange(start, end[, type])`#

Added in: v15.0.0, v14.18.0

- `start` `<string>` | `<net.SocketAddress>` The starting IPv4 or IPv6 address in the range.
- `end` `<string>` | `<net.SocketAddress>` The ending IPv4 or IPv6 address in the range.
- `type` `<string>` Either `'ipv4'` or `'ipv6'`. Default: `'ipv4'`.

Adds a rule to block a range of IP addresses from `start` (inclusive) to `end` (inclusive).

#### `blockList.addSubnet(net, prefix[, type])`#

Added in: v15.0.0, v14.18.0

- `net` `<string>` | `<net.SocketAddress>` The network IPv4 or IPv6 address.
- `prefix` `<number>` The number of CIDR prefix bits. For IPv4, this must be a value between `0` and `32`. For IPv6, this must be between `0` and `128`.
- `type` `<string>` Either `'ipv4'` or `'ipv6'`. Default: `'ipv4'`.

Adds a rule to block a range of IP addresses specified as a subnet mask.

#### `blockList.check(address[, type])`#

Added in: v15.0.0, v14.18.0

- `address` `<string>` | `<net.SocketAddress>` The IP address to check
- `type` `<string>` Either `'ipv4'` or `'ipv6'`. Default: `'ipv4'`.
- Returns: `<boolean>`

Returns `true` if the given IP address matches any of the rules added to the `BlockList`.

    const blockList = new net.BlockList();
    blockList.addAddress('123.123.123.123');
    blockList.addRange('10.0.0.1', '10.0.0.10');
    blockList.addSubnet('8592:757c:efae:4e45::', 64, 'ipv6');

    console.log(blockList.check('123.123.123.123'));  // Prints: true
    console.log(blockList.check('10.0.0.3'));  // Prints: true
    console.log(blockList.check('222.111.111.222'));  // Prints: false

    // IPv6 notation for IPv4 addresses works:
    console.log(blockList.check('::ffff:7b7b:7b7b', 'ipv6')); // Prints: true
    console.log(blockList.check('::ffff:123.123.123.123', 'ipv6')); // Prints: true
    js

#### `blockList.rules`#

Added in: v15.0.0, v14.18.0

- Type: `<string>`[]

The list of rules added to the blocklist.

#### `BlockList.isBlockList(value)`#

Added in: v23.4.0, v22.13.0

- `value` `<any>` Any JS value
- Returns `true` if the `value` is a `net.BlockList`.

#### `blockList.fromJSON(value)`#

Stability: 1.2 - Release candidate

    const blockList = new net.BlockList();
    const data = [
      'Subnet: IPv4 192.168.1.0/24',
      'Address: IPv4 10.0.0.5',
      'Range: IPv4 192.168.2.1-192.168.2.10',
      'Range: IPv4 10.0.0.1-10.0.0.10',
    ];
    blockList.fromJSON(data);
    blockList.fromJSON(JSON.stringify(data));
    js

- `value` Blocklist.rules

#### `blockList.toJSON()`#

Stability: 1.2 - Release candidate

- Returns Blocklist.rules

### Class: `net.SocketAddress`#

Added in: v15.14.0, v14.18.0

#### `new net.SocketAddress([options])`#

Added in: v15.14.0, v14.18.0

- `options` `<Object>`
  - `address` `<string>` The network address as either an IPv4 or IPv6 string. Default: `'127.0.0.1'` if `family` is `'ipv4'`; `'::'` if `family` is `'ipv6'`.
  - `family` `<string>` One of either `'ipv4'` or `'ipv6'`. Default: `'ipv4'`.
  - `flowlabel` `<number>` An IPv6 flow-label used only if `family` is `'ipv6'`.
  - `port` `<number>` An IP port.

#### `socketaddress.address`#

Added in: v15.14.0, v14.18.0

- Type: `<string>`

#### `socketaddress.family`#

Added in: v15.14.0, v14.18.0

- Type: `<string>` Either `'ipv4'` or `'ipv6'`.

#### `socketaddress.flowlabel`#

Added in: v15.14.0, v14.18.0

- Type: `<number>`

#### `socketaddress.port`#

Added in: v15.14.0, v14.18.0

- Type: `<number>`

#### `SocketAddress.parse(input)`#

Added in: v23.4.0, v22.13.0

- `input` `<string>` An input string containing an IP address and optional port, e.g. `123.1.2.3:1234` or `[1::1]:1234`.
- Returns: `<net.SocketAddress>` Returns a `SocketAddress` if parsing was successful. Otherwise returns `undefined`.

### Class: `net.Server`#

Added in: v0.1.90

- Extends: `<EventEmitter>`

This class is used to create a TCP or IPC server.

A listening TCP `net.Server` can be transferred to a worker thread by listing it in the `transferList` of a `worker_threads` `postMessage()` call. This moves the underlying listening socket to the receiving thread, where it resumes accepting connections. See Transferring TCP handles to other threads.

#### `new net.Server([options][, connectionListener])`#

- `options` `<Object>` See `net.createServer([options][, connectionListener])`.
- `connectionListener` `<Function>` Automatically set as a listener for the `'connection'` event.
- Returns: `<net.Server>`

`net.Server` is an `EventEmitter` with the following events:

#### Event: `'close'`#

Added in: v0.5.0

Emitted when the server closes. If connections exist, this event is not emitted until all connections are ended.

#### Event: `'connection'`#

Added in: v0.1.90

- Type: `<net.Socket>` The connection object

Emitted when a new connection is made. `socket` is an instance of `net.Socket`.

#### Event: `'error'`#

Added in: v0.1.90

- Type: `<Error>`

Emitted when an error occurs. Unlike `net.Socket`, the `'close'` event will not be emitted directly following this event unless `server.close()` is manually called. See the example in discussion of `server.listen()`.

#### Event: `'listening'`#

Added in: v0.1.90

Emitted when the server has been bound after calling `server.listen()`.

#### Event: `'drop'`#

Added in: v18.6.0, v16.17.0

When the number of connections reaches the threshold of `server.maxConnections`, the server will drop new connections and emit `'drop'` event instead. If it is a TCP server, the argument is as follows, otherwise the argument is `undefined`.

- `data` `<Object>` The argument passed to event listener.
  - `localAddress` `<string>` Local address.
  - `localPort` `<number>` Local port.
  - `localFamily` `<string>` Local family.
  - `remoteAddress` `<string>` Remote address.
  - `remotePort` `<number>` Remote port.
  - `remoteFamily` `<string>` Remote IP family. `'IPv4'` or `'IPv6'`.

#### `server.address()`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v18.4.0 | The `family` property now returns a string instead of a number.
v18.0.0 | The `family` property now returns a number instead of a string.

- Returns: `<Object>` | `<string>` | `<null>`

Returns the bound `address`, the address `family` name, and `port` of the server as reported by the operating system if listening on an IP socket (useful to find which port was assigned when getting an OS-assigned address): `{ port: 12346, family: 'IPv4', address: '127.0.0.1' }`.

For a server listening on a pipe or Unix domain socket, the name is returned as a string.

    const server = net.createServer((socket) => {
      socket.end('goodbye\n');
    }).on('error', (err) => {
      // Handle errors here.
      throw err;
    });

    // Grab an arbitrary unused port.
    server.listen(() => {
      console.log('opened server on', server.address());
    });
    js

`server.address()` returns `null` before the `'listening'` event has been emitted or after calling `server.close()`.

#### `server.close([callback])`#

Added in: v0.1.90

- `callback` `<Function>` Called when the server is closed.
- Returns: `<net.Server>`

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a `'close'` event. The optional `callback` will be called once the `'close'` event occurs. Unlike that event, it will be called with an `Error` as its only argument if the server was not open when it was closed.

#### `server[Symbol.asyncDispose]()`#

Added in: v20.5.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `server.close()` and returns a promise that fulfills when the server has closed.

#### `server.getConnections(callback)`#

Added in: v0.9.7

- `callback` `<Function>`
- Returns: `<net.Server>`

Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.

Callback should take two arguments `err` and `count`.

#### `server.listen()`#

Start a server listening for connections. A `net.Server` can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

- `server.listen(handle[, backlog][, callback])`
- `server.listen(options[, callback])`
- `server.listen(path[, backlog][, callback])` for IPC servers
- `server.listen([port[, host[, backlog]]][, callback])` for TCP servers

This function is asynchronous. When the server starts listening, the `'listening'` event will be emitted. The last parameter `callback` will be added as a listener for the `'listening'` event.

All `listen()` methods can take a `backlog` parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as `tcp_max_syn_backlog` and `somaxconn` on Linux. The default value of this parameter is 511 (not 512).

All `net.Socket` are set to `SO_REUSEADDR` (see `socket(7)` for details).

The `server.listen()` method can be called again if and only if there was an error during the first `server.listen()` call or `server.close()` has been called. Otherwise, an `ERR_SERVER_ALREADY_LISTEN` error will be thrown.

One of the most common errors raised when listening is `EADDRINUSE`. This happens when another server is already listening on the requested `port`/`path`/`handle`. One way to handle this would be to retry after a certain amount of time:

    server.on('error', (e) => {
      if (e.code === 'EADDRINUSE') {
        console.error('Address in use, retrying...');
        setTimeout(() => {
          server.close();
          server.listen(PORT, HOST);
        }, 1000);
      }
    });
    js

##### `server.listen(handle[, backlog][, callback])`#

Added in: v0.5.10

- `handle` `<Object>`
- `backlog` `<number>` Common parameter of `server.listen()` functions
- `callback` `<Function>`
- Returns: `<net.Server>`

Start a server listening for connections on a given `handle` that has already been bound to a port, a Unix domain socket, or a Windows named pipe.

The `handle` object can be either a server, a socket (anything with an underlying `_handle` member), a `BoundSocket`, or an object with an `fd` member that is a valid file descriptor.

When `handle` is a `BoundSocket`, the server adopts the already-bound socket and starts listening on it. Adoption consumes the bound socket (see ownership transfer).

Listening on a file descriptor is not supported on Windows.

##### `server.listen(options[, callback])`#

Added in: v0.11.14
History

Version | Changes
--- | ---
v23.1.0, v22.12.0 | The `reusePort` option is supported.
v15.6.0 | AbortSignal support was added.
v11.4.0 | The `ipv6Only` option is supported.

- `options` `<Object>` Required. Supports the following properties:
  - `backlog` `<number>` Common parameter of `server.listen()` functions.
  - `exclusive` `<boolean>` Default: `false`
  - `handle` `<net.BoundSocket>` A pre-bound `BoundSocket`. The server adopts the already-bound socket and listens on it, ignoring `host`, `port`, and `path`. Adoption consumes the bound socket (see ownership transfer).
  - `host` `<string>`
  - `ipv6Only` `<boolean>` For TCP servers, setting `ipv6Only` to `true` will disable dual-stack support, i.e., binding to host `::` won't make `0.0.0.0` be bound. Default: `false`.
  - `reusePort` `<boolean>` For TCP servers, setting `reusePort` to `true` allows multiple sockets on the same host to bind to the same port. Incoming connections are distributed by the operating system to listening sockets. This option is available only on some platforms, such as Linux 3.9+, DragonFlyBSD 3.6+, FreeBSD 12.0+, Solaris 11.4, and AIX 7.2.5+. On unsupported platforms, this option raises an error. Default: `false`.
  - `path` `<string>` Will be ignored if `port` is specified. See Identifying paths for IPC connections.
  - `port` `<number>`
  - `readableAll` `<boolean>` For IPC servers makes the pipe readable for all users. Default: `false`.
  - `signal` `<AbortSignal>` An AbortSignal that may be used to close a listening server.
  - `writableAll` `<boolean>` For IPC servers makes the pipe writable for all users. Default: `false`.
- `callback` `<Function>` functions.
- Returns: `<net.Server>`

If `handle` is specified, the server adopts that pre-bound socket. Otherwise, if `port` is specified, it behaves the same as `server.listen([port[, host[, backlog]]][, callback])`. Otherwise, if `path` is specified, it behaves the same as `server.listen(path[, backlog][, callback])`. If none of them is specified, an error will be thrown.

If `exclusive` is `false` (default), then cluster workers will use the same underlying handle, allowing connection handling duties to be shared. When `exclusive` is `true`, the handle is not shared, and attempted port sharing results in an error. An example which listens on an exclusive port is shown below.

    server.listen({
      host: 'localhost',
      port: 80,
      exclusive: true,
    });
    js

When `exclusive` is `true` and the underlying handle is shared, it is possible that several workers query a handle with different backlogs. In this case, the first `backlog` passed to the master process will be used.

Starting an IPC server as root may cause the server path to be inaccessible for unprivileged users. Using `readableAll` and `writableAll` will make the server accessible for all users.

If the `signal` option is enabled, calling `.abort()` on the corresponding `AbortController` is similar to calling `.close()` on the server:

    const controller = new AbortController();
    server.listen({
      host: 'localhost',
      port: 80,
      signal: controller.signal,
    });
    // Later, when you want to close the server.
    controller.abort();
    js

##### `server.listen(path[, backlog][, callback])`#

Added in: v0.1.90

- `path` `<string>` Path the server should listen to. See Identifying paths for IPC connections.
- `backlog` `<number>` Common parameter of `server.listen()` functions.
- `callback` `<Function>`.
- Returns: `<net.Server>`

Start an IPC server listening for connections on the given `path`.

##### `server.listen([port[, host[, backlog]]][, callback])`#

Added in: v0.1.90

- `port` `<number>`
- `host` `<string>`
- `backlog` `<number>` Common parameter of `server.listen()` functions.
- `callback` `<Function>`.
- Returns: `<net.Server>`

Start a TCP server listening for connections on the given `port` and `host`.

If `port` is omitted or is 0, the operating system will assign an arbitrary unused port, which can be retrieved by using `server.address().port` after the `'listening'` event has been emitted.

If `host` is omitted, the server will accept connections on the unspecified IPv6 address (`::`) when IPv6 is available, or the unspecified IPv4 address (`0.0.0.0`) otherwise.

In most operating systems, listening to the unspecified IPv6 address (`::`) may cause the `net.Server` to also listen on the unspecified IPv4 address (`0.0.0.0`).

#### `server.listening`#

Added in: v5.7.0

- Type: `<boolean>` Indicates whether or not the server is listening for connections.

#### `server.maxConnections`#

Added in: v0.2.0
History

Version | Changes
--- | ---
v21.0.0 | Setting `maxConnections` to `0` drops all the incoming connections. Previously, it was interpreted as `Infinity`.

- Type: `<integer>`

When the number of connections reaches the `server.maxConnections` threshold:

1. If the process is not running in cluster mode, Node.js will close the connection.

2. If the process is running in cluster mode, Node.js will, by default, route the connection to another worker process. To close the connection instead, set `server.dropMaxConnection` to `true`.

It is not recommended to use this option once a socket has been sent to a child with `child_process.fork()`.

#### `server.dropMaxConnection`#

Added in: v23.1.0, v22.12.0

- Type: `<boolean>`

Set this property to `true` to begin closing connections once the number of connections reaches the `server.maxConnections` threshold. This setting is only effective in cluster mode.

#### `server.ref()`#

Added in: v0.9.1

- Returns: `<net.Server>`

Opposite of `unref()`, calling `ref()` on a previously `unref`ed server will not let the program exit if it's the only server left (the default behavior). If the server is `ref`ed calling `ref()` again will have no effect.

#### `server.unref()`#

Added in: v0.9.1

- Returns: `<net.Server>`

Calling `unref()` on a server will allow the program to exit if this is the only active server in the event system. If the server is already `unref`ed calling `unref()` again will have no effect.

### Class: `net.Socket`#

Added in: v0.3.4

- Extends: `<stream.Duplex>`

This class is an abstraction of a TCP socket or a streaming IPC endpoint (uses named pipes on Windows, and Unix domain sockets otherwise). It is also an `EventEmitter`.

A `net.Socket` can be created by the user and used directly to interact with a server. For example, it is returned by `net.createConnection()`, so the user can use it to talk to the server.

It can also be created by Node.js and passed to the user when a connection is received. For example, it is passed to the listeners of a `'connection'` event emitted on a `net.Server`, so the user can use it to interact with the client.

#### Transferring TCP handles to other threads#

A connected TCP `net.Socket` can be moved to another thread by listing it in the `transferList` of a `worker_threads` `postMessage()` call. After the transfer, the source socket is destroyed on the sending thread (further use fails with `ERR_STREAM_DESTROYED` rather than silently dropping data), and the socket continues to work on the receiving thread. This makes it possible to accept connections on one thread and distribute them across a pool of worker threads, for example to build a `node:cluster`-like model on top of worker threads.

The socket must be a freshly accepted or created TCP connection: it must still be attached to a live handle, must not be connecting or destroyed, and must not have started reading or have buffered data. Otherwise `postMessage()` throws `ERR_WORKER_HANDLE_NOT_TRANSFERABLE`. Only TCP sockets are supported.

    const net = require('node:net');
    const { Worker } = require('node:worker_threads');

    // worker.js receives `{ socket }` messages and handles each connection.
    const worker = new Worker('./worker.js');

    const server = net.createServer((socket) => {
      // Hand the freshly accepted connection off to the worker thread.
      worker.postMessage({ socket }, [socket]);
    });
    server.listen(8000);
    cjs

A listening `net.Server` can be transferred the same way, which moves the listening socket itself (and its pending accept queue) to the receiving thread.

#### `new net.Socket([options])`#

Added in: v0.3.4
History

Version | Changes
--- | ---
v25.6.0 | Added `typeOfService` option.
v15.14.0 | AbortSignal support was added.
v12.10.0 | Added `onread` option.

- `options` `<Object>` Available options are:
  - `allowHalfOpen` `<boolean>` If set to `false`, then the socket will automatically end the writable side when the readable side ends. See `net.createServer()` and the `'end'` event for details. Default: `false`.
  - `blockList` `<net.BlockList>` `blockList` can be used for disabling outbound access to specific IP addresses, IP ranges, or IP subnets.
  - `fd` `<number>` If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.
  - `handle` `<net.BoundSocket>` If specified, wrap around the bound socket from a `BoundSocket`. A subsequent `socket.connect()` uses the bound socket as the connection's source binding (honoring the bound local address and port). Adoption consumes the bound socket (see ownership transfer).
  - `keepAlive` `<boolean>` If set to `true`, it enables keep-alive functionality on the socket immediately after the connection is established, similarly on what is done in `socket.setKeepAlive()`. Default: `false`.
  - `keepAliveInitialDelay` `<number>` If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket. Default: `0`.
  - `noDelay` `<boolean>` If set to `true`, it disables the use of Nagle's algorithm immediately after the socket is established. Default: `false`.
  - `onread` `<Object>` If specified, incoming data is stored in a single `buffer` and passed to the supplied `callback` when data arrives on the socket. This will cause the streaming functionality to not provide any data. The socket will emit events like `'error'`, `'end'`, and `'close'` as usual. Methods like `pause()` and `resume()` will also behave as expected.
    - `buffer` `<Buffer>` | `<Uint8Array>` | `<Function>` Either a reusable chunk of memory to use for storing incoming data or a function that returns such.
    - `callback` `<Function>` This function is called for every chunk of incoming data. Two arguments are passed to it: the number of bytes written to `buffer` and a reference to `buffer`. Return `false` from this function to implicitly `pause()` the socket. This function will be executed in the global context.
  - `readable` `<boolean>` Allow reads on the socket when an `fd` is passed, otherwise ignored. Default: `false`.
  - `signal` `<AbortSignal>` An Abort signal that may be used to destroy the socket.
  - `typeOfService` `<number>` The initial Type of Service (TOS) value.
  - `writable` `<boolean>` Allow writes on the socket when an `fd` is passed, otherwise ignored. Default: `false`.
- Returns: `<net.Socket>`

Creates a new socket object.

The newly created socket can be either a TCP socket or a streaming IPC endpoint, depending on what it `connect()` to.

#### Event: `'close'`#

Added in: v0.1.90

- `hadError` `<boolean>` `true` if the socket had a transmission error.

Emitted once the socket is fully closed. The argument `hadError` is a boolean which says if the socket was closed due to a transmission error.

#### Event: `'connect'`#

Added in: v0.1.90

Emitted when a socket connection is successfully established. See `net.createConnection()`.

#### Event: `'connectionAttempt'`#

Added in: v21.6.0, v20.12.0

- `ip` `<string>` The IP which the socket is attempting to connect to.
- `port` `<number>` The port which the socket is attempting to connect to.
- `family` `<number>` The family of the IP. It can be `6` for IPv6 or `4` for IPv4.

Emitted when a new connection attempt is started. This may be emitted multiple times if the family autoselection algorithm is enabled in `socket.connect(options)`.

#### Event: `'connectionAttemptFailed'`#

Added in: v21.6.0, v20.12.0

- `ip` `<string>` The IP which the socket attempted to connect to.
- `port` `<number>` The port which the socket attempted to connect to.
- `family` `<number>` The family of the IP. It can be `6` for IPv6 or `4` for IPv4.
- `error` `<Error>` The error associated with the failure.

Emitted when a connection attempt failed. This may be emitted multiple times if the family autoselection algorithm is enabled in `socket.connect(options)`.

#### Event: `'connectionAttemptTimeout'`#

Added in: v21.6.0, v20.12.0

- `ip` `<string>` The IP which the socket attempted to connect to.
- `port` `<number>` The port which the socket attempted to connect to.
- `family` `<number>` The family of the IP. It can be `6` for IPv6 or `4` for IPv4.

Emitted when a connection attempt timed out. This is only emitted (and may be emitted multiple times) if the family autoselection algorithm is enabled in `socket.connect(options)`.

#### Event: `'data'`#

Added in: v0.1.90

- Type: `<Buffer>` | `<string>`

Emitted when data is received. The argument `data` will be a `Buffer` or `String`. Encoding of data is set by `socket.setEncoding()`.

The data will be lost if there is no listener when a `Socket` emits a `'data'` event.

#### Event: `'drain'`#

Added in: v0.1.90

Emitted when the write buffer becomes empty. Can be used to throttle uploads.

See also: the return values of `socket.write()`.

#### Event: `'end'`#

Added in: v0.1.90

Emitted when the other end of the socket signals the end of transmission, thus ending the readable side of the socket.

By default (`allowHalfOpen` is `false`) the socket will send an end of transmission packet back and destroy its file descriptor once it has written out its pending write queue. However, if `allowHalfOpen` is set to `true`, the socket will not automatically `end()` its writable side, allowing the user to write arbitrary amounts of data. The user must call `end()` explicitly to close the connection (i.e. sending a FIN packet back).

#### Event: `'error'`#

Added in: v0.1.90

- Type: `<Error>`

Emitted when an error occurs. The `'close'` event will be called directly following this event.

#### Event: `'lookup'`#

Added in: v0.11.3
History

Version | Changes
--- | ---
v5.10.0 | The `host` parameter is supported now.

Emitted after resolving the host name but before connecting. Not applicable to Unix sockets.

- `err` `<Error>` | `<null>` The error object. See `dns.lookup()`.
- `address` `<string>` The IP address.
- `family` `<number>` | `<null>` The address type. See `dns.lookup()`.
- `host` `<string>` The host name.

#### Event: `'ready'`#

Added in: v9.11.0

Emitted when a socket is ready to be used.

Triggered immediately after `'connect'`.

#### Event: `'timeout'`#

Added in: v0.1.90

Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.

See also: `socket.setTimeout()`.

#### `socket.address()`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v18.4.0 | The `family` property now returns a string instead of a number.
v18.0.0 | The `family` property now returns a number instead of a string.

- Returns: `<Object>`

Returns the bound `address`, the address `family` name and `port` of the socket as reported by the operating system: `{ port: 12346, family: 'IPv4', address: '127.0.0.1' }`

#### `socket.autoSelectFamilyAttemptedAddresses`#

Added in: v19.4.0, v18.18.0

- Type: `<string>`[]

This property is only present if the family autoselection algorithm is enabled in `socket.connect(options)` and it is an array of the addresses that have been attempted.

Each address is a string in the form of `$IP:$PORT`. If the connection was successful, then the last address is the one that the socket is currently connected to.

#### `socket.bufferSize`#

Added in: v0.3.8Deprecated in: v14.6.0

Stability: 0 - Deprecated: Use `writable.writableLength` instead.

- Type: `<integer>`

This property shows the number of characters buffered for writing. The buffer may contain strings whose length after encoding is not yet known. So this number is only an approximation of the number of bytes in the buffer.

`net.Socket` has the property that `socket.write()` always works. This is to help users get up and running quickly. The computer cannot always keep up with the amount of data that is written to a socket. The network connection simply might be too slow. Node.js will internally queue up the data written to a socket and send it out over the wire when it is possible.

The consequence of this internal buffering is that memory may grow. Users who experience large or growing `bufferSize` should attempt to "throttle" the data flows in their program with `socket.pause()` and `socket.resume()`.

#### `socket.bytesRead`#

Added in: v0.5.3

- Type: `<integer>`

The amount of received bytes.

#### `socket.bytesWritten`#

Added in: v0.5.3

- Type: `<integer>`

The amount of bytes sent.

#### `socket.connect()`#

Initiate a connection on a given socket.

Possible signatures:

- `socket.connect(options[, connectListener])`
- `socket.connect(path[, connectListener])` for IPC connections.
- `socket.connect(port[, host][, connectListener])` for TCP connections.
- Returns: `<net.Socket>` The socket itself.

This function is asynchronous. When the connection is established, the `'connect'` event will be emitted. If there is a problem connecting, instead of a `'connect'` event, an `'error'` event will be emitted with the error passed to the `'error'` listener. The last parameter `connectListener`, if supplied, will be added as a listener for the `'connect'` event once.

This function should only be used for reconnecting a socket after `'close'` has been emitted or otherwise it may lead to undefined behavior.

##### `socket.connect(options[, connectListener])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v20.0.0, v18.18.0 | The default value for the autoSelectFamily option is now true. The `--enable-network-family-autoselection` CLI flag has been renamed to `--network-family-autoselection`. The old name is now an alias but it is discouraged.
v19.4.0 | The default value for autoSelectFamily option can be changed at runtime using `setDefaultAutoSelectFamily` or via the command line option `--enable-network-family-autoselection`.
v19.3.0, v18.13.0 | Added the `autoSelectFamily` option.
v17.7.0, v16.15.0 | The `noDelay`, `keepAlive`, and `keepAliveInitialDelay` options are supported now.
v6.0.0 | The `hints` option defaults to `0` in all cases now. Previously, in the absence of the `family` option it would default to `dns.ADDRCONFIG | dns.V4MAPPED`.
v5.11.0 | The `hints` option is supported now.

- `options` `<Object>`
- `connectListener` `<Function>` Common parameter of `socket.connect()` methods. Will be added as a listener for the `'connect'` event once.
- Returns: `<net.Socket>` The socket itself.

Initiate a connection on a given socket. Normally this method is not needed, the socket should be created and opened with `net.createConnection()`. Use this only when implementing a custom Socket.

For TCP connections, available `options` are:

- `autoSelectFamily` `<boolean>`: If set to `true`, it enables a family autodetection algorithm that loosely implements section 5 of RFC 8305. The `all` option passed to lookup is set to `true` and the sockets attempts to connect to all obtained IPv6 and IPv4 addresses, in sequence, until a connection is established. The first returned AAAA address is tried first, then the first returned A address, then the second returned AAAA address and so on. Each connection attempt (but the last one) is given the amount of time specified by the `autoSelectFamilyAttemptTimeout` option before timing out and trying the next address. Ignored if the `family` option is not `0` or if `localAddress` is set. Connection errors are not emitted if at least one connection succeeds. If all connections attempts fails, a single `AggregateError` with all failed attempts is emitted. Default: `net.getDefaultAutoSelectFamily()`.
- `autoSelectFamilyAttemptTimeout` `<number>`: The amount of time in milliseconds to wait for a connection attempt to finish before trying the next address when using the `autoSelectFamily` option. If set to a positive integer less than `10`, then the value `10` will be used instead. Default: `net.getDefaultAutoSelectFamilyAttemptTimeout()`.
- `family` `<number>`: Version of IP stack. Must be `4`, `6`, or `0`. The value `0` indicates that both IPv4 and IPv6 addresses are allowed. Default: `0`.
- `hints` `<number>` Optional `dns.lookup()` hints.
- `host` `<string>` Host the socket should connect to. Default: `'localhost'`.
- `localAddress` `<string>` Local address the socket should connect from.
- `localPort` `<number>` Local port the socket should connect from.
- `lookup` `<Function>` Custom lookup function. Default: `dns.lookup()`.
- `port` `<number>` Required. Port the socket should connect to.

For IPC connections, available `options` are:

- `path` `<string>` Required. Path the client should connect to. See Identifying paths for IPC connections. If provided, the TCP-specific options above are ignored.

##### `socket.connect(path[, connectListener])`#

- `path` `<string>` Path the client should connect to. See Identifying paths for IPC connections.
- `connectListener` `<Function>` Common parameter of `socket.connect()` methods. Will be added as a listener for the `'connect'` event once.
- Returns: `<net.Socket>` The socket itself.

Initiate an IPC connection on the given socket.

Alias to `socket.connect(options[, connectListener])` called with `{ path: path }` as `options`.

##### `socket.connect(port[, host][, connectListener])`#

Added in: v0.1.90

- `port` `<number>` Port the client should connect to.
- `host` `<string>` Host the client should connect to.
- `connectListener` `<Function>` Common parameter of `socket.connect()` methods. Will be added as a listener for the `'connect'` event once.
- Returns: `<net.Socket>` The socket itself.

Initiate a TCP connection on the given socket.

Alias to `socket.connect(options[, connectListener])` called with `{port: port, host: host}` as `options`.

#### `socket.connecting`#

Added in: v6.1.0

- Type: `<boolean>`

If `true`, `socket.connect(options[, connectListener])` was called and has not yet finished. It will stay `true` until the socket becomes connected, then it is set to `false` and the `'connect'` event is emitted. Note that the `socket.connect(options[, connectListener])` callback is a listener for the `'connect'` event.

#### `socket.destroy([error])`#

Added in: v0.1.90

- `error` `<Object>`
- Returns: `<net.Socket>`

Ensures that no more I/O activity happens on this socket. Destroys the stream and closes the connection.

See `writable.destroy()` for further details.

#### `socket.destroyed`#

- Type: `<boolean>` Indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.

See `writable.destroyed` for further details.

#### `socket.destroySoon()`#

Added in: v0.3.4

Destroys the socket after all data is written. If the `'finish'` event was already emitted the socket is destroyed immediately. If the socket is still writable it implicitly calls `socket.end()`.

#### `socket.end([data[, encoding]][, callback])`#

Added in: v0.1.90

- `data` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>` Only used when data is `string`. Default: `'utf8'`.
- `callback` `<Function>` Optional callback for when the socket is finished.
- Returns: `<net.Socket>` The socket itself.

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

See `writable.end()` for further details.

#### `socket.localAddress`#

Added in: v0.9.6

- Type: `<string>`

The string representation of the local IP address the remote client is connecting on. For example, in a server listening on `'0.0.0.0'`, if a client connects on `'192.168.1.1'`, the value of `socket.localAddress` would be `'192.168.1.1'`.

#### `socket.localPort`#

Added in: v0.9.6

- Type: `<integer>`

The numeric representation of the local port. For example, `80` or `21`.

#### `socket.localFamily`#

Added in: v18.8.0, v16.18.0

- Type: `<string>`

The string representation of the local IP family. `'IPv4'` or `'IPv6'`.

#### `socket.pause()`#

- Returns: `<net.Socket>` The socket itself.

Pauses the reading of data. That is, `'data'` events will not be emitted. Useful to throttle back an upload.

#### `socket.pending`#

Added in: v11.2.0, v10.16.0

- Type: `<boolean>`

This is `true` if the socket is not connected yet, either because `.connect()` has not yet been called or because it is still in the process of connecting (see `socket.connecting`).

#### `socket.ref()`#

Added in: v0.9.1

- Returns: `<net.Socket>` The socket itself.

Opposite of `unref()`, calling `ref()` on a previously `unref`ed socket will not let the program exit if it's the only socket left (the default behavior). If the socket is `ref`ed calling `ref` again will have no effect.

#### `socket.remoteAddress`#

Added in: v0.5.10

- Type: `<string>`

The string representation of the remote IP address. For example, `'74.125.127.100'` or `'2001:4860:a005::68'`. Value may be `undefined` if the socket is destroyed (for example, if the client disconnected).

#### `socket.remoteFamily`#

Added in: v0.11.14

- Type: `<string>`

The string representation of the remote IP family. `'IPv4'` or `'IPv6'`. Value may be `undefined` if the socket is destroyed (for example, if the client disconnected).

#### `socket.remotePort`#

Added in: v0.5.10

- Type: `<integer>`

The numeric representation of the remote port. For example, `80` or `21`. Value may be `undefined` if the socket is destroyed (for example, if the client disconnected).

#### `socket.server`#

Added in: v0.3.4

- Type: `<net.Server>` | `<null>`

Reference to the server that accepted the socket. This is `null` for sockets that were not accepted by a server.

#### `socket.resetAndDestroy()`#

Added in: v18.3.0, v16.17.0

- Returns: `<net.Socket>`

Close the TCP connection by sending an RST packet and destroy the stream. If this TCP socket is in connecting status, it will send an RST packet and destroy this TCP socket once it is connected. Otherwise, it will call `socket.destroy` with an `ERR_SOCKET_CLOSED` Error. If this is not a TCP socket (for example, a pipe), calling this method will immediately throw an `ERR_INVALID_HANDLE_TYPE` Error.

#### `socket.resume()`#

- Returns: `<net.Socket>` The socket itself.

Resumes reading after a call to `socket.pause()`.

#### `socket.setEncoding([encoding])`#

Added in: v0.1.90

- `encoding` `<string>`
- Returns: `<net.Socket>` The socket itself.

Set the encoding for the socket as a Readable Stream. See `readable.setEncoding()` for more information.

#### `socket.setKeepAlive()`#

Enable/disable keep-alive functionality, and optionally configure the keepalive probe timing. Returns the socket itself.

Possible signatures:

- `socket.setKeepAlive([options])`
- `socket.setKeepAlive([enable][, initialDelay][, interval][, count])`

Enabling keep-alive sets the initial delay before the first keepalive probe is sent on an idle socket.

Set `initialDelay` (in milliseconds) to set the delay between the last data packet received and the first keepalive probe. Setting `0` for `initialDelay` will leave the value unchanged from the default (or previous) setting.

Set `interval` (in milliseconds) to set the delay between successive keepalive probes once they begin (`TCP_KEEPINTVL`). Set `count` to the number of unacknowledged probes sent before the connection is dropped (`TCP_KEEPCNT`). Both are only applied when keep-alive is enabled. Omitting `interval` or `count` uses the defaults of `1000` ms and `10`. As with `initialDelay`, a non-positive `interval` or `count` leaves the corresponding system default unchanged.

`initialDelay` and `interval` are specified in milliseconds but the underlying socket options are configured in whole seconds; the values are divided by `1000` and rounded down before being applied.

Enabling the keep-alive functionality will set the following socket options:

- `SO_KEEPALIVE=1`
- `TCP_KEEPIDLE=initialDelay / 1000`
- `TCP_KEEPCNT=count`
- `TCP_KEEPINTVL=interval / 1000`

On Windows versions older than build 1709, keep-alive is configured through `SIO_KEEPALIVE_VALS`, which has no probe-count field, so `count` is ignored on those platforms.

##### `socket.setKeepAlive([options])`#

Added in: v26.4.0

- `options` `<Object>`
  - `enable` `<boolean>` Default: `false`
  - `initialDelay` `<number>` Default: `0`
  - `interval` `<number>` Default: `1000`
  - `count` `<number>` Default: `10`
- Returns: `<net.Socket>` The socket itself.

Configure keep-alive using an options object. See `socket.setKeepAlive()` for a description of each property.

    socket.setKeepAlive({ enable: true, initialDelay: 1000, interval: 1000, count: 10 });
    js

##### `socket.setKeepAlive([enable][, initialDelay][, interval][, count])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v26.4.0 | Added the `interval` and `count` arguments to configure `TCP_KEEPINTVL` and `TCP_KEEPCNT`.
v13.12.0, v12.17.0 | New defaults for `TCP_KEEPCNT` and `TCP_KEEPINTVL` socket options were added.

- `enable` `<boolean>` Default: `false`
- `initialDelay` `<number>` Default: `0`
- `interval` `<number>` Default: `1000`
- `count` `<number>` Default: `10`
- Returns: `<net.Socket>` The socket itself.

Configure keep-alive using positional arguments. See `socket.setKeepAlive()` for a description of each argument.

#### `socket.setNoDelay([noDelay])`#

Added in: v0.1.90

- `noDelay` `<boolean>` Default: `true`
- Returns: `<net.Socket>` The socket itself.

Enable/disable the use of Nagle's algorithm.

When a TCP connection is created, it will have Nagle's algorithm enabled.

Nagle's algorithm delays data before it is sent via the network. It attempts to optimize throughput at the expense of latency.

Passing `true` for `noDelay` or not passing an argument will disable Nagle's algorithm for the socket. Passing `false` for `noDelay` will enable Nagle's algorithm.

#### `socket.setTimeout(timeout[, callback])`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `timeout` `<number>`
- `callback` `<Function>`
- Returns: `<net.Socket>` The socket itself.

Sets the socket to timeout after `timeout` milliseconds of inactivity on the socket. By default `net.Socket` do not have a timeout.

When an idle timeout is triggered the socket will receive a `'timeout'` event but the connection will not be severed. The user must manually call `socket.end()` or `socket.destroy()` to end the connection.

    socket.setTimeout(3000);
    socket.on('timeout', () => {
      console.log('socket timeout');
      socket.end();
    });
    js

If `timeout` is 0, then the existing idle timeout is disabled.

The optional `callback` parameter will be added as a one-time listener for the `'timeout'` event.

#### `socket.getTypeOfService()`#

Added in: v25.6.0

- Returns: `<integer>` The current TOS value.

Returns the current Type of Service (TOS) field for IPv4 packets or Traffic Class for IPv6 packets for this socket.

`setTypeOfService()` may be called before the socket is connected; the value will be cached and applied when the socket establishes a connection. `getTypeOfService()` will return the currently set value even before connection.

On some platforms (e.g., Linux), certain TOS/ECN bits may be masked or ignored, and behavior can differ between IPv4 and IPv6 or dual-stack sockets. Callers should verify platform-specific semantics.

#### `socket.setTypeOfService(tos)`#

Added in: v25.6.0

- `tos` `<integer>` The TOS value to set (0-255).
- Returns: `<net.Socket>` The socket itself.

Sets the Type of Service (TOS) field for IPv4 packets or Traffic Class for IPv6 Packets sent from this socket. This can be used to prioritize network traffic.

`setTypeOfService()` may be called before the socket is connected; the value will be cached and applied when the socket establishes a connection. `getTypeOfService()` will return the currently set value even before connection.

On some platforms (e.g., Linux), certain TOS/ECN bits may be masked or ignored, and behavior can differ between IPv4 and IPv6 or dual-stack sockets. Callers should verify platform-specific semantics.

#### `socket.timeout`#

Added in: v10.7.0

- Type: `<number>` | `<undefined>`

The socket timeout in milliseconds as set by `socket.setTimeout()`. It is `undefined` if a timeout has not been set.

#### `socket.unref()`#

Added in: v0.9.1

- Returns: `<net.Socket>` The socket itself.

Calling `unref()` on a socket will allow the program to exit if this is the only active socket in the event system. If the socket is already `unref`ed calling `unref()` again will have no effect.

#### `socket.write(data[, encoding][, callback])`#

Added in: v0.1.90

- `data` `<string>` | `<Buffer>` | `<Uint8Array>`
- `encoding` `<string>` Only used when data is `string`. Default: `utf8`.
- `callback` `<Function>`
- Returns: `<boolean>`

Sends data on the socket. The second parameter specifies the encoding in the case of a string. It defaults to UTF8 encoding.

Returns `true` if the entire data was flushed successfully to the kernel buffer. Returns `false` if all or part of the data was queued in user memory. `'drain'` will be emitted when the buffer is again free.

The optional `callback` parameter will be executed when the data is finally written out, which may not be immediately.

See `Writable` stream `write()` method for more information.

#### `socket.readyState`#

Added in: v0.5.0

- Type: `<string>`

This property represents the state of the connection as a string.

- If the socket is connecting, `socket.readyState` is `opening`.
- If the socket is readable and writable, it is `open`.
- If the socket is readable and not writable, it is `readOnly`.
- If the socket is not readable and writable, it is `writeOnly`.
- Otherwise, it is `closed`.

### Class: `net.BoundSocket`#

Added in: v26.4.0

Allows for the synchronous creation of a pre-bound socket, that can be passed to `listen()` or `new net.Socket()` later on. For `listen()` this enables synchronous port reservation, while for `new net.Socket()`, it allows control over the local egress port/IP, via `bind(2)` semantics.

A `BoundSocket` binds either a TCP endpoint (`host` or `port`) or a Unix domain/named-pipe endpoint (`path`); the two are mutually exclusive. For a `path`, the file system entry is reserved in the constructor, so conflicts such as `EADDRINUSE` throw synchronously exactly as a TCP bind does. On Linux a leading `'\0'` in `path` selects the abstract namespace (no file system entry); an abstract path on any other platform throws `ERR_INVALID_ARG_VALUE`.

Adoption transfers ownership of the socket; afterwards `address()` and `close()` throw `ERR_SOCKET_HANDLE_ADOPTED`. A handle that is never adopted must be closed to avoid leaking the socket. Closing a pipe `BoundSocket` removes its file system entry; abstract and TCP binds have none to remove.

When a pipe `BoundSocket` bound to a source `path` is adopted as a client, that path is reported as the socket's `localAddress` once it connects.

When an adopted `BoundSocket` connects to a numeric IP literal, `connect(2)` is issued synchronously, so `socket.localAddress` is resolved once `socket.connect()` returns. Connection failures are still reported via a deferred `'error'` event.

    import net from 'node:net';

    const bound = new net.BoundSocket();
    const { port } = bound.address();
    console.log(`Reserved port ${port} for server`);

    const server = net.createServer();
    server.listen(bound); // Adopt as a server, or pass to new net.Socket() instead.
    mjs

#### `new net.BoundSocket([options])`#

Added in: v26.4.0
History

Version | Changes
--- | ---
v26.7.0 | The `path` option is supported.

- `options` `<Object>`
  - `host` `<string>` Local address to bind. Must be a numeric IP literal; no DNS resolution is performed. Default: `'0.0.0.0'`, or `'::'` when `ipv6Only` is `true`.
  - `port` `<number>` Local port. `0` requests an OS-assigned ephemeral port. Default: `0`.
  - `ipv6Only` `<boolean>` Sets `IPV6_V6ONLY`, disabling dual-stack support so the socket binds IPv6 only. Only meaningful for IPv6 binds. Default: `false`.
  - `reusePort` `<boolean>` Sets `SO_REUSEPORT`, allowing multiple sockets to bind the same address and port for kernel-level load balancing. Support is platform-dependent. Default: `false`.
  - `path` `<string>` Binds a Unix domain socket (or Windows named pipe) at the given path instead of a TCP endpoint. A leading `'\0'` selects the Linux abstract namespace. Mutually exclusive with `host`, `port`, `ipv6Only`, and `reusePort`; combining them throws `ERR_INVALID_ARG_VALUE`.

#### `boundSocket.address()`#

Added in: v26.4.0
History

Version | Changes
--- | ---
v26.7.0 | The bound path is returned for a pipe bind.

- Returns: `<Object>` | `<string>` For a TCP bind, an object with `address`, `family`, and `port` properties, as `server.address()` returns. For a pipe bind, the bound path string, as `server.address()` returns for a pipe server.

Returns the bound local address. When bound with `port: 0`, `port` is the OS-assigned ephemeral port.

#### `boundSocket.isPipe`#

Added in: v26.7.0

- `<boolean>`

`true` when the socket was bound with a `path` (a Unix domain socket or Windows named pipe), `false` for a TCP bind. The getter's presence on `net.BoundSocket.prototype` also serves as a capability probe for `path` support.

#### `boundSocket.fd()`#

Added in: v26.4.0

- Returns: `<integer>` The underlying OS file descriptor, or `-1` on platforms that do not expose one for sockets (such as Windows).

Returns the file descriptor of the bound socket. Ownership remains with the `BoundSocket`, so the descriptor must not be closed by the caller. The descriptor is only available before the handle is adopted; afterwards it belongs to the adopting `net.Server` or `net.Socket` and `fd()` throws `ERR_SOCKET_HANDLE_ADOPTED`.

#### `boundSocket.close()`#

Added in: v26.4.0

Releases the bound socket. Only needed when the handle is never adopted.

#### `boundSocket[Symbol.dispose]()`#

Added in: v26.4.0

Closes the handle if it has not been adopted or closed; otherwise a no-op.

### `net.connect()`#

Aliases to `net.createConnection()`.

Possible signatures:

- `net.connect(options[, connectListener])`
- `net.connect(path[, connectListener])` for IPC connections.
- `net.connect(port[, host][, connectListener])` for TCP connections.

#### `net.connect(options[, connectListener])`#

Added in: v0.7.0

- `options` `<Object>`
- `connectListener` `<Function>`
- Returns: `<net.Socket>`

Alias to `net.createConnection(options[, connectListener])`.

#### `net.connect(path[, connectListener])`#

Added in: v0.1.90

- `path` `<string>`
- `connectListener` `<Function>`
- Returns: `<net.Socket>`

Alias to `net.createConnection(path[, connectListener])`.

#### `net.connect(port[, host][, connectListener])`#

Added in: v0.1.90

- `port` `<number>`
- `host` `<string>`
- `connectListener` `<Function>`
- Returns: `<net.Socket>`

Alias to `net.createConnection(port[, host][, connectListener])`.

### `net.createConnection()`#

A factory function, which creates a new `net.Socket`, immediately initiates connection with `socket.connect()`, then returns the `net.Socket` that starts the connection.

When the connection is established, a `'connect'` event will be emitted on the returned socket. The last parameter `connectListener`, if supplied, will be added as a listener for the `'connect'` event once.

Possible signatures:

- `net.createConnection(options[, connectListener])`
- `net.createConnection(path[, connectListener])` for IPC connections.
- `net.createConnection(port[, host][, connectListener])` for TCP connections.

The `net.connect()` function is an alias to this function.

#### `net.createConnection(options[, connectListener])`#

Added in: v0.1.90

- `options` `<Object>` Required. Will be passed to both the `new net.Socket([options])` call and the `socket.connect(options[, connectListener])` method.
- `connectListener` `<Function>` Common parameter of the `net.createConnection()` functions. If supplied, will be added as a listener for the `'connect'` event on the returned socket once.
- Returns: `<net.Socket>` The newly created socket used to start the connection.

For available options, see `new net.Socket([options])` and `socket.connect(options[, connectListener])`.

Additional options:

- `handle` `<net.BoundSocket>` A pre-bound `BoundSocket` used as the connection's source binding, honoring its local address and port. Adoption consumes the bound socket (see ownership transfer).
- `timeout` `<number>` If set, will be used to call `socket.setTimeout(timeout)` after the socket is created, but before it starts the connection.

Following is an example of a client of the echo server described in the `net.createServer()` section:

    import net from 'node:net';
    const client = net.createConnection({ port: 8124 }, () => {
      // 'connect' listener.
      console.log('connected to server!');
      client.write('world!\r\n');
    });
    client.on('data', (data) => {
      console.log(data.toString());
      client.end();
    });
    client.on('end', () => {
      console.log('disconnected from server');
    });
    const net = require('node:net');
    const client = net.createConnection({ port: 8124 }, () => {
      // 'connect' listener.
      console.log('connected to server!');
      client.write('world!\r\n');
    });
    client.on('data', (data) => {
      console.log(data.toString());
      client.end();
    });
    client.on('end', () => {
      console.log('disconnected from server');
    });
    javascript

To connect on the socket `/tmp/echo.sock`:

    const client = net.createConnection({ path: '/tmp/echo.sock' });
    js

Following is an example of a client using the `port` and `onread` option. In this case, the `onread` option will be only used to call `new net.Socket([options])` and the `port` option will be used to call `socket.connect(options[, connectListener])`.

    import net from 'node:net';
    import { Buffer } from 'node:buffer';
    net.createConnection({
      port: 8124,
      onread: {
        // Reuses a 4KiB Buffer for every read from the socket.
        buffer: Buffer.alloc(4 * 1024),
        callback: function(nread, buf) {
          // Received data is available in `buf` from 0 to `nread`.
          console.log(buf.toString('utf8', 0, nread));
        },
      },
    });
    const net = require('node:net');
    net.createConnection({
      port: 8124,
      onread: {
        // Reuses a 4KiB Buffer for every read from the socket.
        buffer: Buffer.alloc(4 * 1024),
        callback: function(nread, buf) {
          // Received data is available in `buf` from 0 to `nread`.
          console.log(buf.toString('utf8', 0, nread));
        },
      },
    });
    javascript

#### `net.createConnection(path[, connectListener])`#

Added in: v0.1.90

- `path` `<string>` Path the socket should connect to. Will be passed to `socket.connect(path[, connectListener])`. See Identifying paths for IPC connections.
- `connectListener` `<Function>` Common parameter of the `net.createConnection()` functions, an "once" listener for the `'connect'` event on the initiating socket. Will be passed to `socket.connect(path[, connectListener])`.
- Returns: `<net.Socket>` The newly created socket used to start the connection.

Initiates an IPC connection.

This function creates a new `net.Socket` with all options set to default, immediately initiates connection with `socket.connect(path[, connectListener])`, then returns the `net.Socket` that starts the connection.

#### `net.createConnection(port[, host][, connectListener])`#

Added in: v0.1.90

- `port` `<number>` Port the socket should connect to. Will be passed to `socket.connect(port[, host][, connectListener])`.
- `host` `<string>` Host the socket should connect to. Will be passed to `socket.connect(port[, host][, connectListener])`. Default: `'localhost'`.
- `connectListener` `<Function>` Common parameter of the `net.createConnection()` functions, an "once" listener for the `'connect'` event on the initiating socket. Will be passed to `socket.connect(port[, host][, connectListener])`.
- Returns: `<net.Socket>` The newly created socket used to start the connection.

Initiates a TCP connection.

This function creates a new `net.Socket` with all options set to default, immediately initiates connection with `socket.connect(port[, host][, connectListener])`, then returns the `net.Socket` that starts the connection.

### `net.createServer([options][, connectionListener])`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | The `highWaterMark` option is supported now.
v17.7.0, v16.15.0 | The `noDelay`, `keepAlive`, and `keepAliveInitialDelay` options are supported now.

- `options` `<Object>`

  - `allowHalfOpen` `<boolean>` If set to `false`, then the socket will automatically end the writable side when the readable side ends. Default: `false`.
  - `highWaterMark` `<number>` Optionally overrides all `net.Socket`s' `readableHighWaterMark` and `writableHighWaterMark`. Default: See `stream.getDefaultHighWaterMark()`.
  - `keepAlive` `<boolean>` If set to `true`, it enables keep-alive functionality on the socket immediately after a new incoming connection is received, similarly on what is done in `socket.setKeepAlive()`. Default: `false`.
  - `keepAliveInitialDelay` `<number>` If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket. Default: `0`.
  - `noDelay` `<boolean>` If set to `true`, it disables the use of Nagle's algorithm immediately after a new incoming connection is received. Default: `false`.
  - `pauseOnConnect` `<boolean>` Indicates whether the socket should be paused on incoming connections. Default: `false`.
  - `blockList` `<net.BlockList>` `blockList` can be used for disabling inbound access to specific IP addresses, IP ranges, or IP subnets. This does not work if the server is behind a reverse proxy, NAT, etc. because the address checked against the block list is the address of the proxy, or the one specified by the NAT.

- `connectionListener` `<Function>` Automatically set as a listener for the `'connection'` event.

- Returns: `<net.Server>`

Creates a new TCP or IPC server.

If `allowHalfOpen` is set to `true`, when the other end of the socket signals the end of transmission, the server will only send back the end of transmission when `socket.end()` is explicitly called. For example, in the context of TCP, when a FIN packet is received, a FIN packet is sent back only when `socket.end()` is explicitly called. Until then the connection is half-closed (non-readable but still writable). See `'end'` event and RFC 1122 (section 4.2.2.13) for more information.

If `pauseOnConnect` is set to `true`, then the socket associated with each incoming connection will be paused, and no data will be read from its handle. This allows connections to be passed between processes without any data being read by the original process. To begin reading data from a paused socket, call `socket.resume()`.

The server can be a TCP server or an IPC server, depending on what it `listen()` to.

Here is an example of a TCP echo server which listens for connections on port 8124:

    import net from 'node:net';
    const server = net.createServer((c) => {
      // 'connection' listener.
      console.log('client connected');
      c.on('end', () => {
        console.log('client disconnected');
      });
      c.write('hello\r\n');
      c.pipe(c);
    });
    server.on('error', (err) => {
      throw err;
    });
    server.listen(8124, () => {
      console.log('server bound');
    });
    const net = require('node:net');
    const server = net.createServer((c) => {
      // 'connection' listener.
      console.log('client connected');
      c.on('end', () => {
        console.log('client disconnected');
      });
      c.write('hello\r\n');
      c.pipe(c);
    });
    server.on('error', (err) => {
      throw err;
    });
    server.listen(8124, () => {
      console.log('server bound');
    });
    javascript

Test this by using `telnet`:

    telnet localhost 8124
    bash

To listen on the socket `/tmp/echo.sock`:

    server.listen('/tmp/echo.sock', () => {
      console.log('server bound');
    });
    js

Use `nc` to connect to a Unix domain socket server:

    nc -U /tmp/echo.sock
    bash

### `net.getDefaultAutoSelectFamily()`#

Added in: v19.4.0

Gets the current default value of the `autoSelectFamily` option of `socket.connect(options)`. The initial default value is `true`, unless the command line option `--no-network-family-autoselection` is provided.

- Returns: `<boolean>` The current default value of the `autoSelectFamily` option.

### `net.setDefaultAutoSelectFamily(value)`#

Added in: v19.4.0

Sets the default value of the `autoSelectFamily` option of `socket.connect(options)`.

- `value` `<boolean>` The new default value. The initial default value is `true`, unless the command line option `--no-network-family-autoselection` is provided.

### `net.getDefaultAutoSelectFamilyAttemptTimeout()`#

Added in: v19.8.0, v18.18.0

Gets the current default value of the `autoSelectFamilyAttemptTimeout` option of `socket.connect(options)`. The initial default value is `500` or the value specified via the command line option `--network-family-autoselection-attempt-timeout`.

- Returns: `<number>` The current default value of the `autoSelectFamilyAttemptTimeout` option.

### `net.setDefaultAutoSelectFamilyAttemptTimeout(value)`#

Added in: v19.8.0, v18.18.0

Sets the default value of the `autoSelectFamilyAttemptTimeout` option of `socket.connect(options)`.

- `value` `<number>` The new default value, which must be a positive number. If the number is less than `10`, the value `10` is used instead. The initial default value is `250` or the value specified via the command line option `--network-family-autoselection-attempt-timeout`.

### `net.isIP(input)`#

Added in: v0.3.0

- `input` `<string>`
- Returns: `<integer>`

Returns `6` if `input` is an IPv6 address. Returns `4` if `input` is an IPv4 address in dot-decimal notation with no leading zeroes. Otherwise, returns `0`.

    net.isIP('::1'); // returns 6
    net.isIP('127.0.0.1'); // returns 4
    net.isIP('127.000.000.001'); // returns 0
    net.isIP('127.0.0.1/24'); // returns 0
    net.isIP('fhqwhgads'); // returns 0
    js

### `net.isIPv4(input)`#

Added in: v0.3.0

- `input` `<string>`
- Returns: `<boolean>`

Returns `true` if `input` is an IPv4 address in dot-decimal notation with no leading zeroes. Otherwise, returns `false`.

    net.isIPv4('127.0.0.1'); // returns true
    net.isIPv4('127.000.000.001'); // returns false
    net.isIPv4('127.0.0.1/24'); // returns false
    net.isIPv4('fhqwhgads'); // returns false
    js

### `net.isIPv6(input)`#

Added in: v0.3.0

- `input` `<string>`
- Returns: `<boolean>`

Returns `true` if `input` is an IPv6 address. Otherwise, returns `false`.

    net.isIPv6('::1'); // returns true
    net.isIPv6('fhqwhgads'); // returns false
    js

## Node-API#

Stability: 2 - Stable

Node-API (formerly N-API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (for example, V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one major version to run on later major versions of Node.js without recompilation. The ABI Stability guide provides a more in-depth explanation.

Addons are built/packaged with the same approach/tools outlined in the section titled C++ Addons. The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in Node-API are used.

APIs exposed by Node-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA-262 Language Specification. The APIs have the following properties:

- All Node-API calls return a status code of type `napi_status`. This status indicates whether the API call succeeded or failed.
- The API's return value is passed via an out parameter.
- All JavaScript values are abstracted behind an opaque type named `napi_value`.
- In case of an error status code, additional information can be obtained using `napi_get_last_error_info`. More information can be found in the error handling section Error handling.

### Writing addons in various programming languages#

Node-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. With this stability guarantee, it is possible to write addons in other programming languages on top of Node-API. Refer to language and engine bindings for more programming languages and engines support details.

`node-addon-api` is the official C++ binding that provides a more efficient way to write C++ code that calls Node-API. This wrapper is a header-only library that offers an inlinable C++ API. Binaries built with `node-addon-api` will depend on the symbols of the Node-API C-based functions exported by Node.js. The following code snippet is an example of `node-addon-api`:

    Object obj = Object::New(env);
    obj["foo"] = String::New(env, "bar");
    cpp

The above `node-addon-api` C++ code is equivalent to the following C-based Node-API code:

    napi_status status;
    napi_value object, string;
    status = napi_create_object(env, &object);
    if (status != napi_ok) {
      napi_throw_error(env, ...);
      return;
    }

    status = napi_create_string_utf8(env, "bar", NAPI_AUTO_LENGTH, &string);
    if (status != napi_ok) {
      napi_throw_error(env, ...);
      return;
    }

    status = napi_set_named_property(env, object, "foo", string);
    if (status != napi_ok) {
      napi_throw_error(env, ...);
      return;
    }
    cpp

The end result is that the addon only uses the exported C APIs. Even though the addon is written in C++, it still gets the benefits of the ABI stability provided by the C Node-API.

When using `node-addon-api` instead of the C APIs, start with the API docs for `node-addon-api`.

The Node-API Resource offers an excellent orientation and tips for developers just getting started with Node-API and `node-addon-api`. Additional media resources can be found on the Node-API Media page.

### Implications of ABI stability#

Although Node-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions:

- the Node.js C++ APIs available via any of

      #include <node.h>
      #include <node_buffer.h>
      #include <node_version.h>
      #include <node_object_wrap.h>
      cpp

- the libuv APIs which are also included with Node.js and available via

      #include <uv.h>
      cpp

- the V8 API available via

      #include <v8.h>
      cpp

Thus, for an addon to remain ABI-compatible across Node.js major versions, it must use Node-API exclusively by restricting itself to using

    #include <node_api.h>
    c

and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to Node-API.

#### Enum values in ABI stability#

All enum data types defined in Node-API should be considered as a fixed size `int32_t` value. Bit flag enum types should be explicitly documented, and they work with bit operators like bit-OR (`|`) as a bit value. Unless otherwise documented, an enum type should be considered to be extensible.

A new enum value will be added at the end of the enum definition. An enum value will not be removed or renamed.

For an enum type returned from a Node-API function, or provided as an out parameter of a Node-API function, the value is an integer value and an addon should handle unknown values. New values are allowed to be introduced without a version guard. For example, when checking `napi_status` in switch statements, an addon should include a default branch, as new status codes may be introduced in newer Node.js versions.

For an enum type used in an in-parameter, the result of passing an unknown integer value to Node-API functions is undefined unless otherwise documented. A new value is added with a version guard to indicate the Node-API version in which it was introduced. For example, `napi_get_all_property_names` can be extended with new enum value of `napi_key_filter`.

For an enum type used in both in-parameters and out-parameters, new values are allowed to be introduced without a version guard.

### Building#

Unlike modules written in JavaScript, developing and deploying Node.js native addons using Node-API requires an additional set of tools. Besides the basic tools required to develop for Node.js, the native addon developer requires a toolchain that can compile C and C++ code into a binary. In addition, depending upon how the native addon is deployed, the user of the native addon will also need to have a C/C++ toolchain installed.

For Linux developers, the necessary C/C++ toolchain packages are readily available. GCC is widely used in the Node.js community to build and test across a variety of platforms. For many developers, the LLVM compiler infrastructure is also a good choice.

For Mac developers, Xcode offers all the required compiler tools. However, it is not necessary to install the entire Xcode IDE. The following command installs the necessary toolchain:

    xcode-select --install
    bash

For Windows developers, Visual Studio offers all the required compiler tools. However, it is not necessary to install the entire Visual Studio IDE. The following command installs the necessary toolchain:

    npm install --global windows-build-tools
    bash

The sections below describe the additional tools available for developing and deploying Node.js native addons.

#### Build tools#

Both the tools listed here require that users of the native addon have a C/C++ toolchain installed in order to successfully install the native addon.

##### node-gyp#

node-gyp is a build system based on the gyp-next fork of Google's GYP tool and comes bundled with npm. GYP, and therefore node-gyp, requires that Python be installed.

Historically, node-gyp has been the tool of choice for building native addons. It has widespread adoption and documentation. However, some developers have run into limitations in node-gyp.

##### CMake.js#

CMake.js is an alternative build system based on CMake.

CMake.js is a good choice for projects that already use CMake or for developers affected by limitations in node-gyp. `build_with_cmake` is an example of a CMake-based native addon project.

#### Uploading precompiled binaries#

The three tools listed here permit native addon developers and maintainers to create and upload binaries to public or private servers. These tools are typically integrated with CI/CD build systems like Travis CI and AppVeyor to build and upload binaries for a variety of platforms and architectures. These binaries are then available for download by users who do not need to have a C/C++ toolchain installed.

##### node-pre-gyp#

node-pre-gyp is a tool based on node-gyp that adds the ability to upload binaries to a server of the developer's choice. node-pre-gyp has particularly good support for uploading binaries to Amazon S3.

##### prebuild#

prebuild is a tool that supports builds using either node-gyp or CMake.js. Unlike node-pre-gyp which supports a variety of servers, prebuild uploads binaries only to GitHub releases. prebuild is a good choice for GitHub projects using CMake.js.

##### prebuildify#

prebuildify is a tool based on node-gyp. The advantage of prebuildify is that the built binaries are bundled with the native addon when it's uploaded to npm. The binaries are downloaded from npm and are immediately available to the module user when the native addon is installed.

### Usage#

In order to use the Node-API functions, include the file `node_api.h` which is located in the src directory in the node development tree:

    #include <node_api.h>
    c

This will opt into the default `NAPI_VERSION` for the given release of Node.js. In order to ensure compatibility with specific versions of Node-API, the version can be specified explicitly when including the header:

    #define NAPI_VERSION 3
    #include <node_api.h>
    c

This restricts the Node-API surface to just the functionality that was available in the specified (and earlier) versions.

Some of the Node-API surface is experimental and requires explicit opt-in:

    #define NAPI_EXPERIMENTAL
    #include <node_api.h>
    c

In this case the entire API surface, including any experimental APIs, will be available to the module code.

Occasionally, experimental features are introduced that affect already-released and stable APIs. These features can be disabled by an opt-out:

    #define NAPI_EXPERIMENTAL
    #define NODE_API_EXPERIMENTAL_<FEATURE_NAME>_OPT_OUT
    #include <node_api.h>
    c

where `<FEATURE_NAME>` is the name of an experimental feature that affects both experimental and stable APIs.

### Node-API version matrix#

Up until version 9, Node-API versions were additive and versioned independently from Node.js. This meant that any version was an extension to the previous version in that it had all of the APIs from the previous version with some additions. Each Node.js version only supported a single Node-API version. For example v18.15.0 supports only Node-API version 8. ABI stability was achieved because 8 was a strict superset of all previous versions.

As of version 9, while Node-API versions continue to be versioned independently, an add-on that ran with Node-API version 9 may need code updates to run with Node-API version 10. ABI stability is maintained, however, because Node.js versions that support Node-API versions higher than 8 will support all versions between 8 and the highest version they support and will default to providing the version 8 APIs unless an add-on opts into a higher Node-API version. This approach provides the flexibility of better optimizing existing Node-API functions while maintaining ABI stability. Existing add-ons can continue to run without recompilation using an earlier version of Node-API. If an add-on needs functionality from a newer Node-API version, changes to existing code and recompilation will be needed to use those new functions anyway.

In versions of Node.js that support Node-API version 9 and later, defining `NAPI_VERSION=X` and using the existing add-on initialization macros will bake in the requested Node-API version that will be used at runtime into the add-on. If `NAPI_VERSION` is not set it will default to 8.

This table may not be up to date in older streams, the most up to date information is in the latest API documentation in: Node-API version matrix

Node-API version | Supported In
--- | ---
10 | v22.14.0+, 23.6.0+ and all later versions
9 | v18.17.0+, 20.3.0+, 21.0.0 and all later versions
8 | v12.22.0+, v14.17.0+, v15.12.0+, 16.0.0 and all later versions
7 | v10.23.0+, v12.19.0+, v14.12.0+, 15.0.0 and all later versions
6 | v10.20.0+, v12.17.0+, 14.0.0 and all later versions
5 | v10.17.0+, v12.11.0+, 13.0.0 and all later versions
4 | v10.16.0+, v11.8.0+, 12.0.0 and all later versions
3 | v6.14.2*, 8.11.2+, v9.11.0+*, 10.0.0 and all later versions
2 | v8.10.0+*, v9.3.0+*, 10.0.0 and all later versions
1 | v8.6.0+**, v9.0.0+*, 10.0.0 and all later versions

* Node-API was experimental.

** Node.js 8.0.0 included Node-API as experimental. It was released as Node-API version 1 but continued to evolve until Node.js 8.6.0. The API is different in versions prior to Node.js 8.6.0. We recommend Node-API version 3 or later.

Each API documented for Node-API will have a header named `added in:`, and APIs which are stable will have the additional header `Node-API version:`. APIs are directly usable when using a Node.js version which supports the Node-API version shown in `Node-API version:` or higher. When using a Node.js version that does not support the `Node-API version:` listed or if there is no `Node-API version:` listed, then the API will only be available if `#define NAPI_EXPERIMENTAL` precedes the inclusion of `node_api.h` or `js_native_api.h`. If an API appears not to be available on a version of Node.js which is later than the one shown in `added in:` then this is most likely the reason for the apparent absence.

The Node-APIs associated strictly with accessing ECMAScript features from native code can be found separately in `js_native_api.h` and `js_native_api_types.h`. The APIs defined in these headers are included in `node_api.h` and `node_api_types.h`. The headers are structured in this way in order to allow implementations of Node-API outside of Node.js. For those implementations the Node.js specific APIs may not be applicable.

The Node.js-specific parts of an addon can be separated from the code that exposes the actual functionality to the JavaScript environment so that the latter may be used with multiple implementations of Node-API. In the example below, `addon.c` and `addon.h` refer only to `js_native_api.h`. This ensures that `addon.c` can be reused to compile against either the Node.js implementation of Node-API or any implementation of Node-API outside of Node.js.

`addon_node.c` is a separate file that contains the Node.js specific entry point to the addon and which instantiates the addon by calling into `addon.c` when the addon is loaded into a Node.js environment.

    // addon.h
    #ifndef _ADDON_H_
    #define _ADDON_H_
    #include <js_native_api.h>
    napi_value create_addon(napi_env env);
    #endif  // _ADDON_H_
    c

    // addon.c
    #include "addon.h"

    #define NODE_API_CALL(env, call)                                  \
      do {                                                            \
        napi_status status = (call);                                  \
        if (status != napi_ok) {                                      \
          const napi_extended_error_info* error_info = NULL;          \
          napi_get_last_error_info((env), &error_info);               \
          const char* err_message = error_info->error_message;        \
          bool is_pending;                                            \
          napi_is_exception_pending((env), &is_pending);              \
          /* If an exception is already pending, don't rethrow it */  \
          if (!is_pending) {                                          \
            const char* message = (err_message == NULL)               \
                ? "empty error message"                               \
                : err_message;                                        \
            napi_throw_error((env), NULL, message);                   \
          }                                                           \
          return NULL;                                                \
        }                                                             \
      } while(0)

    static napi_value
    DoSomethingUseful(napi_env env, napi_callback_info info) {
      // Do something useful.
      return NULL;
    }

    napi_value create_addon(napi_env env) {
      napi_value result;
      NODE_API_CALL(env, napi_create_object(env, &result));

      napi_value exported_function;
      NODE_API_CALL(env, napi_create_function(env,
                                              "doSomethingUseful",
                                              NAPI_AUTO_LENGTH,
                                              DoSomethingUseful,
                                              NULL,
                                              &exported_function));

      NODE_API_CALL(env, napi_set_named_property(env,
                                                 result,
                                                 "doSomethingUseful",
                                                 exported_function));

      return result;
    }
    c

    // addon_node.c
    #include <node_api.h>
    #include "addon.h"

    NAPI_MODULE_INIT(/* napi_env env, napi_value exports */) {
      // This function body is expected to return a `napi_value`.
      // The variables `napi_env env` and `napi_value exports` may be used within
      // the body, as they are provided by the definition of `NAPI_MODULE_INIT()`.
      return create_addon(env);
    }
    c

### Environment life cycle APIs#

Section Agents of the ECMAScript Language Specification defines the concept of an "Agent" as a self-contained environment in which JavaScript code runs. Multiple such Agents may be started and terminated either concurrently or in sequence by the process.

A Node.js environment corresponds to an ECMAScript Agent. In the main process, an environment is created at startup, and additional environments can be created on separate threads to serve as worker threads. When Node.js is embedded in another application, the main thread of the application may also construct and destroy a Node.js environment multiple times during the life cycle of the application process such that each Node.js environment created by the application may, in turn, during its life cycle create and destroy additional environments as worker threads.

From the perspective of a native addon this means that the bindings it provides may be called multiple times, from multiple contexts, and even concurrently from multiple threads.

Native addons may need to allocate global state which they use during their life cycle of an Node.js environment such that the state can be unique to each instance of the addon.

To this end, Node-API provides a way to associate data such that its life cycle is tied to the life cycle of a Node.js environment.

#### `napi_set_instance_data`#

Added in: v12.8.0, v10.20.0N-API Version: 6

    napi_status napi_set_instance_data(node_api_basic_env env,
                                       void* data,
                                       napi_finalize finalize_cb,
                                       void* finalize_hint);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] data`: The data item to make available to bindings of this instance.
- `[in] finalize_cb`: The function to call when the environment is being torn down. The function receives `data` so that it might free it. `napi_finalize` provides more details.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.

Returns `napi_ok` if the API succeeded.

This API associates `data` with the currently running Node.js environment. `data` can later be retrieved using `napi_get_instance_data()`. Any existing data associated with the currently running Node.js environment which was set by means of a previous call to `napi_set_instance_data()` will be overwritten. If a `finalize_cb` was provided by the previous call, it will not be called.

#### `napi_get_instance_data`#

Added in: v12.8.0, v10.20.0N-API Version: 6

    napi_status napi_get_instance_data(node_api_basic_env env,
                                       void** data);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[out] data`: The data item that was previously associated with the currently running Node.js environment by a call to `napi_set_instance_data()`.

Returns `napi_ok` if the API succeeded.

This API retrieves data that was previously associated with the currently running Node.js environment via `napi_set_instance_data()`. If no data is set, the call will succeed and `data` will be set to `NULL`.

### Basic Node-API data types#

Node-API exposes the following fundamental data types as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other Node-API calls.

#### `napi_status`#

Added in: v8.0.0N-API Version: 1

Integral status code indicating the success or failure of a Node-API call. Currently, the following status codes are supported.

    typedef enum {
      napi_ok,
      napi_invalid_arg,
      napi_object_expected,
      napi_string_expected,
      napi_name_expected,
      napi_function_expected,
      napi_number_expected,
      napi_boolean_expected,
      napi_array_expected,
      napi_generic_failure,
      napi_pending_exception,
      napi_cancelled,
      napi_escape_called_twice,
      napi_handle_scope_mismatch,
      napi_callback_scope_mismatch,
      napi_queue_full,
      napi_closing,
      napi_bigint_expected,
      napi_date_expected,
      napi_arraybuffer_expected,
      napi_detachable_arraybuffer_expected,
      napi_would_deadlock,  /* unused */
      napi_no_external_buffers_allowed,
      napi_cannot_run_js
    } napi_status;
    c

If additional information is required upon an API returning a failed status, it can be obtained by calling `napi_get_last_error_info`.

#### `napi_extended_error_info`#

Added in: v8.0.0N-API Version: 1

    typedef struct {
      const char* error_message;
      void* engine_reserved;
      uint32_t engine_error_code;
      napi_status error_code;
    } napi_extended_error_info;
    c

- `error_message`: UTF8-encoded string containing a VM-neutral description of the error.
- `engine_reserved`: Reserved for VM-specific error details. This is currently not implemented for any VM.
- `engine_error_code`: VM-specific error code. This is currently not implemented for any VM.
- `error_code`: The Node-API status code that originated with the last error.

See the Error handling section for additional information.

#### `napi_env`#

`napi_env` is used to represent a context that the underlying Node-API implementation can use to persist VM-specific state. This structure is passed to native functions when they're invoked, and it must be passed back when making Node-API calls. Specifically, the same `napi_env` that was passed in when the initial native function was called must be passed to any subsequent nested Node-API calls. Caching the `napi_env` for the purpose of general reuse, and passing the `napi_env` between instances of the same addon running on different `Worker` threads is not allowed. The `napi_env` becomes invalid when an instance of a native addon is unloaded. Notification of this event is delivered through the callbacks given to `napi_add_env_cleanup_hook` and `napi_set_instance_data`.

#### `node_api_basic_env`#

Stability: 1 - Experimental

This variant of `napi_env` is passed to synchronous finalizers (`node_api_basic_finalize`). There is a subset of Node-APIs which accept a parameter of type `node_api_basic_env` as their first argument. These APIs do not access the state of the JavaScript engine and are thus safe to call from synchronous finalizers. Passing a parameter of type `napi_env` to these APIs is allowed, however, passing a parameter of type `node_api_basic_env` to APIs that access the JavaScript engine state is not allowed. Attempting to do so without a cast will produce a compiler warning or an error when add-ons are compiled with flags which cause them to emit warnings and/or errors when incorrect pointer types are passed into a function. Calling such APIs from a synchronous finalizer will ultimately result in the termination of the application.

#### `napi_value`#

This is an opaque pointer that is used to represent a JavaScript value.

#### `napi_threadsafe_function`#

Added in: v10.6.0N-API Version: 4

This is an opaque pointer that represents a JavaScript function which can be called asynchronously from multiple threads via `napi_call_threadsafe_function()`.

#### `napi_threadsafe_function_release_mode`#

Added in: v10.6.0N-API Version: 4

A value to be given to `napi_release_threadsafe_function()` to indicate whether the thread-safe function is to be closed immediately (`napi_tsfn_abort`) or merely released (`napi_tsfn_release`) and thus available for subsequent use via `napi_acquire_threadsafe_function()` and `napi_call_threadsafe_function()`.

    typedef enum {
      napi_tsfn_release,
      napi_tsfn_abort
    } napi_threadsafe_function_release_mode;
    c

#### `napi_threadsafe_function_call_mode`#

Added in: v10.6.0N-API Version: 4

A value to be given to `napi_call_threadsafe_function()` to indicate whether the call should block whenever the queue associated with the thread-safe function is full.

    typedef enum {
      napi_tsfn_nonblocking,
      napi_tsfn_blocking
    } napi_threadsafe_function_call_mode;
    c

#### Node-API memory management types#

##### `napi_handle_scope`#

This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, Node-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, Node-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.

Handle scopes are created using `napi_open_handle_scope` and are destroyed using `napi_close_handle_scope`. Closing the scope can indicate to the GC that all `napi_value`s created during the lifetime of the handle scope are no longer referenced from the current stack frame.

For more details, review the Object lifetime management.

##### `napi_escapable_handle_scope`#

Added in: v8.0.0N-API Version: 1

Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.

##### `napi_ref`#

Added in: v8.0.0N-API Version: 1

This is the abstraction to use to reference a `napi_value`. This allows for users to manage the lifetimes of JavaScript values, including defining their minimum lifetimes explicitly.

For more details, review the Object lifetime management.

##### `napi_type_tag`#

Added in: v14.8.0, v12.19.0N-API Version: 8

A 128-bit value stored as two unsigned 64-bit integers. It serves as a UUID with which JavaScript objects or externals can be "tagged" in order to ensure that they are of a certain type. This is a stronger check than `napi_instanceof`, because the latter can report a false positive if the object's prototype has been manipulated. Type-tagging is most useful in conjunction with `napi_wrap` because it ensures that the pointer retrieved from a wrapped object can be safely cast to the native type corresponding to the type tag that had been previously applied to the JavaScript object.

    typedef struct {
      uint64_t lower;
      uint64_t upper;
    } napi_type_tag;
    c

##### `napi_async_cleanup_hook_handle`#

Added in: v14.10.0, v12.19.0

An opaque value returned by `napi_add_async_cleanup_hook`. It must be passed to `napi_remove_async_cleanup_hook` when the chain of asynchronous cleanup events completes.

#### Node-API callback types#

##### `napi_callback_info`#

Added in: v8.0.0N-API Version: 1

Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.

##### `napi_callback`#

Added in: v8.0.0N-API Version: 1

Function pointer type for user-provided native functions which are to be exposed to JavaScript via Node-API. Callback functions should satisfy the following signature:

    typedef napi_value (*napi_callback)(napi_env, napi_callback_info);
    c

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside a `napi_callback` is not necessary.

##### `node_api_basic_finalize`#

Added in: v21.6.0, v20.12.0, v18.20.0

Stability: 1 - Experimental

Function pointer type for add-on provided functions that allow the user to be notified when externally-owned data is ready to be cleaned up because the object it was associated with has been garbage-collected. The user must provide a function satisfying the following signature which would get called upon the object's collection. Currently, `node_api_basic_finalize` can be used for finding out when objects that have external data are collected.

    typedef void (*node_api_basic_finalize)(node_api_basic_env env,
                                          void* finalize_data,
                                          void* finalize_hint);
    c

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

Since these functions may be called while the JavaScript engine is in a state where it cannot execute JavaScript code, only Node-APIs which accept a `node_api_basic_env` as their first parameter may be called. `node_api_post_finalizer` can be used to schedule Node-API calls that require access to the JavaScript engine's state to run after the current garbage collection cycle has completed.

In the case of `node_api_create_external_string_latin1` and `node_api_create_external_string_utf16` the `env` parameter may be null, because external strings can be collected during the latter part of environment shutdown.

Change History:

- experimental (`NAPI_EXPERIMENTAL`):

  Only Node-API calls that accept a `node_api_basic_env` as their first parameter may be called, otherwise the application will be terminated with an appropriate error message. This feature can be turned off by defining `NODE_API_EXPERIMENTAL_BASIC_ENV_OPT_OUT`.

##### `napi_finalize`#

Added in: v8.0.0N-API Version: 1

Function pointer type for add-on provided function that allow the user to schedule a group of calls to Node-APIs in response to a garbage collection event, after the garbage collection cycle has completed. These function pointers can be used with `node_api_post_finalizer`.

    typedef void (*napi_finalize)(napi_env env,
                                  void* finalize_data,
                                  void* finalize_hint);
    c

Change History:

- experimental (`NAPI_EXPERIMENTAL` is defined):

  A function of this type may no longer be used as a finalizer, except with `node_api_post_finalizer`. `node_api_basic_finalize` must be used instead. This feature can be turned off by defining `NODE_API_EXPERIMENTAL_BASIC_ENV_OPT_OUT`.

##### `napi_async_execute_callback`#

Added in: v8.0.0N-API Version: 1

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

    typedef void (*napi_async_execute_callback)(napi_env env, void* data);
    c

Implementations of this function must avoid making Node-API calls that execute JavaScript or interact with JavaScript objects. Node-API calls should be in the `napi_async_complete_callback` instead. Do not use the `napi_env` parameter as it will likely result in execution of JavaScript.

##### `napi_async_complete_callback`#

Added in: v8.0.0N-API Version: 1

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

    typedef void (*napi_async_complete_callback)(napi_env env,
                                                 napi_status status,
                                                 void* data);
    c

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

##### `napi_threadsafe_function_call_js`#

Added in: v10.6.0N-API Version: 4

Function pointer used with asynchronous thread-safe function calls. The callback will be called on the main thread. Its purpose is to use a data item arriving via the queue from one of the secondary threads to construct the parameters necessary for a call into JavaScript, usually via `napi_call_function`, and then make the call into JavaScript.

The data arriving from the secondary thread via the queue is given in the `data` parameter and the JavaScript function to call is given in the `js_callback` parameter.

Node-API sets up the environment prior to calling this callback, so it is sufficient to call the JavaScript function via `napi_call_function` rather than via `napi_make_callback`.

Callback functions must satisfy the following signature:

    typedef void (*napi_threadsafe_function_call_js)(napi_env env,
                                                     napi_value js_callback,
                                                     void* context,
                                                     void* data);
    c

- `[in] env`: The environment to use for API calls, or `NULL` if the thread-safe function is being torn down and `data` may need to be freed.
- `[in] js_callback`: The JavaScript function to call, or `NULL` if the thread-safe function is being torn down and `data` may need to be freed. It may also be `NULL` if the thread-safe function was created without `js_callback`.
- `[in] context`: The optional data with which the thread-safe function was created.
- `[in] data`: Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with Node-API functions) that can be passed as parameters when `js_callback` is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data.

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

##### `napi_cleanup_hook`#

Added in: v19.2.0, v18.13.0N-API Version: 3

Function pointer used with `napi_add_env_cleanup_hook`. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

    typedef void (*napi_cleanup_hook)(void* data);
    c

- `[in] data`: The data that was passed to `napi_add_env_cleanup_hook`.

##### `napi_async_cleanup_hook`#

Added in: v14.10.0, v12.19.0

Function pointer used with `napi_add_async_cleanup_hook`. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

    typedef void (*napi_async_cleanup_hook)(napi_async_cleanup_hook_handle handle,
                                            void* data);
    c

- `[in] handle`: The handle that must be passed to `napi_remove_async_cleanup_hook` after completion of the asynchronous cleanup.
- `[in] data`: The data that was passed to `napi_add_async_cleanup_hook`.

The body of the function should initiate the asynchronous cleanup actions at the end of which `handle` must be passed in a call to `napi_remove_async_cleanup_hook`.

### Error handling#

Node-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case.

#### Return values#

Added in: v8.0.0N-API Version: 1

All of the Node-API functions share the same error handling pattern. The return type of all API functions is `napi_status`.

The return value will be `napi_ok` if the request was successful and no uncaught JavaScript exception was thrown. If an error occurred AND an exception was thrown, the `napi_status` value for the error will be returned. If an exception was thrown, and no error occurred, `napi_pending_exception` will be returned.

In cases where a return value other than `napi_ok` or `napi_pending_exception` is returned, `napi_is_exception_pending` must be called to check if an exception is pending. See the section on exceptions for more details.

The full set of possible `napi_status` values is defined in `napi_api_types.h`.

The `napi_status` return value provides a VM-independent representation of the error which occurred. In some cases it is useful to be able to get more detailed information, including a string representing the error as well as VM (engine)-specific information.

In order to retrieve this information `napi_get_last_error_info` is provided which returns a `napi_extended_error_info` structure. The format of the `napi_extended_error_info` structure is as follows:

    typedef struct napi_extended_error_info {
      const char* error_message;
      void* engine_reserved;
      uint32_t engine_error_code;
      napi_status error_code;
    };
    c

- `error_message`: Textual representation of the error that occurred.
- `engine_reserved`: Opaque handle reserved for engine use only.
- `engine_error_code`: VM specific error code.
- `error_code`: Node-API status code for the last error.

`napi_get_last_error_info` returns the information for the last Node-API call that was made.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

##### `napi_get_last_error_info`#

Added in: v8.0.0N-API Version: 1

    napi_status
    napi_get_last_error_info(node_api_basic_env env,
                             const napi_extended_error_info** result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: The `napi_extended_error_info` structure with more information about the error.

Returns `napi_ok` if the API succeeded.

This API retrieves a `napi_extended_error_info` structure with information about the last error that occurred.

The content of the `napi_extended_error_info` returned is only valid up until a Node-API function is called on the same `env`. This includes a call to `napi_is_exception_pending` so it may often be necessary to make a copy of the information so that it can be used later. The pointer returned in `error_message` points to a statically-defined string so it is safe to use that pointer if you have copied it out of the `error_message` field (which will be overwritten) before another Node-API function was called.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

This API can be called even if there is a pending JavaScript exception.

#### Exceptions#

Any Node-API function call may result in a pending JavaScript exception. This is the case for any of the API functions, even those that may not cause the execution of JavaScript.

If the `napi_status` returned by a function is `napi_ok` then no exception is pending and no additional action is required. If the `napi_status` returned is anything other than `napi_ok` or `napi_pending_exception`, in order to try to recover and continue instead of simply returning immediately, `napi_is_exception_pending` must be called in order to determine if an exception is pending or not.

In many cases when a Node-API function is called and an exception is already pending, the function will return immediately with a `napi_status` of `napi_pending_exception`. However, this is not the case for all functions. Node-API allows a subset of the functions to be called to allow for some minimal cleanup before returning to JavaScript. In that case, `napi_status` will reflect the status for the function. It will not reflect previous pending exceptions. To avoid confusion, check the error status after every function call.

When an exception is pending one of two approaches can be employed.

The first approach is to do any appropriate cleanup and then return so that execution will return to JavaScript. As part of the transition back to JavaScript, the exception will be thrown at the point in the JavaScript code where the native method was invoked. The behavior of most Node-API calls is unspecified while an exception is pending, and many will simply return `napi_pending_exception`, so do as little as possible and then return to JavaScript where the exception can be handled.

The second approach is to try to handle the exception. There will be cases where the native code can catch the exception, take the appropriate action, and then continue. This is only recommended in specific cases where it is known that the exception can be safely handled. In these cases `napi_get_and_clear_last_exception` can be used to get and clear the exception. On success, result will contain the handle to the last JavaScript `Object` thrown. If it is determined, after retrieving the exception, the exception cannot be handled after all it can be re-thrown it with `napi_throw` where error is the JavaScript value to be thrown.

The following utility functions are also available in case native code needs to throw an exception or determine if a `napi_value` is an instance of a JavaScript `Error` object: `napi_throw_error`, `napi_throw_type_error`, `napi_throw_range_error`, `node_api_throw_syntax_error` and `napi_is_error`.

The following utility functions are also available in case native code needs to create an `Error` object: `napi_create_error`, `napi_create_type_error`, `napi_create_range_error` and `node_api_create_syntax_error`, where result is the `napi_value` that refers to the newly created JavaScript `Error` object.

The Node.js project is adding error codes to all of the errors generated internally. The goal is for applications to use these error codes for all error checking. The associated error messages will remain, but will only be meant to be used for logging and display with the expectation that the message can change without SemVer applying. In order to support this model with Node-API, both in internal functionality and for module specific functionality (as its good practice), the `throw_` and `create_` functions take an optional code parameter which is the string for the code to be added to the error object. If the optional parameter is `NULL` then no code will be associated with the error. If a code is provided, the name associated with the error is also updated to be:

    originalName [code]
    text

where `originalName` is the original name associated with the error and `code` is the code that was provided. For example, if the code is `'ERR_ERROR_1'` and a `TypeError` is being created the name will be:

    TypeError [ERR_ERROR_1]
    text

##### `napi_throw`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] error`: The JavaScript value to be thrown.

Returns `napi_ok` if the API succeeded.

This API throws the JavaScript value provided.

##### `napi_throw_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_throw_error(napi_env env,
                                             const char* code,
                                             const char* msg);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional error code to be set on the error.
- `[in] msg`: C string representing the text to be associated with the error.

Returns `napi_ok` if the API succeeded.

This API throws a JavaScript `Error` with the text provided.

##### `napi_throw_type_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_throw_type_error(napi_env env,
                                                  const char* code,
                                                  const char* msg);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional error code to be set on the error.
- `[in] msg`: C string representing the text to be associated with the error.

Returns `napi_ok` if the API succeeded.

This API throws a JavaScript `TypeError` with the text provided.

##### `napi_throw_range_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_throw_range_error(napi_env env,
                                                   const char* code,
                                                   const char* msg);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional error code to be set on the error.
- `[in] msg`: C string representing the text to be associated with the error.

Returns `napi_ok` if the API succeeded.

This API throws a JavaScript `RangeError` with the text provided.

##### `node_api_throw_syntax_error`#

Added in: v17.2.0, v16.14.0N-API Version: 9

    NAPI_EXTERN napi_status node_api_throw_syntax_error(napi_env env,
                                                        const char* code,
                                                        const char* msg);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional error code to be set on the error.
- `[in] msg`: C string representing the text to be associated with the error.

Returns `napi_ok` if the API succeeded.

This API throws a JavaScript `SyntaxError` with the text provided.

##### `napi_is_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_is_error(napi_env env,
                                          napi_value value,
                                          bool* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The `napi_value` to be checked.
- `[out] result`: Boolean value that is set to true if `napi_value` represents an error, false otherwise.

Returns `napi_ok` if the API succeeded.

This API queries a `napi_value` to check if it represents an error object.

##### `napi_create_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_create_error(napi_env env,
                                              napi_value code,
                                              napi_value msg,
                                              napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional `napi_value` with the string for the error code to be associated with the error.
- `[in] msg`: `napi_value` that references a JavaScript `string` to be used as the message for the `Error`.
- `[out] result`: `napi_value` representing the error created.

Returns `napi_ok` if the API succeeded.

This API returns a JavaScript `Error` with the text provided.

##### `napi_create_type_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_create_type_error(napi_env env,
                                                   napi_value code,
                                                   napi_value msg,
                                                   napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional `napi_value` with the string for the error code to be associated with the error.
- `[in] msg`: `napi_value` that references a JavaScript `string` to be used as the message for the `Error`.
- `[out] result`: `napi_value` representing the error created.

Returns `napi_ok` if the API succeeded.

This API returns a JavaScript `TypeError` with the text provided.

##### `napi_create_range_error`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_create_range_error(napi_env env,
                                                    napi_value code,
                                                    napi_value msg,
                                                    napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional `napi_value` with the string for the error code to be associated with the error.
- `[in] msg`: `napi_value` that references a JavaScript `string` to be used as the message for the `Error`.
- `[out] result`: `napi_value` representing the error created.

Returns `napi_ok` if the API succeeded.

This API returns a JavaScript `RangeError` with the text provided.

##### `node_api_create_syntax_error`#

Added in: v17.2.0, v16.14.0N-API Version: 9

    NAPI_EXTERN napi_status node_api_create_syntax_error(napi_env env,
                                                         napi_value code,
                                                         napi_value msg,
                                                         napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] code`: Optional `napi_value` with the string for the error code to be associated with the error.
- `[in] msg`: `napi_value` that references a JavaScript `string` to be used as the message for the `Error`.
- `[out] result`: `napi_value` representing the error created.

Returns `napi_ok` if the API succeeded.

This API returns a JavaScript `SyntaxError` with the text provided.

##### `napi_get_and_clear_last_exception`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_and_clear_last_exception(napi_env env,
                                                  napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: The exception if one is pending, `NULL` otherwise.

Returns `napi_ok` if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

##### `napi_is_exception_pending`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_is_exception_pending(napi_env env, bool* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: Boolean value that is set to true if an exception is pending.

Returns `napi_ok` if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

##### `napi_fatal_exception`#

Added in: v9.10.0N-API Version: 3

    napi_status napi_fatal_exception(napi_env env, napi_value err);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] err`: The error that is passed to `'uncaughtException'`.

Trigger an `'uncaughtException'` in JavaScript. Useful if an async callback throws an exception with no way to recover.

#### Fatal errors#

In the event of an unrecoverable error in a native addon, a fatal error can be thrown to immediately terminate the process.

##### `napi_fatal_error`#

Added in: v8.2.0N-API Version: 1

    NAPI_NO_RETURN void napi_fatal_error(const char* location,
                                         size_t location_len,
                                         const char* message,
                                         size_t message_len);
    c

- `[in] location`: Optional location at which the error occurred.
- `[in] location_len`: The length of the location in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[in] message`: The message associated with the error.
- `[in] message_len`: The length of the message in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.

The function call does not return, the process will be terminated.

This API can be called even if there is a pending JavaScript exception.

### Object lifetime management#

As Node-API calls are made, handles to objects in the heap for the underlying VM may be returned as `napi_values`. These handles must hold the objects 'live' until they are no longer required by the native code, otherwise the objects could be collected before the native code was finished using them.

As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.

In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the Node-API functions that can be used to change the handle lifespan from the default.

#### Making handle lifespan shorter than that of the native method#

It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:

    for (int i = 0; i < 1000000; i++) {
      napi_value result;
      napi_status status = napi_get_element(env, object, i, &result);
      if (status != napi_ok) {
        break;
      }
      // do something with element
    }
    c

This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.

To handle this case, Node-API provides the ability to establish a new 'scope' to which newly created handles will be associated. Once those handles are no longer required, the scope can be 'closed' and any handles associated with the scope are invalidated. The methods available to open/close scopes are `napi_open_handle_scope` and `napi_close_handle_scope`.

Node-API only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.

Taking the earlier example, adding calls to `napi_open_handle_scope` and `napi_close_handle_scope` would ensure that at most a single handle is valid throughout the execution of the loop:

    for (int i = 0; i < 1000000; i++) {
      napi_handle_scope scope;
      napi_status status = napi_open_handle_scope(env, &scope);
      if (status != napi_ok) {
        break;
      }
      napi_value result;
      status = napi_get_element(env, object, i, &result);
      if (status != napi_ok) {
        break;
      }
      // do something with element
      status = napi_close_handle_scope(env, scope);
      if (status != napi_ok) {
        break;
      }
    }
    c

When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. Node-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope.

The methods available to open/close escapable scopes are `napi_open_escapable_handle_scope` and `napi_close_escapable_handle_scope`.

The request to promote a handle is made through `napi_escape_handle` which can only be called once.

##### `napi_open_handle_scope`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_open_handle_scope(napi_env env,
                                                   napi_handle_scope* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: `napi_value` representing the new scope.

Returns `napi_ok` if the API succeeded.

This API opens a new scope.

##### `napi_close_handle_scope`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_close_handle_scope(napi_env env,
                                                    napi_handle_scope scope);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] scope`: `napi_value` representing the scope to be closed.

Returns `napi_ok` if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

##### `napi_open_escapable_handle_scope`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status
        napi_open_escapable_handle_scope(napi_env env,
                                         napi_handle_scope* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: `napi_value` representing the new scope.

Returns `napi_ok` if the API succeeded.

This API opens a new scope from which one object can be promoted to the outer scope.

##### `napi_close_escapable_handle_scope`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status
        napi_close_escapable_handle_scope(napi_env env,
                                          napi_handle_scope scope);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] scope`: `napi_value` representing the scope to be closed.

Returns `napi_ok` if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

##### `napi_escape_handle`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_escape_handle(napi_env env,
                                   napi_escapable_handle_scope scope,
                                   napi_value escapee,
                                   napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] scope`: `napi_value` representing the current scope.
- `[in] escapee`: `napi_value` representing the JavaScript `Object` to be escaped.
- `[out] result`: `napi_value` representing the handle to the escaped `Object` in the outer scope.

Returns `napi_ok` if the API succeeded.

This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned.

This API can be called even if there is a pending JavaScript exception.

#### References to values with a lifespan longer than that of the native method#

In some cases, an addon will need to be able to create and reference values with a lifespan longer than that of a single native method invocation. For example, to create a constructor and later use that constructor in a request to create instances, it must be possible to reference the constructor object across many different instance creation requests. This would not be possible with a normal handle returned as a `napi_value` as described in the earlier section. The lifespan of a normal handle is managed by scopes and all scopes must be closed before the end of a native method.

Node-API provides methods for creating persistent references to values. Currently Node-API only allows references to be created for a limited set of value types, including object, external, function, and symbol.

Each reference has an associated count with a value of 0 or higher, which determines whether the reference will keep the corresponding value alive. References with a count of 0 do not prevent values from being collected. Values of object (object, function, external) and symbol types are becoming 'weak' references and can still be accessed while they are not collected. Any count greater than 0 will prevent the values from being collected.

Symbol values have different flavors. The true weak reference behavior is only supported by local symbols created with the `napi_create_symbol` function or the JavaScript `Symbol()` constructor calls. Globally registered symbols created with the `node_api_symbol_for` function or JavaScript `Symbol.for()` function calls remain always strong references because the garbage collector does not collect them. The same is true for well-known symbols such as `Symbol.iterator`. They are also never collected by the garbage collector.

References can be created with an initial reference count. The count can then be modified through `napi_reference_ref` and `napi_reference_unref`. If an object is collected while the count for a reference is 0, all subsequent calls to get the object associated with the reference `napi_get_reference_value` will return `NULL` for the returned `napi_value`. An attempt to call `napi_reference_ref` for a reference whose object has been collected results in an error.

References must be deleted once they are no longer required by the addon. When a reference is deleted, it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference results in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.

There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count. Multiple persistent references to the same object can result in unexpectedly keeping alive native memory. The native structures for a persistent reference must be kept alive until finalizers for the referenced object are executed. If a new persistent reference is created for the same object, the finalizers for that object will not be run and the native memory pointed by the earlier persistent reference will not be freed. This can be avoided by calling `napi_delete_reference` in addition to `napi_reference_unref` when possible.

Change History:

- Version 10 (`NAPI_VERSION` is defined as `10` or higher):

  References can be created for all value types. The new supported value types do not support weak reference semantic and the values of these types are released when the reference count becomes 0 and cannot be accessed from the reference anymore.

##### `napi_create_reference`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_create_reference(napi_env env,
                                                  napi_value value,
                                                  uint32_t initial_refcount,
                                                  napi_ref* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The `napi_value` for which a reference is being created.
- `[in] initial_refcount`: Initial reference count for the new reference.
- `[out] result`: `napi_ref` pointing to the new reference.

Returns `napi_ok` if the API succeeded.

This API creates a new reference with the specified reference count to the value passed in.

##### `napi_delete_reference`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] ref`: `napi_ref` to be deleted.

Returns `napi_ok` if the API succeeded.

This API deletes the reference passed in.

This API can be called even if there is a pending JavaScript exception.

##### `napi_reference_ref`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_reference_ref(napi_env env,
                                               napi_ref ref,
                                               uint32_t* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] ref`: `napi_ref` for which the reference count will be incremented.
- `[out] result`: The new reference count.

Returns `napi_ok` if the API succeeded.

This API increments the reference count for the reference passed in and returns the resulting reference count.

##### `napi_reference_unref`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_reference_unref(napi_env env,
                                                 napi_ref ref,
                                                 uint32_t* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] ref`: `napi_ref` for which the reference count will be decremented.
- `[out] result`: The new reference count.

Returns `napi_ok` if the API succeeded.

This API decrements the reference count for the reference passed in and returns the resulting reference count.

##### `napi_get_reference_value`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_get_reference_value(napi_env env,
                                                     napi_ref ref,
                                                     napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] ref`: The `napi_ref` for which the corresponding value is being requested.
- `[out] result`: The `napi_value` referenced by the `napi_ref`.

Returns `napi_ok` if the API succeeded.

If still valid, this API returns the `napi_value` representing the JavaScript value associated with the `napi_ref`. Otherwise, result will be `NULL`.

#### Cleanup on exit of the current Node.js environment#

While a Node.js process typically releases all its resources when exiting, embedders of Node.js, or future Worker support, may require addons to register clean-up hooks that will be run once the current Node.js environment exits.

Node-API provides functions for registering and un-registering such callbacks. When those callbacks are run, all resources that are being held by the addon should be freed up.

##### `napi_add_env_cleanup_hook`#

Added in: v10.2.0N-API Version: 3

    NODE_EXTERN napi_status napi_add_env_cleanup_hook(node_api_basic_env env,
                                                      napi_cleanup_hook fun,
                                                      void* arg);
    c

Registers `fun` as a function to be run with the `arg` parameter once the current Node.js environment exits.

A function can safely be specified multiple times with different `arg` values. In that case, it will be called multiple times as well. Providing the same `fun` and `arg` values multiple times is not allowed and will lead the process to abort.

The hooks will be called in reverse order, i.e. the most recently added one will be called first.

Removing this hook can be done by using `napi_remove_env_cleanup_hook`. Typically, that happens when the resource for which this hook was added is being torn down anyway.

For asynchronous cleanup, `napi_add_async_cleanup_hook` is available.

##### `napi_remove_env_cleanup_hook`#

Added in: v10.2.0N-API Version: 3

    NAPI_EXTERN napi_status napi_remove_env_cleanup_hook(node_api_basic_env env,
                                                         void (*fun)(void* arg),
                                                         void* arg);
    c

Unregisters `fun` as a function to be run with the `arg` parameter once the current Node.js environment exits. Both the argument and the function value need to be exact matches.

The function must have originally been registered with `napi_add_env_cleanup_hook`, otherwise the process will abort.

##### `napi_add_async_cleanup_hook`#

Added in: v14.8.0, v12.19.0N-API Version: 8
History

Version | Changes
--- | ---
v14.10.0, v12.19.0 | Changed signature of the `hook` callback.

    NAPI_EXTERN napi_status napi_add_async_cleanup_hook(
        node_api_basic_env env,
        napi_async_cleanup_hook hook,
        void* arg,
        napi_async_cleanup_hook_handle* remove_handle);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] hook`: The function pointer to call at environment teardown.
- `[in] arg`: The pointer to pass to `hook` when it gets called.
- `[out] remove_handle`: Optional handle that refers to the asynchronous cleanup hook.

Registers `hook`, which is a function of type `napi_async_cleanup_hook`, as a function to be run with the `remove_handle` and `arg` parameters once the current Node.js environment exits.

Unlike `napi_add_env_cleanup_hook`, the hook is allowed to be asynchronous.

Otherwise, behavior generally matches that of `napi_add_env_cleanup_hook`.

If `remove_handle` is not `NULL`, an opaque value will be stored in it that must later be passed to `napi_remove_async_cleanup_hook`, regardless of whether the hook has already been invoked. Typically, that happens when the resource for which this hook was added is being torn down anyway.

##### `napi_remove_async_cleanup_hook`#

Added in: v14.8.0, v12.19.0
History

Version | Changes
--- | ---
v14.10.0, v12.19.0 | Removed `env` parameter.

    NAPI_EXTERN napi_status napi_remove_async_cleanup_hook(
        napi_async_cleanup_hook_handle remove_handle);
    c

- `[in] remove_handle`: The handle to an asynchronous cleanup hook that was created with `napi_add_async_cleanup_hook`.

Unregisters the cleanup hook corresponding to `remove_handle`. This will prevent the hook from being executed, unless it has already started executing. This must be called on any `napi_async_cleanup_hook_handle` value obtained from `napi_add_async_cleanup_hook`.

#### Finalization on the exit of the Node.js environment#

The Node.js environment may be torn down at an arbitrary time as soon as possible with JavaScript execution disallowed, like on the request of `worker.terminate()`. When the environment is being torn down, the registered `napi_finalize` callbacks of JavaScript objects, thread-safe functions and environment instance data are invoked immediately and independently.

The invocation of `napi_finalize` callbacks is scheduled after the manually registered cleanup hooks. In order to ensure a proper order of addon finalization during environment shutdown to avoid use-after-free in the `napi_finalize` callback, addons should register a cleanup hook with `napi_add_env_cleanup_hook` and `napi_add_async_cleanup_hook` to manually release the allocated resource in a proper order.

### Module registration#

Node-API modules are registered in a manner similar to other modules except that instead of using the `NODE_MODULE` macro the following is used:

    NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)
    c

The next difference is the signature for the `Init` method. For a Node-API module it is as follows:

    napi_value Init(napi_env env, napi_value exports);
    c

The return value from `Init` is treated as the `exports` object for the module. The `Init` method is passed an empty object via the `exports` parameter as a convenience. If `Init` returns `NULL`, the parameter passed as `exports` is exported by the module. Node-API modules cannot modify the `module` object but can specify anything as the `exports` property of the module.

To add the method `hello` as a function so that it can be called as a method provided by the addon:

    napi_value Init(napi_env env, napi_value exports) {
      napi_status status;
      napi_property_descriptor desc = {
        "hello",
        NULL,
        Method,
        NULL,
        NULL,
        NULL,
        napi_writable | napi_enumerable | napi_configurable,
        NULL
      };
      status = napi_define_properties(env, exports, 1, &desc);
      if (status != napi_ok) return NULL;
      return exports;
    }
    c

To set a function to be returned by the `require()` for the addon:

    napi_value Init(napi_env env, napi_value exports) {
      napi_value method;
      napi_status status;
      status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method);
      if (status != napi_ok) return NULL;
      return method;
    }
    c

To define a class so that new instances can be created (often used with Object wrap):

    // NOTE: partial example, not all referenced code is included
    napi_value Init(napi_env env, napi_value exports) {
      napi_status status;
      napi_property_descriptor properties[] = {
        { "value", NULL, NULL, GetValue, SetValue, NULL, napi_writable | napi_configurable, NULL },
        DECLARE_NAPI_METHOD("plusOne", PlusOne),
        DECLARE_NAPI_METHOD("multiply", Multiply),
      };

      napi_value cons;
      status =
          napi_define_class(env, "MyObject", New, NULL, 3, properties, &cons);
      if (status != napi_ok) return NULL;

      status = napi_create_reference(env, cons, 1, &constructor);
      if (status != napi_ok) return NULL;

      status = napi_set_named_property(env, exports, "MyObject", cons);
      if (status != napi_ok) return NULL;

      return exports;
    }
    c

You can also use the `NAPI_MODULE_INIT` macro, which acts as a shorthand for `NAPI_MODULE` and defining an `Init` function:

    NAPI_MODULE_INIT(/* napi_env env, napi_value exports */) {
      napi_value answer;
      napi_status result;

      status = napi_create_int64(env, 42, &answer);
      if (status != napi_ok) return NULL;

      status = napi_set_named_property(env, exports, "answer", answer);
      if (status != napi_ok) return NULL;

      return exports;
    }
    c

The parameters `env` and `exports` are provided to the body of the `NAPI_MODULE_INIT` macro.

All Node-API addons are context-aware, meaning they may be loaded multiple times. There are a few design considerations when declaring such a module. The documentation on context-aware addons provides more details.

The variables `env` and `exports` will be available inside the function body following the macro invocation.

For more details on setting properties on objects, see the section on Working with JavaScript properties.

For more details on building addon modules in general, refer to the existing API.

### Working with JavaScript values#

Node-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section language types of the ECMAScript Language Specification.

Fundamentally, these APIs are used to do one of the following:

1. Create a new JavaScript object
2. Convert from a primitive C type to a Node-API value
3. Convert from Node-API value to a primitive C type
4. Get global instances including `undefined` and `null`

Node-API values are represented by the type `napi_value`. Any Node-API call that requires a JavaScript value takes in a `napi_value`. In some cases, the API does check the type of the `napi_value` up-front. However, for better performance, it's better for the caller to make sure that the `napi_value` in question is of the JavaScript type expected by the API.

#### Enum types#

##### `napi_key_collection_mode`#

Added in: v13.7.0, v12.17.0, v10.20.0N-API Version: 6

    typedef enum {
      napi_key_include_prototypes,
      napi_key_own_only
    } napi_key_collection_mode;
    c

Describes the `Keys/Properties` filter enums:

`napi_key_collection_mode` limits the range of collected properties.

`napi_key_own_only` limits the collected properties to the given object only. `napi_key_include_prototypes` will include all keys of the objects's prototype chain as well.

##### `napi_key_filter`#

Added in: v13.7.0, v12.17.0, v10.20.0N-API Version: 6

    typedef enum {
      napi_key_all_properties = 0,
      napi_key_writable = 1,
      napi_key_enumerable = 1 << 1,
      napi_key_configurable = 1 << 2,
      napi_key_skip_strings = 1 << 3,
      napi_key_skip_symbols = 1 << 4
    } napi_key_filter;
    c

Property filter bit flag. This works with bit operators to build a composite filter.

##### `napi_key_conversion`#

Added in: v13.7.0, v12.17.0, v10.20.0N-API Version: 6

    typedef enum {
      napi_key_keep_numbers,
      napi_key_numbers_to_strings
    } napi_key_conversion;
    c

`napi_key_numbers_to_strings` will convert integer indexes to strings. `napi_key_keep_numbers` will return numbers for integer indexes.

##### `napi_valuetype`#

    typedef enum {
      // ES6 types (corresponds to typeof)
      napi_undefined,
      napi_null,
      napi_boolean,
      napi_number,
      napi_string,
      napi_symbol,
      napi_object,
      napi_function,
      napi_external,
      napi_bigint,
    } napi_valuetype;
    c

Describes the type of a `napi_value`. This generally corresponds to the types described in Section language types of the ECMAScript Language Specification. In addition to types in that section, `napi_valuetype` can also represent `Function`s and `Object`s with external data.

A JavaScript value of type `napi_external` appears in JavaScript as a plain object such that no properties can be set on it, and no prototype.

##### `napi_typedarray_type`#

History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | Added `napi_float16_array` for Float16Array support.

    typedef enum {
      napi_int8_array,
      napi_uint8_array,
      napi_uint8_clamped_array,
      napi_int16_array,
      napi_uint16_array,
      napi_int32_array,
      napi_uint32_array,
      napi_float32_array,
      napi_float64_array,
      napi_bigint64_array,
      napi_biguint64_array,
      napi_float16_array,
    } napi_typedarray_type;
    c

This represents the underlying binary scalar datatype of the `TypedArray`. Elements of this enum correspond to Section TypedArray objects of the ECMAScript Language Specification.

#### Object creation functions#

##### `napi_create_array`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_array(napi_env env, napi_value* result)
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[out] result`: A `napi_value` representing a JavaScript `Array`.

Returns `napi_ok` if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript `Array` type. JavaScript arrays are described in Section Array objects of the ECMAScript Language Specification.

##### `napi_create_array_with_length`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_array_with_length(napi_env env,
                                              size_t length,
                                              napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] length`: The initial length of the `Array`.
- `[out] result`: A `napi_value` representing a JavaScript `Array`.

Returns `napi_ok` if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript `Array` type. The `Array`'s length property is set to the passed-in length parameter. However, the underlying buffer is not guaranteed to be pre-allocated by the VM when the array is created. That behavior is left to the underlying VM implementation. If the buffer must be a contiguous block of memory that can be directly read and/or written via C, consider using `napi_create_external_arraybuffer`.

JavaScript arrays are described in Section Array objects of the ECMAScript Language Specification.

##### `napi_create_arraybuffer`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_arraybuffer(napi_env env,
                                        size_t byte_length,
                                        void** data,
                                        napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] length`: The length in bytes of the array buffer to create.
- `[out] data`: Pointer to the underlying byte buffer of the `ArrayBuffer`. `data` can optionally be ignored by passing `NULL`.
- `[out] result`: A `napi_value` representing a JavaScript `ArrayBuffer`.

Returns `napi_ok` if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript `ArrayBuffer`. `ArrayBuffer`s are used to represent fixed-length binary data buffers. They are normally used as a backing-buffer for `TypedArray` objects. The `ArrayBuffer` allocated will have an underlying byte buffer whose size is determined by the `length` parameter that's passed in. The underlying buffer is optionally returned back to the caller in case the caller wants to directly manipulate the buffer. This buffer can only be written to directly from native code. To write to this buffer from JavaScript, a typed array or `DataView` object would need to be created.

JavaScript `ArrayBuffer` objects are described in Section ArrayBuffer objects of the ECMAScript Language Specification.

##### `napi_create_buffer`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_buffer(napi_env env,
                                   size_t size,
                                   void** data,
                                   napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] size`: Size in bytes of the underlying buffer.
- `[out] data`: Raw pointer to the underlying buffer. `data` can optionally be ignored by passing `NULL`.
- `[out] result`: A `napi_value` representing a `node::Buffer`.

Returns `napi_ok` if the API succeeded.

This API allocates a `node::Buffer` object. While this is still a fully-supported data structure, in most cases using a `TypedArray` will suffice.

##### `napi_create_buffer_copy`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_buffer_copy(napi_env env,
                                        size_t length,
                                        const void* data,
                                        void** result_data,
                                        napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] size`: Size in bytes of the input buffer (should be the same as the size of the new buffer).
- `[in] data`: Raw pointer to the underlying buffer to copy from.
- `[out] result_data`: Pointer to the new `Buffer`'s underlying data buffer. `result_data` can optionally be ignored by passing `NULL`.
- `[out] result`: A `napi_value` representing a `node::Buffer`.

Returns `napi_ok` if the API succeeded.

This API allocates a `node::Buffer` object and initializes it with data copied from the passed-in buffer. While this is still a fully-supported data structure, in most cases using a `TypedArray` will suffice.

##### `napi_create_date`#

Added in: v11.11.0, v10.17.0N-API Version: 5

    napi_status napi_create_date(napi_env env,
                                 double time,
                                 napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] time`: ECMAScript time value in milliseconds since 01 January, 1970 UTC.
- `[out] result`: A `napi_value` representing a JavaScript `Date`.

Returns `napi_ok` if the API succeeded.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

This API allocates a JavaScript `Date` object.

JavaScript `Date` objects are described in Section Date objects of the ECMAScript Language Specification.

##### `napi_create_external`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_external(napi_env env,
                                     void* data,
                                     napi_finalize finalize_cb,
                                     void* finalize_hint,
                                     napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] data`: Raw pointer to the external data.
- `[in] finalize_cb`: Optional callback to call when the external value is being collected. `napi_finalize` provides more details.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.
- `[out] result`: A `napi_value` representing an external value.

Returns `napi_ok` if the API succeeded.

This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code using `napi_get_value_external`.

The API adds a `napi_finalize` callback which will be called when the JavaScript object just created has been garbage collected.

The created value is not an object, and therefore does not support additional properties. It is considered a distinct value type: calling `napi_typeof()` with an external value yields `napi_external`.

##### `napi_create_external_arraybuffer`#

Added in: v8.0.0N-API Version: 1

    napi_status
    napi_create_external_arraybuffer(napi_env env,
                                     void* external_data,
                                     size_t byte_length,
                                     napi_finalize finalize_cb,
                                     void* finalize_hint,
                                     napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] external_data`: Pointer to the underlying byte buffer of the `ArrayBuffer`.
- `[in] byte_length`: The length in bytes of the underlying buffer.
- `[in] finalize_cb`: Optional callback to call when the `ArrayBuffer` is being collected. `napi_finalize` provides more details.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.
- `[out] result`: A `napi_value` representing a JavaScript `ArrayBuffer`.

Returns `napi_ok` if the API succeeded.

Some runtimes other than Node.js have dropped support for external buffers. On runtimes other than Node.js this method may return `napi_no_external_buffers_allowed` to indicate that external buffers are not supported. One such runtime is Electron as described in this issue electron/issues/35801.

In order to maintain broadest compatibility with all runtimes you may define `NODE_API_NO_EXTERNAL_BUFFERS_ALLOWED` in your addon before includes for the node-api headers. Doing so will hide the 2 functions that create external buffers. This will ensure a compilation error occurs if you accidentally use one of these methods.

This API returns a Node-API value corresponding to a JavaScript `ArrayBuffer`. The underlying byte buffer of the `ArrayBuffer` is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called.

The API adds a `napi_finalize` callback which will be called when the JavaScript object just created has been garbage collected.

JavaScript `ArrayBuffer`s are described in Section ArrayBuffer objects of the ECMAScript Language Specification.

##### `node_api_create_external_sharedarraybuffer`#

Added in: v26.1.0

    napi_status
    node_api_create_external_sharedarraybuffer(napi_env env,
                                               void* external_data,
                                               size_t byte_length,
                                               node_api_noenv_finalize finalize_cb,
                                               void* finalize_hint,
                                               napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] external_data`: Pointer to the underlying byte buffer of the `SharedArrayBuffer`.
- `[in] byte_length`: The length in bytes of the underlying buffer.
- `[in] finalize_cb`: Optional callback to call when the `SharedArrayBuffer` is being collected. Called on an arbitrary thread. Because a `SharedArrayBuffer` can outlive the environment it's created in, the callback does not receive a reference to `env`.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.
- `[out] result`: A `napi_value` representing a JavaScript `SharedArrayBuffer`.

Returns `napi_ok` if the API succeeded.

Create a `SharedArrayBuffer` with externally managed memory.

See the entry on `napi_create_external_arraybuffer` for runtime compatibility.

##### `napi_create_external_buffer`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_external_buffer(napi_env env,
                                            size_t length,
                                            void* data,
                                            napi_finalize finalize_cb,
                                            void* finalize_hint,
                                            napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] length`: Size in bytes of the input buffer (should be the same as the size of the new buffer).
- `[in] data`: Raw pointer to the underlying buffer to expose to JavaScript.
- `[in] finalize_cb`: Optional callback to call when the `ArrayBuffer` is being collected. `napi_finalize` provides more details.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.
- `[out] result`: A `napi_value` representing a `node::Buffer`.

Returns `napi_ok` if the API succeeded.

Some runtimes other than Node.js have dropped support for external buffers. On runtimes other than Node.js this method may return `napi_no_external_buffers_allowed` to indicate that external buffers are not supported. One such runtime is Electron as described in this issue electron/issues/35801.

In order to maintain broadest compatibility with all runtimes you may define `NODE_API_NO_EXTERNAL_BUFFERS_ALLOWED` in your addon before includes for the node-api headers. Doing so will hide the 2 functions that create external buffers. This will ensure a compilation error occurs if you accidentally use one of these methods.

This API allocates a `node::Buffer` object and initializes it with data backed by the passed in buffer. While this is still a fully-supported data structure, in most cases using a `TypedArray` will suffice.

The API adds a `napi_finalize` callback which will be called when the JavaScript object just created has been garbage collected.

For Node.js >=4 `Buffers` are `Uint8Array`s.

##### `napi_create_object`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_object(napi_env env, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: A `napi_value` representing a JavaScript `Object`.

Returns `napi_ok` if the API succeeded.

This API allocates a default JavaScript `Object`. It is the equivalent of doing `new Object()` in JavaScript.

The JavaScript `Object` type is described in Section object type of the ECMAScript Language Specification.

##### `node_api_create_object_with_properties`#

Added in: v25.2.0, v24.12.0

Stability: 1 - Experimental

    napi_status node_api_create_object_with_properties(napi_env env,
                                                       napi_value prototype_or_null,
                                                       const napi_value* property_names,
                                                       const napi_value* property_values,
                                                       size_t property_count,
                                                       napi_value* result)
    cpp

- `[in] env`: The environment that the API is invoked under.
- `[in] prototype_or_null`: The prototype object for the new object. Can be a `napi_value` representing a JavaScript object to use as the prototype, a `napi_value` representing JavaScript `null`, or a `nullptr` that will be converted to `null`.
- `[in] property_names`: Array of `napi_value` representing the property names.
- `[in] property_values`: Array of `napi_value` representing the property values.
- `[in] property_count`: Number of properties in the arrays.
- `[out] result`: A `napi_value` representing a JavaScript `Object`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `Object` with the specified prototype and properties. This is more efficient than calling `napi_create_object` followed by multiple `napi_set_property` calls, as it can create the object with all properties atomically, avoiding potential V8 map transitions.

The arrays `property_names` and `property_values` must have the same length specified by `property_count`. The properties are added to the object in the order they appear in the arrays.

##### `napi_create_symbol`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_symbol(napi_env env,
                                   napi_value description,
                                   napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] description`: Optional `napi_value` which refers to a JavaScript `string` to be set as the description for the symbol.
- `[out] result`: A `napi_value` representing a JavaScript `symbol`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `symbol` value from a UTF8-encoded C string.

The JavaScript `symbol` type is described in Section symbol type of the ECMAScript Language Specification.

##### `node_api_symbol_for`#

Added in: v17.5.0, v16.15.0N-API Version: 9

    napi_status node_api_symbol_for(napi_env env,
                                    const char* utf8description,
                                    size_t length,
                                    napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] utf8description`: UTF-8 C string representing the text to be used as the description for the symbol.
- `[in] length`: The length of the description string in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing a JavaScript `symbol`.

Returns `napi_ok` if the API succeeded.

This API searches in the global registry for an existing symbol with the given description. If the symbol already exists it will be returned, otherwise a new symbol will be created in the registry.

The JavaScript `symbol` type is described in Section symbol type of the ECMAScript Language Specification.

##### `napi_create_typedarray`#

Added in: v8.0.0N-API Version: 1
History

Version | Changes
--- | ---
v26.2.0 | Added support for `SharedArrayBuffer`.

    napi_status napi_create_typedarray(napi_env env,
                                       napi_typedarray_type type,
                                       size_t length,
                                       napi_value arraybuffer,
                                       size_t byte_offset,
                                       napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] type`: Scalar datatype of the elements within the `TypedArray`.
- `[in] length`: Number of elements in the `TypedArray`.
- `[in] arraybuffer`: `ArrayBuffer` or `SharedArrayBuffer` underlying the typed array.
- `[in] byte_offset`: The byte offset within the `ArrayBuffer` or `SharedArrayBuffer` from which to start projecting the `TypedArray`.
- `[out] result`: A `napi_value` representing a JavaScript `TypedArray`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `TypedArray` object over an existing `ArrayBuffer` or `SharedArrayBuffer`. `TypedArray` objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype.

It is required that `(length * size_of_element) + byte_offset` is less than or equal to the size in bytes of the `ArrayBuffer` or `SharedArrayBuffer` passed in. If not, a `RangeError` exception is raised.

For element sizes greater than 1, `byte_offset` is required to be a multiple of the element size. If not, a `RangeError` exception is raised.

JavaScript `TypedArray` objects are described in Section TypedArray objects of the ECMAScript Language Specification.

##### `node_api_create_buffer_from_arraybuffer`#

Added in: v23.0.0, v22.12.0N-API Version: 10

    napi_status NAPI_CDECL node_api_create_buffer_from_arraybuffer(napi_env env,
                                                                  napi_value arraybuffer,
                                                                  size_t byte_offset,
                                                                  size_t byte_length,
                                                                  napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] arraybuffer`: The `ArrayBuffer` from which the buffer will be created.
- `[in] byte_offset`: The byte offset within the `ArrayBuffer` from which to start creating the buffer.
- `[in] byte_length`: The length in bytes of the buffer to be created from the `ArrayBuffer`.
- `[out] result`: A `napi_value` representing the created JavaScript `Buffer` object.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `Buffer` object from an existing `ArrayBuffer`. The `Buffer` object is a Node.js-specific class that provides a way to work with binary data directly in JavaScript.

The byte range `[byte_offset, byte_offset + byte_length)` must be within the bounds of the `ArrayBuffer`. If `byte_offset + byte_length` exceeds the size of the `ArrayBuffer`, a `RangeError` exception is raised.

##### `napi_create_dataview`#

Added in: v8.3.0N-API Version: 1
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | Added support for `SharedArrayBuffer`.

    napi_status napi_create_dataview(napi_env env,
                                     size_t byte_length,
                                     napi_value arraybuffer,
                                     size_t byte_offset,
                                     napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] length`: Number of elements in the `DataView`.
- `[in] arraybuffer`: `ArrayBuffer` or `SharedArrayBuffer` underlying the `DataView`.
- `[in] byte_offset`: The byte offset within the `ArrayBuffer` from which to start projecting the `DataView`.
- `[out] result`: A `napi_value` representing a JavaScript `DataView`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `DataView` object over an existing `ArrayBuffer` or `SharedArrayBuffer`. `DataView` objects provide an array-like view over an underlying data buffer, but one which allows items of different size and type in the `ArrayBuffer` or `SharedArrayBuffer`.

It is required that `byte_length + byte_offset` is less than or equal to the size in bytes of the array passed in. If not, a `RangeError` exception is raised.

JavaScript `DataView` objects are described in Section DataView objects of the ECMAScript Language Specification.

#### Functions to convert from C types to Node-API#

##### `napi_create_int32`#

Added in: v8.4.0N-API Version: 1

    napi_status napi_create_int32(napi_env env, int32_t value, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: Integer value to be represented in JavaScript.
- `[out] result`: A `napi_value` representing a JavaScript `number`.

Returns `napi_ok` if the API succeeded.

This API is used to convert from the C `int32_t` type to the JavaScript `number` type.

The JavaScript `number` type is described in Section number type of the ECMAScript Language Specification.

##### `napi_create_uint32`#

Added in: v8.4.0N-API Version: 1

    napi_status napi_create_uint32(napi_env env, uint32_t value, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: Unsigned integer value to be represented in JavaScript.
- `[out] result`: A `napi_value` representing a JavaScript `number`.

Returns `napi_ok` if the API succeeded.

This API is used to convert from the C `uint32_t` type to the JavaScript `number` type.

The JavaScript `number` type is described in Section number type of the ECMAScript Language Specification.

##### `napi_create_int64`#

Added in: v8.4.0N-API Version: 1

    napi_status napi_create_int64(napi_env env, int64_t value, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: Integer value to be represented in JavaScript.
- `[out] result`: A `napi_value` representing a JavaScript `number`.

Returns `napi_ok` if the API succeeded.

This API is used to convert from the C `int64_t` type to the JavaScript `number` type.

The JavaScript `number` type is described in Section number type of the ECMAScript Language Specification. Note the complete range of `int64_t` cannot be represented with full precision in JavaScript. Integer values outside the range of `Number.MIN_SAFE_INTEGER` `-(2**53 - 1)` - `Number.MAX_SAFE_INTEGER` `(2**53 - 1)` will lose precision.

##### `napi_create_double`#

Added in: v8.4.0N-API Version: 1

    napi_status napi_create_double(napi_env env, double value, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: Double-precision value to be represented in JavaScript.
- `[out] result`: A `napi_value` representing a JavaScript `number`.

Returns `napi_ok` if the API succeeded.

This API is used to convert from the C `double` type to the JavaScript `number` type.

The JavaScript `number` type is described in Section number type of the ECMAScript Language Specification.

##### `napi_create_bigint_int64`#

Added in: v10.7.0N-API Version: 6

    napi_status napi_create_bigint_int64(napi_env env,
                                         int64_t value,
                                         napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: Integer value to be represented in JavaScript.
- `[out] result`: A `napi_value` representing a JavaScript `BigInt`.

Returns `napi_ok` if the API succeeded.

This API converts the C `int64_t` type to the JavaScript `BigInt` type.

##### `napi_create_bigint_uint64`#

Added in: v10.7.0N-API Version: 6

    napi_status napi_create_bigint_uint64(napi_env env,
                                          uint64_t value,
                                          napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: Unsigned integer value to be represented in JavaScript.
- `[out] result`: A `napi_value` representing a JavaScript `BigInt`.

Returns `napi_ok` if the API succeeded.

This API converts the C `uint64_t` type to the JavaScript `BigInt` type.

##### `napi_create_bigint_words`#

Added in: v10.7.0N-API Version: 6

    napi_status napi_create_bigint_words(napi_env env,
                                         int sign_bit,
                                         size_t word_count,
                                         const uint64_t* words,
                                         napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] sign_bit`: Determines if the resulting `BigInt` will be positive or negative.
- `[in] word_count`: The length of the `words` array.
- `[in] words`: An array of `uint64_t` little-endian 64-bit words.
- `[out] result`: A `napi_value` representing a JavaScript `BigInt`.

Returns `napi_ok` if the API succeeded.

This API converts an array of unsigned 64-bit words into a single `BigInt` value.

The resulting `BigInt` is calculated as: (–1)`sign_bit` (`words[0]` × (264)0 + `words[1]` × (264)1 + …)

##### `napi_create_string_latin1`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_string_latin1(napi_env env,
                                          const char* str,
                                          size_t length,
                                          napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing an ISO-8859-1-encoded string.
- `[in] length`: The length of the string in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing a JavaScript `string`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `string` value from an ISO-8859-1-encoded C string. The native string is copied.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

##### `node_api_create_external_string_latin1`#

Added in: v20.4.0, v18.18.0N-API Version: 10

    napi_status
    node_api_create_external_string_latin1(napi_env env,
                                           char* str,
                                           size_t length,
                                           napi_finalize finalize_callback,
                                           void* finalize_hint,
                                           napi_value* result,
                                           bool* copied);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing an ISO-8859-1-encoded string.
- `[in] length`: The length of the string in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[in] finalize_callback`: The function to call when the string is being collected. The function will be called with the following parameters:
  - `[in] env`: The environment in which the add-on is running. This value may be null if the string is being collected as part of the termination of the worker or the main Node.js instance.
  - `[in] data`: This is the value `str` as a `void*` pointer.
  - `[in] finalize_hint`: This is the value `finalize_hint` that was given to the API. `napi_finalize` provides more details. This parameter is optional. Passing a null value means that the add-on doesn't need to be notified when the corresponding JavaScript string is collected.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.
- `[out] result`: A `napi_value` representing a JavaScript `string`.
- `[out] copied`: Whether the string was copied. If it was, the finalizer will already have been invoked to destroy `str`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `string` value from an ISO-8859-1-encoded C string. The native string may not be copied and must thus exist for the entire life cycle of the JavaScript value.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

##### `napi_create_string_utf16`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_string_utf16(napi_env env,
                                         const char16_t* str,
                                         size_t length,
                                         napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing a UTF16-LE-encoded string.
- `[in] length`: The length of the string in two-byte code units, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing a JavaScript `string`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `string` value from a UTF16-LE-encoded C string. The native string is copied.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

##### `node_api_create_external_string_utf16`#

Added in: v20.4.0, v18.18.0N-API Version: 10

    napi_status
    node_api_create_external_string_utf16(napi_env env,
                                          char16_t* str,
                                          size_t length,
                                          napi_finalize finalize_callback,
                                          void* finalize_hint,
                                          napi_value* result,
                                          bool* copied);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing a UTF16-LE-encoded string.
- `[in] length`: The length of the string in two-byte code units, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[in] finalize_callback`: The function to call when the string is being collected. The function will be called with the following parameters:
  - `[in] env`: The environment in which the add-on is running. This value may be null if the string is being collected as part of the termination of the worker or the main Node.js instance.
  - `[in] data`: This is the value `str` as a `void*` pointer.
  - `[in] finalize_hint`: This is the value `finalize_hint` that was given to the API. `napi_finalize` provides more details. This parameter is optional. Passing a null value means that the add-on doesn't need to be notified when the corresponding JavaScript string is collected.
- `[in] finalize_hint`: Optional hint to pass to the finalize callback during collection.
- `[out] result`: A `napi_value` representing a JavaScript `string`.
- `[out] copied`: Whether the string was copied. If it was, the finalizer will already have been invoked to destroy `str`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `string` value from a UTF16-LE-encoded C string. The native string may not be copied and must thus exist for the entire life cycle of the JavaScript value.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

##### `napi_create_string_utf8`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_string_utf8(napi_env env,
                                        const char* str,
                                        size_t length,
                                        napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing a UTF8-encoded string.
- `[in] length`: The length of the string in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing a JavaScript `string`.

Returns `napi_ok` if the API succeeded.

This API creates a JavaScript `string` value from a UTF8-encoded C string. The native string is copied.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

#### Functions to create optimized property keys#

Many JavaScript engines including V8 use internalized strings as keys to set and get property values. They typically use a hash table to create and lookup such strings. While it adds some cost per key creation, it improves the performance after that by enabling comparison of string pointers instead of the whole strings.

If a new JavaScript string is intended to be used as a property key, then for some JavaScript engines it will be more efficient to use the functions in this section. Otherwise, use the `napi_create_string_utf8` or `node_api_create_external_string_utf8` series functions as there may be additional overhead in creating/storing strings with the property key creation methods.

##### `node_api_create_property_key_latin1`#

Added in: v22.9.0, v20.18.0N-API Version: 10

    napi_status NAPI_CDECL node_api_create_property_key_latin1(napi_env env,
                                                               const char* str,
                                                               size_t length,
                                                               napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing an ISO-8859-1-encoded string.
- `[in] length`: The length of the string in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing an optimized JavaScript `string` to be used as a property key for objects.

Returns `napi_ok` if the API succeeded.

This API creates an optimized JavaScript `string` value from an ISO-8859-1-encoded C string to be used as a property key for objects. The native string is copied. In contrast with `napi_create_string_latin1`, subsequent calls to this function with the same `str` pointer may benefit from a speedup in the creation of the requested `napi_value`, depending on the engine.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

##### `node_api_create_property_key_utf16`#

Added in: v21.7.0, v20.12.0N-API Version: 10

    napi_status NAPI_CDECL node_api_create_property_key_utf16(napi_env env,
                                                              const char16_t* str,
                                                              size_t length,
                                                              napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing a UTF16-LE-encoded string.
- `[in] length`: The length of the string in two-byte code units, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing an optimized JavaScript `string` to be used as a property key for objects.

Returns `napi_ok` if the API succeeded.

This API creates an optimized JavaScript `string` value from a UTF16-LE-encoded C string to be used as a property key for objects. The native string is copied.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

##### `node_api_create_property_key_utf8`#

Added in: v22.9.0, v20.18.0N-API Version: 10

    napi_status NAPI_CDECL node_api_create_property_key_utf8(napi_env env,
                                                             const char* str,
                                                             size_t length,
                                                             napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] str`: Character buffer representing a UTF8-encoded string.
- `[in] length`: The length of the string in two-byte code units, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[out] result`: A `napi_value` representing an optimized JavaScript `string` to be used as a property key for objects.

Returns `napi_ok` if the API succeeded.

This API creates an optimized JavaScript `string` value from a UTF8-encoded C string to be used as a property key for objects. The native string is copied.

The JavaScript `string` type is described in Section string type of the ECMAScript Language Specification.

#### Functions to convert from Node-API to C types#

##### `napi_get_array_length`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_array_length(napi_env env,
                                      napi_value value,
                                      uint32_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing the JavaScript `Array` whose length is being queried.
- `[out] result`: `uint32` representing length of the array.

Returns `napi_ok` if the API succeeded.

This API returns the length of an array.

`Array` length is described in Section Array instance length of the ECMAScript Language Specification.

##### `napi_get_arraybuffer_info`#

Added in: v8.0.0N-API Version: 1
History

Version | Changes
--- | ---
v24.9.0, v22.21.0 | Added support for `SharedArrayBuffer`.

    napi_status napi_get_arraybuffer_info(napi_env env,
                                          napi_value arraybuffer,
                                          void** data,
                                          size_t* byte_length)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] arraybuffer`: `napi_value` representing the `ArrayBuffer` or `SharedArrayBuffer` being queried.
- `[out] data`: The underlying data buffer of the `ArrayBuffer` or `SharedArrayBuffer` is `0`, this may be `NULL` or any other pointer value.
- `[out] byte_length`: Length in bytes of the underlying data buffer.

Returns `napi_ok` if the API succeeded.

This API is used to retrieve the underlying data buffer of an `ArrayBuffer` or `SharedArrayBuffer` and its length.

WARNING: Use caution while using this API. The lifetime of the underlying data buffer is managed by the `ArrayBuffer` or `SharedArrayBuffer` even after it's returned. A possible safe way to use this API is in conjunction with `napi_create_reference`, which can be used to guarantee control over the lifetime of the `ArrayBuffer` or `SharedArrayBuffer`. It's also safe to use the returned data buffer within the same callback as long as there are no calls to other APIs that might trigger a GC.

##### `napi_get_buffer_info`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_buffer_info(napi_env env,
                                     napi_value value,
                                     void** data,
                                     size_t* length)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing the `node::Buffer` or `Uint8Array` being queried.
- `[out] data`: The underlying data buffer of the `node::Buffer` or `Uint8Array`. If length is `0`, this may be `NULL` or any other pointer value.
- `[out] length`: Length in bytes of the underlying data buffer.

Returns `napi_ok` if the API succeeded.

This method returns the identical `data` and `byte_length` as `napi_get_typedarray_info`. And `napi_get_typedarray_info` accepts a `node::Buffer` (a Uint8Array) as the value too.

This API is used to retrieve the underlying data buffer of a `node::Buffer` and its length.

Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.

##### `napi_get_prototype`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_prototype(napi_env env,
                                   napi_value object,
                                   napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] object`: `napi_value` representing JavaScript `Object` whose prototype to return. This returns the equivalent of `Object.getPrototypeOf` (which is not the same as the function's `prototype` property).
- `[out] result`: `napi_value` representing prototype of the given object.

Returns `napi_ok` if the API succeeded.

##### `napi_get_typedarray_info`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_typedarray_info(napi_env env,
                                         napi_value typedarray,
                                         napi_typedarray_type* type,
                                         size_t* length,
                                         void** data,
                                         napi_value* arraybuffer,
                                         size_t* byte_offset)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] typedarray`: `napi_value` representing the `TypedArray` whose properties to query.
- `[out] type`: Scalar datatype of the elements within the `TypedArray`.
- `[out] length`: The number of elements in the `TypedArray`.
- `[out] data`: The data buffer underlying the `TypedArray` adjusted by the `byte_offset` value so that it points to the first element in the `TypedArray`. If the length of the array is `0`, this may be `NULL` or any other pointer value.
- `[out] arraybuffer`: The `ArrayBuffer` or `SharedArrayBuffer` underlying the `TypedArray`.
- `[out] byte_offset`: The byte offset within the underlying native array at which the first element of the arrays is located. The value for the data parameter has already been adjusted so that data points to the first element in the array. Therefore, the first byte of the native array would be at `data - byte_offset`.

Returns `napi_ok` if the API succeeded.

This API returns various properties of a typed array.

Any of the out parameters may be `NULL` if that property is unneeded.

Warning: Use caution while using this API since the underlying data buffer is managed by the VM.

##### `napi_get_dataview_info`#

Added in: v8.3.0N-API Version: 1

    napi_status napi_get_dataview_info(napi_env env,
                                       napi_value dataview,
                                       size_t* byte_length,
                                       void** data,
                                       napi_value* arraybuffer,
                                       size_t* byte_offset)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] dataview`: `napi_value` representing the `DataView` whose properties to query.
- `[out] byte_length`: Number of bytes in the `DataView`.
- `[out] data`: The data buffer underlying the `DataView`. If byte_length is `0`, this may be `NULL` or any other pointer value.
- `[out] arraybuffer`: `ArrayBuffer` underlying the `DataView`.
- `[out] byte_offset`: The byte offset within the data buffer from which to start projecting the `DataView`.

Returns `napi_ok` if the API succeeded.

Any of the out parameters may be `NULL` if that property is unneeded.

This API returns various properties of a `DataView`.

##### `napi_get_date_value`#

Added in: v11.11.0, v10.17.0N-API Version: 5

    napi_status napi_get_date_value(napi_env env,
                                    napi_value value,
                                    double* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing a JavaScript `Date`.
- `[out] result`: Time value as a `double` represented as milliseconds since midnight at the beginning of 01 January, 1970 UTC.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

Returns `napi_ok` if the API succeeded. If a non-date `napi_value` is passed in it returns `napi_date_expected`.

This API returns the C double primitive of time value for the given JavaScript `Date`.

##### `napi_get_value_bool`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `Boolean`.
- `[out] result`: C boolean primitive equivalent of the given JavaScript `Boolean`.

Returns `napi_ok` if the API succeeded. If a non-boolean `napi_value` is passed in it returns `napi_boolean_expected`.

This API returns the C boolean primitive equivalent of the given JavaScript `Boolean`.

##### `napi_get_value_double`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_double(napi_env env,
                                      napi_value value,
                                      double* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `number`.
- `[out] result`: C double primitive equivalent of the given JavaScript `number`.

Returns `napi_ok` if the API succeeded. If a non-number `napi_value` is passed in it returns `napi_number_expected`.

This API returns the C double primitive equivalent of the given JavaScript `number`.

##### `napi_get_value_bigint_int64`#

Added in: v10.7.0N-API Version: 6

    napi_status napi_get_value_bigint_int64(napi_env env,
                                            napi_value value,
                                            int64_t* result,
                                            bool* lossless);
    c

- `[in] env`: The environment that the API is invoked under
- `[in] value`: `napi_value` representing JavaScript `BigInt`.
- `[out] result`: C `int64_t` primitive equivalent of the given JavaScript `BigInt`.
- `[out] lossless`: Indicates whether the `BigInt` value was converted losslessly.

Returns `napi_ok` if the API succeeded. If a non-`BigInt` is passed in it returns `napi_bigint_expected`.

This API returns the C `int64_t` primitive equivalent of the given JavaScript `BigInt`. If needed it will truncate the value, setting `lossless` to `false`.

##### `napi_get_value_bigint_uint64`#

Added in: v10.7.0N-API Version: 6

    napi_status napi_get_value_bigint_uint64(napi_env env,
                                            napi_value value,
                                            uint64_t* result,
                                            bool* lossless);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `BigInt`.
- `[out] result`: C `uint64_t` primitive equivalent of the given JavaScript `BigInt`.
- `[out] lossless`: Indicates whether the `BigInt` value was converted losslessly.

Returns `napi_ok` if the API succeeded. If a non-`BigInt` is passed in it returns `napi_bigint_expected`.

This API returns the C `uint64_t` primitive equivalent of the given JavaScript `BigInt`. If needed it will truncate the value, setting `lossless` to `false`.

##### `napi_get_value_bigint_words`#

Added in: v10.7.0N-API Version: 6

    napi_status napi_get_value_bigint_words(napi_env env,
                                            napi_value value,
                                            int* sign_bit,
                                            size_t* word_count,
                                            uint64_t* words);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `BigInt`.
- `[out] sign_bit`: Integer representing if the JavaScript `BigInt` is positive or negative.
- `[in/out] word_count`: Must be initialized to the length of the `words` array. Upon return, it will be set to the actual number of words that would be needed to store this `BigInt`.
- `[out] words`: Pointer to a pre-allocated 64-bit word array.

Returns `napi_ok` if the API succeeded.

This API converts a single `BigInt` value into a sign bit, 64-bit little-endian array, and the number of elements in the array. `sign_bit` and `words` may be both set to `NULL`, in order to get only `word_count`.

##### `napi_get_value_external`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_external(napi_env env,
                                        napi_value value,
                                        void** result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript external value.
- `[out] result`: Pointer to the data wrapped by the JavaScript external value.

Returns `napi_ok` if the API succeeded. If a non-external `napi_value` is passed in it returns `napi_invalid_arg`.

This API retrieves the external data pointer that was previously passed to `napi_create_external()`.

##### `napi_get_value_int32`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_int32(napi_env env,
                                     napi_value value,
                                     int32_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `number`.
- `[out] result`: C `int32` primitive equivalent of the given JavaScript `number`.

Returns `napi_ok` if the API succeeded. If a non-number `napi_value` is passed in `napi_number_expected`.

This API returns the C `int32` primitive equivalent of the given JavaScript `number`.

If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the value is > 231 - 1.

Non-finite number values (`NaN`, `+Infinity`, or `-Infinity`) set the result to zero.

##### `napi_get_value_int64`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_int64(napi_env env,
                                     napi_value value,
                                     int64_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `number`.
- `[out] result`: C `int64` primitive equivalent of the given JavaScript `number`.

Returns `napi_ok` if the API succeeded. If a non-number `napi_value` is passed in it returns `napi_number_expected`.

This API returns the C `int64` primitive equivalent of the given JavaScript `number`.

`number` values outside the range of `Number.MIN_SAFE_INTEGER` `-(2**53 - 1)` - `Number.MAX_SAFE_INTEGER` `(2**53 - 1)` will lose precision.

Non-finite number values (`NaN`, `+Infinity`, or `-Infinity`) set the result to zero.

##### `napi_get_value_string_latin1`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_string_latin1(napi_env env,
                                             napi_value value,
                                             char* buf,
                                             size_t bufsize,
                                             size_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript string.
- `[in] buf`: Buffer to write the ISO-8859-1-encoded string into. If `NULL` is passed in, the length of the string in bytes and excluding the null terminator is returned in `result`.
- `[in] bufsize`: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated. If this value is zero, then the string is not returned and no changes are done to the buffer.
- `[out] result`: Number of bytes copied into the buffer, excluding the null terminator.

Returns `napi_ok` if the API succeeded. If a non-`string` `napi_value` is passed in it returns `napi_string_expected`.

This API returns the ISO-8859-1-encoded string corresponding the value passed in.

##### `napi_get_value_string_utf8`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_string_utf8(napi_env env,
                                           napi_value value,
                                           char* buf,
                                           size_t bufsize,
                                           size_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript string.
- `[in] buf`: Buffer to write the UTF8-encoded string into. If `NULL` is passed in, the length of the string in bytes and excluding the null terminator is returned in `result`.
- `[in] bufsize`: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated. If this value is zero, then the string is not returned and no changes are done to the buffer.
- `[out] result`: Number of bytes copied into the buffer, excluding the null terminator.

Returns `napi_ok` if the API succeeded. If a non-`string` `napi_value` is passed in it returns `napi_string_expected`.

This API returns the UTF8-encoded string corresponding the value passed in.

##### `napi_get_value_string_utf16`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_string_utf16(napi_env env,
                                            napi_value value,
                                            char16_t* buf,
                                            size_t bufsize,
                                            size_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript string.
- `[in] buf`: Buffer to write the UTF16-LE-encoded string into. If `NULL` is passed in, the length of the string in 2-byte code units and excluding the null terminator is returned.
- `[in] bufsize`: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated. If this value is zero, then the string is not returned and no changes are done to the buffer.
- `[out] result`: Number of 2-byte code units copied into the buffer, excluding the null terminator.

Returns `napi_ok` if the API succeeded. If a non-`string` `napi_value` is passed in it returns `napi_string_expected`.

This API returns the UTF16-encoded string corresponding the value passed in.

##### `napi_get_value_uint32`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_value_uint32(napi_env env,
                                      napi_value value,
                                      uint32_t* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: `napi_value` representing JavaScript `number`.
- `[out] result`: C primitive equivalent of the given `napi_value` as a `uint32_t`.

Returns `napi_ok` if the API succeeded. If a non-number `napi_value` is passed in it returns `napi_number_expected`.

This API returns the C primitive equivalent of the given `napi_value` as a `uint32_t`.

#### Functions to get global instances#

##### `napi_get_boolean`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The value of the boolean to retrieve.
- `[out] result`: `napi_value` representing JavaScript `Boolean` singleton to retrieve.

Returns `napi_ok` if the API succeeded.

This API is used to return the JavaScript singleton object that is used to represent the given boolean value.

##### `napi_get_global`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_global(napi_env env, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: `napi_value` representing JavaScript `global` object.

Returns `napi_ok` if the API succeeded.

This API returns the `global` object.

##### `napi_get_null`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_null(napi_env env, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: `napi_value` representing JavaScript `null` object.

Returns `napi_ok` if the API succeeded.

This API returns the `null` object.

##### `napi_get_undefined`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_undefined(napi_env env, napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: `napi_value` representing JavaScript Undefined value.

Returns `napi_ok` if the API succeeded.

This API returns the Undefined object.

### Working with JavaScript values and abstract operations#

Node-API exposes a set of APIs to perform some abstract operations on JavaScript values.

These APIs support doing one of the following:

1. Coerce JavaScript values to specific JavaScript types (such as `number` or `string`).
2. Check the type of a JavaScript value.
3. Check for equality between two JavaScript values.

#### `napi_coerce_to_bool`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_coerce_to_bool(napi_env env,
                                    napi_value value,
                                    napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to coerce.
- `[out] result`: `napi_value` representing the coerced JavaScript `Boolean`.

Returns `napi_ok` if the API succeeded.

This API implements the abstract operation `ToBoolean()` as defined in Section ToBoolean of the ECMAScript Language Specification.

#### `napi_coerce_to_number`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_coerce_to_number(napi_env env,
                                      napi_value value,
                                      napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to coerce.
- `[out] result`: `napi_value` representing the coerced JavaScript `number`.

Returns `napi_ok` if the API succeeded.

This API implements the abstract operation `ToNumber()` as defined in Section ToNumber of the ECMAScript Language Specification. This function potentially runs JS code if the passed-in value is an object.

#### `napi_coerce_to_object`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_coerce_to_object(napi_env env,
                                      napi_value value,
                                      napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to coerce.
- `[out] result`: `napi_value` representing the coerced JavaScript `Object`.

Returns `napi_ok` if the API succeeded.

This API implements the abstract operation `ToObject()` as defined in Section ToObject of the ECMAScript Language Specification.

#### `napi_coerce_to_string`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_coerce_to_string(napi_env env,
                                      napi_value value,
                                      napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to coerce.
- `[out] result`: `napi_value` representing the coerced JavaScript `string`.

Returns `napi_ok` if the API succeeded.

This API implements the abstract operation `ToString()` as defined in Section ToString of the ECMAScript Language Specification. This function potentially runs JS code if the passed-in value is an object.

#### `napi_typeof`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value whose type to query.
- `[out] result`: The type of the JavaScript value.

Returns `napi_ok` if the API succeeded.

- `napi_invalid_arg` if the type of `value` is not a known ECMAScript type and `value` is not an External value.

This API represents behavior similar to invoking the `typeof` Operator on the object as defined in Section typeof operator of the ECMAScript Language Specification. However, there are some differences:

1. It has support for detecting an External value.
2. It detects `null` as a separate type, while ECMAScript `typeof` would detect `object`.

If `value` has a type that is invalid, an error is returned.

#### `napi_instanceof`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_instanceof(napi_env env,
                                napi_value object,
                                napi_value constructor,
                                bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] object`: The JavaScript value to check.
- `[in] constructor`: The JavaScript function object of the constructor function to check against.
- `[out] result`: Boolean that is set to true if `object instanceof constructor` is true.

Returns `napi_ok` if the API succeeded.

This API represents invoking the `instanceof` Operator on the object as defined in Section instanceof operator of the ECMAScript Language Specification.

#### `napi_is_array`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_is_array(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given object is an array.

Returns `napi_ok` if the API succeeded.

This API represents invoking the `IsArray` operation on the object as defined in Section IsArray of the ECMAScript Language Specification.

#### `napi_is_arraybuffer`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given object is an `ArrayBuffer`.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in is an array buffer.

#### `napi_is_buffer`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given `napi_value` represents a `node::Buffer` or `Uint8Array` object.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in is a buffer or Uint8Array. `napi_is_typedarray` should be preferred if the caller needs to check if the value is a Uint8Array.

#### `napi_is_date`#

Added in: v11.11.0, v10.17.0N-API Version: 5

    napi_status napi_is_date(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given `napi_value` represents a JavaScript `Date` object.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in is a date.

#### `napi_is_error`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_is_error(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given `napi_value` represents an `Error` object.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in is an `Error`.

#### `napi_is_typedarray`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given `napi_value` represents a `TypedArray`.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in is a typed array.

#### `napi_is_dataview`#

Added in: v8.3.0N-API Version: 1

    napi_status napi_is_dataview(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given `napi_value` represents a `DataView`.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in is a `DataView`.

#### `napi_strict_equals`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_strict_equals(napi_env env,
                                   napi_value lhs,
                                   napi_value rhs,
                                   bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] lhs`: The JavaScript value to check.
- `[in] rhs`: The JavaScript value to check against.
- `[out] result`: Whether the two `napi_value` objects are equal.

Returns `napi_ok` if the API succeeded.

This API represents the invocation of the Strict Equality algorithm as defined in Section IsStrictlyEqual of the ECMAScript Language Specification.

#### `napi_detach_arraybuffer`#

Added in: v13.0.0, v12.16.0, v10.22.0N-API Version: 7

    napi_status napi_detach_arraybuffer(napi_env env,
                                        napi_value arraybuffer)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] arraybuffer`: The JavaScript `ArrayBuffer` to be detached.

Returns `napi_ok` if the API succeeded. If a non-detachable `ArrayBuffer` is passed in it returns `napi_detachable_arraybuffer_expected`.

Generally, an `ArrayBuffer` is non-detachable if it has been detached before. The engine may impose additional conditions on whether an `ArrayBuffer` is detachable. For example, V8 requires that the `ArrayBuffer` be external, that is, created with `napi_create_external_arraybuffer`.

This API represents the invocation of the `ArrayBuffer` detach operation as defined in Section detachArrayBuffer of the ECMAScript Language Specification.

#### `napi_is_detached_arraybuffer`#

Added in: v13.3.0, v12.16.0, v10.22.0N-API Version: 7

    napi_status napi_is_detached_arraybuffer(napi_env env,
                                             napi_value arraybuffer,
                                             bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] arraybuffer`: The JavaScript `ArrayBuffer` to be checked.
- `[out] result`: Whether the `arraybuffer` is detached.

Returns `napi_ok` if the API succeeded.

The `ArrayBuffer` is considered detached if its internal data is `null`.

This API represents the invocation of the `ArrayBuffer` `IsDetachedBuffer` operation as defined in Section isDetachedBuffer of the ECMAScript Language Specification.

#### `node_api_is_sharedarraybuffer`#

Added in: v24.9.0, v22.21.0

Stability: 1 - Experimental

    napi_status node_api_is_sharedarraybuffer(napi_env env, napi_value value, bool* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The JavaScript value to check.
- `[out] result`: Whether the given `napi_value` represents a `SharedArrayBuffer`.

Returns `napi_ok` if the API succeeded.

This API checks if the Object passed in is a `SharedArrayBuffer`.

#### `node_api_create_sharedarraybuffer`#

Added in: v24.9.0, v22.21.0

Stability: 1 - Experimental

    napi_status node_api_create_sharedarraybuffer(napi_env env,
                                                 size_t byte_length,
                                                 void** data,
                                                 napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] byte_length`: The length in bytes of the shared array buffer to create.
- `[out] data`: Pointer to the underlying byte buffer of the `SharedArrayBuffer`. `data` can optionally be ignored by passing `NULL`.
- `[out] result`: A `napi_value` representing a JavaScript `SharedArrayBuffer`.

Returns `napi_ok` if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript `SharedArrayBuffer`. `SharedArrayBuffer`s are used to represent fixed-length binary data buffers that can be shared across multiple workers.

The `SharedArrayBuffer` allocated will have an underlying byte buffer whose size is determined by the `byte_length` parameter that's passed in. The underlying buffer is optionally returned back to the caller in case the caller wants to directly manipulate the buffer. This buffer can only be written to directly from native code. To write to this buffer from JavaScript, a typed array or `DataView` object would need to be created.

JavaScript `SharedArrayBuffer` objects are described in Section SharedArrayBuffer objects of the ECMAScript Language Specification.

### Working with JavaScript properties#

Node-API exposes a set of APIs to get and set properties on JavaScript objects.

Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in Node-API can be represented in one of the following forms:

- Named: a simple UTF8-encoded string
- Integer-Indexed: an index value represented by `uint32_t`
- JavaScript value: these are represented in Node-API by `napi_value`. This can be a `napi_value` representing a `string`, `number`, or `symbol`.

Node-API values are represented by the type `napi_value`. Any Node-API call that requires a JavaScript value takes in a `napi_value`. However, it's the caller's responsibility to make sure that the `napi_value` in question is of the JavaScript type expected by the API.

The APIs documented in this section provide a simple interface to get and set properties on arbitrary JavaScript objects represented by `napi_value`.

For instance, consider the following JavaScript code snippet:

    const obj = {};
    obj.myProp = 123;
    js

The equivalent can be done using Node-API values with the following snippet:

    napi_status status = napi_generic_failure;

    // const obj = {}
    napi_value obj, value;
    status = napi_create_object(env, &obj);
    if (status != napi_ok) return status;

    // Create a napi_value for 123
    status = napi_create_int32(env, 123, &value);
    if (status != napi_ok) return status;

    // obj.myProp = 123
    status = napi_set_named_property(env, obj, "myProp", value);
    if (status != napi_ok) return status;
    c

Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:

    const arr = [];
    arr[123] = 'hello';
    js

The equivalent can be done using Node-API values with the following snippet:

    napi_status status = napi_generic_failure;

    // const arr = [];
    napi_value arr, value;
    status = napi_create_array(env, &arr);
    if (status != napi_ok) return status;

    // Create a napi_value for 'hello'
    status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &value);
    if (status != napi_ok) return status;

    // arr[123] = 'hello';
    status = napi_set_element(env, arr, 123, value);
    if (status != napi_ok) return status;
    c

Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:

    const arr = [];
    const value = arr[123];
    js

The following is the approximate equivalent of the Node-API counterpart:

    napi_status status = napi_generic_failure;

    // const arr = []
    napi_value arr, value;
    status = napi_create_array(env, &arr);
    if (status != napi_ok) return status;

    // const value = arr[123]
    status = napi_get_element(env, arr, 123, &value);
    if (status != napi_ok) return status;
    c

Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:

    const obj = {};
    Object.defineProperties(obj, {
      'foo': { value: 123, writable: true, configurable: true, enumerable: true },
      'bar': { value: 456, writable: true, configurable: true, enumerable: true },
    });
    js

The following is the approximate equivalent of the Node-API counterpart:

    napi_status status = napi_status_generic_failure;

    // const obj = {};
    napi_value obj;
    status = napi_create_object(env, &obj);
    if (status != napi_ok) return status;

    // Create napi_values for 123 and 456
    napi_value fooValue, barValue;
    status = napi_create_int32(env, 123, &fooValue);
    if (status != napi_ok) return status;
    status = napi_create_int32(env, 456, &barValue);
    if (status != napi_ok) return status;

    // Set the properties
    napi_property_descriptor descriptors[] = {
      { "foo", NULL, NULL, NULL, NULL, fooValue, napi_writable | napi_configurable, NULL },
      { "bar", NULL, NULL, NULL, NULL, barValue, napi_writable | napi_configurable, NULL }
    }
    status = napi_define_properties(env,
                                    obj,
                                    sizeof(descriptors) / sizeof(descriptors[0]),
                                    descriptors);
    if (status != napi_ok) return status;
    c

#### Structures#

##### `napi_property_attributes`#

History

Version | Changes
--- | ---
v14.12.0 | added `napi_default_method` and `napi_default_property`.

    typedef enum {
      napi_default = 0,
      napi_writable = 1 << 0,
      napi_enumerable = 1 << 1,
      napi_configurable = 1 << 2,

      // Used with napi_define_class to distinguish static properties
      // from instance properties. Ignored by napi_define_properties.
      napi_static = 1 << 10,

      // Default for class methods.
      napi_default_method = napi_writable | napi_configurable,

      // Default for object properties, like in JS obj[prop].
      napi_default_jsproperty = napi_writable |
                              napi_enumerable |
                              napi_configurable,
    } napi_property_attributes;
    c

`napi_property_attributes` are bit flags used to control the behavior of properties set on a JavaScript object. Other than `napi_static` they correspond to the attributes listed in Section property attributes of the ECMAScript Language Specification. They can be one or more of the following bit flags:

- `napi_default`: No explicit attributes are set on the property. By default, a property is read only, not enumerable and not configurable.
- `napi_writable`: The property is writable.
- `napi_enumerable`: The property is enumerable.
- `napi_configurable`: The property is configurable as defined in Section property attributes of the ECMAScript Language Specification.
- `napi_static`: The property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only by `napi_define_class`. It is ignored by `napi_define_properties`.
- `napi_default_method`: Like a method in a JS class, the property is configurable and writable, but not enumerable.
- `napi_default_jsproperty`: Like a property set via assignment in JavaScript, the property is writable, enumerable, and configurable.

##### `napi_property_descriptor`#

    typedef struct {
      // One of utf8name or name should be NULL.
      const char* utf8name;
      napi_value name;

      napi_callback method;
      napi_callback getter;
      napi_callback setter;
      napi_value value;

      napi_property_attributes attributes;
      void* data;
    } napi_property_descriptor;
    c

- `utf8name`: Optional string describing the key for the property, encoded as UTF8. One of `utf8name` or `name` must be provided for the property.
- `name`: Optional `napi_value` that points to a JavaScript string or symbol to be used as the key for the property. One of `utf8name` or `name` must be provided for the property.
- `value`: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, set `getter`, `setter`, `method` and `data` to `NULL` (since these members won't be used).
- `getter`: A function to call when a get access of the property is performed. If this is passed in, set `value` and `method` to `NULL` (since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a Node-API call). `napi_callback` provides more details.
- `setter`: A function to call when a set access of the property is performed. If this is passed in, set `value` and `method` to `NULL` (since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a Node-API call). `napi_callback` provides more details.
- `method`: Set this to make the property descriptor object's `value` property to be a JavaScript function represented by `method`. If this is passed in, set `value`, `getter` and `setter` to `NULL` (since these members won't be used). `napi_callback` provides more details.
- `attributes`: The attributes associated with the particular property. See `napi_property_attributes`.
- `data`: The callback data passed into `method`, `getter` and `setter` if this function is invoked.

#### Functions#

##### `napi_get_property_names`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_property_names(napi_env env,
                                        napi_value object,
                                        napi_value* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to retrieve the properties.
- `[out] result`: A `napi_value` representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate over `result` using `napi_get_array_length` and `napi_get_element`.

Returns `napi_ok` if the API succeeded.

This API returns the names of the enumerable properties of `object` as an array of strings. The properties of `object` whose key is a symbol will not be included.

##### `napi_get_all_property_names`#

Added in: v13.7.0, v12.17.0, v10.20.0N-API Version: 6

    napi_get_all_property_names(napi_env env,
                                napi_value object,
                                napi_key_collection_mode key_mode,
                                napi_key_filter key_filter,
                                napi_key_conversion key_conversion,
                                napi_value* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to retrieve the properties.
- `[in] key_mode`: Whether to retrieve prototype properties as well.
- `[in] key_filter`: Which properties to retrieve (enumerable/readable/writable).
- `[in] key_conversion`: Whether to convert numbered property keys to strings.
- `[out] result`: A `napi_value` representing an array of JavaScript values that represent the property names of the object. `napi_get_array_length` and `napi_get_element` can be used to iterate over `result`.

Returns `napi_ok` if the API succeeded.

This API returns an array containing the names of the available properties of this object.

##### `napi_set_property`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_set_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  napi_value value);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object on which to set the property.
- `[in] key`: The name of the property to set.
- `[in] value`: The property value.

Returns `napi_ok` if the API succeeded.

This API set a property on the `Object` passed in.

##### `napi_get_property`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  napi_value* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to retrieve the property.
- `[in] key`: The name of the property to retrieve.
- `[out] result`: The value of the property.

Returns `napi_ok` if the API succeeded.

This API gets the requested property from the `Object` passed in.

##### `napi_has_property`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_has_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  bool* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to query.
- `[in] key`: The name of the property whose existence to check.
- `[out] result`: Whether the property exists on the object or not.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in has the named property.

##### `napi_delete_property`#

Added in: v8.2.0N-API Version: 1

    napi_status napi_delete_property(napi_env env,
                                     napi_value object,
                                     napi_value key,
                                     bool* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to query.
- `[in] key`: The name of the property to delete.
- `[out] result`: Whether the property deletion succeeded or not. `result` can optionally be ignored by passing `NULL`.

Returns `napi_ok` if the API succeeded.

This API attempts to delete the `key` own property from `object`.

##### `napi_has_own_property`#

Added in: v8.2.0N-API Version: 1

    napi_status napi_has_own_property(napi_env env,
                                      napi_value object,
                                      napi_value key,
                                      bool* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to query.
- `[in] key`: The name of the own property whose existence to check.
- `[out] result`: Whether the own property exists on the object or not.

Returns `napi_ok` if the API succeeded.

This API checks if the `Object` passed in has the named own property. `key` must be a `string` or a `symbol`, or an error will be thrown. Node-API will not perform any conversion between data types.

##### `napi_set_named_property`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_set_named_property(napi_env env,
                                        napi_value object,
                                        const char* utf8Name,
                                        napi_value value);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object on which to set the property.
- `[in] utf8Name`: The name of the property to set.
- `[in] value`: The property value.

Returns `napi_ok` if the API succeeded.

This method is equivalent to calling `napi_set_property` with a `napi_value` created from the string passed in as `utf8Name`.

##### `napi_get_named_property`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_named_property(napi_env env,
                                        napi_value object,
                                        const char* utf8Name,
                                        napi_value* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to retrieve the property.
- `[in] utf8Name`: The name of the property to get.
- `[out] result`: The value of the property.

Returns `napi_ok` if the API succeeded.

This method is equivalent to calling `napi_get_property` with a `napi_value` created from the string passed in as `utf8Name`.

##### `napi_has_named_property`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_has_named_property(napi_env env,
                                        napi_value object,
                                        const char* utf8Name,
                                        bool* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to query.
- `[in] utf8Name`: The name of the property whose existence to check.
- `[out] result`: Whether the property exists on the object or not.

Returns `napi_ok` if the API succeeded.

This method is equivalent to calling `napi_has_property` with a `napi_value` created from the string passed in as `utf8Name`.

##### `napi_set_element`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_set_element(napi_env env,
                                 napi_value object,
                                 uint32_t index,
                                 napi_value value);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to set the properties.
- `[in] index`: The index of the property to set.
- `[in] value`: The property value.

Returns `napi_ok` if the API succeeded.

This API sets an element on the `Object` passed in.

##### `napi_get_element`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_element(napi_env env,
                                 napi_value object,
                                 uint32_t index,
                                 napi_value* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to retrieve the property.
- `[in] index`: The index of the property to get.
- `[out] result`: The value of the property.

Returns `napi_ok` if the API succeeded.

This API gets the element at the requested index.

##### `napi_has_element`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_has_element(napi_env env,
                                 napi_value object,
                                 uint32_t index,
                                 bool* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to query.
- `[in] index`: The index of the property whose existence to check.
- `[out] result`: Whether the property exists on the object or not.

Returns `napi_ok` if the API succeeded.

This API returns if the `Object` passed in has an element at the requested index.

##### `napi_delete_element`#

Added in: v8.2.0N-API Version: 1

    napi_status napi_delete_element(napi_env env,
                                    napi_value object,
                                    uint32_t index,
                                    bool* result);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to query.
- `[in] index`: The index of the property to delete.
- `[out] result`: Whether the element deletion succeeded or not. `result` can optionally be ignored by passing `NULL`.

Returns `napi_ok` if the API succeeded.

This API attempts to delete the specified `index` from `object`.

##### `napi_define_properties`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_define_properties(napi_env env,
                                       napi_value object,
                                       size_t property_count,
                                       const napi_property_descriptor* properties);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object from which to retrieve the properties.
- `[in] property_count`: The number of elements in the `properties` array.
- `[in] properties`: The array of property descriptors.

Returns `napi_ok` if the API succeeded.

This method allows the efficient definition of multiple properties on a given object. The properties are defined using property descriptors (see `napi_property_descriptor`). Given an array of such property descriptors, this API will set the properties on the object one at a time, as defined by `DefineOwnProperty()` (described in Section DefineOwnProperty of the ECMA-262 specification).

##### `napi_object_freeze`#

Added in: v14.14.0, v12.20.0N-API Version: 8

    napi_status napi_object_freeze(napi_env env,
                                   napi_value object);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to freeze.

Returns `napi_ok` if the API succeeded.

This method freezes a given object. This prevents new properties from being added to it, existing properties from being removed, prevents changing the enumerability, configurability, or writability of existing properties, and prevents the values of existing properties from being changed. It also prevents the object's prototype from being changed. This is described in Section 19.1.2.6 of the ECMA-262 specification.

##### `napi_object_seal`#

Added in: v14.14.0, v12.20.0N-API Version: 8

    napi_status napi_object_seal(napi_env env,
                                 napi_value object);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object to seal.

Returns `napi_ok` if the API succeeded.

This method seals a given object. This prevents new properties from being added to it, as well as marking all existing properties as non-configurable. This is described in Section 19.1.2.20 of the ECMA-262 specification.

##### `node_api_set_prototype`#

Added in: v25.4.0, v24.13.1

Stability: 1 - Experimental

    napi_status node_api_set_prototype(napi_env env,
                                       napi_value object,
                                       napi_value value);
    c

- `[in] env`: The environment that the Node-API call is invoked under.
- `[in] object`: The object on which to set the prototype.
- `[in] value`: The prototype value.

Returns `napi_ok` if the API succeeded.

This API sets the prototype of the `Object` passed in.

### Working with JavaScript functions#

Node-API provides a set of APIs that allow JavaScript code to call back into native code. Node-APIs that support calling back into native code take in a callback functions represented by the `napi_callback` type. When the JavaScript VM calls back to native code, the `napi_callback` function provided is invoked. The APIs documented in this section allow the callback function to do the following:

- Get information about the context in which the callback was invoked.
- Get the arguments passed into the callback.
- Return a `napi_value` back from the callback.

Additionally, Node-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.

Any non-`NULL` data which is passed to this API via the `data` field of the `napi_property_descriptor` items can be associated with `object` and freed whenever `object` is garbage-collected by passing both `object` and the data to `napi_add_finalizer`.

#### `napi_call_function`#

Added in: v8.0.0N-API Version: 1

    NAPI_EXTERN napi_status napi_call_function(napi_env env,
                                               napi_value recv,
                                               napi_value func,
                                               size_t argc,
                                               const napi_value* argv,
                                               napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] recv`: The `this` value passed to the called function.
- `[in] func`: `napi_value` representing the JavaScript function to be invoked.
- `[in] argc`: The count of elements in the `argv` array.
- `[in] argv`: Array of `napi_values` representing JavaScript values passed in as arguments to the function.
- `[out] result`: `napi_value` representing the JavaScript object returned.

Returns `napi_ok` if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This is the primary mechanism of calling back from the add-on's native code into JavaScript. For the special case of calling into JavaScript after an async operation, see `napi_make_callback`.

A sample use case might look as follows. Consider the following JavaScript snippet:

    function AddTwo(num) {
      return num + 2;
    }
    global.AddTwo = AddTwo;
    js

Then, the above function can be invoked from a native add-on using the following code:

    // Get the function named "AddTwo" on the global object
    napi_value global, add_two, arg;
    napi_status status = napi_get_global(env, &global);
    if (status != napi_ok) return;

    status = napi_get_named_property(env, global, "AddTwo", &add_two);
    if (status != napi_ok) return;

    // const arg = 1337
    status = napi_create_int32(env, 1337, &arg);
    if (status != napi_ok) return;

    napi_value* argv = &arg;
    size_t argc = 1;

    // AddTwo(arg);
    napi_value return_val;
    status = napi_call_function(env, global, add_two, argc, argv, &return_val);
    if (status != napi_ok) return;

    // Convert the result back to a native type
    int32_t result;
    status = napi_get_value_int32(env, return_val, &result);
    if (status != napi_ok) return;
    c

#### `napi_create_function`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_create_function(napi_env env,
                                     const char* utf8name,
                                     size_t length,
                                     napi_callback cb,
                                     void* data,
                                     napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] utf8Name`: Optional name of the function encoded as UTF8. This is visible within JavaScript as the new function object's `name` property.
- `[in] length`: The length of the `utf8name` in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[in] cb`: The native function which should be called when this function object is invoked. `napi_callback` provides more details.
- `[in] data`: User-provided data context. This will be passed back into the function when invoked later.
- `[out] result`: `napi_value` representing the JavaScript function object for the newly created function.

Returns `napi_ok` if the API succeeded.

This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from JavaScript.

The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.

In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:

    napi_value SayHello(napi_env env, napi_callback_info info) {
      printf("Hello\n");
      return NULL;
    }

    napi_value Init(napi_env env, napi_value exports) {
      napi_status status;

      napi_value fn;
      status = napi_create_function(env, NULL, 0, SayHello, NULL, &fn);
      if (status != napi_ok) return NULL;

      status = napi_set_named_property(env, exports, "sayHello", fn);
      if (status != napi_ok) return NULL;

      return exports;
    }

    NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)
    c

Given the above code, the add-on can be used from JavaScript as follows:

    const myaddon = require('./addon');
    myaddon.sayHello();
    js

The string passed to `require()` is the name of the target in `binding.gyp` responsible for creating the `.node` file.

Any non-`NULL` data which is passed to this API via the `data` parameter can be associated with the resulting JavaScript function (which is returned in the `result` parameter) and freed whenever the function is garbage-collected by passing both the JavaScript function and the data to `napi_add_finalizer`.

JavaScript `Function`s are described in Section Function objects of the ECMAScript Language Specification.

#### `napi_get_cb_info`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_cb_info(napi_env env,
                                 napi_callback_info cbinfo,
                                 size_t* argc,
                                 napi_value* argv,
                                 napi_value* thisArg,
                                 void** data)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] cbinfo`: The callback info passed into the callback function.
- `[in-out] argc`: Specifies the length of the provided `argv` array and receives the actual count of arguments. `argc` can optionally be ignored by passing `NULL`.
- `[out] argv`: C array of `napi_value`s to which the arguments will be copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest of `argv` is filled with `napi_value` values that represent `undefined`. `argv` can optionally be ignored by passing `NULL`.
- `[out] thisArg`: Receives the JavaScript `this` argument for the call. `thisArg` can optionally be ignored by passing `NULL`.
- `[out] data`: Receives the data pointer for the callback. `data` can optionally be ignored by passing `NULL`.

Returns `napi_ok` if the API succeeded.

This method is used within a callback function to retrieve details about the call like the arguments and the `this` pointer from a given callback info.

#### `napi_get_new_target`#

Added in: v8.6.0N-API Version: 1

    napi_status napi_get_new_target(napi_env env,
                                    napi_callback_info cbinfo,
                                    napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] cbinfo`: The callback info passed into the callback function.
- `[out] result`: The `new.target` of the constructor call.

Returns `napi_ok` if the API succeeded.

This API returns the `new.target` of the constructor call. If the current callback is not a constructor call, the result is `NULL`.

#### `napi_new_instance`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_new_instance(napi_env env,
                                  napi_value cons,
                                  size_t argc,
                                  napi_value* argv,
                                  napi_value* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] cons`: `napi_value` representing the JavaScript function to be invoked as a constructor.
- `[in] argc`: The count of elements in the `argv` array.
- `[in] argv`: Array of JavaScript values as `napi_value` representing the arguments to the constructor. If `argc` is zero this parameter may be omitted by passing in `NULL`.
- `[out] result`: `napi_value` representing the JavaScript object returned, which in this case is the constructed object.

This method is used to instantiate a new JavaScript value using a given `napi_value` that represents the constructor for the object. For example, consider the following snippet:

    function MyObject(param) {
      this.param = param;
    }

    const arg = 'hello';
    const value = new MyObject(arg);
    js

The following can be approximated in Node-API using the following snippet:

    // Get the constructor function MyObject
    napi_value global, constructor, arg, value;
    napi_status status = napi_get_global(env, &global);
    if (status != napi_ok) return;

    status = napi_get_named_property(env, global, "MyObject", &constructor);
    if (status != napi_ok) return;

    // const arg = "hello"
    status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &arg);
    if (status != napi_ok) return;

    napi_value* argv = &arg;
    size_t argc = 1;

    // const value = new MyObject(arg)
    status = napi_new_instance(env, constructor, argc, argv, &value);
    c

Returns `napi_ok` if the API succeeded.

### Object wrap#

Node-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.

1. The `napi_define_class` API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the C++ class.
2. When JavaScript code invokes the constructor, the constructor callback uses `napi_wrap` to wrap a new C++ instance in a JavaScript object, then returns the wrapper object.
3. When JavaScript code invokes a method or property accessor on the class, the corresponding `napi_callback` C++ function is invoked. For an instance callback, `napi_unwrap` obtains the C++ instance that is the target of the call.

For wrapped objects it may be difficult to distinguish between a function called on a class prototype and a function called on an instance of a class. A common pattern used to address this problem is to save a persistent reference to the class constructor for later `instanceof` checks.

    napi_value MyClass_constructor = NULL;
    status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);
    assert(napi_ok == status);
    bool is_instance = false;
    status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
    assert(napi_ok == status);
    if (is_instance) {
      // napi_unwrap() ...
    } else {
      // otherwise...
    }
    c

The reference must be freed once it is no longer needed.

There are occasions where `napi_instanceof()` is insufficient for ensuring that a JavaScript object is a wrapper for a certain native type. This is the case especially when wrapped JavaScript objects are passed back into the addon via static methods rather than as the `this` value of prototype methods. In such cases there is a chance that they may be unwrapped incorrectly.

    const myAddon = require('./build/Release/my_addon.node');

    // `openDatabase()` returns a JavaScript object that wraps a native database
    // handle.
    const dbHandle = myAddon.openDatabase();

    // `query()` returns a JavaScript object that wraps a native query handle.
    const queryHandle = myAddon.query(dbHandle, 'Gimme ALL the things!');

    // There is an accidental error in the line below. The first parameter to
    // `myAddon.queryHasRecords()` should be the database handle (`dbHandle`), not
    // the query handle (`query`), so the correct condition for the while-loop
    // should be
    //
    // myAddon.queryHasRecords(dbHandle, queryHandle)
    //
    while (myAddon.queryHasRecords(queryHandle, dbHandle)) {
      // retrieve records
    }
    js

In the above example `myAddon.queryHasRecords()` is a method that accepts two arguments. The first is a database handle and the second is a query handle. Internally, it unwraps the first argument and casts the resulting pointer to a native database handle. It then unwraps the second argument and casts the resulting pointer to a query handle. If the arguments are passed in the wrong order, the casts will work, however, there is a good chance that the underlying database operation will fail, or will even cause an invalid memory access.

To ensure that the pointer retrieved from the first argument is indeed a pointer to a database handle and, similarly, that the pointer retrieved from the second argument is indeed a pointer to a query handle, the implementation of `queryHasRecords()` has to perform a type validation. Retaining the JavaScript class constructor from which the database handle was instantiated and the constructor from which the query handle was instantiated in `napi_ref`s can help, because `napi_instanceof()` can then be used to ensure that the instances passed into `queryHashRecords()` are indeed of the correct type.

Unfortunately, `napi_instanceof()` does not protect against prototype manipulation. For example, the prototype of the database handle instance can be set to the prototype of the constructor for query handle instances. In this case, the database handle instance can appear as a query handle instance, and it will pass the `napi_instanceof()` test for a query handle instance, while still containing a pointer to a database handle.

To this end, Node-API provides type-tagging capabilities.

A type tag is a 128-bit integer unique to the addon. Node-API provides the `napi_type_tag` structure for storing a type tag. When such a value is passed along with a JavaScript object or external stored in a `napi_value` to `napi_type_tag_object()`, the JavaScript object will be "marked" with the type tag. The "mark" is invisible on the JavaScript side. When a JavaScript object arrives into a native binding, `napi_check_object_type_tag()` can be used along with the original type tag to determine whether the JavaScript object was previously "marked" with the type tag. This creates a type-checking capability of a higher fidelity than `napi_instanceof()` can provide, because such type- tagging survives prototype manipulation and addon unloading/reloading.

Continuing the above example, the following skeleton addon implementation illustrates the use of `napi_type_tag_object()` and `napi_check_object_type_tag()`.

    // This value is the type tag for a database handle. The command
    //
    //   uuidgen | sed -r -e 's/-//g' -e 's/(.{16})(.*)/0x\1, 0x\2/'
    //
    // can be used to obtain the two values with which to initialize the structure.
    static const napi_type_tag DatabaseHandleTypeTag = {
      0x1edf75a38336451d, 0xa5ed9ce2e4c00c38
    };

    // This value is the type tag for a query handle.
    static const napi_type_tag QueryHandleTypeTag = {
      0x9c73317f9fad44a3, 0x93c3920bf3b0ad6a
    };

    static napi_value
    openDatabase(napi_env env, napi_callback_info info) {
      napi_status status;
      napi_value result;

      // Perform the underlying action which results in a database handle.
      DatabaseHandle* dbHandle = open_database();

      // Create a new, empty JS object.
      status = napi_create_object(env, &result);
      if (status != napi_ok) return NULL;

      // Tag the object to indicate that it holds a pointer to a `DatabaseHandle`.
      status = napi_type_tag_object(env, result, &DatabaseHandleTypeTag);
      if (status != napi_ok) return NULL;

      // Store the pointer to the `DatabaseHandle` structure inside the JS object.
      status = napi_wrap(env, result, dbHandle, NULL, NULL, NULL);
      if (status != napi_ok) return NULL;

      return result;
    }

    // Later when we receive a JavaScript object purporting to be a database handle
    // we can use `napi_check_object_type_tag()` to ensure that it is indeed such a
    // handle.

    static napi_value
    query(napi_env env, napi_callback_info info) {
      napi_status status;
      size_t argc = 2;
      napi_value argv[2];
      bool is_db_handle;

      status = napi_get_cb_info(env, info, &argc, argv, NULL, NULL);
      if (status != napi_ok) return NULL;

      // Check that the object passed as the first parameter has the previously
      // applied tag.
      status = napi_check_object_type_tag(env,
                                          argv[0],
                                          &DatabaseHandleTypeTag,
                                          &is_db_handle);
      if (status != napi_ok) return NULL;

      // Throw a `TypeError` if it doesn't.
      if (!is_db_handle) {
        // Throw a TypeError.
        return NULL;
      }
    }
    c

#### `napi_define_class`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_define_class(napi_env env,
                                  const char* utf8name,
                                  size_t length,
                                  napi_callback constructor,
                                  void* data,
                                  size_t property_count,
                                  const napi_property_descriptor* properties,
                                  napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] utf8name`: Name of the JavaScript constructor function. For clarity, it is recommended to use the C++ class name when wrapping a C++ class.
- `[in] length`: The length of the `utf8name` in bytes, or `NAPI_AUTO_LENGTH` if it is null-terminated.
- `[in] constructor`: Callback function that handles constructing instances of the class. When wrapping a C++ class, this method must be a static member with the `napi_callback` signature. A C++ class constructor cannot be used. `napi_callback` provides more details.
- `[in] data`: Optional data to be passed to the constructor callback as the `data` property of the callback info.
- `[in] property_count`: Number of items in the `properties` array argument.
- `[in] properties`: Array of property descriptors describing static and instance data properties, accessors, and methods on the class See `napi_property_descriptor`.
- `[out] result`: A `napi_value` representing the constructor function for the class.

Returns `napi_ok` if the API succeeded.

Defines a JavaScript class, including:

- A JavaScript constructor function that has the class name. When wrapping a corresponding C++ class, the callback passed via `constructor` can be used to instantiate a new C++ class instance, which can then be placed inside the JavaScript object instance being constructed using `napi_wrap`.
- Properties on the constructor function whose implementation can call corresponding static data properties, accessors, and methods of the C++ class (defined by property descriptors with the `napi_static` attribute).
- Properties on the constructor function's `prototype` object. When wrapping a C++ class, non-static data properties, accessors, and methods of the C++ class can be called from the static functions given in the property descriptors without the `napi_static` attribute after retrieving the C++ class instance placed inside the JavaScript object instance by using `napi_unwrap`.

When wrapping a C++ class, the C++ constructor callback passed via `constructor` should be a static method on the class that calls the actual class constructor, then wraps the new C++ instance in a JavaScript object, and returns the wrapper object. See `napi_wrap` for details.

The JavaScript constructor function returned from `napi_define_class` is often saved and used later to construct new instances of the class from native code, and/or to check whether provided values are instances of the class. In that case, to prevent the function value from being garbage-collected, a strong persistent reference to it can be created using `napi_create_reference`, ensuring that the reference count is kept >= 1.

Any non-`NULL` data which is passed to this API via the `data` parameter or via the `data` field of the `napi_property_descriptor` array items can be associated with the resulting JavaScript constructor (which is returned in the `result` parameter) and freed whenever the class is garbage-collected by passing both the JavaScript function and the data to `napi_add_finalizer`.

#### `napi_wrap`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_wrap(napi_env env,
                          napi_value js_object,
                          void* native_object,
                          napi_finalize finalize_cb,
                          void* finalize_hint,
                          napi_ref* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] js_object`: The JavaScript object that will be the wrapper for the native object.
- `[in] native_object`: The native instance that will be wrapped in the JavaScript object.
- `[in] finalize_cb`: Optional native callback that can be used to free the native instance when the JavaScript object has been garbage-collected. `napi_finalize` provides more details.
- `[in] finalize_hint`: Optional contextual hint that is passed to the finalize callback.
- `[out] result`: Optional reference to the wrapped object.

Returns `napi_ok` if the API succeeded.

Wraps a native instance in a JavaScript object. The native instance can be retrieved later using `napi_unwrap()`.

When JavaScript code invokes a constructor for a class that was defined using `napi_define_class()`, the `napi_callback` for the constructor is invoked. After constructing an instance of the native class, the callback must then call `napi_wrap()` to wrap the newly constructed instance in the already-created JavaScript object that is the `this` argument to the constructor callback. (That `this` object was created from the constructor function's `prototype`, so it already has definitions of all the instance properties and methods.)

Typically when wrapping a class instance, a finalize callback should be provided that simply deletes the native instance that is received as the `data` argument to the finalize callback.

The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.

Caution: The optional returned reference (if obtained) should be deleted via `napi_delete_reference` ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Finalizer callbacks may be deferred, leaving a window where the object has been garbage collected (and the weak reference is invalid) but the finalizer hasn't been called yet. When using `napi_get_reference_value()` on weak references returned by `napi_wrap()`, you should still handle an empty result.

Calling `napi_wrap()` a second time on an object will return an error. To associate another native instance with the object, use `napi_remove_wrap()` first.

#### `napi_unwrap`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_unwrap(napi_env env,
                            napi_value js_object,
                            void** result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] js_object`: The object associated with the native instance.
- `[out] result`: Pointer to the wrapped native instance.

Returns `napi_ok` if the API succeeded.

Retrieves a native instance that was previously wrapped in a JavaScript object using `napi_wrap()`.

When JavaScript code invokes a method or property accessor on the class, the corresponding `napi_callback` is invoked. If the callback is for an instance method or accessor, then the `this` argument to the callback is the wrapper object; the wrapped C++ instance that is the target of the call can be obtained then by calling `napi_unwrap()` on the wrapper object.

#### `napi_remove_wrap`#

Added in: v8.5.0N-API Version: 1

    napi_status napi_remove_wrap(napi_env env,
                                 napi_value js_object,
                                 void** result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] js_object`: The object associated with the native instance.
- `[out] result`: Pointer to the wrapped native instance.

Returns `napi_ok` if the API succeeded.

Retrieves a native instance that was previously wrapped in the JavaScript object `js_object` using `napi_wrap()` and removes the wrapping. If a finalize callback was associated with the wrapping, it will no longer be called when the JavaScript object becomes garbage-collected.

#### `napi_type_tag_object`#

Added in: v14.8.0, v12.19.0N-API Version: 8

    napi_status napi_type_tag_object(napi_env env,
                                     napi_value js_object,
                                     const napi_type_tag* type_tag);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] js_object`: The JavaScript object or external to be marked.
- `[in] type_tag`: The tag with which the object is to be marked.

Returns `napi_ok` if the API succeeded.

Associates the value of the `type_tag` pointer with the JavaScript object or external. `napi_check_object_type_tag()` can then be used to compare the tag that was attached to the object with one owned by the addon to ensure that the object has the right type.

If the object already has an associated type tag, this API will return `napi_invalid_arg`.

#### `napi_check_object_type_tag`#

Added in: v14.8.0, v12.19.0N-API Version: 8

    napi_status napi_check_object_type_tag(napi_env env,
                                           napi_value js_object,
                                           const napi_type_tag* type_tag,
                                           bool* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] js_object`: The JavaScript object or external whose type tag to examine.
- `[in] type_tag`: The tag with which to compare any tag found on the object.
- `[out] result`: Whether the type tag given matched the type tag on the object. `false` is also returned if no type tag was found on the object.

Returns `napi_ok` if the API succeeded.

Compares the pointer given as `type_tag` with any that can be found on `js_object`. If no tag is found on `js_object` or, if a tag is found but it does not match `type_tag`, then `result` is set to `false`. If a tag is found and it matches `type_tag`, then `result` is set to `true`.

#### `napi_add_finalizer`#

Added in: v8.0.0N-API Version: 5

    napi_status napi_add_finalizer(napi_env env,
                                   napi_value js_object,
                                   void* finalize_data,
                                   node_api_basic_finalize finalize_cb,
                                   void* finalize_hint,
                                   napi_ref* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] js_object`: The JavaScript object to which the native data will be attached.
- `[in] finalize_data`: Optional data to be passed to `finalize_cb`.
- `[in] finalize_cb`: Native callback that will be used to free the native data when the JavaScript object has been garbage-collected. `napi_finalize` provides more details.
- `[in] finalize_hint`: Optional contextual hint that is passed to the finalize callback.
- `[out] result`: Optional reference to the JavaScript object.

Returns `napi_ok` if the API succeeded.

Adds a `napi_finalize` callback which will be called when the JavaScript object in `js_object` has been garbage-collected.

This API can be called multiple times on a single JavaScript object.

Caution: The optional returned reference (if obtained) should be deleted via `napi_delete_reference` ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

##### `node_api_post_finalizer`#

Added in: v21.0.0, v20.10.0, v18.19.0

Stability: 1 - Experimental

    napi_status node_api_post_finalizer(node_api_basic_env env,
                                        napi_finalize finalize_cb,
                                        void* finalize_data,
                                        void* finalize_hint);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] finalize_cb`: Native callback that will be used to free the native data when the JavaScript object has been garbage-collected. `napi_finalize` provides more details.
- `[in] finalize_data`: Optional data to be passed to `finalize_cb`.
- `[in] finalize_hint`: Optional contextual hint that is passed to the finalize callback.

Returns `napi_ok` if the API succeeded.

Schedules a `napi_finalize` callback to be called asynchronously in the event loop.

Normally, finalizers are called while the GC (garbage collector) collects objects. At that point calling any Node-API that may cause changes in the GC state will be disabled and will crash Node.js.

`node_api_post_finalizer` helps to work around this limitation by allowing the add-on to defer calls to such Node-APIs to a point in time outside of the GC finalization.

### Simple asynchronous operations#

Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This allows them to avoid blocking overall execution of the Node.js application.

Node-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.

Node-API defines the `napi_async_work` structure which is used to manage asynchronous workers. Instances are created/deleted with `napi_create_async_work` and `napi_delete_async_work`.

The `execute` and `complete` callbacks are functions that will be invoked when the executor is ready to execute and when it completes its task respectively.

The `execute` function should avoid making any Node-API calls that could result in the execution of JavaScript or interaction with JavaScript objects. Most often, any code that needs to make Node-API calls should be made in `complete` callback instead. Avoid using the `napi_env` parameter in the execute callback as it will likely execute JavaScript.

These functions implement the following interfaces:

    typedef void (*napi_async_execute_callback)(napi_env env,
                                                void* data);
    typedef void (*napi_async_complete_callback)(napi_env env,
                                                 napi_status status,
                                                 void* data);
    c

When these methods are invoked, the `data` parameter passed will be the addon-provided `void*` data that was passed into the `napi_create_async_work` call.

Once created the async worker can be queued for execution using the `napi_queue_async_work` function:

    napi_status napi_queue_async_work(node_api_basic_env env,
                                      napi_async_work work);
    c

`napi_cancel_async_work` can be used if the work needs to be cancelled before the work has started execution.

After calling `napi_cancel_async_work`, the `complete` callback will be invoked with a status value of `napi_cancelled`. The work should not be deleted before the `complete` callback invocation, even when it was cancelled.

#### `napi_create_async_work`#

Added in: v8.0.0N-API Version: 1
History

Version | Changes
--- | ---
v8.6.0 | Added `async_resource` and `async_resource_name` parameters.

    napi_status napi_create_async_work(napi_env env,
                                       napi_value async_resource,
                                       napi_value async_resource_name,
                                       napi_async_execute_callback execute,
                                       napi_async_complete_callback complete,
                                       void* data,
                                       napi_async_work* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] async_resource`: An optional object associated with the async work that will be passed to possible `async_hooks` `init` hooks.
- `[in] async_resource_name`: Identifier for the kind of resource that is being provided for diagnostic information exposed by the `async_hooks` API.
- `[in] execute`: The native function which should be called to execute the logic asynchronously. The given function is called from a worker pool thread and can execute in parallel with the main event loop thread.
- `[in] complete`: The native function which will be called when the asynchronous logic is completed or is cancelled. The given function is called from the main event loop thread. `napi_async_complete_callback` provides more details.
- `[in] data`: User-provided data context. This will be passed back into the execute and complete functions.
- `[out] result`: `napi_async_work*` which is the handle to the newly created async work.

Returns `napi_ok` if the API succeeded.

This API allocates a work object that is used to execute logic asynchronously. It should be freed using `napi_delete_async_work` once the work is no longer required.

`async_resource_name` should be a null-terminated, UTF-8-encoded string.

The `async_resource_name` identifier is provided by the user and should be representative of the type of async work being performed. It is also recommended to apply namespacing to the identifier, e.g. by including the module name. See the `async_hooks` documentation for more information.

#### `napi_delete_async_work`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_delete_async_work(napi_env env,
                                       napi_async_work work);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] work`: The handle returned by the call to `napi_create_async_work`.

Returns `napi_ok` if the API succeeded.

This API frees a previously allocated work object.

This API can be called even if there is a pending JavaScript exception.

#### `napi_queue_async_work`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_queue_async_work(node_api_basic_env env,
                                      napi_async_work work);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] work`: The handle returned by the call to `napi_create_async_work`.

Returns `napi_ok` if the API succeeded.

This API requests that the previously allocated work be scheduled for execution. Once it returns successfully, this API must not be called again with the same `napi_async_work` item or the result will be undefined.

#### `napi_cancel_async_work`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_cancel_async_work(node_api_basic_env env,
                                       napi_async_work work);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] work`: The handle returned by the call to `napi_create_async_work`.

Returns `napi_ok` if the API succeeded.

This API cancels queued work if it has not yet been started. If it has already started executing, it cannot be cancelled and `napi_generic_failure` will be returned. If successful, the `complete` callback will be invoked with a status value of `napi_cancelled`. The work should not be deleted before the `complete` callback invocation, even if it has been successfully cancelled.

This API can be called even if there is a pending JavaScript exception.

### Custom asynchronous operations#

The simple asynchronous work APIs above may not be appropriate for every scenario. When using any other asynchronous mechanism, the following APIs are necessary to ensure an asynchronous operation is properly tracked by the runtime.

#### `napi_async_init`#

Added in: v8.6.0N-API Version: 1
History

Version | Changes
--- | ---
v25.0.0 | The `async_resource` object will now be held as a strong reference.

    napi_status napi_async_init(napi_env env,
                                napi_value async_resource,
                                napi_value async_resource_name,
                                napi_async_context* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] async_resource`: Object associated with the async work that will be passed to possible `async_hooks` `init` hooks and can be accessed by `async_hooks.executionAsyncResource()`.
- `[in] async_resource_name`: Identifier for the kind of resource that is being provided for diagnostic information exposed by the `async_hooks` API.
- `[out] result`: The initialized async context.

Returns `napi_ok` if the API succeeded.

In order to retain ABI compatibility with previous versions, passing `NULL` for `async_resource` does not result in an error. However, this is not recommended as this will result in undesirable behavior with `async_hooks` `init` hooks and `async_hooks.executionAsyncResource()` as the resource is now required by the underlying `async_hooks` implementation in order to provide the linkage between async callbacks.

Previous versions of this API were not maintaining a strong reference to `async_resource` while the `napi_async_context` object existed and instead expected the caller to hold a strong reference. This has been changed, as a corresponding call to `napi_async_destroy` for every call to `napi_async_init()` is a requirement in any case to avoid memory leaks.

#### `napi_async_destroy`#

Added in: v8.6.0N-API Version: 1

    napi_status napi_async_destroy(napi_env env,
                                   napi_async_context async_context);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] async_context`: The async context to be destroyed.

Returns `napi_ok` if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

#### `napi_make_callback`#

Added in: v8.0.0N-API Version: 1
History

Version | Changes
--- | ---
v8.6.0 | Added `async_context` parameter.

    NAPI_EXTERN napi_status napi_make_callback(napi_env env,
                                               napi_async_context async_context,
                                               napi_value recv,
                                               napi_value func,
                                               size_t argc,
                                               const napi_value* argv,
                                               napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] async_context`: Context for the async operation that is invoking the callback. This should normally be a value previously obtained from `napi_async_init`. In order to retain ABI compatibility with previous versions, passing `NULL` for `async_context` does not result in an error. However, this results in incorrect operation of async hooks. Potential issues include loss of async context when using the `AsyncLocalStorage` API.
- `[in] recv`: The `this` value passed to the called function.
- `[in] func`: `napi_value` representing the JavaScript function to be invoked.
- `[in] argc`: The count of elements in the `argv` array.
- `[in] argv`: Array of JavaScript values as `napi_value` representing the arguments to the function. If `argc` is zero this parameter may be omitted by passing in `NULL`.
- `[out] result`: `napi_value` representing the JavaScript object returned.

Returns `napi_ok` if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This API is similar to `napi_call_function`. However, it is used to call from native code back into JavaScript after returning from an async operation (when there is no other script on the stack). It is a fairly simple wrapper around `node::MakeCallback`.

Note it is not necessary to use `napi_make_callback` from within a `napi_async_complete_callback`; in that situation the callback's async context has already been set up, so a direct call to `napi_call_function` is sufficient and appropriate. Use of the `napi_make_callback` function may be required when implementing custom async behavior that does not use `napi_create_async_work`.

Any `process.nextTick`s or Promises scheduled on the microtask queue by JavaScript during the callback are ran before returning back to C/C++.

#### `napi_open_callback_scope`#

Added in: v9.6.0N-API Version: 3

    NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env,
                                                     napi_value resource_object,
                                                     napi_async_context context,
                                                     napi_callback_scope* result)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] resource_object`: An object associated with the async work that will be passed to possible `async_hooks` `init` hooks. This parameter has been deprecated and is ignored at runtime. Use the `async_resource` parameter in `napi_async_init` instead.
- `[in] context`: Context for the async operation that is invoking the callback. This should be a value previously obtained from `napi_async_init`.
- `[out] result`: The newly created scope.

There are cases (for example, resolving promises) where it is necessary to have the equivalent of the scope associated with a callback in place when making certain Node-API calls. If there is no other script on the stack the `napi_open_callback_scope` and `napi_close_callback_scope` functions can be used to open/close the required scope.

#### `napi_close_callback_scope`#

Added in: v9.6.0N-API Version: 3

    NAPI_EXTERN napi_status napi_close_callback_scope(napi_env env,
                                                      napi_callback_scope scope)
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] scope`: The scope to be closed.

This API can be called even if there is a pending JavaScript exception.

### Version management#

#### `napi_get_node_version`#

Added in: v8.4.0N-API Version: 1

    typedef struct {
      uint32_t major;
      uint32_t minor;
      uint32_t patch;
      const char* release;
    } napi_node_version;

    napi_status napi_get_node_version(node_api_basic_env env,
                                      const napi_node_version** version);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] version`: A pointer to version information for Node.js itself.

Returns `napi_ok` if the API succeeded.

This function fills the `version` struct with the major, minor, and patch version of Node.js that is currently running, and the `release` field with the value of `process.release.name`.

The returned buffer is statically allocated and does not need to be freed.

#### `napi_get_version`#

Added in: v8.0.0N-API Version: 1

    napi_status napi_get_version(node_api_basic_env env,
                                 uint32_t* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: The highest version of Node-API supported.

Returns `napi_ok` if the API succeeded.

This API returns the highest Node-API version supported by the Node.js runtime. Node-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:

- Call `napi_get_version()` to determine if the API is available.
- If available, dynamically load a pointer to the function using `uv_dlsym()`.
- Use the dynamically loaded pointer to invoke the function.
- If the function is not available, provide an alternate implementation that does not use the function.

### Memory management#

#### `napi_adjust_external_memory`#

Added in: v8.5.0N-API Version: 1

    NAPI_EXTERN napi_status napi_adjust_external_memory(node_api_basic_env env,
                                                        int64_t change_in_bytes,
                                                        int64_t* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] change_in_bytes`: The change in externally allocated memory that is kept alive by JavaScript objects.
- `[out] result`: The adjusted value. This value should reflect the total amount of external memory with the given `change_in_bytes` included. The absolute value of the returned value should not be depended on. For example, implementations may use a single counter for all addons, or a counter for each addon.

Returns `napi_ok` if the API succeeded.

This function gives the runtime an indication of the amount of externally allocated memory that is kept alive by JavaScript objects (i.e. a JavaScript object that points to its own memory allocated by a native addon). Registering externally allocated memory may, but is not guaranteed to, trigger global garbage collections more often than it would otherwise.

This function is expected to be called in a manner such that an addon does not decrease the external memory more than it has increased the external memory.

### Promises#

Node-API provides facilities for creating `Promise` objects as described in Section Promise objects of the ECMA specification. It implements promises as a pair of objects. When a promise is created by `napi_create_promise()`, a "deferred" object is created and returned alongside the `Promise`. The deferred object is bound to the created `Promise` and is the only means to resolve or reject the `Promise` using `napi_resolve_deferred()` or `napi_reject_deferred()`. The deferred object that is created by `napi_create_promise()` is freed by `napi_resolve_deferred()` or `napi_reject_deferred()`. The `Promise` object may be returned to JavaScript where it can be used in the usual fashion.

For example, to create a promise and pass it to an asynchronous worker:

    napi_deferred deferred;
    napi_value promise;
    napi_status status;

    // Create the promise.
    status = napi_create_promise(env, &deferred, &promise);
    if (status != napi_ok) return NULL;

    // Pass the deferred to a function that performs an asynchronous action.
    do_something_asynchronous(deferred);

    // Return the promise to JS
    return promise;
    c

The above function `do_something_asynchronous()` would perform its asynchronous action and then it would resolve or reject the deferred, thereby concluding the promise and freeing the deferred:

    napi_deferred deferred;
    napi_value undefined;
    napi_status status;

    // Create a value with which to conclude the deferred.
    status = napi_get_undefined(env, &undefined);
    if (status != napi_ok) return NULL;

    // Resolve or reject the promise associated with the deferred depending on
    // whether the asynchronous action succeeded.
    if (asynchronous_action_succeeded) {
      status = napi_resolve_deferred(env, deferred, undefined);
    } else {
      status = napi_reject_deferred(env, deferred, undefined);
    }
    if (status != napi_ok) return NULL;

    // At this point the deferred has been freed, so we should assign NULL to it.
    deferred = NULL;
    c

#### `napi_create_promise`#

Added in: v8.5.0N-API Version: 1

    napi_status napi_create_promise(napi_env env,
                                    napi_deferred* deferred,
                                    napi_value* promise);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] deferred`: A newly created deferred object which can later be passed to `napi_resolve_deferred()` or `napi_reject_deferred()` to resolve resp. reject the associated promise.
- `[out] promise`: The JavaScript promise associated with the deferred object.

Returns `napi_ok` if the API succeeded.

This API creates a deferred object and a JavaScript promise.

#### `napi_resolve_deferred`#

Added in: v8.5.0N-API Version: 1

    napi_status napi_resolve_deferred(napi_env env,
                                      napi_deferred deferred,
                                      napi_value resolution);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] deferred`: The deferred object whose associated promise to resolve.
- `[in] resolution`: The value with which to resolve the promise.

This API resolves a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to resolve JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using `napi_create_promise()` and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

#### `napi_reject_deferred`#

Added in: v8.5.0N-API Version: 1

    napi_status napi_reject_deferred(napi_env env,
                                     napi_deferred deferred,
                                     napi_value rejection);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] deferred`: The deferred object whose associated promise to resolve.
- `[in] rejection`: The value with which to reject the promise.

This API rejects a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to reject JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using `napi_create_promise()` and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

#### `napi_is_promise`#

Added in: v8.5.0N-API Version: 1

    napi_status napi_is_promise(napi_env env,
                                napi_value value,
                                bool* is_promise);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] value`: The value to examine
- `[out] is_promise`: Flag indicating whether `promise` is a native promise object (that is, a promise object created by the underlying engine).

### Script execution#

Node-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine.

#### `napi_run_script`#

Added in: v8.5.0N-API Version: 1

    NAPI_EXTERN napi_status napi_run_script(napi_env env,
                                            napi_value script,
                                            napi_value* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] script`: A JavaScript string containing the script to execute.
- `[out] result`: The value resulting from having executed the script.

This function executes a string of JavaScript code and returns its result with the following caveats:

- Unlike `eval`, this function does not allow the script to access the current lexical scope, and therefore also does not allow to access the module scope, meaning that pseudo-globals such as `require` will not be available.
- The script can access the global scope. Function and `var` declarations in the script will be added to the `global` object. Variable declarations made using `let` and `const` will be visible globally, but will not be added to the `global` object.
- The value of `this` is `global` within the script.

### libuv event loop#

Node-API provides a function for getting the current event loop associated with a specific `napi_env`.

#### `napi_get_uv_event_loop`#

Added in: v9.3.0, v8.10.0N-API Version: 2

    NAPI_EXTERN napi_status napi_get_uv_event_loop(node_api_basic_env env,
                                                   struct uv_loop_s** loop);
    c

- `[in] env`: The environment that the API is invoked under.
- `[out] loop`: The current libuv loop instance.

Note: While libuv only guarantees ABI stability in a major version, its use may result in an addon that does not work across Node.js major versions.

ThreadSafeFunction is an ABI-stable alternative for many use cases to calling into the JavaScript thread from another thread.

### Asynchronous thread-safe function calls#

JavaScript functions can normally only be called from a native addon's main thread. If an addon creates additional threads, then Node-API functions that require a `napi_env`, `napi_value`, or `napi_ref` must not be called from those threads.

When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this.

These APIs provide the type `napi_threadsafe_function` as well as APIs to create, destroy, and call objects of this type. `napi_create_threadsafe_function()` creates a persistent reference to a `napi_value` that holds a JavaScript function which can be called from multiple threads. The calls happen asynchronously. This means that values with which the JavaScript callback is to be called will be placed in a queue, and, for each value in the queue, a call will eventually be made to the JavaScript function.

Upon creation of a `napi_threadsafe_function` a `napi_finalize` callback can be provided. This callback will be invoked on the main thread when the thread-safe function is about to be destroyed. It receives the context and the finalize data given during construction, and provides an opportunity for cleaning up after the threads e.g. by calling `uv_thread_join()`. Aside from the main loop thread, no threads should be using the thread-safe function after the finalize callback completes.

The `context` given during the call to `napi_create_threadsafe_function()` can be retrieved from any thread with a call to `napi_get_threadsafe_function_context()`.

#### Calling a thread-safe function#

`napi_call_threadsafe_function()` can be used for initiating a call into JavaScript. `napi_call_threadsafe_function()` accepts a parameter which controls whether the API behaves blockingly. If set to `napi_tsfn_nonblocking`, the API behaves non-blockingly, returning `napi_queue_full` if the queue was full, preventing data from being successfully added to the queue. If set to `napi_tsfn_blocking`, the API blocks until space becomes available in the queue. `napi_call_threadsafe_function()` never blocks if the thread-safe function was created with a maximum queue size of 0.

`napi_call_threadsafe_function()` should not be called with `napi_tsfn_blocking` from a JavaScript thread, because, if the queue is full, it may cause the JavaScript thread to deadlock.

The actual call into JavaScript is controlled by the callback given via the `call_js_cb` parameter. `call_js_cb` is invoked on the main thread once for each value that was placed into the queue by a successful call to `napi_call_threadsafe_function()`. If such a callback is not given, a default callback will be used, and the resulting JavaScript call will have no arguments. The `call_js_cb` callback receives the JavaScript function to call as a `napi_value` in its parameters, as well as the `void*` context pointer used when creating the `napi_threadsafe_function`, and the next data pointer that was created by one of the secondary threads. The callback can then use an API such as `napi_call_function()` to call into JavaScript.

The callback may also be invoked with `env` and `call_js_cb` both set to `NULL` to indicate that calls into JavaScript are no longer possible, while items remain in the queue that may need to be freed. This normally occurs when the Node.js process exits while there is a thread-safe function still active.

It is not necessary to call into JavaScript via `napi_make_callback()` because Node-API runs `call_js_cb` in a context appropriate for callbacks.

Zero or more queued items may be invoked in each tick of the event loop. Applications should not depend on a specific behavior other than progress in invoking callbacks will be made and events will be invoked as time moves forward.

#### Reference counting of thread-safe functions#

Threads can be added to and removed from a `napi_threadsafe_function` object during its existence. Thus, in addition to specifying an initial number of threads upon creation, `napi_acquire_threadsafe_function` can be called to indicate that a new thread will start making use of the thread-safe function. Similarly, `napi_release_threadsafe_function` can be called to indicate that an existing thread will stop making use of the thread-safe function.

`napi_threadsafe_function` objects are destroyed when every thread which uses the object has called `napi_release_threadsafe_function()` or has received a return status of `napi_closing` in response to a call to `napi_call_threadsafe_function`. The queue is emptied before the `napi_threadsafe_function` is destroyed. `napi_release_threadsafe_function()` should be the last API call made in conjunction with a given `napi_threadsafe_function`, because after the call completes, there is no guarantee that the `napi_threadsafe_function` is still allocated. For the same reason, do not use a thread-safe function after receiving a return value of `napi_closing` in response to a call to `napi_call_threadsafe_function`. Data associated with the `napi_threadsafe_function` can be freed in its `napi_finalize` callback which was passed to `napi_create_threadsafe_function()`. The parameter `initial_thread_count` of `napi_create_threadsafe_function` marks the initial number of acquisitions of the thread-safe functions, instead of calling `napi_acquire_threadsafe_function` multiple times at creation.

Once the number of threads making use of a `napi_threadsafe_function` reaches zero, no further threads can start making use of it by calling `napi_acquire_threadsafe_function()`. In fact, all subsequent API calls associated with it, except `napi_release_threadsafe_function()`, will return an error value of `napi_closing`.

The thread-safe function can be "aborted" by giving a value of `napi_tsfn_abort` to `napi_release_threadsafe_function()`. This will cause all subsequent APIs associated with the thread-safe function except `napi_release_threadsafe_function()` to return `napi_closing` even before its reference count reaches zero. In particular, `napi_call_threadsafe_function()` will return `napi_closing`, thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of `napi_closing` from `napi_call_threadsafe_function()` a thread must not use the thread-safe function anymore because it is no longer guaranteed to be allocated.

#### Deciding whether to keep the process running#

Similarly to libuv handles, thread-safe functions can be "referenced" and "unreferenced". A "referenced" thread-safe function will cause the event loop on the thread on which it is created to remain alive until the thread-safe function is destroyed. In contrast, an "unreferenced" thread-safe function will not prevent the event loop from exiting. The APIs `napi_ref_threadsafe_function` and `napi_unref_threadsafe_function` exist for this purpose.

Neither does `napi_unref_threadsafe_function` mark the thread-safe functions as able to be destroyed nor does `napi_ref_threadsafe_function` prevent it from being destroyed.

#### `napi_create_threadsafe_function`#

Added in: v10.6.0N-API Version: 4
History

Version | Changes
--- | ---
v12.6.0, v10.17.0 | Made `func` parameter optional with custom `call_js_cb`.

    NAPI_EXTERN napi_status
    napi_create_threadsafe_function(napi_env env,
                                    napi_value func,
                                    napi_value async_resource,
                                    napi_value async_resource_name,
                                    size_t max_queue_size,
                                    size_t initial_thread_count,
                                    void* thread_finalize_data,
                                    napi_finalize thread_finalize_cb,
                                    void* context,
                                    napi_threadsafe_function_call_js call_js_cb,
                                    napi_threadsafe_function* result);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] func`: An optional JavaScript function to call from another thread. It must be provided if `NULL` is passed to `call_js_cb`.
- `[in] async_resource`: An optional object associated with the async work that will be passed to possible `async_hooks` `init` hooks.
- `[in] async_resource_name`: A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by the `async_hooks` API.
- `[in] max_queue_size`: Maximum size of the queue. `0` for no limit.
- `[in] initial_thread_count`: The initial number of acquisitions, i.e. the initial number of threads, including the main thread, which will be making use of this function.
- `[in] thread_finalize_data`: Optional data to be passed to `thread_finalize_cb`.
- `[in] thread_finalize_cb`: Optional function to call when the `napi_threadsafe_function` is being destroyed.
- `[in] context`: Optional data to attach to the resulting `napi_threadsafe_function`.
- `[in] call_js_cb`: Optional callback which calls the JavaScript function in response to a call on a different thread. This callback will be called on the main thread. If not given, the JavaScript function will be called with no parameters and with `undefined` as its `this` value. `napi_threadsafe_function_call_js` provides more details.
- `[out] result`: The asynchronous thread-safe JavaScript function.

Change History:

- Version 10 (`NAPI_VERSION` is defined as `10` or higher):

  Uncaught exceptions thrown in `call_js_cb` are handled with the `'uncaughtException'` event, instead of being ignored.

#### `napi_get_threadsafe_function_context`#

Added in: v10.6.0N-API Version: 4

    NAPI_EXTERN napi_status
    napi_get_threadsafe_function_context(napi_threadsafe_function func,
                                         void** result);
    c

- `[in] func`: The thread-safe function for which to retrieve the context.
- `[out] result`: The location where to store the context.

This API may be called from any thread which makes use of `func`.

#### `napi_call_threadsafe_function`#

Added in: v10.6.0N-API Version: 4
History

Version | Changes
--- | ---
v14.5.0 | Support for `napi_would_deadlock` has been reverted.
v14.1.0 | Return `napi_would_deadlock` when called with `napi_tsfn_blocking` from the main thread or a worker thread and the queue is full.

    NAPI_EXTERN napi_status
    napi_call_threadsafe_function(napi_threadsafe_function func,
                                  void* data,
                                  napi_threadsafe_function_call_mode is_blocking);
    c

- `[in] func`: The asynchronous thread-safe JavaScript function to invoke.
- `[in] data`: Data to send into JavaScript via the callback `call_js_cb` provided during the creation of the thread-safe JavaScript function.
- `[in] is_blocking`: Flag whose value can be either `napi_tsfn_blocking` to indicate that the call should block if the queue is full or `napi_tsfn_nonblocking` to indicate that the call should return immediately with a status of `napi_queue_full` whenever the queue is full.

This API should not be called with `napi_tsfn_blocking` from a JavaScript thread, because, if the queue is full, it may cause the JavaScript thread to deadlock.

This API will return `napi_closing` if `napi_release_threadsafe_function()` was called with `abort` set to `napi_tsfn_abort` from any thread. The value is only added to the queue if the API returns `napi_ok`.

This API may be called from any thread which makes use of `func`.

#### `napi_acquire_threadsafe_function`#

Added in: v10.6.0N-API Version: 4

    NAPI_EXTERN napi_status
    napi_acquire_threadsafe_function(napi_threadsafe_function func);
    c

- `[in] func`: The asynchronous thread-safe JavaScript function to start making use of.

A thread should call this API before passing `func` to any other thread-safe function APIs to indicate that it will be making use of `func`. This prevents `func` from being destroyed when all other threads have stopped making use of it.

This API may be called from any thread which will start making use of `func`.

#### `napi_release_threadsafe_function`#

Added in: v10.6.0N-API Version: 4

    NAPI_EXTERN napi_status
    napi_release_threadsafe_function(napi_threadsafe_function func,
                                     napi_threadsafe_function_release_mode mode);
    c

- `[in] func`: The asynchronous thread-safe JavaScript function whose reference count to decrement.
- `[in] mode`: Flag whose value can be either `napi_tsfn_release` to indicate that the current thread will make no further calls to the thread-safe function, or `napi_tsfn_abort` to indicate that in addition to the current thread, no other thread should make any further calls to the thread-safe function. If set to `napi_tsfn_abort`, further calls to `napi_call_threadsafe_function()` will return `napi_closing`, and no further values will be placed in the queue.

A thread should call this API when it stops making use of `func`. Passing `func` to any thread-safe APIs after having called this API has undefined results, as `func` may have been destroyed.

This API may be called from any thread which will stop making use of `func`.

#### `napi_ref_threadsafe_function`#

Added in: v10.6.0N-API Version: 4

    NAPI_EXTERN napi_status
    napi_ref_threadsafe_function(node_api_basic_env env, napi_threadsafe_function func);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] func`: The thread-safe function to reference.

This API is used to indicate that the event loop running on the main thread should not exit until `func` has been destroyed. Similar to `uv_ref` it is also idempotent.

Neither does `napi_unref_threadsafe_function` mark the thread-safe functions as able to be destroyed nor does `napi_ref_threadsafe_function` prevent it from being destroyed. `napi_acquire_threadsafe_function` and `napi_release_threadsafe_function` are available for that purpose.

This API may only be called from the main thread.

#### `napi_unref_threadsafe_function`#

Added in: v10.6.0N-API Version: 4

    NAPI_EXTERN napi_status
    napi_unref_threadsafe_function(node_api_basic_env env, napi_threadsafe_function func);
    c

- `[in] env`: The environment that the API is invoked under.
- `[in] func`: The thread-safe function to unreference.

This API is used to indicate that the event loop running on the main thread may exit before `func` is destroyed. Similar to `uv_unref` it is also idempotent.

This API may only be called from the main thread.

### Miscellaneous utilities#

#### `node_api_get_module_file_name`#

Added in: v15.9.0, v14.18.0, v12.22.0N-API Version: 9

    NAPI_EXTERN napi_status
    node_api_get_module_file_name(node_api_basic_env env, const char** result);

    c

- `[in] env`: The environment that the API is invoked under.
- `[out] result`: A URL containing the absolute path of the location from which the add-on was loaded. For a file on the local file system it will start with `file://`. The string is null-terminated and owned by `env` and must thus not be modified or freed.

`result` may be an empty string if the add-on loading process fails to establish the add-on's file name during loading.

## OS#

Source Code: lib/os.js

Stability: 2 - Stable

The `node:os` module provides operating system-related utility methods and properties. It can be accessed using:

    import os from 'node:os';
    const os = require('node:os');
    javascript

### `os.EOL`#

Added in: v0.7.8

- Type: `<string>`

The operating system-specific end-of-line marker.

- `\n` on POSIX
- `\r\n` on Windows

### `os.availableParallelism()`#

Added in: v19.4.0, v18.14.0

- Returns: `<integer>`

Returns an estimate of the default amount of parallelism a program should use. Always returns a value greater than zero.

This function is a small wrapper about libuv's `uv_available_parallelism()`.

### `os.arch()`#

Added in: v0.5.0

- Returns: `<string>`

Returns the operating system CPU architecture for which the Node.js binary was compiled. Possible values are `'arm'`, `'arm64'`, `'ia32'`, `'loong64'`, `'mips'`, `'mipsel'`, `'ppc64'`, `'riscv64'`, `'s390x'`, and `'x64'`.

The return value is equivalent to `process.arch`.

### `os.constants`#

Added in: v6.3.0

- Type: `<Object>`

Contains commonly used operating system-specific constants for error codes, process signals, and so on. The specific constants defined are described in OS constants.

### `os.cpus()`#

Added in: v0.3.3

- Returns: `<Object>`[]

Returns an array of objects containing information about each logical CPU core. The array will be empty if no CPU information is available, such as if the `/proc` file system is unavailable.

The properties included on each object include:

- `model` `<string>`
- `speed` `<number>` (in MHz)
- `times` `<Object>`
  - `user` `<number>` The number of milliseconds the CPU has spent in user mode.
  - `nice` `<number>` The number of milliseconds the CPU has spent in nice mode.
  - `sys` `<number>` The number of milliseconds the CPU has spent in sys mode.
  - `idle` `<number>` The number of milliseconds the CPU has spent in idle mode.
  - `irq` `<number>` The number of milliseconds the CPU has spent in irq mode.

    [
      {
        model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
        speed: 2926,
        times: {
          user: 252020,
          nice: 0,
          sys: 30340,
          idle: 1070356870,
          irq: 0,
        },
      },
      {
        model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
        speed: 2926,
        times: {
          user: 306960,
          nice: 0,
          sys: 26980,
          idle: 1071569080,
          irq: 0,
        },
      },
      {
        model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
        speed: 2926,
        times: {
          user: 248450,
          nice: 0,
          sys: 21750,
          idle: 1070919370,
          irq: 0,
        },
      },
      {
        model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
        speed: 2926,
        times: {
          user: 256880,
          nice: 0,
          sys: 19430,
          idle: 1070905480,
          irq: 20,
        },
      },
    ];
    js

`nice` values are POSIX-only. On Windows, the `nice` values of all processors are always 0.

`os.cpus().length` should not be used to calculate the amount of parallelism available to an application. Use `os.availableParallelism()` for this purpose.

### `os.devNull`#

Added in: v16.3.0, v14.18.0

- Type: `<string>`

The platform-specific file path of the null device.

- `\\.\nul` on Windows
- `/dev/null` on POSIX

### `os.endianness()`#

Added in: v0.9.4

- Returns: `<string>`

Returns a string identifying the endianness of the CPU for which the Node.js binary was compiled.

Possible values are `'BE'` for big endian and `'LE'` for little endian.

### `os.freemem()`#

Added in: v0.3.3

- Returns: `<integer>`

Returns the amount of free system memory in bytes as an integer.

### `os.getPriority([pid])`#

Added in: v10.10.0

- `pid` `<integer>` The process ID to retrieve scheduling priority for. Default: `0`.
- Returns: `<integer>`

Returns the scheduling priority for the process specified by `pid`. If `pid` is not provided or is `0`, the priority of the current process is returned.

### `os.homedir()`#

Added in: v2.3.0

- Returns: `<string>`

Returns the string path of the current user's home directory.

On POSIX, it uses the `$HOME` environment variable if defined. Otherwise it uses the effective UID to look up the user's home directory.

On Windows, it uses the `USERPROFILE` environment variable if defined. Otherwise it uses the path to the profile directory of the current user.

### `os.hostname()`#

Added in: v0.3.3

- Returns: `<string>`

Returns the host name of the operating system as a string.

### `os.loadavg()`#

Added in: v0.3.3

- Returns: `<number>`[]

Returns an array containing the 1, 5, and 15 minute load averages.

The load average is a measure of system activity calculated by the operating system and expressed as a fractional number.

The load average is a Unix-specific concept. On Windows, the return value is always `[0, 0, 0]`.

### `os.machine()`#

Added in: v18.9.0, v16.18.0

- Returns: `<string>`

Returns the machine type as a string, such as `arm`, `arm64`, `aarch64`, `mips`, `mips64`, `ppc64`, `ppc64le`, `s390x`, `i386`, `i686`, `x86_64`.

On POSIX systems, the machine type is determined by calling `uname(3)`. On Windows, `RtlGetVersion()` is used, and if it is not available, `GetVersionExW()` will be used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

### `os.networkInterfaces()`#

Added in: v0.6.0
History

Version | Changes
--- | ---
v18.4.0 | The `family` property now returns a string instead of a number.
v18.0.0 | The `family` property now returns a number instead of a string.

- Returns: `<Object>`

Returns an object containing network interfaces that have been assigned a network address.

Each key on the returned object identifies a network interface. The associated value is an array of objects that each describe an assigned network address.

The properties available on the assigned network address object include:

- `address` `<string>` The assigned IPv4 or IPv6 address
- `netmask` `<string>` The IPv4 or IPv6 network mask
- `family` `<string>` Either `IPv4` or `IPv6`
- `mac` `<string>` The MAC address of the network interface
- `internal` `<boolean>` `true` if the network interface is a loopback or similar interface that is not remotely accessible; otherwise `false`
- `scopeid` `<number>` The numeric IPv6 scope ID (only specified when `family` is `IPv6`)
- `cidr` `<string>` The assigned IPv4 or IPv6 address with the routing prefix in CIDR notation. If the `netmask` is invalid, this property is set to `null`.

    {
      "lo:": [
        {
          "address:": "127.0.0.1",
          "netmask:": "255.0.0.0",
          "family:": "IPv4",
          "mac:": "00:00:00:00:00:00",
          "internal:": true,
          "cidr:": "127.0.0.1/8"
        },
        {
          "address:": "::1",
          "netmask:": "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff",
          "family:": "IPv6",
          "mac:": "00:00:00:00:00:00",
          "scopeid:": 0,
          "internal:": true,
          "cidr:": "::1/128"
        }
      ],
      "eth0:": [
        {
          "address:": "192.168.1.108",
          "netmask:": "255.255.255.0",
          "family:": "IPv4",
          "mac:": "01:02:03:0a:0b:0c",
          "internal:": false,
          "cidr:": "192.168.1.108/24"
        },
        {
          "address:": "fe80::a00:27ff:fe4e:66a1",
          "netmask:": "ffff:ffff:ffff:ffff::",
          "family:": "IPv6",
          "mac:": "01:02:03:0a:0b:0c",
          "scopeid:": 1,
          "internal:": false,
          "cidr:": "fe80::a00:27ff:fe4e:66a1/64"
        }
      ]
    }
    json

### `os.platform()`#

Added in: v0.5.0

- Returns: `<string>`

Returns a string identifying the operating system platform for which the Node.js binary was compiled. The value is set at compile time. Possible values are `'aix'`, `'darwin'`, `'freebsd'`,`'linux'`, `'openbsd'`, `'sunos'`, and `'win32'`.

The return value is equivalent to `process.platform`.

The value `'android'` may also be returned if Node.js is built on the Android operating system. Android support is experimental.

### `os.release()`#

Added in: v0.3.3

- Returns: `<string>`

Returns the operating system as a string.

On POSIX systems, the operating system release is determined by calling `uname(3)`. On Windows, `GetVersionExW()` is used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

### `os.setPriority([pid, ]priority)`#

Added in: v10.10.0

- `pid` `<integer>` The process ID to set scheduling priority for. Default: `0`.
- `priority` `<integer>` The scheduling priority to assign to the process.

Attempts to set the scheduling priority for the process specified by `pid`. If `pid` is not provided or is `0`, the process ID of the current process is used.

The `priority` input must be an integer between `-20` (high priority) and `19` (low priority). Due to differences between Unix priority levels and Windows priority classes, `priority` is mapped to one of six priority constants in `os.constants.priority`. When retrieving a process priority level, this range mapping may cause the return value to be slightly different on Windows. To avoid confusion, set `priority` to one of the priority constants.

On Windows, setting priority to `PRIORITY_HIGHEST` requires elevated user privileges. Otherwise the set priority will be silently reduced to `PRIORITY_HIGH`.

### `os.tmpdir()`#

Added in: v0.9.9
History

Version | Changes
--- | ---
v2.0.0 | This function is now cross-platform consistent and no longer returns a path with a trailing slash on any platform.

- Returns: `<string>`

Returns the operating system's default directory for temporary files as a string.

On Windows, the result can be overridden by `TEMP` and `TMP` environment variables, and `TEMP` takes precedence over `TMP`. If neither is set, it defaults to `%SystemRoot%\temp` or `%windir%\temp`.

On non-Windows platforms, `TMPDIR`, `TMP` and `TEMP` environment variables will be checked to override the result of this method, in the described order. If none of them is set, it defaults to `/tmp`.

Some operating system distributions would either configure `TMPDIR` (non-Windows) or `TEMP` and `TMP` (Windows) by default without additional configurations by the system administrators. The result of `os.tmpdir()` typically reflects the system preference unless it's explicitly overridden by the users.

### `os.totalmem()`#

Added in: v0.3.3

- Returns: `<integer>`

Returns the total amount of system memory in bytes as an integer.

### `os.type()`#

Added in: v0.3.3

- Returns: `<string>`

Returns the operating system name as returned by `uname(3)`. For example, it returns `'Linux'` on Linux, `'Darwin'` on macOS, and `'Windows_NT'` on Windows.

See https://en.wikipedia.org/wiki/Uname#Examples for additional information about the output of running `uname(3)` on various operating systems.

### `os.uptime()`#

Added in: v0.3.3
History

Version | Changes
--- | ---
v10.0.0 | The result of this function no longer contains a fraction component on Windows.

- Returns: `<integer>`

Returns the system uptime in number of seconds.

### `os.userInfo([options])`#

Added in: v6.0.0

- `options` `<Object>`
  - `encoding` `<string>` Character encoding used to interpret resulting strings. If `encoding` is set to `'buffer'`, the `username`, `shell`, and `homedir` values will be `Buffer` instances. Default: `'utf8'`.
- Returns: `<Object>`

Returns information about the currently effective user. On POSIX platforms, this is typically a subset of the password file. The returned object includes the `username`, `uid`, `gid`, `shell`, and `homedir`. On Windows, the `uid` and `gid` fields are `-1`, and `shell` is `null`.

The value of `homedir` returned by `os.userInfo()` is provided by the operating system. This differs from the result of `os.homedir()`, which queries environment variables for the home directory before falling back to the operating system response.

Throws a `SystemError` if a user has no `username` or `homedir`.

### `os.version()`#

Added in: v13.11.0, v12.17.0

- Returns: `<string>`

Returns a string identifying the kernel version.

On POSIX systems, the operating system release is determined by calling `uname(3)`. On Windows, `RtlGetVersion()` is used, and if it is not available, `GetVersionExW()` will be used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

### OS constants#

The following constants are exported by `os.constants`.

Not all constants will be available on every operating system.

#### Signal constants#

History

Version | Changes
--- | ---
v5.11.0 | Added support for `SIGINFO`.

The following signal constants are exported by `os.constants.signals`.

Constant | Description
--- | ---
`SIGHUP` | Sent to indicate when a controlling terminal is closed or a parent process exits.
`SIGINT` | Sent to indicate when a user wishes to interrupt a process (`Ctrl`+`C`).
`SIGQUIT` | Sent to indicate when a user wishes to terminate a process and perform a core dump.
`SIGILL` | Sent to a process to notify that it has attempted to perform an illegal, malformed, unknown, or privileged instruction.
`SIGTRAP` | Sent to a process when an exception has occurred.
`SIGABRT` | Sent to a process to request that it abort.
`SIGIOT` | Synonym for `SIGABRT`
`SIGBUS` | Sent to a process to notify that it has caused a bus error.
`SIGFPE` | Sent to a process to notify that it has performed an illegal arithmetic operation.
`SIGKILL` | Sent to a process to terminate it immediately.
`SIGUSR1` `SIGUSR2` | Sent to a process to identify user-defined conditions.
`SIGSEGV` | Sent to a process to notify of a segmentation fault.
`SIGPIPE` | Sent to a process when it has attempted to write to a disconnected pipe.
`SIGALRM` | Sent to a process when a system timer elapses.
`SIGTERM` | Sent to a process to request termination.
`SIGCHLD` | Sent to a process when a child process terminates.
`SIGSTKFLT` | Sent to a process to indicate a stack fault on a coprocessor.
`SIGCONT` | Sent to instruct the operating system to continue a paused process.
`SIGSTOP` | Sent to instruct the operating system to halt a process.
`SIGTSTP` | Sent to a process to request it to stop.
`SIGBREAK` | Sent to indicate when a user wishes to interrupt a process.
`SIGTTIN` | Sent to a process when it reads from the TTY while in the background.
`SIGTTOU` | Sent to a process when it writes to the TTY while in the background.
`SIGURG` | Sent to a process when a socket has urgent data to read.
`SIGXCPU` | Sent to a process when it has exceeded its limit on CPU usage.
`SIGXFSZ` | Sent to a process when it grows a file larger than the maximum allowed.
`SIGVTALRM` | Sent to a process when a virtual timer has elapsed.
`SIGPROF` | Sent to a process when a system timer has elapsed.
`SIGWINCH` | Sent to a process when the controlling terminal has changed its size.
`SIGIO` | Sent to a process when I/O is available.
`SIGPOLL` | Synonym for `SIGIO`
`SIGLOST` | Sent to a process when a file lock has been lost.
`SIGPWR` | Sent to a process to notify of a power failure.
`SIGINFO` | Synonym for `SIGPWR`
`SIGSYS` | Sent to a process to notify of a bad argument.
`SIGUNUSED` | Synonym for `SIGSYS`

#### Error constants#

The following error constants are exported by `os.constants.errno`.

##### POSIX error constants#

Constant | Description
--- | ---
`E2BIG` | Indicates that the list of arguments is longer than expected.
`EACCES` | Indicates that the operation did not have sufficient permissions.
`EADDRINUSE` | Indicates that the network address is already in use.
`EADDRNOTAVAIL` | Indicates that the network address is currently unavailable for use.
`EAFNOSUPPORT` | Indicates that the network address family is not supported.
`EAGAIN` | Indicates that there is no data available and to try the operation again later.
`EALREADY` | Indicates that the socket already has a pending connection in progress.
`EBADF` | Indicates that a file descriptor is not valid.
`EBADMSG` | Indicates an invalid data message.
`EBUSY` | Indicates that a device or resource is busy.
`ECANCELED` | Indicates that an operation was canceled.
`ECHILD` | Indicates that there are no child processes.
`ECONNABORTED` | Indicates that the network connection has been aborted.
`ECONNREFUSED` | Indicates that the network connection has been refused.
`ECONNRESET` | Indicates that the network connection has been reset.
`EDEADLK` | Indicates that a resource deadlock has been avoided.
`EDESTADDRREQ` | Indicates that a destination address is required.
`EDOM` | Indicates that an argument is out of the domain of the function.
`EDQUOT` | Indicates that the disk quota has been exceeded.
`EEXIST` | Indicates that the file already exists.
`EFAULT` | Indicates an invalid pointer address.
`EFBIG` | Indicates that the file is too large.
`EHOSTUNREACH` | Indicates that the host is unreachable.
`EIDRM` | Indicates that the identifier has been removed.
`EILSEQ` | Indicates an illegal byte sequence.
`EINPROGRESS` | Indicates that an operation is already in progress.
`EINTR` | Indicates that a function call was interrupted.
`EINVAL` | Indicates that an invalid argument was provided.
`EIO` | Indicates an otherwise unspecified I/O error.
`EISCONN` | Indicates that the socket is connected.
`EISDIR` | Indicates that the path is a directory.
`ELOOP` | Indicates too many levels of symbolic links in a path.
`EMFILE` | Indicates that there are too many open files.
`EMLINK` | Indicates that there are too many hard links to a file.
`EMSGSIZE` | Indicates that the provided message is too long.
`EMULTIHOP` | Indicates that a multihop was attempted.
`ENAMETOOLONG` | Indicates that the filename is too long.
`ENETDOWN` | Indicates that the network is down.
`ENETRESET` | Indicates that the connection has been aborted by the network.
`ENETUNREACH` | Indicates that the network is unreachable.
`ENFILE` | Indicates too many open files in the system.
`ENOBUFS` | Indicates that no buffer space is available.
`ENODATA` | Indicates that no message is available on the stream head read queue.
`ENODEV` | Indicates that there is no such device.
`ENOENT` | Indicates that there is no such file or directory.
`ENOEXEC` | Indicates an exec format error.
`ENOLCK` | Indicates that there are no locks available.
`ENOLINK` | Indications that a link has been severed.
`ENOMEM` | Indicates that there is not enough space.
`ENOMSG` | Indicates that there is no message of the desired type.
`ENOPROTOOPT` | Indicates that a given protocol is not available.
`ENOSPC` | Indicates that there is no space available on the device.
`ENOSR` | Indicates that there are no stream resources available.
`ENOSTR` | Indicates that a given resource is not a stream.
`ENOSYS` | Indicates that a function has not been implemented.
`ENOTCONN` | Indicates that the socket is not connected.
`ENOTDIR` | Indicates that the path is not a directory.
`ENOTEMPTY` | Indicates that the directory is not empty.
`ENOTSOCK` | Indicates that the given item is not a socket.
`ENOTSUP` | Indicates that a given operation is not supported.
`ENOTTY` | Indicates an inappropriate I/O control operation.
`ENXIO` | Indicates no such device or address.
`EOPNOTSUPP` | Indicates that an operation is not supported on the socket. Although `ENOTSUP` and `EOPNOTSUPP` have the same value on Linux, according to POSIX.1 these error values should be distinct.)
`EOVERFLOW` | Indicates that a value is too large to be stored in a given data type.
`EPERM` | Indicates that the operation is not permitted.
`EPIPE` | Indicates a broken pipe.
`EPROTO` | Indicates a protocol error.
`EPROTONOSUPPORT` | Indicates that a protocol is not supported.
`EPROTOTYPE` | Indicates the wrong type of protocol for a socket.
`ERANGE` | Indicates that the results are too large.
`EROFS` | Indicates that the file system is read only.
`ESPIPE` | Indicates an invalid seek operation.
`ESRCH` | Indicates that there is no such process.
`ESTALE` | Indicates that the file handle is stale.
`ETIME` | Indicates an expired timer.
`ETIMEDOUT` | Indicates that the connection timed out.
`ETXTBSY` | Indicates that a text file is busy.
`EWOULDBLOCK` | Indicates that the operation would block.
`EXDEV` | Indicates an improper link.

##### Windows-specific error constants#

The following error codes are specific to the Windows operating system.

Constant | Description
--- | ---
`WSAEINTR` | Indicates an interrupted function call.
`WSAEBADF` | Indicates an invalid file handle.
`WSAEACCES` | Indicates insufficient permissions to complete the operation.
`WSAEFAULT` | Indicates an invalid pointer address.
`WSAEINVAL` | Indicates that an invalid argument was passed.
`WSAEMFILE` | Indicates that there are too many open files.
`WSAEWOULDBLOCK` | Indicates that a resource is temporarily unavailable.
`WSAEINPROGRESS` | Indicates that an operation is currently in progress.
`WSAEALREADY` | Indicates that an operation is already in progress.
`WSAENOTSOCK` | Indicates that the resource is not a socket.
`WSAEDESTADDRREQ` | Indicates that a destination address is required.
`WSAEMSGSIZE` | Indicates that the message size is too long.
`WSAEPROTOTYPE` | Indicates the wrong protocol type for the socket.
`WSAENOPROTOOPT` | Indicates a bad protocol option.
`WSAEPROTONOSUPPORT` | Indicates that the protocol is not supported.
`WSAESOCKTNOSUPPORT` | Indicates that the socket type is not supported.
`WSAEOPNOTSUPP` | Indicates that the operation is not supported.
`WSAEPFNOSUPPORT` | Indicates that the protocol family is not supported.
`WSAEAFNOSUPPORT` | Indicates that the address family is not supported.
`WSAEADDRINUSE` | Indicates that the network address is already in use.
`WSAEADDRNOTAVAIL` | Indicates that the network address is not available.
`WSAENETDOWN` | Indicates that the network is down.
`WSAENETUNREACH` | Indicates that the network is unreachable.
`WSAENETRESET` | Indicates that the network connection has been reset.
`WSAECONNABORTED` | Indicates that the connection has been aborted.
`WSAECONNRESET` | Indicates that the connection has been reset by the peer.
`WSAENOBUFS` | Indicates that there is no buffer space available.
`WSAEISCONN` | Indicates that the socket is already connected.
`WSAENOTCONN` | Indicates that the socket is not connected.
`WSAESHUTDOWN` | Indicates that data cannot be sent after the socket has been shutdown.
`WSAETOOMANYREFS` | Indicates that there are too many references.
`WSAETIMEDOUT` | Indicates that the connection has timed out.
`WSAECONNREFUSED` | Indicates that the connection has been refused.
`WSAELOOP` | Indicates that a name cannot be translated.
`WSAENAMETOOLONG` | Indicates that a name was too long.
`WSAEHOSTDOWN` | Indicates that a network host is down.
`WSAEHOSTUNREACH` | Indicates that there is no route to a network host.
`WSAENOTEMPTY` | Indicates that the directory is not empty.
`WSAEPROCLIM` | Indicates that there are too many processes.
`WSAEUSERS` | Indicates that the user quota has been exceeded.
`WSAEDQUOT` | Indicates that the disk quota has been exceeded.
`WSAESTALE` | Indicates a stale file handle reference.
`WSAEREMOTE` | Indicates that the item is remote.
`WSASYSNOTREADY` | Indicates that the network subsystem is not ready.
`WSAVERNOTSUPPORTED` | Indicates that the `winsock.dll` version is out of range.
`WSANOTINITIALISED` | Indicates that successful WSAStartup has not yet been performed.
`WSAEDISCON` | Indicates that a graceful shutdown is in progress.
`WSAENOMORE` | Indicates that there are no more results.
`WSAECANCELLED` | Indicates that an operation has been canceled.
`WSAEINVALIDPROCTABLE` | Indicates that the procedure call table is invalid.
`WSAEINVALIDPROVIDER` | Indicates an invalid service provider.
`WSAEPROVIDERFAILEDINIT` | Indicates that the service provider failed to initialized.
`WSASYSCALLFAILURE` | Indicates a system call failure.
`WSASERVICE_NOT_FOUND` | Indicates that a service was not found.
`WSATYPE_NOT_FOUND` | Indicates that a class type was not found.
`WSA_E_NO_MORE` | Indicates that there are no more results.
`WSA_E_CANCELLED` | Indicates that the call was canceled.
`WSAEREFUSED` | Indicates that a database query was refused.

#### dlopen constants#

If available on the operating system, the following constants are exported in `os.constants.dlopen`. See `dlopen(3)` for detailed information.

Constant | Description
--- | ---
`RTLD_LAZY` | Perform lazy binding. Node.js sets this flag by default.
`RTLD_NOW` | Resolve all undefined symbols in the library before dlopen(3) returns.
`RTLD_GLOBAL` | Symbols defined by the library will be made available for symbol resolution of subsequently loaded libraries.
`RTLD_LOCAL` | The converse of `RTLD_GLOBAL`. This is the default behavior if neither flag is specified.
`RTLD_DEEPBIND` | Make a self-contained library use its own symbols in preference to symbols from previously loaded libraries.

#### Priority constants#

Added in: v10.10.0

The following process scheduling constants are exported by `os.constants.priority`.

Constant | Description
--- | ---
`PRIORITY_LOW` | The lowest process scheduling priority. This corresponds to `IDLE_PRIORITY_CLASS` on Windows, and a nice value of `19` on all other platforms.
`PRIORITY_BELOW_NORMAL` | The process scheduling priority above `PRIORITY_LOW` and below `PRIORITY_NORMAL`. This corresponds to `BELOW_NORMAL_PRIORITY_CLASS` on Windows, and a nice value of `10` on all other platforms.
`PRIORITY_NORMAL` | The default process scheduling priority. This corresponds to `NORMAL_PRIORITY_CLASS` on Windows, and a nice value of `0` on all other platforms.
`PRIORITY_ABOVE_NORMAL` | The process scheduling priority above `PRIORITY_NORMAL` and below `PRIORITY_HIGH`. This corresponds to `ABOVE_NORMAL_PRIORITY_CLASS` on Windows, and a nice value of `-7` on all other platforms.
`PRIORITY_HIGH` | The process scheduling priority above `PRIORITY_ABOVE_NORMAL` and below `PRIORITY_HIGHEST`. This corresponds to `HIGH_PRIORITY_CLASS` on Windows, and a nice value of `-14` on all other platforms.
`PRIORITY_HIGHEST` | The highest process scheduling priority. This corresponds to `REALTIME_PRIORITY_CLASS` on Windows, and a nice value of `-20` on all other platforms.

#### libuv constants#

Constant | Description
--- | ---
`UV_UDP_REUSEADDR` |

## Path#

Source Code: lib/path.js

Stability: 2 - Stable

The `node:path` module provides utilities for working with file and directory paths. It can be accessed using:

    const path = require('node:path');
    import path from 'node:path';
    javascript

### Windows vs. POSIX#

The default operation of the `node:path` module varies based on the operating system on which a Node.js application is running. Specifically, when running on a Windows operating system, the `node:path` module will assume that Windows-style paths are being used.

So using `path.basename()` might yield different results on POSIX and Windows:

On POSIX:

    path.basename('C:\\temp\\myfile.html');
    // Returns: 'C:\\temp\\myfile.html'
    js

On Windows:

    path.basename('C:\\temp\\myfile.html');
    // Returns: 'myfile.html'
    js

To achieve consistent results when working with Windows file paths on any operating system, use `path.win32`:

On POSIX and Windows:

    path.win32.basename('C:\\temp\\myfile.html');
    // Returns: 'myfile.html'
    js

To achieve consistent results when working with POSIX file paths on any operating system, use `path.posix`:

On POSIX and Windows:

    path.posix.basename('/tmp/myfile.html');
    // Returns: 'myfile.html'
    js

On Windows Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example, `path.resolve('C:\\')` can potentially return a different result than `path.resolve('C:')`. For more information, see this MSDN page.

### `path.basename(path[, suffix])`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v6.0.0 | Passing a non-string as the `path` argument will throw now.

- `path` `<string>`
- `suffix` `<string>` An optional suffix to remove
- Returns: `<string>`

The `path.basename()` method returns the last portion of a `path`, similar to the Unix `basename` command. Trailing directory separators are ignored.

    path.basename('/foo/bar/baz/asdf/quux.html');
    // Returns: 'quux.html'

    path.basename('/foo/bar/baz/asdf/quux.html', '.html');
    // Returns: 'quux'
    js

Although Windows usually treats file names, including file extensions, in a case-insensitive manner, this function does not. For example, `C:\\foo.html` and `C:\\foo.HTML` refer to the same file, but `basename` treats the extension as a case-sensitive string:

    path.win32.basename('C:\\foo.html', '.html');
    // Returns: 'foo'

    path.win32.basename('C:\\foo.HTML', '.html');
    // Returns: 'foo.HTML'
    js

A `TypeError` is thrown if `path` is not a string or if `suffix` is given and is not a string.

### `path.delimiter`#

Added in: v0.9.3

- Type: `<string>`

Provides the platform-specific path delimiter:

- `;` for Windows
- `:` for POSIX

For example, on POSIX:

    console.log(process.env.PATH);
    // Prints: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin'

    process.env.PATH.split(path.delimiter);
    // Returns: ['/usr/bin', '/bin', '/usr/sbin', '/sbin', '/usr/local/bin']
    js

On Windows:

    console.log(process.env.PATH);
    // Prints: 'C:\Windows\system32;C:\Windows;C:\Program Files\node\'

    process.env.PATH.split(path.delimiter);
    // Returns ['C:\\Windows\\system32', 'C:\\Windows', 'C:\\Program Files\\node\\']
    js

### `path.dirname(path)`#

Added in: v0.1.16
History

Version | Changes
--- | ---
v6.0.0 | Passing a non-string as the `path` argument will throw now.

- `path` `<string>`
- Returns: `<string>`

The `path.dirname()` method returns the directory name of a `path`, similar to the Unix `dirname` command. Trailing directory separators are ignored, see `path.sep`.

    path.dirname('/foo/bar/baz/asdf/quux');
    // Returns: '/foo/bar/baz/asdf'
    js

A `TypeError` is thrown if `path` is not a string.

### `path.extname(path)`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v6.0.0 | Passing a non-string as the `path` argument will throw now.

- `path` `<string>`
- Returns: `<string>`

The `path.extname()` method returns the extension of the `path`, from the last occurrence of the `.` (period) character to end of string in the last portion of the `path`. If there is no `.` in the last portion of the `path`, or if there are no `.` characters other than the first character of the basename of `path` (see `path.basename()`) , an empty string is returned.

    path.extname('index.html');
    // Returns: '.html'

    path.extname('index.coffee.md');
    // Returns: '.md'

    path.extname('index.');
    // Returns: '.'

    path.extname('index');
    // Returns: ''

    path.extname('.index');
    // Returns: ''

    path.extname('.index.md');
    // Returns: '.md'
    js

A `TypeError` is thrown if `path` is not a string.

### `path.format(pathObject)`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v19.0.0 | The dot will be added if it is not specified in `ext`.

- `pathObject` `<Object>` Any JavaScript object having the following properties:
  - `dir` `<string>`
  - `root` `<string>`
  - `base` `<string>`
  - `name` `<string>`
  - `ext` `<string>`
- Returns: `<string>`

The `path.format()` method returns a path string from an object. This is the opposite of `path.parse()`.

When providing properties to the `pathObject` remember that there are combinations where one property has priority over another:

- `pathObject.root` is ignored if `pathObject.dir` is provided
- `pathObject.ext` and `pathObject.name` are ignored if `pathObject.base` exists

For example, on POSIX:

    // If `dir`, `root` and `base` are provided,
    // `${dir}${path.sep}${base}`
    // will be returned. `root` is ignored.
    path.format({
      root: '/ignored',
      dir: '/home/user/dir',
      base: 'file.txt',
    });
    // Returns: '/home/user/dir/file.txt'

    // `root` will be used if `dir` is not specified.
    // If only `root` is provided or `dir` is equal to `root` then the
    // platform separator will not be included. `ext` will be ignored.
    path.format({
      root: '/',
      base: 'file.txt',
      ext: 'ignored',
    });
    // Returns: '/file.txt'

    // `name` + `ext` will be used if `base` is not specified.
    path.format({
      root: '/',
      name: 'file',
      ext: '.txt',
    });
    // Returns: '/file.txt'

    // The dot will be added if it is not specified in `ext`.
    path.format({
      root: '/',
      name: 'file',
      ext: 'txt',
    });
    // Returns: '/file.txt'
    js

On Windows:

    path.format({
      dir: 'C:\\path\\dir',
      base: 'file.txt',
    });
    // Returns: 'C:\\path\\dir\\file.txt'
    js

### `path.matchesGlob(path, pattern)`#

Added in: v22.5.0, v20.17.0
History

Version | Changes
--- | ---
v24.8.0, v22.20.0 | Marking the API stable.

- `path` `<string>` The path to glob-match against.
- `pattern` `<string>` The glob to check the path against.
- Returns: `<boolean>` Whether or not the `path` matched the `pattern`.

The `path.matchesGlob()` method determines if `path` matches the `pattern`.

For example:

    path.matchesGlob('/foo/bar', '/foo/*'); // true
    path.matchesGlob('/foo/bar*', 'foo/bird'); // false
    js

A `TypeError` is thrown if `path` or `pattern` are not strings.

### `path.isAbsolute(path)`#

Added in: v0.11.2

- `path` `<string>`
- Returns: `<boolean>`

The `path.isAbsolute()` method determines if the literal `path` is absolute. Therefore, it’s not safe for mitigating path traversals.

If the given `path` is a zero-length string, `false` will be returned.

For example, on POSIX:

    path.isAbsolute('/foo/bar');   // true
    path.isAbsolute('/baz/..');    // true
    path.isAbsolute('/baz/../..'); // true
    path.isAbsolute('qux/');       // false
    path.isAbsolute('.');          // false
    js

On Windows:

    path.isAbsolute('//server');    // true
    path.isAbsolute('\\\\server');  // true
    path.isAbsolute('C:/foo/..');   // true
    path.isAbsolute('C:\\foo\\..'); // true
    path.isAbsolute('bar\\baz');    // false
    path.isAbsolute('bar/baz');     // false
    path.isAbsolute('.');           // false
    js

A `TypeError` is thrown if `path` is not a string.

### `path.join([...paths])`#

Added in: v0.1.16

- `...paths` `<string>` A sequence of path segments
- Returns: `<string>`

The `path.join()` method joins all given `path` segments together using the platform-specific separator as a delimiter, then normalizes the resulting path.

Zero-length `path` segments are ignored. If the joined path string is a zero-length string then `'.'` will be returned, representing the current working directory.

    path.join('/foo', 'bar', 'baz/asdf', 'quux', '..');
    // Returns: '/foo/bar/baz/asdf'

    path.join('foo', {}, 'bar');
    // Throws 'TypeError: Path must be a string. Received {}'
    js

A `TypeError` is thrown if any of the path segments is not a string.

### `path.normalize(path)`#

Added in: v0.1.23

- `path` `<string>`
- Returns: `<string>`

The `path.normalize()` method normalizes the given `path`, resolving `'..'` and `'.'` segments.

When multiple, sequential path segment separation characters are found (e.g. `/` on POSIX and either `\` or `/` on Windows), they are replaced by a single instance of the platform-specific path segment separator (`/` on POSIX and `\` on Windows). Trailing separators are preserved.

If the `path` is a zero-length string, `'.'` is returned, representing the current working directory.

On POSIX, the types of normalization applied by this function do not strictly adhere to the POSIX specification. For example, this function will replace two leading forward slashes with a single slash as if it was a regular absolute path, whereas a few POSIX systems assign special meaning to paths beginning with exactly two forward slashes. Similarly, other substitutions performed by this function, such as removing `..` segments, may change how the underlying system resolves the path.

For example, on POSIX:

    path.normalize('/foo/bar//baz/asdf/quux/..');
    // Returns: '/foo/bar/baz/asdf'
    js

On Windows:

    path.normalize('C:\\temp\\\\foo\\bar\\..\\');
    // Returns: 'C:\\temp\\foo\\'
    js

Since Windows recognizes multiple path separators, both separators will be replaced by instances of the Windows preferred separator (`\`):

    path.win32.normalize('C:////temp\\\\/\\/\\/foo/bar');
    // Returns: 'C:\\temp\\foo\\bar'
    js

A `TypeError` is thrown if `path` is not a string.

### `path.parse(path)`#

Added in: v0.11.15

- `path` `<string>`
- Returns: `<Object>`

The `path.parse()` method returns an object whose properties represent significant elements of the `path`. Trailing directory separators are ignored, see `path.sep`.

The returned object will have the following properties:

- `dir` `<string>`
- `root` `<string>`
- `base` `<string>`
- `name` `<string>`
- `ext` `<string>`

For example, on POSIX:

    path.parse('/home/user/dir/file.txt');
    // Returns:
    // { root: '/',
    //   dir: '/home/user/dir',
    //   base: 'file.txt',
    //   ext: '.txt',
    //   name: 'file' }
    js

    ┌─────────────────────┬────────────┐
    │          dir        │    base    │
    ├──────┬              ├──────┬─────┤
    │ root │              │ name │ ext │
    "  /    home/user/dir / file  .txt "
    └──────┴──────────────┴──────┴─────┘
    (All spaces in the "" line should be ignored. They are purely for formatting.)
    text

On Windows:

    path.parse('C:\\path\\dir\\file.txt');
    // Returns:
    // { root: 'C:\\',
    //   dir: 'C:\\path\\dir',
    //   base: 'file.txt',
    //   ext: '.txt',
    //   name: 'file' }
    js

    ┌─────────────────────┬────────────┐
    │          dir        │    base    │
    ├──────┬              ├──────┬─────┤
    │ root │              │ name │ ext │
    " C:\      path\dir   \ file  .txt "
    └──────┴──────────────┴──────┴─────┘
    (All spaces in the "" line should be ignored. They are purely for formatting.)
    text

A `TypeError` is thrown if `path` is not a string.

### `path.posix`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v15.3.0 | Exposed as `require('path/posix')`.

- Type: `<Object>`

The `path.posix` property provides access to POSIX specific implementations of the `path` methods.

The API is accessible via `require('node:path').posix` or `require('node:path/posix')`.

### `path.relative(from, to)`#

Added in: v0.5.0
History

Version | Changes
--- | ---
v6.8.0 | On Windows, the leading slashes for UNC paths are now included in the return value.

- `from` `<string>`
- `to` `<string>`
- Returns: `<string>`

The `path.relative()` method returns the relative path from `from` to `to` based on the current working directory. If `from` and `to` each resolve to the same path (after calling `path.resolve()` on each), a zero-length string is returned.

If a zero-length string is passed as `from` or `to`, the current working directory will be used instead of the zero-length strings.

For example, on POSIX:

    path.relative('/data/orandea/test/aaa', '/data/orandea/impl/bbb');
    // Returns: '../../impl/bbb'
    js

On Windows:

    path.relative('C:\\orandea\\test\\aaa', 'C:\\orandea\\impl\\bbb');
    // Returns: '..\\..\\impl\\bbb'
    js

A `TypeError` is thrown if either `from` or `to` is not a string.

### `path.resolve([...paths])`#

Added in: v0.3.4

- `...paths` `<string>` A sequence of paths or path segments
- Returns: `<string>`

The `path.resolve()` method resolves a sequence of paths or path segments into an absolute path.

The given sequence of paths is processed from right to left, with each subsequent `path` prepended until an absolute path is constructed. For instance, given the sequence of path segments: `/foo`, `/bar`, `baz`, calling `path.resolve('/foo', '/bar', 'baz')` would return `/bar/baz` because `'baz'` is not an absolute path but `'/bar' + '/' + 'baz'` is.

If, after processing all given `path` segments, an absolute path has not yet been generated, the current working directory is used.

The resulting path is normalized and trailing slashes are removed unless the path is resolved to the root directory.

Zero-length `path` segments are ignored.

If no `path` segments are passed, `path.resolve()` will return the absolute path of the current working directory.

    path.resolve('/foo/bar', './baz');
    // Returns: '/foo/bar/baz'

    path.resolve('/foo/bar', '/tmp/file/');
    // Returns: '/tmp/file'

    path.resolve('wwwroot', 'static_files/png/', '../gif/image.gif');
    // If the current working directory is /home/myself/node,
    // this returns '/home/myself/node/wwwroot/static_files/gif/image.gif'
    js

A `TypeError` is thrown if any of the arguments is not a string.

### `path.sep`#

Added in: v0.7.9

- Type: `<string>`

Provides the platform-specific path segment separator:

- `\` on Windows
- `/` on POSIX

For example, on POSIX:

    'foo/bar/baz'.split(path.sep);
    // Returns: ['foo', 'bar', 'baz']
    js

On Windows:

    'foo\\bar\\baz'.split(path.sep);
    // Returns: ['foo', 'bar', 'baz']
    js

On Windows, both the forward slash (`/`) and backward slash (`\`) are accepted as path segment separators; however, the `path` methods only add backward slashes (`\`).

### `path.toNamespacedPath(path)`#

Added in: v9.0.0

- `path` `<string>`
- Returns: `<string>`

On Windows systems only, returns an equivalent namespace-prefixed path for the given `path`. If `path` is not a string, `path` will be returned without modifications.

This method is meaningful only on Windows systems. On POSIX systems, the method is non-operational and always returns `path` without modifications.

### `path.win32`#

Added in: v0.11.15
History

Version | Changes
--- | ---
v15.3.0 | Exposed as `require('path/win32')`.

- Type: `<Object>`

The `path.win32` property provides access to Windows-specific implementations of the `path` methods.

The API is accessible via `require('node:path').win32` or `require('node:path/win32')`.

## String decoder#

Source Code: lib/string_decoder.js

Stability: 2 - Stable

The `node:string_decoder` module provides an API for decoding `Buffer` objects into strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16 characters. It can be accessed using:

    import { StringDecoder } from 'node:string_decoder';
    const { StringDecoder } = require('node:string_decoder');
    javascript

The following example shows the basic use of the `StringDecoder` class.

    import { StringDecoder } from 'node:string_decoder';
    import { Buffer } from 'node:buffer';
    const decoder = new StringDecoder('utf8');

    const cent = Buffer.from([0xC2, 0xA2]);
    console.log(decoder.write(cent)); // Prints: ¢

    const euro = Buffer.from([0xE2, 0x82, 0xAC]);
    console.log(decoder.write(euro)); // Prints: €
    const { StringDecoder } = require('node:string_decoder');
    const decoder = new StringDecoder('utf8');

    const cent = Buffer.from([0xC2, 0xA2]);
    console.log(decoder.write(cent)); // Prints: ¢

    const euro = Buffer.from([0xE2, 0x82, 0xAC]);
    console.log(decoder.write(euro)); // Prints: €
    javascript

When a `Buffer` instance is written to the `StringDecoder` instance, an internal buffer is used to ensure that the decoded string does not contain any incomplete multibyte characters. These are held in the buffer until the next call to `stringDecoder.write()` or until `stringDecoder.end()` is called.

In the following example, the three UTF-8 encoded bytes of the European Euro symbol (`€`) are written over three separate operations:

    import { StringDecoder } from 'node:string_decoder';
    import { Buffer } from 'node:buffer';
    const decoder = new StringDecoder('utf8');

    decoder.write(Buffer.from([0xE2]));
    decoder.write(Buffer.from([0x82]));
    console.log(decoder.end(Buffer.from([0xAC]))); // Prints: €
    const { StringDecoder } = require('node:string_decoder');
    const decoder = new StringDecoder('utf8');

    decoder.write(Buffer.from([0xE2]));
    decoder.write(Buffer.from([0x82]));
    console.log(decoder.end(Buffer.from([0xAC]))); // Prints: €
    javascript

### Class: `StringDecoder`#

#### `new StringDecoder([encoding])`#

Added in: v0.1.99

- `encoding` `<string>` The character encoding the `StringDecoder` will use. Default: `'utf8'`.

Creates a new `StringDecoder` instance.

#### `stringDecoder.end([buffer])`#

Added in: v0.9.3

- `buffer` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The bytes to decode.
- Returns: `<string>`

Returns any remaining input stored in the internal buffer as a string. Bytes representing incomplete UTF-8 and UTF-16 characters will be replaced with substitution characters appropriate for the character encoding.

If the `buffer` argument is provided, one final call to `stringDecoder.write()` is performed before returning the remaining input. After `end()` is called, the `stringDecoder` object can be reused for new input.

#### `stringDecoder.write(buffer)`#

Added in: v0.1.99
History

Version | Changes
--- | ---
v8.0.0 | Each invalid character is now replaced by a single replacement character instead of one for each individual byte.

- `buffer` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The bytes to decode.
- Returns: `<string>`

Returns a decoded string, ensuring that any incomplete multibyte characters at the end of the `Buffer`, or `TypedArray`, or `DataView` are omitted from the returned string and stored in an internal buffer for the next call to `stringDecoder.write()` or `stringDecoder.end()`.

## Test runner#

Source Code: lib/test.js Added in: v18.0.0, v16.17.0
History

Version | Changes
--- | ---
v20.0.0 | The test runner is now stable.

Stability: 2 - Stable

The `node:test` module facilitates the creation of JavaScript tests. To access it:

    import test from 'node:test';
    const test = require('node:test');
    javascript

This module is only available under the `node:` scheme.

Tests created via the `test` module consist of a single function that is processed in one of three ways:

1. A synchronous function that is considered failing if it throws an exception, and is considered passing otherwise.
2. A function that returns a `Promise` that is considered failing if the `Promise` rejects, and is considered passing if the `Promise` fulfills.
3. A function that receives a callback function. If the callback receives any truthy value as its first argument, the test is considered failing. If a falsy value is passed as the first argument to the callback, the test is considered passing. If the test function receives a callback function and also returns a `Promise`, the test will fail.

The following example illustrates how tests are written using the `test` module.

    test('synchronous passing test', (t) => {
      // This test passes because it does not throw an exception.
      assert.strictEqual(1, 1);
    });

    test('synchronous failing test', (t) => {
      // This test fails because it throws an exception.
      assert.strictEqual(1, 2);
    });

    test('asynchronous passing test', async (t) => {
      // This test passes because the Promise returned by the async
      // function is settled and not rejected.
      assert.strictEqual(1, 1);
    });

    test('asynchronous failing test', async (t) => {
      // This test fails because the Promise returned by the async
      // function is rejected.
      assert.strictEqual(1, 2);
    });

    test('failing test using Promises', (t) => {
      // Promises can be used directly as well.
      return new Promise((resolve, reject) => {
        setImmediate(() => {
          reject(new Error('this will cause the test to fail'));
        });
      });
    });

    test('callback passing test', (t, done) => {
      // done() is the callback function. When the setImmediate() runs, it invokes
      // done() with no arguments.
      setImmediate(done);
    });

    test('callback failing test', (t, done) => {
      // When the setImmediate() runs, done() is invoked with an Error object and
      // the test fails.
      setImmediate(() => {
        done(new Error('callback failure'));
      });
    });
    js

If any tests fail, the process exit code is set to `1`.

### Subtests#

The test context's `test()` method allows subtests to be created. It allows you to structure your tests in a hierarchical manner, where you can create nested tests within a larger test. This method behaves identically to the top level `test()` function. The following example demonstrates the creation of a top level test with two subtests.

    test('top level test', async (t) => {
      await t.test('subtest 1', (t) => {
        assert.strictEqual(1, 1);
      });

      await t.test('subtest 2', (t) => {
        assert.strictEqual(2, 2);
      });
    });
    js

> Note: `beforeEach` and `afterEach` hooks are triggered between each subtest execution.

In this example, `await` is used to ensure that both subtests have completed. This is necessary because tests do not wait for their subtests to complete, unlike tests created within suites. Any subtests that are still outstanding when their parent finishes are cancelled and treated as failures. Any subtest failures cause the parent test to fail.

### Rerunning failed tests#

The test runner supports persisting the state of the run to a file, allowing the test runner to rerun failed tests without having to re-run the entire test suite. Use the `--test-rerun-failures` command-line option to specify a file path where the state of the run is stored. if the state file does not exist, the test runner will create it. the state file is a JSON file that contains an array of run attempts. Each run attempt is an object mapping successful tests to the attempt they have passed in. The key identifying a test in this map is the test file path, with the line and column where the test is defined. in a case where a test defined in a specific location is run multiple times, for example within a function or a loop, a counter will be appended to the key, to disambiguate the test runs. note changing the order of test execution or the location of a test can lead the test runner to consider tests as passed on a previous attempt, meaning `--test-rerun-failures` should be used when tests run in a deterministic order.

example of a state file:

    [
      {
        "test.js:10:5": { "passed_on_attempt": 0, "name": "test 1" }
      },
      {
        "test.js:10:5": { "passed_on_attempt": 0, "name": "test 1" },
        "test.js:20:5": { "passed_on_attempt": 1, "name": "test 2" }
      }
    ]
    json

in this example, there are two run attempts, with two tests defined in `test.js`, the first test succeeded on the first attempt, and the second test succeeded on the second attempt.

When the `--test-rerun-failures` option is used, the test runner will only run tests that have not yet passed.

    node --test-rerun-failures /path/to/state/file
    bash

### `describe()` and `it()` aliases#

Suites and tests can also be written using the `describe()` and `it()` functions. `describe()` is an alias for `suite()`, and `it()` is an alias for `test()`.

    describe('A thing', () => {
      it('should work', () => {
        assert.strictEqual(1, 1);
      });

      it('should be ok', () => {
        assert.strictEqual(2, 2);
      });

      describe('a nested thing', () => {
        it('should work', () => {
          assert.strictEqual(3, 3);
        });
      });
    });
    js

`describe()` and `it()` are imported from the `node:test` module.

    import { describe, it } from 'node:test';
    const { describe, it } = require('node:test');
    javascript

### Skipping tests#

Individual tests can be skipped by passing the `skip` option to the test, or by calling the test context's `skip()` method as shown in the following example.

    // The skip option is used, but no message is provided.
    test('skip option', { skip: true }, (t) => {
      // This code is never executed.
    });

    // The skip option is used, and a message is provided.
    test('skip option with message', { skip: 'this is skipped' }, (t) => {
      // This code is never executed.
    });

    test('skip() method', (t) => {
      // Make sure to return here as well if the test contains additional logic.
      t.skip();
    });

    test('skip() method with message', (t) => {
      // Make sure to return here as well if the test contains additional logic.
      t.skip('this is skipped');
    });
    js

### TODO tests#

Individual tests can be marked as flaky or incomplete by passing the `todo` option to the test, or by calling the test context's `todo()` method, as shown in the following example. These tests represent a pending implementation or bug that needs to be fixed. TODO tests are executed, but are not treated as test failures, and therefore do not affect the process exit code. If a test is marked as both TODO and skipped, the TODO option is ignored.

    // The todo option is used, but no message is provided.
    test('todo option', { todo: true }, (t) => {
      // This code is executed, but not treated as a failure.
      throw new Error('this does not fail the test');
    });

    // The todo option is used, and a message is provided.
    test('todo option with message', { todo: 'this is a todo test' }, (t) => {
      // This code is executed.
    });

    test('todo() method', (t) => {
      t.todo();
    });

    test('todo() method with message', (t) => {
      t.todo('this is a todo test and is not treated as a failure');
      throw new Error('this does not fail the test');
    });
    js

### Expecting tests to fail#

Added in: v25.5.0, v24.14.0

This flips the pass/fail reporting for a specific test or suite: a flagged test case must throw in order to pass, and a flagged test case that does not throw fails.

In each of the following, `doTheThing()` fails to return `true`, but since the tests are flagged `expectFailure`, they pass.

    it.expectFailure('should do the thing', () => {
      assert.strictEqual(doTheThing(), true);
    });

    it('should do the thing', { expectFailure: true }, () => {
      assert.strictEqual(doTheThing(), true);
    });

    it('should do the thing', { expectFailure: 'feature not implemented' }, () => {
      assert.strictEqual(doTheThing(), true);
    });
    js

If the value of `expectFailure` is a `<RegExp>` | `<Function>` | `<Object>` | `<Error>` the tests will pass only if they throw a matching value. See `assert.throws` for how each value type is handled.

Each of the following tests fails despite being flagged `expectFailure` because the failure does not match the specific expected failure.

    it('fails because regex does not match', {
      expectFailure: /expected message/,
    }, () => {
      throw new Error('different message');
    });

    it('fails because object matcher does not match', {
      expectFailure: { code: 'ERR_EXPECTED' },
    }, () => {
      const err = new Error('boom');
      err.code = 'ERR_ACTUAL';
      throw err;
    });
    js

To supply both a reason and specific error for `expectFailure`, use `{ label, match }`.

    it('should fail with specific error and reason', {
      expectFailure: {
        label: 'reason for failure',
        match: /error message/,
      },
    }, () => {
      assert.strictEqual(doTheThing(), true);
    });
    js

`skip` and/or `todo` are mutually exclusive to `expectFailure`, and `skip` or `todo` will "win" when both are applied (`skip` wins against both, and `todo` wins against `expectFailure`).

These tests will be skipped (and not run):

    it.expectFailure('should do the thing', { skip: true }, () => {
      assert.strictEqual(doTheThing(), true);
    });

    it.skip('should do the thing', { expectFailure: true }, () => {
      assert.strictEqual(doTheThing(), true);
    });
    js

These tests will be marked "todo" (silencing errors):

    it.expectFailure('should do the thing', { todo: true }, () => {
      assert.strictEqual(doTheThing(), true);
    });

    it.todo('should do the thing', { expectFailure: true }, () => {
      assert.strictEqual(doTheThing(), true);
    });
    js

### `only` tests#

If Node.js is started with the `--test-only` command-line option, or test isolation is disabled, it is possible to skip all tests except for a selected subset by passing the `only` option to the tests that should run. When a test with the `only` option is set, all subtests are also run. If a suite has the `only` option set, all tests within the suite are run, unless it has descendants with the `only` option set, in which case only those tests are run.

When using subtests within a `test()`/`it()`, it is required to mark all ancestor tests with the `only` option to run only a selected subset of tests.

The test context's `runOnly()` method can be used to implement the same behavior at the subtest level. Tests that are not executed are omitted from the test runner output.

    // Assume Node.js is run with the --test-only command-line option.
    // The suite's 'only' option is set, so these tests are run.
    test('this test is run', { only: true }, async (t) => {
      // Within this test, all subtests are run by default.
      await t.test('running subtest');

      // The test context can be updated to run subtests with the 'only' option.
      t.runOnly(true);
      await t.test('this subtest is now skipped');
      await t.test('this subtest is run', { only: true });

      // Switch the context back to execute all tests.
      t.runOnly(false);
      await t.test('this subtest is now run');

      // Explicitly do not run these tests.
      await t.test('skipped subtest 3', { only: false });
      await t.test('skipped subtest 4', { skip: true });
    });

    // The 'only' option is not set, so this test is skipped.
    test('this test is not run', () => {
      // This code is not run.
      throw new Error('fail');
    });

    describe('a suite', () => {
      // The 'only' option is set, so this test is run.
      it('this test is run', { only: true }, () => {
        // This code is run.
      });

      it('this test is not run', () => {
        // This code is not run.
        throw new Error('fail');
      });
    });

    describe.only('a suite', () => {
      // The 'only' option is set, so this test is run.
      it('this test is run', () => {
        // This code is run.
      });

      it('this test is run', () => {
        // This code is run.
      });
    });
    js

### Filtering tests by name#

The `--test-name-pattern` command-line option can be used to only run tests whose name matches the provided pattern, and the `--test-skip-pattern` option can be used to skip tests whose name matches the provided pattern. Test name patterns are interpreted as JavaScript regular expressions. The `--test-name-pattern` and `--test-skip-pattern` options can be specified multiple times in order to run nested tests. For each test that is executed, any corresponding test hooks, such as `beforeEach()`, are also run. Tests that are not executed are omitted from the test runner output.

Given the following test file, starting Node.js with the `--test-name-pattern="test [1-3]"` option would cause the test runner to execute `test 1`, `test 2`, and `test 3`. If `test 1` did not match the test name pattern, then its subtests would not execute, despite matching the pattern. The same set of tests could also be executed by passing `--test-name-pattern` multiple times (e.g. `--test-name-pattern="test 1"`, `--test-name-pattern="test 2"`, etc.).

    test('test 1', async (t) => {
      await t.test('test 2');
      await t.test('test 3');
    });

    test('Test 4', async (t) => {
      await t.test('Test 5');
      await t.test('test 6');
    });
    js

Test name patterns can also be specified using regular expression literals. This allows regular expression flags to be used. In the previous example, starting Node.js with `--test-name-pattern="/test [4-5]/i"` (or `--test-skip-pattern="/test [4-5]/i"`) would match `Test 4` and `Test 5` because the pattern is case-insensitive.

To match a single test with a pattern, you can prefix it with all its ancestor test names separated by space, to ensure it is unique. For example, given the following test file:

    describe('test 1', (t) => {
      it('some test');
    });

    describe('test 2', (t) => {
      it('some test');
    });
    js

Starting Node.js with `--test-name-pattern="test 1 some test"` would match only `some test` in `test 1`.

Test name patterns do not change the set of files that the test runner executes.

If both `--test-name-pattern` and `--test-skip-pattern` are supplied, tests must satisfy both requirements in order to be executed.

### Test tags#

Added in: v26.2.0

Stability: 1.0 - Early development

Tags annotate tests and suites with arbitrary string labels. The `--experimental-test-tag-filter` CLI flag (or the `testTagFilters` option on `run()`) selects tests whose tag set contains every provided filter value.

Tags are an alternative to encoding metadata into test names. They are useful for cross-cutting axes such as subsystem, speed bucket, flakiness, or environment, where a name pattern would be brittle.

#### Authoring tagged tests#

Pass a `tags` array on any of `test()`, `it()`, `suite()`, or `describe()`. Tags inherit from a suite to its child tests by union—a test inside a suite tagged `['db']` that declares its own `tags: ['integration']` effectively has both tags.

    import { describe, it } from 'node:test';

    describe('database', { tags: ['db'] }, () => {
      it('reads a row');                                            // tags: ['db']
      it('writes a row', { tags: ['integration'] });                // tags: ['db', 'integration']
      it('reconnects after disconnect', { tags: ['flaky'] });       // tags: ['db', 'flaky']
    });
    const { describe, it } = require('node:test');

    describe('database', { tags: ['db'] }, () => {
      it('reads a row');                                            // tags: ['db']
      it('writes a row', { tags: ['integration'] });                // tags: ['db', 'integration']
      it('reconnects after disconnect', { tags: ['flaky'] });       // tags: ['db', 'flaky']
    });
    javascript

Tag values must be non-empty strings. Tags are matched case-insensitively; the canonical form is lowercase. Duplicates within a single `tags` array are collapsed on the lowercased form, preserving the first-seen declaration order.

Hooks (`before`, `after`, `beforeEach`, `afterEach`) do not declare their own tags. They run as part of their owning suite, which carries the suite's tags.

#### Filtering by tag#

Each `--experimental-test-tag-filter` value is a literal tag name. A test runs only when its tag set contains that name. The flag may be specified more than once; tests must match every filter to run. The same applies to the `testTagFilters` array on `run()`. Filters are case-insensitive and AND'd with `--test-name-pattern`, `--test-skip-pattern`, and `.only` filtering.

Untagged tests are excluded under any non-empty filter, since the filter requires the tag to be present.

#### Reading tags from inside a test#

The `TestContext` object exposes the test's tags as a frozen array through `context.tags`, so tests can branch on their own metadata.

#### Errors#

A tag value that violates the validation rules above throws `ERR_INVALID_ARG_VALUE` at the registration site, before any test runs. A non-array `tags` value throws `ERR_INVALID_ARG_TYPE`.

### Extraneous asynchronous activity#

Once a test function finishes executing, the results are reported as quickly as possible while maintaining the order of the tests. However, it is possible for the test function to generate asynchronous activity that outlives the test itself. The test runner handles this type of activity, but does not delay the reporting of test results in order to accommodate it.

In the following example, a test completes with two `setImmediate()` operations still outstanding. The first `setImmediate()` attempts to create a new subtest. Because the parent test has already finished and output its results, the new subtest is immediately marked as failed, and reported later to the `<TestsStream>`.

The second `setImmediate()` creates an `uncaughtException` event. `uncaughtException` and `unhandledRejection` events originating from a completed test are marked as failed by the `test` module and reported as diagnostic warnings at the top level by the `<TestsStream>`.

    test('a test that creates asynchronous activity', (t) => {
      setImmediate(() => {
        t.test('subtest that is created too late', (t) => {
          throw new Error('error1');
        });
      });

      setImmediate(() => {
        throw new Error('error2');
      });

      // The test finishes after this line.
    });
    js

### Watch mode#

Added in: v19.2.0, v18.13.0

Stability: 1 - Experimental

The Node.js test runner supports running in watch mode by passing the `--watch` flag:

    node --test --watch
    bash

In watch mode, the test runner will watch for changes to test files and their dependencies. When a change is detected, the test runner will rerun the tests affected by the change. The test runner will continue to run until the process is terminated.

### Global setup and teardown#

Added in: v24.0.0

Stability: 1.0 - Early development

The test runner supports specifying a module that will be evaluated before all tests are executed and can be used to setup global state or fixtures for tests. This is useful for preparing resources or setting up shared state that is required by multiple tests.

This module can export any of the following:

- A `globalSetup` function which runs once before all tests start
- A `globalTeardown` function which runs once after all tests complete

The module is specified using the `--test-global-setup` flag when running tests from the command line.

    // setup-module.js
    async function globalSetup() {
      // Setup shared resources, state, or environment
      console.log('Global setup executed');
      // Run servers, create files, prepare databases, etc.
    }

    async function globalTeardown() {
      // Clean up resources, state, or environment
      console.log('Global teardown executed');
      // Close servers, remove files, disconnect from databases, etc.
    }

    module.exports = { globalSetup, globalTeardown };
    // setup-module.mjs
    export async function globalSetup() {
      // Setup shared resources, state, or environment
      console.log('Global setup executed');
      // Run servers, create files, prepare databases, etc.
    }

    export async function globalTeardown() {
      // Clean up resources, state, or environment
      console.log('Global teardown executed');
      // Close servers, remove files, disconnect from databases, etc.
    }
    javascript

If the global setup function throws an error, no tests will be run and the process will exit with a non-zero exit code. The global teardown function will not be called in this case.

### Running tests from the command line#

The Node.js test runner can be invoked from the command line by passing the `--test` flag:

    node --test
    bash

By default, Node.js will run all files matching these patterns:

- `**/*.test.{cjs,mjs,js}`
- `**/*-test.{cjs,mjs,js}`
- `**/*_test.{cjs,mjs,js}`
- `**/test-*.{cjs,mjs,js}`
- `**/test.{cjs,mjs,js}`
- `**/test/**/*.{cjs,mjs,js}`

Unless `--no-strip-types` is supplied, the following additional patterns are also matched:

- `**/*.test.{cts,mts,ts}`
- `**/*-test.{cts,mts,ts}`
- `**/*_test.{cts,mts,ts}`
- `**/test-*.{cts,mts,ts}`
- `**/test.{cts,mts,ts}`
- `**/test/**/*.{cts,mts,ts}`

Alternatively, one or more glob patterns can be provided as the final argument(s) to the Node.js command, as shown below. Glob patterns follow the behavior of `glob(7)`. The glob patterns should be enclosed in double quotes on the command line to prevent shell expansion, which can reduce portability across systems.

    node --test "**/*.test.js" "**/*.spec.js"
    bash

#### Randomizing tests execution order#

Added in: v26.1.0

Stability: 1.0 - Early development

The test runner can randomize execution order to help detect order-dependent tests. When enabled, the runner randomizes both discovered test files and queued tests within each file. Use `--test-randomize` to enable this mode.

    node --test --test-randomize
    bash

When randomization is enabled, the test runner prints the seed used for the run as a diagnostic message:

    Randomized test order seed: 12345
    text

Use `--test-random-seed=<number>` to replay the same randomized order deterministically. Supplying `--test-random-seed` also enables randomization, so `--test-randomize` is optional when a seed is provided:

    node --test --test-random-seed=12345
    bash

In most test files, randomization works automatically. One important exception is when subtests are awaited one by one. In that pattern, each subtest starts only after the previous one finishes, so the runner keeps declaration order instead of randomizing it.

Example: this runs sequentially and is not randomized.

    import test from 'node:test';

    test('math', async (t) => {
      for (const name of ['adds', 'subtracts', 'multiplies']) {
        // Sequentially awaiting each subtest preserves declaration order.
        await t.test(name, async () => {});
      }
    });
    const test = require('node:test');

    test('math', async (t) => {
      for (const name of ['adds', 'subtracts', 'multiplies']) {
        // Sequentially awaiting each subtest preserves declaration order.
        await t.test(name, async () => {});
      }
    });
    javascript

Using suite-style APIs such as `describe()`/`it()` or `suite()`/`test()` still allows randomization, because sibling tests are enqueued together.

Example: this remains eligible for randomization.

    import { describe, it } from 'node:test';

    describe('math', () => {
      it('adds', () => {});
      it('subtracts', () => {});
      it('multiplies', () => {});
    });
    const { describe, it } = require('node:test');

    describe('math', () => {
      it('adds', () => {});
      it('subtracts', () => {});
      it('multiplies', () => {});
    });
    javascript

`--test-randomize` and `--test-random-seed` are not supported with `--watch` mode.

Matching files are executed as test files. More information on the test file execution can be found in the test runner execution model section.

#### Test runner execution model#

When process-level test isolation is enabled, each matching test file is executed in a separate child process. The maximum number of child processes running at any time is controlled by the `--test-concurrency` flag. If the child process finishes with an exit code of 0, the test is considered passing. Otherwise, the test is considered to be a failure. Test files must be executable by Node.js, but are not required to use the `node:test` module internally.

Each test file is executed as if it was a regular script. That is, if the test file itself uses `node:test` to define tests, all of those tests will be executed within a single application thread, regardless of the value of the `concurrency` option of `test()`.

When process-level test isolation is disabled, each matching test file is imported into the test runner process. Once all test files have been loaded, the top level tests are executed with a concurrency of one. Because the test files are all run within the same context, it is possible for tests to interact with each other in ways that are not possible when isolation is enabled. For example, if a test relies on global state, it is possible for that state to be modified by a test originating from another file.

##### Child process option inheritance#

When running tests in process isolation mode (the default), spawned child processes inherit Node.js options from the parent process, including those specified in configuration files. However, certain flags are filtered out to enable proper test runner functionality:

- `--test` - Prevented to avoid recursive test execution
- `--experimental-test-coverage` - Managed by the test runner
- `--experimental-test-tag-filter` - Filter values are validated by the parent process and re-emitted to child processes
- `--watch` - Watch mode is handled at the parent level
- `--experimental-default-config-file` - Config file loading is handled by the parent
- `--test-reporter` - Reporting is managed by the parent process
- `--test-reporter-destination` - Output destinations are controlled by the parent
- `--experimental-config-file` - Config file paths are managed by the parent
- `--test-randomize` - Randomization is managed by the parent process and propagated to child processes
- `--test-random-seed` - Randomization seed is managed by the parent process and propagated to child processes

All other Node.js options from command line arguments, environment variables, and configuration files are inherited by the child processes.

### Collecting code coverage#

Stability: 1 - Experimental

When Node.js is started with the `--experimental-test-coverage` command-line flag, code coverage is collected and statistics are reported once all tests have completed. If the `NODE_V8_COVERAGE` environment variable is used to specify a code coverage directory, the generated V8 coverage files are written to that directory. Node.js core modules and files within `node_modules/` directories are, by default, not included in the coverage report. However, they can be explicitly included via the `--test-coverage-include` flag. By default all the matching test files are excluded from the coverage report. Exclusions can be overridden by using the `--test-coverage-exclude` flag. If coverage is enabled, the coverage report is sent to any test reporters via the `'test:coverage'` event.

Coverage can be disabled on a series of lines using the following comment syntax:

    /* node:coverage disable */
    if (anAlwaysFalseCondition) {
      // Code in this branch will never be executed, but the lines are ignored for
      // coverage purposes. All lines following the 'disable' comment are ignored
      // until a corresponding 'enable' comment is encountered.
      console.log('this is never executed');
    }
    /* node:coverage enable */
    js

Coverage can also be disabled for a specified number of lines. After the specified number of lines, coverage will be automatically reenabled. If the number of lines is not explicitly provided, a single line is ignored.

    /* node:coverage ignore next */
    if (anAlwaysFalseCondition) { console.log('this is never executed'); }

    /* node:coverage ignore next 3 */
    if (anAlwaysFalseCondition) {
      console.log('this is never executed');
    }
    js

#### Coverage reporters#

The tap and spec reporters will print a summary of the coverage statistics. There is also an lcov reporter that will generate an lcov file which can be used as an in depth coverage report.

    node --test --experimental-test-coverage --test-reporter=lcov --test-reporter-destination=lcov.info
    bash

- No test results are reported by this reporter.
- This reporter should ideally be used alongside another reporter.

### Mocking#

The `node:test` module supports mocking during testing via a top-level `mock` object. The following example creates a spy on a function that adds two numbers together. The spy is then used to assert that the function was called as expected.

    import assert from 'node:assert';
    import { mock, test } from 'node:test';

    test('spies on a function', () => {
      const sum = mock.fn((a, b) => {
        return a + b;
      });

      assert.strictEqual(sum.mock.callCount(), 0);
      assert.strictEqual(sum(3, 4), 7);
      assert.strictEqual(sum.mock.callCount(), 1);

      const call = sum.mock.calls[0];
      assert.deepStrictEqual(call.arguments, [3, 4]);
      assert.strictEqual(call.result, 7);
      assert.strictEqual(call.error, undefined);

      // Reset the globally tracked mocks.
      mock.reset();
    });
    const assert = require('node:assert');
    const { mock, test } = require('node:test');

    test('spies on a function', () => {
      const sum = mock.fn((a, b) => {
        return a + b;
      });

      assert.strictEqual(sum.mock.callCount(), 0);
      assert.strictEqual(sum(3, 4), 7);
      assert.strictEqual(sum.mock.callCount(), 1);

      const call = sum.mock.calls[0];
      assert.deepStrictEqual(call.arguments, [3, 4]);
      assert.strictEqual(call.result, 7);
      assert.strictEqual(call.error, undefined);

      // Reset the globally tracked mocks.
      mock.reset();
    });
    javascript

The same mocking functionality is also exposed on the `TestContext` object of each test. The following example creates a spy on an object method using the API exposed on the `TestContext`. The benefit of mocking via the test context is that the test runner will automatically restore all mocked functionality once the test finishes.

    test('spies on an object method', (t) => {
      const number = {
        value: 5,
        add(a) {
          return this.value + a;
        },
      };

      t.mock.method(number, 'add');
      assert.strictEqual(number.add.mock.callCount(), 0);
      assert.strictEqual(number.add(3), 8);
      assert.strictEqual(number.add.mock.callCount(), 1);

      const call = number.add.mock.calls[0];

      assert.deepStrictEqual(call.arguments, [3]);
      assert.strictEqual(call.result, 8);
      assert.strictEqual(call.target, undefined);
      assert.strictEqual(call.this, number);
    });
    js

#### Timers#

Mocking timers is a technique commonly used in software testing to simulate and control the behavior of timers, such as `setInterval` and `setTimeout`, without actually waiting for the specified time intervals.

Refer to the `MockTimers` class for a full list of methods and features.

This allows developers to write more reliable and predictable tests for time-dependent functionality.

The example below shows how to mock `setTimeout`. Using `.enable({ apis: ['setTimeout'] });` it will mock the `setTimeout` functions in the node:timers and node:timers/promises modules, as well as from the Node.js global context.

Note: Destructuring functions such as `import { setTimeout } from 'node:timers'` is currently not supported by this API.

    import assert from 'node:assert';
    import { mock, test } from 'node:test';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', () => {
      const fn = mock.fn();

      // Optionally choose what to mock
      mock.timers.enable({ apis: ['setTimeout'] });
      setTimeout(fn, 9999);
      assert.strictEqual(fn.mock.callCount(), 0);

      // Advance in time
      mock.timers.tick(9999);
      assert.strictEqual(fn.mock.callCount(), 1);

      // Reset the globally tracked mocks.
      mock.timers.reset();

      // If you call reset mock instance, it will also reset timers instance
      mock.reset();
    });
    const assert = require('node:assert');
    const { mock, test } = require('node:test');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', () => {
      const fn = mock.fn();

      // Optionally choose what to mock
      mock.timers.enable({ apis: ['setTimeout'] });
      setTimeout(fn, 9999);
      assert.strictEqual(fn.mock.callCount(), 0);

      // Advance in time
      mock.timers.tick(9999);
      assert.strictEqual(fn.mock.callCount(), 1);

      // Reset the globally tracked mocks.
      mock.timers.reset();

      // If you call reset mock instance, it will also reset timers instance
      mock.reset();
    });
    javascript

The same mocking functionality is also exposed in the mock property on the `TestContext` object of each test. The benefit of mocking via the test context is that the test runner will automatically restore all mocked timers functionality once the test finishes.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();

      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout'] });
      setTimeout(fn, 9999);
      assert.strictEqual(fn.mock.callCount(), 0);

      // Advance in time
      context.mock.timers.tick(9999);
      assert.strictEqual(fn.mock.callCount(), 1);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();

      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout'] });
      setTimeout(fn, 9999);
      assert.strictEqual(fn.mock.callCount(), 0);

      // Advance in time
      context.mock.timers.tick(9999);
      assert.strictEqual(fn.mock.callCount(), 1);
    });
    javascript

#### Dates#

The mock timers API also allows the mocking of the `Date` object. This is a useful feature for testing time-dependent functionality, or to simulate internal calendar functions such as `Date.now()`.

The dates implementation is also part of the `MockTimers` class. Refer to it for a full list of methods and features.

Note: Dates and timers are dependent when mocked together. This means that if you have both the `Date` and `setTimeout` mocked, advancing the time will also advance the mocked date as they simulate a single internal clock.

The example below show how to mock the `Date` object and obtain the current `Date.now()` value.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks the Date object', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['Date'] });
      // If not specified, the initial date will be based on 0 in the UNIX epoch
      assert.strictEqual(Date.now(), 0);

      // Advance in time will also advance the date
      context.mock.timers.tick(9999);
      assert.strictEqual(Date.now(), 9999);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks the Date object', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['Date'] });
      // If not specified, the initial date will be based on 0 in the UNIX epoch
      assert.strictEqual(Date.now(), 0);

      // Advance in time will also advance the date
      context.mock.timers.tick(9999);
      assert.strictEqual(Date.now(), 9999);
    });
    javascript

If there is no initial epoch set, the initial date will be based on 0 in the Unix epoch. This is January 1st, 1970, 00:00:00 UTC. You can set an initial date by passing a `now` property to the `.enable()` method. This value will be used as the initial date for the mocked `Date` object. It can either be a positive integer, or another Date object.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks the Date object with initial time', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['Date'], now: 100 });
      assert.strictEqual(Date.now(), 100);

      // Advance in time will also advance the date
      context.mock.timers.tick(200);
      assert.strictEqual(Date.now(), 300);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks the Date object with initial time', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['Date'], now: 100 });
      assert.strictEqual(Date.now(), 100);

      // Advance in time will also advance the date
      context.mock.timers.tick(200);
      assert.strictEqual(Date.now(), 300);
    });
    javascript

You can use the `.setTime()` method to manually move the mocked date to another time. This method only accepts a positive integer.

Note: This method will not execute any mocked timers that are in the past from the new time.

In the below example we are setting a new time for the mocked date.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('sets the time of a date object', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['Date'], now: 100 });
      assert.strictEqual(Date.now(), 100);

      // Advance in time will also advance the date
      context.mock.timers.setTime(1000);
      context.mock.timers.tick(200);
      assert.strictEqual(Date.now(), 1200);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('sets the time of a date object', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['Date'], now: 100 });
      assert.strictEqual(Date.now(), 100);

      // Advance in time will also advance the date
      context.mock.timers.setTime(1000);
      context.mock.timers.tick(200);
      assert.strictEqual(Date.now(), 1200);
    });
    javascript

Timers scheduled in the past will not run when you call `setTime()`. To execute those timers, you can use the `.tick()` method to move forward from the new time.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('setTime does not execute timers', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const fn = context.mock.fn();
      setTimeout(fn, 1000);

      context.mock.timers.setTime(800);
      // Timer is not executed as the time is not yet reached
      assert.strictEqual(fn.mock.callCount(), 0);
      assert.strictEqual(Date.now(), 800);

      context.mock.timers.setTime(1200);
      // Timer is still not executed
      assert.strictEqual(fn.mock.callCount(), 0);
      // Advance in time to execute the timer
      context.mock.timers.tick(0);
      assert.strictEqual(fn.mock.callCount(), 1);
      assert.strictEqual(Date.now(), 1200);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('setTime does not execute timers', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const fn = context.mock.fn();
      setTimeout(fn, 1000);

      context.mock.timers.setTime(800);
      // Timer is not executed as the time is not yet reached
      assert.strictEqual(fn.mock.callCount(), 0);
      assert.strictEqual(Date.now(), 800);

      context.mock.timers.setTime(1200);
      // Timer is still not executed
      assert.strictEqual(fn.mock.callCount(), 0);
      // Advance in time to execute the timer
      context.mock.timers.tick(0);
      assert.strictEqual(fn.mock.callCount(), 1);
      assert.strictEqual(Date.now(), 1200);
    });
    javascript

Using `.runAll()` will execute all timers that are currently in the queue. This will also advance the mocked date to the time of the last timer that was executed as if the time has passed.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('runs timers as setTime passes ticks', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const fn = context.mock.fn();
      setTimeout(fn, 1000);
      setTimeout(fn, 2000);
      setTimeout(fn, 3000);

      context.mock.timers.runAll();
      // All timers are executed as the time is now reached
      assert.strictEqual(fn.mock.callCount(), 3);
      assert.strictEqual(Date.now(), 3000);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('runs timers as setTime passes ticks', (context) => {
      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const fn = context.mock.fn();
      setTimeout(fn, 1000);
      setTimeout(fn, 2000);
      setTimeout(fn, 3000);

      context.mock.timers.runAll();
      // All timers are executed as the time is now reached
      assert.strictEqual(fn.mock.callCount(), 3);
      assert.strictEqual(Date.now(), 3000);
    });
    javascript

### Snapshot testing#

Added in: v22.3.0
History

Version | Changes
--- | ---
v23.4.0 | Snapshot testing is no longer experimental.

Snapshot tests allow arbitrary values to be serialized into string values and compared against a set of known good values. The known good values are known as snapshots, and are stored in a snapshot file. Snapshot files are managed by the test runner, but are designed to be human readable to aid in debugging. Best practice is for snapshot files to be checked into source control along with your test files.

Snapshot files are generated by starting Node.js with the `--test-update-snapshots` command-line flag. A separate snapshot file is generated for each test file. By default, the snapshot file has the same name as the test file with a `.snapshot` file extension. This behavior can be configured using the `snapshot.setResolveSnapshotPath()` function. Each snapshot assertion corresponds to an export in the snapshot file.

An example snapshot test is shown below. The first time this test is executed, it will fail because the corresponding snapshot file does not exist.

    // test.js
    suite('suite of snapshot tests', () => {
      test('snapshot test', (t) => {
        t.assert.snapshot({ value1: 1, value2: 2 });
        t.assert.snapshot(5);
      });
    });
    js

Generate the snapshot file by running the test file with `--test-update-snapshots`. The test should pass, and a file named `test.js.snapshot` is created in the same directory as the test file. The contents of the snapshot file are shown below. Each snapshot is identified by the full name of test and a counter to differentiate between snapshots in the same test.

    exports[`suite of snapshot tests > snapshot test 1`] = `
    {
      "value1": 1,
      "value2": 2
    }
    `;

    exports[`suite of snapshot tests > snapshot test 2`] = `
    5
    `;
    js

Once the snapshot file is created, run the tests again without the `--test-update-snapshots` flag. The tests should pass now.

### Test reporters#

Added in: v19.6.0, v18.15.0
History

Version | Changes
--- | ---
v23.0.0 | The default reporter on non-TTY stdout is changed from `tap` to `spec`, aligning with TTY stdout.
v19.9.0, v18.17.0 | Reporters are now exposed at `node:test/reporters`.

The `node:test` module supports passing `--test-reporter` flags for the test runner to use a specific reporter.

The following built-reporters are supported:

- `spec` The `spec` reporter outputs the test results in a human-readable format. This is the default reporter.

- `tap` The `tap` reporter outputs the test results in the TAP format.

- `dot` The `dot` reporter outputs the test results in a compact format, where each passing test is represented by a `.`, and each failing test is represented by a `X`.

- `junit` The junit reporter outputs test results in a jUnit XML format

- `lcov` The `lcov` reporter outputs test coverage when used with the `--experimental-test-coverage` flag.

The exact output of these reporters is subject to change between versions of Node.js, and should not be relied on programmatically. If programmatic access to the test runner's output is required, use the events emitted by the `<TestsStream>`.

The reporters are available via the `node:test/reporters` module:

    import { tap, spec, dot, junit, lcov } from 'node:test/reporters';
    const { tap, spec, dot, junit, lcov } = require('node:test/reporters');
    javascript

#### Custom reporters#

`--test-reporter` can be used to specify a path to custom reporter. A custom reporter is a module that exports a value accepted by stream.compose. Reporters should transform events emitted by a `<TestsStream>`

Example of a custom reporter using `<stream.Transform>`:

    import { Transform } from 'node:stream';

    const customReporter = new Transform({
      writableObjectMode: true,
      transform(event, encoding, callback) {
        switch (event.type) {
          case 'test:dequeue':
            callback(null, `test ${event.data.name} dequeued`);
            break;
          case 'test:enqueue':
            callback(null, `test ${event.data.name} enqueued`);
            break;
          case 'test:watch:drained':
            callback(null, 'test watch queue drained');
            break;
          case 'test:watch:restarted':
            callback(null, 'test watch restarted due to file change');
            break;
          case 'test:start':
            callback(null, `test ${event.data.name} started`);
            break;
          case 'test:pass':
            callback(null, `test ${event.data.name} passed`);
            break;
          case 'test:fail':
            callback(null, `test ${event.data.name} failed`);
            break;
          case 'test:plan':
            callback(null, 'test plan');
            break;
          case 'test:diagnostic':
          case 'test:stderr':
          case 'test:stdout':
            callback(null, event.data.message);
            break;
          case 'test:coverage': {
            const { totalLineCount } = event.data.summary.totals;
            callback(null, `total line count: ${totalLineCount}\n`);
            break;
          }
        }
      },
    });

    export default customReporter;
    const { Transform } = require('node:stream');

    const customReporter = new Transform({
      writableObjectMode: true,
      transform(event, encoding, callback) {
        switch (event.type) {
          case 'test:dequeue':
            callback(null, `test ${event.data.name} dequeued`);
            break;
          case 'test:enqueue':
            callback(null, `test ${event.data.name} enqueued`);
            break;
          case 'test:watch:drained':
            callback(null, 'test watch queue drained');
            break;
          case 'test:watch:restarted':
            callback(null, 'test watch restarted due to file change');
            break;
          case 'test:start':
            callback(null, `test ${event.data.name} started`);
            break;
          case 'test:pass':
            callback(null, `test ${event.data.name} passed`);
            break;
          case 'test:fail':
            callback(null, `test ${event.data.name} failed`);
            break;
          case 'test:plan':
            callback(null, 'test plan');
            break;
          case 'test:diagnostic':
          case 'test:stderr':
          case 'test:stdout':
            callback(null, event.data.message);
            break;
          case 'test:coverage': {
            const { totalLineCount } = event.data.summary.totals;
            callback(null, `total line count: ${totalLineCount}\n`);
            break;
          }
        }
      },
    });

    module.exports = customReporter;
    javascript

Example of a custom reporter using a generator function:

    export default async function * customReporter(source) {
      for await (const event of source) {
        switch (event.type) {
          case 'test:dequeue':
            yield `test ${event.data.name} dequeued\n`;
            break;
          case 'test:enqueue':
            yield `test ${event.data.name} enqueued\n`;
            break;
          case 'test:watch:drained':
            yield 'test watch queue drained\n';
            break;
          case 'test:watch:restarted':
            yield 'test watch restarted due to file change\n';
            break;
          case 'test:start':
            yield `test ${event.data.name} started\n`;
            break;
          case 'test:pass':
            yield `test ${event.data.name} passed\n`;
            break;
          case 'test:fail':
            yield `test ${event.data.name} failed\n`;
            break;
          case 'test:plan':
            yield 'test plan\n';
            break;
          case 'test:diagnostic':
          case 'test:stderr':
          case 'test:stdout':
            yield `${event.data.message}\n`;
            break;
          case 'test:coverage': {
            const { totalLineCount } = event.data.summary.totals;
            yield `total line count: ${totalLineCount}\n`;
            break;
          }
        }
      }
    }
    module.exports = async function * customReporter(source) {
      for await (const event of source) {
        switch (event.type) {
          case 'test:dequeue':
            yield `test ${event.data.name} dequeued\n`;
            break;
          case 'test:enqueue':
            yield `test ${event.data.name} enqueued\n`;
            break;
          case 'test:watch:drained':
            yield 'test watch queue drained\n';
            break;
          case 'test:watch:restarted':
            yield 'test watch restarted due to file change\n';
            break;
          case 'test:start':
            yield `test ${event.data.name} started\n`;
            break;
          case 'test:pass':
            yield `test ${event.data.name} passed\n`;
            break;
          case 'test:fail':
            yield `test ${event.data.name} failed\n`;
            break;
          case 'test:plan':
            yield 'test plan\n';
            break;
          case 'test:diagnostic':
          case 'test:stderr':
          case 'test:stdout':
            yield `${event.data.message}\n`;
            break;
          case 'test:coverage': {
            const { totalLineCount } = event.data.summary.totals;
            yield `total line count: ${totalLineCount}\n`;
            break;
          }
        }
      }
    };
    javascript

The value provided to `--test-reporter` should be a string like one used in an `import()` in JavaScript code, or a value provided for `--import`.

#### Multiple reporters#

The `--test-reporter` flag can be specified multiple times to report test results in several formats. In this situation it is required to specify a destination for each reporter using `--test-reporter-destination`. Destination can be `stdout`, `stderr`, or a file path. Reporters and destinations are paired according to the order they were specified.

In the following example, the `spec` reporter will output to `stdout`, and the `dot` reporter will output to `file.txt`:

    node --test-reporter=spec --test-reporter=dot --test-reporter-destination=stdout --test-reporter-destination=file.txt
    bash

When a single reporter is specified, the destination will default to `stdout`, unless a destination is explicitly provided.

### `run([options])`#

Added in: v18.9.0, v16.19.0
History

Version | Changes
--- | ---
v26.2.0 | Added the `testTagFilters` option.
v25.6.0, v24.14.0 | Add the `env` option.
v24.7.0 | Added a rerunFailuresFilePath option.
v23.0.0 | Added the `cwd` option.
v23.0.0, v22.10.0 | Added coverage options.
v22.8.0 | Added the `isolation` option.
v22.6.0 | Added the `globPatterns` option.
v22.0.0, v20.14.0 | Added the `forceExit` option.
v20.1.0, v18.17.0 | Add a testNamePatterns option.

- `options` `<Object>` Configuration options for running tests. The following properties are supported:
  - `concurrency` `<number>` | `<boolean>` If a number is provided, then that many test processes would run in parallel, where each process corresponds to one test file. If `true`, it would run `os.availableParallelism() - 1` test files in parallel. If `false`, it would only run one test file at a time. Default: `false`.
  - `cwd` `<string>` Specifies the current working directory to be used by the test runner. Serves as the base path for resolving files as if running tests from the command line from that directory. Default: `process.cwd()`.
  - `files` `<Array>` An array containing the list of files to run. Default: Same as running tests from the command line.
  - `forceExit` `<boolean>` Configures the test runner to exit the process once all known tests have finished executing even if the event loop would otherwise remain active. Default: `false`.
  - `globPatterns` `<Array>` An array containing the list of glob patterns to match test files. This option cannot be used together with `files`. Default: Same as running tests from the command line.
  - `inspectPort` `<number>` | `<Function>` Sets inspector port of test child process. This can be a number, or a function that takes no arguments and returns a number. If a nullish value is provided, each process gets its own port, incremented from the primary's `process.debugPort`. This option is ignored if the `isolation` option is set to `'none'` as no child processes are spawned. Default: `undefined`.
  - `isolation` `<string>` Configures the type of test isolation. If set to `'process'`, each test file is run in a separate child process. If set to `'none'`, all test files run in the current process. Default: `'process'`.
  - `only` `<boolean>` If truthy, the test context will only run tests that have the `only` option set
  - `setup` `<Function>` A function that accepts the `TestsStream` instance and can be used to setup listeners before any tests are run. Default: `undefined`.
  - `execArgv` `<Array>` An array of CLI flags to pass to the `node` executable when spawning the subprocesses. This option has no effect when `isolation` is `'none`'. Default: `[]`
  - `argv` `<Array>` An array of CLI flags to pass to each test file when spawning the subprocesses. This option has no effect when `isolation` is `'none'`. Default: `[]`.
  - `signal` `<AbortSignal>` Allows aborting an in-progress test execution.
  - `testNamePatterns` `<string>` | `<RegExp>` | `<Array>` A String, RegExp or a RegExp Array, that can be used to only run tests whose name matches the provided pattern. Test name patterns are interpreted as JavaScript regular expressions. For each test that is executed, any corresponding test hooks, such as `beforeEach()`, are also run. Default: `undefined`.
  - `testSkipPatterns` `<string>` | `<RegExp>` | `<Array>` A String, RegExp or a RegExp Array, that can be used to exclude running tests whose name matches the provided pattern. Test name patterns are interpreted as JavaScript regular expressions. For each test that is executed, any corresponding test hooks, such as `beforeEach()`, are also run. Default: `undefined`.
  - `testTagFilters` `<string>` | `<string>`[] A tag name, or an array of tag names, used to filter tests by their declared tags. Tests must contain every listed tag to run. Equivalent to passing `--experimental-test-tag-filter` on the command line. See Test tags. Default: `undefined`.
  - `timeout` `<number>` A number of milliseconds the test execution will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.
  - `watch` `<boolean>` Whether to run in watch mode or not. Default: `false`.
  - `shard` `<Object>` Running tests in a specific shard. Default: `undefined`.
    - `index` `<number>` is a positive integer between 1 and `<total>` that specifies the index of the shard to run. This option is required.
    - `total` `<number>` is a positive integer that specifies the total number of shards to split the test files to. This option is required.
  - `randomize` `<boolean>` Randomize execution order for test files and queued tests. This option is not supported with `watch: true`. Default: `false`.
  - `randomSeed` `<number>` Seed used when randomizing execution order. If this option is set, runs can replay the same randomized order deterministically, and setting this option also enables randomization. The value must be an integer between `0` and `4294967295`. Default: `undefined`.
  - `rerunFailuresFilePath` `<string>` A file path where the test runner will store the state of the tests to allow rerunning only the failed tests on a next run. see [Rerunning failed tests][] for more information. Default: `undefined`.
  - `coverage` `<boolean>` enable code coverage collection. Default: `false`.
  - `coverageExcludeGlobs` `<string>` | `<Array>` Excludes specific files from code coverage using a glob pattern, which can match both absolute and relative file paths. This property is only applicable when `coverage` was set to `true`. If both `coverageExcludeGlobs` and `coverageIncludeGlobs` are provided, files must meet both criteria to be included in the coverage report. Default: `undefined`.
  - `coverageIncludeGlobs` `<string>` | `<Array>` Includes specific files in code coverage using a glob pattern, which can match both absolute and relative file paths. This property is only applicable when `coverage` was set to `true`. If both `coverageExcludeGlobs` and `coverageIncludeGlobs` are provided, files must meet both criteria to be included in the coverage report. Default: `undefined`.
  - `coverageIncludeAll` `<boolean>` Includes source files that were never loaded by the test run in the coverage report, where they are reported as having zero coverage. Candidate files are searched for in `cwd`, and are subject to the same `coverageIncludeGlobs` and `coverageExcludeGlobs` filtering as the rest of the report. This property is only applicable when `coverage` was set to `true`. Default: `false`.
  - `lineCoverage` `<number>` Require a minimum percent of covered lines. If code coverage does not reach the threshold specified, the process will exit with code `1`. Default: `0`.
  - `branchCoverage` `<number>` Require a minimum percent of covered branches. If code coverage does not reach the threshold specified, the process will exit with code `1`. Default: `0`.
  - `functionCoverage` `<number>` Require a minimum percent of covered functions. If code coverage does not reach the threshold specified, the process will exit with code `1`. Default: `0`.
  - `env` `<Object>` Specify environment variables to be passed along to the test process. This option is not compatible with `isolation='none'`. These variables will override those from the main process, and are not merged with `process.env`. Default: `process.env`.
- Returns: `<TestsStream>`

Note: `shard` is used to horizontally parallelize test running across machines or processes, ideal for large-scale executions across varied environments. It's incompatible with `watch` mode, tailored for rapid code iteration by automatically rerunning tests on file changes.

    import { tap } from 'node:test/reporters';
    import { run } from 'node:test';
    import process from 'node:process';
    import path from 'node:path';

    run({ files: [path.resolve('./tests/test.js')] })
     .on('test:fail', () => {
       process.exitCode = 1;
     })
     .compose(tap)
     .pipe(process.stdout);
    const { tap } = require('node:test/reporters');
    const { run } = require('node:test');
    const path = require('node:path');

    run({ files: [path.resolve('./tests/test.js')] })
     .on('test:fail', () => {
       process.exitCode = 1;
     })
     .compose(tap)
     .pipe(process.stdout);
    javascript

### `suite([name][, options][, fn])`#

Added in: v22.0.0, v20.13.0

- `name` `<string>` The name of the suite, which is displayed when reporting test results. Default: The `name` property of `fn`, or `'<anonymous>'` if `fn` does not have a name.
- `options` `<Object>` Optional configuration options for the suite. This supports the same options as `test([name][, options][, fn])`.
- `fn` `<Function>` | `<AsyncFunction>` The suite function declaring nested tests and suites. The first argument to this function is a `SuiteContext` object. Default: A no-op function.
- Returns: `<Promise>` Immediately fulfilled with `undefined`.

The `suite()` function is imported from the `node:test` module.

### `suite.skip([name][, options][, fn])`#

Added in: v22.0.0, v20.13.0

Shorthand for skipping a suite. This is the same as `suite([name], { skip: true }[, fn])`.

### `suite.todo([name][, options][, fn])`#

Added in: v22.0.0, v20.13.0

Shorthand for marking a suite as `TODO`. This is the same as `suite([name], { todo: true }[, fn])`.

### `suite.only([name][, options][, fn])`#

Added in: v22.0.0, v20.13.0

Shorthand for marking a suite as `only`. This is the same as `suite([name], { only: true }[, fn])`.

### `test([name][, options][, fn])`#

Added in: v18.0.0, v16.17.0
History

Version | Changes
--- | ---
v26.2.0 | Added the `tags` option.
v20.2.0, v18.17.0 | Added the `skip`, `todo`, and `only` shorthands.
v18.8.0, v16.18.0 | Add a `signal` option.
v18.7.0, v16.17.0 | Add a `timeout` option.

- `name` `<string>` The name of the test, which is displayed when reporting test results. Default: The `name` property of `fn`, or `'<anonymous>'` if `fn` does not have a name.
- `options` `<Object>` Configuration options for the test. The following properties are supported:
  - `concurrency` `<number>` | `<boolean>` If a number is provided, then that many tests would run asynchronously (they are still managed by the single-threaded event loop). If `true`, all scheduled asynchronous tests run concurrently within the thread. If `false`, only one test runs at a time. If unspecified, subtests inherit this value from their parent. Default: `false`.
  - `expectFailure` `<boolean>` | `<string>` | `<RegExp>` | `<Function>` | `<Object>` | `<Error>` If truthy, the test is expected to fail. If a non-empty string is provided, that string is displayed in the test results as the reason why the test is expected to fail. If a `<RegExp>` | `<Function>` | `<Object>` | `<Error>` is provided directly (without wrapping in `{ match: … }`), the test passes only if the thrown error matches, following the behavior of `assert.throws`. To provide both a reason and validation, pass an object with `label` (string) and `match` (RegExp, Function, Object, or Error). Default: `false`.
  - `only` `<boolean>` If truthy, and the test context is configured to run `only` tests, then this test will be run. Otherwise, the test is skipped. Default: `false`.
  - `signal` `<AbortSignal>` Allows aborting an in-progress test.
  - `skip` `<boolean>` | `<string>` If truthy, the test is skipped. If a string is provided, that string is displayed in the test results as the reason for skipping the test. Default: `false`.
  - `tags` `<string>`[] An array of string labels associated with the test. Used together with `--experimental-test-tag-filter` to filter which tests run. Tags inherit from suites to nested tests by union. See Test tags. Default: `[]`.
  - `todo` `<boolean>` | `<string>` If truthy, the test marked as `TODO`. If a string is provided, that string is displayed in the test results as the reason why the test is `TODO`. Default: `false`.
  - `timeout` `<number>` A number of milliseconds the test will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.
  - `plan` `<number>` The number of assertions and subtests expected to be run in the test. If the number of assertions run in the test does not match the number specified in the plan, the test will fail. Default: `undefined`.
  - `fn` `<Function>` | `<AsyncFunction>` The function under test. If provided, it will take precedence over the `fn` parameter.
  - `name` `<string>` The name of the test. If provided, it will take precedence over the `name` parameter.
- `fn` `<Function>` | `<AsyncFunction>` The function under test. The first argument to this function is a `TestContext` object. If the test uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- Returns: `<Promise>` Fulfilled with `undefined` once the test completes, or immediately if the test runs within a suite.

The `test()` function is the value imported from the `test` module. Each invocation of this function results in reporting the test to the `<TestsStream>`.

The `TestContext` object passed to the `fn` argument can be used to perform actions related to the current test. Examples include skipping the test, adding additional diagnostic information, or creating subtests.

`test()` returns a `Promise` that fulfills once the test completes. if `test()` is called within a suite, it fulfills immediately. The return value can usually be discarded for top level tests. However, the return value from subtests should be used to prevent the parent test from finishing first and cancelling the subtest as shown in the following example.

    test('top level test', async (t) => {
      // The setTimeout() in the following subtest would cause it to outlive its
      // parent test if 'await' is removed on the next line. Once the parent test
      // completes, it will cancel any outstanding subtests.
      await t.test('longer running subtest', async (t) => {
        return new Promise((resolve, reject) => {
          setTimeout(resolve, 1000);
        });
      });
    });
    js

The `timeout` option can be used to fail the test if it takes longer than `timeout` milliseconds to complete. However, it is not a reliable mechanism for canceling tests because a running test might block the application thread and thus prevent the scheduled cancellation.

### `test.skip([name][, options][, fn])`#

Shorthand for skipping a test, same as `test([name], { skip: true }[, fn])`.

### `test.todo([name][, options][, fn])`#

Shorthand for marking a test as `TODO`, same as `test([name], { todo: true }[, fn])`.

### `test.only([name][, options][, fn])`#

Shorthand for marking a test as `only`, same as `test([name], { only: true }[, fn])`.

### `describe([name][, options][, fn])`#

Alias for `suite()`.

The `describe()` function is imported from the `node:test` module.

### `describe.skip([name][, options][, fn])`#

Shorthand for skipping a suite. This is the same as `describe([name], { skip: true }[, fn])`.

### `describe.todo([name][, options][, fn])`#

Shorthand for marking a suite as `TODO`. This is the same as `describe([name], { todo: true }[, fn])`.

### `describe.only([name][, options][, fn])`#

Added in: v19.8.0, v18.15.0

Shorthand for marking a suite as `only`. This is the same as `describe([name], { only: true }[, fn])`.

### `it([name][, options][, fn])`#

Added in: v18.6.0, v16.17.0
History

Version | Changes
--- | ---
v19.8.0, v18.16.0 | Calling `it()` is now equivalent to calling `test()`.

Alias for `test()`.

The `it()` function is imported from the `node:test` module.

### `it.skip([name][, options][, fn])`#

Shorthand for skipping a test, same as `it([name], { skip: true }[, fn])`.

### `it.todo([name][, options][, fn])`#

Shorthand for marking a test as `TODO`, same as `it([name], { todo: true }[, fn])`.

### `it.only([name][, options][, fn])`#

Added in: v19.8.0, v18.15.0

Shorthand for marking a test as `only`, same as `it([name], { only: true }[, fn])`.

### `before([fn][, options])`#

Added in: v18.8.0, v16.18.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function creates a hook that runs before executing a suite.

    describe('tests', async () => {
      before(() => console.log('about to run some test'));
      it('is a subtest', () => {
        // Some relevant assertions here
      });
    });
    js

### `after([fn][, options])`#

Added in: v18.8.0, v16.18.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function creates a hook that runs after executing a suite.

    describe('tests', async () => {
      after(() => console.log('finished running tests'));
      it('is a subtest', () => {
        // Some relevant assertion here
      });
    });
    js

Note: The `after` hook is guaranteed to run, even if tests within the suite fail.

### `beforeEach([fn][, options])`#

Added in: v18.8.0, v16.18.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function creates a hook that runs before each test in the current suite.

    describe('tests', async () => {
      beforeEach(() => console.log('about to run a test'));
      it('is a subtest', () => {
        // Some relevant assertion here
      });
    });
    js

### `afterEach([fn][, options])`#

Added in: v18.8.0, v16.18.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function creates a hook that runs after each test in the current suite. The `afterEach()` hook is run even if the test fails.

    describe('tests', async () => {
      afterEach(() => console.log('finished running a test'));
      it('is a subtest', () => {
        // Some relevant assertion here
      });
    });
    js

### `assert`#

Added in: v23.7.0, v22.14.0

An object whose methods are used to configure available assertions on the `TestContext` objects in the current process. The methods from `node:assert` and snapshot testing functions are available by default.

It is possible to apply the same configuration to all files by placing common configuration code in a module preloaded with `--require` or `--import`.

#### `assert.register(name, fn)`#

Added in: v23.7.0, v22.14.0

Defines a new assertion function with the provided name and function. If an assertion already exists with the same name, it is overwritten.

### `snapshot`#

Added in: v22.3.0

An object whose methods are used to configure default snapshot settings in the current process. It is possible to apply the same configuration to all files by placing common configuration code in a module preloaded with `--require` or `--import`.

#### `snapshot.setDefaultSnapshotSerializers(serializers)`#

Added in: v22.3.0

- `serializers` `<Array>` An array of synchronous functions used as the default serializers for snapshot tests.

This function is used to customize the default serialization mechanism used by the test runner. By default, the test runner performs serialization by calling `JSON.stringify(value, null, 2)` on the provided value. `JSON.stringify()` does have limitations regarding circular structures and supported data types. If a more robust serialization mechanism is required, this function should be used.

#### `snapshot.setResolveSnapshotPath(fn)`#

Added in: v22.3.0

- `fn` `<Function>` A function used to compute the location of the snapshot file. The function receives the path of the test file as its only argument. If the test is not associated with a file (for example in the REPL), the input is undefined. `fn()` must return a string specifying the location of the snapshot file.

This function is used to customize the location of the snapshot file used for snapshot testing. By default, the snapshot filename is the same as the entry point filename with a `.snapshot` file extension.

### Class: `MockFunctionContext`#

Added in: v19.1.0, v18.13.0

The `MockFunctionContext` class is used to inspect or manipulate the behavior of mocks created via the `MockTracker` APIs.

#### `ctx.calls`#

Added in: v19.1.0, v18.13.0

- Type: `<Array>`

A getter that returns a copy of the internal array used to track calls to the mock. Each entry in the array is an object with the following properties.

- `arguments` `<Array>` An array of the arguments passed to the mock function.
- `error` `<any>` If the mocked function threw then this property contains the thrown value. Default: `undefined`.
- `result` `<any>` The value returned by the mocked function.
- `stack` `<Error>` An `Error` object whose stack can be used to determine the callsite of the mocked function invocation.
- `target` `<Function>` | `<undefined>` If the mocked function is a constructor, this field contains the class being constructed. Otherwise this will be `undefined`.
- `this` `<any>` The mocked function's `this` value.

#### `ctx.callCount()`#

Added in: v19.1.0, v18.13.0

- Returns: `<integer>` The number of times that this mock has been invoked.

This function returns the number of times that this mock has been invoked. This function is more efficient than checking `ctx.calls.length` because `ctx.calls` is a getter that creates a copy of the internal call tracking array.

#### `ctx.mockImplementation(implementation)`#

Added in: v19.1.0, v18.13.0

- `implementation` `<Function>` | `<AsyncFunction>` The function to be used as the mock's new implementation.

This function is used to change the behavior of an existing mock.

The following example creates a mock function using `t.mock.fn()`, calls the mock function, and then changes the mock implementation to a different function.

    test('changes a mock behavior', (t) => {
      let cnt = 0;

      function addOne() {
        cnt++;
        return cnt;
      }

      function addTwo() {
        cnt += 2;
        return cnt;
      }

      const fn = t.mock.fn(addOne);

      assert.strictEqual(fn(), 1);
      fn.mock.mockImplementation(addTwo);
      assert.strictEqual(fn(), 3);
      assert.strictEqual(fn(), 5);
    });
    js

#### `ctx.mockImplementationOnce(implementation[, onCall])`#

Added in: v19.1.0, v18.13.0

- `implementation` `<Function>` | `<AsyncFunction>` The function to be used as the mock's implementation for the invocation number specified by `onCall`.
- `onCall` `<integer>` The invocation number that will use `implementation`. If the specified invocation has already occurred then an exception is thrown. Default: The number of the next invocation.

This function is used to change the behavior of an existing mock for a single invocation. Once invocation `onCall` has occurred, the mock will revert to whatever behavior it would have used had `mockImplementationOnce()` not been called.

The following example creates a mock function using `t.mock.fn()`, calls the mock function, changes the mock implementation to a different function for the next invocation, and then resumes its previous behavior.

    test('changes a mock behavior once', (t) => {
      let cnt = 0;

      function addOne() {
        cnt++;
        return cnt;
      }

      function addTwo() {
        cnt += 2;
        return cnt;
      }

      const fn = t.mock.fn(addOne);

      assert.strictEqual(fn(), 1);
      fn.mock.mockImplementationOnce(addTwo);
      assert.strictEqual(fn(), 3);
      assert.strictEqual(fn(), 4);
    });
    js

#### `ctx.resetCalls()`#

Added in: v19.3.0, v18.13.0

Resets the call history of the mock function.

#### `ctx.restore()`#

Added in: v19.1.0, v18.13.0

Resets the implementation of the mock function to its original behavior. The mock can still be used after calling this function.

### Class: `MockModuleContext`#

Added in: v22.3.0, v20.18.0

Stability: 1.0 - Early development

The `MockModuleContext` class is used to manipulate the behavior of module mocks created via the `MockTracker` APIs.

#### `ctx.restore()`#

Added in: v22.3.0, v20.18.0

Resets the implementation of the mock module.

### Class: `MockPropertyContext`#

Added in: v24.3.0, v22.20.0

The `MockPropertyContext` class is used to inspect or manipulate the behavior of property mocks created via the `MockTracker` APIs.

#### `ctx.accesses`#

- Type: `<Array>`

A getter that returns a copy of the internal array used to track accesses (get/set) to the mocked property. Each entry in the array is an object with the following properties:

- `type` `<string>` Either `'get'` or `'set'`, indicating the type of access.
- `value` `<any>` The value that was read (for `'get'`) or written (for `'set'`).
- `stack` `<Error>` An `Error` object whose stack can be used to determine the callsite of the mocked function invocation.

#### `ctx.accessCount()`#

- Returns: `<integer>` The number of times that the property was accessed (read or written).

This function returns the number of times that the property was accessed. This function is more efficient than checking `ctx.accesses.length` because `ctx.accesses` is a getter that creates a copy of the internal access tracking array.

#### `ctx.mockImplementation(value)`#

- `value` `<any>` The new value to be set as the mocked property value.

This function is used to change the value returned by the mocked property getter.

#### `ctx.mockImplementationOnce(value[, onAccess])`#

- `value` `<any>` The value to be used as the mock's implementation for the invocation number specified by `onAccess`.
- `onAccess` `<integer>` The invocation number that will use `value`. If the specified invocation has already occurred then an exception is thrown. Default: The number of the next invocation.

This function is used to change the behavior of an existing mock for a single invocation. Once invocation `onAccess` has occurred, the mock will revert to whatever behavior it would have used had `mockImplementationOnce()` not been called.

The following example creates a mock function using `t.mock.property()`, calls the mock property, changes the mock implementation to a different value for the next invocation, and then resumes its previous behavior.

    test('changes a mock behavior once', (t) => {
      const obj = { foo: 1 };

      const prop = t.mock.property(obj, 'foo', 5);

      assert.strictEqual(obj.foo, 5);
      prop.mock.mockImplementationOnce(25);
      assert.strictEqual(obj.foo, 25);
      assert.strictEqual(obj.foo, 5);
    });
    js

##### Caveat#

For consistency with the rest of the mocking API, this function treats both property gets and sets as accesses. If a property set occurs at the same access index, the "once" value will be consumed by the set operation, and the mocked property value will be changed to the "once" value. This may lead to unexpected behavior if you intend the "once" value to only be used for a get operation.

#### `ctx.resetAccesses()`#

Resets the access history of the mocked property.

#### `ctx.restore()`#

Resets the implementation of the mock property to its original behavior. The mock can still be used after calling this function.

### Class: `MockTracker`#

Added in: v19.1.0, v18.13.0

The `MockTracker` class is used to manage mocking functionality. The test runner module provides a top level `mock` export which is a `MockTracker` instance. Each test also provides its own `MockTracker` instance via the test context's `mock` property.

#### `mock.fn([original[, implementation]][, options])`#

Added in: v19.1.0, v18.13.0

- `original` `<Function>` | `<AsyncFunction>` An optional function to create a mock on. Default: A no-op function.
- `implementation` `<Function>` | `<AsyncFunction>` An optional function used as the mock implementation for `original`. This is useful for creating mocks that exhibit one behavior for a specified number of calls and then restore the behavior of `original`. Default: The function specified by `original`.
- `options` `<Object>` Optional configuration options for the mock function. The following properties are supported:
  - `times` `<integer>` The number of times that the mock will use the behavior of `implementation`. Once the mock function has been called `times` times, it will automatically restore the behavior of `original`. This value must be an integer greater than zero. Default: `Infinity`.
- Returns: `<Proxy>` The mocked function. The mocked function contains a special `mock` property, which is an instance of `MockFunctionContext`, and can be used for inspecting and changing the behavior of the mocked function.

This function is used to create a mock function.

The following example creates a mock function that increments a counter by one on each invocation. The `times` option is used to modify the mock behavior such that the first two invocations add two to the counter instead of one.

    test('mocks a counting function', (t) => {
      let cnt = 0;

      function addOne() {
        cnt++;
        return cnt;
      }

      function addTwo() {
        cnt += 2;
        return cnt;
      }

      const fn = t.mock.fn(addOne, addTwo, { times: 2 });

      assert.strictEqual(fn(), 2);
      assert.strictEqual(fn(), 4);
      assert.strictEqual(fn(), 5);
      assert.strictEqual(fn(), 6);
    });
    js

#### `mock.getter(object, methodName[, implementation][, options])`#

Added in: v19.3.0, v18.13.0

This function is syntax sugar for `MockTracker.method` with `options.getter` set to `true`.

#### `mock.method(object, methodName[, implementation][, options])`#

Added in: v19.1.0, v18.13.0

- `object` `<Object>` The object whose method is being mocked.
- `methodName` `<string>` | `<symbol>` The identifier of the method on `object` to mock. If `object[methodName]` is not a function, an error is thrown.
- `implementation` `<Function>` | `<AsyncFunction>` An optional function used as the mock implementation for `object[methodName]`. Default: The original method specified by `object[methodName]`.
- `options` `<Object>` Optional configuration options for the mock method. The following properties are supported:
  - `getter` `<boolean>` If `true`, `object[methodName]` is treated as a getter. This option cannot be used with the `setter` option. Default: false.
  - `setter` `<boolean>` If `true`, `object[methodName]` is treated as a setter. This option cannot be used with the `getter` option. Default: false.
  - `times` `<integer>` The number of times that the mock will use the behavior of `implementation`. Once the mocked method has been called `times` times, it will automatically restore the original behavior. This value must be an integer greater than zero. Default: `Infinity`.
- Returns: `<Proxy>` The mocked method. The mocked method contains a special `mock` property, which is an instance of `MockFunctionContext`, and can be used for inspecting and changing the behavior of the mocked method.

This function is used to create a mock on an existing object method. The following example demonstrates how a mock is created on an existing object method.

    test('spies on an object method', (t) => {
      const number = {
        value: 5,
        subtract(a) {
          return this.value - a;
        },
      };

      t.mock.method(number, 'subtract');
      assert.strictEqual(number.subtract.mock.callCount(), 0);
      assert.strictEqual(number.subtract(3), 2);
      assert.strictEqual(number.subtract.mock.callCount(), 1);

      const call = number.subtract.mock.calls[0];

      assert.deepStrictEqual(call.arguments, [3]);
      assert.strictEqual(call.result, 2);
      assert.strictEqual(call.error, undefined);
      assert.strictEqual(call.target, undefined);
      assert.strictEqual(call.this, number);
    });
    js

#### `mock.module(specifier[, options])`#

Added in: v22.3.0, v20.18.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Support JSON modules.

Stability: 1.0 - Early development

- `specifier` `<string>` | `<URL>` A string identifying the module to mock.
- `options` `<Object>` Optional configuration options for the mock module. The following properties are supported:
  - `cache` `<boolean>` If `false`, each call to `require()` or `import()` generates a new mock module. If `true`, subsequent calls will return the same module mock, and the mock module is inserted into the CommonJS cache. Default: false.
  - `exports` `<Object>` Optional mocked exports. The `default` property, if provided, is used as the mocked module's default export. All other own enumerable properties are used as named exports. This option cannot be used with `defaultExport` or `namedExports`.
    - If the mock is a CommonJS or builtin module, `exports.default` is used as the value of `module.exports`.
    - If `exports.default` is not provided for a CommonJS or builtin mock, `module.exports` defaults to an empty object.
    - If named exports are provided with a non-object default export, the mock throws an exception when used as a CommonJS or builtin module.
  - `defaultExport` `<any>` An optional value used as the mocked module's default export. If this value is not provided, ESM mocks do not include a default export. If the mock is a CommonJS or builtin module, this setting is used as the value of `module.exports`. If this value is not provided, CJS and builtin mocks use an empty object as the value of `module.exports`. This option cannot be used with `options.exports`. This option is deprecated and will be removed in a later version. Prefer `options.exports.default`.
  - `namedExports` `<Object>` An optional object whose keys and values are used to create the named exports of the mock module. If the mock is a CommonJS or builtin module, these values are copied onto `module.exports`. Therefore, if a mock is created with both named exports and a non-object default export, the mock will throw an exception when used as a CJS or builtin module. This option cannot be used with `options.exports`. This option is deprecated and will be removed in a later version. Prefer `options.exports`.
- Returns: `<MockModuleContext>` An object that can be used to manipulate the mock.

This function is used to mock the exports of ECMAScript modules, CommonJS modules, JSON modules, and Node.js builtin modules. Any references to the original module prior to mocking are not impacted. In order to enable module mocking, Node.js must be started with the `--experimental-test-module-mocks` command-line flag.

Note: module customization hooks registered via the synchronous API effect resolution of the `specifier` provided to `mock.module`. Customization hooks registered via the asynchronous API are currently ignored (because the test runner's loader is synchronous, and node does not support multi-chain / cross-chain loading).

The following example demonstrates how a mock is created for a module.

    test('mocks a builtin module in both module systems', async (t) => {
      // Create a mock of 'node:readline' with a named export named 'foo', which
      // does not exist in the original 'node:readline' module.
      const mock = t.mock.module('node:readline', {
        exports: { foo: () => 42 },
      });

      let esmImpl = await import('node:readline');
      let cjsImpl = require('node:readline');

      // cursorTo() is an export of the original 'node:readline' module.
      assert.strictEqual(esmImpl.cursorTo, undefined);
      assert.strictEqual(cjsImpl.cursorTo, undefined);
      assert.strictEqual(esmImpl.foo(), 42);
      assert.strictEqual(cjsImpl.foo(), 42);

      mock.restore();

      // The mock is restored, so the original builtin module is returned.
      esmImpl = await import('node:readline');
      cjsImpl = require('node:readline');

      assert.strictEqual(typeof esmImpl.cursorTo, 'function');
      assert.strictEqual(typeof cjsImpl.cursorTo, 'function');
      assert.strictEqual(esmImpl.foo, undefined);
      assert.strictEqual(cjsImpl.foo, undefined);
    });
    js

#### `mock.property(object, propertyName[, value])`#

Added in: v24.3.0, v22.20.0

- `object` `<Object>` The object whose value is being mocked.
- `propertyName` `<string>` | `<symbol>` The identifier of the property on `object` to mock.
- `value` `<any>` An optional value used as the mock value for `object[propertyName]`. Default: The original property value.
- Returns: `<Proxy>` A proxy to the mocked object. The mocked object contains a special `mock` property, which is an instance of `MockPropertyContext`, and can be used for inspecting and changing the behavior of the mocked property.

Creates a mock for a property value on an object. This allows you to track and control access to a specific property, including how many times it is read (getter) or written (setter), and to restore the original value after mocking.

    test('mocks a property value', (t) => {
      const obj = { foo: 42 };
      const prop = t.mock.property(obj, 'foo', 100);

      assert.strictEqual(obj.foo, 100);
      assert.strictEqual(prop.mock.accessCount(), 1);
      assert.strictEqual(prop.mock.accesses[0].type, 'get');
      assert.strictEqual(prop.mock.accesses[0].value, 100);

      obj.foo = 200;
      assert.strictEqual(prop.mock.accessCount(), 2);
      assert.strictEqual(prop.mock.accesses[1].type, 'set');
      assert.strictEqual(prop.mock.accesses[1].value, 200);

      prop.mock.restore();
      assert.strictEqual(obj.foo, 42);
    });
    js

#### `mock.reset()`#

Added in: v19.1.0, v18.13.0

This function restores the default behavior of all mocks that were previously created by this `MockTracker` and disassociates the mocks from the `MockTracker` instance. Once disassociated, the mocks can still be used, but the `MockTracker` instance can no longer be used to reset their behavior or otherwise interact with them.

After each test completes, this function is called on the test context's `MockTracker`. If the global `MockTracker` is used extensively, calling this function manually is recommended.

#### `mock.restoreAll()`#

Added in: v19.1.0, v18.13.0

This function restores the default behavior of all mocks that were previously created by this `MockTracker`. Unlike `mock.reset()`, `mock.restoreAll()` does not disassociate the mocks from the `MockTracker` instance.

#### `mock.setter(object, methodName[, implementation][, options])`#

Added in: v19.3.0, v18.13.0

This function is syntax sugar for `MockTracker.method` with `options.setter` set to `true`.

### Class: `MockTimers`#

Added in: v20.4.0, v18.19.0
History

Version | Changes
--- | ---
v23.1.0 | The Mock Timers is now stable.

Mocking timers is a technique commonly used in software testing to simulate and control the behavior of timers, such as `setInterval` and `setTimeout`, without actually waiting for the specified time intervals.

MockTimers is also able to mock the `Date` object.

The `MockTracker` provides a top-level `timers` export which is a `MockTimers` instance.

#### `timers.enable([enableOptions])`#

Added in: v20.4.0, v18.19.0
History

Version | Changes
--- | ---
v21.2.0, v20.11.0 | Updated parameters to be an option object with available APIs and the default initial epoch.

Enables timer mocking for the specified timers.

- `enableOptions` `<Object>` Optional configuration options for enabling timer mocking. The following properties are supported:
  - `apis` `<Array>` An optional array containing the timers to mock. The currently supported timer values are `'setInterval'`, `'setTimeout'`, `'setImmediate'`, and `'Date'`. Default: `['setInterval', 'setTimeout', 'setImmediate', 'Date']`. If no array is provided, all time related APIs (`'setInterval'`, `'clearInterval'`, `'setTimeout'`, `'clearTimeout'`, `'setImmediate'`, `'clearImmediate'`, and `'Date'`) will be mocked by default.
  - `now` `<number>` | `<Date>` An optional number or Date object representing the initial time (in milliseconds) to use as the value for `Date.now()`. Default: `0`.

Note: When you enable mocking for a specific timer, its associated clear function will also be implicitly mocked.

Note: Mocking `Date` will affect the behavior of the mocked timers as they use the same internal clock.

Example usage without setting initial time:

    import { mock } from 'node:test';
    mock.timers.enable({ apis: ['setInterval'] });
    const { mock } = require('node:test');
    mock.timers.enable({ apis: ['setInterval'] });
    javascript

The above example enables mocking for the `setInterval` timer and implicitly mocks the `clearInterval` function. Only the `setInterval` and `clearInterval` functions from node:timers, node:timers/promises, and `globalThis` will be mocked.

Example usage with initial time set

    import { mock } from 'node:test';
    mock.timers.enable({ apis: ['Date'], now: 1000 });
    const { mock } = require('node:test');
    mock.timers.enable({ apis: ['Date'], now: 1000 });
    javascript

Example usage with initial Date object as time set

    import { mock } from 'node:test';
    mock.timers.enable({ apis: ['Date'], now: new Date() });
    const { mock } = require('node:test');
    mock.timers.enable({ apis: ['Date'], now: new Date() });
    javascript

Alternatively, if you call `mock.timers.enable()` without any parameters:

All timers (`'setInterval'`, `'clearInterval'`, `'setTimeout'`, `'clearTimeout'`, `'setImmediate'`, and `'clearImmediate'`) will be mocked. The `setInterval`, `clearInterval`, `setTimeout`, `clearTimeout`, `setImmediate`, and `clearImmediate` functions from `node:timers`, `node:timers/promises`, and `globalThis` will be mocked. As well as the global `Date` object.

#### `timers.reset()`#

Added in: v20.4.0, v18.19.0

This function restores the default behavior of all mocks that were previously created by this `MockTimers` instance and disassociates the mocks from the `MockTracker` instance.

Note: After each test completes, this function is called on the test context's `MockTracker`.

    import { mock } from 'node:test';
    mock.timers.reset();
    const { mock } = require('node:test');
    mock.timers.reset();
    javascript

#### `timers[Symbol.dispose]()`#

Calls `timers.reset()`.

#### `timers.tick([milliseconds])`#

Added in: v20.4.0, v18.19.0

Advances time for all mocked timers.

- `milliseconds` `<number>` The amount of time, in milliseconds, to advance the timers. Default: `1`.

Note: This diverges from how `setTimeout` in Node.js behaves and accepts only positive numbers. In Node.js, `setTimeout` with negative numbers is only supported for web compatibility reasons.

The following example mocks a `setTimeout` function and by using `.tick` advances in time triggering all pending timers.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();

      context.mock.timers.enable({ apis: ['setTimeout'] });

      setTimeout(fn, 9999);

      assert.strictEqual(fn.mock.callCount(), 0);

      // Advance in time
      context.mock.timers.tick(9999);

      assert.strictEqual(fn.mock.callCount(), 1);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();
      context.mock.timers.enable({ apis: ['setTimeout'] });

      setTimeout(fn, 9999);
      assert.strictEqual(fn.mock.callCount(), 0);

      // Advance in time
      context.mock.timers.tick(9999);

      assert.strictEqual(fn.mock.callCount(), 1);
    });
    javascript

Alternatively, the `.tick` function can be called many times

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();
      context.mock.timers.enable({ apis: ['setTimeout'] });
      const nineSecs = 9000;
      setTimeout(fn, nineSecs);

      const threeSeconds = 3000;
      context.mock.timers.tick(threeSeconds);
      context.mock.timers.tick(threeSeconds);
      context.mock.timers.tick(threeSeconds);

      assert.strictEqual(fn.mock.callCount(), 1);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();
      context.mock.timers.enable({ apis: ['setTimeout'] });
      const nineSecs = 9000;
      setTimeout(fn, nineSecs);

      const threeSeconds = 3000;
      context.mock.timers.tick(threeSeconds);
      context.mock.timers.tick(threeSeconds);
      context.mock.timers.tick(threeSeconds);

      assert.strictEqual(fn.mock.callCount(), 1);
    });
    javascript

Advancing time using `.tick` will also advance the time for any `Date` object created after the mock was enabled (if `Date` was also set to be mocked).

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();

      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      setTimeout(fn, 9999);

      assert.strictEqual(fn.mock.callCount(), 0);
      assert.strictEqual(Date.now(), 0);

      // Advance in time
      context.mock.timers.tick(9999);
      assert.strictEqual(fn.mock.callCount(), 1);
      assert.strictEqual(Date.now(), 9999);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });

      setTimeout(fn, 9999);
      assert.strictEqual(fn.mock.callCount(), 0);
      assert.strictEqual(Date.now(), 0);

      // Advance in time
      context.mock.timers.tick(9999);
      assert.strictEqual(fn.mock.callCount(), 1);
      assert.strictEqual(Date.now(), 9999);
    });
    javascript

##### Using clear functions#

As mentioned, all clear functions from timers (`clearTimeout`, `clearInterval`,and `clearImmediate`) are implicitly mocked. Take a look at this example using `setTimeout`:

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();

      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout'] });
      const id = setTimeout(fn, 9999);

      // Implicitly mocked as well
      clearTimeout(id);
      context.mock.timers.tick(9999);

      // As that setTimeout was cleared the mock function will never be called
      assert.strictEqual(fn.mock.callCount(), 0);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', (context) => {
      const fn = context.mock.fn();

      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout'] });
      const id = setTimeout(fn, 9999);

      // Implicitly mocked as well
      clearTimeout(id);
      context.mock.timers.tick(9999);

      // As that setTimeout was cleared the mock function will never be called
      assert.strictEqual(fn.mock.callCount(), 0);
    });
    javascript

##### Working with Node.js timers modules#

Once you enable mocking timers, node:timers, node:timers/promises modules, and timers from the Node.js global context are enabled:

Note: Destructuring functions such as `import { setTimeout } from 'node:timers'` is currently not supported by this API.

    import assert from 'node:assert';
    import { test } from 'node:test';
    import nodeTimers from 'node:timers';
    import nodeTimersPromises from 'node:timers/promises';

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', async (context) => {
      const globalTimeoutObjectSpy = context.mock.fn();
      const nodeTimerSpy = context.mock.fn();
      const nodeTimerPromiseSpy = context.mock.fn();

      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout'] });
      setTimeout(globalTimeoutObjectSpy, 9999);
      nodeTimers.setTimeout(nodeTimerSpy, 9999);

      const promise = nodeTimersPromises.setTimeout(9999).then(nodeTimerPromiseSpy);

      // Advance in time
      context.mock.timers.tick(9999);
      assert.strictEqual(globalTimeoutObjectSpy.mock.callCount(), 1);
      assert.strictEqual(nodeTimerSpy.mock.callCount(), 1);
      await promise;
      assert.strictEqual(nodeTimerPromiseSpy.mock.callCount(), 1);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');
    const nodeTimers = require('node:timers');
    const nodeTimersPromises = require('node:timers/promises');

    test('mocks setTimeout to be executed synchronously without having to actually wait for it', async (context) => {
      const globalTimeoutObjectSpy = context.mock.fn();
      const nodeTimerSpy = context.mock.fn();
      const nodeTimerPromiseSpy = context.mock.fn();

      // Optionally choose what to mock
      context.mock.timers.enable({ apis: ['setTimeout'] });
      setTimeout(globalTimeoutObjectSpy, 9999);
      nodeTimers.setTimeout(nodeTimerSpy, 9999);

      const promise = nodeTimersPromises.setTimeout(9999).then(nodeTimerPromiseSpy);

      // Advance in time
      context.mock.timers.tick(9999);
      assert.strictEqual(globalTimeoutObjectSpy.mock.callCount(), 1);
      assert.strictEqual(nodeTimerSpy.mock.callCount(), 1);
      await promise;
      assert.strictEqual(nodeTimerPromiseSpy.mock.callCount(), 1);
    });
    javascript

In Node.js, `setInterval` from node:timers/promises is an `AsyncGenerator` and is also supported by this API:

    import assert from 'node:assert';
    import { test } from 'node:test';
    import nodeTimersPromises from 'node:timers/promises';
    test('should tick five times testing a real use case', async (context) => {
      context.mock.timers.enable({ apis: ['setInterval'] });

      const expectedIterations = 3;
      const interval = 1000;
      const startedAt = Date.now();
      async function run() {
        const times = [];
        for await (const time of nodeTimersPromises.setInterval(interval, startedAt)) {
          times.push(time);
          if (times.length === expectedIterations) break;
        }
        return times;
      }

      const r = run();
      context.mock.timers.tick(interval);
      context.mock.timers.tick(interval);
      context.mock.timers.tick(interval);

      const timeResults = await r;
      assert.strictEqual(timeResults.length, expectedIterations);
      for (let it = 1; it < expectedIterations; it++) {
        assert.strictEqual(timeResults[it - 1], startedAt + (interval * it));
      }
    });
    const assert = require('node:assert');
    const { test } = require('node:test');
    const nodeTimersPromises = require('node:timers/promises');
    test('should tick five times testing a real use case', async (context) => {
      context.mock.timers.enable({ apis: ['setInterval'] });

      const expectedIterations = 3;
      const interval = 1000;
      const startedAt = Date.now();
      async function run() {
        const times = [];
        for await (const time of nodeTimersPromises.setInterval(interval, startedAt)) {
          times.push(time);
          if (times.length === expectedIterations) break;
        }
        return times;
      }

      const r = run();
      context.mock.timers.tick(interval);
      context.mock.timers.tick(interval);
      context.mock.timers.tick(interval);

      const timeResults = await r;
      assert.strictEqual(timeResults.length, expectedIterations);
      for (let it = 1; it < expectedIterations; it++) {
        assert.strictEqual(timeResults[it - 1], startedAt + (interval * it));
      }
    });
    javascript

#### `timers.runAll()`#

Added in: v20.4.0, v18.19.0

Triggers all pending mocked timers immediately. If the `Date` object is also mocked, it will also advance the `Date` object to the furthest timer's time.

The example below triggers all pending timers immediately, causing them to execute without any delay.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('runAll functions following the given order', (context) => {
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const results = [];
      setTimeout(() => results.push(1), 9999);

      // Notice that if both timers have the same timeout,
      // the order of execution is guaranteed
      setTimeout(() => results.push(3), 8888);
      setTimeout(() => results.push(2), 8888);

      assert.deepStrictEqual(results, []);

      context.mock.timers.runAll();
      assert.deepStrictEqual(results, [3, 2, 1]);
      // The Date object is also advanced to the furthest timer's time
      assert.strictEqual(Date.now(), 9999);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('runAll functions following the given order', (context) => {
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const results = [];
      setTimeout(() => results.push(1), 9999);

      // Notice that if both timers have the same timeout,
      // the order of execution is guaranteed
      setTimeout(() => results.push(3), 8888);
      setTimeout(() => results.push(2), 8888);

      assert.deepStrictEqual(results, []);

      context.mock.timers.runAll();
      assert.deepStrictEqual(results, [3, 2, 1]);
      // The Date object is also advanced to the furthest timer's time
      assert.strictEqual(Date.now(), 9999);
    });
    javascript

Note: The `runAll()` function is specifically designed for triggering timers in the context of timer mocking. It does not have any effect on real-time system clocks or actual timers outside of the mocking environment.

#### `timers.setTime(milliseconds)`#

Added in: v21.2.0, v20.11.0

Sets the current Unix timestamp that will be used as reference for any mocked `Date` objects.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('runAll functions following the given order', (context) => {
      const now = Date.now();
      const setTime = 1000;
      // Date.now is not mocked
      assert.deepStrictEqual(Date.now(), now);

      context.mock.timers.enable({ apis: ['Date'] });
      context.mock.timers.setTime(setTime);
      // Date.now is now 1000
      assert.strictEqual(Date.now(), setTime);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('setTime replaces current time', (context) => {
      const now = Date.now();
      const setTime = 1000;
      // Date.now is not mocked
      assert.deepStrictEqual(Date.now(), now);

      context.mock.timers.enable({ apis: ['Date'] });
      context.mock.timers.setTime(setTime);
      // Date.now is now 1000
      assert.strictEqual(Date.now(), setTime);
    });
    javascript

##### Dates and Timers working together#

Dates and timer objects are dependent on each other. If you use `setTime()` to pass the current time to the mocked `Date` object, the set timers with `setTimeout` and `setInterval` will not be affected.

However, the `tick` method will advance the mocked `Date` object.

    import assert from 'node:assert';
    import { test } from 'node:test';

    test('runAll functions following the given order', (context) => {
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const results = [];
      setTimeout(() => results.push(1), 9999);

      assert.deepStrictEqual(results, []);
      context.mock.timers.setTime(12000);
      assert.deepStrictEqual(results, []);
      // The date is advanced but the timers don't tick
      assert.strictEqual(Date.now(), 12000);
    });
    const assert = require('node:assert');
    const { test } = require('node:test');

    test('runAll functions following the given order', (context) => {
      context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
      const results = [];
      setTimeout(() => results.push(1), 9999);

      assert.deepStrictEqual(results, []);
      context.mock.timers.setTime(12000);
      assert.deepStrictEqual(results, []);
      // The date is advanced but the timers don't tick
      assert.strictEqual(Date.now(), 12000);
    });
    javascript

### Class: `TestsStream`#

Added in: v18.9.0, v16.19.0
History

Version | Changes
--- | ---
v26.6.0 | Added `entryFile` to events forwarded from child processes when tests run with process isolation.
v26.3.0 | Added `parentId` to test events that carry a `testId`.
v20.0.0, v19.9.0, v18.17.0 | added type to test:pass and test:fail events for when the test is a suite.

- Extends `<Readable>`

A successful call to `run()` method will return a new `<TestsStream>` object, streaming a series of events representing the execution of the tests. `TestsStream` will emit events, in the order of the tests definition

Some of the events are guaranteed to be emitted in the same order as the tests are defined, while others are emitted in the order that the tests execute.

The following tables summarize all events by scope.

Test scoped events are emitted once per test or suite. Most of them come in pairs: a declaration ordered event, buffered so that events are emitted in the same order as the tests are defined, and one or more corresponding execution ordered events, emitted immediately as the tests execute.

Declaration ordered (buffered) | Execution ordered (immediate)
--- | ---
`'test:start'` | `'test:enqueue'` followed by `'test:dequeue'`
`'test:pass'` | `'test:complete'` (`details.passed` is `true`)
`'test:fail'` | `'test:complete'` (`details.passed` is `false`)
`'test:plan'` |
`'test:diagnostic'` |
| `'test:log'`

`'test:log'` is deliberately execution ordered only: it is the live counterpart of `'test:diagnostic'`'s buffered reporting.

File scoped and global events are always emitted immediately, in execution order.

File scoped events are emitted once per test file:

Event | Notes
--- | ---
`'test:stderr'` | Only emitted if the `--test` flag is passed.
`'test:stdout'` | Only emitted if the `--test` flag is passed.
`'test:summary'` | Per file, only when process isolation is used.

Global events are emitted once per test run:

Event | Notes
--- | ---
`'test:summary'` | The final cumulative summary.
`'test:coverage'` | Only when code coverage is enabled.
`'test:interrupted'` | Only when the run receives `SIGINT`.
`'test:watch:drained'` | Watch mode only.
`'test:watch:restarted'` | Watch mode only.

The root test also emits `'test:plan'` and `'test:diagnostic'` events at the end of the run to report run level totals.

#### Event: `'test:coverage'`#

- `data` `<Object>`
  - `summary` `<Object>` An object containing the coverage report.
    - `files` `<Array>` An array of coverage reports for individual files. Each report is an object with the following schema:
      - `path` `<string>` The absolute path of the file.
      - `totalLineCount` `<number>` The total number of lines.
      - `totalBranchCount` `<number>` The total number of branches.
      - `totalFunctionCount` `<number>` The total number of functions.
      - `coveredLineCount` `<number>` The number of covered lines.
      - `coveredBranchCount` `<number>` The number of covered branches.
      - `coveredFunctionCount` `<number>` The number of covered functions.
      - `coveredLinePercent` `<number>` The percentage of lines covered.
      - `coveredBranchPercent` `<number>` The percentage of branches covered.
      - `coveredFunctionPercent` `<number>` The percentage of functions covered.
      - `functions` `<Array>` An array of functions representing function coverage.
        - `name` `<string>` The name of the function.
        - `line` `<number>` The line number where the function is defined.
        - `count` `<number>` The number of times the function was called.
      - `branches` `<Array>` An array of branches representing branch coverage.
        - `line` `<number>` The line number where the branch is defined.
        - `count` `<number>` The number of times the branch was taken.
      - `lines` `<Array>` An array of lines representing line numbers and the number of times they were covered.
        - `line` `<number>` The line number.
        - `count` `<number>` The number of times the line was covered.
    - `thresholds` `<Object>` An object containing whether or not the coverage for each coverage type.
      - `function` `<number>` The function coverage threshold.
      - `branch` `<number>` The branch coverage threshold.
      - `line` `<number>` The line coverage threshold.
    - `totals` `<Object>` An object containing a summary of coverage for all files.
      - `totalLineCount` `<number>` The total number of lines.
      - `totalBranchCount` `<number>` The total number of branches.
      - `totalFunctionCount` `<number>` The total number of functions.
      - `coveredLineCount` `<number>` The number of covered lines.
      - `coveredBranchCount` `<number>` The number of covered branches.
      - `coveredFunctionCount` `<number>` The number of covered functions.
      - `coveredLinePercent` `<number>` The percentage of lines covered.
      - `coveredBranchPercent` `<number>` The percentage of branches covered.
      - `coveredFunctionPercent` `<number>` The percentage of functions covered.
    - `workingDirectory` `<string>` The working directory when code coverage began. This is useful for displaying relative path names in case the tests changed the working directory of the Node.js process.
  - `nesting` `<number>` The nesting level of the test.

Emitted when code coverage is enabled and all tests have completed.

#### Event: `'test:complete'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `details` `<Object>` Additional execution metadata.
    - `passed` `<boolean>` Whether the test passed or not.
    - `duration_ms` `<number>` The duration of the test in milliseconds.
    - `error` `<Error>` | `<undefined>` An error wrapping the error thrown by the test if it did not pass.
      - `cause` `<Error>` The actual error thrown by the test.
    - `type` `<string>` | `<undefined>` The type of the test, used to denote whether this is a suite.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests. Lets custom reporters track lineage when concurrent siblings at the same nesting level interleave.
  - `tags` `<string>`[] The flattened lowercased tags declared on the test and its ancestor suites, in declaration order. Empty for untagged tests. See Test tags.
  - `testId` `<number>` A numeric identifier for this test instance, unique within the test file's process. Consistent across all events for the same test instance, enabling reliable correlation in custom reporters.
  - `testNumber` `<number>` The ordinal number of the test.
  - `todo` `<string>` | `<boolean>` | `<undefined>` Present if `context.todo` is called
  - `skip` `<string>` | `<boolean>` | `<undefined>` Present if `context.skip` is called

Emitted when a test completes its execution. This event is not emitted in the same order as the tests are defined. The corresponding declaration ordered events are `'test:pass'` and `'test:fail'`.

#### Event: `'test:dequeue'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests. Lets custom reporters track lineage when concurrent siblings at the same nesting level interleave.
  - `tags` `<string>`[] The flattened lowercased tags declared on the test and its ancestor suites, in declaration order. Empty for untagged tests. See Test tags.
  - `testId` `<number>` A numeric identifier for this test instance, unique within the test file's process. Consistent across all events for the same test instance, enabling reliable correlation in custom reporters.
  - `type` `<string>` The test type. Either `'suite'` or `'test'`.

Emitted when a test is dequeued, right before it is executed. This event is not guaranteed to be emitted in the same order as the tests are defined. The corresponding declaration ordered event is `'test:start'`.

#### Event: `'test:diagnostic'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `message` `<string>` The diagnostic message.
  - `nesting` `<number>` The nesting level of the test.
  - `level` `<string>` The severity level of the diagnostic message. Possible values are:
    - `'info'`: Informational messages.
    - `'warn'`: Warnings.
    - `'error'`: Errors.

Emitted when `context.diagnostic` is called. This event is guaranteed to be emitted in the same order as the tests are defined.

#### Event: `'test:enqueue'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests. Lets custom reporters track lineage when concurrent siblings at the same nesting level interleave.
  - `tags` `<string>`[] The flattened lowercased tags declared on the test and its ancestor suites, in declaration order. Empty for untagged tests. See Test tags.
  - `testId` `<number>` A numeric identifier for this test instance, unique within the test file's process. Consistent across all events for the same test instance, enabling reliable correlation in custom reporters.
  - `type` `<string>` The test type. Either `'suite'` or `'test'`.

Emitted when a test is enqueued for execution.

#### Event: `'test:fail'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `details` `<Object>` Additional execution metadata.
    - `duration_ms` `<number>` The duration of the test in milliseconds.
    - `error` `<Error>` An error wrapping the error thrown by the test.
      - `cause` `<Error>` The actual error thrown by the test.
    - `type` `<string>` | `<undefined>` The type of the test, used to denote whether this is a suite.
    - `attempt` `<number>` | `<undefined>` The attempt number of the test run, present only when using the `--test-rerun-failures` flag.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests. Lets custom reporters track lineage when concurrent siblings at the same nesting level interleave.
  - `tags` `<string>`[] The flattened lowercased tags declared on the test and its ancestor suites, in declaration order. Empty for untagged tests. See Test tags.
  - `testId` `<number>` A numeric identifier for this test instance, unique within the test file's process. Consistent across all events for the same test instance, enabling reliable correlation in custom reporters.
  - `testNumber` `<number>` The ordinal number of the test.
  - `todo` `<string>` | `<boolean>` | `<undefined>` Present if `context.todo` is called
  - `skip` `<string>` | `<boolean>` | `<undefined>` Present if `context.skip` is called

Emitted when a test fails. This event is guaranteed to be emitted in the same order as the tests are defined. The corresponding execution ordered event is `'test:complete'`.

#### Event: `'test:interrupted'`#

Added in: v25.7.0

- `data` `<Object>`
  - `tests` `<Array>` An array of objects containing information about the interrupted tests.
    - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
    - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
    - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
    - `name` `<string>` The test name.
    - `nesting` `<number>` The nesting level of the test.

Emitted when the test runner is interrupted by a `SIGINT` signal (e.g., when pressing `Ctrl`+`C`). The event contains information about the tests that were running at the time of interruption.

When using process isolation (the default), the test name will be the file path since the parent runner only knows about file-level tests. When using `--test-isolation=none`, the actual test name is shown.

#### Event: `'test:log'`#

Added in: v26.6.0

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `data` `<any>` The structured payload passed to `context.log`, or `undefined` if none was provided. The test runner does not interpret this value.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `message` `<string>` The log message.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests.
  - `testId` `<number>` A numeric identifier for the test instance that emitted the log message.

Emitted when `context.log` is called. Unlike `'test:diagnostic'`, this event is emitted immediately, in the order that the tests execute, making it suitable for reporters that render test output unbuffered.

#### Event: `'test:pass'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `details` `<Object>` Additional execution metadata.
    - `duration_ms` `<number>` The duration of the test in milliseconds.
    - `type` `<string>` | `<undefined>` The type of the test, used to denote whether this is a suite.
    - `attempt` `<number>` | `<undefined>` The attempt number of the test run, present only when using the `--test-rerun-failures` flag.
    - `passed_on_attempt` `<number>` | `<undefined>` The attempt number the test passed on, present only when using the `--test-rerun-failures` flag.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests. Lets custom reporters track lineage when concurrent siblings at the same nesting level interleave.
  - `tags` `<string>`[] The flattened lowercased tags declared on the test and its ancestor suites, in declaration order. Empty for untagged tests. See Test tags.
  - `testId` `<number>` A numeric identifier for this test instance, unique within the test file's process. Consistent across all events for the same test instance, enabling reliable correlation in custom reporters.
  - `testNumber` `<number>` The ordinal number of the test.
  - `todo` `<string>` | `<boolean>` | `<undefined>` Present if `context.todo` is called
  - `skip` `<string>` | `<boolean>` | `<undefined>` Present if `context.skip` is called

Emitted when a test passes. This event is guaranteed to be emitted in the same order as the tests are defined. The corresponding execution ordered event is `'test:complete'`.

#### Event: `'test:plan'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `nesting` `<number>` The nesting level of the test.
  - `count` `<number>` The number of subtests that have ran.

Emitted when all subtests have completed for a given test. This event is guaranteed to be emitted in the same order as the tests are defined.

#### Event: `'test:start'`#

- `data` `<Object>`
  - `column` `<number>` | `<undefined>` The column number where the test is defined, or `undefined` if the test was run through the REPL.
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation. May differ from `file` when the test is defined in a module imported by the entry file.
  - `file` `<string>` | `<undefined>` The path of the test file, `undefined` if test was run through the REPL.
  - `line` `<number>` | `<undefined>` The line number where the test is defined, or `undefined` if the test was run through the REPL.
  - `name` `<string>` The test name.
  - `nesting` `<number>` The nesting level of the test.
  - `parentId` `<number>` | `<undefined>` The `testId` of the enclosing test, or `undefined` for top-level tests. Lets custom reporters track lineage when concurrent siblings at the same nesting level interleave.
  - `tags` `<string>`[] The flattened lowercased tags declared on the test and its ancestor suites, in declaration order. Empty for untagged tests. See Test tags.
  - `testId` `<number>` A numeric identifier for this test instance, unique within the test file's process. Consistent across all events for the same test instance, enabling reliable correlation in custom reporters.

Emitted when a test starts reporting its own and its subtests status. This event is guaranteed to be emitted in the same order as the tests are defined. The corresponding execution ordered event is `'test:dequeue'`.

#### Event: `'test:stderr'`#

- `data` `<Object>`
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation.
  - `file` `<string>` The path of the test file.
  - `message` `<string>` The message written to `stderr`.

Emitted when a running test writes to `stderr`. This event is only emitted if `--test` flag is passed. This event is not guaranteed to be emitted in the same order as the tests are defined.

#### Event: `'test:stdout'`#

- `data` `<Object>`
  - `entryFile` `<string>` | `<undefined>` The path of the test file that was executed as the entry point of the child process that emitted this event. Only present when tests run with process isolation.
  - `file` `<string>` The path of the test file.
  - `message` `<string>` The message written to `stdout`.

Emitted when a running test writes to `stdout`. This event is only emitted if `--test` flag is passed. This event is not guaranteed to be emitted in the same order as the tests are defined.

#### Event: `'test:summary'`#

- `data` `<Object>`
  - `counts` `<Object>` An object containing the counts of various test results.
    - `cancelled` `<number>` The total number of cancelled tests.
    - `failed` `<number>` The total number of failed tests.
    - `passed` `<number>` The total number of passed tests.
    - `skipped` `<number>` The total number of skipped tests.
    - `suites` `<number>` The total number of suites run.
    - `tests` `<number>` The total number of tests run, excluding suites.
    - `todo` `<number>` The total number of TODO tests.
    - `topLevel` `<number>` The total number of top level tests and suites.
  - `duration_ms` `<number>` The duration of the test run in milliseconds.
  - `file` `<string>` | `<undefined>` The path of the test file that generated the summary. If the summary corresponds to multiple files, this value is `undefined`.
  - `success` `<boolean>` Indicates whether or not the test run is considered successful or not. If any error condition occurs, such as a failing test or unmet coverage threshold, this value will be set to `false`.

Emitted when a test run completes. This event contains metrics pertaining to the completed test run, and is useful for determining if a test run passed or failed. If process-level test isolation is used, a `'test:summary'` event is generated for each test file in addition to a final cumulative summary.

#### Event: `'test:watch:drained'`#

Emitted when no more tests are queued for execution in watch mode.

#### Event: `'test:watch:restarted'`#

Emitted when one or more tests are restarted due to a file change in watch mode.

### `getTestContext()`#

Added in: v26.1.0

- Returns: `<TestContext>` | `<SuiteContext>` | `<undefined>`

Returns the `TestContext` or `SuiteContext` object associated with the currently executing test or suite, or `undefined` if called outside of a test or suite. This function can be used to access context information from within the test or suite function or any async operations within them.

    import { getTestContext } from 'node:test';

    test('example test', async () => {
      const ctx = getTestContext();
      console.log(`Running test: ${ctx.name}`);
    });

    describe('example suite', () => {
      const ctx = getTestContext();
      console.log(`Running suite: ${ctx.name}`);
    });
    mjs

When called from a test, returns a `TestContext`. When called from a suite, returns a `SuiteContext`.

If called from outside a test or suite (e.g., at the top level of a module or in a setTimeout callback after execution has completed), this function returns `undefined`.

When called from within a hook (before, beforeEach, after, afterEach), this function returns the context of the test or suite that the hook is associated with.

### Test instrumentation and OpenTelemetry#

Added in: v26.1.0

The test runner publishes test execution events through the Node.js `diagnostics_channel` module, enabling integration with observability tools like OpenTelemetry without requiring changes to the test runner itself.

#### Tracing events#

The test runner publishes events to the `'node.test'` tracing channel. Subscribers can use the `TracingChannel` API to bind context or perform custom instrumentation.

##### Channel: `'tracing:node.test:start'`#

- `data` `<Object>`
  - `name` `<string>` The name of the test.
  - `nesting` `<number>` The nesting level of the test.
  - `file` `<string>` | `<undefined>` The path to the test file, or `undefined` when running in the REPL.
  - `type` `<string>` The type of test. Either `'test'` or `'suite'`.

Emitted when a test or suite starts execution. The test's span encompasses all of its before, beforeEach, and afterEach hooks, as well as the test body.

##### Channel: `'tracing:node.test:end'`#

- `data` `<Object>`
  - `name` `<string>` The name of the test.
  - `nesting` `<number>` The nesting level of the test.
  - `file` `<string>` | `<undefined>` The path to the test file, or `undefined` when running in the REPL.
  - `type` `<string>` The type of test. Either `'test'` or `'suite'`.

Emitted when a test or suite finishes execution.

##### Channel: `'tracing:node.test:error'`#

- `data` `<Object>`
  - `name` `<string>` The name of the test.
  - `nesting` `<number>` The nesting level of the test.
  - `file` `<string>` | `<undefined>` The path to the test file, or `undefined` when running in the REPL.
  - `type` `<string>` The type of test. Either `'test'` or `'suite'`.
  - `error` `<Error>` The error that was thrown.

Emitted when a test or suite throws an error.

#### Context propagation with `bindStore()`#

The tracing channel can be used to propagate context through test execution by binding an `AsyncLocalStorage` instance. This allows context to be automatically available in the test function and all async operations within the test.

    import dc from 'node:diagnostics_channel';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const testStorage = new AsyncLocalStorage();
    const testChannel = dc.tracingChannel('node.test');

    // Bind context to test execution — the returned value becomes the store
    testChannel.start.bindStore(testStorage, (data) => {
      return { testName: data.name, startTime: Date.now() };
    });

    // Optionally handle errors and cleanup
    testChannel.error.subscribe((data) => {
      const store = testStorage.getStore();
      console.log(`Test "${data.name}" failed after ${Date.now() - store.startTime}ms`);
    });

    testChannel.end.subscribe((data) => {
      const store = testStorage.getStore();
      console.log(`Test "${data.name}" completed in ${Date.now() - store.startTime}ms`);
    });
    mjs

When using `bindStore()`, the context provided will be automatically propagated to the test function and all async operations within the test, without requiring any additional instrumentation in the test code.

### Class: `TestContext`#

Added in: v18.0.0, v16.17.0
History

Version | Changes
--- | ---
v20.1.0, v18.17.0 | The `before` function was added to TestContext.

An instance of `TestContext` is passed to each test function in order to interact with the test runner. However, the `TestContext` constructor is not exposed as part of the API.

#### `context.before([fn][, options])`#

Added in: v20.1.0, v18.17.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. The first argument to this function is a `TestContext` object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function registers a hook that runs before any subtests of the current test.

#### `context.beforeEach([fn][, options])`#

Added in: v18.8.0, v16.18.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. The first argument to this function is a `TestContext` object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function registers a hook that runs before each subtest of the current test.

    test('top level test', async (t) => {
      t.beforeEach((t) => t.diagnostic(`about to run ${t.name}`));
      await t.test(
        'This is a subtest',
        (t) => {
          // Some relevant assertion here
        },
      );
    });
    js

#### `context.after([fn][, options])`#

Added in: v19.3.0, v18.13.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. The first argument to this function is a `TestContext` object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function registers a hook that runs after the current test finishes.

    test('top level test', async (t) => {
      t.after((t) => t.diagnostic(`finished running ${t.name}`));
      // Some relevant assertion here
    });
    js

#### `context.afterEach([fn][, options])`#

Added in: v18.8.0, v16.18.0

- `fn` `<Function>` | `<AsyncFunction>` The hook function. The first argument to this function is a `TestContext` object. If the hook uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- `options` `<Object>` Configuration options for the hook. The following properties are supported:
  - `signal` `<AbortSignal>` Allows aborting an in-progress hook.
  - `timeout` `<number>` A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.

This function registers a hook that runs after each subtest of the current test.

    test('top level test', async (t) => {
      t.afterEach((t) => t.diagnostic(`finished running ${t.name}`));
      await t.test(
        'This is a subtest',
        (t) => {
          // Some relevant assertion here
        },
      );
    });
    js

#### `context.assert`#

Added in: v22.2.0, v20.15.0

An object containing assertion methods bound to `context`. The top-level functions from the `node:assert` module are exposed here for the purpose of creating test plans.

    test('test', (t) => {
      t.plan(1);
      t.assert.strictEqual(true, true);
    });
    js

##### `context.assert.fileSnapshot(value, path[, options])`#

Added in: v23.7.0, v22.14.0

- `value` `<any>` A value to serialize to a string. If Node.js was started with the `--test-update-snapshots` flag, the serialized value is written to `path`. Otherwise, the serialized value is compared to the contents of the existing snapshot file.
- `path` `<string>` The file where the serialized `value` is written.
- `options` `<Object>` Optional configuration options. The following properties are supported:
  - `serializers` `<Array>` An array of synchronous functions used to serialize `value` into a string. `value` is passed as the only argument to the first serializer function. The return value of each serializer is passed as input to the next serializer. Once all serializers have run, the resulting value is coerced to a string. Default: If no serializers are provided, the test runner's default serializers are used.

This function serializes `value` and writes it to the file specified by `path`.

    test('snapshot test with default serialization', (t) => {
      t.assert.fileSnapshot({ value1: 1, value2: 2 }, './snapshots/snapshot.json');
    });
    js

This function differs from `context.assert.snapshot()` in the following ways:

- The snapshot file path is explicitly provided by the user.
- Each snapshot file is limited to a single snapshot value.
- No additional escaping is performed by the test runner.

These differences allow snapshot files to better support features such as syntax highlighting.

##### `context.assert.snapshot(value[, options])`#

Added in: v22.3.0

- `value` `<any>` A value to serialize to a string. If Node.js was started with the `--test-update-snapshots` flag, the serialized value is written to the snapshot file. Otherwise, the serialized value is compared to the corresponding value in the existing snapshot file.
- `options` `<Object>` Optional configuration options. The following properties are supported:
  - `serializers` `<Array>` An array of synchronous functions used to serialize `value` into a string. `value` is passed as the only argument to the first serializer function. The return value of each serializer is passed as input to the next serializer. Once all serializers have run, the resulting value is coerced to a string. Default: If no serializers are provided, the test runner's default serializers are used.

This function implements assertions for snapshot testing.

    test('snapshot test with default serialization', (t) => {
      t.assert.snapshot({ value1: 1, value2: 2 });
    });

    test('snapshot test with custom serialization', (t) => {
      t.assert.snapshot({ value3: 3, value4: 4 }, {
        serializers: [(value) => JSON.stringify(value)],
      });
    });
    js

#### `context.diagnostic(message)`#

Added in: v18.0.0, v16.17.0

- `message` `<string>` Message to be reported.

This function is used to write diagnostics to the output. Any diagnostic information is included at the end of the test's results. This function does not return a value.

    test('top level test', (t) => {
      t.diagnostic('A diagnostic message');
    });
    js

#### `context.log(message[, data])`#

Added in: v26.6.0

- `message` `<string>` Message to be reported.
- `data` `<any>` Optional structured payload attached to the message. The test runner passes it through untouched. When tests run with process isolation, this value must be compatible with the HTML structured clone algorithm.

This function is used to write a log message to the output. Unlike `context.diagnostic`, the resulting `'test:log'` event is emitted immediately, in the order that the tests execute, rather than being buffered until the test reports its results. This function does not return a value.

    test('top level test', (t) => {
      t.log('fetched user', { userId: 42 });
      t.log('retrying flaky endpoint', { attempt: 3 });
    });
    js

#### `context.filePath`#

Added in: v22.6.0, v20.16.0

The absolute path of the test file that created the current test. If a test file imports additional modules that generate tests, the imported tests will return the path of the root test file.

#### `context.fullName`#

Added in: v22.3.0, v20.16.0

The name of the test and each of its ancestors, separated by `>`.

#### `context.name`#

Added in: v18.8.0, v16.18.0

The name of the test.

#### `context.passed`#

Added in: v21.7.0, v20.12.0

- Type: `<boolean>` `false` before the test is executed, e.g. in a `beforeEach` hook.

Indicated whether the test succeeded.

#### `context.error`#

Added in: v21.7.0, v20.12.0

- Type: `<Error>` | `<null>`

The failure reason for the test/case; wrapped and available via `context.error.cause`.

#### `context.attempt`#

Added in: v25.0.0

- Type: `<number>`

The attempt number of the test. This value is zero-based, so the first attempt is `0`, the second attempt is `1`, and so on. This property is useful in conjunction with the `--test-rerun-failures` option to determine which attempt the test is currently running.

#### `context.tags`#

Added in: v26.2.0

Stability: 1.0 - Early development

- Type: `<string>`[]

A frozen array of the test's flattened lowercased tags, in declaration order, including any tags inherited from ancestor suites. Empty when the test has no tags. See Test tags.

#### `context.workerId`#

Added in: v25.8.0

- Type: `<number>` | `<undefined>`

The unique identifier of the worker running the current test file. This value is derived from the `NODE_TEST_WORKER_ID` environment variable. When running tests with `--test-isolation=process` (the default), each test file runs in a separate child process and is assigned a worker ID from 1 to N, where N is the number of concurrent workers. When running with `--test-isolation=none`, all tests run in the same process and the worker ID is always 1. This value is `undefined` when not running in a test context.

This property is useful for splitting resources (like database connections or server ports) across concurrent test files:

    import { test } from 'node:test';
    import { process } from 'node:process';

    test('database operations', async (t) => {
      // Worker ID is available via context
      console.log(`Running in worker ${t.workerId}`);

      // Or via environment variable (available at import time)
      const workerId = process.env.NODE_TEST_WORKER_ID;
      // Use workerId to allocate separate resources per worker
    });
    mjs

#### `context.plan(count[,options])`#

Added in: v22.2.0, v20.15.0
History

Version | Changes
--- | ---
v23.9.0, v22.15.0 | Add the `options` parameter.
v23.4.0, v22.13.0 | This function is no longer experimental.

- `count` `<number>` The number of assertions and subtests that are expected to run.
- `options` `<Object>` Additional options for the plan.
  - `wait` `<boolean>` | `<number>` The wait time for the plan:
    - If `true`, the plan waits indefinitely for all assertions and subtests to run.
    - If `false`, the plan performs an immediate check after the test function completes, without waiting for any pending assertions or subtests. Any assertions or subtests that complete after this check will not be counted towards the plan.
    - If a number, it specifies the maximum wait time in milliseconds before timing out while waiting for expected assertions and subtests to be matched. If the timeout is reached, the test will fail. Default: `false`.

This function is used to set the number of assertions and subtests that are expected to run within the test. If the number of assertions and subtests that run does not match the expected count, the test will fail.

> Note: To make sure assertions are tracked, `t.assert` must be used instead of `assert` directly.

    test('top level test', (t) => {
      t.plan(2);
      t.assert.ok('some relevant assertion here');
      t.test('subtest', () => {});
    });
    js

When working with asynchronous code, the `plan` function can be used to ensure that the correct number of assertions are run:

    test('planning with streams', (t, done) => {
      function* generate() {
        yield 'a';
        yield 'b';
        yield 'c';
      }
      const expected = ['a', 'b', 'c'];
      t.plan(expected.length);
      const stream = Readable.from(generate());
      stream.on('data', (chunk) => {
        t.assert.strictEqual(chunk, expected.shift());
      });

      stream.on('end', () => {
        done();
      });
    });
    js

When using the `wait` option, you can control how long the test will wait for the expected assertions. For example, setting a maximum wait time ensures that the test will wait for asynchronous assertions to complete within the specified timeframe:

    test('plan with wait: 2000 waits for async assertions', (t) => {
      t.plan(1, { wait: 2000 }); // Waits for up to 2 seconds for the assertion to complete.

      const asyncActivity = () => {
        setTimeout(() => {
          t.assert.ok(true, 'Async assertion completed within the wait time');
        }, 1000); // Completes after 1 second, within the 2-second wait time.
      };

      asyncActivity(); // The test will pass because the assertion is completed in time.
    });
    js

Note: If a `wait` timeout is specified, it begins counting down only after the test function finishes executing.

#### `context.runOnly(shouldRunOnlyTests)`#

Added in: v18.0.0, v16.17.0

- `shouldRunOnlyTests` `<boolean>` Whether or not to run `only` tests.

If `shouldRunOnlyTests` is truthy, the test context will only run tests that have the `only` option set. Otherwise, all tests are run. If Node.js was not started with the `--test-only` command-line option, this function is a no-op.

    test('top level test', (t) => {
      // The test context can be set to run subtests with the 'only' option.
      t.runOnly(true);
      return Promise.all([
        t.test('this subtest is now skipped'),
        t.test('this subtest is run', { only: true }),
      ]);
    });
    js

#### `context.signal`#

Added in: v18.7.0, v16.17.0

- Type: `<AbortSignal>`

Can be used to abort test subtasks when the test has been aborted.

    test('top level test', async (t) => {
      await fetch('some/uri', { signal: t.signal });
    });
    js

#### `context.skip([message])`#

Added in: v18.0.0, v16.17.0

- `message` `<string>` Optional skip message.

This function causes the test's output to indicate the test as skipped. If `message` is provided, it is included in the output. Calling `skip()` does not terminate execution of the test function. This function does not return a value.

    test('top level test', (t) => {
      // Make sure to return here as well if the test contains additional logic.
      t.skip('this is skipped');
    });
    js

#### `context.todo([message])`#

Added in: v18.0.0, v16.17.0

- `message` `<string>` Optional `TODO` message.

This function adds a `TODO` directive to the test's output. If `message` is provided, it is included in the output. Calling `todo()` does not terminate execution of the test function. This function does not return a value.

    test('top level test', (t) => {
      // This test is marked as `TODO`
      t.todo('this is a todo');
    });
    js

#### `context.test([name][, options][, fn])`#

Added in: v18.0.0, v16.17.0
History

Version | Changes
--- | ---
v26.2.0 | Added the `tags` option.
v18.8.0, v16.18.0 | Add a `signal` option.
v18.7.0, v16.17.0 | Add a `timeout` option.

- `name` `<string>` The name of the subtest, which is displayed when reporting test results. Default: The `name` property of `fn`, or `'<anonymous>'` if `fn` does not have a name.
- `options` `<Object>` Configuration options for the subtest. The following properties are supported:
  - `concurrency` `<number>` | `<boolean>` | `<null>` If a number is provided, then that many tests would run asynchronously (they are still managed by the single-threaded event loop). If `true`, it would run all subtests in parallel. If `false`, it would only run one test at a time. If unspecified, subtests inherit this value from their parent. Default: `null`.
  - `only` `<boolean>` If truthy, and the test context is configured to run `only` tests, then this test will be run. Otherwise, the test is skipped. Default: `false`.
  - `signal` `<AbortSignal>` Allows aborting an in-progress test.
  - `skip` `<boolean>` | `<string>` If truthy, the test is skipped. If a string is provided, that string is displayed in the test results as the reason for skipping the test. Default: `false`.
  - `tags` `<string>`[] An array of string labels associated with the subtest. Used together with `--experimental-test-tag-filter` to filter which tests run. Tags inherit from the parent test or suite by union. See Test tags. Default: `[]`.
  - `todo` `<boolean>` | `<string>` If truthy, the test marked as `TODO`. If a string is provided, that string is displayed in the test results as the reason why the test is `TODO`. Default: `false`.
  - `timeout` `<number>` A number of milliseconds the test will fail after. If unspecified, subtests inherit this value from their parent. Default: `Infinity`.
  - `plan` `<number>` The number of assertions and subtests expected to be run in the test. If the number of assertions run in the test does not match the number specified in the plan, the test will fail. Default: `undefined`.
- `fn` `<Function>` | `<AsyncFunction>` The function under test. The first argument to this function is a `TestContext` object. If the test uses callbacks, the callback function is passed as the second argument. Default: A no-op function.
- Returns: `<Promise>` Fulfilled with `undefined` once the test completes.

This function is used to create subtests under the current test. This function behaves in the same fashion as the top level `test()` function.

    test('top level test', async (t) => {
      await t.test(
        'This is a subtest',
        { only: false, skip: false, concurrency: 1, todo: false, plan: 1 },
        (t) => {
          t.assert.ok('some relevant assertion here');
        },
      );
    });
    js

#### `context.waitFor(condition[, options])`#

Added in: v23.7.0, v22.14.0

- `condition` `<Function>` | `<AsyncFunction>` An assertion function that is invoked periodically until it completes successfully or the defined polling timeout elapses. Successful completion is defined as not throwing or rejecting. This function does not accept any arguments, and is allowed to return any value.
- `options` `<Object>` An optional configuration object for the polling operation. The following properties are supported:
  - `interval` `<number>` The number of milliseconds to wait after an unsuccessful invocation of `condition` before trying again. Default: `50`.
  - `timeout` `<number>` The poll timeout in milliseconds. If `condition` has not succeeded by the time this elapses, an error occurs. Default: `1000`.
- Returns: `<Promise>` Fulfilled with the value returned by `condition`.

This method polls a `condition` function until that function either returns successfully or the operation times out.

### Class: `SuiteContext`#

Added in: v18.7.0, v16.17.0

An instance of `SuiteContext` is passed to each suite function in order to interact with the test runner. However, the `SuiteContext` constructor is not exposed as part of the API.

#### `context.filePath`#

Added in: v22.6.0

The absolute path of the test file that created the current suite. If a test file imports additional modules that generate suites, the imported suites will return the path of the root test file.

#### `context.fullName`#

Added in: v22.3.0, v20.16.0

The name of the suite and each of its ancestors, separated by `>`.

#### `context.name`#

Added in: v18.8.0, v16.18.0

The name of the suite.

#### `context.signal`#

Added in: v18.7.0, v16.17.0

- Type: `<AbortSignal>`

Can be used to abort test subtasks when the test has been aborted.

#### `context.passed`#

Added in: v26.1.0

- Type: `<boolean>`

Indicates whether the suite and all of its subtests have passed.

#### `context.attempt`#

Added in: v26.1.0

- Type: `<number>`

The attempt number of the suite. This value is zero-based, so the first attempt is `0`, the second attempt is `1`, and so on. This property is useful in conjunction with the `--test-rerun-failures` option to determine the attempt number of the current run.

#### `context.diagnostic(message)`#

Added in: v26.1.0

- `message` `<string>` A diagnostic message to output.

Output a diagnostic message. This is typically used for logging information about the current suite or its tests.

    test.describe('my suite', (suite) => {
      suite.diagnostic('Suite diagnostic message');
    });
    js

#### `context.log(message[, data])`#

Added in: v26.6.0

- `message` `<string>` Message to be reported.
- `data` `<any>` Optional structured payload attached to the message. The test runner passes it through untouched.

Write a log message to the output. The resulting `'test:log'` event is emitted immediately, in the order that the tests execute.

    test.describe('my suite', (suite) => {
      suite.log('Suite log message');
    });
    js

## Timers#

Source Code: lib/timers.js

Stability: 2 - Stable

The `timer` module exposes a global API for scheduling functions to be called at some future period of time. Because the timer functions are globals, there is no need to call `require('node:timers')` to use the API.

The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.

### Class: `Immediate`#

This object is created internally and is returned from `setImmediate()`. It can be passed to `clearImmediate()` in order to cancel the scheduled actions.

By default, when an immediate is scheduled, the Node.js event loop will continue running as long as the immediate is active. The `Immediate` object returned by `setImmediate()` exports both `immediate.ref()` and `immediate.unref()` functions that can be used to control this default behavior.

#### `immediate.hasRef()`#

Added in: v11.0.0

- Returns: `<boolean>`

If true, the `Immediate` object will keep the Node.js event loop active.

#### `immediate.ref()`#

Added in: v9.7.0

- Returns: `<Immediate>` a reference to `immediate`

When called, requests that the Node.js event loop not exit so long as the `Immediate` is active. Calling `immediate.ref()` multiple times will have no effect.

By default, all `Immediate` objects are "ref'ed", making it normally unnecessary to call `immediate.ref()` unless `immediate.unref()` had been called previously.

#### `immediate.unref()`#

Added in: v9.7.0

- Returns: `<Immediate>` a reference to `immediate`

When called, the active `Immediate` object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the `Immediate` object's callback is invoked. Calling `immediate.unref()` multiple times will have no effect.

#### `immediate[Symbol.dispose]()`#

Added in: v20.5.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Cancels the immediate. This is similar to calling `clearImmediate()`.

### Class: `Timeout`#

This object is created internally and is returned from `setTimeout()` and `setInterval()`. It can be passed to either `clearTimeout()` or `clearInterval()` in order to cancel the scheduled actions.

By default, when a timer is scheduled using either `setTimeout()` or `setInterval()`, the Node.js event loop will continue running as long as the timer is active. Each of the `Timeout` objects returned by these functions export both `timeout.ref()` and `timeout.unref()` functions that can be used to control this default behavior.

#### `timeout.close()`#

Added in: v0.9.1

Stability: 3 - Legacy: Use `clearTimeout()` instead.

- Returns: `<Timeout>` a reference to `timeout`

Cancels the timeout.

#### `timeout.hasRef()`#

Added in: v11.0.0

- Returns: `<boolean>`

If true, the `Timeout` object will keep the Node.js event loop active.

#### `timeout.ref()`#

Added in: v0.9.1

- Returns: `<Timeout>` a reference to `timeout`

When called, requests that the Node.js event loop not exit so long as the `Timeout` is active. Calling `timeout.ref()` multiple times will have no effect.

By default, all `Timeout` objects are "ref'ed", making it normally unnecessary to call `timeout.ref()` unless `timeout.unref()` had been called previously.

#### `timeout.refresh()`#

Added in: v10.2.0

- Returns: `<Timeout>` a reference to `timeout`

Sets the timer's start time to the current time, and reschedules the timer to call its callback at the previously specified duration adjusted to the current time. This is useful for refreshing a timer without allocating a new JavaScript object.

Using this on a timer that has already called its callback will reactivate the timer.

#### `timeout.unref()`#

Added in: v0.9.1

- Returns: `<Timeout>` a reference to `timeout`

When called, the active `Timeout` object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the `Timeout` object's callback is invoked. Calling `timeout.unref()` multiple times will have no effect.

#### `timeout[Symbol.toPrimitive]()`#

Added in: v14.9.0, v12.19.0

- Returns: `<integer>` a number that can be used to reference this `timeout`

Coerce a `Timeout` to a primitive. The primitive can be used to clear the `Timeout`. The primitive can only be used in the same thread where the timeout was created. Therefore, to use it across `worker_threads` it must first be passed to the correct thread. This allows enhanced compatibility with browser `setTimeout()` and `setInterval()` implementations.

#### `timeout[Symbol.dispose]()`#

Added in: v20.5.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Cancels the timeout.

### Scheduling timers#

A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.

#### `setImmediate(callback[, ...args])`#

Added in: v0.9.1
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `callback` `<Function>` The function to call at the end of this turn of the Node.js Event Loop
- `...args` `<any>` Optional arguments to pass when the `callback` is called.
- Returns: `<Immediate>` for use with `clearImmediate()`

Schedules the "immediate" execution of the `callback` after I/O events' callbacks.

When multiple calls to `setImmediate()` are made, the `callback` functions are queued for execution in the order in which they are created. The entire callback queue is processed every event loop iteration. If an immediate timer is queued from inside an executing callback, that timer will not be triggered until the next event loop iteration.

If `callback` is not a function, a `TypeError` will be thrown.

This method has a custom variant for promises that is available using `timersPromises.setImmediate()`.

#### `setInterval(callback[, delay[, ...args]])`#

Added in: v0.0.1
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `callback` `<Function>` The function to call when the timer elapses.
- `delay` `<number>` The number of milliseconds to wait before calling the `callback`. Default: `1`.
- `...args` `<any>` Optional arguments to pass when the `callback` is called.
- Returns: `<Timeout>` for use with `clearInterval()`

Schedules repeated execution of `callback` every `delay` milliseconds.

When `delay` is larger than `2147483647` or less than `1` or `NaN`, the `delay` will be set to `1`. Non-integer delays are truncated to an integer.

If `callback` is not a function, a `TypeError` will be thrown.

This method has a custom variant for promises that is available using `timersPromises.setInterval()`.

#### `setTimeout(callback[, delay[, ...args]])`#

Added in: v0.0.1
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `callback` `<Function>` The function to call when the timer elapses.
- `delay` `<number>` The number of milliseconds to wait before calling the `callback`. Default: `1`.
- `...args` `<any>` Optional arguments to pass when the `callback` is called.
- Returns: `<Timeout>` for use with `clearTimeout()`

Schedules execution of a one-time `callback` after `delay` milliseconds.

The `callback` will likely not be invoked in precisely `delay` milliseconds. Node.js makes no guarantees about the exact timing of when callbacks will fire, nor of their ordering. The callback will be called as close as possible to the time specified.

When `delay` is larger than `2147483647` or less than `1` or `NaN`, the `delay` will be set to `1`. Non-integer delays are truncated to an integer.

If `callback` is not a function, a `TypeError` will be thrown.

This method has a custom variant for promises that is available using `timersPromises.setTimeout()`.

### Cancelling timers#

The `setImmediate()`, `setInterval()`, and `setTimeout()` methods each return objects that represent the scheduled timers. These can be used to cancel the timer and prevent it from triggering.

For the promisified variants of `setImmediate()` and `setTimeout()`, an `AbortController` may be used to cancel the timer. When canceled, the returned Promises will be rejected with an `'AbortError'`.

For `setImmediate()`:

    import { setImmediate as setImmediatePromise } from 'node:timers/promises';

    const ac = new AbortController();
    const signal = ac.signal;

    // We do not `await` the promise so `ac.abort()` is called concurrently.
    setImmediatePromise('foobar', { signal })
      .then(console.log)
      .catch((err) => {
        if (err.name === 'AbortError')
          console.error('The immediate was aborted');
      });

    ac.abort();
    const { setImmediate: setImmediatePromise } = require('node:timers/promises');

    const ac = new AbortController();
    const signal = ac.signal;

    setImmediatePromise('foobar', { signal })
      .then(console.log)
      .catch((err) => {
        if (err.name === 'AbortError')
          console.error('The immediate was aborted');
      });

    ac.abort();
    javascript

For `setTimeout()`:

    import { setTimeout as setTimeoutPromise } from 'node:timers/promises';

    const ac = new AbortController();
    const signal = ac.signal;

    // We do not `await` the promise so `ac.abort()` is called concurrently.
    setTimeoutPromise(1000, 'foobar', { signal })
      .then(console.log)
      .catch((err) => {
        if (err.name === 'AbortError')
          console.error('The timeout was aborted');
      });

    ac.abort();
    const { setTimeout: setTimeoutPromise } = require('node:timers/promises');

    const ac = new AbortController();
    const signal = ac.signal;

    setTimeoutPromise(1000, 'foobar', { signal })
      .then(console.log)
      .catch((err) => {
        if (err.name === 'AbortError')
          console.error('The timeout was aborted');
      });

    ac.abort();
    javascript

#### `clearImmediate(immediate)`#

Added in: v0.9.1

- `immediate` `<Immediate>` An `Immediate` object as returned by `setImmediate()`.

Cancels an `Immediate` object created by `setImmediate()`.

#### `clearInterval(timeout)`#

Added in: v0.0.1

- `timeout` `<Timeout>` | `<string>` | `<number>` A `Timeout` object as returned by `setInterval()` or the primitive of the `Timeout` object as a string or a number.

Cancels a `Timeout` object created by `setInterval()`.

#### `clearTimeout(timeout)`#

Added in: v0.0.1

- `timeout` `<Timeout>` | `<string>` | `<number>` A `Timeout` object as returned by `setTimeout()` or the primitive of the `Timeout` object as a string or a number.

Cancels a `Timeout` object created by `setTimeout()`.

### Timers Promises API#

Added in: v15.0.0
History

Version | Changes
--- | ---
v16.0.0 | Graduated from experimental.

The `timers/promises` API provides an alternative set of timer functions that return `Promise` objects. The API is accessible via `require('node:timers/promises')`.

    import {
      setTimeout,
      setImmediate,
      setInterval,
    } from 'node:timers/promises';
    const {
      setTimeout,
      setImmediate,
      setInterval,
    } = require('node:timers/promises');
    javascript

#### `timersPromises.setTimeout([delay[, value[, options]]])`#

Added in: v15.0.0

- `delay` `<number>` The number of milliseconds to wait before fulfilling the promise. Default: `1`.
- `value` `<any>` A value with which the promise is fulfilled.
- `options` `<Object>`
  - `ref` `<boolean>` Set to `false` to indicate that the scheduled `Timeout` should not require the Node.js event loop to remain active. Default: `true`.
  - `signal` `<AbortSignal>` An optional `AbortSignal` that can be used to cancel the scheduled `Timeout`.

    import {
      setTimeout,
    } from 'node:timers/promises';

    const res = await setTimeout(100, 'result');

    console.log(res);  // Prints 'result'
    const {
      setTimeout,
    } = require('node:timers/promises');

    setTimeout(100, 'result').then((res) => {
      console.log(res);  // Prints 'result'
    });
    javascript

#### `timersPromises.setImmediate([value[, options]])`#

Added in: v15.0.0

- `value` `<any>` A value with which the promise is fulfilled.
- `options` `<Object>`
  - `ref` `<boolean>` Set to `false` to indicate that the scheduled `Immediate` should not require the Node.js event loop to remain active. Default: `true`.
  - `signal` `<AbortSignal>` An optional `AbortSignal` that can be used to cancel the scheduled `Immediate`.

    import {
      setImmediate,
    } from 'node:timers/promises';

    const res = await setImmediate('result');

    console.log(res);  // Prints 'result'
    const {
      setImmediate,
    } = require('node:timers/promises');

    setImmediate('result').then((res) => {
      console.log(res);  // Prints 'result'
    });
    javascript

#### `timersPromises.setInterval([delay[, value[, options]]])`#

Added in: v15.9.0

Returns an async iterator that generates values in an interval of `delay` ms. If `ref` is `true`, you need to call `next()` of async iterator explicitly or implicitly to keep the event loop alive.

- `delay` `<number>` The number of milliseconds to wait between iterations. Default: `1`.
- `value` `<any>` A value with which the iterator returns.
- `options` `<Object>`
  - `ref` `<boolean>` Set to `false` to indicate that the scheduled `Timeout` between iterations should not require the Node.js event loop to remain active. Default: `true`.
  - `signal` `<AbortSignal>` An optional `AbortSignal` that can be used to cancel the scheduled `Timeout` between operations.

    import {
      setInterval,
    } from 'node:timers/promises';

    const interval = 100;
    for await (const startTime of setInterval(interval, Date.now())) {
      const now = Date.now();
      console.log(now);
      if ((now - startTime) > 1000)
        break;
    }
    console.log(Date.now());
    const {
      setInterval,
    } = require('node:timers/promises');
    const interval = 100;

    (async function() {
      for await (const startTime of setInterval(interval, Date.now())) {
        const now = Date.now();
        console.log(now);
        if ((now - startTime) > 1000)
          break;
      }
      console.log(Date.now());
    })();
    javascript

#### `timersPromises.scheduler.wait(delay[, options])`#

Added in: v17.3.0, v16.14.0

Stability: 1 - Experimental

- `delay` `<number>` The number of milliseconds to wait before resolving the promise.
- `options` `<Object>`
  - `ref` `<boolean>` Set to `false` to indicate that the scheduled `Timeout` should not require the Node.js event loop to remain active. Default: `true`.
  - `signal` `<AbortSignal>` An optional `AbortSignal` that can be used to cancel waiting.
- Returns: `<Promise>`

An experimental API defined by the Scheduling APIs draft specification being developed as a standard Web Platform API.

Calling `timersPromises.scheduler.wait(delay, options)` is equivalent to calling `timersPromises.setTimeout(delay, undefined, options)`.

    import { scheduler } from 'node:timers/promises';

    await scheduler.wait(1000); // Wait one second before continuing
    mjs

#### `timersPromises.scheduler.yield()`#

Added in: v17.3.0, v16.14.0

Stability: 1 - Experimental

- Returns: `<Promise>`

An experimental API defined by the Scheduling APIs draft specification being developed as a standard Web Platform API.

Calling `timersPromises.scheduler.yield()` is equivalent to calling `timersPromises.setImmediate()` with no arguments.

## TLS (SSL)#

Source Code: lib/tls.js

Stability: 2 - Stable

The `node:tls` module provides an implementation of the Transport Layer Security (TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL. The module can be accessed using:

    import tls from 'node:tls';
    const tls = require('node:tls');
    javascript

### Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the `node:crypto` module. In such cases, attempting to `import` from `tls` or calling `require('node:tls')` will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

    let tls;
    try {
      tls = require('node:tls');
    } catch (err) {
      console.error('tls support is disabled!');
    }
    cjs

When using the lexical ESM `import` keyword, the error can only be caught if a handler for `process.on('uncaughtException')` is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the `import()` function instead of the lexical `import` keyword:

    let tls;
    try {
      tls = await import('node:tls');
    } catch (err) {
      console.error('tls support is disabled!');
    }
    mjs

### TLS/SSL concepts#

TLS/SSL is a set of protocols that rely on a public key infrastructure (PKI) to enable secure communication between a client and a server. For most common cases, each server must have a private key.

Private keys can be generated in multiple ways. The example below illustrates use of the OpenSSL command-line interface to generate a 2048-bit RSA private key:

    openssl genrsa -out ryans-key.pem 2048
    bash

With TLS/SSL, all servers (and some clients) must have a certificate. Certificates are public keys that correspond to a private key, and that are digitally signed either by a Certificate Authority or by the owner of the private key (such certificates are referred to as "self-signed"). The first step to obtaining a certificate is to create a Certificate Signing Request (CSR) file.

The OpenSSL command-line interface can be used to generate a CSR for a private key:

    openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem
    bash

Once the CSR file is generated, it can either be sent to a Certificate Authority for signing or used to generate a self-signed certificate.

Creating a self-signed certificate using the OpenSSL command-line interface is illustrated in the example below:

    openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem
    bash

Once the certificate is generated, it can be used to generate a `.pfx` or `.p12` file:

    openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
          -certfile ca-cert.pem -out ryans.pfx
    bash

Where:

- `in`: is the signed certificate
- `inkey`: is the associated private key
- `certfile`: is a concatenation of all Certificate Authority (CA) certs into a single file, e.g. `cat ca1-cert.pem ca2-cert.pem > ca-cert.pem`

#### Perfect forward secrecy#

The term forward secrecy or perfect forward secrecy describes a feature of key-agreement (i.e., key-exchange) methods. That is, the server and client keys are used to negotiate new temporary keys that are used specifically and only for the current communication session. Practically, this means that even if the server's private key is compromised, communication can only be decrypted by eavesdroppers if the attacker manages to obtain the key-pair specifically generated for the session.

Perfect forward secrecy is achieved by randomly generating a key pair for key-agreement on every TLS/SSL handshake (in contrast to using the same key for all sessions). Methods implementing this technique are called "ephemeral".

Currently two methods are commonly used to achieve perfect forward secrecy (note the character "E" appended to the traditional abbreviations):

- ECDHE: An ephemeral version of the Elliptic Curve Diffie-Hellman key-agreement protocol.
- DHE: An ephemeral version of the Diffie-Hellman key-agreement protocol.

Perfect forward secrecy using ECDHE is enabled by default. The `ecdhCurve` option can be used when creating a TLS server to customize the list of supported ECDH curves for TLSv1.2 and below, and the list of supported TLS groups for TLSv1.3. See `tls.createServer()` for more info.

DHE is disabled by default but can be enabled alongside ECDHE by setting the `dhparam` option to `'auto'`. Custom DHE parameters are also supported but discouraged in favor of automatically selected, well-known parameters.

Perfect forward secrecy was optional up to TLSv1.2. As of TLSv1.3, (EC)DHE is always used (with the exception of PSK-only connections).

#### ALPN and SNI#

ALPN (Application-Layer Protocol Negotiation Extension) and SNI (Server Name Indication) are TLS handshake extensions:

- ALPN: Allows the use of one TLS server for multiple protocols (HTTP, HTTP/2)
- SNI: Allows the use of one TLS server for multiple hostnames with different certificates.

#### Pre-shared keys#

TLS-PSK support is available as an alternative to normal certificate-based authentication. It uses a pre-shared key instead of certificates to authenticate a TLS connection, providing mutual authentication. TLS-PSK and public key infrastructure are not mutually exclusive. Clients and servers can accommodate both, choosing either of them during the normal cipher negotiation step.

TLS-PSK is only a good choice where means exist to securely share a key with every connecting machine, so it does not replace the public key infrastructure (PKI) for the majority of TLS uses. The TLS-PSK implementation in OpenSSL has seen many security flaws in recent years, mostly because it is used only by a minority of applications. Please consider all alternative solutions before switching to PSK ciphers. Upon generating PSK it is of critical importance to use sufficient entropy as discussed in RFC 4086. Deriving a shared secret from a password or other low-entropy sources is not secure.

PSK ciphers are disabled by default, and using TLS-PSK thus requires explicitly specifying a cipher suite with the `ciphers` option. The list of available ciphers can be retrieved via `openssl ciphers -v 'PSK'`. All TLS 1.3 ciphers are eligible for PSK and can be retrieved via `openssl ciphers -v -s -tls1_3 -psk`. On the client connection, a custom `checkServerIdentity` should be passed because the default one will fail in the absence of a certificate.

According to the RFC 4279, PSK identities up to 128 bytes in length and PSKs up to 64 bytes in length must be supported. As of OpenSSL 1.1.0 maximum identity size is 128 bytes, and maximum PSK length is 256 bytes.

The current implementation doesn't support asynchronous PSK callbacks due to the limitations of the underlying OpenSSL API.

To use TLS-PSK, client and server must specify the `pskCallback` option, a function that returns the PSK to use (which must be compatible with the selected cipher's digest).

It will be called first on the client:

- `hint` `<string>` optional message sent from the server to help the client decide which identity to use during negotiation. Always `null` if TLS 1.3 is used.
- Returns: `<Object>` in the form `{ psk: <Buffer|TypedArray|DataView>, identity: <string> }` or `null`.

Then on the server:

- `socket` `<tls.TLSSocket>` the server socket instance, equivalent to `this`.
- `identity` `<string>` identity parameter sent from the client.
- Returns: `<Buffer>` | `<TypedArray>` | `<DataView>` the PSK (or `null`).

A return value of `null` stops the negotiation process and sends an `unknown_psk_identity` alert message to the other party. If the server wishes to hide the fact that the PSK identity was not known, the callback must provide some random data as `psk` to make the connection fail with `decrypt_error` before negotiation is finished.

#### Client-initiated renegotiation attack mitigation#

The TLS protocol allows clients to renegotiate certain aspects of the TLS session. Unfortunately, session renegotiation requires a disproportionate amount of server-side resources, making it a potential vector for denial-of-service attacks.

To mitigate the risk, renegotiation is limited to three times every ten minutes. An `'error'` event is emitted on the `tls.TLSSocket` instance when this threshold is exceeded. The limits are configurable:

- `tls.CLIENT_RENEG_LIMIT` `<number>` Specifies the number of renegotiation requests. Default: `3`.
- `tls.CLIENT_RENEG_WINDOW` `<number>` Specifies the time renegotiation window in seconds. Default: `600` (10 minutes).

The default renegotiation limits should not be modified without a full understanding of the implications and risks.

TLSv1.3 does not support renegotiation.

#### Session resumption#

Establishing a TLS session can be relatively slow. The process can be sped up by saving and later reusing the session state. There are several mechanisms to do so, discussed here from oldest to newest (and preferred).

##### Session identifiers#

Servers generate a unique ID for new connections and send it to the client. Clients and servers save the session state. When reconnecting, clients send the ID of their saved session state and if the server also has the state for that ID, it can agree to use it. Otherwise, the server will create a new session. See RFC 2246 for more information, page 23 and 30.

Resumption using session identifiers is supported by most web browsers when making HTTPS requests.

For Node.js, clients wait for the `'session'` event to get the session data, and provide the data to the `session` option of a subsequent `tls.connect()` to reuse the session. Servers must implement handlers for the `'newSession'` and `'resumeSession'` events to save and restore the session data using the session ID as the lookup key to reuse sessions. To reuse sessions across load balancers or cluster workers, servers must use a shared session cache (such as Redis) in their session handlers.

##### Session tickets#

The servers encrypt the entire session state and send it to the client as a "ticket". When reconnecting, the state is sent to the server in the initial connection. This mechanism avoids the need for a server-side session cache. If the server doesn't use the ticket, for any reason (failure to decrypt it, it's too old, etc.), it will create a new session and send a new ticket. See RFC 5077 for more information.

Resumption using session tickets is becoming commonly supported by many web browsers when making HTTPS requests.

For Node.js, clients use the same APIs for resumption with session identifiers as for resumption with session tickets. For debugging, if `tls.TLSSocket.getTLSTicket()` returns a value, the session data contains a ticket, otherwise it contains client-side session state.

With TLSv1.3, be aware that multiple tickets may be sent by the server, resulting in multiple `'session'` events, see `'session'` for more information.

Single process servers need no specific implementation to use session tickets. To use session tickets across server restarts or load balancers, servers must all have the same ticket keys. There are three 16-byte keys internally, but the tls API exposes them as a single 48-byte buffer for convenience.

It's possible to get the ticket keys by calling `server.getTicketKeys()` on one server instance and then distribute them, but it is more reasonable to securely generate 48 bytes of secure random data and set them with the `ticketKeys` option of `tls.createServer()`. The keys should be regularly regenerated and server's keys can be reset with `server.setTicketKeys()`.

Session ticket keys are cryptographic keys, and they must be stored securely. With TLS 1.2 and below, if they are compromised all sessions that used tickets encrypted with them can be decrypted. They should not be stored on disk, and they should be regenerated regularly.

If clients advertise support for tickets, the server will send them. The server can disable tickets by supplying `require('node:constants').SSL_OP_NO_TICKET` in `secureOptions`.

Both session identifiers and session tickets timeout, causing the server to create new sessions. The timeout can be configured with the `sessionTimeout` option of `tls.createServer()`.

For all the mechanisms, when resumption fails, servers will create new sessions. Since failing to resume the session does not cause TLS/HTTPS connection failures, it is easy to not notice unnecessarily poor TLS performance. The OpenSSL CLI can be used to verify that servers are resuming sessions. Use the `-reconnect` option to `openssl s_client`, for example:

    openssl s_client -connect localhost:443 -reconnect
    bash

Read through the debug output. The first connection should say "New", for example:

    New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
    text

Subsequent connections should say "Reused", for example:

    Reused, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
    text

### Modifying the default TLS cipher suite#

Node.js is built with a default suite of enabled and disabled TLS ciphers. This default cipher list can be configured when building Node.js to allow distributions to provide their own default list.

The following command can be used to show the default cipher suite:

    node -p crypto.constants.defaultCoreCipherList | tr ':' '\n'
    TLS_AES_256_GCM_SHA384
    TLS_CHACHA20_POLY1305_SHA256
    TLS_AES_128_GCM_SHA256
    ECDHE-RSA-AES128-GCM-SHA256
    ECDHE-ECDSA-AES128-GCM-SHA256
    ECDHE-RSA-AES256-GCM-SHA384
    ECDHE-ECDSA-AES256-GCM-SHA384
    DHE-RSA-AES128-GCM-SHA256
    ECDHE-RSA-AES128-SHA256
    DHE-RSA-AES128-SHA256
    ECDHE-RSA-AES256-SHA384
    DHE-RSA-AES256-SHA384
    ECDHE-RSA-AES256-SHA256
    DHE-RSA-AES256-SHA256
    HIGH
    !aNULL
    !eNULL
    !EXPORT
    !DES
    !RC4
    !MD5
    !PSK
    !SRP
    !CAMELLIA
    console

This default can be replaced entirely using the `--tls-cipher-list` command-line switch (directly, or via the `NODE_OPTIONS` environment variable). For instance, the following makes `ECDHE-RSA-AES128-GCM-SHA256:!RC4` the default TLS cipher suite:

    node --tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4' server.js

    export NODE_OPTIONS=--tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4'
    node server.js
    bash

To verify, use the following command to show the set cipher list, note the difference between `defaultCoreCipherList` and `defaultCipherList`:

    node --tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4' -p crypto.constants.defaultCipherList | tr ':' '\n'
    ECDHE-RSA-AES128-GCM-SHA256
    !RC4
    bash

i.e. the `defaultCoreCipherList` list is set at compilation time and the `defaultCipherList` is set at runtime.

To modify the default cipher suites from within the runtime, modify the `tls.DEFAULT_CIPHERS` variable, this must be performed before listening on any sockets, it will not affect sockets already opened. For example:

    // Remove Obsolete CBC Ciphers and RSA Key Exchange based Ciphers as they don't provide Forward Secrecy
    tls.DEFAULT_CIPHERS +=
      ':!ECDHE-RSA-AES128-SHA:!ECDHE-RSA-AES128-SHA256:!ECDHE-RSA-AES256-SHA:!ECDHE-RSA-AES256-SHA384' +
      ':!ECDHE-ECDSA-AES128-SHA:!ECDHE-ECDSA-AES128-SHA256:!ECDHE-ECDSA-AES256-SHA:!ECDHE-ECDSA-AES256-SHA384' +
      ':!kRSA';
    js

The default can also be replaced on a per client or server basis using the `ciphers` option from `tls.createSecureContext()`, which is also available in `tls.createServer()`, `tls.connect()`, and when creating new `tls.TLSSocket`s.

The ciphers list can contain a mixture of TLSv1.3 cipher suite names, the ones that start with `'TLS_'`, and specifications for TLSv1.2 and below cipher suites. The TLSv1.2 ciphers support a legacy specification format, consult the OpenSSL cipher list format documentation for details, but those specifications do not apply to TLSv1.3 ciphers. The TLSv1.3 suites can only be enabled by including their full name in the cipher list. They cannot, for example, be enabled or disabled by using the legacy TLSv1.2 `'EECDH'` or `'!EECDH'` specification.

Despite the relative order of TLSv1.3 and TLSv1.2 cipher suites, the TLSv1.3 protocol is significantly more secure than TLSv1.2, and will always be chosen over TLSv1.2 if the handshake indicates it is supported, and if any TLSv1.3 cipher suites are enabled.

The default cipher suite included within Node.js has been carefully selected to reflect current security best practices and risk mitigation. Changing the default cipher suite can have a significant impact on the security of an application. The `--tls-cipher-list` switch and `ciphers` option should by used only if absolutely necessary.

The default cipher suite prefers GCM ciphers for Chrome's 'modern cryptography' setting and also prefers ECDHE and DHE ciphers for perfect forward secrecy, while offering some backward compatibility.

Old clients that rely on insecure and deprecated RC4 or DES-based ciphers (like Internet Explorer 6) cannot complete the handshaking process with the default configuration. If these clients must be supported, the TLS recommendations may offer a compatible cipher suite. For more details on the format, see the OpenSSL cipher list format documentation.

There are only five TLSv1.3 cipher suites:

- `'TLS_AES_256_GCM_SHA384'`
- `'TLS_CHACHA20_POLY1305_SHA256'`
- `'TLS_AES_128_GCM_SHA256'`
- `'TLS_AES_128_CCM_SHA256'`
- `'TLS_AES_128_CCM_8_SHA256'`

The first three are enabled by default. The two `CCM`-based suites are supported by TLSv1.3 because they may be more performant on constrained systems, but they are not enabled by default since they offer less security.

### OpenSSL security level#

The OpenSSL library enforces security levels to control the minimum acceptable level of security for cryptographic operations. OpenSSL's security levels range from 0 to 5, with each level imposing stricter security requirements. The default security level is 2, which is generally suitable for most modern applications. However, some legacy features and protocols, such as TLSv1, require a lower security level (`SECLEVEL=0`) to function properly. For more detailed information, please refer to the OpenSSL documentation on security levels.

#### Setting security levels#

To adjust the security level in your Node.js application, you can include `@SECLEVEL=X` within a cipher string, where `X` is the desired security level. For example, to set the security level to 0 while using the default OpenSSL cipher list, you could use:

    import { createServer, connect } from 'node:tls';
    const port = 443;

    createServer({ ciphers: 'DEFAULT@SECLEVEL=0', minVersion: 'TLSv1' }, function(socket) {
      console.log('Client connected with protocol:', socket.getProtocol());
      socket.end();
      this.close();
    })
    .listen(port, () => {
      connect(port, { ciphers: 'DEFAULT@SECLEVEL=0', maxVersion: 'TLSv1' });
    });
    const { createServer, connect } = require('node:tls');
    const port = 443;

    createServer({ ciphers: 'DEFAULT@SECLEVEL=0', minVersion: 'TLSv1' }, function(socket) {
      console.log('Client connected with protocol:', socket.getProtocol());
      socket.end();
      this.close();
    })
    .listen(port, () => {
      connect(port, { ciphers: 'DEFAULT@SECLEVEL=0', maxVersion: 'TLSv1' });
    });
    javascript

This approach sets the security level to 0, allowing the use of legacy features while still leveraging the default OpenSSL ciphers.

#### Using `--tls-cipher-list`#

You can also set the security level and ciphers from the command line using the `--tls-cipher-list=DEFAULT@SECLEVEL=X` as described in Modifying the default TLS cipher suite. However, it is generally discouraged to use the command line option for setting ciphers and it is preferable to configure the ciphers for individual contexts within your application code, as this approach provides finer control and reduces the risk of globally downgrading the security level.

### X509 certificate error codes#

Multiple functions can fail due to certificate errors that are reported by OpenSSL. In such a case, the function provides an `<Error>` via its callback that has the property `code` which can take one of the following values:

- `'UNABLE_TO_GET_ISSUER_CERT'`: Unable to get issuer certificate.
- `'UNABLE_TO_GET_CRL'`: Unable to get certificate CRL.
- `'UNABLE_TO_DECRYPT_CERT_SIGNATURE'`: Unable to decrypt certificate's signature.
- `'UNABLE_TO_DECRYPT_CRL_SIGNATURE'`: Unable to decrypt CRL's signature.
- `'UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY'`: Unable to decode issuer public key.
- `'CERT_SIGNATURE_FAILURE'`: Certificate signature failure.
- `'CRL_SIGNATURE_FAILURE'`: CRL signature failure.
- `'CERT_NOT_YET_VALID'`: Certificate is not yet valid.
- `'CERT_HAS_EXPIRED'`: Certificate has expired.
- `'CRL_NOT_YET_VALID'`: CRL is not yet valid.
- `'CRL_HAS_EXPIRED'`: CRL has expired.
- `'ERROR_IN_CERT_NOT_BEFORE_FIELD'`: Format error in certificate's notBefore field.
- `'ERROR_IN_CERT_NOT_AFTER_FIELD'`: Format error in certificate's notAfter field.
- `'ERROR_IN_CRL_LAST_UPDATE_FIELD'`: Format error in CRL's lastUpdate field.
- `'ERROR_IN_CRL_NEXT_UPDATE_FIELD'`: Format error in CRL's nextUpdate field.
- `'OUT_OF_MEM'`: Out of memory.
- `'DEPTH_ZERO_SELF_SIGNED_CERT'`: Self signed certificate.
- `'SELF_SIGNED_CERT_IN_CHAIN'`: Self signed certificate in certificate chain.
- `'UNABLE_TO_GET_ISSUER_CERT_LOCALLY'`: Unable to get local issuer certificate.
- `'UNABLE_TO_VERIFY_LEAF_SIGNATURE'`: Unable to verify the first certificate.
- `'CERT_CHAIN_TOO_LONG'`: Certificate chain too long.
- `'CERT_REVOKED'`: Certificate revoked.
- `'INVALID_CA'`: Invalid CA certificate.
- `'PATH_LENGTH_EXCEEDED'`: Path length constraint exceeded.
- `'INVALID_PURPOSE'`: Unsupported certificate purpose.
- `'CERT_UNTRUSTED'`: Certificate not trusted.
- `'CERT_REJECTED'`: Certificate rejected.
- `'HOSTNAME_MISMATCH'`: Hostname mismatch.

When certificate errors like `UNABLE_TO_VERIFY_LEAF_SIGNATURE`, `DEPTH_ZERO_SELF_SIGNED_CERT`, or `UNABLE_TO_GET_ISSUER_CERT` occur, Node.js appends a hint suggesting that if the root CA is installed locally, try running with the `--use-system-ca` flag to direct developers towards a secure solution, to prevent unsafe workarounds.

### Class: `tls.Server`#

Added in: v0.3.2

- Extends: `<net.Server>`

Accepts encrypted connections using TLS or SSL.

#### Event: `'connection'`#

Added in: v0.3.2

- `socket` `<stream.Duplex>`

This event is emitted when a new TCP stream is established, before the TLS handshake begins. `socket` is typically an object of type `net.Socket` but will not receive events unlike the socket created from the `net.Server` `'connection'` event. Usually users will not want to access this event.

This event can also be explicitly emitted by users to inject connections into the TLS server. In that case, any `Duplex` stream can be passed.

#### Event: `'keylog'`#

Added in: v12.3.0, v10.20.0

- `line` `<Buffer>` Line of ASCII text, in NSS `SSLKEYLOGFILE` format.
- `tlsSocket` `<tls.TLSSocket>` The `tls.TLSSocket` instance on which it was generated.

The `keylog` event is emitted when key material is generated or received by a connection to this server (typically before handshake has completed, but not necessarily). This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times for each socket.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

    const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
    // ...
    server.on('keylog', (line, tlsSocket) => {
      if (tlsSocket.remoteAddress !== '...')
        return; // Only log keys for a particular IP
      logFile.write(line);
    });
    js

#### Event: `'newSession'`#

Added in: v0.9.2
History

Version | Changes
--- | ---
v0.11.12 | The `callback` argument is now supported.

The `'newSession'` event is emitted upon creation of a new TLS session. This may be used to store sessions in external storage. The data should be provided to the `'resumeSession'` callback.

The listener callback is passed three arguments when called:

- `sessionId` `<Buffer>` The TLS session identifier
- `sessionData` `<Buffer>` The TLS session data
- `callback` `<Function>` A callback function taking no arguments that must be invoked in order for data to be sent or received over the secure connection.

Listening for this event will have an effect only on connections established after the addition of the event listener.

#### Event: `'OCSPRequest'`#

Added in: v0.11.13

The `'OCSPRequest'` event is emitted when the client sends a certificate status request. The listener callback is passed three arguments when called:

- `certificate` `<Buffer>` The server certificate
- `issuer` `<Buffer>` The issuer's certificate
- `callback` `<Function>` A callback function that must be invoked to provide the results of the OCSP request.

The server's current certificate can be parsed to obtain the OCSP URL and certificate ID; after obtaining an OCSP response, `callback(null, resp)` is then invoked, where `resp` is a `Buffer` instance containing the OCSP response. Both `certificate` and `issuer` are `Buffer` DER-representations of the primary and issuer's certificates. These can be used to obtain the OCSP certificate ID and OCSP endpoint URL.

Alternatively, `callback(null, null)` may be called, indicating that there was no OCSP response.

Calling `callback(err)` will result in a `socket.destroy(err)` call.

The typical flow of an OCSP request is as follows:

1. Client connects to the server and sends an `'OCSPRequest'` (via the status info extension in ClientHello).
2. Server receives the request and emits the `'OCSPRequest'` event, calling the listener if registered.
3. Server extracts the OCSP URL from either the `certificate` or `issuer` and performs an OCSP request to the CA.
4. Server receives `'OCSPResponse'` from the CA and sends it back to the client via the `callback` argument
5. Client validates the response and either destroys the socket or performs a handshake.

The `issuer` can be `null` if the certificate is either self-signed or the issuer is not in the root certificates list. (An issuer may be provided via the `ca` option when establishing the TLS connection.)

Listening for this event will have an effect only on connections established after the addition of the event listener.

An npm module like asn1.js may be used to parse the certificates.

#### Event: `'resumeSession'`#

Added in: v0.9.2

The `'resumeSession'` event is emitted when the client requests to resume a previous TLS session. The listener callback is passed two arguments when called:

- `sessionId` `<Buffer>` The TLS session identifier
- `callback` `<Function>` A callback function to be called when the prior session has been recovered: `callback([err[, sessionData]])`
  - `err` `<Error>`
  - `sessionData` `<Buffer>`

The event listener should perform a lookup in external storage for the `sessionData` saved by the `'newSession'` event handler using the given `sessionId`. If found, call `callback(null, sessionData)` to resume the session. If not found, the session cannot be resumed. `callback()` must be called without `sessionData` so that the handshake can continue and a new session can be created. It is possible to call `callback(err)` to terminate the incoming connection and destroy the socket.

Listening for this event will have an effect only on connections established after the addition of the event listener.

The following illustrates resuming a TLS session:

    const tlsSessionStore = {};
    server.on('newSession', (id, data, cb) => {
      tlsSessionStore[id.toString('hex')] = data;
      cb();
    });
    server.on('resumeSession', (id, cb) => {
      cb(null, tlsSessionStore[id.toString('hex')] || null);
    });
    js

#### Event: `'secureConnection'`#

Added in: v0.3.2

The `'secureConnection'` event is emitted after the handshaking process for a new connection has successfully completed. The listener callback is passed a single argument when called:

- `tlsSocket` `<tls.TLSSocket>` The established TLS socket.

The `tlsSocket.authorized` property is a `boolean` indicating whether the client has been verified by one of the supplied Certificate Authorities for the server. If `tlsSocket.authorized` is `false`, then `socket.authorizationError` is set to describe how authorization failed. Depending on the settings of the TLS server, unauthorized connections may still be accepted.

The `tls.TLSSocket.servername` and `tls.TLSSocket.alpnProtocol` properties can be used to check which server name was requested, and which protocol was negotiated.

#### Event: `'tlsClientError'`#

Added in: v6.0.0

The `'tlsClientError'` event is emitted when an error occurs before a secure connection is established. The listener callback is passed two arguments when called:

- `exception` `<Error>` The `Error` object describing the error
- `tlsSocket` `<tls.TLSSocket>` The `tls.TLSSocket` instance from which the error originated.

#### `server.addContext(hostname, context)`#

Added in: v0.5.3

- `hostname` `<string>` A SNI host name or wildcard (e.g. `'*'`)
- `context` `<Object>` | `<tls.SecureContext>` An object containing any of the possible properties from the `tls.createSecureContext()` `options` arguments (e.g. `key`, `cert`, `ca`, etc), or a TLS context object created with `tls.createSecureContext()` itself.

The `server.addContext()` method adds a secure context that will be used if the client request's SNI name matches the supplied `hostname` (or wildcard).

When there are multiple matching contexts, the most recently added one is used.

#### `server.address()`#

Added in: v0.6.0

- Returns: `<Object>`

Returns the bound address, the address family name, and port of the server as reported by the operating system. See `net.Server.address()` for more information.

#### `server.close([callback])`#

Added in: v0.3.2

- `callback` `<Function>` A listener callback that will be registered to listen for the server instance's `'close'` event.
- Returns: `<tls.Server>`

The `server.close()` method stops the server from accepting new connections.

This function operates asynchronously. The `'close'` event will be emitted when the server has no more open connections.

#### `server.getTicketKeys()`#

Added in: v3.0.0

- Returns: `<Buffer>` A 48-byte buffer containing the session ticket keys.

Returns the session ticket keys.

See Session Resumption for more information.

#### `server.listen()`#

Starts the server listening for encrypted connections. This method is identical to `server.listen()` from `net.Server`.

#### `server.setSecureContext(options)`#

Added in: v11.0.0

- `options` `<Object>` An object containing any of the possible properties from the `tls.createSecureContext()` `options` arguments (e.g. `key`, `cert`, `ca`, etc).

The `server.setSecureContext()` method replaces the secure context of an existing server. Existing connections to the server are not interrupted.

#### `server.setTicketKeys(keys)`#

Added in: v3.0.0

- `keys` `<Buffer>` | `<TypedArray>` | `<DataView>` A 48-byte buffer containing the session ticket keys.

Sets the session ticket keys.

Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.

See Session Resumption for more information.

### Class: `tls.TLSSocket`#

Added in: v0.11.4

- Extends: `<net.Socket>`

Performs transparent encryption of written data and all required TLS negotiation.

Instances of `tls.TLSSocket` implement the duplex Stream interface.

Methods that return TLS connection metadata (e.g. `tls.TLSSocket.getPeerCertificate()`) will only return data while the connection is open.

#### `new tls.TLSSocket(socket[, options])`#

Added in: v0.11.4
History

Version | Changes
--- | ---
v12.2.0 | The `enableTrace` option is now supported.
v5.0.0 | ALPN options are supported now.

- `socket` `<net.Socket>` | `<stream.Duplex>` On the server side, any `Duplex` stream. On the client side, any instance of `net.Socket` (for generic `Duplex` stream support on the client side, `tls.connect()` must be used).
- `options` `<Object>`
  - `enableTrace`: See `tls.createServer()`
  - `isServer`: The SSL/TLS protocol is asymmetrical, TLSSockets must know if they are to behave as a server or a client. If `true` the TLS socket will be instantiated as a server. Default: `false`.
  - `server` `<net.Server>` A `net.Server` instance.
  - `requestCert`: Whether to authenticate the remote peer by requesting a certificate. Clients always request a server certificate. Servers (`isServer` is true) may set `requestCert` to true to request a client certificate.
  - `rejectUnauthorized`: See `tls.createServer()`
  - `ALPNProtocols`: See `tls.createServer()`
  - `SNICallback`: See `tls.createServer()`
  - `ALPNCallback`: See `tls.createServer()`
  - `session` `<Buffer>` A `Buffer` instance containing a TLS session.
  - `requestOCSP` `<boolean>` If `true`, specifies that the OCSP status request extension will be added to the client hello and an `'OCSPResponse'` event will be emitted on the socket before establishing a secure communication
  - `secureContext`: TLS context object created with `tls.createSecureContext()`. If a `secureContext` is not provided, one will be created by passing the entire `options` object to `tls.createSecureContext()`.
  - ...: `tls.createSecureContext()` options that are used if the `secureContext` option is missing. Otherwise, they are ignored.

Construct a new `tls.TLSSocket` object from an existing TCP socket.

#### Event: `'keylog'`#

Added in: v12.3.0, v10.20.0

- `line` `<Buffer>` Line of ASCII text, in NSS `SSLKEYLOGFILE` format.

The `keylog` event is emitted on a `tls.TLSSocket` when key material is generated or received by the socket. This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times, before or after the handshake completes.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

    const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
    // ...
    tlsSocket.on('keylog', (line) => logFile.write(line));
    js

#### Event: `'OCSPResponse'`#

Added in: v0.11.13

The `'OCSPResponse'` event is emitted if the `requestOCSP` option was set when the `tls.TLSSocket` was created and an OCSP response has been received. The listener callback is passed a single argument when called:

- `response` `<Buffer>` The server's OCSP response

Typically, the `response` is a digitally signed object from the server's CA that contains information about server's certificate revocation status.

#### Event: `'secure'`#

Added in: v0.11.4

The `'secure'` event is emitted after the TLS handshake has successfully completed and a secure connection has been established.

This event is emitted on both client and server `<tls.TLSSocket>` instances, including sockets created using the `new tls.TLSSocket()` constructor.

#### Event: `'secureConnect'`#

Added in: v0.11.4

The `'secureConnect'` event is emitted after the handshaking process for a new connection has successfully completed. The listener callback will be called regardless of whether or not the server's certificate has been authorized. It is the client's responsibility to check the `tlsSocket.authorized` property to determine if the server certificate was signed by one of the specified CAs. If `tlsSocket.authorized === false`, then the error can be found by examining the `tlsSocket.authorizationError` property. If ALPN was used, the `tlsSocket.alpnProtocol` property can be checked to determine the negotiated protocol.

The `'secureConnect'` event is not emitted when a `<tls.TLSSocket>` is created using the `new tls.TLSSocket()` constructor.

#### Event: `'session'`#

Added in: v11.10.0

- `session` `<Buffer>`

The `'session'` event is emitted on a client `tls.TLSSocket` when a new session or TLS ticket is available. This may or may not be before the handshake is complete, depending on the TLS protocol version that was negotiated. The event is not emitted on the server, or if a new session was not created, for example, when the connection was resumed. For some TLS protocol versions the event may be emitted multiple times, in which case all the sessions can be used for resumption.

On the client, the `session` can be provided to the `session` option of `tls.connect()` to resume the connection.

See Session Resumption for more information.

For TLSv1.2 and below, `tls.TLSSocket.getSession()` can be called once the handshake is complete. For TLSv1.3, only ticket-based resumption is allowed by the protocol, multiple tickets are sent, and the tickets aren't sent until after the handshake completes. So it is necessary to wait for the `'session'` event to get a resumable session. Applications should use the `'session'` event instead of `getSession()` to ensure they will work for all TLS versions. Applications that only expect to get or use one session should listen for this event only once:

    tlsSocket.once('session', (session) => {
      // The session can be used immediately or later.
      tls.connect({
        session: session,
        // Other connect options...
      });
    });
    js

#### `tlsSocket.address()`#

Added in: v0.11.4
History

Version | Changes
--- | ---
v18.4.0 | The `family` property now returns a string instead of a number.
v18.0.0 | The `family` property now returns a number instead of a string.

- Returns: `<Object>`

Returns the bound `address`, the address `family` name, and `port` of the underlying socket as reported by the operating system: `{ port: 12346, family: 'IPv4', address: '127.0.0.1' }`.

#### `tlsSocket.alpnProtocol`#

Added in: v6.0.0

- Type: `<string>` | `<boolean>` | `<null>`

The negotiated ALPN protocol. This is `null` before the handshake completes. Once the handshake completes, it settles as either the negotiated protocol name, or `false` if the peers did not negotiate an ALPN protocol.

#### `tlsSocket.authorizationError`#

Added in: v0.11.4

Returns the reason why the peer's certificate was not been verified. This property is set only when `tlsSocket.authorized === false`.

#### `tlsSocket.authorized`#

Added in: v0.11.4

- Type: `<boolean>`

This property is `true` if the peer certificate was signed by one of the CAs specified when creating the `tls.TLSSocket` instance, otherwise `false`.

The peer certificate is only verified during a full TLS handshake. When a connection is established by resuming a previous session (see Session Resumption), verification is not repeated. If the client presented a certificate in the original handshake, `authorized` and `authorizationError` carry the result stored with the session, including any verification error. On TLS 1.3, a client that sent no certificate at all can resume a session and report `authorized` as `true`, while `tls.TLSSocket.getPeerCertificate()` returns an empty object. Servers that authorize clients manually with `rejectUnauthorized: false` should therefore also check `tls.TLSSocket.isSessionReused()` and that a peer certificate is present.

#### `tlsSocket.disableRenegotiation()`#

Added in: v8.4.0

Disables TLS renegotiation for this `TLSSocket` instance. Once called, attempts to renegotiate will trigger an `'error'` event on the `TLSSocket`.

#### `tlsSocket.enableTrace()`#

Added in: v12.2.0

When enabled, TLS packet trace information is written to `stderr`. This can be used to debug TLS connection problems.

The format of the output is identical to the output of `openssl s_client -trace` or `openssl s_server -trace`. While it is produced by OpenSSL's `SSL_trace()` function, the format is undocumented, can change without notice, and should not be relied on.

#### `tlsSocket.encrypted`#

Added in: v0.11.4

Always returns `true`. This may be used to distinguish TLS sockets from regular `net.Socket` instances.

#### `tlsSocket.exportKeyingMaterial(length, label[, context])`#

Added in: v13.10.0, v12.17.0

- `length` `<number>` number of bytes to retrieve from keying material

- `label` `<string>` an application specific label, typically this will be a value from the IANA Exporter Label Registry.

- `context` `<Buffer>` Optionally provide a context.

- Returns: `<Buffer>` requested bytes of the keying material

Keying material is used for validations to prevent different kind of attacks in network protocols, for example in the specifications of IEEE 802.1X.

Example

    const keyingMaterial = tlsSocket.exportKeyingMaterial(
      128,
      'client finished');

    /*
     Example return value of keyingMaterial:
     <Buffer 76 26 af 99 c5 56 8e 42 09 91 ef 9f 93 cb ad 6c 7b 65 f8 53 f1 d8 d9
        12 5a 33 b8 b5 25 df 7b 37 9f e0 e2 4f b8 67 83 a3 2f cd 5d 41 42 4c 91
        74 ef 2c ... 78 more bytes>
    */
    js

See the OpenSSL `SSL_export_keying_material` documentation for more information.

#### `tlsSocket.getCertificate()`#

Added in: v11.2.0

- Returns: `<Object>`

Returns an object representing the local certificate. The returned object has some properties corresponding to the fields of the certificate.

See `tls.TLSSocket.getPeerCertificate()` for an example of the certificate structure.

If there is no local certificate, an empty object will be returned. If the socket has been destroyed, `null` will be returned.

#### `tlsSocket.getCipher()`#

Added in: v0.11.4
History

Version | Changes
--- | ---
v13.4.0, v12.16.0 | Return the IETF cipher name as `standardName`.
v12.0.0 | Return the minimum cipher version, instead of a fixed string (`'TLSv1/SSLv3'`).

- Returns: `<Object>`
  - `name` `<string>` OpenSSL name for the cipher suite.
  - `standardName` `<string>` IETF name for the cipher suite.
  - `version` `<string>` The minimum TLS protocol version supported by this cipher suite. For the actual negotiated protocol, see `tls.TLSSocket.getProtocol()`.

Returns an object containing information on the negotiated cipher suite.

For example, a TLSv1.2 protocol with AES256-SHA cipher:

    {
        "name": "AES256-SHA",
        "standardName": "TLS_RSA_WITH_AES_256_CBC_SHA",
        "version": "SSLv3"
    }
    json

See SSL_CIPHER_get_name for more information.

#### `tlsSocket.getEphemeralKeyInfo()`#

Added in: v5.0.0

- Returns: `<Object>`

Returns an object describing ephemeral key agreement in perfect forward secrecy on a client connection. It returns an empty object when the key agreement is not ephemeral. As this is only supported on a client socket; `null` is returned if called on a server socket. The supported types are `'DH'`, `'ECDH'`, and `'TLSGroup'`. For `'DH'` and `'ECDH'`, the object describes peer temporary key parameters. For `'TLSGroup'`, the object identifies the negotiated TLS Supported Group used for key agreement when a peer temporary key object is not available.

The `name` property is available only when type is `'ECDH'` or `'TLSGroup'`. The `size` property is not available when type is `'TLSGroup'`. For `'TLSGroup'`, `name` is the negotiated TLS Supported Group name. Standardized TLS group names and code points are listed in the IANA TLS Supported Groups registry.

For example: `{ type: 'ECDH', name: 'prime256v1', size: 256 }`.

#### `tlsSocket.getFinished()`#

Added in: v9.9.0

- Returns: `<Buffer>` | `<undefined>` The latest `Finished` message that has been sent to the socket as part of an SSL/TLS handshake, or `undefined` if no `Finished` message has been sent yet.

As the `Finished` messages are message digests of the complete handshake (with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can be used for external authentication procedures when the authentication provided by SSL/TLS is not desired or is not enough.

Corresponds to the `SSL_get_finished` routine in OpenSSL and may be used to implement the `tls-unique` channel binding from RFC 5929.

#### `tlsSocket.getPeerCertificate([detailed])`#

Added in: v0.11.4

- `detailed` `<boolean>` Include the full certificate chain if `true`, otherwise include just the peer's certificate.
- Returns: `<Object>` A certificate object.

Returns an object representing the peer's certificate. If the peer does not provide a certificate, an empty object will be returned. If the socket has been destroyed, `null` will be returned.

If the full certificate chain was requested, each certificate will include an `issuerCertificate` property containing an object representing its issuer's certificate.

##### Certificate object#

History

Version | Changes
--- | ---
v19.1.0, v18.13.0 | Add "ca" property.
v17.2.0, v16.14.0 | Add fingerprint512.
v11.4.0 | Support Elliptic Curve public key info.

A certificate object has properties corresponding to the fields of the certificate.

- `ca` `<boolean>` `true` if a Certificate Authority (CA), `false` otherwise.
- `raw` `<Buffer>` The DER encoded X.509 certificate data.
- `subject` `<Object>` The certificate subject, described in terms of Country (`C`), StateOrProvince (`ST`), Locality (`L`), Organization (`O`), OrganizationalUnit (`OU`), and CommonName (`CN`). The CommonName is typically a DNS name with TLS certificates. Example: `{C: 'UK', ST: 'BC', L: 'Metro', O: 'Node Fans', OU: 'Docs', CN: 'example.com'}`.
- `issuer` `<Object>` The certificate issuer, described in the same terms as the `subject`.
- `valid_from` `<string>` The date-time the certificate is valid from.
- `valid_to` `<string>` The date-time the certificate is valid to.
- `serialNumber` `<string>` The certificate serial number, as a hex string. Example: `'B9B0D332A1AA5635'`.
- `fingerprint` `<string>` The SHA-1 digest of the DER encoded certificate. It is returned as a `:` separated hexadecimal string. Example: `'2A:7A:C2:DD:...'`.
- `fingerprint256` `<string>` The SHA-256 digest of the DER encoded certificate. It is returned as a `:` separated hexadecimal string. Example: `'2A:7A:C2:DD:...'`.
- `fingerprint512` `<string>` The SHA-512 digest of the DER encoded certificate. It is returned as a `:` separated hexadecimal string. Example: `'2A:7A:C2:DD:...'`.
- `ext_key_usage` `<Array>` (Optional) The extended key usage, a set of OIDs.
- `subjectaltname` `<string>` (Optional) A string containing concatenated names for the subject, an alternative to the `subject` names.
- `infoAccess` `<Array>` (Optional) An array describing the AuthorityInfoAccess, used with OCSP.
- `issuerCertificate` `<Object>` (Optional) The issuer certificate object. For self-signed certificates, this may be a circular reference.

The certificate may contain information about the public key, depending on the key type.

For RSA keys, the following properties may be defined:

- `bits` `<number>` The RSA bit size. Example: `1024`.
- `exponent` `<string>` The RSA exponent, as a string in hexadecimal number notation. Example: `'0x010001'`.
- `modulus` `<string>` The RSA modulus, as a hexadecimal string. Example: `'B56CE45CB7...'`.
- `pubkey` `<Buffer>` The public key.

For EC keys, the following properties may be defined:

- `pubkey` `<Buffer>` The public key.
- `bits` `<number>` The key size in bits. Example: `256`.
- `asn1Curve` `<string>` (Optional) The ASN.1 name of the OID of the elliptic curve. Well-known curves are identified by an OID. While it is unusual, it is possible that the curve is identified by its mathematical properties, in which case it will not have an OID. Example: `'prime256v1'`.
- `nistCurve` `<string>` (Optional) The NIST name for the elliptic curve, if it has one (not all well-known curves have been assigned names by NIST). Example: `'P-256'`.

Example certificate:

    { subject:
       { OU: [ 'Domain Control Validated', 'PositiveSSL Wildcard' ],
         CN: '*.nodejs.org' },
      issuer:
       { C: 'GB',
         ST: 'Greater Manchester',
         L: 'Salford',
         O: 'COMODO CA Limited',
         CN: 'COMODO RSA Domain Validation Secure Server CA' },
      subjectaltname: 'DNS:*.nodejs.org, DNS:nodejs.org',
      infoAccess:
       { 'CA Issuers - URI':
          [ 'http://crt.comodoca.com/COMODORSADomainValidationSecureServerCA.crt' ],
         'OCSP - URI': [ 'http://ocsp.comodoca.com' ] },
      modulus: 'B56CE45CB740B09A13F64AC543B712FF9EE8E4C284B542A1708A27E82A8D151CA178153E12E6DDA15BF70FFD96CB8A88618641BDFCCA03527E665B70D779C8A349A6F88FD4EF6557180BD4C98192872BCFE3AF56E863C09DDD8BC1EC58DF9D94F914F0369102B2870BECFA1348A0838C9C49BD1C20124B442477572347047506B1FCD658A80D0C44BCC16BC5C5496CFE6E4A8428EF654CD3D8972BF6E5BFAD59C93006830B5EB1056BBB38B53D1464FA6E02BFDF2FF66CD949486F0775EC43034EC2602AEFBF1703AD221DAA2A88353C3B6A688EFE8387811F645CEED7B3FE46E1F8B9F59FAD028F349B9BC14211D5830994D055EEA3D547911E07A0ADDEB8A82B9188E58720D95CD478EEC9AF1F17BE8141BE80906F1A339445A7EB5B285F68039B0F294598A7D1C0005FC22B5271B0752F58CCDEF8C8FD856FB7AE21C80B8A2CE983AE94046E53EDE4CB89F42502D31B5360771C01C80155918637490550E3F555E2EE75CC8C636DDE3633CFEDD62E91BF0F7688273694EEEBA20C2FC9F14A2A435517BC1D7373922463409AB603295CEB0BB53787A334C9CA3CA8B30005C5A62FC0715083462E00719A8FA3ED0A9828C3871360A73F8B04A4FC1E71302844E9BB9940B77E745C9D91F226D71AFCAD4B113AAF68D92B24DDB4A2136B55A1CD1ADF39605B63CB639038ED0F4C987689866743A68769CC55847E4A06D6E2E3F1',
      exponent: '0x10001',
      pubkey: <Buffer ... >,
      valid_from: 'Aug 14 00:00:00 2017 GMT',
      valid_to: 'Nov 20 23:59:59 2019 GMT',
      fingerprint: '01:02:59:D9:C3:D2:0D:08:F7:82:4E:44:A4:B4:53:C5:E2:3A:87:4D',
      fingerprint256: '69:AE:1A:6A:D4:3D:C6:C1:1B:EA:C6:23:DE:BA:2A:14:62:62:93:5C:7A:EA:06:41:9B:0B:BC:87:CE:48:4E:02',
      fingerprint512: '19:2B:3E:C3:B3:5B:32:E8:AE:BB:78:97:27:E4:BA:6C:39:C9:92:79:4F:31:46:39:E2:70:E5:5F:89:42:17:C9:E8:64:CA:FF:BB:72:56:73:6E:28:8A:92:7E:A3:2A:15:8B:C2:E0:45:CA:C3:BC:EA:40:52:EC:CA:A2:68:CB:32',
      ext_key_usage: [ '1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2' ],
      serialNumber: '66593D57F20CBC573E433381B5FEC280',
      raw: <Buffer ... > }
    js

#### `tlsSocket.getPeerFinished()`#

Added in: v9.9.0

- Returns: `<Buffer>` | `<undefined>` The latest `Finished` message that is expected or has actually been received from the socket as part of an SSL/TLS handshake, or `undefined` if there is no `Finished` message so far.

As the `Finished` messages are message digests of the complete handshake (with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can be used for external authentication procedures when the authentication provided by SSL/TLS is not desired or is not enough.

Corresponds to the `SSL_get_peer_finished` routine in OpenSSL and may be used to implement the `tls-unique` channel binding from RFC 5929.

#### `tlsSocket.getPeerX509Certificate()`#

Added in: v15.9.0

- Returns: `<X509Certificate>`

Returns the peer certificate as an `<X509Certificate>` object.

If there is no peer certificate, or the socket has been destroyed, `undefined` will be returned.

#### `tlsSocket.getProtocol()`#

Added in: v5.7.0

- Returns: `<string>` | `<null>`

Returns a string containing the negotiated SSL/TLS protocol version of the current connection. The value `'unknown'` will be returned for connected sockets that have not completed the handshaking process. The value `null` will be returned for server sockets or disconnected client sockets.

Protocol versions are:

- `'SSLv3'`
- `'TLSv1'`
- `'TLSv1.1'`
- `'TLSv1.2'`
- `'TLSv1.3'`

See the OpenSSL `SSL_get_version` documentation for more information.

#### `tlsSocket.getSession()`#

Added in: v0.11.4

- Type: `<Buffer>`

Returns the TLS session data or `undefined` if no session was negotiated. On the client, the data can be provided to the `session` option of `tls.connect()` to resume the connection. On the server, it may be useful for debugging.

See Session Resumption for more information.

Note: `getSession()` works only for TLSv1.2 and below. For TLSv1.3, applications must use the `'session'` event (it also works for TLSv1.2 and below).

#### `tlsSocket.getSharedSigalgs()`#

Added in: v12.11.0

- Returns: `<Array>` List of signature algorithms shared between the server and the client in the order of decreasing preference.

See SSL_get_shared_sigalgs for more information.

#### `tlsSocket.getTLSTicket()`#

Added in: v0.11.4

- Type: `<Buffer>`

For a client, returns the TLS session ticket if one is available, or `undefined`. For a server, always returns `undefined`.

It may be useful for debugging.

See Session Resumption for more information.

#### `tlsSocket.getX509Certificate()`#

Added in: v15.9.0

- Returns: `<X509Certificate>`

Returns the local certificate as an `<X509Certificate>` object.

If there is no local certificate, or the socket has been destroyed, `undefined` will be returned.

#### `tlsSocket.isSessionReused()`#

Added in: v0.5.6

- Returns: `<boolean>` `true` if the session was reused, `false` otherwise.

See Session Resumption for more information.

#### `tlsSocket.localAddress`#

Added in: v0.11.4

- Type: `<string>`

Returns the string representation of the local IP address.

#### `tlsSocket.localPort`#

Added in: v0.11.4

- Type: `<integer>`

Returns the numeric representation of the local port.

#### `tlsSocket.remoteAddress`#

Added in: v0.11.4

- Type: `<string>`

Returns the string representation of the remote IP address. For example, `'74.125.127.100'` or `'2001:4860:a005::68'`.

#### `tlsSocket.remoteFamily`#

Added in: v0.11.4

- Type: `<string>`

Returns the string representation of the remote IP family. `'IPv4'` or `'IPv6'`.

#### `tlsSocket.remotePort`#

Added in: v0.11.4

- Type: `<integer>`

Returns the numeric representation of the remote port. For example, `443`.

#### `tlsSocket.renegotiate(options, callback)`#

Added in: v0.11.8
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `options` `<Object>`

  - `rejectUnauthorized` `<boolean>` If not `false`, the server certificate is verified against the list of supplied CAs. An `'error'` event is emitted if verification fails; `err.code` contains the OpenSSL error code. Default: `true`.
  - `requestCert`

- `callback` `<Function>` If `renegotiate()` returned `true`, callback is attached once to the `'secure'` event. If `renegotiate()` returned `false`, `callback` will be called in the next tick with an error, unless the `tlsSocket` has been destroyed, in which case `callback` will not be called at all.

- Returns: `<boolean>` `true` if renegotiation was initiated, `false` otherwise.

The `tlsSocket.renegotiate()` method initiates a TLS renegotiation process. Upon completion, the `callback` function will be passed a single argument that is either an `Error` (if the request failed) or `null`.

This method can be used to request a peer's certificate after the secure connection has been established.

When running as the server, the socket will be destroyed with an error after `handshakeTimeout` timeout.

For TLSv1.3, renegotiation cannot be initiated, it is not supported by the protocol.

#### `tlsSocket.servername`#

Added in: v0.11.3

- Type: `<string>` | `<boolean>` | `<null>`

The SNI (Server Name Indication) host name associated with the socket. This is `null` before the handshake completes. Once the handshake completes it settles as either the host name string, or `false` if SNI was not used.

#### `tlsSocket.setKeyCert(context)`#

Added in: v22.5.0, v20.17.0

- `context` `<Object>` | `<tls.SecureContext>` An object containing at least `key` and `cert` properties from the `tls.createSecureContext()` `options`, or a TLS context object created with `tls.createSecureContext()` itself.

The `tlsSocket.setKeyCert()` method sets the private key and certificate to use for the socket. This is mainly useful if you wish to select a server certificate from a TLS server's `ALPNCallback`.

#### `tlsSocket.setMaxSendFragment(size)`#

Added in: v0.11.11

- `size` `<number>` The maximum TLS fragment size. The maximum value is `16384`. Default: `16384`.
- Returns: `<boolean>`

The `tlsSocket.setMaxSendFragment()` method sets the maximum TLS fragment size. Returns `true` if setting the limit succeeded; `false` otherwise.

Smaller fragment sizes decrease the buffering latency on the client: larger fragments are buffered by the TLS layer until the entire fragment is received and its integrity is verified; large fragments can span multiple roundtrips and their processing can be delayed due to packet loss or reordering. However, smaller fragments add extra TLS framing bytes and CPU overhead, which may decrease overall server throughput.

### `tls.checkServerIdentity(hostname, cert)`#

Added in: v0.8.4
History

Version | Changes
--- | ---
v17.3.1, v16.13.2, v14.18.3, v12.22.9 | Support for `uniformResourceIdentifier` subject alternative names has been disabled in response to CVE-2021-44531.

- `hostname` `<string>` The host name or IP address to verify the certificate against.
- `cert` `<Object>` A certificate object representing the peer's certificate.
- Returns: `<Error>` | `<undefined>`

Verifies the certificate `cert` is issued to `hostname`.

Returns `<Error>` object, populating it with `reason`, `host`, and `cert` on failure. On success, returns `<undefined>`.

This function is intended to be used in combination with the `checkServerIdentity` option that can be passed to `tls.connect()` and as such operates on a certificate object. For other purposes, consider using `x509.checkHost()` instead.

This function can be overwritten by providing an alternative function as the `options.checkServerIdentity` option that is passed to `tls.connect()`. The overwriting function can call `tls.checkServerIdentity()` of course, to augment the checks done with additional verification.

This function is only called if the certificate passed all other checks, such as being issued by trusted CA (`options.ca`).

Earlier versions of Node.js incorrectly accepted certificates for a given `hostname` if a matching `uniformResourceIdentifier` subject alternative name was present (see CVE-2021-44531). Applications that wish to accept `uniformResourceIdentifier` subject alternative names can use a custom `options.checkServerIdentity` function that implements the desired behavior.

### `tls.connect(options[, callback])`#

Added in: v0.11.3
History

Version | Changes
--- | ---
v15.1.0, v14.18.0 | Added `onread` option.
v14.1.0, v13.14.0 | The `highWaterMark` option is accepted now.
v13.6.0, v12.16.0 | The `pskCallback` option is now supported.
v12.9.0 | Support the `allowHalfOpen` option.
v12.4.0 | The `hints` option is now supported.
v12.2.0 | The `enableTrace` option is now supported.
v11.8.0, v10.16.0 | The `timeout` option is supported now.
v8.0.0 | The `lookup` option is supported now.
v8.0.0 | The `ALPNProtocols` option can be a `TypedArray` or `DataView` now.
v5.3.0, v4.7.0 | The `secureContext` option is supported now.
v5.0.0 | ALPN options are supported now.

- `options` `<Object>`
  - `enableTrace`: See `tls.createServer()`
  - `host` `<string>` Host the client should connect to. Default: `'localhost'`.
  - `port` `<number>` Port the client should connect to.
  - `path` `<string>` Creates Unix socket connection to path. If this option is specified, `host` and `port` are ignored.
  - `socket` `<stream.Duplex>` Establish secure connection on a given socket rather than creating a new socket. Typically, this is an instance of `net.Socket`, but any `Duplex` stream is allowed. If this option is specified, `path`, `host`, and `port` are ignored, except for certificate validation. Usually, a socket is already connected when passed to `tls.connect()`, but it can be connected later. Connection/disconnection/destruction of `socket` is the user's responsibility; calling `tls.connect()` will not cause `net.connect()` to be called.
  - `allowHalfOpen` `<boolean>` If set to `false`, then the socket will automatically end the writable side when the readable side ends. If the `socket` option is set, this option has no effect. See the `allowHalfOpen` option of `net.Socket` for details. Default: `false`.
  - `rejectUnauthorized` `<boolean>` If not `false`, the server certificate is verified against the list of supplied CAs. An `'error'` event is emitted if verification fails; `err.code` contains the OpenSSL error code. Default: `true`.
  - `pskCallback` `<Function>` For TLS-PSK negotiation, see Pre-shared keys.
  - `ALPNProtocols` `<string>`[] | `<Buffer>` | `<TypedArray>` | `<DataView>` An array of strings, or a single `Buffer`, `TypedArray`, or `DataView` containing the supported ALPN protocols. Buffers should have the format `[len][name][len][name]...` e.g. `'\x08http/1.1\x08http/1.0'`, where the `len` byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. `['http/1.1', 'http/1.0']`. Protocols earlier in the list have higher preference than those later.
  - `servername` `<string>` Server name for the SNI (Server Name Indication) TLS extension. It is the name of the host being connected to, and must be a host name, and not an IP address. It can be used by a multi-homed server to choose the correct certificate to present to the client, see the `SNICallback` option to `tls.createServer()`.
  - `checkServerIdentity(servername, cert)` `<Function>` A callback function to be used (instead of the builtin `tls.checkServerIdentity()` function) when checking the server's host name (or the provided `servername` when explicitly set) against the certificate. This should return an `<Error>` if verification fails. The method should return `undefined` if the `servername` and `cert` are verified.
  - `session` `<Buffer>` A `Buffer` instance, containing TLS session.
  - `requestOCSP` `<boolean>` If `true`, specifies that the OCSP status request extension will be added to the client hello and an `'OCSPResponse'` event will be emitted on the socket before establishing a secure communication.
  - `minDHSize` `<number>` Minimum size of the DH parameter in bits to accept a TLS connection. When a server offers a DH parameter with a size less than `minDHSize`, the TLS connection is destroyed and an error is thrown. Default: `1024`.
  - `highWaterMark` `<number>` Consistent with the readable stream `highWaterMark` parameter. Default: `16 * 1024`.
  - `timeout`: `<number>` If set and if a socket is created internally, will call `socket.setTimeout(timeout)` after the socket is created, but before it starts the connection.
  - `secureContext`: TLS context object created with `tls.createSecureContext()`. If a `secureContext` is not provided, one will be created by passing the entire `options` object to `tls.createSecureContext()`.
  - `onread` `<Object>` If the `socket` option is missing, incoming data is stored in a single `buffer` and passed to the supplied `callback` when data arrives on the socket, otherwise the option is ignored. See the `onread` option of `net.Socket` for details.
  - ...: `tls.createSecureContext()` options that are used if the `secureContext` option is missing, otherwise they are ignored.
  - ...: Any `socket.connect()` option not already listed.
- `callback` `<Function>`
- Returns: `<tls.TLSSocket>`

The `callback` function, if specified, will be added as a listener for the `'secureConnect'` event.

`tls.connect()` returns a `tls.TLSSocket` object.

Unlike the `https` API, `tls.connect()` does not enable the SNI (Server Name Indication) extension by default, which may cause some servers to return an incorrect certificate or reject the connection altogether. To enable SNI, set the `servername` option in addition to `host`.

The following illustrates a client for the echo server example from `tls.createServer()`:

    // Assumes an echo server that is listening on port 8000.
    import { connect } from 'node:tls';
    import { readFileSync } from 'node:fs';
    import { stdin } from 'node:process';

    const options = {
      // Necessary only if the server requires client certificate authentication.
      key: readFileSync('client-key.pem'),
      cert: readFileSync('client-cert.pem'),

      // Necessary only if the server uses a self-signed certificate.
      ca: [ readFileSync('server-cert.pem') ],

      // Necessary only if the server's cert isn't for "localhost".
      checkServerIdentity: () => { return null; },
    };

    const socket = connect(8000, options, () => {
      console.log('client connected',
                  socket.authorized ? 'authorized' : 'unauthorized');
      stdin.pipe(socket);
      stdin.resume();
    });
    socket.setEncoding('utf8');
    socket.on('data', (data) => {
      console.log(data);
    });
    socket.on('end', () => {
      console.log('server ends connection');
    });
    // Assumes an echo server that is listening on port 8000.
    const { connect } = require('node:tls');
    const { readFileSync } = require('node:fs');

    const options = {
      // Necessary only if the server requires client certificate authentication.
      key: readFileSync('client-key.pem'),
      cert: readFileSync('client-cert.pem'),

      // Necessary only if the server uses a self-signed certificate.
      ca: [ readFileSync('server-cert.pem') ],

      // Necessary only if the server's cert isn't for "localhost".
      checkServerIdentity: () => { return null; },
    };

    const socket = connect(8000, options, () => {
      console.log('client connected',
                  socket.authorized ? 'authorized' : 'unauthorized');
      process.stdin.pipe(socket);
      process.stdin.resume();
    });
    socket.setEncoding('utf8');
    socket.on('data', (data) => {
      console.log(data);
    });
    socket.on('end', () => {
      console.log('server ends connection');
    });
    javascript

To generate the certificate and key for this example, run:

    openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
      -keyout client-key.pem -out client-cert.pem
    bash

Then, to generate the `server-cert.pem` certificate for this example, run:

    openssl pkcs12 -certpbe AES-256-CBC -export -out server-cert.pem \
      -inkey client-key.pem -in client-cert.pem
    bash

### `tls.connect(path[, options][, callback])`#

Added in: v0.11.3

- `path` `<string>` Default value for `options.path`.
- `options` `<Object>` See `tls.connect()`.
- `callback` `<Function>` See `tls.connect()`.
- Returns: `<tls.TLSSocket>`

Same as `tls.connect()` except that `path` can be provided as an argument instead of an option.

A path option, if specified, will take precedence over the path argument.

### `tls.connect(port[, host][, options][, callback])`#

Added in: v0.11.3

- `port` `<number>` Default value for `options.port`.
- `host` `<string>` Default value for `options.host`.
- `options` `<Object>` See `tls.connect()`.
- `callback` `<Function>` See `tls.connect()`.
- Returns: `<tls.TLSSocket>`

Same as `tls.connect()` except that `port` and `host` can be provided as arguments instead of options.

A port or host option, if specified, will take precedence over any port or host argument.

### `tls.createSecureContext([options])`#

Added in: v0.11.13
History

Version | Changes
--- | ---
v26.4.0 | The `certificateCompression` option has been added.
v22.9.0, v20.18.0 | The `allowPartialTrustChain` option has been added.
v22.4.0, v20.16.0 | The `clientCertEngine`, `privateKeyEngine` and `privateKeyIdentifier` options depend on custom engine support in OpenSSL which is deprecated in OpenSSL 3.
v19.8.0, v18.16.0 | The `dhparam` option can now be set to `'auto'` to enable DHE with appropriate well-known parameters.
v12.12.0 | Added `privateKeyIdentifier` and `privateKeyEngine` options to get private key from an OpenSSL engine.
v12.11.0 | Added `sigalgs` option to override supported signature algorithms.
v12.0.0 | TLSv1.3 support added.
v11.5.0 | The `ca:` option now supports `BEGIN TRUSTED CERTIFICATE`.
v11.4.0, v10.16.0 | The `minVersion` and `maxVersion` can be used to restrict the allowed TLS protocol versions.
v10.0.0 | The `ecdhCurve` cannot be set to `false` anymore due to a change in OpenSSL.
v9.3.0 | The `options` parameter can now include `clientCertEngine`.
v9.0.0 | The `ecdhCurve` option can now be multiple `':'` separated curve names or `'auto'`.
v7.3.0 | If the `key` option is an array, individual entries do not need a `passphrase` property anymore. `Array` entries can also just be `string`s or `Buffer`s now.
v5.2.0 | The `ca` option can now be a single string containing multiple CA certificates.

- `options` `<Object>`
  - `allowPartialTrustChain` `<boolean>` Treat intermediate (non-self-signed) certificates in the trust CA certificate list as trusted.
  - `ca` `<string>` | `<string>`[] | `<Buffer>` | `<Buffer>`[] Optionally override the trusted CA certificates. If not specified, the CA certificates trusted by default are the same as the ones returned by `tls.getCACertificates()` using the `default` type. If specified, the default list would be completely replaced (instead of being concatenated) by the certificates in the `ca` option. Users need to concatenate manually if they wish to add additional certificates instead of completely overriding the default. The value can be a string or `Buffer`, or an `Array` of strings and/or `Buffer`s. Any string or `Buffer` can contain multiple PEM CAs concatenated together. The peer's certificate must be chainable to a CA trusted by the server for the connection to be authenticated. When using certificates that are not chainable to a well-known CA, the certificate's CA must be explicitly specified as a trusted or the connection will fail to authenticate. If the peer uses a certificate that doesn't match or chain to one of the default CAs, use the `ca` option to provide a CA certificate that the peer's certificate can match or chain to. For self-signed certificates, the certificate is its own CA, and must be provided. For PEM encoded certificates, supported types are "TRUSTED CERTIFICATE", "X509 CERTIFICATE", and "CERTIFICATE".
  - `cert` `<string>` | `<string>`[] | `<Buffer>` | `<Buffer>`[] Cert chains in PEM format. One cert chain should be provided per private key. Each cert chain should consist of the PEM formatted certificate for a provided private `key`, followed by the PEM formatted intermediate certificates (if any), in order, and not including the root CA (the root CA must be pre-known to the peer, see `ca`). When providing multiple cert chains, they do not have to be in the same order as their private keys in `key`. If the intermediate certificates are not provided, the peer will not be able to validate the certificate, and the handshake will fail.
  - `certificateCompression` `<string>`[] An array of supported certificate compression algorithm names, in preference order. Supported values are `'zlib'`, `'brotli'`, and `'zstd'`. When set, enables TLS certificate compression (RFC 8879) which compresses certificates during the TLS handshake, reducing handshake size. Only effective with TLSv1.3. Default: `[]` (disabled).
  - `sigalgs` `<string>` Colon-separated list of supported signature algorithms. The list can contain digest algorithms (`SHA256`, `MD5` etc.), public key algorithms (`RSA-PSS`, `ECDSA` etc.), combination of both (e.g 'RSA+SHA384') or TLS v1.3 scheme names (e.g. `rsa_pss_pss_sha512`). See OpenSSL man pages for more info.
  - `ciphers` `<string>` Cipher suite specification, replacing the default. For more information, see Modifying the default TLS cipher suite. Permitted ciphers can be obtained via `tls.getCiphers()`. Cipher names must be uppercased in order for OpenSSL to accept them.
  - `clientCertEngine` `<string>` Name of an OpenSSL engine which can provide the client certificate. Deprecated.
  - `crl` `<string>` | `<string>`[] | `<Buffer>` | `<Buffer>`[] PEM formatted CRLs (Certificate Revocation Lists).
  - `dhparam` `<string>` | `<Buffer>` `'auto'` or custom Diffie-Hellman parameters, required for non-ECDHE perfect forward secrecy. If omitted or invalid, the parameters are silently discarded and DHE ciphers will not be available. ECDHE-based perfect forward secrecy will still be available.
  - `ecdhCurve` `<string>` A string describing a named curve, TLS group, or colon-separated list of named curves or TLS groups to use for key agreement, for example `P-521:P-384:P-256`, `X25519`, or `X25519MLKEM768`. The historical name of this option refers to ECDH key agreement in TLSv1.2 and below. In TLSv1.3, this option configures the TLS Supported Groups and key share groups offered or accepted by the TLS stack. Set to `auto` to select the group automatically. Use `crypto.getCurves()` to obtain a list of available elliptic curve names. For TLS group names, use `openssl list -tls-groups` or consult the IANA TLS Supported Groups registry. Default: `tls.DEFAULT_ECDH_CURVE`.
  - `honorCipherOrder` `<boolean>` Attempt to use the server's cipher suite preferences instead of the client's. When `true`, causes `SSL_OP_CIPHER_SERVER_PREFERENCE` to be set in `secureOptions`, see OpenSSL Options for more information.
  - `key` `<string>` | `<string>`[] | `<Buffer>` | `<Buffer>`[] | `<Object>`[] Private keys in PEM format. PEM allows the option of private keys being encrypted. Encrypted keys will be decrypted with `options.passphrase`. Multiple keys using different algorithms can be provided either as an array of unencrypted key strings or buffers, or an array of objects in the form `{pem: <string|buffer>[, passphrase: <string>]}`. The object form can only occur in an array. `object.passphrase` is optional. Encrypted keys will be decrypted with `object.passphrase` if provided, or `options.passphrase` if it is not.
  - `privateKeyEngine` `<string>` Name of an OpenSSL engine to get private key from. Should be used together with `privateKeyIdentifier`. Deprecated.
  - `privateKeyIdentifier` `<string>` Identifier of a private key managed by an OpenSSL engine. Should be used together with `privateKeyEngine`. Should not be set together with `key`, because both options define a private key in different ways. Deprecated.
  - `maxVersion` `<string>` Optionally set the maximum TLS version to allow. One of `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`. Cannot be specified along with the `secureProtocol` option; use one or the other. Default: `tls.DEFAULT_MAX_VERSION`.
  - `minVersion` `<string>` Optionally set the minimum TLS version to allow. One of `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`. Cannot be specified along with the `secureProtocol` option; use one or the other. Avoid setting to less than TLSv1.2, but it may be required for interoperability. Versions before TLSv1.2 may require downgrading the OpenSSL Security Level. Default: `tls.DEFAULT_MIN_VERSION`.
  - `passphrase` `<string>` Shared passphrase used for a single private key and/or a PFX.
  - `pfx` `<string>` | `<string>`[] | `<Buffer>` | `<Buffer>`[] | `<Object>`[] PFX or PKCS12 encoded private key and certificate chain. `pfx` is an alternative to providing `key` and `cert` individually. PFX is usually encrypted, if it is, `passphrase` will be used to decrypt it. Multiple PFX can be provided either as an array of unencrypted PFX buffers, or an array of objects in the form `{buf: <string|buffer>[, passphrase: <string>]}`. The object form can only occur in an array. `object.passphrase` is optional. Encrypted PFX will be decrypted with `object.passphrase` if provided, or `options.passphrase` if it is not.
  - `secureOptions` `<number>` Optionally affect the OpenSSL protocol behavior, which is not usually necessary. This should be used carefully if at all! Value is a numeric bitmask of the `SSL_OP_*` options from OpenSSL Options.
  - `secureProtocol` `<string>` Legacy mechanism to select the TLS protocol version to use, it does not support independent control of the minimum and maximum version, and does not support limiting the protocol to TLSv1.3. Use `minVersion` and `maxVersion` instead. The possible values are listed as SSL_METHODS, use the function names as strings. For example, use `'TLSv1_1_method'` to force TLS version 1.1, or `'TLS_method'` to allow any TLS protocol version up to TLSv1.3. It is not recommended to use TLS versions less than 1.2, but it may be required for interoperability. Default: none, see `minVersion`.
  - `sessionIdContext` `<string>` Opaque identifier used by servers to ensure session state is not shared between applications. Unused by clients.
  - `ticketKeys` `<Buffer>` 48-bytes of cryptographically strong pseudorandom data. See Session Resumption for more information.
  - `sessionTimeout` `<number>` The number of seconds after which a TLS session created by the server will no longer be resumable. See Session Resumption for more information. Default: `300`.

`tls.createServer()` sets the default value of the `honorCipherOrder` option to `true`, other APIs that create secure contexts leave it unset.

`tls.createServer()` uses a 128 bit truncated SHA1 hash value generated from `process.argv` as the default value of the `sessionIdContext` option, other APIs that create secure contexts have no default value.

The `tls.createSecureContext()` method creates a `SecureContext` object. It is usable as an argument to several `tls` APIs, such as `server.addContext()`, but has no public methods. The `tls.Server` constructor and the `tls.createServer()` method do not support the `secureContext` option.

A key is required for ciphers that use certificates. Either `key` or `pfx` can be used to provide it.

If the `ca` option is not given, then Node.js will default to using Mozilla's publicly trusted list of CAs.

Custom DHE parameters are discouraged in favor of the new `dhparam: 'auto'` option. When set to `'auto'`, well-known DHE parameters of sufficient strength will be selected automatically. Otherwise, if necessary, `openssl dhparam` can be used to create custom parameters. The key length must be greater than or equal to 1024 bits or else an error will be thrown. Although 1024 bits is permissible, use 2048 bits or larger for stronger security.

### `tls.createServer([options][, secureConnectionListener])`#

Added in: v0.3.2
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | The `clientCertEngine` option depends on custom engine support in OpenSSL which is deprecated in OpenSSL 3.
v20.4.0, v18.19.0 | The `options` parameter can now include `ALPNCallback`.
v19.0.0 | If `ALPNProtocols` is set, incoming connections that send an ALPN extension with no supported protocols are terminated with a fatal `no_application_protocol` alert.
v12.3.0 | The `options` parameter now supports `net.createServer()` options.
v9.3.0 | The `options` parameter can now include `clientCertEngine`.
v8.0.0 | The `ALPNProtocols` option can be a `TypedArray` or `DataView` now.
v5.0.0 | ALPN options are supported now.

- `options` `<Object>`
  - `ALPNProtocols` `<string>`[] | `<Buffer>` | `<TypedArray>` | `<DataView>` An array of strings, or a single `Buffer`, `TypedArray`, or `DataView` containing the supported ALPN protocols. Buffers should have the format `[len][name][len][name]...` e.g. `0x05hello0x05world`, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. `['hello', 'world']`. (Protocols should be ordered by their priority.)
  - `ALPNCallback` `<Function>` If set, this will be called when a client opens a connection using the ALPN extension. One argument will be passed to the callback: an object containing `servername` and `protocols` fields, respectively containing the server name from the SNI extension (if any) and an array of ALPN protocol name strings. The callback must return either one of the strings listed in `protocols`, which will be returned to the client as the selected ALPN protocol, or `undefined`, to reject the connection with a fatal alert. If a string is returned that does not match one of the client's ALPN protocols, an error will be thrown. This option cannot be used with the `ALPNProtocols` option, and setting both options will throw an error.
  - `clientCertEngine` `<string>` Name of an OpenSSL engine which can provide the client certificate. Deprecated.
  - `enableTrace` `<boolean>` If `true`, `tls.TLSSocket.enableTrace()` will be called on new connections. Tracing can be enabled after the secure connection is established, but this option must be used to trace the secure connection setup. Default: `false`.
  - `handshakeTimeout` `<number>` Abort the connection if the SSL/TLS handshake does not finish in the specified number of milliseconds. A `'tlsClientError'` is emitted on the `tls.Server` object whenever a handshake times out. Default: `120000` (120 seconds).
  - `rejectUnauthorized` `<boolean>` If not `false` the server will reject any connection which is not authorized with the list of supplied CAs. This option only has an effect if `requestCert` is `true`. Default: `true`.
  - `requestCert` `<boolean>` If `true` the server will request a certificate from clients that connect and attempt to verify that certificate. Default: `false`.
  - `sessionTimeout` `<number>` The number of seconds after which a TLS session created by the server will no longer be resumable. See Session Resumption for more information. Default: `300`.
  - `SNICallback(servername, callback)` `<Function>` A function that will be called if the client supports SNI TLS extension. Two arguments will be passed when called: `servername` and `callback`. `callback` is an error-first callback that takes two optional arguments: `error` and `ctx`. `ctx`, if provided, is a `SecureContext` instance. `tls.createSecureContext()` can be used to get a proper `SecureContext`. If `callback` is called with a falsy `ctx` argument, the default secure context of the server will be used. If `SNICallback` wasn't provided the default callback with high-level API will be used (see below).
  - `ticketKeys` `<Buffer>` 48-bytes of cryptographically strong pseudorandom data. See Session Resumption for more information.
  - `pskCallback` `<Function>` For TLS-PSK negotiation, see Pre-shared keys.
  - `pskIdentityHint` `<string>` optional hint to send to a client to help with selecting the identity during TLS-PSK negotiation. Will be ignored in TLS 1.3. Upon failing to set pskIdentityHint `'tlsClientError'` will be emitted with `'ERR_TLS_PSK_SET_IDENTITY_HINT_FAILED'` code.
  - ...: Any `tls.createSecureContext()` option can be provided. For servers, the identity options (`pfx`, `key`/`cert`, or `pskCallback`) are usually required.
  - ...: Any `net.createServer()` option can be provided.
- `secureConnectionListener` `<Function>`
- Returns: `<tls.Server>`

Creates a new `tls.Server`. The `secureConnectionListener`, if provided, is automatically set as a listener for the `'secureConnection'` event.

The `ticketKeys` option is automatically shared between `node:cluster` module workers.

The following illustrates a simple echo server:

    import { createServer } from 'node:tls';
    import { readFileSync } from 'node:fs';

    const options = {
      key: readFileSync('server-key.pem'),
      cert: readFileSync('server-cert.pem'),

      // This is necessary only if using client certificate authentication.
      requestCert: true,

      // This is necessary only if the client uses a self-signed certificate.
      ca: [ readFileSync('client-cert.pem') ],
    };

    const server = createServer(options, (socket) => {
      console.log('server connected',
                  socket.authorized ? 'authorized' : 'unauthorized');
      socket.write('welcome!\n');
      socket.setEncoding('utf8');
      socket.pipe(socket);
    });
    server.listen(8000, () => {
      console.log('server bound');
    });
    const { createServer } = require('node:tls');
    const { readFileSync } = require('node:fs');

    const options = {
      key: readFileSync('server-key.pem'),
      cert: readFileSync('server-cert.pem'),

      // This is necessary only if using client certificate authentication.
      requestCert: true,

      // This is necessary only if the client uses a self-signed certificate.
      ca: [ readFileSync('client-cert.pem') ],
    };

    const server = createServer(options, (socket) => {
      console.log('server connected',
                  socket.authorized ? 'authorized' : 'unauthorized');
      socket.write('welcome!\n');
      socket.setEncoding('utf8');
      socket.pipe(socket);
    });
    server.listen(8000, () => {
      console.log('server bound');
    });
    javascript

To generate the certificate and key for this example, run:

    openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
      -keyout server-key.pem -out server-cert.pem
    bash

Then, to generate the `client-cert.pem` certificate for this example, run:

    openssl pkcs12 -certpbe AES-256-CBC -export -out client-cert.pem \
      -inkey server-key.pem -in server-cert.pem
    bash

The server can be tested by connecting to it using the example client from `tls.connect()`.

### `tls.setDefaultCACertificates(certs)`#

Added in: v24.5.0, v22.19.0

- `certs` `<string>`[] | `<ArrayBufferView>`[] An array of CA certificates in PEM format.

Sets the default CA certificates used by Node.js TLS clients. If the provided certificates are parsed successfully, they will become the default CA certificate list returned by `tls.getCACertificates()` and used by subsequent TLS connections that don't specify their own CA certificates. The certificates will be deduplicated before being set as the default.

This function only affects the current Node.js thread. Previous sessions cached by the HTTPS agent won't be affected by this change, so this method should be called before any unwanted cacheable TLS connections are made.

To use system CA certificates as the default:

    const tls = require('node:tls');
    tls.setDefaultCACertificates(tls.getCACertificates('system'));
    import tls from 'node:tls';
    tls.setDefaultCACertificates(tls.getCACertificates('system'));
    javascript

This function completely replaces the default CA certificate list. To add additional certificates to the existing defaults, get the current certificates and append to them:

    const tls = require('node:tls');
    const currentCerts = tls.getCACertificates('default');
    const additionalCerts = ['-----BEGIN CERTIFICATE-----\n...'];
    tls.setDefaultCACertificates([...currentCerts, ...additionalCerts]);
    import tls from 'node:tls';
    const currentCerts = tls.getCACertificates('default');
    const additionalCerts = ['-----BEGIN CERTIFICATE-----\n...'];
    tls.setDefaultCACertificates([...currentCerts, ...additionalCerts]);
    javascript

### `tls.getCACertificates([type])`#

Added in: v23.10.0, v22.15.0

- `type` `<string>` | `<undefined>` The type of CA certificates that will be returned. Valid values are `"default"`, `"system"`, `"bundled"` and `"extra"`. Default: `"default"`.
- Returns: `<string>`[] An array of PEM-encoded certificates. The array may contain duplicates if the same certificate is repeatedly stored in multiple sources.

Returns an array containing the CA certificates from various sources, depending on `type`:

- `"default"`: return the CA certificates that will be used by the Node.js TLS clients by default.
  - When `--use-bundled-ca` is enabled (default), or `--use-openssl-ca` is not enabled, this would include CA certificates from the bundled Mozilla CA store.
  - When `--use-system-ca` is enabled, this would also include certificates from the system's trusted store.
  - When `NODE_EXTRA_CA_CERTS` is used, this would also include certificates loaded from the specified file.
- `"system"`: return the CA certificates that are loaded from the system's trusted store, according to rules set by `--use-system-ca`. This can be used to get the certificates from the system when `--use-system-ca` is not enabled.
- `"bundled"`: return the CA certificates from the bundled Mozilla CA store. This would be the same as `tls.rootCertificates`.
- `"extra"`: return the CA certificates loaded from `NODE_EXTRA_CA_CERTS`. It's an empty array if `NODE_EXTRA_CA_CERTS` is not set.

### `tls.getCiphers()`#

Added in: v0.10.2

- Returns: `<string>`[]

Returns an array with the names of the supported TLS ciphers. The names are lower-case for historical reasons, but must be uppercased to be used in the `ciphers` option of `tls.createSecureContext()`.

Not all supported ciphers are enabled by default. See Modifying the default TLS cipher suite.

Cipher names that start with `'tls_'` are for TLSv1.3, all the others are for TLSv1.2 and below.

    console.log(tls.getCiphers()); // ['aes128-gcm-sha256', 'aes128-sha', ...]
    js

### `tls.getCertificateCompressionAlgorithms()`#

Added in: v26.4.0

- Returns: `<string>`[]

Returns an array with the names of the RFC 8879 certificate compression algorithms supported by the current OpenSSL build, suitable for use in the `certificateCompression` option of `tls.createSecureContext()`. Possible values include `'zlib'`, `'brotli'`, and `'zstd'`.

The array is empty when certificate compression is unavailable.

    console.log(tls.getCertificateCompressionAlgorithms()); // ['zlib', 'brotli', 'zstd']
    js

### `tls.rootCertificates`#

Added in: v12.3.0

- Type: `<string>`[]

An immutable array of strings representing the root certificates (in PEM format) from the bundled Mozilla CA store as supplied by the current Node.js version.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.

To get the actual CA certificates used by the current Node.js instance, which may include certificates loaded from the system store (if `--use-system-ca` is used) or loaded from a file indicated by `NODE_EXTRA_CA_CERTS`, use `tls.getCACertificates()`.

### `tls.DEFAULT_ECDH_CURVE`#

Added in: v0.11.13
History

Version | Changes
--- | ---
v10.0.0 | Default value changed to `'auto'`.

The default named curve or TLS group list to use for key agreement in a TLS server. The default value is `'auto'`. See `tls.createSecureContext()` for further information.

### `tls.DEFAULT_MAX_VERSION`#

Added in: v11.4.0

- Type: `<string>` The default value of the `maxVersion` option of `tls.createSecureContext()`. It can be assigned any of the supported TLS protocol versions, `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`. Default: `'TLSv1.3'`, unless changed using CLI options. Using `--tls-max-v1.2` sets the default to `'TLSv1.2'`. Using `--tls-max-v1.3` sets the default to `'TLSv1.3'`. If multiple of the options are provided, the highest maximum is used.

### `tls.DEFAULT_MIN_VERSION`#

Added in: v11.4.0

- Type: `<string>` The default value of the `minVersion` option of `tls.createSecureContext()`. It can be assigned any of the supported TLS protocol versions, `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`. Versions before TLSv1.2 may require downgrading the OpenSSL Security Level. Default: `'TLSv1.2'`, unless changed using CLI options. Using `--tls-min-v1.0` sets the default to `'TLSv1'`. Using `--tls-min-v1.1` sets the default to `'TLSv1.1'`. Using `--tls-min-v1.3` sets the default to `'TLSv1.3'`. If multiple of the options are provided, the lowest minimum is used.

### `tls.DEFAULT_CIPHERS`#

Added in: v0.11.3

- Type: `<string>` The default value of the `ciphers` option of `tls.createSecureContext()`. It can be assigned any of the supported OpenSSL ciphers. Defaults to the content of `crypto.constants.defaultCoreCipherList`, unless changed using CLI options using `--tls-default-ciphers`.

## Trace events#

Source Code: lib/trace_events.js

Stability: 1 - Experimental

The `node:trace_events` module provides a mechanism to centralize tracing information generated by V8, Node.js core, and userspace code.

Tracing can be enabled with the `--trace-event-categories` command-line flag or by using the `node:trace_events` module. The `--trace-event-categories` flag accepts a list of comma-separated category names.

The available categories are:

- `node`: An empty placeholder.
- `node.async_hooks`: Enables capture of detailed `async_hooks` trace data. The `async_hooks` events have a unique `asyncId` and a special `triggerId` `triggerAsyncId` property.
- `node.bootstrap`: Enables capture of Node.js bootstrap milestones.
- `node.console`: Enables capture of `console.time()` and `console.count()` output.
- `node.threadpoolwork.sync`: Enables capture of trace data for threadpool synchronous operations, such as `blob`, `zlib`, `crypto` and `node_api`.
- `node.threadpoolwork.async`: Enables capture of trace data for threadpool asynchronous operations, such as `blob`, `zlib`, `crypto` and `node_api`.
- `node.dns.native`: Enables capture of trace data for DNS queries.
- `node.net.native`: Enables capture of trace data for network.
- `node.environment`: Enables capture of Node.js Environment milestones.
- `node.fs.sync`: Enables capture of trace data for file system sync methods.
- `node.fs_dir.sync`: Enables capture of trace data for file system sync directory methods.
- `node.fs.async`: Enables capture of trace data for file system async methods.
- `node.fs_dir.async`: Enables capture of trace data for file system async directory methods.
- `node.perf`: Enables capture of Performance API measurements.
  - `node.perf.usertiming`: Enables capture of only Performance API User Timing measures and marks.
  - `node.perf.timerify`: Enables capture of only Performance API timerify measurements.
- `node.promises.rejections`: Enables capture of trace data tracking the number of unhandled Promise rejections and handled-after-rejections.
- `node.vm.script`: Enables capture of trace data for the `node:vm` module's `runInNewContext()`, `runInContext()`, and `runInThisContext()` methods.
- `v8`: The V8 events are GC, compiling, and execution related.
- `node.http`: Enables capture of trace data for http request / response.
- `node.module_timer`: Enables capture of trace data for CJS Module loading.

By default the `node`, `node.async_hooks`, and `v8` categories are enabled.

    node --trace-event-categories v8,node,node.async_hooks server.js
    bash

Prior versions of Node.js required the use of the `--trace-events-enabled` flag to enable trace events. This requirement has been removed. However, the `--trace-events-enabled` flag may still be used and will enable the `node`, `node.async_hooks`, and `v8` trace event categories by default.

    node --trace-events-enabled

    # is equivalent to

    node --trace-event-categories v8,node,node.async_hooks
    bash

Alternatively, trace events may be enabled using the `node:trace_events` module:

    import { createTracing } from 'node:trace_events';
    const tracing = createTracing({ categories: ['node.perf'] });
    tracing.enable();  // Enable trace event capture for the 'node.perf' category

    // do work

    tracing.disable();  // Disable trace event capture for the 'node.perf' category
    const { createTracing } = require('node:trace_events');
    const tracing = createTracing({ categories: ['node.perf'] });
    tracing.enable();  // Enable trace event capture for the 'node.perf' category

    // do work

    tracing.disable();  // Disable trace event capture for the 'node.perf' category
    javascript

Running Node.js with tracing enabled will produce log files that can be opened in the `chrome://tracing` tab of Chrome.

The logging file is by default called `node_trace.${rotation}.log`, where `${rotation}` is an incrementing log-rotation id. The filepath pattern can be specified with `--trace-event-file-pattern` that accepts a template string that supports `${rotation}` and `${pid}`:

    node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js
    bash

To guarantee that the log file is properly generated after signal events like `SIGINT`, `SIGTERM`, or `SIGBREAK`, make sure to have the appropriate handlers in your code, such as:

    process.on('SIGINT', function onSigint() {
      console.info('Received SIGINT.');
      process.exit(130);  // Or applicable exit code depending on OS and signal
    });
    js

The tracing system uses the same time source as the one used by `process.hrtime()`. However the trace-event timestamps are expressed in microseconds, unlike `process.hrtime()` which returns nanoseconds.

The features from this module are not available in `Worker` threads.

### The `node:trace_events` module#

Added in: v10.0.0

#### `Tracing` object#

Added in: v10.0.0

The `Tracing` object is used to enable or disable tracing for sets of categories. Instances are created using the `trace_events.createTracing()` method.

When created, the `Tracing` object is disabled. Calling the `tracing.enable()` method adds the categories to the set of enabled trace event categories. Calling `tracing.disable()` will remove the categories from the set of enabled trace event categories.

##### `tracing.categories`#

Added in: v10.0.0

- Type: `<string>`

A comma-separated list of the trace event categories covered by this `Tracing` object.

##### `tracing.disable()`#

Added in: v10.0.0

Disables this `Tracing` object.

Only trace event categories not covered by other enabled `Tracing` objects and not specified by the `--trace-event-categories` flag will be disabled.

    import { createTracing, getEnabledCategories } from 'node:trace_events';
    const t1 = createTracing({ categories: ['node', 'v8'] });
    const t2 = createTracing({ categories: ['node.perf', 'node'] });
    t1.enable();
    t2.enable();

    // Prints 'node,node.perf,v8'
    console.log(getEnabledCategories());

    t2.disable(); // Will only disable emission of the 'node.perf' category

    // Prints 'node,v8'
    console.log(getEnabledCategories());
    const { createTracing, getEnabledCategories } = require('node:trace_events');
    const t1 = createTracing({ categories: ['node', 'v8'] });
    const t2 = createTracing({ categories: ['node.perf', 'node'] });
    t1.enable();
    t2.enable();

    // Prints 'node,node.perf,v8'
    console.log(getEnabledCategories());

    t2.disable(); // Will only disable emission of the 'node.perf' category

    // Prints 'node,v8'
    console.log(getEnabledCategories());
    javascript

##### `tracing.enable()`#

Added in: v10.0.0

Enables this `Tracing` object for the set of categories covered by the `Tracing` object.

##### `tracing.enabled`#

Added in: v10.0.0

- Type: `<boolean>` `true` only if the `Tracing` object has been enabled.

#### `trace_events.createTracing(options)`#

Added in: v10.0.0

- `options` `<Object>`
  - `categories` `<string>`[] An array of trace category names. Values included in the array are coerced to a string when possible. An error will be thrown if the value cannot be coerced.
- Returns: `<Tracing>`.

Creates and returns a `Tracing` object for the given set of `categories`.

    import { createTracing } from 'node:trace_events';
    const categories = ['node.perf', 'node.async_hooks'];
    const tracing = createTracing({ categories });
    tracing.enable();
    // do stuff
    tracing.disable();
    const { createTracing } = require('node:trace_events');
    const categories = ['node.perf', 'node.async_hooks'];
    const tracing = createTracing({ categories });
    tracing.enable();
    // do stuff
    tracing.disable();
    javascript

#### `trace_events.getEnabledCategories()`#

Added in: v10.0.0

- Returns: `<string>`

Returns a comma-separated list of all currently-enabled trace event categories. The current set of enabled trace event categories is determined by the union of all currently-enabled `Tracing` objects and any categories enabled using the `--trace-event-categories` flag.

Given the file `test.js` below, the command `node --trace-event-categories node.perf test.js` will print `'node.async_hooks,node.perf'` to the console.

    import { createTracing, getEnabledCategories } from 'node:trace_events';
    const t1 = createTracing({ categories: ['node.async_hooks'] });
    const t2 = createTracing({ categories: ['node.perf'] });
    const t3 = createTracing({ categories: ['v8'] });

    t1.enable();
    t2.enable();

    console.log(getEnabledCategories());
    const { createTracing, getEnabledCategories } = require('node:trace_events');
    const t1 = createTracing({ categories: ['node.async_hooks'] });
    const t2 = createTracing({ categories: ['node.perf'] });
    const t3 = createTracing({ categories: ['v8'] });

    t1.enable();
    t2.enable();

    console.log(getEnabledCategories());
    javascript

### Examples#

#### Collect trace events data by inspector#

    import { Session } from 'node:inspector';
    const session = new Session();
    session.connect();

    function post(message, data) {
      return new Promise((resolve, reject) => {
        session.post(message, data, (err, result) => {
          if (err)
            reject(new Error(JSON.stringify(err)));
          else
            resolve(result);
        });
      });
    }

    async function collect() {
      const data = [];
      session.on('NodeTracing.dataCollected', (chunk) => data.push(chunk));
      session.on('NodeTracing.tracingComplete', () => {
        // done
      });
      const traceConfig = { includedCategories: ['v8'] };
      await post('NodeTracing.start', { traceConfig });
      // do something
      setTimeout(() => {
        post('NodeTracing.stop').then(() => {
          session.disconnect();
          console.log(data);
        });
      }, 1000);
    }

    collect();
    const { Session } = require('node:inspector');
    const session = new Session();
    session.connect();

    function post(message, data) {
      return new Promise((resolve, reject) => {
        session.post(message, data, (err, result) => {
          if (err)
            reject(new Error(JSON.stringify(err)));
          else
            resolve(result);
        });
      });
    }

    async function collect() {
      const data = [];
      session.on('NodeTracing.dataCollected', (chunk) => data.push(chunk));
      session.on('NodeTracing.tracingComplete', () => {
        // done
      });
      const traceConfig = { includedCategories: ['v8'] };
      await post('NodeTracing.start', { traceConfig });
      // do something
      setTimeout(() => {
        post('NodeTracing.stop').then(() => {
          session.disconnect();
          console.log(data);
        });
      }, 1000);
    }

    collect();
    javascript

## TTY#

Source Code: lib/tty.js

Stability: 2 - Stable

The `node:tty` module provides the `tty.ReadStream` and `tty.WriteStream` classes. In most cases, it will not be necessary or possible to use this module directly. However, it can be accessed using:

    const tty = require('node:tty');
    js

When Node.js detects that it is being run with a text terminal ("TTY") attached, `process.stdin` will, by default, be initialized as an instance of `tty.ReadStream` and both `process.stdout` and `process.stderr` will, by default, be instances of `tty.WriteStream`. The preferred method of determining whether Node.js is being run within a TTY context is to check that the value of the `process.stdout.isTTY` property is `true`:

    $ node -p -e "Boolean(process.stdout.isTTY)"
    true
    $ node -p -e "Boolean(process.stdout.isTTY)" | cat
    false
    console

In most cases, there should be little to no reason for an application to manually create instances of the `tty.ReadStream` and `tty.WriteStream` classes.

### Class: `tty.ReadStream`#

Added in: v0.5.8

- Extends: `<net.Socket>`

Represents the readable side of a TTY. In normal circumstances `process.stdin` will be the only `tty.ReadStream` instance in a Node.js process and there should be no reason to create additional instances.

#### `readStream.isRaw`#

Added in: v0.7.7

A `boolean` that is `true` if the TTY is currently configured to operate as a raw device.

This flag is always `false` when a process starts, even if the terminal is operating in raw mode. Its value will change with subsequent calls to `setRawMode`.

#### `readStream.isTTY`#

Added in: v0.5.8

A `boolean` that is always `true` for `tty.ReadStream` instances.

#### `readStream.setRawMode(mode)`#

Added in: v0.7.7

- `mode` `<boolean>` If `true`, configures the `tty.ReadStream` to operate as a raw device. If `false`, configures the `tty.ReadStream` to operate in its default mode. The `readStream.isRaw` property will be set to the resulting mode.
- Returns: `<this>` The read stream instance.

Allows configuration of `tty.ReadStream` so that it operates as a raw device.

When in raw mode, input is always available character-by-character, not including modifiers. Additionally, all special processing of input characters by the terminal is disabled, including echoing input characters. `Ctrl`+`C` will no longer cause a `SIGINT` when in this mode. This mode does not affect terminal output processing, such as newline translation on Unix terminals.

### Class: `tty.WriteStream`#

Added in: v0.5.8

- Extends: `<net.Socket>`

Represents the writable side of a TTY. In normal circumstances, `process.stdout` and `process.stderr` will be the only `tty.WriteStream` instances created for a Node.js process and there should be no reason to create additional instances.

#### `new tty.ReadStream(fd[, options])`#

Added in: v0.5.8
History

Version | Changes
--- | ---
v0.9.4 | The `options` argument is supported.

- `fd` `<number>` A file descriptor associated with a TTY.
- `options` `<Object>` Options passed to parent `net.Socket`, see `options` of `net.Socket` constructor.
- Returns: `<tty.ReadStream>`

Creates a `ReadStream` for `fd` associated with a TTY.

#### `new tty.WriteStream(fd)`#

Added in: v0.5.8

- `fd` `<number>` A file descriptor associated with a TTY.
- Returns: `<tty.WriteStream>`

Creates a `WriteStream` for `fd` associated with a TTY.

#### Event: `'resize'`#

Added in: v0.7.7

The `'resize'` event is emitted whenever either of the `writeStream.columns` or `writeStream.rows` properties have changed. No arguments are passed to the listener callback when called.

    process.stdout.on('resize', () => {
      console.log('screen size has changed!');
      console.log(`${process.stdout.columns}x${process.stdout.rows}`);
    });
    js

#### `writeStream.clearLine(dir[, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v12.7.0 | The stream's write() callback and return value are exposed.

- `dir` `<number>`
  - `-1`: to the left from cursor
  - `1`: to the right from cursor
  - `0`: the entire line
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if the stream wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

`writeStream.clearLine()` clears the current line of this `WriteStream` in a direction identified by `dir`.

#### `writeStream.clearScreenDown([callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v12.7.0 | The stream's write() callback and return value are exposed.

- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if the stream wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

`writeStream.clearScreenDown()` clears this `WriteStream` from the current cursor down.

#### `writeStream.columns`#

Added in: v0.7.7

A `number` specifying the number of columns the TTY currently has. This property is updated whenever the `'resize'` event is emitted.

#### `writeStream.cursorTo(x[, y][, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v12.7.0 | The stream's write() callback and return value are exposed.

- `x` `<number>`
- `y` `<number>`
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if the stream wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

`writeStream.cursorTo()` moves this `WriteStream`'s cursor to the specified position.

#### `writeStream.getColorDepth([env])`#

Added in: v9.9.0

- `env` `<Object>` An object containing the environment variables to check. This enables simulating the usage of a specific terminal. Default: `process.env`.
- Returns: `<number>`

Returns:

- `1` for 2,
- `4` for 16,
- `8` for 256,
- `24` for 16,777,216 colors supported.

Use this to determine what colors the terminal supports. Due to the nature of colors in terminals it is possible to either have false positives or false negatives. It depends on process information and the environment variables that may lie about what terminal is used. It is possible to pass in an `env` object to simulate the usage of a specific terminal. This can be useful to check how specific environment settings behave.

To enforce a specific color support, use one of the below environment settings.

- 2 colors: `FORCE_COLOR = 0` (Disables colors)
- 16 colors: `FORCE_COLOR = 1`
- 256 colors: `FORCE_COLOR = 2`
- 16,777,216 colors: `FORCE_COLOR = 3`

Disabling color support is also possible by using the `NO_COLOR` and `NODE_DISABLE_COLORS` environment variables.

#### `writeStream.getWindowSize()`#

Added in: v0.7.7

- Returns: `<number>`[]

`writeStream.getWindowSize()` returns the size of the TTY corresponding to this `WriteStream`. The array is of the type `[numColumns, numRows]` where `numColumns` and `numRows` represent the number of columns and rows in the corresponding TTY.

#### `writeStream.hasColors([count][, env])`#

Added in: v11.13.0, v10.16.0

- `count` `<integer>` The number of colors that are requested (minimum 2). Default: 16.
- `env` `<Object>` An object containing the environment variables to check. This enables simulating the usage of a specific terminal. Default: `process.env`.
- Returns: `<boolean>`

Returns `true` if the `writeStream` supports at least as many colors as provided in `count`. Minimum support is 2 (black and white).

This has the same false positives and negatives as described in `writeStream.getColorDepth()`.

    process.stdout.hasColors();
    // Returns true or false depending on if `stdout` supports at least 16 colors.
    process.stdout.hasColors(256);
    // Returns true or false depending on if `stdout` supports at least 256 colors.
    process.stdout.hasColors({ TMUX: '1' });
    // Returns true.
    process.stdout.hasColors(2 ** 24, { TMUX: '1' });
    // Returns false (the environment setting pretends to support 2 ** 8 colors).
    js

#### `writeStream.isTTY`#

Added in: v0.5.8

A `boolean` that is always `true`.

#### `writeStream.moveCursor(dx, dy[, callback])`#

Added in: v0.7.7
History

Version | Changes
--- | ---
v12.7.0 | The stream's write() callback and return value are exposed.

- `dx` `<number>`
- `dy` `<number>`
- `callback` `<Function>` Invoked once the operation completes.
- Returns: `<boolean>` `false` if the stream wishes for the calling code to wait for the `'drain'` event to be emitted before continuing to write additional data; otherwise `true`.

`writeStream.moveCursor()` moves this `WriteStream`'s cursor relative to its current position.

#### `writeStream.rows`#

Added in: v0.7.7

A `number` specifying the number of rows the TTY currently has. This property is updated whenever the `'resize'` event is emitted.

### `tty.isatty(fd)`#

Added in: v0.5.8

- `fd` `<number>` A numeric file descriptor
- Returns: `<boolean>`

The `tty.isatty()` method returns `true` if the given `fd` is associated with a TTY and `false` if it is not, including whenever `fd` is not a non-negative integer.

## UDP/datagram sockets#

Source Code: lib/dgram.js

Stability: 2 - Stable

The `node:dgram` module provides an implementation of UDP datagram sockets.

    import dgram from 'node:dgram';

    const server = dgram.createSocket('udp4');

    server.on('error', (err) => {
      console.error(`server error:\n${err.stack}`);
      server.close();
    });

    server.on('message', (msg, rinfo) => {
      console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
    });

    server.on('listening', () => {
      const address = server.address();
      console.log(`server listening ${address.address}:${address.port}`);
    });

    server.bind(41234);
    // Prints: server listening 0.0.0.0:41234
    const dgram = require('node:dgram');
    const server = dgram.createSocket('udp4');

    server.on('error', (err) => {
      console.error(`server error:\n${err.stack}`);
      server.close();
    });

    server.on('message', (msg, rinfo) => {
      console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
    });

    server.on('listening', () => {
      const address = server.address();
      console.log(`server listening ${address.address}:${address.port}`);
    });

    server.bind(41234);
    // Prints: server listening 0.0.0.0:41234
    javascript

### Class: `dgram.Socket`#

Added in: v0.1.99

- Extends: `<EventEmitter>`

Encapsulates the datagram functionality.

New instances of `dgram.Socket` are created using `dgram.createSocket()`. The `new` keyword is not to be used to create `dgram.Socket` instances.

#### Event: `'close'`#

Added in: v0.1.99

The `'close'` event is emitted after a socket is closed with `close()`. Once triggered, no new `'message'` events will be emitted on this socket.

#### Event: `'connect'`#

Added in: v12.0.0

The `'connect'` event is emitted after a socket is associated to a remote address as a result of a successful `connect()` call.

#### Event: `'error'`#

Added in: v0.1.99

- `exception` `<Error>`

The `'error'` event is emitted whenever any error occurs. The event handler function is passed a single `Error` object.

#### Event: `'listening'`#

Added in: v0.1.99

The `'listening'` event is emitted once the `dgram.Socket` is addressable and can receive data. This happens either explicitly with `socket.bind()` or implicitly the first time data is sent using `socket.send()`. Until the `dgram.Socket` is listening, the underlying system resources do not exist and calls such as `socket.address()` and `socket.setTTL()` will fail.

#### Event: `'message'`#

Added in: v0.1.99
History

Version | Changes
--- | ---
v18.4.0 | The `family` property now returns a string instead of a number.
v18.0.0 | The `family` property now returns a number instead of a string.

The `'message'` event is emitted when a new datagram is available on a socket. The event handler function is passed two arguments: `msg` and `rinfo`.

- `msg` `<Buffer>` The message.
- `rinfo` `<Object>` Remote address information.
  - `address` `<string>` The sender address.
  - `family` `<string>` The address family (`'IPv4'` or `'IPv6'`).
  - `port` `<number>` The sender port.
  - `size` `<number>` The message size.

If the source address of the incoming packet is an IPv6 link-local address, the interface name is added to the `address`. For example, a packet received on the `en0` interface might have the address field set to `'fe80::2618:1234:ab11:3b9c%en0'`, where `'%en0'` is the interface name as a zone ID suffix.

#### `socket.addMembership(multicastAddress[, multicastInterface])`#

Added in: v0.6.9

- `multicastAddress` `<string>`
- `multicastInterface` `<string>`

Tells the kernel to join a multicast group at the given `multicastAddress` and `multicastInterface` using the `IP_ADD_MEMBERSHIP` socket option. If the `multicastInterface` argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call `addMembership` multiple times, once per interface.

When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.

When sharing a UDP socket across multiple `cluster` workers, the `socket.addMembership()` function must be called only once or an `EADDRINUSE` error will occur:

    import cluster from 'node:cluster';
    import dgram from 'node:dgram';

    if (cluster.isPrimary) {
      cluster.fork(); // Works ok.
      cluster.fork(); // Fails with EADDRINUSE.
    } else {
      const s = dgram.createSocket('udp4');
      s.bind(1234, () => {
        s.addMembership('224.0.0.114');
      });
    }
    const cluster = require('node:cluster');
    const dgram = require('node:dgram');

    if (cluster.isPrimary) {
      cluster.fork(); // Works ok.
      cluster.fork(); // Fails with EADDRINUSE.
    } else {
      const s = dgram.createSocket('udp4');
      s.bind(1234, () => {
        s.addMembership('224.0.0.114');
      });
    }
    javascript

#### `socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])`#

Added in: v13.1.0, v12.16.0

- `sourceAddress` `<string>`
- `groupAddress` `<string>`
- `multicastInterface` `<string>`

Tells the kernel to join a source-specific multicast channel at the given `sourceAddress` and `groupAddress`, using the `multicastInterface` with the `IP_ADD_SOURCE_MEMBERSHIP` socket option. If the `multicastInterface` argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call `socket.addSourceSpecificMembership()` multiple times, once per interface.

When called on an unbound socket, this method will implicitly bind to a random port, listening on all interfaces.

#### `socket.address()`#

Added in: v0.1.99

- Returns: `<Object>`

Returns an object containing the address information for a socket. For UDP sockets, this object will contain `address`, `family`, and `port` properties.

This method throws `EBADF` if called on an unbound socket.

#### `socket.bind([port][, address][, callback])`#

Added in: v0.1.99
History

Version | Changes
--- | ---
v0.9.1 | The method was changed to an asynchronous execution model. Legacy code would need to be changed to pass a callback function to the method call.

- `port` `<integer>`
- `address` `<string>`
- `callback` `<Function>` with no parameters. Called when binding is complete.

For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a named `port` and optional `address`. If `port` is not specified or is `0`, the operating system will attempt to bind to a random port. If `address` is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a `'listening'` event is emitted and the optional `callback` function is called.

Specifying both a `'listening'` event listener and passing a `callback` to the `socket.bind()` method is not harmful but not very useful.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an `'error'` event is generated. In rare cases (e.g., attempting to bind with a closed socket), an `Error` may be thrown.

Example of a UDP server listening on port 41234:

    import dgram from 'node:dgram';

    const server = dgram.createSocket('udp4');

    server.on('error', (err) => {
      console.error(`server error:\n${err.stack}`);
      server.close();
    });

    server.on('message', (msg, rinfo) => {
      console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
    });

    server.on('listening', () => {
      const address = server.address();
      console.log(`server listening ${address.address}:${address.port}`);
    });

    server.bind(41234);
    // Prints: server listening 0.0.0.0:41234
    const dgram = require('node:dgram');
    const server = dgram.createSocket('udp4');

    server.on('error', (err) => {
      console.error(`server error:\n${err.stack}`);
      server.close();
    });

    server.on('message', (msg, rinfo) => {
      console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
    });

    server.on('listening', () => {
      const address = server.address();
      console.log(`server listening ${address.address}:${address.port}`);
    });

    server.bind(41234);
    // Prints: server listening 0.0.0.0:41234
    javascript

#### `socket.bind(options[, callback])`#

Added in: v0.11.14

- `options` `<Object>` Required. Supports the following properties:
  - `port` `<integer>`
  - `address` `<string>`
  - `exclusive` `<boolean>`
  - `fd` `<integer>`
- `callback` `<Function>`

For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a named `port` and optional `address` that are passed as properties of an `options` object passed as the first argument. If `port` is not specified or is `0`, the operating system will attempt to bind to a random port. If `address` is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a `'listening'` event is emitted and the optional `callback` function is called.

The `options` object may contain a `fd` property. When a `fd` greater than `0` is set, it will wrap around an existing socket with the given file descriptor. In this case, the properties of `port` and `address` will be ignored.

Specifying both a `'listening'` event listener and passing a `callback` to the `socket.bind()` method is not harmful but not very useful.

The `options` object may contain an additional `exclusive` property that is used when using `dgram.Socket` objects with the `cluster` module. When `exclusive` is set to `false` (the default), cluster workers will use the same underlying socket handle allowing connection handling duties to be shared. When `exclusive` is `true`, however, the handle is not shared and attempted port sharing results in an error. Creating a `dgram.Socket` with the `reusePort` option set to `true` causes `exclusive` to always be `true` when `socket.bind()` is called.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an `'error'` event is generated. In rare cases (e.g., attempting to bind with a closed socket), an `Error` may be thrown.

An example socket listening on an exclusive port is shown below.

    socket.bind({
      address: 'localhost',
      port: 8000,
      exclusive: true,
    });
    js

#### `socket.bindSync([options])`#

Added in: v26.4.0

- `options` `<Object>`
  - `port` `<integer>` If omitted or `0`, the operating system will assign an arbitrary unused port. Default: `0`.
  - `address` `<string>` A numeric IP address to bind to. Unlike `socket.bind()`, no DNS resolution is performed, so a host name is not accepted. If omitted, the operating system binds to all addresses (`'0.0.0.0'` for `udp4` sockets, `'::'` for `udp6`).
- Returns: `<Object>` The bound address as returned by `socket.address()`.

The synchronous counterpart of `socket.bind()`. `bind(2)` is a local, non-blocking system call, so the bind is performed inline and the resolved address is returned immediately, including the operating-system-assigned ephemeral port when `port` is `0`:

    const dgram = require('node:dgram');

    const socket = dgram.createSocket('udp4');
    const address = socket.bindSync({ address: '0.0.0.0', port: 0 });
    console.log(address); // e.g. { address: '0.0.0.0', family: 'IPv4', port: 53124 }
    js

A bind failure such as `EADDRINUSE` is thrown synchronously rather than emitted as an `'error'` event. After `bindSync()` returns, `socket.address()` is valid synchronously and the `'listening'` event is emitted on the next tick.

`address` must be a numeric IP literal; `bindSync()` never performs DNS resolution (asynchronous name resolution being the only genuinely blocking part of binding). Incoming datagrams continue to be delivered asynchronously via the `'message'` event. `bindSync()` always binds the socket's own handle and does not participate in `cluster` handle sharing.

#### `socket.close([callback])`#

Added in: v0.1.99

- `callback` `<Function>` Called when the socket has been closed.

Close the underlying socket and stop listening for data on it. If a callback is provided, it is added as a listener for the `'close'` event.

#### `socket[Symbol.asyncDispose]()`#

Added in: v20.5.0, v18.18.0
History

Version | Changes
--- | ---
v24.2.0 | No longer experimental.

Calls `socket.close()` and returns a promise that fulfills when the socket has closed.

#### `socket.connect(port[, address][, callback])`#

Added in: v12.0.0

- `port` `<integer>`
- `address` `<string>`
- `callback` `<Function>` Called when the connection is completed or on error.

Associates the `dgram.Socket` to a remote address and port. Every message sent by this handle is automatically sent to that destination. Also, the socket will only receive messages from that remote peer. Trying to call `connect()` on an already connected socket will result in an `ERR_SOCKET_DGRAM_IS_CONNECTED` exception. If `address` is not provided, `'127.0.0.1'` (for `udp4` sockets) or `'::1'` (for `udp6` sockets) will be used by default. Once the connection is complete, a `'connect'` event is emitted and the optional `callback` function is called. In case of failure, the `callback` is called or, failing this, an `'error'` event is emitted.

#### `socket.connectSync(port[, address])`#

Added in: v26.4.0

- `port` `<integer>`
- `address` `<string>` A numeric IP address to connect to. Unlike `socket.connect()`, no DNS resolution is performed, so a host name is not accepted. If omitted, `'127.0.0.1'` (for `udp4` sockets) or `'::1'` (for `udp6` sockets) is used.

The synchronous counterpart of `socket.connect()`. For a UDP socket `connect(2)` only records the default peer address and is a local, non-blocking system call, so the association is performed inline. Any error raised by the call itself (for example `EAFNOSUPPORT` for a mismatched address family) is thrown synchronously rather than reported via the `'error'` event. Because `connect(2)` does not probe reachability, errors such as `ECONNREFUSED` are still surfaced asynchronously on a later send or receive, exactly as for `socket.connect()`:

    const dgram = require('node:dgram');

    const socket = dgram.createSocket('udp4');
    socket.connectSync(41234, '127.0.0.1');
    console.log(socket.remoteAddress()); // { address: '127.0.0.1', family: 'IPv4', port: 41234 }
    js

If the socket is still unbound it is bound synchronously first. After `connectSync()` returns, `socket.remoteAddress()` is valid synchronously and the `'connect'` event is emitted on the next tick. Trying to call `connectSync()` on an already connected socket throws an `ERR_SOCKET_DGRAM_IS_CONNECTED` exception, and calling it while an asynchronous `socket.bind()` is still in progress throws an `ERR_SOCKET_ALREADY_BOUND` exception.

`address` must be a numeric IP literal; `connectSync()` never performs DNS resolution (asynchronous name resolution being the only genuinely blocking part of connecting).

#### `socket.disconnect()`#

Added in: v12.0.0

A synchronous function that disassociates a connected `dgram.Socket` from its remote address. Trying to call `disconnect()` on an unbound or already disconnected socket will result in an `ERR_SOCKET_DGRAM_NOT_CONNECTED` exception.

#### `socket.dropMembership(multicastAddress[, multicastInterface])`#

Added in: v0.6.9

- `multicastAddress` `<string>`
- `multicastInterface` `<string>`

Instructs the kernel to leave a multicast group at `multicastAddress` using the `IP_DROP_MEMBERSHIP` socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If `multicastInterface` is not specified, the operating system will attempt to drop membership on all valid interfaces.

#### `socket.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])`#

Added in: v13.1.0, v12.16.0

- `sourceAddress` `<string>`
- `groupAddress` `<string>`
- `multicastInterface` `<string>`

Instructs the kernel to leave a source-specific multicast channel at the given `sourceAddress` and `groupAddress` using the `IP_DROP_SOURCE_MEMBERSHIP` socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If `multicastInterface` is not specified, the operating system will attempt to drop membership on all valid interfaces.

#### `socket.getRecvBufferSize()`#

Added in: v8.7.0

- Returns: `<number>` the `SO_RCVBUF` socket receive buffer size in bytes.

This method throws `ERR_SOCKET_BUFFER_SIZE` if called on an unbound socket.

#### `socket.getSendBufferSize()`#

Added in: v8.7.0

- Returns: `<number>` the `SO_SNDBUF` socket send buffer size in bytes.

This method throws `ERR_SOCKET_BUFFER_SIZE` if called on an unbound socket.

#### `socket.getSendQueueSize()`#

Added in: v18.8.0, v16.19.0

- Returns: `<number>` Number of bytes queued for sending.

#### `socket.getSendQueueCount()`#

Added in: v18.8.0, v16.19.0

- Returns: `<number>` Number of send requests currently in the queue awaiting to be processed.

#### `socket.ref()`#

Added in: v0.9.1

- Returns: `<dgram.Socket>`

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The `socket.unref()` method can be used to exclude the socket from the reference counting that keeps the Node.js process active. The `socket.ref()` method adds the socket back to the reference counting and restores the default behavior.

Calling `socket.ref()` multiple times will have no additional effect.

The `socket.ref()` method returns a reference to the socket so calls can be chained.

#### `socket.remoteAddress()`#

Added in: v12.0.0

- Returns: `<Object>`

Returns an object containing the `address`, `family`, and `port` of the remote endpoint. This method throws an `ERR_SOCKET_DGRAM_NOT_CONNECTED` exception if the socket is not connected.

#### `socket.send(msg[, offset, length][, port][, address][, callback])`#

Added in: v0.1.99
History

Version | Changes
--- | ---
v17.0.0 | The `address` parameter now only accepts a `string`, `null` or `undefined`.
v14.5.0, v12.19.0 | The `msg` parameter can now be any `TypedArray` or `DataView`.
v12.0.0 | Added support for sending data on connected sockets.
v8.0.0 | The `msg` parameter can be an `Uint8Array` now.
v8.0.0 | The `address` parameter is always optional now.
v6.0.0 | On success, `callback` will now be called with an `error` argument of `null` rather than `0`.
v5.7.0 | The `msg` parameter can be an array now. Also, the `offset` and `length` parameters are optional now.

- `msg` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<string>` | `<Array>` Message to be sent.
- `offset` `<integer>` Offset in the buffer where the message starts.
- `length` `<integer>` Number of bytes in the message.
- `port` `<integer>` Destination port.
- `address` `<string>` Destination host name or IP address.
- `callback` `<Function>` Called when the message has been sent.

Broadcasts a datagram on the socket. For connectionless sockets, the destination `port` and `address` must be specified. Connected sockets, on the other hand, will use their associated remote endpoint, so the `port` and `address` arguments must not be set.

The `msg` argument contains the message to be sent. Depending on its type, different behavior can apply. If `msg` is a `Buffer`, any `TypedArray` or a `DataView`, the `offset` and `length` specify the offset within the `Buffer` where the message begins and the number of bytes in the message, respectively. If `msg` is a `String`, then it is automatically converted to a `Buffer` with `'utf8'` encoding. With messages that contain multi-byte characters, `offset` and `length` will be calculated with respect to byte length and not the character position. If `msg` is an array, `offset` and `length` must not be specified.

The `address` argument is a string. If the value of `address` is a host name, DNS will be used to resolve the address of the host. If `address` is not provided or otherwise nullish, `'127.0.0.1'` (for `udp4` sockets) or `'::1'` (for `udp6` sockets) will be used by default.

If the socket has not been previously bound with a call to `bind`, the socket is assigned a random port number and is bound to the "all interfaces" address (`'0.0.0.0'` for `udp4` sockets, `'::0'` for `udp6` sockets.)

An optional `callback` function may be specified to as a way of reporting DNS errors or for determining when it is safe to reuse the `buf` object. DNS lookups delay the time to send for at least one tick of the Node.js event loop.

The only way to know for sure that the datagram has been sent is by using a `callback`. If an error occurs and a `callback` is given, the error will be passed as the first argument to the `callback`. If a `callback` is not given, the error is emitted as an `'error'` event on the `socket` object.

Offset and length are optional but both must be set if either are used. They are supported only when the first argument is a `Buffer`, a `TypedArray`, or a `DataView`.

This method throws `ERR_SOCKET_BAD_PORT` if called on an unbound socket.

Example of sending a UDP packet to a port on `localhost`;

    import dgram from 'node:dgram';
    import { Buffer } from 'node:buffer';

    const message = Buffer.from('Some bytes');
    const client = dgram.createSocket('udp4');
    client.send(message, 41234, 'localhost', (err) => {
      client.close();
    });
    const dgram = require('node:dgram');
    const { Buffer } = require('node:buffer');

    const message = Buffer.from('Some bytes');
    const client = dgram.createSocket('udp4');
    client.send(message, 41234, 'localhost', (err) => {
      client.close();
    });
    javascript

Example of sending a UDP packet composed of multiple buffers to a port on `127.0.0.1`;

    import dgram from 'node:dgram';
    import { Buffer } from 'node:buffer';

    const buf1 = Buffer.from('Some ');
    const buf2 = Buffer.from('bytes');
    const client = dgram.createSocket('udp4');
    client.send([buf1, buf2], 41234, (err) => {
      client.close();
    });
    const dgram = require('node:dgram');
    const { Buffer } = require('node:buffer');

    const buf1 = Buffer.from('Some ');
    const buf2 = Buffer.from('bytes');
    const client = dgram.createSocket('udp4');
    client.send([buf1, buf2], 41234, (err) => {
      client.close();
    });
    javascript

Sending multiple buffers might be faster or slower, depending on the application and operating system. Run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.

Example of sending a UDP packet using a socket connected to a port on `localhost`:

    import dgram from 'node:dgram';
    import { Buffer } from 'node:buffer';

    const message = Buffer.from('Some bytes');
    const client = dgram.createSocket('udp4');
    client.connect(41234, 'localhost', (err) => {
      client.send(message, (err) => {
        client.close();
      });
    });
    const dgram = require('node:dgram');
    const { Buffer } = require('node:buffer');

    const message = Buffer.from('Some bytes');
    const client = dgram.createSocket('udp4');
    client.connect(41234, 'localhost', (err) => {
      client.send(message, (err) => {
        client.close();
      });
    });
    javascript

##### Note about UDP datagram size#

The maximum size of an IPv4/v6 datagram depends on the `MTU` (Maximum Transmission Unit) and on the `Payload Length` field size.

- The `Payload Length` field is 16 bits wide, which means that a normal payload cannot exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks.

- The `MTU` is the largest size a given link layer technology can support for datagram messages. For any link, IPv4 mandates a minimum `MTU` of 68 octets, while the recommended `MTU` for IPv4 is 576 (typically recommended as the `MTU` for dial-up type applications), whether they arrive whole or in fragments.

  For IPv6, the minimum `MTU` is 1280 octets. However, the mandatory minimum fragment reassembly buffer size is 1500 octets. The value of 68 octets is very small, since most current link layer technologies, like Ethernet, have a minimum `MTU` of 1500.

It is impossible to know in advance the MTU of each link through which a packet might travel. Sending a datagram greater than the receiver `MTU` will not work because the packet will get silently dropped without informing the source that the data did not reach its intended recipient.

#### `socket.setBroadcast(flag)`#

Added in: v0.6.9

- `flag` `<boolean>`

Sets or clears the `SO_BROADCAST` socket option. When set to `true`, UDP packets may be sent to a local interface's broadcast address.

This method throws `EBADF` if called on an unbound socket.

#### `socket.setMulticastInterface(multicastInterface)`#

Added in: v8.6.0

- `multicastInterface` `<string>`

All references to scope in this section are referring to IPv6 Zone Indexes, which are defined by RFC 4007. In string form, an IP with a scope index is written as `'IP%scope'` where scope is an interface name or interface number.

Sets the default outgoing multicast interface of the socket to a chosen interface or back to system interface selection. The `multicastInterface` must be a valid string representation of an IP from the socket's family.

For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.

For IPv6 sockets, `multicastInterface` should include a scope to indicate the interface as in the examples that follow. In IPv6, individual `send` calls can also use explicit scope in addresses, so only packets sent to a multicast address without specifying an explicit scope are affected by the most recent successful use of this call.

This method throws `EBADF` if called on an unbound socket.

##### Example: IPv6 outgoing multicast interface#

On most systems, where scope format uses the interface name:

    const socket = dgram.createSocket('udp6');

    socket.bind(1234, () => {
      socket.setMulticastInterface('::%eth1');
    });
    js

On Windows, where scope format uses an interface number:

    const socket = dgram.createSocket('udp6');

    socket.bind(1234, () => {
      socket.setMulticastInterface('::%2');
    });
    js

##### Example: IPv4 outgoing multicast interface#

All systems use an IP of the host on the desired physical interface:

    const socket = dgram.createSocket('udp4');

    socket.bind(1234, () => {
      socket.setMulticastInterface('10.0.0.2');
    });
    js

##### Call results#

A call on a socket that is not ready to send or no longer open may throw a Not running `Error`.

If `multicastInterface` cannot be parsed into an IP then an EINVAL `System Error` is thrown.

On IPv4, if `multicastInterface` is a valid address but does not match any interface, or if the address does not match the family then a `System Error` such as `EADDRNOTAVAIL` or `EPROTONOSUP` is thrown.

On IPv6, most errors with specifying or omitting scope will result in the socket continuing to use (or returning to) the system's default interface selection.

A socket's address family's ANY address (IPv4 `'0.0.0.0'` or IPv6 `'::'`) can be used to return control of the sockets default outgoing interface to the system for future multicast packets.

#### `socket.setMulticastLoopback(flag)`#

Added in: v0.3.8

- `flag` `<boolean>`

Sets or clears the `IP_MULTICAST_LOOP` socket option. When set to `true`, multicast packets will also be received on the local interface.

This method throws `EBADF` if called on an unbound socket.

#### `socket.setMulticastTTL(ttl)`#

Added in: v0.3.8

- `ttl` `<integer>`

Sets the `IP_MULTICAST_TTL` socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through, specifically for multicast traffic. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.

The `ttl` argument may be between 0 and 255. The default on most systems is `1`.

This method throws `EBADF` if called on an unbound socket.

#### `socket.setRecvBufferSize(size)`#

Added in: v8.7.0

- `size` `<integer>`

Sets the `SO_RCVBUF` socket option. Sets the maximum socket receive buffer in bytes.

This method throws `ERR_SOCKET_BUFFER_SIZE` if called on an unbound socket.

#### `socket.setSendBufferSize(size)`#

Added in: v8.7.0

- `size` `<integer>`

Sets the `SO_SNDBUF` socket option. Sets the maximum socket send buffer in bytes.

This method throws `ERR_SOCKET_BUFFER_SIZE` if called on an unbound socket.

#### `socket.setTTL(ttl)`#

Added in: v0.1.101

- `ttl` `<integer>`

Sets the `IP_TTL` socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded. Changing TTL values is typically done for network probes or when multicasting.

The `ttl` argument may be between 1 and 255. The default on most systems is 64.

This method throws `EBADF` if called on an unbound socket.

#### `socket.unref()`#

Added in: v0.9.1

- Returns: `<dgram.Socket>`

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The `socket.unref()` method can be used to exclude the socket from the reference counting that keeps the Node.js process active, allowing the process to exit even if the socket is still listening.

Calling `socket.unref()` multiple times will have no additional effect.

The `socket.unref()` method returns a reference to the socket so calls can be chained.

### `node:dgram` module functions#

#### `dgram.createSocket(options[, callback])`#

Added in: v0.11.13
History

Version | Changes
--- | ---
v23.1.0, v22.12.0 | The `reusePort` option is supported.
v15.8.0 | AbortSignal support was added.
v11.4.0 | The `ipv6Only` option is supported.
v8.7.0 | The `recvBufferSize` and `sendBufferSize` options are supported now.
v8.6.0 | The `lookup` option is supported.

- `options` `<Object>` Available options are:
  - `type` `<string>` The family of socket. Must be either `'udp4'` or `'udp6'`. Required.
  - `reuseAddr` `<boolean>` When `true` `socket.bind()` will reuse the address, even if another process has already bound a socket on it, but only one socket can receive the data. Default: `false`.
  - `reusePort` `<boolean>` When `true` `socket.bind()` will reuse the port, even if another process has already bound a socket on it. Incoming datagrams are distributed to listening sockets. The option is available only on some platforms, such as Linux 3.9+, DragonFlyBSD 3.6+, FreeBSD 12.0+, Solaris 11.4, and AIX 7.2.5+. On unsupported platforms, this option raises an error when the socket is bound. Default: `false`.
  - `ipv6Only` `<boolean>` Setting `ipv6Only` to `true` will disable dual-stack support, i.e., binding to address `::` won't make `0.0.0.0` be bound. Default: `false`.
  - `recvBufferSize` `<number>` Sets the `SO_RCVBUF` socket value.
  - `sendBufferSize` `<number>` Sets the `SO_SNDBUF` socket value.
  - `lookup` `<Function>` Custom lookup function. Default: `dns.lookup()`. When the default is used, a literal IP address of the socket's family resolves to itself without calling `dns.lookup()`.
  - `signal` `<AbortSignal>` An AbortSignal that may be used to close a socket.
  - `receiveBlockList` `<net.BlockList>` `receiveBlockList` can be used for discarding inbound datagram to specific IP addresses, IP ranges, or IP subnets. This does not work if the server is behind a reverse proxy, NAT, etc. because the address checked against the blocklist is the address of the proxy, or the one specified by the NAT.
  - `sendBlockList` `<net.BlockList>` `sendBlockList` can be used for disabling outbound access to specific IP addresses, IP ranges, or IP subnets.
- `callback` `<Function>` Attached as a listener for `'message'` events. Optional.
- Returns: `<dgram.Socket>`

Creates a `dgram.Socket` object. Once the socket is created, calling `socket.bind()` will instruct the socket to begin listening for datagram messages. When `address` and `port` are not passed to `socket.bind()` the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both `udp4` and `udp6` sockets). The bound address and port can be retrieved using `socket.address().address` and `socket.address().port`.

If the `signal` option is enabled, calling `.abort()` on the corresponding `AbortController` is similar to calling `.close()` on the socket:

    const controller = new AbortController();
    const { signal } = controller;
    const server = dgram.createSocket({ type: 'udp4', signal });
    server.on('message', (msg, rinfo) => {
      console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
    });
    // Later, when you want to close the server.
    controller.abort();
    js

#### `dgram.createSocket(type[, callback])`#

Added in: v0.1.99

- `type` `<string>` Either `'udp4'` or `'udp6'`.
- `callback` `<Function>` Attached as a listener to `'message'` events.
- Returns: `<dgram.Socket>`

Creates a `dgram.Socket` object of the specified `type`.

Once the socket is created, calling `socket.bind()` will instruct the socket to begin listening for datagram messages. When `address` and `port` are not passed to `socket.bind()` the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both `udp4` and `udp6` sockets). The bound address and port can be retrieved using `socket.address().address` and `socket.address().port`.

## URL#

Source Code: lib/url.js

Stability: 2 - Stable

The `node:url` module provides utilities for URL resolution and parsing. It can be accessed using:

    import url from 'node:url';
    const url = require('node:url');
    javascript

### URL strings and URL objects#

A URL string is a structured string containing multiple meaningful components. When parsed, a URL object is returned containing properties for each of these components.

The `node:url` module provides two APIs for working with URLs: a legacy API that is Node.js specific, and a newer API that implements the same WHATWG URL Standard used by web browsers.

A comparison between the WHATWG and legacy APIs is provided below. Above the URL `'https://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash'`, properties of an object returned by the legacy `url.parse()` are shown. Below it are properties of a WHATWG `URL` object.

WHATWG URL's `origin` property includes `protocol` and `host`, but not `username` or `password`.

    ┌────────────────────────────────────────────────────────────────────────────────────────────────┐
    │                                              href                                              │
    ├──────────┬──┬─────────────────────┬────────────────────────┬───────────────────────────┬───────┤
    │ protocol │  │        auth         │          host          │           path            │ hash  │
    │          │  │                     ├─────────────────┬──────┼──────────┬────────────────┤       │
    │          │  │                     │    hostname     │ port │ pathname │     search     │       │
    │          │  │                     │                 │      │          ├─┬──────────────┤       │
    │          │  │                     │                 │      │          │ │    query     │       │
    "  https:   //    user   :   pass   @ sub.example.com : 8080   /p/a/t/h  ?  query=string   #hash "
    │          │  │          │          │    hostname     │ port │          │                │       │
    │          │  │          │          ├─────────────────┴──────┤          │                │       │
    │ protocol │  │ username │ password │          host          │          │                │       │
    ├──────────┴──┼──────────┴──────────┼────────────────────────┤          │                │       │
    │   origin    │                     │         origin         │ pathname │     search     │ hash  │
    ├─────────────┴─────────────────────┴────────────────────────┴──────────┴────────────────┴───────┤
    │                                              href                                              │
    └────────────────────────────────────────────────────────────────────────────────────────────────┘
    (All spaces in the "" line should be ignored. They are purely for formatting.)
    text

Parsing the URL string using the WHATWG API:

    const myURL =
      new URL('https://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash');
    js

Parsing the URL string using the legacy API:

    import url from 'node:url';
    const myURL =
      url.parse('https://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash');
    const url = require('node:url');
    const myURL =
      url.parse('https://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash');
    javascript

#### Constructing a URL from component parts and getting the constructed string#

It is possible to construct a WHATWG URL from component parts using either the property setters or a template literal string:

    const myURL = new URL('https://example.org');
    myURL.pathname = '/a/b/c';
    myURL.search = '?d=e';
    myURL.hash = '#fgh';
    js

    const pathname = '/a/b/c';
    const search = '?d=e';
    const hash = '#fgh';
    const myURL = new URL(`https://example.org${pathname}${search}${hash}`);
    js

To get the constructed URL string, use the `href` property accessor:

    console.log(myURL.href);
    js

### The WHATWG URL API#

#### Class: `URL`#

Added in: v7.0.0, v6.13.0
History

Version | Changes
--- | ---
v10.0.0 | The class is now available on the global object.

Browser-compatible `URL` class, implemented by following the WHATWG URL Standard. Examples of parsed URLs may be found in the Standard itself. The `URL` class is also available on the global object.

In accordance with browser conventions, all properties of `URL` objects are implemented as getters and setters on the class prototype, rather than as data properties on the object itself. Thus, unlike legacy `urlObject`s, using the `delete` keyword on any properties of `URL` objects (e.g. `delete myURL.protocol`, `delete myURL.pathname`, etc) has no effect but will still return `true`.

##### `new URL(input[, base])`#

History

Version | Changes
--- | ---
v20.0.0, v18.17.0 | ICU requirement is removed.

- `input` `<string>` The absolute or relative input URL to parse. If `input` is relative, then `base` is required. If `input` is absolute, the `base` is ignored. If `input` is not a string, it is converted to a string first.
- `base` `<string>` The base URL to resolve against if the `input` is not absolute. If `base` is not a string, it is converted to a string first.

Creates a new `URL` object by parsing the `input` relative to the `base`. If `base` is passed as a string, it will be parsed equivalent to `new URL(base)`.

    const myURL = new URL('/foo', 'https://example.org/');
    // https://example.org/foo
    js

The URL constructor is accessible as a property on the global object. It can also be imported from the built-in url module:

    import { URL } from 'node:url';
    console.log(URL === globalThis.URL); // Prints 'true'.
    console.log(URL === require('node:url').URL); // Prints 'true'.
    javascript

A `TypeError` will be thrown if the `input` or `base` are not valid URLs. Note that an effort will be made to coerce the given values into strings. For instance:

    const myURL = new URL({ toString: () => 'https://example.org/' });
    // https://example.org/
    js

Unicode characters appearing within the host name of `input` will be automatically converted to ASCII using the Punycode algorithm.

    const myURL = new URL('https://測試');
    // https://xn--g6w251d/
    js

In cases where it is not known in advance if `input` is an absolute URL and a `base` is provided, it is advised to validate that the `origin` of the `URL` object is what is expected.

    let myURL = new URL('http://Example.com/', 'https://example.org/');
    // http://example.com/

    myURL = new URL('https://Example.com/', 'https://example.org/');
    // https://example.com/

    myURL = new URL('foo://Example.com/', 'https://example.org/');
    // foo://Example.com/

    myURL = new URL('http:Example.com/', 'https://example.org/');
    // http://example.com/

    myURL = new URL('https:Example.com/', 'https://example.org/');
    // https://example.org/Example.com/

    myURL = new URL('foo:Example.com/', 'https://example.org/');
    // foo:Example.com/
    js

##### `url.hash`#

- Type: `<string>`

Gets and sets the fragment portion of the URL.

    const myURL = new URL('https://example.org/foo#bar');
    console.log(myURL.hash);
    // Prints #bar

    myURL.hash = 'baz';
    console.log(myURL.href);
    // Prints https://example.org/foo#baz
    js

Invalid URL characters included in the value assigned to the `hash` property are percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the `url.parse()` and `url.format()` methods would produce.

##### `url.host`#

- Type: `<string>`

Gets and sets the host portion of the URL.

    const myURL = new URL('https://example.org:81/foo');
    console.log(myURL.host);
    // Prints example.org:81

    myURL.host = 'example.com:82';
    console.log(myURL.href);
    // Prints https://example.com:82/foo
    js

Invalid host values assigned to the `host` property are ignored.

##### `url.hostname`#

- Type: `<string>`

Gets and sets the host name portion of the URL. The key difference between `url.host` and `url.hostname` is that `url.hostname` does not include the port.

    const myURL = new URL('https://example.org:81/foo');
    console.log(myURL.hostname);
    // Prints example.org

    // Setting the hostname does not change the port
    myURL.hostname = 'example.com';
    console.log(myURL.href);
    // Prints https://example.com:81/foo

    // Use myURL.host to change the hostname and port
    myURL.host = 'example.org:82';
    console.log(myURL.href);
    // Prints https://example.org:82/foo
    js

Invalid host name values assigned to the `hostname` property are ignored.

##### `url.href`#

- Type: `<string>`

Gets and sets the serialized URL.

    const myURL = new URL('https://example.org/foo');
    console.log(myURL.href);
    // Prints https://example.org/foo

    myURL.href = 'https://example.com/bar';
    console.log(myURL.href);
    // Prints https://example.com/bar
    js

Getting the value of the `href` property is equivalent to calling `url.toString()`.

Setting the value of this property to a new value is equivalent to creating a new `URL` object using `new URL(value)`. Each of the `URL` object's properties will be modified.

If the value assigned to the `href` property is not a valid URL, a `TypeError` will be thrown.

##### `url.origin`#

History

Version | Changes
--- | ---
v15.0.0 | The scheme "gopher" is no longer special and `url.origin` now returns `'null'` for it.

- Type: `<string>`

Gets the read-only serialization of the URL's origin.

    const myURL = new URL('https://example.org/foo/bar?baz');
    console.log(myURL.origin);
    // Prints https://example.org
    js

    const idnURL = new URL('https://測試');
    console.log(idnURL.origin);
    // Prints https://xn--g6w251d

    console.log(idnURL.hostname);
    // Prints xn--g6w251d
    js

##### `url.password`#

- Type: `<string>`

Gets and sets the password portion of the URL.

    const myURL = new URL('https://abc:xyz@example.com');
    console.log(myURL.password);
    // Prints xyz

    myURL.password = '123';
    console.log(myURL.href);
    // Prints https://abc:123@example.com/
    js

Invalid URL characters included in the value assigned to the `password` property are percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the `url.parse()` and `url.format()` methods would produce.

##### `url.pathname`#

- Type: `<string>`

Gets and sets the path portion of the URL.

    const myURL = new URL('https://example.org/abc/xyz?123');
    console.log(myURL.pathname);
    // Prints /abc/xyz

    myURL.pathname = '/abcdef';
    console.log(myURL.href);
    // Prints https://example.org/abcdef?123
    js

Invalid URL characters included in the value assigned to the `pathname` property are percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the `url.parse()` and `url.format()` methods would produce.

##### `url.port`#

History

Version | Changes
--- | ---
v15.0.0 | The scheme "gopher" is no longer special.

- Type: `<string>`

Gets and sets the port portion of the URL.

The port value may be a number or a string containing a number in the range `0` to `65535` (inclusive). Setting the value to the default port of the `URL` objects given `protocol` will result in the `port` value becoming the empty string (`''`).

The port value can be an empty string in which case the port depends on the protocol/scheme:

protocol | port
--- | ---
"ftp" | 21
"file" |
"http" | 80
"https" | 443
"ws" | 80
"wss" | 443

Upon assigning a value to the port, the value will first be converted to a string using `.toString()`.

If that string is invalid but it begins with a number, the leading number is assigned to `port`. If the number lies outside the range denoted above, it is ignored.

    const myURL = new URL('https://example.org:8888');
    console.log(myURL.port);
    // Prints 8888

    // Default ports are automatically transformed to the empty string
    // (HTTPS protocol's default port is 443)
    myURL.port = '443';
    console.log(myURL.port);
    // Prints the empty string
    console.log(myURL.href);
    // Prints https://example.org/

    myURL.port = 1234;
    console.log(myURL.port);
    // Prints 1234
    console.log(myURL.href);
    // Prints https://example.org:1234/

    // Completely invalid port strings are ignored
    myURL.port = 'abcd';
    console.log(myURL.port);
    // Prints 1234

    // Leading numbers are treated as a port number
    myURL.port = '5678abcd';
    console.log(myURL.port);
    // Prints 5678

    // Non-integers are truncated
    myURL.port = 1234.5678;
    console.log(myURL.port);
    // Prints 1234

    // Out-of-range numbers which are not represented in scientific notation
    // will be ignored.
    myURL.port = 1e10; // 10000000000, will be range-checked as described below
    console.log(myURL.port);
    // Prints 1234
    js

Numbers which contain a decimal point, such as floating-point numbers or numbers in scientific notation, are not an exception to this rule. Leading numbers up to the decimal point will be set as the URL's port, assuming they are valid:

    myURL.port = 4.567e21;
    console.log(myURL.port);
    // Prints 4 (because it is the leading number in the string '4.567e21')
    js

##### `url.protocol`#

- Type: `<string>`

Gets and sets the protocol portion of the URL.

    const myURL = new URL('https://example.org');
    console.log(myURL.protocol);
    // Prints https:

    myURL.protocol = 'ftp';
    console.log(myURL.href);
    // Prints ftp://example.org/
    js

Invalid URL protocol values assigned to the `protocol` property are ignored.

###### Special schemes#

History

Version | Changes
--- | ---
v15.0.0 | The scheme "gopher" is no longer special.

The WHATWG URL Standard considers a handful of URL protocol schemes to be special in terms of how they are parsed and serialized. When a URL is parsed using one of these special protocols, the `url.protocol` property may be changed to another special protocol but cannot be changed to a non-special protocol, and vice versa.

For instance, changing from `http` to `https` works:

    const u = new URL('http://example.org');
    u.protocol = 'https';
    console.log(u.href);
    // https://example.org/
    js

However, changing from `http` to a hypothetical `fish` protocol does not because the new protocol is not special.

    const u = new URL('http://example.org');
    u.protocol = 'fish';
    console.log(u.href);
    // http://example.org/
    js

Likewise, changing from a non-special protocol to a special protocol is also not permitted:

    const u = new URL('fish://example.org');
    u.protocol = 'http';
    console.log(u.href);
    // fish://example.org
    js

According to the WHATWG URL Standard, special protocol schemes are `ftp`, `file`, `http`, `https`, `ws`, and `wss`.

##### `url.search`#

- Type: `<string>`

Gets and sets the serialized query portion of the URL.

    const myURL = new URL('https://example.org/abc?123');
    console.log(myURL.search);
    // Prints ?123

    myURL.search = 'abc=xyz';
    console.log(myURL.href);
    // Prints https://example.org/abc?abc=xyz
    js

Any invalid URL characters appearing in the value assigned the `search` property will be percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the `url.parse()` and `url.format()` methods would produce.

##### `url.searchParams`#

- Type: `<URLSearchParams>`

Gets the `URLSearchParams` object representing the query parameters of the URL. This property is read-only but the `URLSearchParams` object it provides can be used to mutate the URL instance; to replace the entirety of query parameters of the URL, use the `url.search` setter. See `URLSearchParams` documentation for details.

Use care when using `.searchParams` to modify the `URL` because, per the WHATWG specification, the `URLSearchParams` object uses different rules to determine which characters to percent-encode. For instance, the `URL` object will not percent encode the ASCII tilde (`~`) character, while `URLSearchParams` will always encode it:

    const myURL = new URL('https://example.org/abc?foo=~bar');

    console.log(myURL.search);  // prints ?foo=~bar

    // Modify the URL via searchParams...
    myURL.searchParams.sort();

    console.log(myURL.search);  // prints ?foo=%7Ebar
    js

##### `url.username`#

- Type: `<string>`

Gets and sets the username portion of the URL.

    const myURL = new URL('https://abc:xyz@example.com');
    console.log(myURL.username);
    // Prints abc

    myURL.username = '123';
    console.log(myURL.href);
    // Prints https://123:xyz@example.com/
    js

Any invalid URL characters appearing in the value assigned the `username` property will be percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the `url.parse()` and `url.format()` methods would produce.

##### `url.toString()`#

- Returns: `<string>`

The `toString()` method on the `URL` object returns the serialized URL. The value returned is equivalent to that of `url.href` and `url.toJSON()`.

##### `url.toJSON()`#

Added in: v7.7.0, v6.13.0

- Returns: `<string>`

The `toJSON()` method on the `URL` object returns the serialized URL. The value returned is equivalent to that of `url.href` and `url.toString()`.

This method is automatically called when an `URL` object is serialized with `JSON.stringify()`.

    const myURLs = [
      new URL('https://www.example.com'),
      new URL('https://test.example.org'),
    ];
    console.log(JSON.stringify(myURLs));
    // Prints ["https://www.example.com/","https://test.example.org/"]
    js

##### `URL.createObjectURL(blob)`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `blob` `<Blob>`
- Returns: `<string>`

Creates a `'blob:nodedata:...'` URL string that represents the given `<Blob>` object and can be used to retrieve the `Blob` later.

    const {
      Blob,
      resolveObjectURL,
    } = require('node:buffer');

    const blob = new Blob(['hello']);
    const id = URL.createObjectURL(blob);

    // later...

    const otherBlob = resolveObjectURL(id);
    console.log(otherBlob.size);
    js

The data stored by the registered `<Blob>` will be retained in memory until `URL.revokeObjectURL()` is called to remove it.

`Blob` objects are registered within the current thread. If using Worker Threads, `Blob` objects registered within one Worker will not be available to other workers or the main thread.

##### `URL.revokeObjectURL(id)`#

Added in: v16.7.0
History

Version | Changes
--- | ---
v24.0.0, v22.17.0 | Marking the API stable.

- `id` `<string>` A `'blob:nodedata:...` URL string returned by a prior call to `URL.createObjectURL()`.

Removes the stored `<Blob>` identified by the given ID. Attempting to revoke a ID that isn't registered will silently fail.

##### `URL.canParse(input[, base])`#

Added in: v19.9.0, v18.17.0

- `input` `<string>` The absolute or relative input URL to parse. If `input` is relative, then `base` is required. If `input` is absolute, the `base` is ignored. If `input` is not a string, it is converted to a string first.
- `base` `<string>` The base URL to resolve against if the `input` is not absolute. If `base` is not a string, it is converted to a string first.
- Returns: `<boolean>`

Checks if an `input` relative to the `base` can be parsed to a `URL`.

    const isValid = URL.canParse('/foo', 'https://example.org/'); // true

    const isNotValid = URL.canParse('/foo'); // false
    js

##### `URL.parse(input[, base])`#

Added in: v22.1.0

- `input` `<string>` The absolute or relative input URL to parse. If `input` is relative, then `base` is required. If `input` is absolute, the `base` is ignored. If `input` is not a string, it is converted to a string first.
- `base` `<string>` The base URL to resolve against if the `input` is not absolute. If `base` is not a string, it is converted to a string first.
- Returns: `<URL>` | `<null>`

Parses a string as a URL. If `base` is provided, it will be used as the base URL for the purpose of resolving non-absolute `input` URLs. Returns `null` if the parameters can't be resolved to a valid URL.

#### Class: `URLPattern`#

Added in: v23.8.0

Stability: 1 - Experimental

The `URLPattern` API provides an interface to match URLs or parts of URLs against a pattern.

    const myPattern = new URLPattern('https://nodejs.org/docs/latest/api/*.html');
    console.log(myPattern.exec('https://nodejs.org/docs/latest/api/dns.html'));
    // Prints:
    // {
    //  "hash": { "groups": {  "0": "" },  "input": "" },
    //  "hostname": { "groups": {}, "input": "nodejs.org" },
    //  "inputs": [
    //    "https://nodejs.org/docs/latest/api/dns.html"
    //  ],
    //  "password": { "groups": { "0": "" }, "input": "" },
    //  "pathname": { "groups": { "0": "dns" }, "input": "/docs/latest/api/dns.html" },
    //  "port": { "groups": {}, "input": "" },
    //  "protocol": { "groups": {}, "input": "https" },
    //  "search": { "groups": { "0": "" }, "input": "" },
    //  "username": { "groups": { "0": "" }, "input": "" }
    // }

    console.log(myPattern.test('https://nodejs.org/docs/latest/api/dns.html'));
    // Prints: true
    js

##### `new URLPattern()`#

Instantiate a new empty `URLPattern` object.

##### `new URLPattern(string[, baseURL][, options])`#

- `string` `<string>` A URL string
- `baseURL` `<string>` | `<undefined>` A base URL string
- `options` `<Object>` Options

Parse the `string` as a URL, and use it to instantiate a new `URLPattern` object.

If `baseURL` is not specified, it defaults to `undefined`.

An option can have `ignoreCase` boolean attribute which enables case-insensitive matching if set to true.

The constructor can throw a `TypeError` to indicate parsing failure.

##### `new URLPattern(obj[, baseURL][, options])`#

- `obj` `<Object>` An input pattern
- `baseURL` `<string>` | `<undefined>` A base URL string
- `options` `<Object>` Options

Parse the `Object` as an input pattern, and use it to instantiate a new `URLPattern` object. The object members can be any of `protocol`, `username`, `password`, `hostname`, `port`, `pathname`, `search`, `hash` or `baseURL`.

If `baseURL` is not specified, it defaults to `undefined`.

An option can have `ignoreCase` boolean attribute which enables case-insensitive matching if set to true.

The constructor can throw a `TypeError` to indicate parsing failure.

##### `urlPattern.exec(input[, baseURL])`#

- `input` `<string>` | `<Object>` A URL or URL parts
- `baseURL` `<string>` | `<undefined>` A base URL string

Input can be a string or an object providing the individual URL parts. The object members can be any of `protocol`, `username`, `password`, `hostname`, `port`, `pathname`, `search`, `hash` or `baseURL`.

If `baseURL` is not specified, it will default to `undefined`.

Returns an object with an `inputs` key containing the array of arguments passed into the function and keys of the URL components which contains the matched input and matched groups.

    const myPattern = new URLPattern('https://nodejs.org/docs/latest/api/*.html');
    console.log(myPattern.exec('https://nodejs.org/docs/latest/api/dns.html'));
    // Prints:
    // {
    //  "hash": { "groups": {  "0": "" },  "input": "" },
    //  "hostname": { "groups": {}, "input": "nodejs.org" },
    //  "inputs": [
    //    "https://nodejs.org/docs/latest/api/dns.html"
    //  ],
    //  "password": { "groups": { "0": "" }, "input": "" },
    //  "pathname": { "groups": { "0": "dns" }, "input": "/docs/latest/api/dns.html" },
    //  "port": { "groups": {}, "input": "" },
    //  "protocol": { "groups": {}, "input": "https" },
    //  "search": { "groups": { "0": "" }, "input": "" },
    //  "username": { "groups": { "0": "" }, "input": "" }
    // }
    js

##### `urlPattern.test(input[, baseURL])`#

- `input` `<string>` | `<Object>` A URL or URL parts
- `baseURL` `<string>` | `<undefined>` A base URL string

Input can be a string or an object providing the individual URL parts. The object members can be any of `protocol`, `username`, `password`, `hostname`, `port`, `pathname`, `search`, `hash` or `baseURL`.

If `baseURL` is not specified, it will default to `undefined`.

Returns a boolean indicating if the input matches the current pattern.

    const myPattern = new URLPattern('https://nodejs.org/docs/latest/api/*.html');
    console.log(myPattern.test('https://nodejs.org/docs/latest/api/dns.html'));
    // Prints: true
    js

#### Class: `URLSearchParams`#

Added in: v7.5.0, v6.13.0
History

Version | Changes
--- | ---
v10.0.0 | The class is now available on the global object.

The `URLSearchParams` API provides read and write access to the query of a `URL`. The `URLSearchParams` class can also be used standalone with one of the four following constructors. The `URLSearchParams` class is also available on the global object.

The WHATWG `URLSearchParams` interface and the `querystring` module have similar purpose, but the purpose of the `querystring` module is more general, as it allows the customization of delimiter characters (`&` and `=`). On the other hand, this API is designed purely for URL query strings.

    const myURL = new URL('https://example.org/?abc=123');
    console.log(myURL.searchParams.get('abc'));
    // Prints 123

    myURL.searchParams.append('abc', 'xyz');
    console.log(myURL.href);
    // Prints https://example.org/?abc=123&abc=xyz

    myURL.searchParams.delete('abc');
    myURL.searchParams.set('a', 'b');
    console.log(myURL.href);
    // Prints https://example.org/?a=b

    const newSearchParams = new URLSearchParams(myURL.searchParams);
    // The above is equivalent to
    // const newSearchParams = new URLSearchParams(myURL.search);

    newSearchParams.append('a', 'c');
    console.log(myURL.href);
    // Prints https://example.org/?a=b
    console.log(newSearchParams.toString());
    // Prints a=b&a=c

    // newSearchParams.toString() is implicitly called
    myURL.search = newSearchParams;
    console.log(myURL.href);
    // Prints https://example.org/?a=b&a=c
    newSearchParams.delete('a');
    console.log(myURL.href);
    // Prints https://example.org/?a=b&a=c
    js

##### `new URLSearchParams()`#

Instantiate a new empty `URLSearchParams` object.

##### `new URLSearchParams(string)`#

- `string` `<string>` A query string

Parse the `string` as a query string, and use it to instantiate a new `URLSearchParams` object. A leading `'?'`, if present, is ignored.

    let params;

    params = new URLSearchParams('user=abc&query=xyz');
    console.log(params.get('user'));
    // Prints 'abc'
    console.log(params.toString());
    // Prints 'user=abc&query=xyz'

    params = new URLSearchParams('?user=abc&query=xyz');
    console.log(params.toString());
    // Prints 'user=abc&query=xyz'
    js

##### `new URLSearchParams(obj)`#

Added in: v7.10.0, v6.13.0

- `obj` `<Object>` An object representing a collection of key-value pairs

Instantiate a new `URLSearchParams` object with a query hash map. The key and value of each property of `obj` are always coerced to strings.

Unlike `querystring` module, duplicate keys in the form of array values are not allowed. Arrays are stringified using `array.toString()`, which simply joins all array elements with commas.

    const params = new URLSearchParams({
      user: 'abc',
      query: ['first', 'second'],
    });
    console.log(params.getAll('query'));
    // Prints [ 'first,second' ]
    console.log(params.toString());
    // Prints 'user=abc&query=first%2Csecond'
    js

##### `new URLSearchParams(iterable)`#

Added in: v7.10.0, v6.13.0

- `iterable` `<Iterable>` An iterable object whose elements are key-value pairs

Instantiate a new `URLSearchParams` object with an iterable map in a way that is similar to `<Map>`'s constructor. `iterable` can be an `Array` or any iterable object. That means `iterable` can be another `URLSearchParams`, in which case the constructor will simply create a clone of the provided `URLSearchParams`. Elements of `iterable` are key-value pairs, and can themselves be any iterable object.

Duplicate keys are allowed.

    let params;

    // Using an array
    params = new URLSearchParams([
      ['user', 'abc'],
      ['query', 'first'],
      ['query', 'second'],
    ]);
    console.log(params.toString());
    // Prints 'user=abc&query=first&query=second'

    // Using a Map object
    const map = new Map();
    map.set('user', 'abc');
    map.set('query', 'xyz');
    params = new URLSearchParams(map);
    console.log(params.toString());
    // Prints 'user=abc&query=xyz'

    // Using a generator function
    function* getQueryPairs() {
      yield ['user', 'abc'];
      yield ['query', 'first'];
      yield ['query', 'second'];
    }
    params = new URLSearchParams(getQueryPairs());
    console.log(params.toString());
    // Prints 'user=abc&query=first&query=second'

    // Each key-value pair must have exactly two elements
    new URLSearchParams([
      ['user', 'abc', 'error'],
    ]);
    // Throws TypeError [ERR_INVALID_TUPLE]:
    //        Each query pair must be an iterable [name, value] tuple
    js

##### `urlSearchParams.append(name, value)`#

- `name` `<string>`
- `value` `<string>`

Append a new name-value pair to the query string.

##### `urlSearchParams.delete(name[, value])`#

History

Version | Changes
--- | ---
v20.2.0, v18.18.0 | Add support for optional `value` argument.

- `name` `<string>`
- `value` `<string>`

If `value` is provided, removes all name-value pairs where name is `name` and value is `value`..

If `value` is not provided, removes all name-value pairs whose name is `name`.

##### `urlSearchParams.entries()`#

- Returns: `<Iterator>`

Returns an ES6 `Iterator` over each of the name-value pairs in the query. Each item of the iterator is a JavaScript `Array`. The first item of the `Array` is the `name`, the second item of the `Array` is the `value`.

Alias for `urlSearchParams[Symbol.iterator]()`.

##### `urlSearchParams.forEach(fn[, thisArg])`#

History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `fn` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `fn` `<Function>` Invoked for each name-value pair in the query
- `thisArg` `<Object>` To be used as `this` value for when `fn` is called

Iterates over each name-value pair in the query and invokes the given function.

    const myURL = new URL('https://example.org/?a=b&c=d');
    myURL.searchParams.forEach((value, name, searchParams) => {
      console.log(name, value, myURL.searchParams === searchParams);
    });
    // Prints:
    //   a b true
    //   c d true
    js

##### `urlSearchParams.get(name)`#

- `name` `<string>`
- Returns: `<string>` | `<null>` A string or `null` if there is no name-value pair with the given `name`.

Returns the value of the first name-value pair whose name is `name`. If there are no such pairs, `null` is returned.

##### `urlSearchParams.getAll(name)`#

- `name` `<string>`
- Returns: `<string>`[]

Returns the values of all name-value pairs whose name is `name`. If there are no such pairs, an empty array is returned.

##### `urlSearchParams.has(name[, value])`#

History

Version | Changes
--- | ---
v20.2.0, v18.18.0 | Add support for optional `value` argument.

- `name` `<string>`
- `value` `<string>`
- Returns: `<boolean>`

Checks if the `URLSearchParams` object contains key-value pair(s) based on `name` and an optional `value` argument.

If `value` is provided, returns `true` when name-value pair with same `name` and `value` exists.

If `value` is not provided, returns `true` if there is at least one name-value pair whose name is `name`.

##### `urlSearchParams.keys()`#

- Returns: `<Iterator>`

Returns an ES6 `Iterator` over the names of each name-value pair.

    const params = new URLSearchParams('foo=bar&foo=baz');
    for (const name of params.keys()) {
      console.log(name);
    }
    // Prints:
    //   foo
    //   foo
    js

##### `urlSearchParams.set(name, value)`#

- `name` `<string>`
- `value` `<string>`

Sets the value in the `URLSearchParams` object associated with `name` to `value`. If there are any pre-existing name-value pairs whose names are `name`, set the first such pair's value to `value` and remove all others. If not, append the name-value pair to the query string.

    const params = new URLSearchParams();
    params.append('foo', 'bar');
    params.append('foo', 'baz');
    params.append('abc', 'def');
    console.log(params.toString());
    // Prints foo=bar&foo=baz&abc=def

    params.set('foo', 'def');
    params.set('xyz', 'opq');
    console.log(params.toString());
    // Prints foo=def&abc=def&xyz=opq
    js

##### `urlSearchParams.size`#

Added in: v19.8.0, v18.16.0

The total number of parameter entries.

##### `urlSearchParams.sort()`#

Added in: v7.7.0, v6.13.0

Sort all existing name-value pairs in-place by their names. Sorting is done with a stable sorting algorithm, so relative order between name-value pairs with the same name is preserved.

This method can be used, in particular, to increase cache hits.

    const params = new URLSearchParams('query[]=abc&type=search&query[]=123');
    params.sort();
    console.log(params.toString());
    // Prints query%5B%5D=abc&query%5B%5D=123&type=search
    js

##### `urlSearchParams.toString()`#

- Returns: `<string>`

Returns the search parameters serialized as a string, with characters percent-encoded where necessary.

##### `urlSearchParams.values()`#

- Returns: `<Iterator>`

Returns an ES6 `Iterator` over the values of each name-value pair.

##### `urlSearchParams[Symbol.iterator]()`#

- Returns: `<Iterator>`

Returns an ES6 `Iterator` over each of the name-value pairs in the query string. Each item of the iterator is a JavaScript `Array`. The first item of the `Array` is the `name`, the second item of the `Array` is the `value`.

Alias for `urlSearchParams.entries()`.

    const params = new URLSearchParams('foo=bar&xyz=baz');
    for (const [name, value] of params) {
      console.log(name, value);
    }
    // Prints:
    //   foo bar
    //   xyz baz
    js

#### `url.domainToASCII(domain)`#

Added in: v7.4.0, v6.13.0
History

Version | Changes
--- | ---
v20.0.0, v18.17.0 | ICU requirement is removed.

- `domain` `<string>`
- Returns: `<string>`

Returns the Punycode ASCII serialization of the `domain`. If `domain` is an invalid domain, the empty string is returned.

It performs the inverse operation to `url.domainToUnicode()`.

    import url from 'node:url';

    console.log(url.domainToASCII('español.com'));
    // Prints xn--espaol-zwa.com
    console.log(url.domainToASCII('中文.com'));
    // Prints xn--fiq228c.com
    console.log(url.domainToASCII('xn--iñvalid.com'));
    // Prints an empty string
    const url = require('node:url');

    console.log(url.domainToASCII('español.com'));
    // Prints xn--espaol-zwa.com
    console.log(url.domainToASCII('中文.com'));
    // Prints xn--fiq228c.com
    console.log(url.domainToASCII('xn--iñvalid.com'));
    // Prints an empty string
    javascript

#### `url.domainToUnicode(domain)`#

Added in: v7.4.0, v6.13.0
History

Version | Changes
--- | ---
v20.0.0, v18.17.0 | ICU requirement is removed.

- `domain` `<string>`
- Returns: `<string>`

Returns the Unicode serialization of the `domain`. If `domain` is an invalid domain, the empty string is returned.

It performs the inverse operation to `url.domainToASCII()`.

    import url from 'node:url';

    console.log(url.domainToUnicode('xn--espaol-zwa.com'));
    // Prints español.com
    console.log(url.domainToUnicode('xn--fiq228c.com'));
    // Prints 中文.com
    console.log(url.domainToUnicode('xn--iñvalid.com'));
    // Prints an empty string
    const url = require('node:url');

    console.log(url.domainToUnicode('xn--espaol-zwa.com'));
    // Prints español.com
    console.log(url.domainToUnicode('xn--fiq228c.com'));
    // Prints 中文.com
    console.log(url.domainToUnicode('xn--iñvalid.com'));
    // Prints an empty string
    javascript

#### `url.fileURLToPath(url[, options])`#

Added in: v10.12.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `options` argument can now be used to determine how to parse the `path` argument.

- `url` `<URL>` | `<string>` The file URL string or URL object to convert to a path.
- `options` `<Object>`
  - `windows` `<boolean>` | `<undefined>` `true` if the `path` should be return as a windows filepath, `false` for posix, and `undefined` for the system default. Default: `undefined`.
- Returns: `<string>` The fully-resolved platform-specific Node.js file path.

This function ensures the correct decodings of percent-encoded characters as well as ensuring a cross-platform valid absolute path string.

Security Considerations:

This function decodes percent-encoded characters, including encoded dot-segments (`%2e` as `.` and `%2e%2e` as `..`), and then normalizes the resulting path. This means that encoded directory traversal sequences (such as `%2e%2e`) are decoded and processed as actual path traversal, even though encoded slashes (`%2F`, `%5C`) are correctly rejected.

Applications must not rely on `fileURLToPath()` alone to prevent directory traversal attacks. Always perform explicit path validation and security checks on the returned path value to ensure it remains within expected boundaries before using it for file system operations.

    import { fileURLToPath } from 'node:url';

    const __filename = fileURLToPath(import.meta.url);

    new URL('file:///C:/path/').pathname;      // Incorrect: /C:/path/
    fileURLToPath('file:///C:/path/');         // Correct:   C:\path\ (Windows)

    new URL('file://nas/foo.txt').pathname;    // Incorrect: /foo.txt
    fileURLToPath('file://nas/foo.txt');       // Correct:   \\nas\foo.txt (Windows)

    new URL('file:///你好.txt').pathname;      // Incorrect: /%E4%BD%A0%E5%A5%BD.txt
    fileURLToPath('file:///你好.txt');         // Correct:   /你好.txt (POSIX)

    new URL('file:///hello world').pathname;   // Incorrect: /hello%20world
    fileURLToPath('file:///hello world');      // Correct:   /hello world (POSIX)
    const { fileURLToPath } = require('node:url');
    new URL('file:///C:/path/').pathname;      // Incorrect: /C:/path/
    fileURLToPath('file:///C:/path/');         // Correct:   C:\path\ (Windows)

    new URL('file://nas/foo.txt').pathname;    // Incorrect: /foo.txt
    fileURLToPath('file://nas/foo.txt');       // Correct:   \\nas\foo.txt (Windows)

    new URL('file:///你好.txt').pathname;      // Incorrect: /%E4%BD%A0%E5%A5%BD.txt
    fileURLToPath('file:///你好.txt');         // Correct:   /你好.txt (POSIX)

    new URL('file:///hello world').pathname;   // Incorrect: /hello%20world
    fileURLToPath('file:///hello world');      // Correct:   /hello world (POSIX)
    javascript

#### `url.fileURLToPathBuffer(url[, options])`#

- `url` `<URL>` | `<string>` The file URL string or URL object to convert to a path.
- `options` `<Object>`
  - `windows` `<boolean>` | `<undefined>` `true` if the `path` should be return as a windows filepath, `false` for posix, and `undefined` for the system default. Default: `undefined`.
- Returns: `<Buffer>` The fully-resolved platform-specific Node.js file path as a `<Buffer>`.

Like `url.fileURLToPath(...)` except that instead of returning a string representation of the path, a `Buffer` is returned. This conversion is helpful when the input URL contains percent-encoded segments that are not valid UTF-8 / Unicode sequences.

Security Considerations:

This function has the same security considerations as `url.fileURLToPath()`. It decodes percent-encoded characters, including encoded dot-segments (`%2e` as `.` and `%2e%2e` as `..`), and normalizes the path. Applications must not rely on this function alone to prevent directory traversal attacks. Always perform explicit path validation on the returned buffer value before using it for file system operations.

#### `url.format(URL[, options])`#

Added in: v7.6.0

- `URL` `<URL>` A WHATWG URL object
- `options` `<Object>`
  - `auth` `<boolean>` `true` if the serialized URL string should include the username and password, `false` otherwise. Default: `true`.
  - `fragment` `<boolean>` `true` if the serialized URL string should include the fragment, `false` otherwise. Default: `true`.
  - `search` `<boolean>` `true` if the serialized URL string should include the search query, `false` otherwise. Default: `true`.
  - `unicode` `<boolean>` `true` if Unicode characters appearing in the host component of the URL string should be encoded directly as opposed to being Punycode encoded. Default: `false`.
- Returns: `<string>`

Returns a customizable serialization of a URL `String` representation of a WHATWG URL object.

The URL object has both a `toString()` method and `href` property that return string serializations of the URL. These are not, however, customizable in any way. The `url.format(URL[, options])` method allows for basic customization of the output.

    import url from 'node:url';
    const myURL = new URL('https://a:b@測試?abc#foo');

    console.log(myURL.href);
    // Prints https://a:b@xn--g6w251d/?abc#foo

    console.log(myURL.toString());
    // Prints https://a:b@xn--g6w251d/?abc#foo

    console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
    // Prints 'https://測試/?abc'
    const url = require('node:url');
    const myURL = new URL('https://a:b@測試?abc#foo');

    console.log(myURL.href);
    // Prints https://a:b@xn--g6w251d/?abc#foo

    console.log(myURL.toString());
    // Prints https://a:b@xn--g6w251d/?abc#foo

    console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
    // Prints 'https://測試/?abc'
    javascript

#### `url.pathToFileURL(path[, options])`#

Added in: v10.12.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `options` argument can now be used to determine how to return the `path` value.

- `path` `<string>` The path to convert to a File URL.
- `options` `<Object>`
  - `windows` `<boolean>` | `<undefined>` `true` if the `path` should be treated as a windows filepath, `false` for posix, and `undefined` for the system default. Default: `undefined`.
- Returns: `<URL>` The file URL object.

This function ensures that `path` is resolved absolutely, and that the URL control characters are correctly encoded when converting into a File URL.

    import { pathToFileURL } from 'node:url';

    new URL('/foo#1', 'file:');           // Incorrect: file:///foo#1
    pathToFileURL('/foo#1');              // Correct:   file:///foo%231 (POSIX)

    new URL('/some/path%.c', 'file:');    // Incorrect: file:///some/path%.c
    pathToFileURL('/some/path%.c');       // Correct:   file:///some/path%25.c (POSIX)
    const { pathToFileURL } = require('node:url');
    new URL(__filename);                  // Incorrect: throws (POSIX)
    new URL(__filename);                  // Incorrect: C:\... (Windows)
    pathToFileURL(__filename);            // Correct:   file:///... (POSIX)
    pathToFileURL(__filename);            // Correct:   file:///C:/... (Windows)

    new URL('/foo#1', 'file:');           // Incorrect: file:///foo#1
    pathToFileURL('/foo#1');              // Correct:   file:///foo%231 (POSIX)

    new URL('/some/path%.c', 'file:');    // Incorrect: file:///some/path%.c
    pathToFileURL('/some/path%.c');       // Correct:   file:///some/path%25.c (POSIX)
    javascript

#### `url.urlToHttpOptions(url)`#

Added in: v15.7.0, v14.18.0
History

Version | Changes
--- | ---
v19.9.0, v18.17.0 | The returned object will also contain all the own enumerable properties of the `url` argument.

- `url` `<URL>` The WHATWG URL object to convert to an options object.
- Returns: `<Object>` Options object
  - `protocol` `<string>` Protocol to use.
  - `hostname` `<string>` A domain name or IP address of the server to issue the request to.
  - `hash` `<string>` The fragment portion of the URL.
  - `search` `<string>` The serialized query portion of the URL.
  - `pathname` `<string>` The path portion of the URL.
  - `path` `<string>` Request path. Should include query string if any. E.G. `'/index.html?page=12'`. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future.
  - `href` `<string>` The serialized URL.
  - `port` `<number>` Port of remote server.
  - `auth` `<string>` Basic authentication i.e. `'user:password'` to compute an Authorization header.

This utility function converts a URL object into an ordinary options object as expected by the `http.request()` and `https.request()` APIs.

    import { urlToHttpOptions } from 'node:url';
    const myURL = new URL('https://a:b@測試?abc#foo');

    console.log(urlToHttpOptions(myURL));
    /*
    {
      protocol: 'https:',
      hostname: 'xn--g6w251d',
      hash: '#foo',
      search: '?abc',
      pathname: '/',
      path: '/?abc',
      href: 'https://a:b@xn--g6w251d/?abc#foo',
      auth: 'a:b'
    }
    */
    const { urlToHttpOptions } = require('node:url');
    const myURL = new URL('https://a:b@測試?abc#foo');

    console.log(urlToHttpOptions(myURL));
    /*
    {
      protocol: 'https:',
      hostname: 'xn--g6w251d',
      hash: '#foo',
      search: '?abc',
      pathname: '/',
      path: '/?abc',
      href: 'https://a:b@xn--g6w251d/?abc#foo',
      auth: 'a:b'
    }
    */
    javascript

### Legacy URL API#

History

Version | Changes
--- | ---
v15.13.0, v14.17.0 | Deprecation revoked. Status changed to "Legacy".
v11.0.0 | This API is deprecated.

Stability: 3 - Legacy: Use the WHATWG URL API instead.

#### Legacy `urlObject`#

History

Version | Changes
--- | ---
v15.13.0, v14.17.0 | Deprecation revoked. Status changed to "Legacy".
v11.0.0 | The Legacy URL API is deprecated. Use the WHATWG URL API.

The legacy `urlObject` (`require('node:url').Url` or `import { Url } from 'node:url'`) is created and returned by the `url.parse()` function.

##### `urlObject.auth`#

The `auth` property is the username and password portion of the URL, also referred to as userinfo. This string subset follows the `protocol` and double slashes (if present) and precedes the `host` component, delimited by `@`. The string is either the username, or it is the username and password separated by `:`.

For example: `'user:pass'`.

##### `urlObject.hash`#

The `hash` property is the fragment identifier portion of the URL including the leading `#` character.

For example: `'#hash'`.

##### `urlObject.host`#

The `host` property is the full lower-cased host portion of the URL, including the `port` if specified.

For example: `'sub.example.com:8080'`.

##### `urlObject.hostname`#

The `hostname` property is the lower-cased host name portion of the `host` component without the `port` included.

For example: `'sub.example.com'`.

##### `urlObject.href`#

The `href` property is the full URL string that was parsed with both the `protocol` and `host` components converted to lower-case.

For example: `'http://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash'`.

##### `urlObject.path`#

The `path` property is a concatenation of the `pathname` and `search` components.

For example: `'/p/a/t/h?query=string'`.

No decoding of the `path` is performed.

##### `urlObject.pathname`#

The `pathname` property consists of the entire path section of the URL. This is everything following the `host` (including the `port`) and before the start of the `query` or `hash` components, delimited by either the ASCII question mark (`?`) or hash (`#`) characters.

For example: `'/p/a/t/h'`.

No decoding of the path string is performed.

##### `urlObject.port`#

The `port` property is the numeric port portion of the `host` component.

For example: `'8080'`.

##### `urlObject.protocol`#

The `protocol` property identifies the URL's lower-cased protocol scheme.

For example: `'http:'`.

##### `urlObject.query`#

The `query` property is either the query string without the leading ASCII question mark (`?`), or an object returned by the `querystring` module's `parse()` method. Whether the `query` property is a string or object is determined by the `parseQueryString` argument passed to `url.parse()`.

For example: `'query=string'` or `{'query': 'string'}`.

If returned as a string, no decoding of the query string is performed. If returned as an object, both keys and values are decoded.

##### `urlObject.search`#

The `search` property consists of the entire "query string" portion of the URL, including the leading ASCII question mark (`?`) character.

For example: `'?query=string'`.

No decoding of the query string is performed.

##### `urlObject.slashes`#

The `slashes` property is a `boolean` with a value of `true` if two ASCII forward-slash characters (`/`) are required following the colon in the `protocol`.

#### `url.format(urlObject)`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v17.0.0 | Now throws an `ERR_INVALID_URL` exception when Punycode conversion of a hostname introduces changes that could cause the URL to be re-parsed differently.
v15.13.0, v14.17.0 | Deprecation revoked. Status changed to "Legacy".
v11.0.0 | The Legacy URL API is deprecated. Use the WHATWG URL API.
v7.0.0 | URLs with a `file:` scheme will now always use the correct number of slashes regardless of `slashes` option. A falsy `slashes` option with no protocol is now also respected at all times.

- `urlObject` `<Object>` A URL object (as returned by `url.parse()` or constructed otherwise).
- Returns: `<string>`

The `url.format()` method returns a formatted URL string derived from `urlObject`.

    const url = require('node:url');
    url.format({
      protocol: 'https',
      hostname: 'example.com',
      pathname: '/some/path',
      query: {
        page: 1,
        format: 'json',
      },
    });

    // => 'https://example.com/some/path?page=1&format=json'
    js

If `urlObject` is not an object or a string, `url.format()` will throw a `TypeError`.

The formatting process operates as follows:

- A new empty string `result` is created.
- If `urlObject.protocol` is a string, it is appended as-is to `result`.
- Otherwise, if `urlObject.protocol` is not `undefined` and is not a string, an `Error` is thrown.
- For all string values of `urlObject.protocol` that do not end with an ASCII colon (`:`) character, the literal string `:` will be appended to `result`.
- If either of the following conditions is true, then the literal string `//` will be appended to `result`:
  - `urlObject.slashes` property is true;
  - `urlObject.protocol` begins with `http`, `https`, `ftp`, `gopher`, or `file`;
- If the value of the `urlObject.auth` property is truthy, and either `urlObject.host` or `urlObject.hostname` are not `undefined`, the value of `urlObject.auth` will be coerced into a string and appended to `result` followed by the literal string `@`.
- If the `urlObject.host` property is `undefined` then:
  - If the `urlObject.hostname` is a string, it is appended to `result`.
  - Otherwise, if `urlObject.hostname` is not `undefined` and is not a string, an `Error` is thrown.
  - If the `urlObject.port` property value is truthy, and `urlObject.hostname` is not `undefined`:
    - The literal string `:` is appended to `result`, and
    - The value of `urlObject.port` is coerced to a string and appended to `result`.
- Otherwise, if the `urlObject.host` property value is truthy, the value of `urlObject.host` is coerced to a string and appended to `result`.
- If the `urlObject.pathname` property is a string that is not an empty string:
  - If the `urlObject.pathname` does not start with an ASCII forward slash (`/`), then the literal string `'/'` is appended to `result`.
  - The value of `urlObject.pathname` is appended to `result`.
- Otherwise, if `urlObject.pathname` is not `undefined` and is not a string, an `Error` is thrown.
- If the `urlObject.search` property is `undefined` and if the `urlObject.query` property is an `Object`, the literal string `?` is appended to `result` followed by the output of calling the `querystring` module's `stringify()` method passing the value of `urlObject.query`.
- Otherwise, if `urlObject.search` is a string:
  - If the value of `urlObject.search` does not start with the ASCII question mark (`?`) character, the literal string `?` is appended to `result`.
  - The value of `urlObject.search` is appended to `result`.
- Otherwise, if `urlObject.search` is not `undefined` and is not a string, an `Error` is thrown.
- If the `urlObject.hash` property is a string:
  - If the value of `urlObject.hash` does not start with the ASCII hash (`#`) character, the literal string `#` is appended to `result`.
  - The value of `urlObject.hash` is appended to `result`.
- Otherwise, if the `urlObject.hash` property is not `undefined` and is not a string, an `Error` is thrown.
- `result` is returned.

An automated migration is available (source).

    npx codemod@latest @nodejs/node-url-to-whatwg-url
    bash

#### `url.format(urlString)`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v24.0.0 | Application deprecation.

Stability: 0 - Deprecated: Use the WHATWG URL API instead.

- `urlString` `<string>` A string that will be passed to `url.parse()` and then formatted.
- Returns: `<string>`

`url.format(urlString)` is shorthand for `url.format(url.parse(urlString))`.

Because it invokes the deprecated `url.parse()` internally, passing a string argument to `url.format()` is itself deprecated.

Canonicalizing a URL string can be performed using the WHATWG URL API, by constructing a new URL object and calling `url.toString()`.

    import { URL } from 'node:url';

    const unformatted = 'http://[fe80:0:0:0:0:0:0:1]:/a/b?a=b#abc';
    const formatted = new URL(unformatted).toString();

    console.log(formatted); // Prints: http://[fe80::1]/a/b?a=b#abc
    const { URL } = require('node:url');

    const unformatted = 'http://[fe80:0:0:0:0:0:0:1]:/a/b?a=b#abc';
    const formatted = new URL(unformatted).toString();

    console.log(formatted); // Prints: http://[fe80::1]/a/b?a=b#abc
    javascript

#### `url.parse(urlString[, parseQueryString[, slashesDenoteHost]])`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v24.0.0 | Application deprecation.
v19.9.0, v18.17.0 | Added support for `--pending-deprecation`.
v19.0.0, v18.13.0 | Documentation-only deprecation.
v15.13.0, v14.17.0 | Deprecation revoked. Status changed to "Legacy".
v11.14.0 | The `pathname` property on the returned URL object is now `/` when there is no path and the protocol scheme is `ws:` or `wss:`.
v11.0.0 | The Legacy URL API is deprecated. Use the WHATWG URL API.
v9.0.0 | The `search` property on the returned URL object is now `null` when no query string is present.

Stability: 0 - Deprecated: Use the WHATWG URL API instead.

- `urlString` `<string>` The URL string to parse.
- `parseQueryString` `<boolean>` If `true`, the `query` property will always be set to an object returned by the `querystring` module's `parse()` method. If `false`, the `query` property on the returned URL object will be an unparsed, undecoded string. Default: `false`.
- `slashesDenoteHost` `<boolean>` If `true`, the first token after the literal string `//` and preceding the next `/` will be interpreted as the `host`. For instance, given `//foo/bar`, the result would be `{host: 'foo', pathname: '/bar'}` rather than `{pathname: '//foo/bar'}`. Default: `false`.

The `url.parse()` method takes a URL string, parses it, and returns a URL object.

A `TypeError` is thrown if `urlString` is not a string.

A `URIError` is thrown if the `auth` property is present but cannot be decoded.

`url.parse()` uses a lenient, non-standard algorithm for parsing URL strings. It is prone to security issues such as host name spoofing and incorrect handling of usernames and passwords. Do not use with untrusted input. CVEs are not issued for `url.parse()` vulnerabilities. Use the WHATWG URL API instead, for example:

    function getURL(req) {
      const proto = req.headers['x-forwarded-proto'] || 'https';
      const host = req.headers['x-forwarded-host'] || req.headers.host || 'example.com';
      return new URL(`${proto}://${host}${req.url || '/'}`);
    }
    js

The example above assumes well-formed headers are forwarded from a reverse proxy to your Node.js server. If you are not using a reverse proxy, you should use the example below:

    function getURL(req) {
      return new URL(`https://example.com${req.url || '/'}`);
    }
    js

An automated migration is available (source).

    npx codemod@latest @nodejs/node-url-to-whatwg-url
    bash

#### `url.resolve(from, to)`#

Added in: v0.1.25
History

Version | Changes
--- | ---
v24.0.0 | Deprecated again through DEP0169.
v15.13.0, v14.17.0 | Deprecation revoked. Status changed to "Legacy".
v11.0.0 | The Legacy URL API is deprecated. Use the WHATWG URL API.
v6.6.0 | The `auth` fields are now kept intact when `from` and `to` refer to the same host.
v6.5.0, v4.6.2 | The `port` field is copied correctly now.
v6.0.0 | The `auth` fields is cleared now the `to` parameter contains a hostname.

Stability: 0 - Deprecated: Use the WHATWG URL API instead.

- `from` `<string>` The base URL to use if `to` is a relative URL.
- `to` `<string>` The target URL to resolve.

The `url.resolve()` method resolves a target URL relative to a base URL in a manner similar to that of a web browser resolving an anchor tag.

    const url = require('node:url');
    url.resolve('/one/two/three', 'four');         // '/one/two/four'
    url.resolve('http://example.com/', '/one');    // 'http://example.com/one'
    url.resolve('http://example.com/one', '/two'); // 'http://example.com/two'
    js

Because it invokes the deprecated `url.parse()` internally, `url.resolve()` is itself deprecated.

To achieve the same result using the WHATWG URL API:

    function resolve(from, to) {
      const resolvedUrl = new URL(to, new URL(from, 'resolve://'));
      if (resolvedUrl.protocol === 'resolve:') {
        // `from` is a relative URL.
        const { pathname, search, hash } = resolvedUrl;
        return pathname + search + hash;
      }
      return resolvedUrl.toString();
    }

    resolve('/one/two/three', 'four');         // '/one/two/four'
    resolve('http://example.com/', '/one');    // 'http://example.com/one'
    resolve('http://example.com/one', '/two'); // 'http://example.com/two'
    js

### Percent-encoding in URLs#

URLs are permitted to only contain a certain range of characters. Any character falling outside of that range must be encoded. How such characters are encoded, and which characters to encode depends entirely on where the character is located within the structure of the URL.

#### Legacy API#

Within the Legacy API, spaces (`' '`) and the following characters will be automatically escaped in the properties of URL objects:

    < > " ` \r \n \t { } | \ ^ '
    text

For example, the ASCII space character (`' '`) is encoded as `%20`. The ASCII forward slash (`/`) character is encoded as `%3C`.

#### WHATWG API#

The WHATWG URL Standard uses a more selective and fine grained approach to selecting encoded characters than that used by the Legacy API.

The WHATWG algorithm defines four "percent-encode sets" that describe ranges of characters that must be percent-encoded:

- The C0 control percent-encode set includes code points in range U+0000 to U+001F (inclusive) and all code points greater than U+007E (~).

- The fragment percent-encode set includes the C0 control percent-encode set and code points U+0020 SPACE, U+0022 ("), U+003C (`<>`<`<), U+003E (>`>), and U+0060 (`).

- The path percent-encode set includes the C0 control percent-encode set and code points U+0020 SPACE, U+0022 ("), U+0023 (#), U+003C (`<>`<`<), U+003E (>`

  ), U+003F (?), U+0060 (`), U+007B ({), and U+007D (}).

- The userinfo encode set includes the path percent-encode set and code points U+002F (/), U+003A (:), U+003B (;), U+003D (=), U+0040 (@), U+005B ([) to U+005E(^), and U+007C (|).

The userinfo percent-encode set is used exclusively for username and passwords encoded within the URL. The path percent-encode set is used for the path of most URLs. The fragment percent-encode set is used for URL fragments. The C0 control percent-encode set is used for host and path under certain specific conditions, in addition to all other cases.

When non-ASCII characters appear within a host name, the host name is encoded using the Punycode algorithm. Note, however, that a host name may contain both Punycode encoded and percent-encoded characters:

    const myURL = new URL('https://%CF%80.example.com/foo');
    console.log(myURL.href);
    // Prints https://xn--1xa.example.com/foo
    console.log(myURL.origin);
    // Prints https://xn--1xa.example.com
    js

## Usage and example#

### Usage#

`node [options] [V8 options] [script.js | -e "script" | - ] [arguments]`

Please see the Command-line options document for more information.

### Example#

An example of a web server written with Node.js which responds with `'Hello, World!'`:

Commands in this document start with `$` or `>` to replicate how they would appear in a user's terminal. Do not include the `$` and `>` characters. They are there to show the start of each command.

Lines that don't start with `$` or `>` character show the output of the previous command.

First, make sure to have downloaded and installed Node.js. See Installing Node.js via package manager for further install information.

Now, create an empty project folder called `projects`, then navigate into it.

Linux and Mac:

    mkdir ~/projects
    cd ~/projects
    bash

Windows CMD:

    mkdir %USERPROFILE%\projects
    cd %USERPROFILE%\projects
    powershell

Windows PowerShell:

    mkdir $env:USERPROFILE\projects
    cd $env:USERPROFILE\projects
    powershell

Next, create a new source file in the `projects` folder and call it `hello-world.js`.

Open `hello-world.js` in any preferred text editor and paste in the following content:

    const http = require('node:http');

    const hostname = '127.0.0.1';
    const port = 3000;

    const server = http.createServer((req, res) => {
      res.statusCode = 200;
      res.setHeader('Content-Type', 'text/plain');
      res.end('Hello, World!\n');
    });

    server.listen(port, hostname, () => {
      console.log(`Server running at http://${hostname}:${port}/`);
    });
    js

Save the file. Then, in the terminal window, to run the `hello-world.js` file, enter:

    node hello-world.js
    bash

Output like this should appear in the terminal:

    Server running at http://127.0.0.1:3000/
    console

Now, open any preferred web browser and visit `http://127.0.0.1:3000`.

If the browser displays the string `Hello, World!`, that indicates the server is working.

## Util#

Source Code: lib/util.js

Stability: 2 - Stable

The `node:util` module supports the needs of Node.js internal APIs. Many of the utilities are useful for application and module developers as well. To access it:

    import util from 'node:util';
    const util = require('node:util');
    javascript

### `util.callbackify(original)`#

Added in: v8.2.0

- `original` `<Function>` An `async` function
- Returns: `<Function>` a callback style function

Takes an `async` function (or a function that returns a `Promise`) and returns a function following the error-first callback style, i.e. taking an `(err, value) => ...` callback as the last argument. In the callback, the first argument will be the rejection reason (or `null` if the `Promise` resolved), and the second argument will be the resolved value.

    import { callbackify } from 'node:util';

    async function fn() {
      return 'hello world';
    }
    const callbackFunction = callbackify(fn);

    callbackFunction((err, ret) => {
      if (err) throw err;
      console.log(ret);
    });
    const { callbackify } = require('node:util');

    async function fn() {
      return 'hello world';
    }
    const callbackFunction = callbackify(fn);

    callbackFunction((err, ret) => {
      if (err) throw err;
      console.log(ret);
    });
    javascript

Will print:

    hello world
    text

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an `'uncaughtException'` event, and if not handled will exit.

Since `null` has a special meaning as the first argument to a callback, if a wrapped function rejects a `Promise` with a falsy value as a reason, the value is wrapped in an `Error` with the original value stored in a field named `reason`.

    import util from 'node:util';

    function fn() {
      return Promise.reject(null);
    }
    const callbackFunction = util.callbackify(fn);

    callbackFunction((err, ret) => {
      // When the Promise was rejected with `null` it is wrapped with an Error and
      // the original value is stored in `reason`.
      err && Object.hasOwn(err, 'reason') && err.reason === null;  // true
    });
    const util = require('node:util');

    function fn() {
      return Promise.reject(null);
    }
    const callbackFunction = util.callbackify(fn);

    callbackFunction((err, ret) => {
      // When the Promise was rejected with `null` it is wrapped with an Error and
      // the original value is stored in `reason`.
      err && Object.hasOwn(err, 'reason') && err.reason === null;  // true
    });
    javascript

### `util.convertProcessSignalToExitCode(signal)`#

Added in: v25.4.0, v24.14.0

- `signal` `<string>` A signal name (e.g. `'SIGTERM'`)
- Returns: `<number>` The exit code corresponding to `signal`

The `util.convertProcessSignalToExitCode()` method converts a signal name to its corresponding POSIX exit code. Following the POSIX standard, the exit code for a process terminated by a signal is calculated as `128 + signal number`.

If `signal` is not a valid signal name, then an error will be thrown. See `signal(7)` for a list of valid signals.

    import { convertProcessSignalToExitCode } from 'node:util';

    console.log(convertProcessSignalToExitCode('SIGTERM')); // 143 (128 + 15)
    console.log(convertProcessSignalToExitCode('SIGKILL')); // 137 (128 + 9)
    const { convertProcessSignalToExitCode } = require('node:util');

    console.log(convertProcessSignalToExitCode('SIGTERM')); // 143 (128 + 15)
    console.log(convertProcessSignalToExitCode('SIGKILL')); // 137 (128 + 9)
    javascript

This is particularly useful when working with processes to determine the exit code based on the signal that terminated the process.

### `util.debuglog(section[, callback])`#

Added in: v0.11.3

- `section` `<string>` A string identifying the portion of the application for which the `debuglog` function is being created.
- `callback` `<Function>` A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.
- Returns: `<Function>` The logging function

The `util.debuglog()` method is used to create a function that conditionally writes debug messages to `stderr` based on the existence of the `NODE_DEBUG` environment variable. If the `section` name appears within the value of that environment variable, then the returned function operates similar to `console.error()`. If not, then the returned function is a no-op.

    import { debuglog } from 'node:util';
    const log = debuglog('foo');

    log('hello from foo [%d]', 123);
    const { debuglog } = require('node:util');
    const log = debuglog('foo');

    log('hello from foo [%d]', 123);
    javascript

If this program is run with `NODE_DEBUG=foo` in the environment, then it will output something like:

    FOO 3245: hello from foo [123]
    console

where `3245` is the process id. If it is not run with that environment variable set, then it will not print anything.

The `section` supports wildcard also:

    import { debuglog } from 'node:util';
    const log = debuglog('foo-bar');

    log('hi there, it\'s foo-bar [%d]', 2333);
    const { debuglog } = require('node:util');
    const log = debuglog('foo-bar');

    log('hi there, it\'s foo-bar [%d]', 2333);
    javascript

if it is run with `NODE_DEBUG=foo*` in the environment, then it will output something like:

    FOO-BAR 3257: hi there, it's foo-bar [2333]
    console

Multiple comma-separated `section` names may be specified in the `NODE_DEBUG` environment variable: `NODE_DEBUG=fs,net,tls`.

The optional `callback` argument can be used to replace the logging function with a different function that doesn't have any initialization or unnecessary wrapping.

    import { debuglog } from 'node:util';
    let log = debuglog('internals', (debug) => {
      // Replace with a logging function that optimizes out
      // testing if the section is enabled
      log = debug;
    });
    const { debuglog } = require('node:util');
    let log = debuglog('internals', (debug) => {
      // Replace with a logging function that optimizes out
      // testing if the section is enabled
      log = debug;
    });
    javascript

#### `debuglog().enabled`#

Added in: v14.9.0

- Type: `<boolean>`

The `util.debuglog().enabled` getter is used to create a test that can be used in conditionals based on the existence of the `NODE_DEBUG` environment variable. If the `section` name appears within the value of that environment variable, then the returned value will be `true`. If not, then the returned value will be `false`.

    import { debuglog } from 'node:util';
    const enabled = debuglog('foo').enabled;
    if (enabled) {
      console.log('hello from foo [%d]', 123);
    }
    const { debuglog } = require('node:util');
    const enabled = debuglog('foo').enabled;
    if (enabled) {
      console.log('hello from foo [%d]', 123);
    }
    javascript

If this program is run with `NODE_DEBUG=foo` in the environment, then it will output something like:

    hello from foo [123]
    console

### `util.debug(section)`#

Added in: v14.9.0

Alias for `util.debuglog`. Usage allows for readability of that doesn't imply logging when only using `util.debuglog().enabled`.

### `util.deprecate(fn, msg[, code[, options]])`#

Added in: v0.8.0
History

Version | Changes
--- | ---
v25.2.0, v24.12.0 | Add options object with modifyPrototype to conditionally modify the prototype of the deprecated object.
v10.0.0 | Deprecation warnings are only emitted once for each code.

- `fn` `<Function>` The function that is being deprecated.
- `msg` `<string>` A warning message to display when the deprecated function is invoked.
- `code` `<string>` A deprecation code. See the list of deprecated APIs for a list of codes.
- `options` `<Object>`
  - `modifyPrototype` `<boolean>` When false do not change the prototype of object while emitting the deprecation warning. Default: `true`.
- Returns: `<Function>` The deprecated function wrapped to emit a warning.

The `util.deprecate()` method wraps `fn` (which may be a function or class) in such a way that it is marked as deprecated.

    import { deprecate } from 'node:util';

    export const obsoleteFunction = deprecate(() => {
      // Do something here.
    }, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');
    const { deprecate } = require('node:util');

    exports.obsoleteFunction = deprecate(() => {
      // Do something here.
    }, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');
    javascript

When called, `util.deprecate()` will return a function that will emit a `DeprecationWarning` using the `'warning'` event. The warning will be emitted and printed to `stderr` the first time the returned function is called. After the warning is emitted, the wrapped function is called without emitting a warning.

If the same optional `code` is supplied in multiple calls to `util.deprecate()`, the warning will be emitted only once for that `code`.

    import { deprecate } from 'node:util';

    const fn1 = deprecate(
      () => 'a value',
      'deprecation message',
      'DEP0001',
    );
    const fn2 = deprecate(
      () => 'a  different value',
      'other dep message',
      'DEP0001',
    );
    fn1(); // Emits a deprecation warning with code DEP0001
    fn2(); // Does not emit a deprecation warning because it has the same code
    const { deprecate } = require('node:util');

    const fn1 = deprecate(
      function() {
        return 'a value';
      },
      'deprecation message',
      'DEP0001',
    );
    const fn2 = deprecate(
      function() {
        return 'a  different value';
      },
      'other dep message',
      'DEP0001',
    );
    fn1(); // Emits a deprecation warning with code DEP0001
    fn2(); // Does not emit a deprecation warning because it has the same code
    javascript

If either the `--no-deprecation` or `--no-warnings` command-line flags are used, or if the `process.noDeprecation` property is set to `true` prior to the first deprecation warning, the `util.deprecate()` method does nothing.

If the `--trace-deprecation` or `--trace-warnings` command-line flags are set, or the `process.traceDeprecation` property is set to `true`, a warning and a stack trace are printed to `stderr` the first time the deprecated function is called.

If the `--throw-deprecation` command-line flag is set, or the `process.throwDeprecation` property is set to `true`, then an exception will be thrown when the deprecated function is called.

The `--throw-deprecation` command-line flag and `process.throwDeprecation` property take precedence over `--trace-deprecation` and `process.traceDeprecation`.

### `util.diff(actual, expected)`#

Added in: v23.11.0, v22.15.0

Stability: 1 - Experimental

- `actual` `<Array>` | `<string>` The first value to compare

- `expected` `<Array>` | `<string>` The second value to compare

- Returns: `<Array>` An array of difference entries. Each entry is an array with two elements:

  - `0` `<number>` Operation code: `-1` for delete, `0` for no-op/unchanged, `1` for insert
  - `1` `<string>` The value associated with the operation

- Algorithm complexity: O(N*D), where:

- N is the total length of the two sequences combined (N = actual.length + expected.length)

- D is the edit distance (the minimum number of operations required to transform one sequence into the other).

`util.diff()` compares two string or array values and returns an array of difference entries. It uses the Myers diff algorithm to compute minimal differences, which is the same algorithm used internally by assertion error messages.

If the values are equal, an empty array is returned.

    const { diff } = require('node:util');

    // Comparing strings
    const actualString = '12345678';
    const expectedString = '12!!5!7!';
    console.log(diff(actualString, expectedString));
    // [
    //   [0, '1'],
    //   [0, '2'],
    //   [1, '3'],
    //   [1, '4'],
    //   [-1, '!'],
    //   [-1, '!'],
    //   [0, '5'],
    //   [1, '6'],
    //   [-1, '!'],
    //   [0, '7'],
    //   [1, '8'],
    //   [-1, '!'],
    // ]
    // Comparing arrays
    const actualArray = ['1', '2', '3'];
    const expectedArray = ['1', '3', '4'];
    console.log(diff(actualArray, expectedArray));
    // [
    //   [0, '1'],
    //   [1, '2'],
    //   [0, '3'],
    //   [-1, '4'],
    // ]
    // Equal values return empty array
    console.log(diff('same', 'same'));
    // []
    js

### `util.format(format[, ...args])`#

Added in: v0.5.3
History

Version | Changes
--- | ---
v12.11.0 | The `%c` specifier is ignored now.
v12.0.0 | The `format` argument is now only taken as such if it actually contains format specifiers.
v12.0.0 | If the `format` argument is not a format string, the output string's formatting is no longer dependent on the type of the first argument. This change removes previously present quotes from strings that were being output when the first argument was not a string.
v11.4.0 | The `%d`, `%f`, and `%i` specifiers now support Symbols properly.
v11.4.0 | The `%o` specifier's `depth` has default depth of 4 again.
v11.0.0 | The `%o` specifier's `depth` option will now fall back to the default depth.
v10.12.0 | The `%d` and `%i` specifiers now support BigInt.
v8.4.0 | The `%o` and `%O` specifiers are supported now.

- `format` `<string>` A `printf`-like format string.

The `util.format()` method returns a formatted string using the first argument as a `printf`-like format string which can contain zero or more format specifiers. Each specifier is replaced with the converted value from the corresponding argument. Supported specifiers are:

- `%s`: `String` will be used to convert all values except `BigInt`, `Object` and `-0`. `BigInt` values will be represented with an `n` and Objects that have neither a user defined `toString` function nor `Symbol.toPrimitive` function are inspected using `util.inspect()` with options `{ depth: 0, colors: false, compact: 3 }`.
- `%d`: `Number` will be used to convert all values except `BigInt` and `Symbol`.
- `%i`: `parseInt(value, 10)` is used for all values except `BigInt` and `Symbol`.
- `%f`: `parseFloat(value)` is used for all values except `Symbol`.
- `%j`: JSON. Replaced with the string `'[Circular]'` if the argument contains circular references.
- `%o`: `Object`. A string representation of an object with generic JavaScript object formatting. Similar to `util.inspect()` with options `{ showHidden: true, showProxy: true }`. This will show the full object including non-enumerable properties and proxies.
- `%O`: `Object`. A string representation of an object with generic JavaScript object formatting. Similar to `util.inspect()` without options. This will show the full object not including non-enumerable properties and proxies.
- `%c`: `CSS`. This specifier is ignored and will skip any CSS passed in.
- `%%`: single percent sign (`'%'`). This does not consume an argument.
- Returns: `<string>` The formatted string

If a specifier does not have a corresponding argument, it is not replaced:

    util.format('%s:%s', 'foo');
    // Returns: 'foo:%s'
    js

Values that are not part of the format string are formatted using `util.inspect()` if their type is not `string`.

If there are more arguments passed to the `util.format()` method than the number of specifiers, the extra arguments are concatenated to the returned string, separated by spaces:

    util.format('%s:%s', 'foo', 'bar', 'baz');
    // Returns: 'foo:bar baz'
    js

If the first argument does not contain a valid format specifier, `util.format()` returns a string that is the concatenation of all arguments separated by spaces:

    util.format(1, 2, 3);
    // Returns: '1 2 3'
    js

If only one argument is passed to `util.format()`, it is returned as it is without any formatting:

    util.format('%% %s');
    // Returns: '%% %s'
    js

`util.format()` is a synchronous method that is intended as a debugging tool. Some input values can have a significant performance overhead that can block the event loop. Use this function with care and never in a hot code path.

### `util.formatWithOptions(inspectOptions, format[, ...args])`#

Added in: v10.0.0

- `inspectOptions` `<Object>`
- `format` `<string>`

This function is identical to `util.format()`, except in that it takes an `inspectOptions` argument which specifies options that are passed along to `util.inspect()`.

    util.formatWithOptions({ colors: true }, 'See object %O', { foo: 42 });
    // Returns 'See object { foo: 42 }', where `42` is colored as a number
    // when printed to a terminal.
    js

### `util.getCallSites([frameCount][, options])`#

Added in: v22.9.0
History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Property `column` is deprecated in favor of `columnNumber`.
v23.7.0, v22.14.0 | Property `CallSite.scriptId` is exposed.
v23.3.0, v22.12.0 | The API is renamed from `util.getCallSite` to `util.getCallSites()`.

Stability: 1.1 - Active development

- `frameCount` `<integer>` Optional number of frames to capture as call site objects. Default: `10`. Allowable range is between 1 and 200.
- `options` `<Object>` Optional
  - `sourceMap` `<boolean>` Reconstruct the original location in the stacktrace from the source-map. Enabled by default with the flag `--enable-source-maps`.
- Returns: `<Object>`[] An array of call site objects
  - `functionName` `<string>` Returns the name of the function associated with this call site.
  - `scriptName` `<string>` Returns the name of the resource that contains the script for the function for this call site.
  - `scriptId` `<string>` Returns the unique id of the script, as in Chrome DevTools protocol `Runtime.ScriptId`.
  - `lineNumber` `<number>` Returns the JavaScript script line number (1-based).
  - `columnNumber` `<number>` Returns the JavaScript script column number (1-based).

Returns an array of call site objects containing the stack of the caller function.

Unlike accessing an `error.stack`, the result returned from this API is not interfered with `Error.prepareStackTrace`.

    import { getCallSites } from 'node:util';

    function exampleFunction() {
      const callSites = getCallSites();

      console.log('Call Sites:');
      callSites.forEach((callSite, index) => {
        console.log(`CallSite ${index + 1}:`);
        console.log(`Function Name: ${callSite.functionName}`);
        console.log(`Script Name: ${callSite.scriptName}`);
        console.log(`Line Number: ${callSite.lineNumber}`);
        console.log(`Column Number: ${callSite.columnNumber}`);
      });
      // CallSite 1:
      // Function Name: exampleFunction
      // Script Name: /home/example.js
      // Line Number: 5
      // Column Number: 26

      // CallSite 2:
      // Function Name: anotherFunction
      // Script Name: /home/example.js
      // Line Number: 22
      // Column Number: 3

      // ...
    }

    // A function to simulate another stack layer
    function anotherFunction() {
      exampleFunction();
    }

    anotherFunction();
    const { getCallSites } = require('node:util');

    function exampleFunction() {
      const callSites = getCallSites();

      console.log('Call Sites:');
      callSites.forEach((callSite, index) => {
        console.log(`CallSite ${index + 1}:`);
        console.log(`Function Name: ${callSite.functionName}`);
        console.log(`Script Name: ${callSite.scriptName}`);
        console.log(`Line Number: ${callSite.lineNumber}`);
        console.log(`Column Number: ${callSite.columnNumber}`);
      });
      // CallSite 1:
      // Function Name: exampleFunction
      // Script Name: /home/example.js
      // Line Number: 5
      // Column Number: 26

      // CallSite 2:
      // Function Name: anotherFunction
      // Script Name: /home/example.js
      // Line Number: 22
      // Column Number: 3

      // ...
    }

    // A function to simulate another stack layer
    function anotherFunction() {
      exampleFunction();
    }

    anotherFunction();
    javascript

It is possible to reconstruct the original locations by setting the option `sourceMap` to `true`. If the source map is not available, the original location will be the same as the current location. When the `--enable-source-maps` flag is enabled,`sourceMap` will be true by default.

    import { getCallSites } from 'node:util';

    interface Foo {
      foo: string;
    }

    const callSites = getCallSites({ sourceMap: true });

    // With sourceMap:
    // Function Name: ''
    // Script Name: example.js
    // Line Number: 7
    // Column Number: 26

    // Without sourceMap:
    // Function Name: ''
    // Script Name: example.js
    // Line Number: 2
    // Column Number: 26
    ts

    const { getCallSites } = require('node:util');

    const callSites = getCallSites({ sourceMap: true });

    // With sourceMap:
    // Function Name: ''
    // Script Name: example.js
    // Line Number: 7
    // Column Number: 26

    // Without sourceMap:
    // Function Name: ''
    // Script Name: example.js
    // Line Number: 2
    // Column Number: 26
    cjs

### `util.getSystemErrorName(err)`#

Added in: v9.7.0

- `err` `<number>`
- Returns: `<string>`

Returns the string name for a numeric error code that comes from a Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

    fs.access('file/that/does/not/exist', (err) => {
      const name = util.getSystemErrorName(err.errno);
      console.error(name);  // ENOENT
    });
    js

### `util.getSystemErrorMap()`#

Added in: v16.0.0, v14.17.0

- Returns: `<Map>`

Returns a Map of all system error codes available from the Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

    fs.access('file/that/does/not/exist', (err) => {
      const errorMap = util.getSystemErrorMap();
      const name = errorMap.get(err.errno);
      console.error(name);  // ENOENT
    });
    js

### `util.getSystemErrorMessage(err)`#

Added in: v23.1.0, v22.12.0

- `err` `<number>`
- Returns: `<string>`

Returns the string message for a numeric error code that comes from a Node.js API. The mapping between error codes and string messages is platform-dependent.

    fs.access('file/that/does/not/exist', (err) => {
      const message = util.getSystemErrorMessage(err.errno);
      console.error(message);  // No such file or directory
    });
    js

### `util.setTraceSigInt(enable)`#

Added in: v24.6.0, v22.19.0

- `enable` `<boolean>`

Enable or disable printing a stack trace on `SIGINT`. The API is only available on the main thread.

### `util.inherits(constructor, superConstructor)`#

Added in: v0.3.0
History

Version | Changes
--- | ---
v5.0.0 | The `constructor` parameter can refer to an ES6 class now.

Stability: 3 - Legacy: Use ES2015 class syntax and `extends` keyword instead.

- `constructor` `<Function>`
- `superConstructor` `<Function>`

Usage of `util.inherits()` is discouraged. Please use the ES6 `class` and `extends` keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

Inherit the prototype methods from one constructor into another. The prototype of `constructor` will be set to a new object created from `superConstructor`.

This mainly adds some input validation on top of `Object.setPrototypeOf(constructor.prototype, superConstructor.prototype)`. As an additional convenience, `superConstructor` will be accessible through the `constructor.super_` property.

    const util = require('node:util');
    const EventEmitter = require('node:events');

    function MyStream() {
      EventEmitter.call(this);
    }

    util.inherits(MyStream, EventEmitter);

    MyStream.prototype.write = function(data) {
      this.emit('data', data);
    };

    const stream = new MyStream();

    console.log(stream instanceof EventEmitter); // true
    console.log(MyStream.super_ === EventEmitter); // true

    stream.on('data', (data) => {
      console.log(`Received data: "${data}"`);
    });
    stream.write('It works!'); // Received data: "It works!"
    js

ES6 example using `class` and `extends`:

    import EventEmitter from 'node:events';

    class MyStream extends EventEmitter {
      write(data) {
        this.emit('data', data);
      }
    }

    const stream = new MyStream();

    stream.on('data', (data) => {
      console.log(`Received data: "${data}"`);
    });
    stream.write('With ES6');
    const EventEmitter = require('node:events');

    class MyStream extends EventEmitter {
      write(data) {
        this.emit('data', data);
      }
    }

    const stream = new MyStream();

    stream.on('data', (data) => {
      console.log(`Received data: "${data}"`);
    });
    stream.write('With ES6');
    javascript

### `util.inspect(object[, options])`#

### `util.inspect(object[, showHidden[, depth[, colors]]])`#

Added in: v0.3.0
History

Version | Changes
--- | ---
v25.0.0 | The util.inspect.styles.regexp style is now a method that is invoked for coloring the stringified regular expression.
v17.3.0, v16.14.0 | The `numericSeparator` option is supported now.
v16.18.0 | add support for `maxArrayLength` when inspecting `Set` and `Map`.
v14.6.0, v12.19.0 | If `object` is from a different `vm.Context` now, a custom inspection function on it will not receive context-specific arguments anymore.
v13.13.0, v12.17.0 | The `maxStringLength` option is supported now.
v13.5.0, v12.16.0 | User defined prototype properties are inspected in case `showHidden` is `true`.
v13.0.0 | Circular references now include a marker to the reference.
v12.0.0 | The `compact` options default is changed to `3` and the `breakLength` options default is changed to `80`.
v12.0.0 | Internal properties no longer appear in the context argument of a custom inspection function.
v11.11.0 | The `compact` option accepts numbers for a new output mode.
v11.7.0 | ArrayBuffers now also show their binary contents.
v11.5.0 | The `getters` option is supported now.
v11.4.0 | The `depth` default changed back to `2`.
v11.0.0 | The `depth` default changed to `20`.
v11.0.0 | The inspection output is now limited to about 128 MiB. Data above that size will not be fully inspected.
v10.12.0 | The `sorted` option is supported now.
v10.6.0 | Inspecting linked lists and similar objects is now possible up to the maximum call stack size.
v10.0.0 | The `WeakMap` and `WeakSet` entries can now be inspected as well.
v9.9.0 | The `compact` option is supported now.
v6.6.0 | Custom inspection functions can now return `this`.
v6.3.0 | The `breakLength` option is supported now.
v6.1.0 | The `maxArrayLength` option is supported now; in particular, long arrays are truncated by default.
v6.1.0 | The `showProxy` option is supported now.

- `object` `<any>` Any JavaScript primitive or `Object`.
- `options` `<Object>`
  - `showHidden` `<boolean>` If `true`, `object`'s non-enumerable symbols and properties are included in the formatted result. `<WeakMap>` and `<WeakSet>` entries are also included as well as user defined prototype properties (excluding method properties). Default: `false`.
  - `depth` `<number>` Specifies the number of times to recurse while formatting `object`. This is useful for inspecting large objects. To recurse up to the maximum call stack size pass `Infinity` or `null`. Default: `2`.
  - `colors` `<boolean>` If `true`, the output is styled with ANSI color codes. Colors are customizable. See Customizing `util.inspect` colors. Default: `false`.
  - `customInspect` `<boolean>` If `false`, `[util.inspect.custom](depth, opts, inspect)` functions are not invoked. Default: `true`.
  - `showProxy` `<boolean>` If `true`, `Proxy` inspection includes the `target` and `handler` objects. Default: `false`.
  - `maxArrayLength` `<integer>` Specifies the maximum number of `Array`, `<TypedArray>`, `<Map>`, `<WeakMap>`, and `<WeakSet>` elements to include when formatting. Set to `null` or `Infinity` to show all elements. Set to `0` or negative to show no elements. Default: `100`.
  - `maxStringLength` `<integer>` Specifies the maximum number of characters to include when formatting. Set to `null` or `Infinity` to show all elements. Set to `0` or negative to show no characters. Default: `10000`.
  - `breakLength` `<integer>` The length at which input values are split across multiple lines. Set to `Infinity` to format the input as a single line (in combination with `compact` set to `true` or any number >= `1`). Default: `80`.
  - `compact` `<boolean>` | `<integer>` Setting this to `false` causes each object key to be displayed on a new line. It will break on new lines in text that is longer than `breakLength`. If set to a number, the most `n` inner elements are united on a single line as long as all properties fit into `breakLength`. Short array elements are also grouped together. For more information, see the example below. Default: `3`.
  - `sorted` `<boolean>` | `<Function>` If set to `true` or a function, all properties of an object, and `Set` and `Map` entries are sorted in the resulting string. If set to `true` the default sort is used. If set to a function, it is used as a compare function.
  - `getters` `<boolean>` | `<string>` If set to `true`, getters are inspected. If set to `'get'`, only getters without a corresponding setter are inspected. If set to `'set'`, only getters with a corresponding setter are inspected. This might cause side effects depending on the getter function. Default: `false`.
  - `numericSeparator` `<boolean>` If set to `true`, an underscore is used to separate every three digits in all bigints and numbers. Default: `false`.
- Returns: `<string>` The representation of `object`.

The `util.inspect()` method returns a string representation of `object` that is intended for debugging. The output of `util.inspect` may change at any time and should not be depended upon programmatically. Additional `options` may be passed that alter the result. `util.inspect()` will use the constructor's name and/or `Symbol.toStringTag` property to make an identifiable tag for an inspected value.

    class Foo {
      get [Symbol.toStringTag]() {
        return 'bar';
      }
    }

    class Bar {}

    const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

    util.inspect(new Foo()); // 'Foo [bar] {}'
    util.inspect(new Bar()); // 'Bar {}'
    util.inspect(baz);       // '[foo] {}'
    js

Circular references point to their anchor by using a reference index:

    import { inspect } from 'node:util';

    const obj = {};
    obj.a = [obj];
    obj.b = {};
    obj.b.inner = obj.b;
    obj.b.obj = obj;

    console.log(inspect(obj));
    // <ref *1> {
    //   a: [ [Circular *1] ],
    //   b: <ref *2> { inner: [Circular *2], obj: [Circular *1] }
    // }
    const { inspect } = require('node:util');

    const obj = {};
    obj.a = [obj];
    obj.b = {};
    obj.b.inner = obj.b;
    obj.b.obj = obj;

    console.log(inspect(obj));
    // <ref *1> {
    //   a: [ [Circular *1] ],
    //   b: <ref *2> { inner: [Circular *2], obj: [Circular *1] }
    // }
    javascript

The following example inspects all properties of the `util` object:

    import util from 'node:util';

    console.log(util.inspect(util, { showHidden: true, depth: null }));
    const util = require('node:util');

    console.log(util.inspect(util, { showHidden: true, depth: null }));
    javascript

The following example highlights the effect of the `compact` option:

    import { inspect } from 'node:util';

    const o = {
      a: [1, 2, [[
        'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
          'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
        'test',
        'foo']], 4],
      b: new Map([['za', 1], ['zb', 'test']]),
    };
    console.log(inspect(o, { compact: true, depth: 5, breakLength: 80 }));

    // { a:
    //   [ 1,
    //     2,
    //     [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
    //           'test',
    //           'foo' ] ],
    //     4 ],
    //   b: Map(2) { 'za' => 1, 'zb' => 'test' } }

    // Setting `compact` to false or an integer creates more reader friendly output.
    console.log(inspect(o, { compact: false, depth: 5, breakLength: 80 }));

    // {
    //   a: [
    //     1,
    //     2,
    //     [
    //       [
    //         'Lorem ipsum dolor sit amet,\n' +
    //           'consectetur adipiscing elit, sed do eiusmod \n' +
    //           'tempor incididunt ut labore et dolore magna aliqua.',
    //         'test',
    //         'foo'
    //       ]
    //     ],
    //     4
    //   ],
    //   b: Map(2) {
    //     'za' => 1,
    //     'zb' => 'test'
    //   }
    // }

    // Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
    // single line.
    const { inspect } = require('node:util');

    const o = {
      a: [1, 2, [[
        'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
          'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
        'test',
        'foo']], 4],
      b: new Map([['za', 1], ['zb', 'test']]),
    };
    console.log(inspect(o, { compact: true, depth: 5, breakLength: 80 }));

    // { a:
    //   [ 1,
    //     2,
    //     [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
    //           'test',
    //           'foo' ] ],
    //     4 ],
    //   b: Map(2) { 'za' => 1, 'zb' => 'test' } }

    // Setting `compact` to false or an integer creates more reader friendly output.
    console.log(inspect(o, { compact: false, depth: 5, breakLength: 80 }));

    // {
    //   a: [
    //     1,
    //     2,
    //     [
    //       [
    //         'Lorem ipsum dolor sit amet,\n' +
    //           'consectetur adipiscing elit, sed do eiusmod \n' +
    //           'tempor incididunt ut labore et dolore magna aliqua.',
    //         'test',
    //         'foo'
    //       ]
    //     ],
    //     4
    //   ],
    //   b: Map(2) {
    //     'za' => 1,
    //     'zb' => 'test'
    //   }
    // }

    // Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
    // single line.
    javascript

The `showHidden` option allows `<WeakMap>` and `<WeakSet>` entries to be inspected. If there are more entries than `maxArrayLength`, there is no guarantee which entries are displayed. That means retrieving the same `<WeakSet>` entries twice may result in different output. Furthermore, entries with no remaining strong references may be garbage collected at any time.

    import { inspect } from 'node:util';

    const obj = { a: 1 };
    const obj2 = { b: 2 };
    const weakSet = new WeakSet([obj, obj2]);

    console.log(inspect(weakSet, { showHidden: true }));
    // WeakSet { { a: 1 }, { b: 2 } }
    const { inspect } = require('node:util');

    const obj = { a: 1 };
    const obj2 = { b: 2 };
    const weakSet = new WeakSet([obj, obj2]);

    console.log(inspect(weakSet, { showHidden: true }));
    // WeakSet { { a: 1 }, { b: 2 } }
    javascript

The `sorted` option ensures that an object's property insertion order does not impact the result of `util.inspect()`.

    import { inspect } from 'node:util';
    import assert from 'node:assert';

    const o1 = {
      b: [2, 3, 1],
      a: '`a` comes before `b`',
      c: new Set([2, 3, 1]),
    };
    console.log(inspect(o1, { sorted: true }));
    // { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
    console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
    // { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

    const o2 = {
      c: new Set([2, 1, 3]),
      a: '`a` comes before `b`',
      b: [2, 3, 1],
    };
    assert.strict.equal(
      inspect(o1, { sorted: true }),
      inspect(o2, { sorted: true }),
    );
    const { inspect } = require('node:util');
    const assert = require('node:assert');

    const o1 = {
      b: [2, 3, 1],
      a: '`a` comes before `b`',
      c: new Set([2, 3, 1]),
    };
    console.log(inspect(o1, { sorted: true }));
    // { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
    console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
    // { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

    const o2 = {
      c: new Set([2, 1, 3]),
      a: '`a` comes before `b`',
      b: [2, 3, 1],
    };
    assert.strict.equal(
      inspect(o1, { sorted: true }),
      inspect(o2, { sorted: true }),
    );
    javascript

The `numericSeparator` option adds an underscore every three digits to all numbers.

    import { inspect } from 'node:util';

    const thousand = 1000;
    const million = 1000000;
    const bigNumber = 123456789n;
    const bigDecimal = 1234.12345;

    console.log(inspect(thousand, { numericSeparator: true }));
    // 1_000
    console.log(inspect(million, { numericSeparator: true }));
    // 1_000_000
    console.log(inspect(bigNumber, { numericSeparator: true }));
    // 123_456_789n
    console.log(inspect(bigDecimal, { numericSeparator: true }));
    // 1_234.123_45
    const { inspect } = require('node:util');

    const thousand = 1000;
    const million = 1000000;
    const bigNumber = 123456789n;
    const bigDecimal = 1234.12345;

    console.log(inspect(thousand, { numericSeparator: true }));
    // 1_000
    console.log(inspect(million, { numericSeparator: true }));
    // 1_000_000
    console.log(inspect(bigNumber, { numericSeparator: true }));
    // 123_456_789n
    console.log(inspect(bigDecimal, { numericSeparator: true }));
    // 1_234.123_45
    javascript

`util.inspect()` is a synchronous method intended for debugging. Its maximum output length is approximately 128 MiB. Inputs that result in longer output will be truncated.

#### Customizing `util.inspect` colors#

Color output (if enabled) of `util.inspect` is customizable globally via the `util.inspect.styles` and `util.inspect.colors` properties.

`util.inspect.styles` is a map associating a style name to a color from `util.inspect.colors`.

The default styles and associated colors are:

- `bigint`: `yellow`
- `boolean`: `yellow`
- `date`: `magenta`
- `module`: `underline`
- `name`: (no styling)
- `null`: `bold`
- `number`: `yellow`
- `regexp`: A method that colors character classes, groups, assertions, and other parts for improved readability. To customize the coloring, change the `colors` property. It is set to `['red', 'green', 'yellow', 'cyan', 'magenta']` by default and may be adjusted as needed. The array is repetitively iterated through depending on the "depth".
- `special`: `cyan` (e.g., `Proxies`)
- `string`: `green`
- `symbol`: `green`
- `undefined`: `grey`

Color styling uses ANSI control codes that may not be supported on all terminals. To verify color support use `tty.hasColors()`.

Predefined control codes are listed below (grouped as "Modifiers", "Foreground colors", and "Background colors").

##### Complex custom coloring#

It is possible to define a method as style. It receives the stringified value of the input. It is invoked in case coloring is active and the type is inspected.

Example: `util.inspect.styles.regexp(value)`

- `value` `<string>` The string representation of the input type.
- Returns: `<string>` The adjusted representation of `object`.

##### Modifiers#

Modifier support varies throughout different terminals. They will mostly be ignored, if not supported.

- `reset` - Resets all (color) modifiers to their defaults
- bold - Make text bold
- italic - Make text italic
- underline - Make text underlined
- strikethrough - Puts a horizontal line through the center of the text (Alias: `strikeThrough`, `crossedout`, `crossedOut`)
- `hidden` - Prints the text, but makes it invisible (Alias: conceal)
- dim - Decreased color intensity (Alias: `faint`)
- overlined - Make text overlined
- blink - Hides and shows the text in an interval
- inverse - Swap foreground and background colors (Alias: `swapcolors`, `swapColors`)
- doubleunderline - Make text double underlined (Alias: `doubleUnderline`)
- framed - Draw a frame around the text

##### Foreground colors#

- `black`
- `red`
- `green`
- `yellow`
- `blue`
- `magenta`
- `cyan`
- `white`
- `gray` (alias: `grey`, `blackBright`)
- `redBright`
- `greenBright`
- `yellowBright`
- `blueBright`
- `magentaBright`
- `cyanBright`
- `whiteBright`

##### Background colors#

- `bgBlack`
- `bgRed`
- `bgGreen`
- `bgYellow`
- `bgBlue`
- `bgMagenta`
- `bgCyan`
- `bgWhite`
- `bgGray` (alias: `bgGrey`, `bgBlackBright`)
- `bgRedBright`
- `bgGreenBright`
- `bgYellowBright`
- `bgBlueBright`
- `bgMagentaBright`
- `bgCyanBright`
- `bgWhiteBright`

#### Custom inspection functions on objects#

Added in: v0.1.97
History

Version | Changes
--- | ---
v17.3.0, v16.14.0 | The inspect argument is added for more interoperability.

Objects may also define their own `[util.inspect.custom](depth, opts, inspect)` function, which `util.inspect()` will invoke and use the result of when inspecting the object.

    import { inspect } from 'node:util';

    class Box {
      constructor(value) {
        this.value = value;
      }

      [inspect.custom](depth, options, inspect) {
        if (depth < 0) {
          return options.stylize('[Box]', 'special');
        }

        const newOptions = Object.assign({}, options, {
          depth: options.depth === null ? null : options.depth - 1,
        });

        // Five space padding because that's the size of "Box< ".
        const padding = ' '.repeat(5);
        const inner = inspect(this.value, newOptions)
                      .replace(/\n/g, `\n${padding}`);
        return `${options.stylize('Box', 'special')}< ${inner} >`;
      }
    }

    const box = new Box(true);

    console.log(inspect(box));
    // "Box< true >"
    const { inspect } = require('node:util');

    class Box {
      constructor(value) {
        this.value = value;
      }

      [inspect.custom](depth, options, inspect) {
        if (depth < 0) {
          return options.stylize('[Box]', 'special');
        }

        const newOptions = Object.assign({}, options, {
          depth: options.depth === null ? null : options.depth - 1,
        });

        // Five space padding because that's the size of "Box< ".
        const padding = ' '.repeat(5);
        const inner = inspect(this.value, newOptions)
                      .replace(/\n/g, `\n${padding}`);
        return `${options.stylize('Box', 'special')}< ${inner} >`;
      }
    }

    const box = new Box(true);

    console.log(inspect(box));
    // "Box< true >"
    javascript

Custom `[util.inspect.custom](depth, opts, inspect)` functions typically return a string but may return a value of any type that will be formatted accordingly by `util.inspect()`.

    import { inspect } from 'node:util';

    const obj = { foo: 'this will not show up in the inspect() output' };
    obj[inspect.custom] = (depth) => {
      return { bar: 'baz' };
    };

    console.log(inspect(obj));
    // "{ bar: 'baz' }"
    const { inspect } = require('node:util');

    const obj = { foo: 'this will not show up in the inspect() output' };
    obj[inspect.custom] = (depth) => {
      return { bar: 'baz' };
    };

    console.log(inspect(obj));
    // "{ bar: 'baz' }"
    javascript

#### `util.inspect.custom`#

Added in: v6.6.0
History

Version | Changes
--- | ---
v10.12.0 | This is now defined as a shared symbol.

- Type: `<symbol>` that can be used to declare custom inspect functions.

In addition to being accessible through `util.inspect.custom`, this symbol is registered globally and can be accessed in any environment as `Symbol.for('nodejs.util.inspect.custom')`.

Using this allows code to be written in a portable fashion, so that the custom inspect function is used in a Node.js environment and ignored in the browser. The `util.inspect()` function itself is passed as third argument to the custom inspect function to allow further portability.

    const customInspectSymbol = Symbol.for('nodejs.util.inspect.custom');

    class Password {
      constructor(value) {
        this.value = value;
      }

      toString() {
        return 'xxxxxxxx';
      }

      [customInspectSymbol](depth, inspectOptions, inspect) {
        return `Password <${this.toString()}>`;
      }
    }

    const password = new Password('r0sebud');
    console.log(password);
    // Prints Password <xxxxxxxx>
    js

See Custom inspection functions on Objects for more details.

#### `util.inspect.defaultOptions`#

Added in: v6.4.0

The `defaultOptions` value allows customization of the default options used by `util.inspect`. This is useful for functions like `console.log` or `util.format` which implicitly call into `util.inspect`. It shall be set to an object containing one or more valid `util.inspect()` options. Setting option properties directly is also supported.

    import { inspect } from 'node:util';
    const arr = Array(156).fill(0);

    console.log(arr); // Logs the truncated array
    inspect.defaultOptions.maxArrayLength = null;
    console.log(arr); // logs the full array
    const { inspect } = require('node:util');
    const arr = Array(156).fill(0);

    console.log(arr); // Logs the truncated array
    inspect.defaultOptions.maxArrayLength = null;
    console.log(arr); // logs the full array
    javascript

### `util.isDeepStrictEqual(val1, val2[, options])`#

Added in: v9.0.0
History

Version | Changes
--- | ---
v24.9.0 | Added `options` parameter to allow skipping prototype comparison.

- `val1` `<any>`
- `val2` `<any>`
- `skipPrototype` `<boolean>` If `true`, prototype and constructor comparison is skipped during deep strict equality check. Default: `false`.
- Returns: `<boolean>`

Returns `true` if there is deep strict equality between `val1` and `val2`. Otherwise, returns `false`.

By default, deep strict equality includes comparison of object prototypes and constructors. When `skipPrototype` is `true`, objects with different prototypes or constructors can still be considered equal if their enumerable properties are deeply strictly equal.

    const util = require('node:util');

    class Foo {
      constructor(a) {
        this.a = a;
      }
    }

    class Bar {
      constructor(a) {
        this.a = a;
      }
    }

    const foo = new Foo(1);
    const bar = new Bar(1);

    // Different constructors, same properties
    console.log(util.isDeepStrictEqual(foo, bar));
    // false

    console.log(util.isDeepStrictEqual(foo, bar, true));
    // true
    js

See `assert.deepStrictEqual()` for more information about deep strict equality.

### Class: `util.MIMEType`#

Added in: v19.1.0, v18.13.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

An implementation of the MIMEType class.

In accordance with browser conventions, all properties of `MIMEType` objects are implemented as getters and setters on the class prototype, rather than as data properties on the object itself.

A MIME string is a structured string containing multiple meaningful components. When parsed, a `MIMEType` object is returned containing properties for each of these components.

#### `new MIMEType(input)`#

- `input` `<string>` The input MIME to parse

Creates a new `MIMEType` object by parsing the `input`.

    import { MIMEType } from 'node:util';

    const myMIME = new MIMEType('text/plain');
    const { MIMEType } = require('node:util');

    const myMIME = new MIMEType('text/plain');
    javascript

A `TypeError` will be thrown if the `input` is not a valid MIME. Note that an effort will be made to coerce the given values into strings. For instance:

    import { MIMEType } from 'node:util';
    const myMIME = new MIMEType({ toString: () => 'text/plain' });
    console.log(String(myMIME));
    // Prints: text/plain
    const { MIMEType } = require('node:util');
    const myMIME = new MIMEType({ toString: () => 'text/plain' });
    console.log(String(myMIME));
    // Prints: text/plain
    javascript

#### `mime.type`#

- Type: `<string>`

Gets and sets the type portion of the MIME.

    import { MIMEType } from 'node:util';

    const myMIME = new MIMEType('text/javascript');
    console.log(myMIME.type);
    // Prints: text
    myMIME.type = 'application';
    console.log(myMIME.type);
    // Prints: application
    console.log(String(myMIME));
    // Prints: application/javascript
    const { MIMEType } = require('node:util');

    const myMIME = new MIMEType('text/javascript');
    console.log(myMIME.type);
    // Prints: text
    myMIME.type = 'application';
    console.log(myMIME.type);
    // Prints: application
    console.log(String(myMIME));
    // Prints: application/javascript
    javascript

#### `mime.subtype`#

- Type: `<string>`

Gets and sets the subtype portion of the MIME.

    import { MIMEType } from 'node:util';

    const myMIME = new MIMEType('text/ecmascript');
    console.log(myMIME.subtype);
    // Prints: ecmascript
    myMIME.subtype = 'javascript';
    console.log(myMIME.subtype);
    // Prints: javascript
    console.log(String(myMIME));
    // Prints: text/javascript
    const { MIMEType } = require('node:util');

    const myMIME = new MIMEType('text/ecmascript');
    console.log(myMIME.subtype);
    // Prints: ecmascript
    myMIME.subtype = 'javascript';
    console.log(myMIME.subtype);
    // Prints: javascript
    console.log(String(myMIME));
    // Prints: text/javascript
    javascript

#### `mime.essence`#

- Type: `<string>`

Gets the essence of the MIME. This property is read only. Use `mime.type` or `mime.subtype` to alter the MIME.

    import { MIMEType } from 'node:util';

    const myMIME = new MIMEType('text/javascript;key=value');
    console.log(myMIME.essence);
    // Prints: text/javascript
    myMIME.type = 'application';
    console.log(myMIME.essence);
    // Prints: application/javascript
    console.log(String(myMIME));
    // Prints: application/javascript;key=value
    const { MIMEType } = require('node:util');

    const myMIME = new MIMEType('text/javascript;key=value');
    console.log(myMIME.essence);
    // Prints: text/javascript
    myMIME.type = 'application';
    console.log(myMIME.essence);
    // Prints: application/javascript
    console.log(String(myMIME));
    // Prints: application/javascript;key=value
    javascript

#### `mime.params`#

- Type: `<MIMEParams>`

Gets the `MIMEParams` object representing the parameters of the MIME. This property is read-only. See `MIMEParams` documentation for details.

#### `mime.toString()`#

- Returns: `<string>`

The `toString()` method on the `MIMEType` object returns the serialized MIME.

Because of the need for standard compliance, this method does not allow users to customize the serialization process of the MIME.

#### `mime.toJSON()`#

- Returns: `<string>`

Alias for `mime.toString()`.

This method is automatically called when an `MIMEType` object is serialized with `JSON.stringify()`.

    import { MIMEType } from 'node:util';

    const myMIMES = [
      new MIMEType('image/png'),
      new MIMEType('image/gif'),
    ];
    console.log(JSON.stringify(myMIMES));
    // Prints: ["image/png", "image/gif"]
    const { MIMEType } = require('node:util');

    const myMIMES = [
      new MIMEType('image/png'),
      new MIMEType('image/gif'),
    ];
    console.log(JSON.stringify(myMIMES));
    // Prints: ["image/png", "image/gif"]
    javascript

### Class: `util.MIMEParams`#

Added in: v19.1.0, v18.13.0

The `MIMEParams` API provides read and write access to the parameters of a `MIMEType`.

#### `new MIMEParams()`#

Creates a new `MIMEParams` object by with empty parameters

    import { MIMEParams } from 'node:util';

    const myParams = new MIMEParams();
    const { MIMEParams } = require('node:util');

    const myParams = new MIMEParams();
    javascript

#### `mimeParams.delete(name)`#

- `name` `<string>`

Remove all name-value pairs whose name is `name`.

#### `mimeParams.entries()`#

- Returns: `<Iterator>`

Returns an iterator over each of the name-value pairs in the parameters. Each item of the iterator is a JavaScript `Array`. The first item of the array is the `name`, the second item of the array is the `value`.

#### `mimeParams.get(name)`#

- `name` `<string>`
- Returns: `<string>` | `<null>` A string or `null` if there is no name-value pair with the given `name`.

Returns the value of the first name-value pair whose name is `name`. If there are no such pairs, `null` is returned.

#### `mimeParams.has(name)`#

- `name` `<string>`
- Returns: `<boolean>`

Returns `true` if there is at least one name-value pair whose name is `name`.

#### `mimeParams.keys()`#

- Returns: `<Iterator>`

Returns an iterator over the names of each name-value pair.

    import { MIMEType } from 'node:util';

    const { params } = new MIMEType('text/plain;foo=0;bar=1');
    for (const name of params.keys()) {
      console.log(name);
    }
    // Prints:
    //   foo
    //   bar
    const { MIMEType } = require('node:util');

    const { params } = new MIMEType('text/plain;foo=0;bar=1');
    for (const name of params.keys()) {
      console.log(name);
    }
    // Prints:
    //   foo
    //   bar
    javascript

#### `mimeParams.set(name, value)`#

- `name` `<string>`
- `value` `<string>`

Sets the value in the `MIMEParams` object associated with `name` to `value`. If there are any pre-existing name-value pairs whose names are `name`, set the first such pair's value to `value`.

    import { MIMEType } from 'node:util';

    const { params } = new MIMEType('text/plain;foo=0;bar=1');
    params.set('foo', 'def');
    params.set('baz', 'xyz');
    console.log(params.toString());
    // Prints: foo=def;bar=1;baz=xyz
    const { MIMEType } = require('node:util');

    const { params } = new MIMEType('text/plain;foo=0;bar=1');
    params.set('foo', 'def');
    params.set('baz', 'xyz');
    console.log(params.toString());
    // Prints: foo=def;bar=1;baz=xyz
    javascript

#### `mimeParams.values()`#

- Returns: `<Iterator>`

Returns an iterator over the values of each name-value pair.

#### `mimeParams[Symbol.iterator]()`#

- Returns: `<Iterator>`

Alias for `mimeParams.entries()`.

    import { MIMEType } from 'node:util';

    const { params } = new MIMEType('text/plain;foo=bar;xyz=baz');
    for (const [name, value] of params) {
      console.log(name, value);
    }
    // Prints:
    //   foo bar
    //   xyz baz
    const { MIMEType } = require('node:util');

    const { params } = new MIMEType('text/plain;foo=bar;xyz=baz');
    for (const [name, value] of params) {
      console.log(name, value);
    }
    // Prints:
    //   foo bar
    //   xyz baz
    javascript

### `util.parseArgs([config])`#

Added in: v18.3.0, v16.17.0
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | add support for allowing negative options in input `config`.
v20.0.0 | The API is no longer experimental.
v18.11.0, v16.19.0 | Add support for default values in input `config`.
v18.7.0, v16.17.0 | add support for returning detailed parse information using `tokens` in input `config` and returned properties.

- `config` `<Object>` Used to provide arguments for parsing and to configure the parser. `config` supports the following properties:

  - `args` `<string>`[] array of argument strings. Default: `process.argv` with `execPath` and `filename` removed.
  - `options` `<Object>` Used to describe arguments known to the parser. Keys of `options` are the long names of options and values are an `<Object>` accepting the following properties:
    - `type` `<string>` Type of argument, which must be either `boolean` or `string`.
    - `multiple` `<boolean>` Whether this option can be provided multiple times. If `true`, all values will be collected in an array. If `false`, values for the option are last-wins. Default: `false`.
    - `short` `<string>` A single character alias for the option.
    - `default` `<string>` | `<boolean>` | `<string>`[] | `<boolean>`[] The value to assign to the option if it does not appear in the arguments to be parsed. The value must match the type specified by the `type` property. If `multiple` is `true`, it must be an array. No default value is applied when the option does appear in the arguments to be parsed, even if the provided value is falsy.
  - `strict` `<boolean>` Should an error be thrown when unknown arguments are encountered, or when arguments are passed that do not match the `type` configured in `options`. Default: `true`.
  - `allowPositionals` `<boolean>` Whether this command accepts positional arguments. Default: `false` if `strict` is `true`, otherwise `true`.
  - `allowNegative` `<boolean>` If `true`, allows explicitly setting boolean options to `false` by prefixing the option name with `--no-`. Default: `false`.
  - `tokens` `<boolean>` Return the parsed tokens. This is useful for extending the built-in behavior, from adding additional checks through to reprocessing the tokens in different ways. Default: `false`.

- Returns: `<Object>` The parsed command line arguments:

  - `values` `<Object>` A mapping of parsed option names with their `<string>` or `<boolean>` values.
  - `positionals` `<string>`[] Positional arguments.
  - `tokens` `<Object>`[] | `<undefined>` See parseArgs tokens section. Only returned if `config` includes `tokens: true`.

Provides a higher level API for command-line argument parsing than interacting with `process.argv` directly. Takes a specification for the expected arguments and returns a structured object with the parsed options and positionals.

    import { parseArgs } from 'node:util';
    const args = ['-f', '--bar', 'b'];
    const options = {
      foo: {
        type: 'boolean',
        short: 'f',
      },
      bar: {
        type: 'string',
      },
    };
    const {
      values,
      positionals,
    } = parseArgs({ args, options });
    console.log(values, positionals);
    // Prints: [Object: null prototype] { foo: true, bar: 'b' } []
    const { parseArgs } = require('node:util');
    const args = ['-f', '--bar', 'b'];
    const options = {
      foo: {
        type: 'boolean',
        short: 'f',
      },
      bar: {
        type: 'string',
      },
    };
    const {
      values,
      positionals,
    } = parseArgs({ args, options });
    console.log(values, positionals);
    // Prints: [Object: null prototype] { foo: true, bar: 'b' } []
    javascript

#### `parseArgs` `tokens`#

Detailed parse information is available for adding custom behaviors by specifying `tokens: true` in the configuration. The returned tokens have properties describing:

- all tokens
  - `kind` `<string>` One of 'option', 'positional', or 'option-terminator'.
  - `index` `<number>` Index of element in `args` containing token. So the source argument for a token is `args[token.index]`.
- option tokens
  - `name` `<string>` Long name of option.
  - `rawName` `<string>` How option used in args, like `-f` of `--foo`.
  - `value` `<string>` | `<undefined>` Option value specified in args. Undefined for boolean options.
  - `inlineValue` `<boolean>` | `<undefined>` Whether option value specified inline, like `--foo=bar`.
- positional tokens
  - `value` `<string>` The value of the positional argument in args (i.e. `args[index]`).
- option-terminator token

The returned tokens are in the order encountered in the input args. Options that appear more than once in args produce a token for each use. Short option groups like `-xy` expand to a token for each option. So `-xxx` produces three tokens.

For example, to add support for a negated option like `--no-color` (which `allowNegative` supports when the option is of `boolean` type), the returned tokens can be reprocessed to change the value stored for the negated option.

    import { parseArgs } from 'node:util';

    const options = {
      'color': { type: 'boolean' },
      'no-color': { type: 'boolean' },
      'logfile': { type: 'string' },
      'no-logfile': { type: 'boolean' },
    };
    const { values, tokens } = parseArgs({ options, tokens: true });

    // Reprocess the option tokens and overwrite the returned values.
    tokens
      .filter((token) => token.kind === 'option')
      .forEach((token) => {
        if (token.name.startsWith('no-')) {
          // Store foo:false for --no-foo
          const positiveName = token.name.slice(3);
          values[positiveName] = false;
          delete values[token.name];
        } else {
          // Resave value so last one wins if both --foo and --no-foo.
          values[token.name] = token.value ?? true;
        }
      });

    const color = values.color;
    const logfile = values.logfile ?? 'default.log';

    console.log({ logfile, color });
    const { parseArgs } = require('node:util');

    const options = {
      'color': { type: 'boolean' },
      'no-color': { type: 'boolean' },
      'logfile': { type: 'string' },
      'no-logfile': { type: 'boolean' },
    };
    const { values, tokens } = parseArgs({ options, tokens: true });

    // Reprocess the option tokens and overwrite the returned values.
    tokens
      .filter((token) => token.kind === 'option')
      .forEach((token) => {
        if (token.name.startsWith('no-')) {
          // Store foo:false for --no-foo
          const positiveName = token.name.slice(3);
          values[positiveName] = false;
          delete values[token.name];
        } else {
          // Resave value so last one wins if both --foo and --no-foo.
          values[token.name] = token.value ?? true;
        }
      });

    const color = values.color;
    const logfile = values.logfile ?? 'default.log';

    console.log({ logfile, color });
    javascript

Example usage showing negated options, and when an option is used multiple ways then last one wins.

    $ node negate.js
    { logfile: 'default.log', color: undefined }
    $ node negate.js --no-logfile --no-color
    { logfile: false, color: false }
    $ node negate.js --logfile=test.log --color
    { logfile: 'test.log', color: true }
    $ node negate.js --no-logfile --logfile=test.log --color --no-color
    { logfile: 'test.log', color: false }
    console

### `util.parseEnv(content)`#

Added in: v21.7.0, v20.12.0
History

Version | Changes
--- | ---
v24.10.0, v22.21.0 | This API is no longer experimental.

- `content` `<string>`

The raw contents of a `.env` file.

- Returns: `<Object>`

Given an example `.env` file:

    const { parseEnv } = require('node:util');

    parseEnv('HELLO=world\nHELLO=oh my\n');
    // Returns: { HELLO: 'oh my' }
    import { parseEnv } from 'node:util';

    parseEnv('HELLO=world\nHELLO=oh my\n');
    // Returns: { HELLO: 'oh my' }
    javascript

### `util.promisify(original)`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v20.8.0 | Calling `promisify` on a function that returns a `Promise` is deprecated.

- `original` `<Function>`
- Returns: `<Function>`

Takes a function following the common error-first callback style, i.e. taking an `(err, value) => ...` callback as the last argument, and returns a version that returns promises.

    import { promisify } from 'node:util';
    import { stat } from 'node:fs';

    const promisifiedStat = promisify(stat);
    promisifiedStat('.').then((stats) => {
      // Do something with `stats`
    }).catch((error) => {
      // Handle the error.
    });
    const { promisify } = require('node:util');
    const { stat } = require('node:fs');

    const promisifiedStat = promisify(stat);
    promisifiedStat('.').then((stats) => {
      // Do something with `stats`
    }).catch((error) => {
      // Handle the error.
    });
    javascript

Or, equivalently using `async function`s:

    import { promisify } from 'node:util';
    import { stat } from 'node:fs';

    const promisifiedStat = promisify(stat);

    async function callStat() {
      const stats = await promisifiedStat('.');
      console.log(`This directory is owned by ${stats.uid}`);
    }

    callStat();
    const { promisify } = require('node:util');
    const { stat } = require('node:fs');

    const promisifiedStat = promisify(stat);

    async function callStat() {
      const stats = await promisifiedStat('.');
      console.log(`This directory is owned by ${stats.uid}`);
    }

    callStat();
    javascript

If there is an `original[util.promisify.custom]` property present, `promisify` will return its value, see Custom promisified functions.

`promisify()` assumes that `original` is a function taking a callback as its final argument in all cases. If `original` is not a function, `promisify()` will throw an error. If `original` is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using `promisify()` on class methods or other methods that use `this` may not work as expected unless handled specially:

    import { promisify } from 'node:util';

    class Foo {
      constructor() {
        this.a = 42;
      }

      bar(callback) {
        callback(null, this.a);
      }
    }

    const foo = new Foo();

    const naiveBar = promisify(foo.bar);
    // TypeError: Cannot read properties of undefined (reading 'a')
    // naiveBar().then(a => console.log(a));

    naiveBar.call(foo).then((a) => console.log(a)); // '42'

    const bindBar = naiveBar.bind(foo);
    bindBar().then((a) => console.log(a)); // '42'
    const { promisify } = require('node:util');

    class Foo {
      constructor() {
        this.a = 42;
      }

      bar(callback) {
        callback(null, this.a);
      }
    }

    const foo = new Foo();

    const naiveBar = promisify(foo.bar);
    // TypeError: Cannot read properties of undefined (reading 'a')
    // naiveBar().then(a => console.log(a));

    naiveBar.call(foo).then((a) => console.log(a)); // '42'

    const bindBar = naiveBar.bind(foo);
    bindBar().then((a) => console.log(a)); // '42'
    javascript

#### Custom promisified functions#

Using the `util.promisify.custom` symbol one can override the return value of `util.promisify()`:

    import { promisify } from 'node:util';

    function doSomething(foo, callback) {
      // ...
    }

    doSomething[promisify.custom] = (foo) => {
      return getPromiseSomehow();
    };

    const promisified = promisify(doSomething);
    console.log(promisified === doSomething[promisify.custom]);
    // prints 'true'
    const { promisify } = require('node:util');

    function doSomething(foo, callback) {
      // ...
    }

    doSomething[promisify.custom] = (foo) => {
      return getPromiseSomehow();
    };

    const promisified = promisify(doSomething);
    console.log(promisified === doSomething[promisify.custom]);
    // prints 'true'
    javascript

This can be useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument.

For example, with a function that takes in `(foo, onSuccessCallback, onErrorCallback)`:

    doSomething[util.promisify.custom] = (foo) => {
      return new Promise((resolve, reject) => {
        doSomething(foo, resolve, reject);
      });
    };
    js

If `promisify.custom` is defined but is not a function, `promisify()` will throw an error.

#### `util.promisify.custom`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v13.12.0, v12.16.2 | This is now defined as a shared symbol.

- Type: `<symbol>` that can be used to declare custom promisified variants of functions, see Custom promisified functions.

In addition to being accessible through `util.promisify.custom`, this symbol is registered globally and can be accessed in any environment as `Symbol.for('nodejs.util.promisify.custom')`.

For example, with a function that takes in `(foo, onSuccessCallback, onErrorCallback)`:

    const kCustomPromisifiedSymbol = Symbol.for('nodejs.util.promisify.custom');

    doSomething[kCustomPromisifiedSymbol] = (foo) => {
      return new Promise((resolve, reject) => {
        doSomething(foo, resolve, reject);
      });
    };
    js

### `util.stripVTControlCharacters(str)`#

Added in: v16.11.0

- `str` `<string>`
- Returns: `<string>`

Returns `str` with any ANSI escape codes removed.

    console.log(util.stripVTControlCharacters('\u001B[4mvalue\u001B[0m'));
    // Prints "value"
    js

### `util.styleText(format, text[, options])`#

Added in: v21.7.0, v20.12.0
History

Version | Changes
--- | ---
v26.1.0 | Add support for hexadecimal colors.
v24.2.0, v22.17.0 | Added the `'none'` format as a non-op format.
v23.5.0, v22.13.0 | styleText is now stable.
v22.8.0, v20.18.0 | Respect isTTY and environment variables such as NO_COLOR, NODE_DISABLE_COLORS, and FORCE_COLOR.

- `format` `<string>` | `<Array>` A text format or an Array of text formats defined in `util.inspect.colors`, or a hex color in `#RGB` or `#RRGGBB` form.
- `text` `<string>` The text to be formatted.
- `options` `<Object>`
  - `validateStream` `<boolean>` When true, `stream` is checked to see if it can handle colors. Default: `true`.
  - `stream` `<Stream>` A stream that will be validated if it can be colored. Default: `process.stdout`.

This function returns a formatted text considering the `format` passed for printing in a terminal. It is aware of the terminal's capabilities and acts according to the configuration set via `NO_COLOR`, `NODE_DISABLE_COLORS` and `FORCE_COLOR` environment variables.

    import { styleText } from 'node:util';
    import { stderr } from 'node:process';

    const successMessage = styleText('green', 'Success!');
    console.log(successMessage);

    const errorMessage = styleText(
      'red',
      'Error! Error!',
      // Validate if process.stderr has TTY
      { stream: stderr },
    );
    console.error(errorMessage);
    const { styleText } = require('node:util');
    const { stderr } = require('node:process');

    const successMessage = styleText('green', 'Success!');
    console.log(successMessage);

    const errorMessage = styleText(
      'red',
      'Error! Error!',
      // Validate if process.stderr has TTY
      { stream: stderr },
    );
    console.error(errorMessage);
    javascript

`util.inspect.colors` also provides text formats such as `italic`, and `underline` and you can combine both:

    console.log(
      util.styleText(['underline', 'italic'], 'My italic underlined message'),
    );
    cjs

When passing an array of formats, the order of the format applied is left to right so the following style might overwrite the previous one.

    console.log(
      util.styleText(['red', 'green'], 'text'), // green
    );
    cjs

The special format value `none` applies no additional styling to the text.

In addition to predefined color names, `util.styleText()` supports hex color strings using ANSI TrueColor (24-bit) escape sequences. Hex colors can be specified in either 3-digit (`#RGB`) or 6-digit (`#RRGGBB`) format:

    import { styleText } from 'node:util';

    // 6-digit hex color
    console.log(styleText('#ff5733', 'Orange text'));

    // 3-digit hex color (shorthand)
    console.log(styleText('#f00', 'Red text'));
    const { styleText } = require('node:util');

    // 6-digit hex color
    console.log(styleText('#ff5733', 'Orange text'));

    // 3-digit hex color (shorthand)
    console.log(styleText('#f00', 'Red text'));
    javascript

The full list of formats can be found in modifiers.

### Class: `util.TextDecoder`#

Added in: v8.3.0
History

Version | Changes
--- | ---
v11.0.0 | The class is now available on the global object.

An implementation of the WHATWG Encoding Standard `TextDecoder` API.

    const decoder = new TextDecoder();
    const u8arr = new Uint8Array([72, 101, 108, 108, 111]);
    console.log(decoder.decode(u8arr)); // Hello
    js

#### WHATWG supported encodings#

Per the WHATWG Encoding Standard, the encodings supported by the `TextDecoder` API are outlined in the tables below. For each encoding, one or more aliases may be used.

Different Node.js build configurations support different sets of encodings. (see Internationalization)

##### Encodings supported by default (with full ICU data)#

Encoding | Aliases
--- | ---
`'ibm866'` | `'866'`, `'cp866'`, `'csibm866'`
`'iso-8859-2'` | `'csisolatin2'`, `'iso-ir-101'`, `'iso8859-2'`, `'iso88592'`, `'iso_8859-2'`, `'iso_8859-2:1987'`, `'l2'`, `'latin2'`
`'iso-8859-3'` | `'csisolatin3'`, `'iso-ir-109'`, `'iso8859-3'`, `'iso88593'`, `'iso_8859-3'`, `'iso_8859-3:1988'`, `'l3'`, `'latin3'`
`'iso-8859-4'` | `'csisolatin4'`, `'iso-ir-110'`, `'iso8859-4'`, `'iso88594'`, `'iso_8859-4'`, `'iso_8859-4:1988'`, `'l4'`, `'latin4'`
`'iso-8859-5'` | `'csisolatincyrillic'`, `'cyrillic'`, `'iso-ir-144'`, `'iso8859-5'`, `'iso88595'`, `'iso_8859-5'`, `'iso_8859-5:1988'`
`'iso-8859-6'` | `'arabic'`, `'asmo-708'`, `'csiso88596e'`, `'csiso88596i'`, `'csisolatinarabic'`, `'ecma-114'`, `'iso-8859-6-e'`, `'iso-8859-6-i'`, `'iso-ir-127'`, `'iso8859-6'`, `'iso88596'`, `'iso_8859-6'`, `'iso_8859-6:1987'`
`'iso-8859-7'` | `'csisolatingreek'`, `'ecma-118'`, `'elot_928'`, `'greek'`, `'greek8'`, `'iso-ir-126'`, `'iso8859-7'`, `'iso88597'`, `'iso_8859-7'`, `'iso_8859-7:1987'`, `'sun_eu_greek'`
`'iso-8859-8'` | `'csiso88598e'`, `'csisolatinhebrew'`, `'hebrew'`, `'iso-8859-8-e'`, `'iso-ir-138'`, `'iso8859-8'`, `'iso88598'`, `'iso_8859-8'`, `'iso_8859-8:1988'`, `'visual'`
`'iso-8859-8-i'` | `'csiso88598i'`, `'logical'`
`'iso-8859-10'` | `'csisolatin6'`, `'iso-ir-157'`, `'iso8859-10'`, `'iso885910'`, `'l6'`, `'latin6'`
`'iso-8859-13'` | `'iso8859-13'`, `'iso885913'`
`'iso-8859-14'` | `'iso8859-14'`, `'iso885914'`
`'iso-8859-15'` | `'csisolatin9'`, `'iso8859-15'`, `'iso885915'`, `'iso_8859-15'`, `'l9'`
`'koi8-r'` | `'cskoi8r'`, `'koi'`, `'koi8'`, `'koi8_r'`
`'koi8-u'` | `'koi8-ru'`
`'macintosh'` | `'csmacintosh'`, `'mac'`, `'x-mac-roman'`
`'windows-874'` | `'dos-874'`, `'iso-8859-11'`, `'iso8859-11'`, `'iso885911'`, `'tis-620'`
`'windows-1250'` | `'cp1250'`, `'x-cp1250'`
`'windows-1251'` | `'cp1251'`, `'x-cp1251'`
`'windows-1252'` | `'ansi_x3.4-1968'`, `'ascii'`, `'cp1252'`, `'cp819'`, `'csisolatin1'`, `'ibm819'`, `'iso-8859-1'`, `'iso-ir-100'`, `'iso8859-1'`, `'iso88591'`, `'iso_8859-1'`, `'iso_8859-1:1987'`, `'l1'`, `'latin1'`, `'us-ascii'`, `'x-cp1252'`
`'windows-1253'` | `'cp1253'`, `'x-cp1253'`
`'windows-1254'` | `'cp1254'`, `'csisolatin5'`, `'iso-8859-9'`, `'iso-ir-148'`, `'iso8859-9'`, `'iso88599'`, `'iso_8859-9'`, `'iso_8859-9:1989'`, `'l5'`, `'latin5'`, `'x-cp1254'`
`'windows-1255'` | `'cp1255'`, `'x-cp1255'`
`'windows-1256'` | `'cp1256'`, `'x-cp1256'`
`'windows-1257'` | `'cp1257'`, `'x-cp1257'`
`'windows-1258'` | `'cp1258'`, `'x-cp1258'`
`'x-mac-cyrillic'` | `'x-mac-ukrainian'`
`'gbk'` | `'chinese'`, `'csgb2312'`, `'csiso58gb231280'`, `'gb2312'`, `'gb_2312'`, `'gb_2312-80'`, `'iso-ir-58'`, `'x-gbk'`
`'gb18030'` |
`'big5'` | `'big5-hkscs'`, `'cn-big5'`, `'csbig5'`, `'x-x-big5'`
`'euc-jp'` | `'cseucpkdfmtjapanese'`, `'x-euc-jp'`
`'iso-2022-jp'` | `'csiso2022jp'`
`'shift_jis'` | `'csshiftjis'`, `'ms932'`, `'ms_kanji'`, `'shift-jis'`, `'sjis'`, `'windows-31j'`, `'x-sjis'`
`'euc-kr'` | `'cseuckr'`, `'csksc56011987'`, `'iso-ir-149'`, `'korean'`, `'ks_c_5601-1987'`, `'ks_c_5601-1989'`, `'ksc5601'`, `'ksc_5601'`, `'windows-949'`

##### Encodings supported when Node.js is built with the `small-icu` option#

Encoding | Aliases
--- | ---
`'utf-8'` | `'unicode-1-1-utf-8'`, `'utf8'`
`'utf-16le'` | `'utf-16'`
`'utf-16be'` |

##### Encodings supported when ICU is disabled#

Encoding | Aliases
--- | ---
`'utf-8'` | `'unicode-1-1-utf-8'`, `'utf8'`
`'utf-16le'` | `'utf-16'`

The `'iso-8859-16'` encoding listed in the WHATWG Encoding Standard is not supported.

#### `new TextDecoder([encoding[, options]])`#

- `encoding` `<string>` Identifies the `encoding` that this `TextDecoder` instance supports. Default: `'utf-8'`.
- `options` `<Object>`
  - `fatal` `<boolean>` `true` if decoding failures are fatal. This option is not supported when ICU is disabled (see Internationalization). Default: `false`.
  - `ignoreBOM` `<boolean>` When `true`, the `TextDecoder` will include the byte order mark in the decoded result. When `false`, the byte order mark will be removed from the output. This option is only used when `encoding` is `'utf-8'`, `'utf-16be'`, or `'utf-16le'`. Default: `false`.

Creates a new `TextDecoder` instance. The `encoding` may specify one of the supported encodings or an alias.

The `TextDecoder` class is also available on the global object.

#### `textDecoder.decode([input[, options]])`#

- `input` `<ArrayBuffer>` | `<DataView>` | `<TypedArray>` An `ArrayBuffer`, `DataView`, or `TypedArray` instance containing the encoded data.
- `options` `<Object>`
  - `stream` `<boolean>` `true` if additional chunks of data are expected. Default: `false`.
- Returns: `<string>`

Decodes the `input` and returns a string. If `options.stream` is `true`, any incomplete byte sequences occurring at the end of the `input` are buffered internally and emitted after the next call to `textDecoder.decode()`.

If `textDecoder.fatal` is `true`, decoding errors that occur will result in a `TypeError` being thrown.

#### `textDecoder.encoding`#

- Type: `<string>`

The encoding supported by the `TextDecoder` instance.

#### `textDecoder.fatal`#

- Type: `<boolean>`

The value will be `true` if decoding errors result in a `TypeError` being thrown.

#### `textDecoder.ignoreBOM`#

- Type: `<boolean>`

The value will be `true` if the decoding result will include the byte order mark.

### Class: `util.TextEncoder`#

Added in: v8.3.0
History

Version | Changes
--- | ---
v11.0.0 | The class is now available on the global object.

An implementation of the WHATWG Encoding Standard `TextEncoder` API. All instances of `TextEncoder` only support UTF-8 encoding.

    const encoder = new TextEncoder();
    const uint8array = encoder.encode('this is some data');
    js

The `TextEncoder` class is also available on the global object.

#### `textEncoder.encode([input])`#

- `input` `<string>` The text to encode. Default: an empty string.
- Returns: `<Uint8Array>`

UTF-8 encodes the `input` string and returns a `Uint8Array` containing the encoded bytes.

#### `textEncoder.encodeInto(src, dest)`#

Added in: v12.11.0

- `src` `<string>` The text to encode.
- `dest` `<Uint8Array>` The array to hold the encode result.
- Returns: `<Object>`
  - `read` `<number>` The read Unicode code units of src.
  - `written` `<number>` The written UTF-8 bytes of dest.

UTF-8 encodes the `src` string to the `dest` Uint8Array and returns an object containing the read Unicode code units and written UTF-8 bytes.

    const encoder = new TextEncoder();
    const src = 'this is some data';
    const dest = new Uint8Array(10);
    const { read, written } = encoder.encodeInto(src, dest);
    js

#### `textEncoder.encoding`#

- Type: `<string>`

The encoding supported by the `TextEncoder` instance. Always set to `'utf-8'`.

### `util.toUSVString(string)`#

Added in: v16.8.0, v14.18.0

- `string` `<string>`

Returns the `string` after replacing any surrogate code points (or equivalently, any unpaired surrogate code units) with the Unicode "replacement character" U+FFFD.

### `util.transferableAbortController()`#

Added in: v18.11.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

Creates and returns an `<AbortController>` instance whose `<AbortSignal>` is marked as transferable and can be used with `structuredClone()` or `postMessage()`.

### `util.transferableAbortSignal(signal)`#

Added in: v18.11.0
History

Version | Changes
--- | ---
v23.11.0, v22.15.0 | Marking the API stable.

- `signal` `<AbortSignal>`
- Returns: `<AbortSignal>`

Marks the given `<AbortSignal>` as transferable so that it can be used with `structuredClone()` and `postMessage()`.

    const signal = transferableAbortSignal(AbortSignal.timeout(100));
    const channel = new MessageChannel();
    channel.port2.postMessage(signal, [signal]);
    js

### `util.aborted(signal, resource)`#

Added in: v19.7.0, v18.16.0
History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Change stability index for this feature from Experimental to Stable.

- `signal` `<AbortSignal>`
- `resource` `<Object>` Any non-null object tied to the abortable operation and held weakly. If `resource` is garbage collected before the `signal` aborts, the promise remains pending, allowing Node.js to stop tracking it. This helps prevent memory leaks in long-running or non-cancelable operations.
- Returns: `<Promise>`

Listens to abort event on the provided `signal` and returns a promise that resolves when the `signal` is aborted. If `resource` is provided, it weakly references the operation's associated object, so if `resource` is garbage collected before the `signal` aborts, then returned promise shall remain pending. This prevents memory leaks in long-running or non-cancelable operations.

    const { aborted } = require('node:util');

    // Obtain an object with an abortable signal, like a custom resource or operation.
    const dependent = obtainSomethingAbortable();

    // Pass `dependent` as the resource, indicating the promise should only resolve
    // if `dependent` is still in memory when the signal is aborted.
    aborted(dependent.signal, dependent).then(() => {

      // This code runs when `dependent` is aborted.
      console.log('Dependent resource was aborted.');
    });

    // Simulate an event that triggers the abort.
    dependent.on('event', () => {
      dependent.abort(); // This will cause the `aborted` promise to resolve.
    });
    import { aborted } from 'node:util';

    // Obtain an object with an abortable signal, like a custom resource or operation.
    const dependent = obtainSomethingAbortable();

    // Pass `dependent` as the resource, indicating the promise should only resolve
    // if `dependent` is still in memory when the signal is aborted.
    aborted(dependent.signal, dependent).then(() => {

      // This code runs when `dependent` is aborted.
      console.log('Dependent resource was aborted.');
    });

    // Simulate an event that triggers the abort.
    dependent.on('event', () => {
      dependent.abort(); // This will cause the `aborted` promise to resolve.
    });
    javascript

### `util.types`#

Added in: v10.0.0
History

Version | Changes
--- | ---
v15.3.0 | Exposed as `require('util/types')`.

`util.types` provides type checks for different kinds of built-in objects. Unlike `instanceof` or `Object.prototype.toString.call(value)`, these checks do not inspect properties of the object that are accessible from JavaScript (like their prototype), and usually have the overhead of calling into C++.

The result generally does not make any guarantees about what kinds of properties or behavior a value exposes in JavaScript. They are primarily useful for addon developers who prefer to do type checking in JavaScript.

The API is accessible via `require('node:util').types` or `require('node:util/types')`.

#### `util.types.isAnyArrayBuffer(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<ArrayBuffer>` or `<SharedArrayBuffer>` instance.

See also `util.types.isArrayBuffer()` and `util.types.isSharedArrayBuffer()`.

    util.types.isAnyArrayBuffer(new ArrayBuffer());  // Returns true
    util.types.isAnyArrayBuffer(new SharedArrayBuffer());  // Returns true
    js

#### `util.types.isArrayBufferView(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is an instance of one of the `<ArrayBuffer>` views, such as typed array objects or `<DataView>`. Equivalent to `ArrayBuffer.isView()`.

    util.types.isArrayBufferView(new Int8Array());  // true
    util.types.isArrayBufferView(Buffer.from('hello world')); // true
    util.types.isArrayBufferView(new DataView(new ArrayBuffer(16)));  // true
    util.types.isArrayBufferView(new ArrayBuffer());  // false
    js

#### `util.types.isArgumentsObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is an `arguments` object.

    function foo() {
      util.types.isArgumentsObject(arguments);  // Returns true
    }
    js

#### `util.types.isArrayBuffer(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<ArrayBuffer>` instance. This does not include `<SharedArrayBuffer>` instances. Usually, it is desirable to test for both; See `util.types.isAnyArrayBuffer()` for that.

    util.types.isArrayBuffer(new ArrayBuffer());  // Returns true
    util.types.isArrayBuffer(new SharedArrayBuffer());  // Returns false
    js

#### `util.types.isAsyncFunction(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is an async function. This only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

    util.types.isAsyncFunction(function foo() {});  // Returns false
    util.types.isAsyncFunction(async function foo() {});  // Returns true
    js

#### `util.types.isBigInt64Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a `BigInt64Array` instance.

    util.types.isBigInt64Array(new BigInt64Array());   // Returns true
    util.types.isBigInt64Array(new BigUint64Array());  // Returns false
    js

#### `util.types.isBigIntObject(value)`#

Added in: v10.4.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a BigInt object, e.g. created by `Object(BigInt(123))`.

    util.types.isBigIntObject(Object(BigInt(123)));   // Returns true
    util.types.isBigIntObject(BigInt(123));   // Returns false
    util.types.isBigIntObject(123);  // Returns false
    js

#### `util.types.isBigUint64Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a `BigUint64Array` instance.

    util.types.isBigUint64Array(new BigInt64Array());   // Returns false
    util.types.isBigUint64Array(new BigUint64Array());  // Returns true
    js

#### `util.types.isBooleanObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a boolean object, e.g. created by `new Boolean()`.

    util.types.isBooleanObject(false);  // Returns false
    util.types.isBooleanObject(true);   // Returns false
    util.types.isBooleanObject(new Boolean(false)); // Returns true
    util.types.isBooleanObject(new Boolean(true));  // Returns true
    util.types.isBooleanObject(Boolean(false)); // Returns false
    util.types.isBooleanObject(Boolean(true));  // Returns false
    js

#### `util.types.isBoxedPrimitive(value)`#

Added in: v10.11.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is any boxed primitive object, e.g. created by `new Boolean()`, `new String()` or `Object(Symbol())`.

For example:

    util.types.isBoxedPrimitive(false); // Returns false
    util.types.isBoxedPrimitive(new Boolean(false)); // Returns true
    util.types.isBoxedPrimitive(Symbol('foo')); // Returns false
    util.types.isBoxedPrimitive(Object(Symbol('foo'))); // Returns true
    util.types.isBoxedPrimitive(Object(BigInt(5))); // Returns true
    js

#### `util.types.isCryptoKey(value)`#

Added in: v16.2.0

- `value` `<Object>`
- Returns: `<boolean>`

Returns `true` if `value` is a `<CryptoKey>`, `false` otherwise.

#### `util.types.isDataView(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<DataView>` instance.

    const ab = new ArrayBuffer(20);
    util.types.isDataView(new DataView(ab));  // Returns true
    util.types.isDataView(new Float64Array());  // Returns false
    js

See also `ArrayBuffer.isView()`.

#### `util.types.isDate(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Date>` instance.

    util.types.isDate(new Date());  // Returns true
    js

#### `util.types.isExternal(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a native `External` value.

A native `External` value is a special type of object that contains a raw C++ pointer (`void*`) for access from native code, and has no other properties. Such objects are created either by Node.js internals or native addons. In JavaScript, they are frozen objects with a `null` prototype.

    import native from 'napi_addon.node';
    import { types } from 'node:util';

    const data = native.myNapi();
    types.isExternal(data); // returns true
    types.isExternal(0); // returns false
    types.isExternal(new String('foo')); // returns false
    const native = require('napi_addon.node');
    const { types } = require('node:util');

    const data = native.myNapi();
    types.isExternal(data); // returns true
    types.isExternal(0); // returns false
    types.isExternal(new String('foo')); // returns false
    javascript

For further information on `napi_create_external`, refer to `napi_create_external()`.

#### `util.types.isFloat16Array(value)`#

Added in: v24.0.0, v22.16.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Float16Array>` instance.

    util.types.isFloat16Array(new ArrayBuffer());  // Returns false
    util.types.isFloat16Array(new Float16Array());  // Returns true
    util.types.isFloat16Array(new Float32Array());  // Returns false
    js

#### `util.types.isFloat32Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Float32Array>` instance.

    util.types.isFloat32Array(new ArrayBuffer());  // Returns false
    util.types.isFloat32Array(new Float32Array());  // Returns true
    util.types.isFloat32Array(new Float64Array());  // Returns false
    js

#### `util.types.isFloat64Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Float64Array>` instance.

    util.types.isFloat64Array(new ArrayBuffer());  // Returns false
    util.types.isFloat64Array(new Uint8Array());  // Returns false
    util.types.isFloat64Array(new Float64Array());  // Returns true
    js

#### `util.types.isGeneratorFunction(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a generator function. This only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

    util.types.isGeneratorFunction(function foo() {});  // Returns false
    util.types.isGeneratorFunction(function* foo() {});  // Returns true
    js

#### `util.types.isGeneratorObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a generator object as returned from a built-in generator function. This only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

    function* foo() {}
    const generator = foo();
    util.types.isGeneratorObject(generator);  // Returns true
    js

#### `util.types.isInt8Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Int8Array>` instance.

    util.types.isInt8Array(new ArrayBuffer());  // Returns false
    util.types.isInt8Array(new Int8Array());  // Returns true
    util.types.isInt8Array(new Float64Array());  // Returns false
    js

#### `util.types.isInt16Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Int16Array>` instance.

    util.types.isInt16Array(new ArrayBuffer());  // Returns false
    util.types.isInt16Array(new Int16Array());  // Returns true
    util.types.isInt16Array(new Float64Array());  // Returns false
    js

#### `util.types.isInt32Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Int32Array>` instance.

    util.types.isInt32Array(new ArrayBuffer());  // Returns false
    util.types.isInt32Array(new Int32Array());  // Returns true
    util.types.isInt32Array(new Float64Array());  // Returns false
    js

#### `util.types.isKeyObject(value)`#

Added in: v16.2.0

- `value` `<Object>`
- Returns: `<boolean>`

Returns `true` if `value` is a `<KeyObject>`, `false` otherwise.

#### `util.types.isMap(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Map>` instance.

    util.types.isMap(new Map());  // Returns true
    js

#### `util.types.isMapIterator(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is an iterator returned for a built-in `<Map>` instance.

    const map = new Map();
    util.types.isMapIterator(map.keys());  // Returns true
    util.types.isMapIterator(map.values());  // Returns true
    util.types.isMapIterator(map.entries());  // Returns true
    util.types.isMapIterator(map[Symbol.iterator]());  // Returns true
    js

#### `util.types.isModuleNamespaceObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is an instance of a Module Namespace Object.

    import * as ns from './a.js';

    util.types.isModuleNamespaceObject(ns);  // Returns true
    mjs

#### `util.types.isNativeError(value)`#

Added in: v10.0.0Deprecated in: v24.2.0

Stability: 0 - Deprecated: Use `Error.isError` instead.

Note: As of Node.js 24, `Error.isError()` is currently slower than `util.types.isNativeError()`. If performance is critical, consider benchmarking both in your environment.

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value was returned by the constructor of a built-in `Error` type.

    console.log(util.types.isNativeError(new Error()));  // true
    console.log(util.types.isNativeError(new TypeError()));  // true
    console.log(util.types.isNativeError(new RangeError()));  // true
    js

Subclasses of the native error types are also native errors:

    class MyError extends Error {}
    console.log(util.types.isNativeError(new MyError()));  // true
    js

A value being `instanceof` a native error class is not equivalent to `isNativeError()` returning `true` for that value. `isNativeError()` returns `true` for errors which come from a different realm while `instanceof Error` returns `false` for these errors:

    import { createContext, runInContext } from 'node:vm';
    import { types } from 'node:util';

    const context = createContext({});
    const myError = runInContext('new Error()', context);
    console.log(types.isNativeError(myError)); // true
    console.log(myError instanceof Error); // false
    const { createContext, runInContext } = require('node:vm');
    const { types } = require('node:util');

    const context = createContext({});
    const myError = runInContext('new Error()', context);
    console.log(types.isNativeError(myError)); // true
    console.log(myError instanceof Error); // false
    javascript

Conversely, `isNativeError()` returns `false` for all objects which were not returned by the constructor of a native error. That includes values which are `instanceof` native errors:

    const myError = { __proto__: Error.prototype };
    console.log(util.types.isNativeError(myError)); // false
    console.log(myError instanceof Error); // true
    js

#### `util.types.isNumberObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a number object, e.g. created by `new Number()`.

    util.types.isNumberObject(0);  // Returns false
    util.types.isNumberObject(new Number(0));   // Returns true
    js

#### `util.types.isPromise(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Promise>`.

    util.types.isPromise(Promise.resolve(42));  // Returns true
    js

#### `util.types.isProxy(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a `<Proxy>` instance.

    const target = {};
    const proxy = new Proxy(target, {});
    util.types.isProxy(target);  // Returns false
    util.types.isProxy(proxy);  // Returns true
    js

#### `util.types.isRegExp(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a regular expression object.

    util.types.isRegExp(/abc/);  // Returns true
    util.types.isRegExp(new RegExp('abc'));  // Returns true
    js

#### `util.types.isSet(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Set>` instance.

    util.types.isSet(new Set());  // Returns true
    js

#### `util.types.isSetIterator(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is an iterator returned for a built-in `<Set>` instance.

    const set = new Set();
    util.types.isSetIterator(set.keys());  // Returns true
    util.types.isSetIterator(set.values());  // Returns true
    util.types.isSetIterator(set.entries());  // Returns true
    util.types.isSetIterator(set[Symbol.iterator]());  // Returns true
    js

#### `util.types.isSharedArrayBuffer(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<SharedArrayBuffer>` instance. This does not include `<ArrayBuffer>` instances. Usually, it is desirable to test for both; See `util.types.isAnyArrayBuffer()` for that.

    util.types.isSharedArrayBuffer(new ArrayBuffer());  // Returns false
    util.types.isSharedArrayBuffer(new SharedArrayBuffer());  // Returns true
    js

#### `util.types.isStringObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a string object, e.g. created by `new String()`.

    util.types.isStringObject('foo');  // Returns false
    util.types.isStringObject(new String('foo'));   // Returns true
    js

#### `util.types.isSymbolObject(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a symbol object, created by calling `Object()` on a `Symbol` primitive.

    const symbol = Symbol('foo');
    util.types.isSymbolObject(symbol);  // Returns false
    util.types.isSymbolObject(Object(symbol));   // Returns true
    js

#### `util.types.isTypedArray(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<TypedArray>` instance.

    util.types.isTypedArray(new ArrayBuffer());  // Returns false
    util.types.isTypedArray(new Uint8Array());  // Returns true
    util.types.isTypedArray(new Float64Array());  // Returns true
    js

See also `ArrayBuffer.isView()`.

#### `util.types.isUint8Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Uint8Array>` instance.

    util.types.isUint8Array(new ArrayBuffer());  // Returns false
    util.types.isUint8Array(new Uint8Array());  // Returns true
    util.types.isUint8Array(new Float64Array());  // Returns false
    js

#### `util.types.isUint8ClampedArray(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Uint8ClampedArray>` instance.

    util.types.isUint8ClampedArray(new ArrayBuffer());  // Returns false
    util.types.isUint8ClampedArray(new Uint8ClampedArray());  // Returns true
    util.types.isUint8ClampedArray(new Float64Array());  // Returns false
    js

#### `util.types.isUint16Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Uint16Array>` instance.

    util.types.isUint16Array(new ArrayBuffer());  // Returns false
    util.types.isUint16Array(new Uint16Array());  // Returns true
    util.types.isUint16Array(new Float64Array());  // Returns false
    js

#### `util.types.isUint32Array(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<Uint32Array>` instance.

    util.types.isUint32Array(new ArrayBuffer());  // Returns false
    util.types.isUint32Array(new Uint32Array());  // Returns true
    util.types.isUint32Array(new Float64Array());  // Returns false
    js

#### `util.types.isWeakMap(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<WeakMap>` instance.

    util.types.isWeakMap(new WeakMap());  // Returns true
    js

#### `util.types.isWeakSet(value)`#

Added in: v10.0.0

- `value` `<any>`
- Returns: `<boolean>`

Returns `true` if the value is a built-in `<WeakSet>` instance.

    util.types.isWeakSet(new WeakSet());  // Returns true
    js

### Deprecated APIs#

The following APIs are deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.

#### `util._extend(target, source)`#

Added in: v0.7.5Deprecated in: v6.0.0

Stability: 0 - Deprecated: Use `Object.assign()` instead.

- `target` `<Object>`
- `source` `<Object>`

The `util._extend()` method was never intended to be used outside of internal Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very similar built-in functionality through `Object.assign()`.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-extend-to-object-assign
    bash

#### `util.isArray(object)`#

Added in: v0.6.0Deprecated in: v4.0.0

Stability: 0 - Deprecated: Use `Array.isArray()` instead.

- `object` `<any>`
- Returns: `<boolean>`

Alias for `Array.isArray()`.

Returns `true` if the given `object` is an `Array`. Otherwise, returns `false`.

    const util = require('node:util');

    util.isArray([]);
    // Returns: true
    util.isArray(new Array());
    // Returns: true
    util.isArray({});
    // Returns: false
    js

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

## Console#

Source Code: lib/console.js

Stability: 2 - Stable

The `node:console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

- A `Console` class with methods such as `console.log()`, `console.error()`, and `console.warn()` that can be used to write to any Node.js stream.
- A global `console` instance configured to write to `process.stdout` and `process.stderr`. The global `console` can be used without calling `require('node:console')`.

Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. Programs that desire to depend on the synchronous / asynchronous behavior of the console functions should first figure out the nature of console's backing stream. This is because the stream is dependent on the underlying platform and standard stream configuration of the current process. See the note on process I/O for more information.

Example using the global `console`:

    console.log('hello world');
    // Prints: hello world, to stdout
    console.log('hello %s', 'world');
    // Prints: hello world, to stdout
    console.error(new Error('Whoops, something bad happened'));
    // Prints error message and stack trace to stderr:
    //   Error: Whoops, something bad happened
    //     at [eval]:5:15
    //     at Script.runInThisContext (node:vm:132:18)
    //     at Object.runInThisContext (node:vm:309:38)
    //     at node:internal/process/execution:77:19
    //     at [eval]-wrapper:6:22
    //     at evalScript (node:internal/process/execution:76:60)
    //     at node:internal/main/eval_string:23:3

    const name = 'Will Robinson';
    console.warn(`Danger ${name}! Danger!`);
    // Prints: Danger Will Robinson! Danger!, to stderr
    js

Example using the `Console` class:

    const out = getStreamSomehow();
    const err = getStreamSomehow();
    const myConsole = new console.Console(out, err);

    myConsole.log('hello world');
    // Prints: hello world, to out
    myConsole.log('hello %s', 'world');
    // Prints: hello world, to out
    myConsole.error(new Error('Whoops, something bad happened'));
    // Prints: [Error: Whoops, something bad happened], to err

    const name = 'Will Robinson';
    myConsole.warn(`Danger ${name}! Danger!`);
    // Prints: Danger Will Robinson! Danger!, to err
    js

### Class: `Console`#

History

Version | Changes
--- | ---
v8.0.0 | Errors that occur while writing to the underlying streams will now be ignored by default.

The `Console` class can be used to create a simple logger with configurable output streams and can be accessed using either `require('node:console').Console` or `console.Console` (or their destructured counterparts):

    import { Console } from 'node:console';
    const { Console } = require('node:console');
    javascript

    const { Console } = console;
    js

#### `new Console(stdout[, stderr][, ignoreErrors])`#

#### `new Console(options)`#

History

Version | Changes
--- | ---
v24.10.0 | The `inspectOptions` option can be a `Map` from stream to options.
v14.2.0, v12.17.0 | The `groupIndentation` option was introduced.
v11.7.0 | The `inspectOptions` option is introduced.
v10.0.0 | The `Console` constructor now supports an `options` argument, and the `colorMode` option was introduced.
v8.0.0 | The `ignoreErrors` option was introduced.

- `options` `<Object>`
  - `stdout` `<stream.Writable>`
  - `stderr` `<stream.Writable>`
  - `ignoreErrors` `<boolean>` Ignore errors when writing to the underlying streams. Default: `true`.
  - `colorMode` `<boolean>` | `<string>` Set color support for this `Console` instance. Setting to `true` enables coloring while inspecting values. Setting to `false` disables coloring while inspecting values. Setting to `'auto'` makes color support depend on the value of the `isTTY` property and the value returned by `getColorDepth()` on the respective stream. This option can not be used, if `inspectOptions.colors` is set as well. Default: `'auto'`.
  - `inspectOptions` `<Object>` | `<Map>` Specifies options that are passed along to `util.inspect()`. Can be an options object or, if different options for stdout and stderr are desired, a `Map` from stream objects to options.
  - `groupIndentation` `<number>` Set group indentation. Default: `2`.

Creates a new `Console` with one or two writable stream instances. `stdout` is a writable stream to print log or info output. `stderr` is used for warning or error output. If `stderr` is not provided, `stdout` is used for `stderr`.

    import { createWriteStream } from 'node:fs';
    import { Console } from 'node:console';
    // Alternatively
    // const { Console } = console;

    const output = createWriteStream('./stdout.log');
    const errorOutput = createWriteStream('./stderr.log');
    // Custom simple logger
    const logger = new Console({ stdout: output, stderr: errorOutput });
    // use it like console
    const count = 5;
    logger.log('count: %d', count);
    // In stdout.log: count 5
    const fs = require('node:fs');
    const { Console } = require('node:console');
    // Alternatively
    // const { Console } = console;

    const output = fs.createWriteStream('./stdout.log');
    const errorOutput = fs.createWriteStream('./stderr.log');
    // Custom simple logger
    const logger = new Console({ stdout: output, stderr: errorOutput });
    // use it like console
    const count = 5;
    logger.log('count: %d', count);
    // In stdout.log: count 5
    javascript

The global `console` is a special `Console` whose output is sent to `process.stdout` and `process.stderr`. It is equivalent to calling:

    new Console({ stdout: process.stdout, stderr: process.stderr });
    js

#### `console.assert(value[, ...message])`#

Added in: v0.1.101
History

Version | Changes
--- | ---
v10.0.0 | The implementation is now spec compliant and does not throw anymore.

- `value` `<any>` The value tested for being truthy.
- `...message` `<any>` All arguments besides `value` are used as error message.

`console.assert()` writes a message if `value` is falsy or omitted. It only writes a message and does not otherwise affect execution. The output always starts with `"Assertion failed"`. If provided, `message` is formatted using `util.format()`.

If `value` is truthy, nothing happens.

    console.assert(true, 'does nothing');

    console.assert(false, 'Whoops %s work', 'didn\'t');
    // Assertion failed: Whoops didn't work

    console.assert();
    // Assertion failed
    js

#### `console.clear()`#

Added in: v8.3.0

When `stdout` is a TTY, calling `console.clear()` will attempt to clear the TTY. When `stdout` is not a TTY, this method does nothing.

The specific operation of `console.clear()` can vary across operating systems and terminal types. For most Linux operating systems, `console.clear()` operates similarly to the `clear` shell command. On Windows, `console.clear()` will clear only the output in the current terminal viewport for the Node.js binary.

#### `console.count([label])`#

Added in: v8.3.0

- `label` `<string>` The display label for the counter. Default: `'default'`.

Maintains an internal counter specific to `label` and outputs to `stdout` the number of times `console.count()` has been called with the given `label`.

    > console.count()
    default: 1
    undefined
    > console.count('default')
    default: 2
    undefined
    > console.count('abc')
    abc: 1
    undefined
    > console.count('xyz')
    xyz: 1
    undefined
    > console.count('abc')
    abc: 2
    undefined
    > console.count()
    default: 3
    undefined
    >
    console

#### `console.countReset([label])`#

Added in: v8.3.0

- `label` `<string>` The display label for the counter. Default: `'default'`.

Resets the internal counter specific to `label`.

    > console.count('abc');
    abc: 1
    undefined
    > console.countReset('abc');
    undefined
    > console.count('abc');
    abc: 1
    undefined
    >
    console

#### `console.debug(data[, ...args])`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v8.10.0 | `console.debug` is now an alias for `console.log`.

- `data` `<any>`
- `...args` `<any>`

The `console.debug()` function is an alias for `console.log()`.

#### `console.dir(obj[, options])`#

Added in: v0.1.101

- `obj` `<any>`
- `options` `<Object>`
  - `showHidden` `<boolean>` If `true` then the object's non-enumerable and symbol properties will be shown too. Default: `false`.
  - `depth` `<number>` Tells `util.inspect()` how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. To make it recurse indefinitely, pass `null`. Default: `2`.
  - `colors` `<boolean>` If `true`, then the output will be styled with ANSI color codes. Colors are customizable; see customizing `util.inspect()` colors. Default: `false`.

Uses `util.inspect()` on `obj` and prints the resulting string to `stdout`. This function bypasses any custom `inspect()` function defined on `obj`.

#### `console.dirxml(...data)`#

Added in: v8.0.0
History

Version | Changes
--- | ---
v9.3.0 | `console.dirxml` now calls `console.log` for its arguments.

- `...data` `<any>`

This method calls `console.log()` passing it the arguments received. This method does not produce any XML formatting.

#### `console.error([data][, ...args])`#

Added in: v0.1.100

- `data` `<any>`
- `...args` `<any>`

Prints to `stderr` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to `printf(3)` (the arguments are all passed to `util.format()`).

    const code = 5;
    console.error('error #%d', code);
    // Prints: error #5, to stderr
    console.error('error', code);
    // Prints: error 5, to stderr
    js

If formatting elements (e.g. `%d`) are not found in the first string then `util.inspect()` is called on each argument and the resulting string values are concatenated. See `util.format()` for more information.

#### `console.group([...label])`#

Added in: v8.5.0

- `...label` `<any>`

Increases indentation of subsequent lines by spaces for `groupIndentation` length.

If one or more `label`s are provided, those are printed first without the additional indentation.

#### `console.groupCollapsed()`#

Added in: v8.5.0

An alias for `console.group()`.

#### `console.groupEnd()`#

Added in: v8.5.0

Decreases indentation of subsequent lines by spaces for `groupIndentation` length.

#### `console.info([data][, ...args])`#

Added in: v0.1.100

- `data` `<any>`
- `...args` `<any>`

The `console.info()` function is an alias for `console.log()`.

#### `console.log([data][, ...args])`#

Added in: v0.1.100

- `data` `<any>`
- `...args` `<any>`

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to `printf(3)` (the arguments are all passed to `util.format()`).

    const count = 5;
    console.log('count: %d', count);
    // Prints: count: 5, to stdout
    console.log('count:', count);
    // Prints: count: 5, to stdout
    js

See `util.format()` for more information.

#### `console.table(tabularData[, properties])`#

Added in: v10.0.0

- `tabularData` `<any>`
- `properties` `<string>`[] Alternate properties for constructing the table.

Try to construct a table with the columns of the properties of `tabularData` (or use `properties`) and rows of `tabularData` and log it. Falls back to just logging the argument if it can't be parsed as tabular.

    // These can't be parsed as tabular data
    console.table(Symbol());
    // Symbol()

    console.table(undefined);
    // undefined

    console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]);
    // ┌─────────┬─────┬─────┐
    // │ (index) │ a   │ b   │
    // ├─────────┼─────┼─────┤
    // │ 0       │ 1   │ 'Y' │
    // │ 1       │ 'Z' │ 2   │
    // └─────────┴─────┴─────┘

    console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']);
    // ┌─────────┬─────┐
    // │ (index) │ a   │
    // ├─────────┼─────┤
    // │ 0       │ 1   │
    // │ 1       │ 'Z' │
    // └─────────┴─────┘
    js

#### `console.time([label])`#

Added in: v0.1.104

- `label` `<string>` Default: `'default'`

Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique `label`. Use the same `label` when calling `console.timeEnd()` to stop the timer and output the elapsed time in suitable time units to `stdout`. For example, if the elapsed time is 3869ms, `console.timeEnd()` displays "3.869s".

#### `console.timeEnd([label])`#

Added in: v0.1.104
History

Version | Changes
--- | ---
v13.0.0 | The elapsed time is displayed with a suitable time unit.
v6.0.0 | This method no longer supports multiple calls that don't map to individual `console.time()` calls; see below for details.

- `label` `<string>` Default: `'default'`

Stops a timer that was previously started by calling `console.time()` and prints the result to `stdout`:

    console.time('bunch-of-stuff');
    // Do a bunch of stuff.
    console.timeEnd('bunch-of-stuff');
    // Prints: bunch-of-stuff: 225.438ms
    js

#### `console.timeLog([label][, ...data])`#

Added in: v10.7.0

- `label` `<string>` Default: `'default'`
- `...data` `<any>`

For a timer that was previously started by calling `console.time()`, prints the elapsed time and other `data` arguments to `stdout`:

    console.time('process');
    const value = expensiveProcess1(); // Returns 42
    console.timeLog('process', value);
    // Prints "process: 365.227ms 42".
    doExpensiveProcess2(value);
    console.timeEnd('process');
    js

#### `console.trace([message][, ...args])`#

Added in: v0.1.104

- `message` `<any>`
- `...args` `<any>`

Prints to `stderr` the string `'Trace: '`, followed by the `util.format()` formatted message and stack trace to the current position in the code.

    console.trace('Show me');
    // Prints: (stack trace will vary based on where trace is called)
    //  Trace: Show me
    //    at repl:2:9
    //    at REPLServer.defaultEval (repl.js:248:27)
    //    at bound (domain.js:287:14)
    //    at REPLServer.runBound [as eval] (domain.js:300:12)
    //    at REPLServer.<anonymous> (repl.js:412:12)
    //    at emitOne (events.js:82:20)
    //    at REPLServer.emit (events.js:169:7)
    //    at REPLServer.Interface._onLine (readline.js:210:10)
    //    at REPLServer.Interface._line (readline.js:549:8)
    //    at REPLServer.Interface._ttyWrite (readline.js:826:14)
    js

#### `console.warn([data][, ...args])`#

Added in: v0.1.100

- `data` `<any>`
- `...args` `<any>`

The `console.warn()` function is an alias for `console.error()`.

### Inspector only methods#

The following methods are exposed by the V8 engine in the general API but do not display anything unless used in conjunction with the inspector (`--inspect` flag).

#### `console.profile([label])`#

Added in: v8.0.0

- `label` `<string>`

This method does not display anything unless used in the inspector. The `console.profile()` method starts a JavaScript CPU profile with an optional label until `console.profileEnd()` is called. The profile is then added to the Profile panel of the inspector.

    console.profile('MyLabel');
    // Some code
    console.profileEnd('MyLabel');
    // Adds the profile 'MyLabel' to the Profiles panel of the inspector.
    js

#### `console.profileEnd([label])`#

Added in: v8.0.0

- `label` `<string>`

This method does not display anything unless used in the inspector. Stops the current JavaScript CPU profiling session if one has been started and prints the report to the Profiles panel of the inspector. See `console.profile()` for an example.

If this method is called without a label, the most recently started profile is stopped.

#### `console.timeStamp([label])`#

Added in: v8.0.0

- `label` `<string>`

This method does not display anything unless used in the inspector. The `console.timeStamp()` method adds an event with the label `'label'` to the Timeline panel of the inspector.

## Crypto#

Source Code: lib/crypto.js

Stability: 2 - Stable

The `node:crypto` module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify functions.

    const { createHmac } = await import('node:crypto');

    const secret = 'abcdefg';
    const hash = createHmac('sha256', secret)
                   .update('I love cupcakes')
                   .digest('hex');
    console.log(hash);
    // Prints:
    //   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e
    const { createHmac } = require('node:crypto');

    const secret = 'abcdefg';
    const hash = createHmac('sha256', secret)
                   .update('I love cupcakes')
                   .digest('hex');
    console.log(hash);
    // Prints:
    //   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e
    javascript

### Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the `node:crypto` module. In such cases, attempting to `import` from `crypto` or calling `require('node:crypto')` will result in an error being thrown.

When using CommonJS, the error thrown can be caught using try/catch:

    let crypto;
    try {
      crypto = require('node:crypto');
    } catch (err) {
      console.error('crypto support is disabled!');
    }
    cjs

When using the lexical ESM `import` keyword, the error can only be caught if a handler for `process.on('uncaughtException')` is registered before any attempt to load the module is made (using, for instance, a preload module).

When using ESM, if there is a chance that the code may be run on a build of Node.js where crypto support is not enabled, consider using the `import()` function instead of the lexical `import` keyword:

    let crypto;
    try {
      crypto = await import('node:crypto');
    } catch (err) {
      console.error('crypto support is disabled!');
    }
    mjs

### Asymmetric key types#

The following lists group the asymmetric key types recognized by the `KeyObject` API by the complete set of formats supported for importing and exporting each type.

Formats: `'pem'`, `'der'`

- `'dh'` (Diffie-Hellman) — OID `1.2.840.113549.1.3.1`
- `'dsa'` — OID `1.2.840.10040.4.1`
- `'rsa-pss'` — OID `1.2.840.113549.1.1.10`

Formats: `'pem'`, `'der'`, `'jwk'`

- `'rsa'` — OID `1.2.840.113549.1.1.1`

Formats: `'pem'`, `'der'`, `'jwk'`, `'raw-public'`, `'raw-private'`

- `'ec'` (Elliptic curve) — OID `1.2.840.10045.2.1`
- `'ed25519'` — OID `1.3.101.112`
- `'ed448'` — OID `1.3.101.113`
- `'slh-dsa-sha2-128f'`1 — OID `2.16.840.1.101.3.4.3.21`
- `'slh-dsa-sha2-128s'`1 — OID `2.16.840.1.101.3.4.3.20`
- `'slh-dsa-sha2-192f'`1 — OID `2.16.840.1.101.3.4.3.23`
- `'slh-dsa-sha2-192s'`1 — OID `2.16.840.1.101.3.4.3.22`
- `'slh-dsa-sha2-256f'`1 — OID `2.16.840.1.101.3.4.3.25`
- `'slh-dsa-sha2-256s'`1 — OID `2.16.840.1.101.3.4.3.24`
- `'slh-dsa-shake-128f'`1 — OID `2.16.840.1.101.3.4.3.27`
- `'slh-dsa-shake-128s'`1 — OID `2.16.840.1.101.3.4.3.26`
- `'slh-dsa-shake-192f'`1 — OID `2.16.840.1.101.3.4.3.29`
- `'slh-dsa-shake-192s'`1 — OID `2.16.840.1.101.3.4.3.28`
- `'slh-dsa-shake-256f'`1 — OID `2.16.840.1.101.3.4.3.31`
- `'slh-dsa-shake-256s'`1 — OID `2.16.840.1.101.3.4.3.30`
- `'x25519'` — OID `1.3.101.110`
- `'x448'` — OID `1.3.101.111`

Formats: `'pem'`, `'der'`, `'jwk'`, `'raw-public'`, `'raw-seed'`

- `'ml-dsa-44'`1 — OID `2.16.840.1.101.3.4.3.17`
- `'ml-dsa-65'`1 — OID `2.16.840.1.101.3.4.3.18`
- `'ml-dsa-87'`1 — OID `2.16.840.1.101.3.4.3.19`
- `'ml-kem-512'`1 — OID `2.16.840.1.101.3.4.4.1`
- `'ml-kem-768'`1 — OID `2.16.840.1.101.3.4.4.2`
- `'ml-kem-1024'`1 — OID `2.16.840.1.101.3.4.4.3`

#### Key formats#

Asymmetric keys can be represented in several formats. The recommended approach is to import key material into a `KeyObject` once and reuse it for all subsequent operations, as this avoids repeated parsing and delivers the best performance.

When a `KeyObject` is not practical - for example, when key material arrives in a protocol message and is used only once - most cryptographic functions also accept a PEM string or an object specifying the format and key material directly. See `crypto.createPublicKey()`, `crypto.createPrivateKey()`, and `keyObject.export()` for the full options accepted by each format.

##### KeyObject#

A `KeyObject` is the in-memory representation of a parsed key. It is created by `crypto.createPublicKey()`, `crypto.createPrivateKey()`, `crypto.createSecretKey()`, or key generation functions such as `crypto.generateKeyPair()`. The first cryptographic operation with a given `KeyObject` may be slower than subsequent ones because OpenSSL lazily initializes internal caches on first use.

##### PEM and DER#

PEM and DER are the traditional encoding formats for asymmetric keys based on ASN.1 structures.

- PEM is a text encoding that wraps Base64-encoded DER data between header and footer lines (e.g. `-----BEGIN PUBLIC KEY-----`). PEM strings can be passed directly to most cryptographic operations.
- DER is the binary encoding of the same ASN.1 structures. When providing DER input, the `type` (typically `'spki'` or `'pkcs8'`) must be specified explicitly.

##### JSON Web Key (JWK)#

JSON Web Key (JWK) is a JSON-based key representation defined in RFC 7517. JWK encodes each key component as an individual Base64url-encoded value inside a JSON object. For RSA keys, JWK avoids ASN.1 parsing overhead and is the fastest serialized import format.

##### Raw key formats#

Stability: 1.1 - Active development

The `'raw-public'`, `'raw-private'`, and `'raw-seed'` key formats allow importing and exporting raw key material without any encoding wrapper. See `keyObject.export()`, `crypto.createPublicKey()`, and `crypto.createPrivateKey()` for usage details.

`'raw-public'` is generally the fastest way to import a public key. `'raw-private'` and `'raw-seed'` are not always faster than other formats because they only contain the private scalar or seed - importing them requires deriving the public key component (e.g. elliptic curve point multiplication or seed expansion), which can be expensive. Other formats include both private and public components, avoiding that computation.

#### Choosing a key format#

Always prefer a `KeyObject` - create one from whatever format you have and reuse it. The guidance below applies only when choosing between serialization formats, either for importing into a `KeyObject` or for passing key material inline when a `KeyObject` is not practical.

##### Importing keys#

When creating a `KeyObject` for repeated use, the import cost is paid once, so choosing a faster format reduces startup latency.

The import cost breaks down into two parts: parsing overhead (decoding the serialization wrapper) and key computation (any mathematical work needed to reconstruct the full key, such as deriving a public key from a private scalar or expanding a seed). Which part dominates depends on the key type. For example:

- Public keys - `'raw-public'` is the fastest serialized format because the raw format skips all ASN.1 and Base64 decoding.
- EC private keys - `'raw-private'` is faster than PEM or DER because it avoids ASN.1 parsing. However, for larger curves (e.g. P-384, P-521) the required derivation of the public point from the private scalar becomes expensive, reducing the advantage.
- RSA keys - `'jwk'` is the fastest serialized format. JWK represents RSA key components as individual Base64url-encoded integers, avoiding the overhead of ASN.1 parsing entirely.

##### Inline key material in operations#

When a `KeyObject` cannot be reused (e.g. the key arrives as raw bytes in a protocol message and is used only once), most cryptographic functions also accept a PEM string or an object specifying the format and key material directly. In this case the total cost is the sum of key import and the cryptographic computation itself.

For operations where the cryptographic computation dominates - such as signing with RSA or ECDH key agreement with P-384 or P-521 - the serialization format has negligible impact on overall throughput, so choose whichever format is most convenient. For lightweight operations like Ed25519 signing or verification, the import cost is a larger fraction of the total, so a faster format like `'raw-public'` or `'raw-private'` can meaningfully improve throughput.

Even if the same key material is used only a few times, it is worth importing it into a `KeyObject` rather than passing the raw or PEM representation repeatedly.

#### Examples#

Example: Reusing a `KeyObject` across sign and verify operations:

    import { promisify } from 'node:util';
    const { generateKeyPair, sign, verify } = await import('node:crypto');

    const { publicKey, privateKey } = await promisify(generateKeyPair)('ed25519');

    // A KeyObject holds the parsed key in memory and can be reused
    // across multiple operations without re-parsing.
    const data = new TextEncoder().encode('message to sign');
    const signature = sign(null, data, privateKey);
    verify(null, data, publicKey, signature);
    mjs

Example: Importing keys of various formats into `KeyObject`s:

    import { promisify } from 'node:util';
    const {
      createPrivateKey, createPublicKey, generateKeyPair,
    } = await import('node:crypto');

    const generated = await promisify(generateKeyPair)('ed25519');

    // PEM
    const privatePem = generated.privateKey.export({ format: 'pem', type: 'pkcs8' });
    const publicPem = generated.publicKey.export({ format: 'pem', type: 'spki' });
    createPrivateKey(privatePem);
    createPublicKey(publicPem);

    // DER - requires explicit type
    const privateDer = generated.privateKey.export({ format: 'der', type: 'pkcs8' });
    const publicDer = generated.publicKey.export({ format: 'der', type: 'spki' });
    createPrivateKey({ key: privateDer, format: 'der', type: 'pkcs8' });
    createPublicKey({ key: publicDer, format: 'der', type: 'spki' });

    // JWK
    const privateJwk = generated.privateKey.export({ format: 'jwk' });
    const publicJwk = generated.publicKey.export({ format: 'jwk' });
    createPrivateKey({ key: privateJwk, format: 'jwk' });
    createPublicKey({ key: publicJwk, format: 'jwk' });

    // Raw
    const rawPriv = generated.privateKey.export({ format: 'raw-private' });
    const rawPub = generated.publicKey.export({ format: 'raw-public' });
    createPrivateKey({ key: rawPriv, format: 'raw-private', asymmetricKeyType: 'ed25519' });
    createPublicKey({ key: rawPub, format: 'raw-public', asymmetricKeyType: 'ed25519' });
    mjs

Example: Passing key material directly to `crypto.sign()` and `crypto.verify()` without creating a `KeyObject` first:

    import { promisify } from 'node:util';
    const { generateKeyPair, sign, verify } = await import('node:crypto');

    const generated = await promisify(generateKeyPair)('ed25519');

    const data = new TextEncoder().encode('message to sign');

    // PEM strings
    const privatePem = generated.privateKey.export({ format: 'pem', type: 'pkcs8' });
    const publicPem = generated.publicKey.export({ format: 'pem', type: 'spki' });
    const sig1 = sign(null, data, privatePem);
    verify(null, data, publicPem, sig1);

    // JWK objects
    const privateJwk = generated.privateKey.export({ format: 'jwk' });
    const publicJwk = generated.publicKey.export({ format: 'jwk' });
    const sig2 = sign(null, data, { key: privateJwk, format: 'jwk' });
    verify(null, data, { key: publicJwk, format: 'jwk' }, sig2);

    // Raw key bytes
    const rawPriv = generated.privateKey.export({ format: 'raw-private' });
    const rawPub = generated.publicKey.export({ format: 'raw-public' });
    const sig3 = sign(null, data, {
      key: rawPriv, format: 'raw-private', asymmetricKeyType: 'ed25519',
    });
    verify(null, data, {
      key: rawPub, format: 'raw-public', asymmetricKeyType: 'ed25519',
    }, sig3);
    mjs

Example: For EC keys, the `namedCurve` option is required when importing raw keys:

    import { promisify } from 'node:util';
    const {
      createPrivateKey, createPublicKey, generateKeyPair, sign, verify,
    } = await import('node:crypto');

    const generated = await promisify(generateKeyPair)('ec', {
      namedCurve: 'P-256',
    });

    // Export the raw EC public key (uncompressed by default).
    const rawPublicKey = generated.publicKey.export({ format: 'raw-public' });

    // The following is equivalent.
    const rawPublicKeyUncompressed = generated.publicKey.export({
      format: 'raw-public',
      type: 'uncompressed',
    });

    // Export compressed point format.
    const rawPublicKeyCompressed = generated.publicKey.export({
      format: 'raw-public',
      type: 'compressed',
    });

    // Export the raw EC private key.
    const rawPrivateKey = generated.privateKey.export({ format: 'raw-private' });

    // Import the raw EC keys.
    // Both compressed and uncompressed point formats are accepted.
    const publicKey = createPublicKey({
      key: rawPublicKey,
      format: 'raw-public',
      asymmetricKeyType: 'ec',
      namedCurve: 'P-256',
    });
    const privateKey = createPrivateKey({
      key: rawPrivateKey,
      format: 'raw-private',
      asymmetricKeyType: 'ec',
      namedCurve: 'P-256',
    });

    const data = new TextEncoder().encode('message to sign');
    const signature = sign('sha256', data, privateKey);
    verify('sha256', data, publicKey, signature);
    mjs

Example: Exporting raw seeds and importing them:

    import { promisify } from 'node:util';
    const {
      createPrivateKey, decapsulate, encapsulate, generateKeyPair,
    } = await import('node:crypto');

    const generated = await promisify(generateKeyPair)('ml-kem-768');

    // Export the raw seed (64 bytes for ML-KEM).
    const seed = generated.privateKey.export({ format: 'raw-seed' });

    // Import the raw seed.
    const privateKey = createPrivateKey({
      key: seed,
      format: 'raw-seed',
      asymmetricKeyType: 'ml-kem-768',
    });

    const { ciphertext } = encapsulate(generated.publicKey);
    decapsulate(privateKey, ciphertext);
    mjs

### Class: `Certificate`#

Added in: v0.11.8

SPKAC is a Certificate Signing Request mechanism originally implemented by Netscape and was specified formally as part of HTML5's `keygen` element.

`<keygen>` is deprecated since HTML 5.2 and new projects should not use this element anymore.

The `node:crypto` module provides the `Certificate` class for working with SPKAC data. The most common usage is handling output generated by the HTML5 `<keygen>` element. Node.js uses OpenSSL's SPKAC implementation internally.

#### Static method: `Certificate.exportChallenge(spkac[, encoding])`#

Added in: v9.0.0
History

Version | Changes
--- | ---
v15.0.0 | The spkac argument can be an ArrayBuffer. Limited the size of the spkac argument to a maximum of 2**31 - 1 bytes.

- `spkac` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `spkac` string.
- Returns: `<Buffer>` The challenge component of the `spkac` data structure, which includes a public key and a challenge.

    const { Certificate } = await import('node:crypto');
    const spkac = getSpkacSomehow();
    const challenge = Certificate.exportChallenge(spkac);
    console.log(challenge.toString('utf8'));
    // Prints: the challenge as a UTF8 string
    const { Certificate } = require('node:crypto');
    const spkac = getSpkacSomehow();
    const challenge = Certificate.exportChallenge(spkac);
    console.log(challenge.toString('utf8'));
    // Prints: the challenge as a UTF8 string
    javascript

#### Static method: `Certificate.exportPublicKey(spkac[, encoding])`#

Added in: v9.0.0
History

Version | Changes
--- | ---
v15.0.0 | The spkac argument can be an ArrayBuffer. Limited the size of the spkac argument to a maximum of 2**31 - 1 bytes.

- `spkac` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `spkac` string.
- Returns: `<Buffer>` The public key component of the `spkac` data structure, which includes a public key and a challenge.

    const { Certificate } = await import('node:crypto');
    const spkac = getSpkacSomehow();
    const publicKey = Certificate.exportPublicKey(spkac);
    console.log(publicKey);
    // Prints: the public key as <Buffer ...>
    const { Certificate } = require('node:crypto');
    const spkac = getSpkacSomehow();
    const publicKey = Certificate.exportPublicKey(spkac);
    console.log(publicKey);
    // Prints: the public key as <Buffer ...>
    javascript

#### Static method: `Certificate.verifySpkac(spkac[, encoding])`#

Added in: v9.0.0
History

Version | Changes
--- | ---
v15.0.0 | The spkac argument can be an ArrayBuffer. Added encoding. Limited the size of the spkac argument to a maximum of 2**31 - 1 bytes.

- `spkac` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `spkac` string.
- Returns: `<boolean>` `true` if the given `spkac` data structure is valid, `false` otherwise.

    import { Buffer } from 'node:buffer';
    const { Certificate } = await import('node:crypto');

    const spkac = getSpkacSomehow();
    console.log(Certificate.verifySpkac(Buffer.from(spkac)));
    // Prints: true or false
    const { Buffer } = require('node:buffer');
    const { Certificate } = require('node:crypto');

    const spkac = getSpkacSomehow();
    console.log(Certificate.verifySpkac(Buffer.from(spkac)));
    // Prints: true or false
    javascript

#### Legacy API#

Stability: 0 - Deprecated

As a legacy interface, it is possible to create new instances of the `crypto.Certificate` class as illustrated in the examples below.

##### `new crypto.Certificate()`#

Instances of the `Certificate` class can be created using the `new` keyword or by calling `crypto.Certificate()` as a function:

    const { Certificate } = await import('node:crypto');

    const cert1 = new Certificate();
    const cert2 = Certificate();
    const { Certificate } = require('node:crypto');

    const cert1 = new Certificate();
    const cert2 = Certificate();
    javascript

##### `certificate.exportChallenge(spkac[, encoding])`#

Added in: v0.11.8

- `spkac` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `spkac` string.
- Returns: `<Buffer>` The challenge component of the `spkac` data structure, which includes a public key and a challenge.

    const { Certificate } = await import('node:crypto');
    const cert = Certificate();
    const spkac = getSpkacSomehow();
    const challenge = cert.exportChallenge(spkac);
    console.log(challenge.toString('utf8'));
    // Prints: the challenge as a UTF8 string
    const { Certificate } = require('node:crypto');
    const cert = Certificate();
    const spkac = getSpkacSomehow();
    const challenge = cert.exportChallenge(spkac);
    console.log(challenge.toString('utf8'));
    // Prints: the challenge as a UTF8 string
    javascript

##### `certificate.exportPublicKey(spkac[, encoding])`#

Added in: v0.11.8

- `spkac` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `spkac` string.
- Returns: `<Buffer>` The public key component of the `spkac` data structure, which includes a public key and a challenge.

    const { Certificate } = await import('node:crypto');
    const cert = Certificate();
    const spkac = getSpkacSomehow();
    const publicKey = cert.exportPublicKey(spkac);
    console.log(publicKey);
    // Prints: the public key as <Buffer ...>
    const { Certificate } = require('node:crypto');
    const cert = Certificate();
    const spkac = getSpkacSomehow();
    const publicKey = cert.exportPublicKey(spkac);
    console.log(publicKey);
    // Prints: the public key as <Buffer ...>
    javascript

##### `certificate.verifySpkac(spkac[, encoding])`#

Added in: v0.11.8

- `spkac` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `spkac` string.
- Returns: `<boolean>` `true` if the given `spkac` data structure is valid, `false` otherwise.

    import { Buffer } from 'node:buffer';
    const { Certificate } = await import('node:crypto');

    const cert = Certificate();
    const spkac = getSpkacSomehow();
    console.log(cert.verifySpkac(Buffer.from(spkac)));
    // Prints: true or false
    const { Buffer } = require('node:buffer');
    const { Certificate } = require('node:crypto');

    const cert = Certificate();
    const spkac = getSpkacSomehow();
    console.log(cert.verifySpkac(Buffer.from(spkac)));
    // Prints: true or false
    javascript

### Class: `Cipheriv`#

Added in: v0.1.94

- Extends: `<stream.Transform>`

Instances of the `Cipheriv` class are used to encrypt data. The class can be used in one of two ways:

- As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
- Using the `cipher.update()` and `cipher.final()` methods to produce the encrypted data.

The `crypto.createCipheriv()` method is used to create `Cipheriv` instances. `Cipheriv` objects are not to be created directly using the `new` keyword.

Example: Using `Cipheriv` objects as streams:

    const {
      scrypt,
      randomFill,
      createCipheriv,
    } = await import('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';

    // First, we'll generate the key. The key length is dependent on the algorithm.
    // In this case for aes192, it is 24 bytes (192 bits).
    scrypt(password, 'salt', 24, (err, key) => {
      if (err) throw err;
      // Then, we'll generate a random initialization vector
      randomFill(new Uint8Array(16), (err, iv) => {
        if (err) throw err;

        // Once we have the key and iv, we can create and use the cipher...
        const cipher = createCipheriv(algorithm, key, iv);

        let encrypted = '';
        cipher.setEncoding('hex');

        cipher.on('data', (chunk) => encrypted += chunk);
        cipher.on('end', () => console.log(encrypted));

        cipher.write('some clear text data');
        cipher.end();
      });
    });
    const {
      scrypt,
      randomFill,
      createCipheriv,
    } = require('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';

    // First, we'll generate the key. The key length is dependent on the algorithm.
    // In this case for aes192, it is 24 bytes (192 bits).
    scrypt(password, 'salt', 24, (err, key) => {
      if (err) throw err;
      // Then, we'll generate a random initialization vector
      randomFill(new Uint8Array(16), (err, iv) => {
        if (err) throw err;

        // Once we have the key and iv, we can create and use the cipher...
        const cipher = createCipheriv(algorithm, key, iv);

        let encrypted = '';
        cipher.setEncoding('hex');

        cipher.on('data', (chunk) => encrypted += chunk);
        cipher.on('end', () => console.log(encrypted));

        cipher.write('some clear text data');
        cipher.end();
      });
    });
    javascript

Example: Using `Cipheriv` and piped streams:

    import {
      createReadStream,
      createWriteStream,
    } from 'node:fs';

    import {
      pipeline,
    } from 'node:stream';

    const {
      scrypt,
      randomFill,
      createCipheriv,
    } = await import('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';

    // First, we'll generate the key. The key length is dependent on the algorithm.
    // In this case for aes192, it is 24 bytes (192 bits).
    scrypt(password, 'salt', 24, (err, key) => {
      if (err) throw err;
      // Then, we'll generate a random initialization vector
      randomFill(new Uint8Array(16), (err, iv) => {
        if (err) throw err;

        const cipher = createCipheriv(algorithm, key, iv);

        const input = createReadStream('test.js');
        const output = createWriteStream('test.enc');

        pipeline(input, cipher, output, (err) => {
          if (err) throw err;
        });
      });
    });
    const {
      createReadStream,
      createWriteStream,
    } = require('node:fs');

    const {
      pipeline,
    } = require('node:stream');

    const {
      scrypt,
      randomFill,
      createCipheriv,
    } = require('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';

    // First, we'll generate the key. The key length is dependent on the algorithm.
    // In this case for aes192, it is 24 bytes (192 bits).
    scrypt(password, 'salt', 24, (err, key) => {
      if (err) throw err;
      // Then, we'll generate a random initialization vector
      randomFill(new Uint8Array(16), (err, iv) => {
        if (err) throw err;

        const cipher = createCipheriv(algorithm, key, iv);

        const input = createReadStream('test.js');
        const output = createWriteStream('test.enc');

        pipeline(input, cipher, output, (err) => {
          if (err) throw err;
        });
      });
    });
    javascript

Example: Using the `cipher.update()` and `cipher.final()` methods:

    const {
      scrypt,
      randomFill,
      createCipheriv,
    } = await import('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';

    // First, we'll generate the key. The key length is dependent on the algorithm.
    // In this case for aes192, it is 24 bytes (192 bits).
    scrypt(password, 'salt', 24, (err, key) => {
      if (err) throw err;
      // Then, we'll generate a random initialization vector
      randomFill(new Uint8Array(16), (err, iv) => {
        if (err) throw err;

        const cipher = createCipheriv(algorithm, key, iv);

        let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
        encrypted += cipher.final('hex');
        console.log(encrypted);
      });
    });
    const {
      scrypt,
      randomFill,
      createCipheriv,
    } = require('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';

    // First, we'll generate the key. The key length is dependent on the algorithm.
    // In this case for aes192, it is 24 bytes (192 bits).
    scrypt(password, 'salt', 24, (err, key) => {
      if (err) throw err;
      // Then, we'll generate a random initialization vector
      randomFill(new Uint8Array(16), (err, iv) => {
        if (err) throw err;

        const cipher = createCipheriv(algorithm, key, iv);

        let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
        encrypted += cipher.final('hex');
        console.log(encrypted);
      });
    });
    javascript

#### `cipher.final([outputEncoding])`#

Added in: v0.1.94

- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>` Any remaining enciphered contents. If `outputEncoding` is specified, a string is returned. If an `outputEncoding` is not provided, a `Buffer` is returned.

If an output encoding was specified in a previous call to `cipher.update()`, `outputEncoding` must use the same encoding.

Once the `cipher.final()` method has been called, the `Cipheriv` object can no longer be used to encrypt data. Attempts to call `cipher.final()` more than once will result in an error being thrown.

#### `cipher.getAuthTag()`#

Added in: v1.0.0

- Returns: `<Buffer>` When using an authenticated encryption mode (`GCM`, `CCM`, `OCB`, and `chacha20-poly1305` are currently supported), the `cipher.getAuthTag()` method returns a `Buffer` containing the authentication tag that has been computed from the given data.

The `cipher.getAuthTag()` method should only be called after encryption has been completed using the `cipher.final()` method.

If the `authTagLength` option was set during the `cipher` instance's creation, this function will return exactly `authTagLength` bytes.

#### `cipher.setAAD(buffer[, options])`#

Added in: v1.0.0

- `buffer` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>` `stream.transform` options
  - `plaintextLength` `<number>`
  - `encoding` `<string>` The string encoding to use when `buffer` is a string.
- Returns: `<Cipheriv>` The same `Cipheriv` instance for method chaining.

When using an authenticated encryption mode (`GCM`, `CCM`, `OCB`, and `chacha20-poly1305` are currently supported), the `cipher.setAAD()` method sets the value used for the additional authenticated data (AAD) input parameter.

The `plaintextLength` option is optional for `GCM` and `OCB`. When using `CCM`, the `plaintextLength` option must be specified and its value must match the length of the plaintext in bytes. See CCM mode.

The `cipher.setAAD()` method must be called before `cipher.update()`.

#### `cipher.setAutoPadding([autoPadding])`#

Added in: v0.7.1

- `autoPadding` `<boolean>` Default: `true`
- Returns: `<Cipheriv>` The same `Cipheriv` instance for method chaining.

When using block encryption algorithms, the `Cipheriv` class will automatically add padding to the input data to the appropriate block size. To disable the default padding call `cipher.setAutoPadding(false)`.

When `autoPadding` is `false`, the length of the entire input data must be a multiple of the cipher's block size or `cipher.final()` will throw an error. Disabling automatic padding is useful for non-standard padding, for instance using `0x0` instead of PKCS padding.

The `cipher.setAutoPadding()` method must be called before `cipher.final()`.

#### `cipher.update(data[, inputEncoding][, outputEncoding])`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the data.
- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Updates the cipher with `data`. If the `inputEncoding` argument is given, the `data` argument is a string using the specified encoding. If the `inputEncoding` argument is not given, `data` must be a `Buffer`, `TypedArray`, or `DataView`. If `data` is a `Buffer`, `TypedArray`, or `DataView`, then `inputEncoding` is ignored.

The `outputEncoding` specifies the output format of the enciphered data. If the `outputEncoding` is specified, a string using the specified encoding is returned. If no `outputEncoding` is provided, a `Buffer` is returned. When `outputEncoding` is specified, it must use the same encoding as previous calls to `cipher.update()`.

The `cipher.update()` method can be called multiple times with new data until `cipher.final()` is called. Calling `cipher.update()` after `cipher.final()` will result in an error being thrown.

### Class: `Decipheriv`#

Added in: v0.1.94

- Extends: `<stream.Transform>`

Instances of the `Decipheriv` class are used to decrypt data. The class can be used in one of two ways:

- As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
- Using the `decipher.update()` and `decipher.final()` methods to produce the unencrypted data.

The `crypto.createDecipheriv()` method is used to create `Decipheriv` instances. `Decipheriv` objects are not to be created directly using the `new` keyword.

Example: Using `Decipheriv` objects as streams:

    import { Buffer } from 'node:buffer';
    const {
      scryptSync,
      createDecipheriv,
    } = await import('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';
    // Key length is dependent on the algorithm. In this case for aes192, it is
    // 24 bytes (192 bits).
    // Use the async `crypto.scrypt()` instead.
    const key = scryptSync(password, 'salt', 24);
    // The IV is usually passed along with the ciphertext.
    const iv = Buffer.alloc(16, 0); // Initialization vector.

    const decipher = createDecipheriv(algorithm, key, iv);

    let decrypted = '';
    decipher.on('readable', () => {
      let chunk;
      while (null !== (chunk = decipher.read())) {
        decrypted += chunk.toString('utf8');
      }
    });
    decipher.on('end', () => {
      console.log(decrypted);
      // Prints: some clear text data
    });

    // Encrypted with same algorithm, key and iv.
    const encrypted =
      'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
    decipher.write(encrypted, 'hex');
    decipher.end();
    const {
      scryptSync,
      createDecipheriv,
    } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';
    // Key length is dependent on the algorithm. In this case for aes192, it is
    // 24 bytes (192 bits).
    // Use the async `crypto.scrypt()` instead.
    const key = scryptSync(password, 'salt', 24);
    // The IV is usually passed along with the ciphertext.
    const iv = Buffer.alloc(16, 0); // Initialization vector.

    const decipher = createDecipheriv(algorithm, key, iv);

    let decrypted = '';
    decipher.on('readable', () => {
      let chunk;
      while (null !== (chunk = decipher.read())) {
        decrypted += chunk.toString('utf8');
      }
    });
    decipher.on('end', () => {
      console.log(decrypted);
      // Prints: some clear text data
    });

    // Encrypted with same algorithm, key and iv.
    const encrypted =
      'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
    decipher.write(encrypted, 'hex');
    decipher.end();
    javascript

Example: Using `Decipheriv` and piped streams:

    import {
      createReadStream,
      createWriteStream,
    } from 'node:fs';
    import { Buffer } from 'node:buffer';
    const {
      scryptSync,
      createDecipheriv,
    } = await import('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';
    // Use the async `crypto.scrypt()` instead.
    const key = scryptSync(password, 'salt', 24);
    // The IV is usually passed along with the ciphertext.
    const iv = Buffer.alloc(16, 0); // Initialization vector.

    const decipher = createDecipheriv(algorithm, key, iv);

    const input = createReadStream('test.enc');
    const output = createWriteStream('test.js');

    input.pipe(decipher).pipe(output);
    const {
      createReadStream,
      createWriteStream,
    } = require('node:fs');
    const {
      scryptSync,
      createDecipheriv,
    } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';
    // Use the async `crypto.scrypt()` instead.
    const key = scryptSync(password, 'salt', 24);
    // The IV is usually passed along with the ciphertext.
    const iv = Buffer.alloc(16, 0); // Initialization vector.

    const decipher = createDecipheriv(algorithm, key, iv);

    const input = createReadStream('test.enc');
    const output = createWriteStream('test.js');

    input.pipe(decipher).pipe(output);
    javascript

Example: Using the `decipher.update()` and `decipher.final()` methods:

    import { Buffer } from 'node:buffer';
    const {
      scryptSync,
      createDecipheriv,
    } = await import('node:crypto');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';
    // Use the async `crypto.scrypt()` instead.
    const key = scryptSync(password, 'salt', 24);
    // The IV is usually passed along with the ciphertext.
    const iv = Buffer.alloc(16, 0); // Initialization vector.

    const decipher = createDecipheriv(algorithm, key, iv);

    // Encrypted using same algorithm, key and iv.
    const encrypted =
      'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
    let decrypted = decipher.update(encrypted, 'hex', 'utf8');
    decrypted += decipher.final('utf8');
    console.log(decrypted);
    // Prints: some clear text data
    const {
      scryptSync,
      createDecipheriv,
    } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const algorithm = 'aes-192-cbc';
    const password = 'Password used to generate key';
    // Use the async `crypto.scrypt()` instead.
    const key = scryptSync(password, 'salt', 24);
    // The IV is usually passed along with the ciphertext.
    const iv = Buffer.alloc(16, 0); // Initialization vector.

    const decipher = createDecipheriv(algorithm, key, iv);

    // Encrypted using same algorithm, key and iv.
    const encrypted =
      'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
    let decrypted = decipher.update(encrypted, 'hex', 'utf8');
    decrypted += decipher.final('utf8');
    console.log(decrypted);
    // Prints: some clear text data
    javascript

#### `decipher.final([outputEncoding])`#

Added in: v0.1.94

- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>` Any remaining deciphered contents. If `outputEncoding` is specified, a string is returned. If an `outputEncoding` is not provided, a `Buffer` is returned.

If an output encoding was specified in a previous call to `decipher.update()`, `outputEncoding` must use the same encoding.

Once the `decipher.final()` method has been called, the `Decipheriv` object can no longer be used to decrypt data. Attempts to call `decipher.final()` more than once will result in an error being thrown.

#### `decipher.setAAD(buffer[, options])`#

Added in: v1.0.0
History

Version | Changes
--- | ---
v15.0.0 | The buffer argument can be a string or ArrayBuffer and is limited to no more than 2 ** 31 - 1 bytes.
v7.2.0 | This method now returns a reference to `decipher`.

- `buffer` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `options` `<Object>` `stream.transform` options
  - `plaintextLength` `<number>`
  - `encoding` `<string>` String encoding to use when `buffer` is a string.
- Returns: `<Decipheriv>` The same `Decipheriv` instance for method chaining.

When using an authenticated encryption mode (`GCM`, `CCM`, `OCB`, and `chacha20-poly1305` are currently supported), the `decipher.setAAD()` method sets the value used for the additional authenticated data (AAD) input parameter.

The `options` argument is optional for `GCM`. When using `CCM`, the `plaintextLength` option must be specified and its value must match the length of the ciphertext in bytes. See CCM mode.

The `decipher.setAAD()` method must be called before `decipher.update()`.

When passing a string as the `buffer`, please consider caveats when using strings as inputs to cryptographic APIs.

#### `decipher.setAuthTag(buffer[, encoding])`#

Added in: v1.0.0
History

Version | Changes
--- | ---
v26.0.0 | Using GCM tag lengths other than 128 bits without specifying the `authTagLength` option when creating `decipher` is not allowed anymore.
v22.0.0, v20.13.0 | Using GCM tag lengths other than 128 bits without specifying the `authTagLength` option when creating `decipher` is deprecated.
v15.0.0 | The buffer argument can be a string or ArrayBuffer and is limited to no more than 2 ** 31 - 1 bytes.
v11.0.0 | This method now throws if the GCM tag length is invalid.
v7.2.0 | This method now returns a reference to `decipher`.

- `buffer` `<string>` | `<Buffer>` | `<ArrayBuffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` String encoding to use when `buffer` is a string.
- Returns: `<Decipheriv>` The same `Decipheriv` instance for method chaining.

When using an authenticated encryption mode (`GCM`, `CCM`, `OCB`, and `chacha20-poly1305` are currently supported), the `decipher.setAuthTag()` method is used to pass in the received authentication tag. If no tag is provided, or if the cipher text has been tampered with, `decipher.final()` will throw, indicating that the cipher text should be discarded due to failed authentication. If the tag length is invalid according to NIST SP 800-38D or does not match the value of the `authTagLength` option, `decipher.setAuthTag()` will throw an error.

The `decipher.setAuthTag()` method must be called before `decipher.update()` for `CCM` mode or before `decipher.final()` for `GCM` and `OCB` modes and `chacha20-poly1305`. `decipher.setAuthTag()` can only be called once.

When passing a string as the authentication tag, please consider caveats when using strings as inputs to cryptographic APIs.

#### `decipher.setAutoPadding([autoPadding])`#

Added in: v0.7.1

- `autoPadding` `<boolean>` Default: `true`
- Returns: `<Decipheriv>` The same `Decipheriv` instance for method chaining.

When data has been encrypted without standard block padding, calling `decipher.setAutoPadding(false)` will disable automatic padding to prevent `decipher.final()` from checking for and removing padding.

Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.

The `decipher.setAutoPadding()` method must be called before `decipher.final()`.

#### `decipher.update(data[, inputEncoding][, outputEncoding])`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the `data` string.
- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Updates the decipher with `data`. If the `inputEncoding` argument is given, the `data` argument is a string using the specified encoding. If the `inputEncoding` argument is not given, `data` must be a `Buffer`. If `data` is a `Buffer` then `inputEncoding` is ignored.

The `outputEncoding` specifies the output format of the enciphered data. If the `outputEncoding` is specified, a string using the specified encoding is returned. If no `outputEncoding` is provided, a `Buffer` is returned. When `outputEncoding` is specified, it must use the same encoding as previous calls to `decipher.update()`.

The `decipher.update()` method can be called multiple times with new data until `decipher.final()` is called. Calling `decipher.update()` after `decipher.final()` will result in an error being thrown.

Even if the underlying cipher implements authentication, the authenticity and integrity of the plaintext returned from this function may be uncertain at this time. For authenticated encryption algorithms, authenticity is generally only established when the application calls `decipher.final()`.

### Class: `DiffieHellman`#

Added in: v0.5.0

The `DiffieHellman` class is a utility for creating Diffie-Hellman key exchanges.

Instances of the `DiffieHellman` class can be created using the `crypto.createDiffieHellman()` function.

    import assert from 'node:assert';

    const {
      createDiffieHellman,
    } = await import('node:crypto');

    // Generate Alice's keys...
    const alice = createDiffieHellman(2048);
    const aliceKey = alice.generateKeys();

    // Generate Bob's keys...
    const bob = createDiffieHellman(alice.getPrime(), alice.getGenerator());
    const bobKey = bob.generateKeys();

    // Exchange and generate the secret...
    const aliceSecret = alice.computeSecret(bobKey);
    const bobSecret = bob.computeSecret(aliceKey);

    // OK
    assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
    const assert = require('node:assert');

    const {
      createDiffieHellman,
    } = require('node:crypto');

    // Generate Alice's keys...
    const alice = createDiffieHellman(2048);
    const aliceKey = alice.generateKeys();

    // Generate Bob's keys...
    const bob = createDiffieHellman(alice.getPrime(), alice.getGenerator());
    const bobKey = bob.generateKeys();

    // Exchange and generate the secret...
    const aliceSecret = alice.computeSecret(bobKey);
    const bobSecret = bob.computeSecret(aliceKey);

    // OK
    assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
    javascript

#### `diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])`#

Added in: v0.5.0

- `otherPublicKey` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of an `otherPublicKey` string.
- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Computes the shared secret using `otherPublicKey` as the other party's public key and returns the computed shared secret. The supplied key is interpreted using the specified `inputEncoding`, and secret is encoded using specified `outputEncoding`. If the `inputEncoding` is not provided, `otherPublicKey` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

If `outputEncoding` is given a string is returned; otherwise, a `Buffer` is returned.

#### `diffieHellman.generateKeys([encoding])`#

Added in: v0.5.0

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Generates private and public Diffie-Hellman key values unless they have been generated or computed already, and returns the public key in the specified `encoding`. This key should be transferred to the other party. If `encoding` is provided a string is returned; otherwise a `Buffer` is returned.

This function is a thin wrapper around `DH_generate_key()`. In particular, once a private key has been generated or set, calling this function only recomputes the public key from the existing private key. Since the public key is determined by the private key, the result will be the same unless the private key has been changed via `diffieHellman.setPrivateKey()`.

#### `diffieHellman.getGenerator([encoding])`#

Added in: v0.5.0

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Returns the Diffie-Hellman generator in the specified `encoding`. If `encoding` is provided a string is returned; otherwise a `Buffer` is returned.

#### `diffieHellman.getPrime([encoding])`#

Added in: v0.5.0

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Returns the Diffie-Hellman prime in the specified `encoding`. If `encoding` is provided a string is returned; otherwise a `Buffer` is returned.

#### `diffieHellman.getPrivateKey([encoding])`#

Added in: v0.5.0

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Returns the Diffie-Hellman private key in the specified `encoding`. If `encoding` is provided a string is returned; otherwise a `Buffer` is returned.

#### `diffieHellman.getPublicKey([encoding])`#

Added in: v0.5.0

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Returns the Diffie-Hellman public key in the specified `encoding`. If `encoding` is provided a string is returned; otherwise a `Buffer` is returned.

#### `diffieHellman.setPrivateKey(privateKey[, encoding])`#

Added in: v0.5.0

- `privateKey` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `privateKey` string.

Sets the Diffie-Hellman private key. If the `encoding` argument is provided, `privateKey` is expected to be a string. If no `encoding` is provided, `privateKey` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

This function does not automatically compute the associated public key. Either `diffieHellman.setPublicKey()` or `diffieHellman.generateKeys()` can be used to manually provide the public key or to automatically derive it.

#### `diffieHellman.setPublicKey(publicKey[, encoding])`#

Added in: v0.5.0

- `publicKey` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `publicKey` string.

Sets the Diffie-Hellman public key. If the `encoding` argument is provided, `publicKey` is expected to be a string. If no `encoding` is provided, `publicKey` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

#### `diffieHellman.verifyError`#

Added in: v0.11.12

A bit field containing any warnings and/or errors resulting from a check performed during initialization of the `DiffieHellman` object.

The following values are valid for this property (as defined in `node:constants` module):

- `DH_CHECK_P_NOT_SAFE_PRIME`
- `DH_CHECK_P_NOT_PRIME`
- `DH_UNABLE_TO_CHECK_GENERATOR`
- `DH_NOT_SUITABLE_GENERATOR`

### Class: `DiffieHellmanGroup`#

Added in: v0.7.5

The `DiffieHellmanGroup` class takes a well-known modp group as its argument. It works the same as `DiffieHellman`, except that it does not allow changing its keys after creation. In other words, it does not implement `setPublicKey()` or `setPrivateKey()` methods.

    const { createDiffieHellmanGroup } = await import('node:crypto');
    const dh = createDiffieHellmanGroup('modp16');
    const { createDiffieHellmanGroup } = require('node:crypto');
    const dh = createDiffieHellmanGroup('modp16');
    javascript

The following groups are supported:

- `'modp14'` (2048 bits, RFC 3526 Section 3)
- `'modp15'` (3072 bits, RFC 3526 Section 4)
- `'modp16'` (4096 bits, RFC 3526 Section 5)
- `'modp17'` (6144 bits, RFC 3526 Section 6)
- `'modp18'` (8192 bits, RFC 3526 Section 7)

The following groups are still supported but deprecated (see Caveats):

- `'modp1'` (768 bits, RFC 2409 Section 6.1)
- `'modp2'` (1024 bits, RFC 2409 Section 6.2)
- `'modp5'` (1536 bits, RFC 3526 Section 2)

These deprecated groups might be removed in future versions of Node.js.

### Class: `ECDH`#

Added in: v0.11.14

The `ECDH` class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH) key exchanges.

Instances of the `ECDH` class can be created using the `crypto.createECDH()` function.

    import assert from 'node:assert';

    const {
      createECDH,
    } = await import('node:crypto');

    // Generate Alice's keys...
    const alice = createECDH('secp521r1');
    const aliceKey = alice.generateKeys();

    // Generate Bob's keys...
    const bob = createECDH('secp521r1');
    const bobKey = bob.generateKeys();

    // Exchange and generate the secret...
    const aliceSecret = alice.computeSecret(bobKey);
    const bobSecret = bob.computeSecret(aliceKey);

    assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
    // OK
    const assert = require('node:assert');

    const {
      createECDH,
    } = require('node:crypto');

    // Generate Alice's keys...
    const alice = createECDH('secp521r1');
    const aliceKey = alice.generateKeys();

    // Generate Bob's keys...
    const bob = createECDH('secp521r1');
    const bobKey = bob.generateKeys();

    // Exchange and generate the secret...
    const aliceSecret = alice.computeSecret(bobKey);
    const bobSecret = bob.computeSecret(aliceKey);

    assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
    // OK
    javascript

#### Static method: `ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])`#

Added in: v10.0.0

- `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `curve` `<string>`
- `inputEncoding` `<string>` The encoding of the `key` string.
- `outputEncoding` `<string>` The encoding of the return value.
- `format` `<string>` Default: `'uncompressed'`
- Returns: `<Buffer>` | `<string>`

Converts the EC Diffie-Hellman public key specified by `key` and `curve` to the format specified by `format`. The `format` argument specifies point encoding and can be `'compressed'`, `'uncompressed'` or `'hybrid'`. The supplied key is interpreted using the specified `inputEncoding`, and the returned key is encoded using the specified `outputEncoding`.

Use `crypto.getCurves()` to obtain a list of available curve names. On recent OpenSSL releases, `openssl ecparam -list_curves` will also display the name and description of each available elliptic curve.

If `format` is not specified the point will be returned in `'uncompressed'` format.

If the `inputEncoding` is not provided, `key` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

Example (uncompressing a key):

    const {
      createECDH,
      ECDH,
    } = await import('node:crypto');

    const ecdh = createECDH('secp256k1');
    ecdh.generateKeys();

    const compressedKey = ecdh.getPublicKey('hex', 'compressed');

    const uncompressedKey = ECDH.convertKey(compressedKey,
                                            'secp256k1',
                                            'hex',
                                            'hex',
                                            'uncompressed');

    // The converted key and the uncompressed public key should be the same
    console.log(uncompressedKey === ecdh.getPublicKey('hex'));
    const {
      createECDH,
      ECDH,
    } = require('node:crypto');

    const ecdh = createECDH('secp256k1');
    ecdh.generateKeys();

    const compressedKey = ecdh.getPublicKey('hex', 'compressed');

    const uncompressedKey = ECDH.convertKey(compressedKey,
                                            'secp256k1',
                                            'hex',
                                            'hex',
                                            'uncompressed');

    // The converted key and the uncompressed public key should be the same
    console.log(uncompressedKey === ecdh.getPublicKey('hex'));
    javascript

#### `ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])`#

Added in: v0.11.14
History

Version | Changes
--- | ---
v10.0.0 | Changed error format to better support invalid public key error.
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `otherPublicKey` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the `otherPublicKey` string.
- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Computes the shared secret using `otherPublicKey` as the other party's public key and returns the computed shared secret. The supplied key is interpreted using specified `inputEncoding`, and the returned secret is encoded using the specified `outputEncoding`. If the `inputEncoding` is not provided, `otherPublicKey` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

If `outputEncoding` is given a string will be returned; otherwise a `Buffer` is returned.

`ecdh.computeSecret` will throw an `ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY` error when `otherPublicKey` lies outside of the elliptic curve. Since `otherPublicKey` is usually supplied from a remote user over an insecure network, be sure to handle this exception accordingly.

#### `ecdh.generateKeys([encoding[, format]])`#

Added in: v0.11.14

- `encoding` `<string>` The encoding of the return value.
- `format` `<string>` Default: `'uncompressed'`
- Returns: `<Buffer>` | `<string>`

Generates private and public EC Diffie-Hellman key values, and returns the public key in the specified `format` and `encoding`. This key should be transferred to the other party.

The `format` argument specifies point encoding and can be `'compressed'` or `'uncompressed'`. If `format` is not specified, the point will be returned in `'uncompressed'` format.

If `encoding` is provided a string is returned; otherwise a `Buffer` is returned.

#### `ecdh.getPrivateKey([encoding])`#

Added in: v0.11.14

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>` The EC Diffie-Hellman in the specified `encoding`.

If `encoding` is specified, a string is returned; otherwise a `Buffer` is returned.

#### `ecdh.getPublicKey([encoding][, format])`#

Added in: v0.11.14

- `encoding` `<string>` The encoding of the return value.
- `format` `<string>` Default: `'uncompressed'`
- Returns: `<Buffer>` | `<string>` The EC Diffie-Hellman public key in the specified `encoding` and `format`.

The `format` argument specifies point encoding and can be `'compressed'` or `'uncompressed'`. If `format` is not specified the point will be returned in `'uncompressed'` format.

If `encoding` is specified, a string is returned; otherwise a `Buffer` is returned.

#### `ecdh.setPrivateKey(privateKey[, encoding])`#

Added in: v0.11.14

- `privateKey` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `privateKey` string.

Sets the EC Diffie-Hellman private key. If `encoding` is provided, `privateKey` is expected to be a string; otherwise `privateKey` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

If `privateKey` is not valid for the curve specified when the `ECDH` object was created, an error is thrown. Upon setting the private key, the associated public point (key) is also generated and set in the `ECDH` object.

#### `ecdh.setPublicKey(publicKey[, encoding])`#

Added in: v0.11.14Deprecated in: v5.2.0

Stability: 0 - Deprecated

- `publicKey` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The encoding of the `publicKey` string.

Sets the EC Diffie-Hellman public key. If `encoding` is provided `publicKey` is expected to be a string; otherwise a `Buffer`, `TypedArray`, or `DataView` is expected.

There is not normally a reason to call this method because `ECDH` only requires a private key and the other party's public key to compute the shared secret. Typically either `ecdh.generateKeys()` or `ecdh.setPrivateKey()` will be called. The `ecdh.setPrivateKey()` method attempts to generate the public point/key associated with the private key being set.

Example (obtaining a shared secret):

    const {
      createECDH,
      createHash,
    } = await import('node:crypto');

    const alice = createECDH('secp256k1');
    const bob = createECDH('secp256k1');

    // This is a shortcut way of specifying one of Alice's previous private
    // keys. It would be unwise to use such a predictable private key in a real
    // application.
    alice.setPrivateKey(
      createHash('sha256').update('alice', 'utf8').digest(),
    );

    // Bob uses a newly generated cryptographically strong
    // pseudorandom key pair
    bob.generateKeys();

    const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
    const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

    // aliceSecret and bobSecret should be the same shared secret value
    console.log(aliceSecret === bobSecret);
    const {
      createECDH,
      createHash,
    } = require('node:crypto');

    const alice = createECDH('secp256k1');
    const bob = createECDH('secp256k1');

    // This is a shortcut way of specifying one of Alice's previous private
    // keys. It would be unwise to use such a predictable private key in a real
    // application.
    alice.setPrivateKey(
      createHash('sha256').update('alice', 'utf8').digest(),
    );

    // Bob uses a newly generated cryptographically strong
    // pseudorandom key pair
    bob.generateKeys();

    const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
    const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

    // aliceSecret and bobSecret should be the same shared secret value
    console.log(aliceSecret === bobSecret);
    javascript

### Class: `Hash`#

Added in: v0.1.92

- Extends: `<stream.Transform>`

The `Hash` class is a utility for creating hash digests of data. It can be used in one of two ways:

- As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
- Using the `hash.update()` and `hash.digest()` methods to produce the computed hash.

The `crypto.createHash()` method is used to create `Hash` instances. `Hash` objects are not to be created directly using the `new` keyword.

Example: Using `Hash` objects as streams:

    const {
      createHash,
    } = await import('node:crypto');

    const hash = createHash('sha256');

    hash.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = hash.read();
      if (data) {
        console.log(data.toString('hex'));
        // Prints:
        //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
      }
    });

    hash.write('some data to hash');
    hash.end();
    const {
      createHash,
    } = require('node:crypto');

    const hash = createHash('sha256');

    hash.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = hash.read();
      if (data) {
        console.log(data.toString('hex'));
        // Prints:
        //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
      }
    });

    hash.write('some data to hash');
    hash.end();
    javascript

Example: Using `Hash` and piped streams:

    import { createReadStream } from 'node:fs';
    import { stdout } from 'node:process';
    const { createHash } = await import('node:crypto');

    const hash = createHash('sha256');

    const input = createReadStream('test.js');
    input.pipe(hash).setEncoding('hex').pipe(stdout);
    const { createReadStream } = require('node:fs');
    const { createHash } = require('node:crypto');
    const { stdout } = require('node:process');

    const hash = createHash('sha256');

    const input = createReadStream('test.js');
    input.pipe(hash).setEncoding('hex').pipe(stdout);
    javascript

Example: Using the `hash.update()` and `hash.digest()` methods:

    const {
      createHash,
    } = await import('node:crypto');

    const hash = createHash('sha256');

    hash.update('some data to hash');
    console.log(hash.digest('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
    const {
      createHash,
    } = require('node:crypto');

    const hash = createHash('sha256');

    hash.update('some data to hash');
    console.log(hash.digest('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
    javascript

#### `hash.copy([options])`#

Added in: v13.1.0

- `options` `<Object>` `stream.transform` options
- Returns: `<Hash>`

Creates a new `Hash` object that contains a deep copy of the internal state of the current `Hash` object.

The optional `options` argument controls stream behavior. For XOF hash functions such as `'shake256'`, the `outputLength` option can be used to specify the desired output length in bytes.

An error is thrown when an attempt is made to copy the `Hash` object after its `hash.digest()` method has been called.

    // Calculate a rolling hash.
    const {
      createHash,
    } = await import('node:crypto');

    const hash = createHash('sha256');

    hash.update('one');
    console.log(hash.copy().digest('hex'));

    hash.update('two');
    console.log(hash.copy().digest('hex'));

    hash.update('three');
    console.log(hash.copy().digest('hex'));

    // Etc.
    // Calculate a rolling hash.
    const {
      createHash,
    } = require('node:crypto');

    const hash = createHash('sha256');

    hash.update('one');
    console.log(hash.copy().digest('hex'));

    hash.update('two');
    console.log(hash.copy().digest('hex'));

    hash.update('three');
    console.log(hash.copy().digest('hex'));

    // Etc.
    javascript

#### `hash.digest([encoding])`#

Added in: v0.1.92

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Calculates the digest of all of the data passed to be hashed (using the `hash.update()` method). If `encoding` is provided a string will be returned; otherwise a `Buffer` is returned.

The `Hash` object can not be used again after `hash.digest()` method has been called. Multiple calls will cause an error to be thrown.

#### `hash.update(data[, inputEncoding])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the `data` string.

Updates the hash content with the given `data`, the encoding of which is given in `inputEncoding`. If `encoding` is not provided, and the `data` is a string, an encoding of `'utf8'` is enforced. If `data` is a `Buffer`, `TypedArray`, or `DataView`, then `inputEncoding` is ignored.

This can be called many times with new data as it is streamed.

### Class: `Hmac`#

Added in: v0.1.94

- Extends: `<stream.Transform>`

The `Hmac` class is a utility for creating cryptographic HMAC digests. It can be used in one of two ways:

- As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
- Using the `hmac.update()` and `hmac.digest()` methods to produce the computed HMAC digest.

The `crypto.createHmac()` method is used to create `Hmac` instances. `Hmac` objects are not to be created directly using the `new` keyword.

Example: Using `Hmac` objects as streams:

    const {
      createHmac,
    } = await import('node:crypto');

    const hmac = createHmac('sha256', 'a secret');

    hmac.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = hmac.read();
      if (data) {
        console.log(data.toString('hex'));
        // Prints:
        //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
      }
    });

    hmac.write('some data to hash');
    hmac.end();
    const {
      createHmac,
    } = require('node:crypto');

    const hmac = createHmac('sha256', 'a secret');

    hmac.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = hmac.read();
      if (data) {
        console.log(data.toString('hex'));
        // Prints:
        //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
      }
    });

    hmac.write('some data to hash');
    hmac.end();
    javascript

Example: Using `Hmac` and piped streams:

    import { createReadStream } from 'node:fs';
    import { stdout } from 'node:process';
    const {
      createHmac,
    } = await import('node:crypto');

    const hmac = createHmac('sha256', 'a secret');

    const input = createReadStream('test.js');
    input.pipe(hmac).pipe(stdout);
    const {
      createReadStream,
    } = require('node:fs');
    const {
      createHmac,
    } = require('node:crypto');
    const { stdout } = require('node:process');

    const hmac = createHmac('sha256', 'a secret');

    const input = createReadStream('test.js');
    input.pipe(hmac).pipe(stdout);
    javascript

Example: Using the `hmac.update()` and `hmac.digest()` methods:

    const {
      createHmac,
    } = await import('node:crypto');

    const hmac = createHmac('sha256', 'a secret');

    hmac.update('some data to hash');
    console.log(hmac.digest('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
    const {
      createHmac,
    } = require('node:crypto');

    const hmac = createHmac('sha256', 'a secret');

    hmac.update('some data to hash');
    console.log(hmac.digest('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
    javascript

#### `hmac.digest([encoding])`#

Added in: v0.1.94

- `encoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Calculates the HMAC digest of all of the data passed using `hmac.update()`. If `encoding` is provided a string is returned; otherwise a `Buffer` is returned;

The `Hmac` object can not be used again after `hmac.digest()` has been called. Multiple calls to `hmac.digest()` will result in an error being thrown.

#### `hmac.update(data[, inputEncoding])`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the `data` string.

Updates the `Hmac` content with the given `data`, the encoding of which is given in `inputEncoding`. If `encoding` is not provided, and the `data` is a string, an encoding of `'utf8'` is enforced. If `data` is a `Buffer`, `TypedArray`, or `DataView`, then `inputEncoding` is ignored.

This can be called many times with new data as it is streamed.

### Class: `KeyObject`#

Added in: v11.6.0
History

Version | Changes
--- | ---
v24.6.0 | Add support for ML-DSA keys.
v14.5.0, v12.19.0 | Instances of this class can now be passed to worker threads using `postMessage`.
v11.13.0 | This class is now exported.

Node.js uses a `KeyObject` class to represent a symmetric or asymmetric key, and each kind of key exposes different functions. The `crypto.createSecretKey()`, `crypto.createPublicKey()` and `crypto.createPrivateKey()` methods are used to create `KeyObject` instances. `KeyObject` objects are not to be created directly using the `new` keyword.

Most applications should consider using the new `KeyObject` API instead of passing keys as strings or `Buffer`s due to improved security features.

`KeyObject` instances can be passed to other threads via `postMessage()`. The receiver obtains a cloned `KeyObject`, and the `KeyObject` does not need to be listed in the `transferList` argument.

#### Static method: `KeyObject.from(key)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v26.0.0 | Passing a non-extractable CryptoKey as `key` is deprecated.

- `key` `<CryptoKey>`
- Returns: `<KeyObject>`

Returns the underlying `<KeyObject>` of a `<CryptoKey>`. The returned `<KeyObject>` does not retain any of the restrictions imposed by the Web Crypto API on the original `<CryptoKey>`, such as the allowed key usages, the algorithm or hash algorithm bindings, and the extractability flag. In particular, the underlying key material of the returned `<KeyObject>` can always be exported.

    const { KeyObject } = await import('node:crypto');
    const { subtle } = globalThis.crypto;

    const key = await subtle.generateKey({
      name: 'HMAC',
      hash: 'SHA-256',
      length: 256,
    }, true, ['sign', 'verify']);

    const keyObject = KeyObject.from(key);
    console.log(keyObject.symmetricKeySize);
    // Prints: 32 (symmetric key size in bytes)
    const { KeyObject } = require('node:crypto');
    const { subtle } = globalThis.crypto;

    (async function() {
      const key = await subtle.generateKey({
        name: 'HMAC',
        hash: 'SHA-256',
        length: 256,
      }, true, ['sign', 'verify']);

      const keyObject = KeyObject.from(key);
      console.log(keyObject.symmetricKeySize);
      // Prints: 32 (symmetric key size in bytes)
    })();
    javascript

#### `keyObject.asymmetricKeyDetails`#

Added in: v15.7.0
History

Version | Changes
--- | ---
v16.9.0 | Expose `RSASSA-PSS-params` sequence parameters for RSA-PSS keys.

- Type: `<Object>`
  - `modulusLength` `<number>` Key size in bits (RSA, DSA).
  - `publicExponent` `<bigint>` Public exponent (RSA).
  - `hashAlgorithm` `<string>` Name of the message digest (RSA-PSS).
  - `mgf1HashAlgorithm` `<string>` Name of the message digest used by MGF1 (RSA-PSS).
  - `saltLength` `<number>` Minimal salt length in bytes (RSA-PSS).
  - `divisorLength` `<number>` Size of `q` in bits (DSA).
  - `namedCurve` `<string>` Name of the curve (EC).

This property exists only on asymmetric keys. Depending on the type of the key, this object contains information about the key. None of the information obtained through this property can be used to uniquely identify a key or to compromise the security of the key.

For RSA-PSS keys, if the key material contains a `RSASSA-PSS-params` sequence, the `hashAlgorithm`, `mgf1HashAlgorithm`, and `saltLength` properties will be set.

Other key details might be exposed via this API using additional attributes.

#### `keyObject.asymmetricKeyType`#

Added in: v11.6.0
History

Version | Changes
--- | ---
v24.8.0 | Add support for SLH-DSA keys.
v24.7.0 | Add support for ML-KEM keys.
v24.6.0 | Add support for ML-DSA keys.
v13.9.0, v12.17.0 | Added support for `'dh'`.
v12.0.0 | Added support for `'rsa-pss'`.
v12.0.0 | This property now returns `undefined` for KeyObject instances of unrecognized type instead of aborting.
v12.0.0 | Added support for `'x25519'` and `'x448'`.
v12.0.0 | Added support for `'ed25519'` and `'ed448'`.

- Type: `<string>`

For asymmetric keys, this property represents the type of the key. See the supported asymmetric key types.

This property is `undefined` for unrecognized `KeyObject` types and symmetric keys.

#### `keyObject.equals(otherKeyObject)`#

Added in: v17.7.0, v16.15.0

- `otherKeyObject` `<KeyObject>` A `KeyObject` with which to compare `keyObject`.
- Returns: `<boolean>`

Returns `true` or `false` depending on whether the keys have exactly the same type, value, and parameters. This method is not constant time.

#### `keyObject.export([options])`#

Added in: v11.6.0
History

Version | Changes
--- | ---
v26.1.0 | Added JWK format support for ML-KEM and SLH-DSA key types.
v26.0.0 | Added support for `'raw-public'`, `'raw-private'`, and `'raw-seed'` formats.
v26.0.0 | ML-KEM and ML-DSA private key `'pkcs8'` export now uses seed-only format by default when a seed is available.
v15.9.0 | Added support for `'jwk'` format.

- `options` `<Object>`
- Returns: `<string>` | `<Buffer>` | `<Object>`

For symmetric keys, the following encoding options can be used:

- `format` `<string>` Must be `'buffer'` (default) or `'jwk'`.

For public keys, the following encoding options can be used:

- `format` `<string>` Must be `'pem'`, `'der'`, `'jwk'`, or `'raw-public'`. See asymmetric key types for format support.
- `type` `<string>` When `format` is `'pem'` or `'der'`, must be `'pkcs1'` (RSA only) or `'spki'`. For EC keys with `'raw-public'` format, may be `'uncompressed'` (default) or `'compressed'`. Ignored when `format` is `'jwk'`.

For private keys, the following encoding options can be used:

- `format` `<string>` Must be `'pem'`, `'der'`, `'jwk'`, `'raw-private'`, or `'raw-seed'`. See asymmetric key types for format support.
- `type` `<string>` When `format` is `'pem'` or `'der'`, must be `'pkcs1'` (RSA only), `'pkcs8'`, or `'sec1'` (EC only). Ignored when `format` is `'jwk'`, `'raw-private'`, or `'raw-seed'`.
- `cipher` `<string>` If specified, the private key will be encrypted with the given `cipher` and `passphrase` using PKCS#5 v2.0 password based encryption. Ignored when `format` is `'jwk'`, `'raw-private'`, or `'raw-seed'`.
- `passphrase` `<string>` | `<Buffer>` The passphrase to use for encryption. Required when `cipher` is specified.

The result type depends on the selected encoding format, when PEM the result is a string, when DER it will be a buffer containing the data encoded as DER, when JWK it will be an object. Raw formats return a `<Buffer>` containing the raw key material.

Private keys can be encrypted by specifying a `cipher` and `passphrase`. The PKCS#8 `type` supports encryption with both PEM and DER `format` for any key algorithm. PKCS#1 and SEC1 can only be encrypted when the PEM `format` is used. For maximum compatibility, use PKCS#8 for encrypted private keys. Since PKCS#8 defines its own encryption mechanism, PEM-level encryption is not supported when encrypting a PKCS#8 key. See RFC 5208 for PKCS#8 encryption and RFC 1421 for PKCS#1 and SEC1 encryption.

#### `keyObject.symmetricKeySize`#

Added in: v11.6.0

- Type: `<number>`

For secret keys, this property represents the size of the key in bytes. This property is `undefined` for asymmetric keys.

#### `keyObject.toCryptoKey(algorithm, extractable, keyUsages)`#

Added in: v23.0.0, v22.10.0

- `algorithm` `<string>` | `<Algorithm>` | `<RsaHashedImportParams>` | `<EcKeyImportParams>` | `<HmacImportParams>`

- `extractable` `<boolean>`
- `keyUsages` `<string>`[] See Key usages.
- Returns: `<CryptoKey>`

Converts a `KeyObject` instance to a `CryptoKey`.

#### `keyObject.type`#

Added in: v11.6.0

- Type: `<string>`

Depending on the type of this `KeyObject`, this property is either `'secret'` for secret (symmetric) keys, `'public'` for public (asymmetric) keys or `'private'` for private (asymmetric) keys.

### Class: `Sign`#

Added in: v0.1.92

- Extends: `<stream.Writable>`

The `Sign` class is a utility for generating signatures. It can be used in one of two ways:

- As a writable stream, where data to be signed is written and the `sign.sign()` method is used to generate and return the signature, or
- Using the `sign.update()` and `sign.sign()` methods to produce the signature.

The `crypto.createSign()` method is used to create `Sign` instances. The argument is the string name of the hash function to use. `Sign` objects are not to be created directly using the `new` keyword.

Example: Using `Sign` and `Verify` objects as streams:

    const {
      generateKeyPairSync,
      createSign,
      createVerify,
    } = await import('node:crypto');

    const { privateKey, publicKey } = generateKeyPairSync('ec', {
      namedCurve: 'sect239k1',
    });

    const sign = createSign('SHA256');
    sign.write('some data to sign');
    sign.end();
    const signature = sign.sign(privateKey, 'hex');

    const verify = createVerify('SHA256');
    verify.write('some data to sign');
    verify.end();
    console.log(verify.verify(publicKey, signature, 'hex'));
    // Prints: true
    const {
      generateKeyPairSync,
      createSign,
      createVerify,
    } = require('node:crypto');

    const { privateKey, publicKey } = generateKeyPairSync('ec', {
      namedCurve: 'sect239k1',
    });

    const sign = createSign('SHA256');
    sign.write('some data to sign');
    sign.end();
    const signature = sign.sign(privateKey, 'hex');

    const verify = createVerify('SHA256');
    verify.write('some data to sign');
    verify.end();
    console.log(verify.verify(publicKey, signature, 'hex'));
    // Prints: true
    javascript

Example: Using the `sign.update()` and `verify.update()` methods:

    const {
      generateKeyPairSync,
      createSign,
      createVerify,
    } = await import('node:crypto');

    const { privateKey, publicKey } = generateKeyPairSync('rsa', {
      modulusLength: 2048,
    });

    const sign = createSign('SHA256');
    sign.update('some data to sign');
    sign.end();
    const signature = sign.sign(privateKey);

    const verify = createVerify('SHA256');
    verify.update('some data to sign');
    verify.end();
    console.log(verify.verify(publicKey, signature));
    // Prints: true
    const {
      generateKeyPairSync,
      createSign,
      createVerify,
    } = require('node:crypto');

    const { privateKey, publicKey } = generateKeyPairSync('rsa', {
      modulusLength: 2048,
    });

    const sign = createSign('SHA256');
    sign.update('some data to sign');
    sign.end();
    const signature = sign.sign(privateKey);

    const verify = createVerify('SHA256');
    verify.update('some data to sign');
    verify.end();
    console.log(verify.verify(publicKey, signature));
    // Prints: true
    javascript

#### `sign.sign(privateKey[, outputEncoding])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v15.0.0 | The privateKey can also be an ArrayBuffer and CryptoKey.
v13.2.0, v12.16.0 | This function now supports IEEE-P1363 DSA and ECDSA signatures.
v12.0.0 | This function now supports RSA-PSS keys.
v11.6.0 | This function now supports key objects.
v8.0.0 | Support for RSASSA-PSS and additional options was added.

- `privateKey` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>` | `<URL>`
  - `dsaEncoding` `<string>`
  - `padding` `<integer>`
  - `saltLength` `<integer>`
- `outputEncoding` `<string>` The encoding of the return value.
- Returns: `<Buffer>` | `<string>`

Calculates the signature on all the data passed through using either `sign.update()` or `sign.write()`.

If `privateKey` is not a `KeyObject`, this function behaves as if `privateKey` had been passed to `crypto.createPrivateKey()`. When `privateKey` is a string, `ArrayBuffer`, `Buffer`, `TypedArray`, or `DataView`, it must contain PEM-encoded key material. If it is an object, the following additional properties can be passed:

- `dsaEncoding` `<string>` For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

  - `'der'` (default): DER-encoded ASN.1 signature structure encoding `(r, s)`.
  - `'ieee-p1363'`: Signature format `r || s` as proposed in IEEE-P1363.

- `padding` `<integer>` Optional padding value for RSA, one of the following:

  - `crypto.constants.RSA_PKCS1_PADDING` (default)
  - `crypto.constants.RSA_PKCS1_PSS_PADDING`

  `RSA_PKCS1_PSS_PADDING` will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055.

- `saltLength` `<integer>` Salt length for when padding is `RSA_PKCS1_PSS_PADDING`. The special value `crypto.constants.RSA_PSS_SALTLEN_DIGEST` sets the salt length to the digest size, `crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN` (default) sets it to the maximum permissible value.

If `outputEncoding` is provided a string is returned; otherwise a `Buffer` is returned.

The `Sign` object can not be again used after `sign.sign()` method has been called. Multiple calls to `sign.sign()` will result in an error being thrown.

#### `sign.update(data[, inputEncoding])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the `data` string.

Updates the `Sign` content with the given `data`, the encoding of which is given in `inputEncoding`. If `encoding` is not provided, and the `data` is a string, an encoding of `'utf8'` is enforced. If `data` is a `Buffer`, `TypedArray`, or `DataView`, then `inputEncoding` is ignored.

This can be called many times with new data as it is streamed.

### Class: `Verify`#

Added in: v0.1.92

- Extends: `<stream.Writable>`

The `Verify` class is a utility for verifying signatures. It can be used in one of two ways:

- As a writable stream where written data is used to validate against the supplied signature, or
- Using the `verify.update()` and `verify.verify()` methods to verify the signature.

The `crypto.createVerify()` method is used to create `Verify` instances. `Verify` objects are not to be created directly using the `new` keyword.

See `Sign` for examples.

#### `verify.update(data[, inputEncoding])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v6.0.0 | The default `inputEncoding` changed from `binary` to `utf8`.

- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `inputEncoding` `<string>` The encoding of the `data` string.

Updates the `Verify` content with the given `data`, the encoding of which is given in `inputEncoding`. If `inputEncoding` is not provided, and the `data` is a string, an encoding of `'utf8'` is enforced. If `data` is a `Buffer`, `TypedArray`, or `DataView`, then `inputEncoding` is ignored.

This can be called many times with new data as it is streamed.

#### `verify.verify(key, signature[, signatureEncoding])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v15.0.0 | The key can also be an ArrayBuffer and CryptoKey.
v13.2.0, v12.16.0 | This function now supports IEEE-P1363 DSA and ECDSA signatures.
v12.0.0 | This function now supports RSA-PSS keys.
v11.7.0 | The key can now be a private key.
v8.0.0 | Support for RSASSA-PSS and additional options was added.

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
  - `dsaEncoding` `<string>`
  - `padding` `<integer>`
  - `saltLength` `<integer>`
- `signature` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `signatureEncoding` `<string>` The encoding of the `signature` string.
- Returns: `<boolean>` `true` or `false` depending on the validity of the signature for the data and public key.

Verifies the provided data using the given `key` and `signature`.

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPublicKey()`. When `key` is a string, `ArrayBuffer`, `Buffer`, `TypedArray`, or `DataView`, it must contain PEM-encoded key material. If it is an object, the following additional properties can be passed:

- `dsaEncoding` `<string>` For DSA and ECDSA, this option specifies the format of the signature. It can be one of the following:

  - `'der'` (default): DER-encoded ASN.1 signature structure encoding `(r, s)`.
  - `'ieee-p1363'`: Signature format `r || s` as proposed in IEEE-P1363.

- `padding` `<integer>` Optional padding value for RSA, one of the following:

  - `crypto.constants.RSA_PKCS1_PADDING` (default)
  - `crypto.constants.RSA_PKCS1_PSS_PADDING`

  `RSA_PKCS1_PSS_PADDING` will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055.

- `saltLength` `<integer>` Salt length for when padding is `RSA_PKCS1_PSS_PADDING`. The special value `crypto.constants.RSA_PSS_SALTLEN_DIGEST` sets the salt length to the digest size, `crypto.constants.RSA_PSS_SALTLEN_AUTO` (default) causes it to be determined automatically.

The `signature` argument is the previously calculated signature for the data, in the `signatureEncoding`. If a `signatureEncoding` is specified, the `signature` is expected to be a string; otherwise `signature` is expected to be a `Buffer`, `TypedArray`, or `DataView`.

The `verify` object can not be used again after `verify.verify()` has been called. Multiple calls to `verify.verify()` will result in an error being thrown.

Because public keys can be derived from private keys, a private key may be passed instead of a public key.

### Class: `X509Certificate`#

Added in: v15.6.0

Encapsulates an X509 certificate and provides read-only access to its information.

    const { X509Certificate } = await import('node:crypto');

    const x509 = new X509Certificate('{... pem encoded cert ...}');

    console.log(x509.subject);
    const { X509Certificate } = require('node:crypto');

    const x509 = new X509Certificate('{... pem encoded cert ...}');

    console.log(x509.subject);
    javascript

#### `new X509Certificate(buffer)`#

Added in: v15.6.0

- `buffer` `<string>` | `<TypedArray>` | `<Buffer>` | `<DataView>` A PEM or DER encoded X509 Certificate.

#### `x509.ca`#

Added in: v15.6.0

- Type: `<boolean>` Will be `true` if this is a Certificate Authority (CA) certificate.

#### `x509.checkEmail(email[, options])`#

Added in: v15.6.0
History

Version | Changes
--- | ---
v18.0.0 | The subject option now defaults to `'default'`.
v17.5.0, v16.14.1 | The `wildcards`, `partialWildcards`, `multiLabelWildcards`, and `singleLabelSubdomains` options have been removed since they had no effect.
v17.5.0, v16.15.0 | The subject option can now be set to `'default'`.

- `email` `<string>`
- `options` `<Object>`
  - `subject` `<string>` `'default'`, `'always'`, or `'never'`. Default: `'default'`.
- Returns: `<string>` | `<undefined>` Returns `email` if the certificate matches, `undefined` if it does not.

Checks whether the certificate matches the given email address.

If the `'subject'` option is undefined or set to `'default'`, the certificate subject is only considered if the subject alternative name extension either does not exist or does not contain any email addresses.

If the `'subject'` option is set to `'always'` and if the subject alternative name extension either does not exist or does not contain a matching email address, the certificate subject is considered.

If the `'subject'` option is set to `'never'`, the certificate subject is never considered, even if the certificate contains no subject alternative names.

#### `x509.checkHost(name[, options])`#

Added in: v15.6.0
History

Version | Changes
--- | ---
v18.0.0 | The subject option now defaults to `'default'`.
v17.5.0, v16.15.0 | The subject option can now be set to `'default'`.

- `name` `<string>`
- `options` `<Object>`
  - `subject` `<string>` `'default'`, `'always'`, or `'never'`. Default: `'default'`.
  - `wildcards` `<boolean>` Default: `true`.
  - `partialWildcards` `<boolean>` Default: `true`.
  - `multiLabelWildcards` `<boolean>` Default: `false`.
  - `singleLabelSubdomains` `<boolean>` Default: `false`.
- Returns: `<string>` | `<undefined>` Returns a subject name that matches `name`, or `undefined` if no subject name matches `name`.

Checks whether the certificate matches the given host name.

If the certificate matches the given host name, the matching subject name is returned. The returned name might be an exact match (e.g., `foo.example.com`) or it might contain wildcards (e.g., `*.example.com`). Because host name comparisons are case-insensitive, the returned subject name might also differ from the given `name` in capitalization.

If the `'subject'` option is undefined or set to `'default'`, the certificate subject is only considered if the subject alternative name extension either does not exist or does not contain any DNS names. This behavior is consistent with RFC 2818 ("HTTP Over TLS").

If the `'subject'` option is set to `'always'` and if the subject alternative name extension either does not exist or does not contain a matching DNS name, the certificate subject is considered.

If the `'subject'` option is set to `'never'`, the certificate subject is never considered, even if the certificate contains no subject alternative names.

#### `x509.checkIP(ip)`#

Added in: v15.6.0
History

Version | Changes
--- | ---
v17.5.0, v16.14.1 | The `options` argument has been removed since it had no effect.

- `ip` `<string>`
- Returns: `<string>` | `<undefined>` Returns `ip` if the certificate matches, `undefined` if it does not.

Checks whether the certificate matches the given IP address (IPv4 or IPv6).

Only RFC 5280 `iPAddress` subject alternative names are considered, and they must match the given `ip` address exactly. Other subject alternative names as well as the subject field of the certificate are ignored.

#### `x509.checkIssued(otherCert)`#

Added in: v15.6.0

- `otherCert` `<X509Certificate>`
- Returns: `<boolean>`

Checks whether this certificate was potentially issued by the given `otherCert` by comparing the certificate metadata.

This is useful for pruning a list of possible issuer certificates which have been selected using a more rudimentary filtering routine, i.e. just based on subject and issuer names.

Finally, to verify that this certificate's signature was produced by a private key corresponding to `otherCert`'s public key use `x509.verify(publicKey)` with `otherCert`'s public key represented as a `KeyObject` like so

    if (!x509.verify(otherCert.publicKey)) {
      throw new Error('otherCert did not issue x509');
    }
    js

#### `x509.checkPrivateKey(privateKey)`#

Added in: v15.6.0

- `privateKey` `<KeyObject>` A private key.
- Returns: `<boolean>`

Checks whether the public key for this certificate is consistent with the given private key.

#### `x509.fingerprint`#

Added in: v15.6.0

- Type: `<string>`

The SHA-1 fingerprint of this certificate.

Because SHA-1 is cryptographically broken and because the security of SHA-1 is significantly worse than that of algorithms that are commonly used to sign certificates, consider using `x509.fingerprint256` instead.

#### `x509.fingerprint256`#

Added in: v15.6.0

- Type: `<string>`

The SHA-256 fingerprint of this certificate.

#### `x509.fingerprint512`#

Added in: v17.2.0, v16.14.0

- Type: `<string>`

The SHA-512 fingerprint of this certificate.

Because computing the SHA-256 fingerprint is usually faster and because it is only half the size of the SHA-512 fingerprint, `x509.fingerprint256` may be a better choice. While SHA-512 presumably provides a higher level of security in general, the security of SHA-256 matches that of most algorithms that are commonly used to sign certificates.

#### `x509.infoAccess`#

Added in: v15.6.0
History

Version | Changes
--- | ---
v17.3.1, v16.13.2 | Parts of this string may be encoded as JSON string literals in response to CVE-2021-44532.

- Type: `<string>`

A textual representation of the certificate's authority information access extension.

This is a line feed separated list of access descriptions. Each line begins with the access method and the kind of the access location, followed by a colon and the value associated with the access location.

After the prefix denoting the access method and the kind of the access location, the remainder of each line might be enclosed in quotes to indicate that the value is a JSON string literal. For backward compatibility, Node.js only uses JSON string literals within this property when necessary to avoid ambiguity. Third-party code should be prepared to handle both possible entry formats.

#### `x509.issuer`#

Added in: v15.6.0

- Type: `<string>`

The issuer identification included in this certificate.

#### `x509.issuerCertificate`#

Added in: v15.9.0

- Type: `<X509Certificate>`

The issuer certificate or `undefined` if the issuer certificate is not available.

#### `x509.keyUsage`#

Added in: v15.6.0

- Type: `<string>`[]

An array detailing the key extended usages for this certificate.

#### `x509.publicKey`#

Added in: v15.6.0

- Type: `<KeyObject>`

The public key `<KeyObject>` for this certificate.

#### `x509.raw`#

Added in: v15.6.0

- Type: `<Buffer>`

A `Buffer` containing the DER encoding of this certificate.

#### `x509.serialNumber`#

Added in: v15.6.0

- Type: `<string>`

The serial number of this certificate.

Serial numbers are assigned by certificate authorities and do not uniquely identify certificates. Consider using `x509.fingerprint256` as a unique identifier instead.

#### `x509.subject`#

Added in: v15.6.0

- Type: `<string>`

The complete subject of this certificate.

#### `x509.subjectAltName`#

Added in: v15.6.0
History

Version | Changes
--- | ---
v17.3.1, v16.13.2 | Parts of this string may be encoded as JSON string literals in response to CVE-2021-44532.

- Type: `<string>`

The subject alternative name specified for this certificate.

This is a comma-separated list of subject alternative names. Each entry begins with a string identifying the kind of the subject alternative name followed by a colon and the value associated with the entry.

Earlier versions of Node.js incorrectly assumed that it is safe to split this property at the two-character sequence `', '` (see CVE-2021-44532). However, both malicious and legitimate certificates can contain subject alternative names that include this sequence when represented as a string.

After the prefix denoting the type of the entry, the remainder of each entry might be enclosed in quotes to indicate that the value is a JSON string literal. For backward compatibility, Node.js only uses JSON string literals within this property when necessary to avoid ambiguity. Third-party code should be prepared to handle both possible entry formats.

#### `x509.toJSON()`#

Added in: v15.6.0

- Type: `<string>`

There is no standard JSON encoding for X509 certificates. The `toJSON()` method returns a string containing the PEM encoded certificate.

#### `x509.toLegacyObject()`#

Added in: v15.6.0

- Type: `<Object>`

Returns information about this certificate using the legacy certificate object encoding.

#### `x509.toString()`#

Added in: v15.6.0

- Type: `<string>`

Returns the PEM-encoded certificate.

#### `x509.validFrom`#

Added in: v15.6.0

- Type: `<string>`

The date/time from which this certificate is valid.

#### `x509.validFromDate`#

Added in: v23.0.0, v22.10.0

- Type: `<Date>`

The date/time from which this certificate is valid, encapsulated in a `Date` object.

#### `x509.validTo`#

Added in: v15.6.0

- Type: `<string>`

The date/time until which this certificate is valid.

#### `x509.validToDate`#

Added in: v23.0.0, v22.10.0

- Type: `<Date>`

The date/time until which this certificate is valid, encapsulated in a `Date` object.

#### `x509.signatureAlgorithm`#

Added in: v24.9.0

- Type: `<string>` | `<undefined>`

The algorithm used to sign the certificate or `undefined` if the signature algorithm is unknown by OpenSSL.

#### `x509.signatureAlgorithmOid`#

Added in: v24.9.0

- Type: `<string>`

The OID of the algorithm used to sign the certificate.

#### `x509.verify(publicKey)`#

Added in: v15.6.0

- `publicKey` `<KeyObject>` A public key.
- Returns: `<boolean>`

Verifies that this certificate was signed by the given public key. Does not perform any other validation checks on the certificate.

### `node:crypto` module methods and properties#

#### `crypto.argon2(algorithm, parameters, callback)`#

Added in: v24.7.0

- `algorithm` `<string>` Variant of Argon2, one of `"argon2d"`, `"argon2i"` or `"argon2id"`.
- `parameters` `<Object>`
  - `message` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` REQUIRED, this is the password for password hashing applications of Argon2.
  - `nonce` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` REQUIRED, must be at least 8 bytes long. This is the salt for password hashing applications of Argon2.
  - `parallelism` `<number>` REQUIRED, degree of parallelism determines how many computational chains (lanes) can be run. Must be at least `1` and at most `2**24-1`.
  - `tagLength` `<number>` REQUIRED, the length of the key to generate. Must be at least `4` and at most `2**32-1`.
  - `memory` `<number>` REQUIRED, memory cost in 1KiB blocks. Must be at least `8 * parallelism` and at most `2**32-1`. The actual number of blocks is rounded down to the nearest multiple of `4 * parallelism`.
  - `passes` `<number>` REQUIRED, number of passes (iterations). Must be at least `1` and at most `2**32-1`.
  - `secret` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<undefined>` OPTIONAL, Random additional input, similar to the salt, that should NOT be stored with the derived key. This is known as pepper in password hashing applications. If used, must have a length not greater than `2**32-1` bytes.
  - `associatedData` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<undefined>` OPTIONAL, Additional data to be added to the hash, functionally equivalent to salt or secret, but meant for non-random data. If used, must have a length not greater than `2**32-1` bytes.
- `callback` `<Function>`
  - `err` `<Error>`
  - `derivedKey` `<Buffer>`

Provides an asynchronous Argon2 implementation. Argon2 is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The `nonce` should be as unique as possible. It is recommended that a nonce is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for `message`, `nonce`, `secret` or `associatedData`, please consider caveats when using strings as inputs to cryptographic APIs.

The `callback` function is called with two arguments: `err` and `derivedKey`. `err` is an exception object when key derivation fails, otherwise `err` is `null`. `derivedKey` is passed to the callback as a `Buffer`.

An exception is thrown when any of the input arguments specify invalid values or types.

    const { argon2, randomBytes } = await import('node:crypto');

    const parameters = {
      message: 'password',
      nonce: randomBytes(16),
      parallelism: 4,
      tagLength: 64,
      memory: 65536,
      passes: 3,
    };

    argon2('argon2id', parameters, (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // 'af91dad...9520f15'
    });
    const { argon2, randomBytes } = require('node:crypto');

    const parameters = {
      message: 'password',
      nonce: randomBytes(16),
      parallelism: 4,
      tagLength: 64,
      memory: 65536,
      passes: 3,
    };

    argon2('argon2id', parameters, (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // 'af91dad...9520f15'
    });
    javascript

#### `crypto.argon2Sync(algorithm, parameters)`#

Added in: v24.7.0

- `algorithm` `<string>` Variant of Argon2, one of `"argon2d"`, `"argon2i"` or `"argon2id"`.
- `parameters` `<Object>`
  - `message` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` REQUIRED, this is the password for password hashing applications of Argon2.
  - `nonce` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` REQUIRED, must be at least 8 bytes long. This is the salt for password hashing applications of Argon2.
  - `parallelism` `<number>` REQUIRED, degree of parallelism determines how many computational chains (lanes) can be run. Must be at least 1 and at most `2**24-1`.
  - `tagLength` `<number>` REQUIRED, the length of the key to generate. Must be at least `4` and at most `2**32-1`.
  - `memory` `<number>` REQUIRED, memory cost in 1KiB blocks. Must be at least `8 * parallelism` and at most `2**32-1`. The actual number of blocks is rounded down to the nearest multiple of `4 * parallelism`.
  - `passes` `<number>` REQUIRED, number of passes (iterations). Must be at least `1` and at most `2**32-1`.
  - `secret` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<undefined>` OPTIONAL, Random additional input, similar to the salt, that should NOT be stored with the derived key. This is known as pepper in password hashing applications. If used, must have a length not greater than `2**32-1` bytes.
  - `associatedData` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<undefined>` OPTIONAL, Additional data to be added to the hash, functionally equivalent to salt or secret, but meant for non-random data. If used, must have a length not greater than `2**32-1` bytes.
- Returns: `<Buffer>`

Provides a synchronous Argon2 implementation. Argon2 is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The `nonce` should be as unique as possible. It is recommended that a nonce is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for `message`, `nonce`, `secret` or `associatedData`, please consider caveats when using strings as inputs to cryptographic APIs.

An exception is thrown when key derivation fails, otherwise the derived key is returned as a `Buffer`.

An exception is thrown when any of the input arguments specify invalid values or types.

    const { argon2Sync, randomBytes } = await import('node:crypto');

    const parameters = {
      message: 'password',
      nonce: randomBytes(16),
      parallelism: 4,
      tagLength: 64,
      memory: 65536,
      passes: 3,
    };

    const derivedKey = argon2Sync('argon2id', parameters);
    console.log(derivedKey.toString('hex'));  // 'af91dad...9520f15'
    const { argon2Sync, randomBytes } = require('node:crypto');

    const parameters = {
      message: 'password',
      nonce: randomBytes(16),
      parallelism: 4,
      tagLength: 64,
      memory: 65536,
      passes: 3,
    };

    const derivedKey = argon2Sync('argon2id', parameters);
    console.log(derivedKey.toString('hex'));  // 'af91dad...9520f15'
    javascript

#### `crypto.checkPrime(candidate[, options], callback)`#

Added in: v15.8.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `candidate` `<ArrayBuffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<Buffer>` | `<DataView>` | `<bigint>` A possible prime encoded as a sequence of big endian octets of arbitrary length.
- `options` `<Object>`
  - `checks` `<number>` The number of Miller-Rabin probabilistic primality iterations to perform. When the value is `0` (zero), a number of checks is used that yields a false positive rate of at most 2-64 for random input. Care must be used when selecting a number of checks. Refer to the OpenSSL documentation for the `BN_is_prime_ex` function `nchecks` options for more details. Default: `0`
- `callback` `<Function>`
  - `err` `<Error>` Set to an `<Error>` object if an error occurred during check.
  - `result` `<boolean>` `true` if the candidate is a prime with an error probability less than `0.25 ** options.checks`.

Checks the primality of the `candidate`.

#### `crypto.checkPrimeSync(candidate[, options])`#

Added in: v15.8.0

- `candidate` `<ArrayBuffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<Buffer>` | `<DataView>` | `<bigint>` A possible prime encoded as a sequence of big endian octets of arbitrary length.
- `options` `<Object>`
  - `checks` `<number>` The number of Miller-Rabin probabilistic primality iterations to perform. When the value is `0` (zero), a number of checks is used that yields a false positive rate of at most 2-64 for random input. Care must be used when selecting a number of checks. Refer to the OpenSSL documentation for the `BN_is_prime_ex` function `nchecks` options for more details. Default: `0`
- Returns: `<boolean>` `true` if the candidate is a prime with an error probability less than `0.25 ** options.checks`.

Checks the primality of the `candidate`.

#### `crypto.constants`#

Added in: v6.3.0

- Type: `<Object>`

An object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto constants.

#### `crypto.createCipheriv(algorithm, key, iv[, options])`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v26.0.0 | Passing a CryptoKey as `key` is deprecated.
v17.9.0, v16.17.0 | The `authTagLength` option is now optional when using the `chacha20-poly1305` cipher and defaults to 16 bytes.
v15.0.0 | The password and iv arguments can be an ArrayBuffer and are each limited to a maximum of 2 ** 31 - 1 bytes.
v11.6.0 | The `key` argument can now be a `KeyObject`.
v11.2.0, v10.17.0 | The cipher `chacha20-poly1305` (the IETF variant of ChaCha20-Poly1305) is now supported.
v10.10.0 | Ciphers in OCB mode are now supported.
v10.2.0 | The `authTagLength` option can now be used to produce shorter authentication tags in GCM mode and defaults to 16 bytes.
v9.9.0 | The `iv` parameter may now be `null` for ciphers which do not need an initialization vector.

- `algorithm` `<string>`
- `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
- `iv` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<null>`
- `options` `<Object>` `stream.transform` options
- Returns: `<Cipheriv>`

Creates and returns a `Cipheriv` object, with the given `algorithm`, `key` and initialization vector (`iv`).

The `options` argument controls stream behavior and is optional except when a cipher in CCM or OCB mode (e.g. `'aes-128-ccm'`) is used. In that case, the `authTagLength` option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the `authTagLength` option is not required but can be used to set the length of the authentication tag that will be returned by `getAuthTag()` and defaults to 16 bytes. For `chacha20-poly1305`, the `authTagLength` option defaults to 16 bytes.

The `algorithm` is dependent on OpenSSL, examples are `'aes192'`, etc. On recent OpenSSL releases, `openssl list -cipher-algorithms` will display the available cipher algorithms.

The `key` is the raw key used by the `algorithm` and `iv` is an initialization vector. Both arguments must be `'utf8'` encoded strings, Buffers, `TypedArray`, or `DataView`s. The `key` may optionally be a `KeyObject` of type `secret`. If the cipher does not need an initialization vector, `iv` may be `null`.

When passing strings for `key` or `iv`, please consider caveats when using strings as inputs to cryptographic APIs.

Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.

#### `crypto.createDecipheriv(algorithm, key, iv[, options])`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v26.0.0 | Passing a CryptoKey as `key` is deprecated.
v17.9.0, v16.17.0 | The `authTagLength` option is now optional when using the `chacha20-poly1305` cipher and defaults to 16 bytes.
v11.6.0 | The `key` argument can now be a `KeyObject`.
v11.2.0, v10.17.0 | The cipher `chacha20-poly1305` (the IETF variant of ChaCha20-Poly1305) is now supported.
v10.10.0 | Ciphers in OCB mode are now supported.
v10.2.0 | The `authTagLength` option can now be used to restrict accepted GCM authentication tag lengths.
v9.9.0 | The `iv` parameter may now be `null` for ciphers which do not need an initialization vector.

- `algorithm` `<string>`
- `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
- `iv` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<null>`
- `options` `<Object>` `stream.transform` options
- Returns: `<Decipheriv>`

Creates and returns a `Decipheriv` object that uses the given `algorithm`, `key` and initialization vector (`iv`).

The `options` argument controls stream behavior and is optional except when a cipher in CCM or OCB mode (e.g. `'aes-128-ccm'`) is used. In that case, the `authTagLength` option is required and specifies the length of the authentication tag in bytes, see CCM mode. For AES-GCM and `chacha20-poly1305`, the `authTagLength` option defaults to 16 bytes and must be set to a different value if a different length is used.

The `algorithm` is dependent on OpenSSL, examples are `'aes192'`, etc. On recent OpenSSL releases, `openssl list -cipher-algorithms` will display the available cipher algorithms.

The `key` is the raw key used by the `algorithm` and `iv` is an initialization vector. Both arguments must be `'utf8'` encoded strings, Buffers, `TypedArray`, or `DataView`s. The `key` may optionally be a `KeyObject` of type `secret`. If the cipher does not need an initialization vector, `iv` may be `null`.

When passing strings for `key` or `iv`, please consider caveats when using strings as inputs to cryptographic APIs.

Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.

#### `crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])`#

Added in: v0.11.12
History

Version | Changes
--- | ---
v8.0.0 | The `prime` argument can be any `TypedArray` or `DataView` now.
v8.0.0 | The `prime` argument can be a `Uint8Array` now.
v6.0.0 | The default for the encoding parameters changed from `binary` to `utf8`.

- `prime` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `primeEncoding` `<string>` The encoding of the `prime` string.
- `generator` `<number>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Default: `2`
- `generatorEncoding` `<string>` The encoding of the `generator` string.
- Returns: `<DiffieHellman>`

Creates a `DiffieHellman` key exchange object using the supplied `prime` and an optional specific `generator`.

The `generator` argument can be a number, string, or `Buffer`. If `generator` is not specified, the value `2` is used.

If `primeEncoding` is specified, `prime` is expected to be a string; otherwise a `Buffer`, `TypedArray`, or `DataView` is expected.

If `generatorEncoding` is specified, `generator` is expected to be a string; otherwise a number, `Buffer`, `TypedArray`, or `DataView` is expected.

#### `crypto.createDiffieHellman(primeLength[, generator])`#

Added in: v0.5.0

- `primeLength` `<number>`
- `generator` `<number>` Default: `2`
- Returns: `<DiffieHellman>`

Creates a `DiffieHellman` key exchange object and generates a prime of `primeLength` bits using an optional specific numeric `generator`. If `generator` is not specified, the value `2` is used.

#### `crypto.createDiffieHellmanGroup(name)`#

Added in: v0.9.3

- `name` `<string>`
- Returns: `<DiffieHellmanGroup>`

An alias for `crypto.getDiffieHellman()`

#### `crypto.createECDH(curveName)`#

Added in: v0.11.14

- `curveName` `<string>`
- Returns: `<ECDH>`

Creates an Elliptic Curve Diffie-Hellman (`ECDH`) key exchange object using a predefined curve specified by the `curveName` string. Use `crypto.getCurves()` to obtain a list of available curve names. On recent OpenSSL releases, `openssl ecparam -list_curves` will also display the name and description of each available elliptic curve.

#### `crypto.createHash(algorithm[, options])`#

Added in: v0.1.92
History

Version | Changes
--- | ---
v12.8.0 | The `outputLength` option was added for XOF hash functions.

- `algorithm` `<string>`
- `options` `<Object>` `stream.transform` options
- Returns: `<Hash>`

Creates and returns a `Hash` object that can be used to generate hash digests using the given `algorithm`. Optional `options` argument controls stream behavior. For XOF hash functions such as `'shake256'`, the `outputLength` option can be used to specify the desired output length in bytes.

When the data is small (< 5MB) and readily available, `crypto.hash()` is usually faster.

The `algorithm` is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are `'sha256'`, `'sha512'`, etc. On recent releases of OpenSSL, `openssl list -digest-algorithms` will display the available digest algorithms.

Example: generating the sha256 sum of a file

    import {
      createReadStream,
    } from 'node:fs';
    import { argv } from 'node:process';
    const {
      createHash,
    } = await import('node:crypto');

    const filename = argv[2];

    const hash = createHash('sha256');

    const input = createReadStream(filename);
    input.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = input.read();
      if (data)
        hash.update(data);
      else {
        console.log(`${hash.digest('hex')} ${filename}`);
      }
    });
    const {
      createReadStream,
    } = require('node:fs');
    const {
      createHash,
    } = require('node:crypto');
    const { argv } = require('node:process');

    const filename = argv[2];

    const hash = createHash('sha256');

    const input = createReadStream(filename);
    input.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = input.read();
      if (data)
        hash.update(data);
      else {
        console.log(`${hash.digest('hex')} ${filename}`);
      }
    });
    javascript

#### `crypto.createHmac(algorithm, key[, options])`#

Added in: v0.1.94
History

Version | Changes
--- | ---
v26.0.0 | Passing a CryptoKey as `key` is deprecated.
v15.0.0 | The key can also be an ArrayBuffer or CryptoKey. The encoding option was added. The key cannot contain more than 2 ** 32 - 1 bytes.
v11.6.0 | The `key` argument can now be a `KeyObject`.

- `algorithm` `<string>`
- `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
- `options` `<Object>` `stream.transform` options
  - `encoding` `<string>` The string encoding to use when `key` is a string.
- Returns: `<Hmac>`

Creates and returns an `Hmac` object that uses the given `algorithm` and `key`. Optional `options` argument controls stream behavior.

The `algorithm` is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are `'sha256'`, `'sha512'`, etc. On recent releases of OpenSSL, `openssl list -digest-algorithms` will display the available digest algorithms.

The `key` is the HMAC key used to generate the cryptographic HMAC hash. If it is a `KeyObject`, its type must be `secret`. If it is a string, please consider caveats when using strings as inputs to cryptographic APIs. If it was obtained from a cryptographically secure source of entropy, such as `crypto.randomBytes()` or `crypto.generateKey()`, its length should not exceed the block size of `algorithm` (e.g., 512 bits for SHA-256).

Example: generating the sha256 HMAC of a file

    import {
      createReadStream,
    } from 'node:fs';
    import { argv } from 'node:process';
    const {
      createHmac,
    } = await import('node:crypto');

    const filename = argv[2];

    const hmac = createHmac('sha256', 'a secret');

    const input = createReadStream(filename);
    input.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = input.read();
      if (data)
        hmac.update(data);
      else {
        console.log(`${hmac.digest('hex')} ${filename}`);
      }
    });
    const {
      createReadStream,
    } = require('node:fs');
    const {
      createHmac,
    } = require('node:crypto');
    const { argv } = require('node:process');

    const filename = argv[2];

    const hmac = createHmac('sha256', 'a secret');

    const input = createReadStream(filename);
    input.on('readable', () => {
      // Only one element is going to be produced by the
      // hash stream.
      const data = input.read();
      if (data)
        hmac.update(data);
      else {
        console.log(`${hmac.digest('hex')} ${filename}`);
      }
    });
    javascript

#### `crypto.createPrivateKey(key)`#

Added in: v11.6.0
History

Version | Changes
--- | ---
v26.7.0 | The key can also be a URL referencing an object for an OpenSSL STORE loader. The `properties` option was added.
v26.7.0 | Passing a CryptoKey as `key` is no longer supported.
v26.1.0 | Added JWK format support for ML-KEM and SLH-DSA key types.
v26.0.0 | Passing a CryptoKey as `key` is deprecated.
v26.0.0 | Added support for `'raw-private'` and `'raw-seed'` formats.
v24.6.0 | Add support for ML-DSA keys.
v15.12.0 | The key can also be a JWK object.
v15.0.0 | The key can also be an ArrayBuffer. The encoding option was added. The key cannot contain more than 2 ** 32 - 1 bytes.

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<URL>`
  - `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<Object>` | `<URL>` The key material, either in PEM, DER, JWK, or raw format, or a `<URL>` referencing an object for an OpenSSL STORE loader.
  - `format` `<string>` Must be `'pem'`, `'der'`, `'jwk'`, `'raw-private'`, or `'raw-seed'`. Default: `'pem'`.
  - `type` `<string>` Must be `'pkcs1'`, `'pkcs8'` or `'sec1'`. This option is required only if the `format` is `'der'` and ignored otherwise.
  - `passphrase` `<string>` | `<Buffer>` The passphrase to use for decryption. When `key` is a `<URL>`, this is the optional PIN/passphrase forwarded to the STORE loader.
  - `properties` `<string>` The optional OpenSSL property query used when fetching the STORE loader for a `<URL>` key.
  - `encoding` `<string>` The string encoding to use when `key` is a string.
  - `asymmetricKeyType` `<string>` Required when `format` is `'raw-private'` or `'raw-seed'` and ignored otherwise. Must be a supported key type.
  - `namedCurve` `<string>` Name of the curve to use. Required when `asymmetricKeyType` is `'ec'` and ignored otherwise.
- Returns: `<KeyObject>`

Creates and returns a new key object containing a private key. If `key` is a string or `Buffer`, `format` is assumed to be `'pem'`; otherwise, `key` must be an object with the properties described above.

If the private key is encrypted, a `passphrase` must be specified. The length of the passphrase is limited to 1024 bytes.

##### Private keys from OpenSSL STORE loaders#

Stability: 1.1 - Active development

If `key` is a `<URL>` (or an object whose `key` is a `<URL>`), the private key is loaded through an OpenSSL STORE loader. The URL is passed to OpenSSL as a URI, for example a `file:` URI or a provider-backed scheme such as `pkcs11:`. When the Permission Model is enabled, `--allow-openssl-store` is required.

> Warning: A URI scheme does not pin an OpenSSL STORE loader or prove where the returned key came from. Node.js forwards the URI to OpenSSL, which chooses loaders according to its version and configuration. For example, OpenSSL may offer an opaque URI such as `pkcs11:object=...` (one without `//` after the scheme) to its `file` loader before trying the `pkcs11` loader. If the complete URI is a valid local path and that file exists, it may be loaded instead. Node.js does not verify which loader supplied the key. Do not rely on a provider-specific URI scheme as proof that a key came from that provider or from a hardware device.

Configured OpenSSL STORE loaders have broad authority and may access files, devices, tokens, or the network. Access performed by a loader is not constrained by the `fs.read`, `fs.write`, or `net` permission scopes.

When a `<URL>` is used, `format`, `type`, `asymmetricKeyType`, and `namedCurve` are ignored even when those options would otherwise depend on each other, such as `type` with `format: 'der'` or `namedCurve` with `asymmetricKeyType: 'ec'`. The input is passed to the STORE loader as a URI, not handled as PEM, DER, JWK, or raw key material. `passphrase` is still used as the optional PIN/passphrase passed to the loader, and `encoding` applies if that `passphrase` is a string.

Use `passphrase` instead of embedding credentials in the URI passed to the STORE loader. Node.js redacts the URI from its own permission-denial resource and diagnostics. Errors reported by OpenSSL or a provider after loading begins may include the URI.

When `properties` is specified with a `<URL>` key, it is passed to OpenSSL as the property query for selecting the STORE loader. It is not appended to the URL and is distinct from provider-specific URI parameters.

#### `crypto.createPublicKey(key)`#

Added in: v11.6.0
History

Version | Changes
--- | ---
v26.1.0 | Added JWK format support for ML-KEM and SLH-DSA key types.
v26.0.0 | Passing a CryptoKey as `key` is deprecated.
v26.0.0 | Added support for `'raw-public'` format.
v24.6.0 | Add support for ML-DSA keys.
v15.12.0 | The key can also be a JWK object.
v15.0.0 | The key can also be an ArrayBuffer. The encoding option was added. The key cannot contain more than 2 ** 32 - 1 bytes.
v11.13.0 | The `key` argument can now be a `KeyObject` with type `private`.
v11.7.0 | The `key` argument can now be a private key.

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
  - `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<Object>` The key material, either in PEM, DER, JWK, or raw format.
  - `format` `<string>` Must be `'pem'`, `'der'`, `'jwk'`, or `'raw-public'`. Default: `'pem'`.
  - `type` `<string>` Must be `'pkcs1'` or `'spki'`. This option is required only if the `format` is `'der'` and ignored otherwise.
  - `encoding` `<string>` The string encoding to use when `key` is a string.
  - `asymmetricKeyType` `<string>` Required when `format` is `'raw-public'` and ignored otherwise. Must be a supported key type.
  - `namedCurve` `<string>` Name of the curve to use. Required when `asymmetricKeyType` is `'ec'` and ignored otherwise.
- Returns: `<KeyObject>`

Creates and returns a new key object containing a public key. If `key` is a string or `Buffer`, `format` is assumed to be `'pem'`; if `key` is a `KeyObject` with type `'private'`, the public key is derived from the given private key; otherwise, `key` must be an object with the properties described above.

If the format is `'pem'`, the `'key'` may also be an X.509 certificate.

Because public keys can be derived from private keys, a private key may be passed instead of a public key. In that case, this function behaves as if `crypto.createPrivateKey()` had been called, except that the type of the returned `KeyObject` will be `'public'` and that the private key cannot be extracted from the returned `KeyObject`. Similarly, if a `KeyObject` with type `'private'` is given, a new `KeyObject` with type `'public'` will be returned and it will be impossible to extract the private key from the returned object.

A store-backed private key can be used as a public key by first loading it with `crypto.createPrivateKey()`; a `<URL>` cannot be passed to `crypto.createPublicKey()` directly.

#### `crypto.createSecretKey(key[, encoding])`#

Added in: v11.6.0
History

Version | Changes
--- | ---
v18.8.0, v16.18.0 | The key can now be zero-length.
v15.0.0 | The key can also be an ArrayBuffer or string. The encoding argument was added. The key cannot contain more than 2 ** 32 - 1 bytes.

- `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `encoding` `<string>` The string encoding when `key` is a string.
- Returns: `<KeyObject>`

Creates and returns a new key object containing a secret key for symmetric encryption or `Hmac`.

#### `crypto.createSign(algorithm[, options])`#

Added in: v0.1.92

- `algorithm` `<string>`
- `options` `<Object>` `stream.Writable` options
- Returns: `<Sign>`

Creates and returns a `Sign` object that uses the given `algorithm`. Use `crypto.getHashes()` to obtain the names of the available digest algorithms. Optional `options` argument controls the `stream.Writable` behavior.

In some cases, a `Sign` instance can be created using the name of a signature algorithm, such as `'RSA-SHA256'`, instead of a digest algorithm. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as `'ecdsa-with-SHA256'`, so it is best to always use digest algorithm names.

#### `crypto.createVerify(algorithm[, options])`#

Added in: v0.1.92

- `algorithm` `<string>`
- `options` `<Object>` `stream.Writable` options
- Returns: `<Verify>`

Creates and returns a `Verify` object that uses the given algorithm. Use `crypto.getHashes()` to obtain an array of names of the available signing algorithms. Optional `options` argument controls the `stream.Writable` behavior.

In some cases, a `Verify` instance can be created using the name of a signature algorithm, such as `'RSA-SHA256'`, instead of a digest algorithm. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as `'ecdsa-with-SHA256'`, so it is best to always use digest algorithm names.

#### `crypto.decapsulate(key, ciphertext[, callback])`#

Added in: v24.7.0

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<URL>` Private Key
- `ciphertext` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `sharedKey` `<Buffer>`
- Returns: `<Buffer>` if the `callback` function is not provided.

Key decapsulation using a KEM algorithm with a private key.

Supported key types and their KEM algorithms are:

- `'rsa'`2 RSA Secret Value Encapsulation
- `'ec'`3 DHKEM(P-256, HKDF-SHA256), DHKEM(P-384, HKDF-SHA256), DHKEM(P-521, HKDF-SHA256)
- `'x25519'`3 DHKEM(X25519, HKDF-SHA256)
- `'x448'`3 DHKEM(X448, HKDF-SHA512)
- `'ml-kem-512'`1 ML-KEM
- `'ml-kem-768'`1 ML-KEM
- `'ml-kem-1024'`1 ML-KEM

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPrivateKey()`.

If the `callback` function is provided this function uses libuv's threadpool.

#### `crypto.diffieHellman(options[, callback])`#

Added in: v13.9.0, v12.17.0
History

Version | Changes
--- | ---
v26.1.0 | Accept key data in addition to KeyObject instances.
v23.11.0 | Optional callback argument added.

- `options` `<Object>`
  - `privateKey` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<URL>`
  - `publicKey` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `secret` `<Buffer>`
- Returns: `<Buffer>` if the `callback` function is not provided.

Computes the Diffie-Hellman shared secret based on a `privateKey` and a `publicKey`. Both keys must represent the same asymmetric key type and must support either the DH or ECDH operation.

If `options.privateKey` is not a `KeyObject`, this function behaves as if `options.privateKey` had been passed to `crypto.createPrivateKey()`.

If `options.publicKey` is not a `KeyObject`, this function behaves as if `options.publicKey` had been passed to `crypto.createPublicKey()`.

If the `callback` function is provided this function uses libuv's threadpool.

#### `crypto.encapsulate(key[, callback])`#

Added in: v24.7.0

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` Public Key
- `callback` `<Function>`
  - `err` `<Error>`
  - `result` `<Object>`
    - `sharedKey` `<Buffer>`
    - `ciphertext` `<Buffer>`
- Returns: `<Object>` if the `callback` function is not provided.
  - `sharedKey` `<Buffer>`
  - `ciphertext` `<Buffer>`

Key encapsulation using a KEM algorithm with a public key.

Supported key types and their KEM algorithms are:

- `'rsa'`2 RSA Secret Value Encapsulation
- `'ec'`3 DHKEM(P-256, HKDF-SHA256), DHKEM(P-384, HKDF-SHA256), DHKEM(P-521, HKDF-SHA256)
- `'x25519'`3 DHKEM(X25519, HKDF-SHA256)
- `'x448'`3 DHKEM(X448, HKDF-SHA512)
- `'ml-kem-512'`1 ML-KEM
- `'ml-kem-768'`1 ML-KEM
- `'ml-kem-1024'`1 ML-KEM

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPublicKey()`.

If the `callback` function is provided this function uses libuv's threadpool.

#### `crypto.fips`#

Added in: v6.0.0Deprecated in: v10.0.0

Stability: 0 - Deprecated

Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.

This property is deprecated. Please use `crypto.setFips()` and `crypto.getFips()` instead.

#### `crypto.generateKey(type, options, callback)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `type` `<string>` The intended use of the generated secret key. Currently accepted values are `'hmac'` and `'aes'`.
- `options` `<Object>`
  - `length` `<number>` The bit length of the key to generate. This must be a value greater than 0.
    - If `type` is `'hmac'`, the minimum is 8, and the maximum length is 231-1. If the value is not a multiple of 8, the generated key will be truncated to `Math.floor(length / 8)`.
    - If `type` is `'aes'`, the length must be one of `128`, `192`, or `256`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `key` `<KeyObject>`

Asynchronously generates a new random secret key of the given `length`. The `type` will determine which validations will be performed on the `length`.

    const {
      generateKey,
    } = await import('node:crypto');

    generateKey('hmac', { length: 512 }, (err, key) => {
      if (err) throw err;
      console.log(key.export().toString('hex'));  // 46e..........620
    });
    const {
      generateKey,
    } = require('node:crypto');

    generateKey('hmac', { length: 512 }, (err, key) => {
      if (err) throw err;
      console.log(key.export().toString('hex'));  // 46e..........620
    });
    javascript

The size of a generated HMAC key should not exceed the block size of the underlying hash function. See `crypto.createHmac()` for more information.

#### `crypto.generateKeyPair(type, options, callback)`#

Added in: v10.12.0
History

Version | Changes
--- | ---
v24.8.0 | Add support for SLH-DSA key pairs.
v24.7.0 | Add support for ML-KEM key pairs.
v24.6.0 | Add support for ML-DSA key pairs.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v16.10.0 | Add ability to define `RSASSA-PSS-params` sequence parameters for RSA-PSS keys pairs.
v13.9.0, v12.17.0 | Add support for Diffie-Hellman.
v12.0.0 | Add support for RSA-PSS key pairs.
v12.0.0 | Add ability to generate X25519 and X448 key pairs.
v12.0.0 | Add ability to generate Ed25519 and Ed448 key pairs.
v11.6.0 | The `generateKeyPair` and `generateKeyPairSync` functions now produce key objects if no encoding was specified.

- `type` `<string>` The asymmetric key type to generate. See the supported asymmetric key types.
- `options` `<Object>`
  - `modulusLength` `<number>` Key size in bits (RSA, DSA).
  - `publicExponent` `<number>` Public exponent (RSA). Default: `0x10001`.
  - `hashAlgorithm` `<string>` Name of the message digest (RSA-PSS).
  - `mgf1HashAlgorithm` `<string>` Name of the message digest used by MGF1 (RSA-PSS).
  - `saltLength` `<number>` Minimal salt length in bytes (RSA-PSS).
  - `divisorLength` `<number>` Size of `q` in bits (DSA).
  - `namedCurve` `<string>` Name of the curve to use (EC).
  - `prime` `<Buffer>` The prime parameter (DH).
  - `primeLength` `<number>` Prime length in bits (DH).
  - `generator` `<number>` Custom generator (DH). Default: `2`.
  - `groupName` `<string>` Diffie-Hellman group name (DH). See `crypto.getDiffieHellman()`.
  - `paramEncoding` `<string>` Must be `'named'` or `'explicit'` (EC). Default: `'named'`.
  - `publicKeyEncoding` `<Object>` See `keyObject.export()`.
  - `privateKeyEncoding` `<Object>` See `keyObject.export()`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `publicKey` `<string>` | `<Buffer>` | `<KeyObject>`
  - `privateKey` `<string>` | `<Buffer>` | `<KeyObject>`

Generates a new asymmetric key pair of the given `type`. See the supported asymmetric key types.

If a `publicKeyEncoding` or `privateKeyEncoding` was specified, this function behaves as if `keyObject.export()` had been called on its result. Otherwise, the respective part of the key is returned as a `KeyObject`.

It is recommended to encode public keys as `'spki'` and private keys as `'pkcs8'` with encryption for long-term storage:

    const {
      generateKeyPair,
    } = await import('node:crypto');

    generateKeyPair('rsa', {
      modulusLength: 4096,
      publicKeyEncoding: {
        type: 'spki',
        format: 'pem',
      },
      privateKeyEncoding: {
        type: 'pkcs8',
        format: 'pem',
        cipher: 'aes-256-cbc',
        passphrase: 'top secret',
      },
    }, (err, publicKey, privateKey) => {
      // Handle errors and use the generated key pair.
    });
    const {
      generateKeyPair,
    } = require('node:crypto');

    generateKeyPair('rsa', {
      modulusLength: 4096,
      publicKeyEncoding: {
        type: 'spki',
        format: 'pem',
      },
      privateKeyEncoding: {
        type: 'pkcs8',
        format: 'pem',
        cipher: 'aes-256-cbc',
        passphrase: 'top secret',
      },
    }, (err, publicKey, privateKey) => {
      // Handle errors and use the generated key pair.
    });
    javascript

On completion, `callback` will be called with `err` set to `undefined` and `publicKey` / `privateKey` representing the generated key pair.

If this method is invoked as its `util.promisify()`ed version, it returns a `Promise` for an `Object` with `publicKey` and `privateKey` properties.

#### `crypto.generateKeyPairSync(type, options)`#

Added in: v10.12.0
History

Version | Changes
--- | ---
v24.8.0 | Add support for SLH-DSA key pairs.
v24.7.0 | Add support for ML-KEM key pairs.
v24.6.0 | Add support for ML-DSA key pairs.
v16.10.0 | Add ability to define `RSASSA-PSS-params` sequence parameters for RSA-PSS keys pairs.
v13.9.0, v12.17.0 | Add support for Diffie-Hellman.
v12.0.0 | Add support for RSA-PSS key pairs.
v12.0.0 | Add ability to generate X25519 and X448 key pairs.
v12.0.0 | Add ability to generate Ed25519 and Ed448 key pairs.
v11.6.0 | The `generateKeyPair` and `generateKeyPairSync` functions now produce key objects if no encoding was specified.

- `type` `<string>` The asymmetric key type to generate. See the supported asymmetric key types.
- `options` `<Object>`
  - `modulusLength` `<number>` Key size in bits (RSA, DSA).
  - `publicExponent` `<number>` Public exponent (RSA). Default: `0x10001`.
  - `hashAlgorithm` `<string>` Name of the message digest (RSA-PSS).
  - `mgf1HashAlgorithm` `<string>` Name of the message digest used by MGF1 (RSA-PSS).
  - `saltLength` `<number>` Minimal salt length in bytes (RSA-PSS).
  - `divisorLength` `<number>` Size of `q` in bits (DSA).
  - `namedCurve` `<string>` Name of the curve to use (EC).
  - `prime` `<Buffer>` The prime parameter (DH).
  - `primeLength` `<number>` Prime length in bits (DH).
  - `generator` `<number>` Custom generator (DH). Default: `2`.
  - `groupName` `<string>` Diffie-Hellman group name (DH). See `crypto.getDiffieHellman()`.
  - `paramEncoding` `<string>` Must be `'named'` or `'explicit'` (EC). Default: `'named'`.
  - `publicKeyEncoding` `<Object>` See `keyObject.export()`.
  - `privateKeyEncoding` `<Object>` See `keyObject.export()`.
- Returns: `<Object>`
  - `publicKey` `<string>` | `<Buffer>` | `<KeyObject>`
  - `privateKey` `<string>` | `<Buffer>` | `<KeyObject>`

Generates a new asymmetric key pair of the given `type`. See the supported asymmetric key types.

If a `publicKeyEncoding` or `privateKeyEncoding` was specified, this function behaves as if `keyObject.export()` had been called on its result. Otherwise, the respective part of the key is returned as a `KeyObject`.

When encoding public keys, it is recommended to use `'spki'`. When encoding private keys, it is recommended to use `'pkcs8'` with a strong passphrase, and to keep the passphrase confidential.

    const {
      generateKeyPairSync,
    } = await import('node:crypto');

    const {
      publicKey,
      privateKey,
    } = generateKeyPairSync('rsa', {
      modulusLength: 4096,
      publicKeyEncoding: {
        type: 'spki',
        format: 'pem',
      },
      privateKeyEncoding: {
        type: 'pkcs8',
        format: 'pem',
        cipher: 'aes-256-cbc',
        passphrase: 'top secret',
      },
    });
    const {
      generateKeyPairSync,
    } = require('node:crypto');

    const {
      publicKey,
      privateKey,
    } = generateKeyPairSync('rsa', {
      modulusLength: 4096,
      publicKeyEncoding: {
        type: 'spki',
        format: 'pem',
      },
      privateKeyEncoding: {
        type: 'pkcs8',
        format: 'pem',
        cipher: 'aes-256-cbc',
        passphrase: 'top secret',
      },
    });
    javascript

The return value `{ publicKey, privateKey }` represents the generated key pair. When PEM encoding was selected, the respective key will be a string, otherwise it will be a buffer containing the data encoded as DER.

#### `crypto.generateKeySync(type, options)`#

Added in: v15.0.0

- `type` `<string>` The intended use of the generated secret key. Currently accepted values are `'hmac'` and `'aes'`.
- `options` `<Object>`
  - `length` `<number>` The bit length of the key to generate.
    - If `type` is `'hmac'`, the minimum is 8, and the maximum length is 231-1. If the value is not a multiple of 8, the generated key will be truncated to `Math.floor(length / 8)`.
    - If `type` is `'aes'`, the length must be one of `128`, `192`, or `256`.
- Returns: `<KeyObject>`

Synchronously generates a new random secret key of the given `length`. The `type` will determine which validations will be performed on the `length`.

    const {
      generateKeySync,
    } = await import('node:crypto');

    const key = generateKeySync('hmac', { length: 512 });
    console.log(key.export().toString('hex'));  // e89..........41e
    const {
      generateKeySync,
    } = require('node:crypto');

    const key = generateKeySync('hmac', { length: 512 });
    console.log(key.export().toString('hex'));  // e89..........41e
    javascript

The size of a generated HMAC key should not exceed the block size of the underlying hash function. See `crypto.createHmac()` for more information.

#### `crypto.generatePrime(size[, options], callback)`#

Added in: v15.8.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `size` `<number>` The size (in bits) of the prime to generate.
- `options` `<Object>`
  - `add` `<ArrayBuffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<Buffer>` | `<DataView>` | `<bigint>`
  - `rem` `<ArrayBuffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<Buffer>` | `<DataView>` | `<bigint>`
  - `safe` `<boolean>` Default: `false`.
  - `bigint` `<boolean>` When `true`, the generated prime is returned as a `bigint`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `prime` `<ArrayBuffer>` | `<bigint>`

Generates a pseudorandom prime of `size` bits.

If `options.safe` is `true`, the prime will be a safe prime -- that is, `(prime - 1) / 2` will also be a prime.

The `options.add` and `options.rem` parameters can be used to enforce additional requirements, e.g., for Diffie-Hellman:

- If `options.add` and `options.rem` are both set, the prime will satisfy the condition that `prime % add = rem`.
- If only `options.add` is set and `options.safe` is not `true`, the prime will satisfy the condition that `prime % add = 1`.
- If only `options.add` is set and `options.safe` is set to `true`, the prime will instead satisfy the condition that `prime % add = 3`. This is necessary because `prime % add = 1` for `options.add > 2` would contradict the condition enforced by `options.safe`.
- `options.rem` is ignored if `options.add` is not given.

Both `options.add` and `options.rem` must be encoded as big-endian sequences if given as an `ArrayBuffer`, `SharedArrayBuffer`, `TypedArray`, `Buffer`, or `DataView`.

By default, the prime is encoded as a big-endian sequence of octets in an `<ArrayBuffer>`. If the `bigint` option is `true`, then a `<bigint>` is provided.

The `size` of the prime will have a direct impact on how long it takes to generate the prime. The larger the size, the longer it will take. Because we use OpenSSL's `BN_generate_prime_ex` function, which provides only minimal control over our ability to interrupt the generation process, it is not recommended to generate overly large primes, as doing so may make the process unresponsive.

#### `crypto.generatePrimeSync(size[, options])`#

Added in: v15.8.0

- `size` `<number>` The size (in bits) of the prime to generate.
- `options` `<Object>`
  - `add` `<ArrayBuffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<Buffer>` | `<DataView>` | `<bigint>`
  - `rem` `<ArrayBuffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<Buffer>` | `<DataView>` | `<bigint>`
  - `safe` `<boolean>` Default: `false`.
  - `bigint` `<boolean>` When `true`, the generated prime is returned as a `bigint`.
- Returns: `<ArrayBuffer>` | `<bigint>`

Generates a pseudorandom prime of `size` bits.

If `options.safe` is `true`, the prime will be a safe prime -- that is, `(prime - 1) / 2` will also be a prime.

The `options.add` and `options.rem` parameters can be used to enforce additional requirements, e.g., for Diffie-Hellman:

- If `options.add` and `options.rem` are both set, the prime will satisfy the condition that `prime % add = rem`.
- If only `options.add` is set and `options.safe` is not `true`, the prime will satisfy the condition that `prime % add = 1`.
- If only `options.add` is set and `options.safe` is set to `true`, the prime will instead satisfy the condition that `prime % add = 3`. This is necessary because `prime % add = 1` for `options.add > 2` would contradict the condition enforced by `options.safe`.
- `options.rem` is ignored if `options.add` is not given.

Both `options.add` and `options.rem` must be encoded as big-endian sequences if given as an `ArrayBuffer`, `SharedArrayBuffer`, `TypedArray`, `Buffer`, or `DataView`.

By default, the prime is encoded as a big-endian sequence of octets in an `<ArrayBuffer>`. If the `bigint` option is `true`, then a `<bigint>` is provided.

The `size` of the prime will have a direct impact on how long it takes to generate the prime. The larger the size, the longer it will take. Because we use OpenSSL's `BN_generate_prime_ex` function, which provides only minimal control over our ability to interrupt the generation process, it is not recommended to generate overly large primes, as doing so may make the process unresponsive.

#### `crypto.getCipherInfo(nameOrNid[, options])`#

Added in: v15.0.0

- `nameOrNid` `<string>` | `<number>` The name or nid of the cipher to query.
- `options` `<Object>`
  - `keyLength` `<number>` A test key length.
  - `ivLength` `<number>` A test IV length.
- Returns: `<Object>`
  - `name` `<string>` The name of the cipher
  - `nid` `<number>` The nid of the cipher
  - `blockSize` `<number>` The block size of the cipher in bytes. This property is omitted when `mode` is `'stream'`.
  - `ivLength` `<number>` The expected or default initialization vector length in bytes. This property is omitted if the cipher does not use an initialization vector.
  - `keyLength` `<number>` The expected or default key length in bytes.
  - `mode` `<string>` The cipher mode. One of `'cbc'`, `'ccm'`, `'cfb'`, `'ctr'`, `'ecb'`, `'gcm'`, `'ocb'`, `'ofb'`, `'stream'`, `'wrap'`, `'xts'`.

Returns information about a given cipher.

Some ciphers accept variable length keys and initialization vectors. By default, the `crypto.getCipherInfo()` method will return the default values for these ciphers. To test if a given key length or iv length is acceptable for given cipher, use the `keyLength` and `ivLength` options. If the given values are unacceptable, `undefined` will be returned.

#### `crypto.getCiphers()`#

Added in: v0.9.3

- Returns: `<string>`[] An array with the names of the supported cipher algorithms.

    const {
      getCiphers,
    } = await import('node:crypto');

    console.log(getCiphers()); // ['aes-128-cbc', 'aes-128-ccm', ...]
    const {
      getCiphers,
    } = require('node:crypto');

    console.log(getCiphers()); // ['aes-128-cbc', 'aes-128-ccm', ...]
    javascript

#### `crypto.getCurves()`#

Added in: v2.3.0

- Returns: `<string>`[] An array with the names of the supported elliptic curves.

    const {
      getCurves,
    } = await import('node:crypto');

    console.log(getCurves()); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]
    const {
      getCurves,
    } = require('node:crypto');

    console.log(getCurves()); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]
    javascript

#### `crypto.getDiffieHellman(groupName)`#

Added in: v0.7.5

- `groupName` `<string>`
- Returns: `<DiffieHellmanGroup>`

Creates a predefined `DiffieHellmanGroup` key exchange object. The supported groups are listed in the documentation for `DiffieHellmanGroup`.

The returned object mimics the interface of objects created by `crypto.createDiffieHellman()`, but will not allow changing the keys (with `diffieHellman.setPublicKey()`, for example). The advantage of using this method is that the parties do not have to generate nor exchange a group modulus beforehand, saving both processor and communication time.

Example (obtaining a shared secret):

    const {
      getDiffieHellman,
    } = await import('node:crypto');
    const alice = getDiffieHellman('modp14');
    const bob = getDiffieHellman('modp14');

    alice.generateKeys();
    bob.generateKeys();

    const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
    const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

    /* aliceSecret and bobSecret should be the same */
    console.log(aliceSecret === bobSecret);
    const {
      getDiffieHellman,
    } = require('node:crypto');

    const alice = getDiffieHellman('modp14');
    const bob = getDiffieHellman('modp14');

    alice.generateKeys();
    bob.generateKeys();

    const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
    const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

    /* aliceSecret and bobSecret should be the same */
    console.log(aliceSecret === bobSecret);
    javascript

#### `crypto.getFips()`#

Added in: v10.0.0

- Returns: `<number>` `1` if and only if a FIPS compliant crypto provider is currently in use, `0` otherwise. A future semver-major release may change the return type of this API to a `<boolean>`.

#### `crypto.getHashes()`#

Added in: v0.9.3

- Returns: `<string>`[] An array of the names of the supported hash algorithms, such as `'RSA-SHA256'`. Hash algorithms are also called "digest" algorithms.

    const {
      getHashes,
    } = await import('node:crypto');

    console.log(getHashes()); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]
    const {
      getHashes,
    } = require('node:crypto');

    console.log(getHashes()); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]
    javascript

#### `crypto.getRandomValues(typedArray)`#

Added in: v17.4.0

- `typedArray` `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>`
- Returns: `<Buffer>` | `<TypedArray>` | `<DataView>` | `<ArrayBuffer>` Returns `typedArray`.

A convenient alias for `crypto.webcrypto.getRandomValues()`. This implementation is not compliant with the Web Crypto spec, to write web-compatible code use `crypto.webcrypto.getRandomValues()` instead.

#### `crypto.hash(algorithm, data[, options])`#

Added in: v21.7.0, v20.12.0
History

Version | Changes
--- | ---
v25.5.0, v24.13.1 | This API is no longer experimental.
v24.4.0 | The `outputLength` option was added for XOF hash functions.

- `algorithm` `<string>` | `<undefined>`
- `data` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>` When `data` is a string, it will be encoded as UTF-8 before being hashed. If a different input encoding is desired for a string input, user could encode the string into a `TypedArray` using either `TextEncoder` or `Buffer.from()` and passing the encoded `TypedArray` into this API instead.
- `options` `<Object>` | `<string>`
  - `outputEncoding` `<string>` Encoding used to encode the returned digest. Default: `'hex'`.
  - `outputLength` `<number>` For XOF hash functions such as 'shake256', the outputLength option can be used to specify the desired output length in bytes.
- Returns: `<string>` | `<Buffer>`

A utility for creating one-shot hash digests of data. It can be faster than the object-based `crypto.createHash()` when hashing a smaller amount of data (<= 5MB) that's readily available. If the data can be big or if it is streamed, it's still recommended to use `crypto.createHash()` instead.

The `algorithm` is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are `'sha256'`, `'sha512'`, etc. On recent releases of OpenSSL, `openssl list -digest-algorithms` will display the available digest algorithms.

If `options` is a string, then it specifies the `outputEncoding`.

Example:

    const crypto = require('node:crypto');
    const { Buffer } = require('node:buffer');

    // Hashing a string and return the result as a hex-encoded string.
    const string = 'Node.js';
    // 10b3493287f831e81a438811a1ffba01f8cec4b7
    console.log(crypto.hash('sha1', string));

    // Encode a base64-encoded string into a Buffer, hash it and return
    // the result as a buffer.
    const base64 = 'Tm9kZS5qcw==';
    // <Buffer 10 b3 49 32 87 f8 31 e8 1a 43 88 11 a1 ff ba 01 f8 ce c4 b7>
    console.log(crypto.hash('sha1', Buffer.from(base64, 'base64'), 'buffer'));
    import crypto from 'node:crypto';
    import { Buffer } from 'node:buffer';

    // Hashing a string and return the result as a hex-encoded string.
    const string = 'Node.js';
    // 10b3493287f831e81a438811a1ffba01f8cec4b7
    console.log(crypto.hash('sha1', string));

    // Encode a base64-encoded string into a Buffer, hash it and return
    // the result as a buffer.
    const base64 = 'Tm9kZS5qcw==';
    // <Buffer 10 b3 49 32 87 f8 31 e8 1a 43 88 11 a1 ff ba 01 f8 ce c4 b7>
    console.log(crypto.hash('sha1', Buffer.from(base64, 'base64'), 'buffer'));
    javascript

#### `crypto.hkdf(digest, ikm, salt, info, keylen, callback)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v18.8.0, v16.18.0 | The input keying material can now be zero-length.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `digest` `<string>` The digest algorithm to use.
- `ikm` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` The input keying material. Must be provided but can be zero-length.
- `salt` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The salt value. Must be provided but can be zero-length.
- `info` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Additional info value. Must be provided but can be zero-length, and cannot be more than 1024 bytes.
- `keylen` `<number>` The length of the key to generate. Must be greater than 0. The maximum allowable value is `255` times the number of bytes produced by the selected digest function (e.g. `sha512` generates 64-byte hashes, making the maximum HKDF output 16320 bytes).
- `callback` `<Function>`
  - `err` `<Error>`
  - `derivedKey` `<ArrayBuffer>`

HKDF is a simple key derivation function defined in RFC 5869. The given `ikm`, `salt` and `info` are used with the `digest` to derive a key of `keylen` bytes.

The supplied `callback` function is called with two arguments: `err` and `derivedKey`. If an error occurs while deriving the key, `err` will be set; otherwise `err` will be `null`. The successfully generated `derivedKey` will be passed to the callback as an `<ArrayBuffer>`. An error will be thrown if any of the input arguments specify invalid values or types.

    import { Buffer } from 'node:buffer';
    const {
      hkdf,
    } = await import('node:crypto');

    hkdf('sha512', 'key', 'salt', 'info', 64, (err, derivedKey) => {
      if (err) throw err;
      console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'
    });
    const {
      hkdf,
    } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    hkdf('sha512', 'key', 'salt', 'info', 64, (err, derivedKey) => {
      if (err) throw err;
      console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'
    });
    javascript

#### `crypto.hkdfSync(digest, ikm, salt, info, keylen)`#

Added in: v15.0.0
History

Version | Changes
--- | ---
v18.8.0, v16.18.0 | The input keying material can now be zero-length.

- `digest` `<string>` The digest algorithm to use.
- `ikm` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` The input keying material. Must be provided but can be zero-length.
- `salt` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The salt value. Must be provided but can be zero-length.
- `info` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Additional info value. Must be provided but can be zero-length, and cannot be more than 1024 bytes.
- `keylen` `<number>` The length of the key to generate. Must be greater than 0. The maximum allowable value is `255` times the number of bytes produced by the selected digest function (e.g. `sha512` generates 64-byte hashes, making the maximum HKDF output 16320 bytes).
- Returns: `<ArrayBuffer>`

Provides a synchronous HKDF key derivation function as defined in RFC 5869. The given `ikm`, `salt` and `info` are used with the `digest` to derive a key of `keylen` bytes.

The successfully generated `derivedKey` will be returned as an `<ArrayBuffer>`.

An error will be thrown if any of the input arguments specify invalid values or types, or if the derived key cannot be generated.

    import { Buffer } from 'node:buffer';
    const {
      hkdfSync,
    } = await import('node:crypto');

    const derivedKey = hkdfSync('sha512', 'key', 'salt', 'info', 64);
    console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'
    const {
      hkdfSync,
    } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const derivedKey = hkdfSync('sha512', 'key', 'salt', 'info', 64);
    console.log(Buffer.from(derivedKey).toString('hex'));  // '24156e2...5391653'
    javascript

#### `crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)`#

Added in: v0.5.5
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v15.0.0 | The password and salt arguments can also be ArrayBuffer instances.
v14.0.0 | The `iterations` parameter is now restricted to positive values. Earlier releases treated other values as one.
v8.0.0 | The `digest` parameter is always required now.
v6.0.0 | Calling this function without passing the `digest` parameter is deprecated now and will emit a warning.
v6.0.0 | The default encoding for `password` if it is a string changed from `binary` to `utf8`.

- `password` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `salt` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `iterations` `<number>`
- `keylen` `<number>`
- `digest` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `derivedKey` `<Buffer>`

Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by `digest` is applied to derive a key of the requested byte length (`keylen`) from the `password`, `salt` and `iterations`.

The supplied `callback` function is called with two arguments: `err` and `derivedKey`. If an error occurs while deriving the key, `err` will be set; otherwise `err` will be `null`. By default, the successfully generated `derivedKey` will be passed to the callback as a `Buffer`. An error will be thrown if any of the input arguments specify invalid values or types.

The `iterations` argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The `salt` should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for `password` or `salt`, please consider caveats when using strings as inputs to cryptographic APIs.

    const {
      pbkdf2,
    } = await import('node:crypto');

    pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
    });
    const {
      pbkdf2,
    } = require('node:crypto');

    pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
    });
    javascript

An array of supported digest functions can be retrieved using `crypto.getHashes()`.

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the `UV_THREADPOOL_SIZE` documentation for more information.

#### `crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)`#

Added in: v0.9.3
History

Version | Changes
--- | ---
v14.0.0 | The `iterations` parameter is now restricted to positive values. Earlier releases treated other values as one.
v6.0.0 | Calling this function without passing the `digest` parameter is deprecated now and will emit a warning.
v6.0.0 | The default encoding for `password` if it is a string changed from `binary` to `utf8`.

- `password` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `salt` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `iterations` `<number>`
- `keylen` `<number>`
- `digest` `<string>`
- Returns: `<Buffer>`

Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by `digest` is applied to derive a key of the requested byte length (`keylen`) from the `password`, `salt` and `iterations`.

If an error occurs an `Error` will be thrown, otherwise the derived key will be returned as a `Buffer`.

The `iterations` argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The `salt` should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for `password` or `salt`, please consider caveats when using strings as inputs to cryptographic APIs.

    const {
      pbkdf2Sync,
    } = await import('node:crypto');

    const key = pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
    console.log(key.toString('hex'));  // '3745e48...08d59ae'
    const {
      pbkdf2Sync,
    } = require('node:crypto');

    const key = pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
    console.log(key.toString('hex'));  // '3745e48...08d59ae'
    javascript

An array of supported digest functions can be retrieved using `crypto.getHashes()`.

#### `crypto.privateDecrypt(privateKey, buffer)`#

Added in: v0.11.14
History

Version | Changes
--- | ---
v21.6.2, v20.11.1, v18.19.1 | The `RSA_PKCS1_PADDING` padding was disabled unless the OpenSSL build supports implicit rejection.
v15.0.0 | Added string, ArrayBuffer, and CryptoKey as allowable key types. The oaepLabel can be an ArrayBuffer. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.
v12.11.0 | The `oaepLabel` option was added.
v12.9.0 | The `oaepHash` option was added.
v11.6.0 | This function now supports key objects.

- `privateKey` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>` | `<URL>`
  - `oaepHash` `<string>` The hash function to use for OAEP padding and MGF1. Default: `'sha1'`
  - `oaepLabel` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The label to use for OAEP padding. If not specified, no label is used.
  - `padding` `<crypto.constants>` An optional padding value defined in `crypto.constants`, which may be: `crypto.constants.RSA_NO_PADDING`, `crypto.constants.RSA_PKCS1_PADDING`, or `crypto.constants.RSA_PKCS1_OAEP_PADDING`.
- `buffer` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Buffer>` A new `Buffer` with the decrypted content.

Decrypts `buffer` with `privateKey`. `buffer` was previously encrypted using the corresponding public key, for example using `crypto.publicEncrypt()`.

If `privateKey` is not a `KeyObject`, this function behaves as if `privateKey` had been passed to `crypto.createPrivateKey()`. If it is an object, the `padding` property can be passed. Otherwise, this function uses `RSA_PKCS1_OAEP_PADDING`.

Using `crypto.constants.RSA_PKCS1_PADDING` in `crypto.privateDecrypt()` requires OpenSSL to support implicit rejection (`rsa_pkcs1_implicit_rejection`). If the version of OpenSSL used by Node.js does not support this feature, attempting to use `RSA_PKCS1_PADDING` will fail.

#### `crypto.privateEncrypt(privateKey, buffer)`#

Added in: v1.1.0
History

Version | Changes
--- | ---
v15.0.0 | Added string, ArrayBuffer, and CryptoKey as allowable key types. The passphrase can be an ArrayBuffer. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.
v11.6.0 | This function now supports key objects.

- `privateKey` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>` | `<URL>`
  - `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>` | `<URL>` The private key material, a `<KeyObject>`, or a `<URL>` referencing an object for an OpenSSL STORE loader.
  - `passphrase` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` An optional passphrase for the private key.
  - `padding` `<crypto.constants>` An optional padding value defined in `crypto.constants`, which may be: `crypto.constants.RSA_NO_PADDING` or `crypto.constants.RSA_PKCS1_PADDING`.
  - `encoding` `<string>` The string encoding to use when `buffer`, `key`, or `passphrase` are strings.
- `buffer` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Buffer>` A new `Buffer` with the encrypted content.

Encrypts `buffer` with `privateKey`. The returned data can be decrypted using the corresponding public key, for example using `crypto.publicDecrypt()`.

If `privateKey` is not a `KeyObject`, this function behaves as if `privateKey` had been passed to `crypto.createPrivateKey()`. If it is an object, the `padding` property can be passed. Otherwise, this function uses `RSA_PKCS1_PADDING`.

#### `crypto.publicDecrypt(key, buffer)`#

Added in: v1.1.0
History

Version | Changes
--- | ---
v15.0.0 | Added string, ArrayBuffer, and CryptoKey as allowable key types. The passphrase can be an ArrayBuffer. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.
v11.6.0 | This function now supports key objects.

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
  - `passphrase` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` An optional passphrase for the private key.
  - `padding` `<crypto.constants>` An optional padding value defined in `crypto.constants`, which may be: `crypto.constants.RSA_NO_PADDING` or `crypto.constants.RSA_PKCS1_PADDING`.
  - `encoding` `<string>` The string encoding to use when `buffer`, `key`, or `passphrase` are strings.
- `buffer` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Buffer>` A new `Buffer` with the decrypted content.

Decrypts `buffer` with `key`. `buffer` was previously encrypted using the corresponding private key, for example using `crypto.privateEncrypt()`.

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPublicKey()`. If it is an object, the `padding` property can be passed. Otherwise, this function uses `RSA_PKCS1_PADDING`.

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

#### `crypto.publicEncrypt(key, buffer)`#

Added in: v0.11.14
History

Version | Changes
--- | ---
v15.0.0 | Added string, ArrayBuffer, and CryptoKey as allowable key types. The oaepLabel and passphrase can be ArrayBuffers. The buffer can be a string or ArrayBuffer. All types that accept buffers are limited to a maximum of 2 ** 31 - 1 bytes.
v12.11.0 | The `oaepLabel` option was added.
v12.9.0 | The `oaepHash` option was added.
v11.6.0 | This function now supports key objects.

- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
  - `key` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>` A PEM encoded public or private key, `<KeyObject>`, or `<CryptoKey>`.
  - `oaepHash` `<string>` The hash function to use for OAEP padding and MGF1. Default: `'sha1'`
  - `oaepLabel` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The label to use for OAEP padding. If not specified, no label is used.
  - `passphrase` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` An optional passphrase for the private key.
  - `padding` `<crypto.constants>` An optional padding value defined in `crypto.constants`, which may be: `crypto.constants.RSA_NO_PADDING`, `crypto.constants.RSA_PKCS1_PADDING`, or `crypto.constants.RSA_PKCS1_OAEP_PADDING`.
  - `encoding` `<string>` The string encoding to use when `buffer`, `key`, `oaepLabel`, or `passphrase` are strings.
- `buffer` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<Buffer>` A new `Buffer` with the encrypted content.

Encrypts the content of `buffer` with `key` and returns a new `Buffer` with encrypted content. The returned data can be decrypted using the corresponding private key, for example using `crypto.privateDecrypt()`.

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPublicKey()`. If it is an object, the `padding` property can be passed. Otherwise, this function uses `RSA_PKCS1_OAEP_PADDING`.

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

#### `crypto.randomBytes(size[, callback])`#

Added in: v0.5.8
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v9.0.0 | Passing `null` as the `callback` argument now throws `ERR_INVALID_CALLBACK`.

- `size` `<number>` The number of bytes to generate. The `size` must not be larger than `2**31 - 1`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `buf` `<Buffer>`
- Returns: `<Buffer>` if the `callback` function is not provided.

Generates cryptographically strong pseudorandom data. The `size` argument is a number indicating the number of bytes to generate.

If a `callback` function is provided, the bytes are generated asynchronously and the `callback` function is invoked with two arguments: `err` and `buf`. If an error occurs, `err` will be an `Error` object; otherwise it is `null`. The `buf` argument is a `Buffer` containing the generated bytes.

    // Asynchronous
    const {
      randomBytes,
    } = await import('node:crypto');

    randomBytes(256, (err, buf) => {
      if (err) throw err;
      console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
    });
    // Asynchronous
    const {
      randomBytes,
    } = require('node:crypto');

    randomBytes(256, (err, buf) => {
      if (err) throw err;
      console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
    });
    javascript

If the `callback` function is not provided, the random bytes are generated synchronously and returned as a `Buffer`. An error will be thrown if there is a problem generating the bytes.

    // Synchronous
    const {
      randomBytes,
    } = await import('node:crypto');

    const buf = randomBytes(256);
    console.log(
      `${buf.length} bytes of random data: ${buf.toString('hex')}`);
    // Synchronous
    const {
      randomBytes,
    } = require('node:crypto');

    const buf = randomBytes(256);
    console.log(
      `${buf.length} bytes of random data: ${buf.toString('hex')}`);
    javascript

The `crypto.randomBytes()` method will not complete until there is sufficient entropy available. This should normally never take longer than a few milliseconds. The only time when generating the random bytes may conceivably block for a longer period of time is right after boot, when the whole system is still low on entropy.

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the `UV_THREADPOOL_SIZE` documentation for more information.

The asynchronous version of `crypto.randomBytes()` is carried out in a single threadpool request. To minimize threadpool task length variation, partition large `randomBytes` requests when doing so as part of fulfilling a client request.

#### `crypto.randomFill(buffer[, offset][, size], callback)`#

Added in: v7.10.0, v6.13.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v9.0.0 | The `buffer` argument may be any `TypedArray` or `DataView`.

- `buffer` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Must be supplied. The size of the provided `buffer` must not be larger than `2**31 - 1`.
- `offset` `<number>` Default: `0`
- `size` `<number>` Default: `buffer.length - offset`. The `size` must not be larger than `2**31 - 1`.
- `callback` `<Function>` `function(err, buf) {}`.

This function is similar to `crypto.randomBytes()` but requires the first argument to be a `Buffer` that will be filled. It also requires that a callback is passed in.

If the `callback` function is not provided, an error will be thrown.

    import { Buffer } from 'node:buffer';
    const { randomFill } = await import('node:crypto');

    const buf = Buffer.alloc(10);
    randomFill(buf, (err, buf) => {
      if (err) throw err;
      console.log(buf.toString('hex'));
    });

    randomFill(buf, 5, (err, buf) => {
      if (err) throw err;
      console.log(buf.toString('hex'));
    });

    // The above is equivalent to the following:
    randomFill(buf, 5, 5, (err, buf) => {
      if (err) throw err;
      console.log(buf.toString('hex'));
    });
    const { randomFill } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const buf = Buffer.alloc(10);
    randomFill(buf, (err, buf) => {
      if (err) throw err;
      console.log(buf.toString('hex'));
    });

    randomFill(buf, 5, (err, buf) => {
      if (err) throw err;
      console.log(buf.toString('hex'));
    });

    // The above is equivalent to the following:
    randomFill(buf, 5, 5, (err, buf) => {
      if (err) throw err;
      console.log(buf.toString('hex'));
    });
    javascript

Any `ArrayBuffer`, `TypedArray`, or `DataView` instance may be passed as `buffer`.

While this includes instances of `Float32Array` and `Float64Array`, this function should not be used to generate random floating-point numbers. The result may contain `+Infinity`, `-Infinity`, and `NaN`, and even if the array contains finite numbers only, they are not drawn from a uniform random distribution and have no meaningful lower or upper bounds.

    import { Buffer } from 'node:buffer';
    const { randomFill } = await import('node:crypto');

    const a = new Uint32Array(10);
    randomFill(a, (err, buf) => {
      if (err) throw err;
      console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
        .toString('hex'));
    });

    const b = new DataView(new ArrayBuffer(10));
    randomFill(b, (err, buf) => {
      if (err) throw err;
      console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
        .toString('hex'));
    });

    const c = new ArrayBuffer(10);
    randomFill(c, (err, buf) => {
      if (err) throw err;
      console.log(Buffer.from(buf).toString('hex'));
    });
    const { randomFill } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const a = new Uint32Array(10);
    randomFill(a, (err, buf) => {
      if (err) throw err;
      console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
        .toString('hex'));
    });

    const b = new DataView(new ArrayBuffer(10));
    randomFill(b, (err, buf) => {
      if (err) throw err;
      console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
        .toString('hex'));
    });

    const c = new ArrayBuffer(10);
    randomFill(c, (err, buf) => {
      if (err) throw err;
      console.log(Buffer.from(buf).toString('hex'));
    });
    javascript

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the `UV_THREADPOOL_SIZE` documentation for more information.

The asynchronous version of `crypto.randomFill()` is carried out in a single threadpool request. To minimize threadpool task length variation, partition large `randomFill` requests when doing so as part of fulfilling a client request.

#### `crypto.randomFillSync(buffer[, offset][, size])`#

Added in: v7.10.0, v6.13.0
History

Version | Changes
--- | ---
v9.0.0 | The `buffer` argument may be any `TypedArray` or `DataView`.

- `buffer` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` Must be supplied. The size of the provided `buffer` must not be larger than `2**31 - 1`.
- `offset` `<number>` Default: `0`
- `size` `<number>` Default: `buffer.length - offset`. The `size` must not be larger than `2**31 - 1`.
- Returns: `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` The object passed as `buffer` argument.

Synchronous version of `crypto.randomFill()`.

    import { Buffer } from 'node:buffer';
    const { randomFillSync } = await import('node:crypto');

    const buf = Buffer.alloc(10);
    console.log(randomFillSync(buf).toString('hex'));

    randomFillSync(buf, 5);
    console.log(buf.toString('hex'));

    // The above is equivalent to the following:
    randomFillSync(buf, 5, 5);
    console.log(buf.toString('hex'));
    const { randomFillSync } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const buf = Buffer.alloc(10);
    console.log(randomFillSync(buf).toString('hex'));

    randomFillSync(buf, 5);
    console.log(buf.toString('hex'));

    // The above is equivalent to the following:
    randomFillSync(buf, 5, 5);
    console.log(buf.toString('hex'));
    javascript

Any `ArrayBuffer`, `TypedArray` or `DataView` instance may be passed as `buffer`.

    import { Buffer } from 'node:buffer';
    const { randomFillSync } = await import('node:crypto');

    const a = new Uint32Array(10);
    console.log(Buffer.from(randomFillSync(a).buffer,
                            a.byteOffset, a.byteLength).toString('hex'));

    const b = new DataView(new ArrayBuffer(10));
    console.log(Buffer.from(randomFillSync(b).buffer,
                            b.byteOffset, b.byteLength).toString('hex'));

    const c = new ArrayBuffer(10);
    console.log(Buffer.from(randomFillSync(c)).toString('hex'));
    const { randomFillSync } = require('node:crypto');
    const { Buffer } = require('node:buffer');

    const a = new Uint32Array(10);
    console.log(Buffer.from(randomFillSync(a).buffer,
                            a.byteOffset, a.byteLength).toString('hex'));

    const b = new DataView(new ArrayBuffer(10));
    console.log(Buffer.from(randomFillSync(b).buffer,
                            b.byteOffset, b.byteLength).toString('hex'));

    const c = new ArrayBuffer(10);
    console.log(Buffer.from(randomFillSync(c)).toString('hex'));
    javascript

#### `crypto.randomInt([min, ]max[, callback])`#

Added in: v14.10.0, v12.19.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `min` `<integer>` Start of random range (inclusive). Default: `0`.
- `max` `<integer>` End of random range (exclusive).
- `callback` `<Function>` `function(err, n) {}`.

Return a random integer `n` such that `min <= n < max`. This implementation avoids modulo bias.

The range (`max - min`) must be less than 248. `min` and `max` must be safe integers.

If the `callback` function is not provided, the random integer is generated synchronously.

    // Asynchronous
    const {
      randomInt,
    } = await import('node:crypto');

    randomInt(3, (err, n) => {
      if (err) throw err;
      console.log(`Random number chosen from (0, 1, 2): ${n}`);
    });
    // Asynchronous
    const {
      randomInt,
    } = require('node:crypto');

    randomInt(3, (err, n) => {
      if (err) throw err;
      console.log(`Random number chosen from (0, 1, 2): ${n}`);
    });
    javascript

    // Synchronous
    const {
      randomInt,
    } = await import('node:crypto');

    const n = randomInt(3);
    console.log(`Random number chosen from (0, 1, 2): ${n}`);
    // Synchronous
    const {
      randomInt,
    } = require('node:crypto');

    const n = randomInt(3);
    console.log(`Random number chosen from (0, 1, 2): ${n}`);
    javascript

    // With `min` argument
    const {
      randomInt,
    } = await import('node:crypto');

    const n = randomInt(1, 7);
    console.log(`The dice rolled: ${n}`);
    // With `min` argument
    const {
      randomInt,
    } = require('node:crypto');

    const n = randomInt(1, 7);
    console.log(`The dice rolled: ${n}`);
    javascript

#### `crypto.randomUUID([options])`#

Added in: v15.6.0, v14.17.0

- `options` `<Object>`
  - `disableEntropyCache` `<boolean>` By default, to improve performance, Node.js generates and caches enough random data to generate up to 128 random UUIDs. To generate a UUID without using the cache, set `disableEntropyCache` to `true`. Default: `false`.
- Returns: `<string>`

Generates a random RFC 4122 version 4 UUID. The UUID is generated using a cryptographic pseudorandom number generator.

#### `crypto.randomUUIDv7([options])`#

Added in: v26.1.0

- `options` `<Object>`
  - `disableEntropyCache` `<boolean>` By default, to improve performance, Node.js generates and caches enough random data to generate up to 128 random UUIDs. To generate a UUID without using the cache, set `disableEntropyCache` to `true`. Default: `false`.
- Returns: `<string>`

Generates a random RFC 9562 version 7 UUID. The UUID contains a millisecond precision Unix timestamp in the most significant 48 bits, followed by cryptographically secure random bits for the remaining fields, making it suitable for use as a database key with time-based sorting. The embedded timestamp relies on a non-monotonic clock and is not guaranteed to be strictly increasing.

#### `crypto.scrypt(password, salt, keylen[, options], callback)`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v15.0.0 | The password and salt arguments can also be ArrayBuffer instances.
v12.8.0, v10.17.0 | The `maxmem` value can now be any safe integer.
v10.9.0 | The `cost`, `blockSize` and `parallelization` option names have been added.

- `password` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `salt` `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `keylen` `<number>`
- `options` `<Object>`
  - `cost` `<number>` CPU/memory cost parameter. Must be a power of two greater than one. Default: `16384`.
  - `blockSize` `<number>` Block size parameter. Default: `8`.
  - `parallelization` `<number>` Parallelization parameter. Default: `1`.
  - `N` `<number>` Alias for `cost`. Only one of both may be specified.
  - `r` `<number>` Alias for `blockSize`. Only one of both may be specified.
  - `p` `<number>` Alias for `parallelization`. Only one of both may be specified.
  - `maxmem` `<number>` Memory upper bound. It is an error when (approximately) `128 * N * r > maxmem`. Default: `32 * 1024 * 1024`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `derivedKey` `<Buffer>`

Provides an asynchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The `salt` should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for `password` or `salt`, please consider caveats when using strings as inputs to cryptographic APIs.

The `callback` function is called with two arguments: `err` and `derivedKey`. `err` is an exception object when key derivation fails, otherwise `err` is `null`. `derivedKey` is passed to the callback as a `Buffer`.

An exception is thrown when any of the input arguments specify invalid values or types.

    const {
      scrypt,
    } = await import('node:crypto');

    // Using the factory defaults.
    scrypt('password', 'salt', 64, (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
    });
    // Using a custom N parameter. Must be a power of two.
    scrypt('password', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // '3745e48...aa39b34'
    });
    const {
      scrypt,
    } = require('node:crypto');

    // Using the factory defaults.
    scrypt('password', 'salt', 64, (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
    });
    // Using a custom N parameter. Must be a power of two.
    scrypt('password', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
      if (err) throw err;
      console.log(derivedKey.toString('hex'));  // '3745e48...aa39b34'
    });
    javascript

#### `crypto.scryptSync(password, salt, keylen[, options])`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v12.8.0, v10.17.0 | The `maxmem` value can now be any safe integer.
v10.9.0 | The `cost`, `blockSize` and `parallelization` option names have been added.

- `password` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `salt` `<string>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `keylen` `<number>`
- `options` `<Object>`
  - `cost` `<number>` CPU/memory cost parameter. Must be a power of two greater than one. Default: `16384`.
  - `blockSize` `<number>` Block size parameter. Default: `8`.
  - `parallelization` `<number>` Parallelization parameter. Default: `1`.
  - `N` `<number>` Alias for `cost`. Only one of both may be specified.
  - `r` `<number>` Alias for `blockSize`. Only one of both may be specified.
  - `p` `<number>` Alias for `parallelization`. Only one of both may be specified.
  - `maxmem` `<number>` Memory upper bound. It is an error when (approximately) `128 * N * r > maxmem`. Default: `32 * 1024 * 1024`.
- Returns: `<Buffer>`

Provides a synchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The `salt` should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

When passing strings for `password` or `salt`, please consider caveats when using strings as inputs to cryptographic APIs.

An exception is thrown when key derivation fails, otherwise the derived key is returned as a `Buffer`.

An exception is thrown when any of the input arguments specify invalid values or types.

    const {
      scryptSync,
    } = await import('node:crypto');
    // Using the factory defaults.

    const key1 = scryptSync('password', 'salt', 64);
    console.log(key1.toString('hex'));  // '3745e48...08d59ae'
    // Using a custom N parameter. Must be a power of two.
    const key2 = scryptSync('password', 'salt', 64, { N: 1024 });
    console.log(key2.toString('hex'));  // '3745e48...aa39b34'
    const {
      scryptSync,
    } = require('node:crypto');
    // Using the factory defaults.

    const key1 = scryptSync('password', 'salt', 64);
    console.log(key1.toString('hex'));  // '3745e48...08d59ae'
    // Using a custom N parameter. Must be a power of two.
    const key2 = scryptSync('password', 'salt', 64, { N: 1024 });
    console.log(key2.toString('hex'));  // '3745e48...aa39b34'
    javascript

#### `crypto.secureHeapUsed()`#

Added in: v15.6.0

- Returns: `<Object>`
  - `total` `<number>` The total allocated secure heap size as specified using the `--secure-heap=n` command-line flag.
  - `min` `<number>` The minimum allocation from the secure heap as specified using the `--secure-heap-min` command-line flag.
  - `used` `<number>` The total number of bytes currently allocated from the secure heap.
  - `utilization` `<number>` The calculated ratio of `used` to `total` allocated bytes.

#### `crypto.setEngine(engine[, flags])`#

Added in: v0.11.11
History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | Custom engine support in OpenSSL 3 is deprecated.

- `engine` `<string>`
- `flags` `<crypto.constants>` Default: `crypto.constants.ENGINE_METHOD_ALL`

Load and set the `engine` for some or all OpenSSL functions (selected by flags). Support for custom engines in OpenSSL is deprecated from OpenSSL 3.

`engine` could be either an id or a path to the engine's shared library.

The optional `flags` argument uses `ENGINE_METHOD_ALL` by default. The `flags` is a bit field taking one of or a mix of the following flags (defined in `crypto.constants`):

- `crypto.constants.ENGINE_METHOD_RSA`
- `crypto.constants.ENGINE_METHOD_DSA`
- `crypto.constants.ENGINE_METHOD_DH`
- `crypto.constants.ENGINE_METHOD_RAND`
- `crypto.constants.ENGINE_METHOD_EC`
- `crypto.constants.ENGINE_METHOD_CIPHERS`
- `crypto.constants.ENGINE_METHOD_DIGESTS`
- `crypto.constants.ENGINE_METHOD_PKEY_METHS`
- `crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS`
- `crypto.constants.ENGINE_METHOD_ALL`
- `crypto.constants.ENGINE_METHOD_NONE`

#### `crypto.setFips(bool)`#

Added in: v10.0.0

- `bool` `<boolean>` `true` to enable FIPS mode.

Enables the FIPS compliant crypto provider in a FIPS-enabled Node.js build. Throws an error if FIPS mode is not available.

#### `crypto.sign(algorithm, data, key[, callback])`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v26.0.0 | Add support for Ed25519 context parameter.
v24.8.0 | Add support for ML-DSA, Ed448, and SLH-DSA context parameter.
v24.8.0 | Add support for SLH-DSA signing.
v24.6.0 | Add support for ML-DSA signing.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v15.12.0 | Optional callback argument added.
v13.2.0, v12.16.0 | This function now supports IEEE-P1363 DSA and ECDSA signatures.

- `algorithm` `<string>` | `<null>` | `<undefined>`
- `data` `<ArrayBuffer>` | `<Buffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<string>`
- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>` | `<URL>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `signature` `<Buffer>`
- Returns: `<Buffer>` if the `callback` function is not provided.

Calculates and returns the signature for `data` using the given private key and algorithm. If `algorithm` is `null` or `undefined`, then the algorithm is dependent upon the key type.

`algorithm` is required to be `null` or `undefined` for Ed25519, Ed448, and ML-DSA.

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPrivateKey()`. When `key` is a string, `ArrayBuffer`, `Buffer`, `TypedArray`, or `DataView`, it must contain PEM-encoded key material. If it is an object, the following additional properties can be passed:

- `dsaEncoding` `<string>` For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

  - `'der'` (default): DER-encoded ASN.1 signature structure encoding `(r, s)`.
  - `'ieee-p1363'`: Signature format `r || s` as proposed in IEEE-P1363.

- `padding` `<integer>` Optional padding value for RSA, one of the following:

  - `crypto.constants.RSA_PKCS1_PADDING` (default)
  - `crypto.constants.RSA_PKCS1_PSS_PADDING`

  `RSA_PKCS1_PSS_PADDING` will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

- `saltLength` `<integer>` Salt length for when padding is `RSA_PKCS1_PSS_PADDING`. The special value `crypto.constants.RSA_PSS_SALTLEN_DIGEST` sets the salt length to the digest size, `crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN` (default) sets it to the maximum permissible value.

- `context` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` For Ed255193 (using Ed25519ctx from RFC 8032), Ed448, ML-DSA, and SLH-DSA, this option specifies the optional context to differentiate signatures generated for different purposes with the same key.

If the `callback` function is provided this function uses libuv's threadpool.

#### `crypto.subtle`#

Added in: v17.4.0

- Type: `<SubtleCrypto>`

A convenient alias for `crypto.webcrypto.subtle`.

#### `crypto.timingSafeEqual(a, b)`#

Added in: v6.6.0
History

Version | Changes
--- | ---
v15.0.0 | The a and b arguments can also be ArrayBuffer.

- `a` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- `b` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>`
- Returns: `<boolean>`

This function compares the underlying bytes that represent the given `ArrayBuffer`, `TypedArray`, or `DataView` instances using a constant-time algorithm.

This function does not leak timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls.

`a` and `b` must both be `Buffer`s, `TypedArray`s, or `DataView`s, and they must have the same byte length. An error is thrown if `a` and `b` have different byte lengths.

If at least one of `a` and `b` is a `TypedArray` with more than one byte per entry, such as `Uint16Array`, the result will be computed using the platform byte order.

When both of the inputs are `Float32Array`s or `Float64Array`s, this function might return unexpected results due to IEEE 754 encoding of floating-point numbers. In particular, neither `x === y` nor `Object.is(x, y)` implies that the byte representations of two floating-point numbers `x` and `y` are equal.

Use of `crypto.timingSafeEqual` does not guarantee that the surrounding code is timing-safe. Care should be taken to ensure that the surrounding code does not introduce timing vulnerabilities.

#### `crypto.verify(algorithm, data, key, signature[, callback])`#

Added in: v12.0.0
History

Version | Changes
--- | ---
v26.0.0 | Add support for Ed25519 context parameter.
v24.8.0 | Add support for ML-DSA, Ed448, and SLH-DSA context parameter.
v24.8.0 | Add support for SLH-DSA signature verification.
v24.6.0 | Add support for ML-DSA signature verification.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v15.12.0 | Optional callback argument added.
v15.0.0 | The data, key, and signature arguments can also be ArrayBuffer.
v13.2.0, v12.16.0 | This function now supports IEEE-P1363 DSA and ECDSA signatures.

- `algorithm` `<string>` | `<null>` | `<undefined>`
- `data` `<ArrayBuffer>` | `<Buffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<DataView>` | `<string>`
- `key` `<Object>` | `<string>` | `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` | `<KeyObject>` | `<CryptoKey>`
- `signature` `<ArrayBuffer>` | `<Buffer>` | `<SharedArrayBuffer>` | `<TypedArray>` | `<DataView>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `result` `<boolean>`
- Returns: `<boolean>` `true` or `false` depending on the validity of the signature for the data and public key if the `callback` function is not provided.

Verifies the given signature for `data` using the given key and algorithm. If `algorithm` is `null` or `undefined`, then the algorithm is dependent upon the key type.

`algorithm` is required to be `null` or `undefined` for Ed25519, Ed448, and ML-DSA.

If `key` is not a `KeyObject`, this function behaves as if `key` had been passed to `crypto.createPublicKey()`. When `key` is a string, `ArrayBuffer`, `Buffer`, `TypedArray`, or `DataView`, it must contain PEM-encoded key material. If it is an object, the following additional properties can be passed:

- `dsaEncoding` `<string>` For DSA and ECDSA, this option specifies the format of the signature. It can be one of the following:

  - `'der'` (default): DER-encoded ASN.1 signature structure encoding `(r, s)`.
  - `'ieee-p1363'`: Signature format `r || s` as proposed in IEEE-P1363.

- `padding` `<integer>` Optional padding value for RSA, one of the following:

  - `crypto.constants.RSA_PKCS1_PADDING` (default)
  - `crypto.constants.RSA_PKCS1_PSS_PADDING`

  `RSA_PKCS1_PSS_PADDING` will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

- `saltLength` `<integer>` Salt length for when padding is `RSA_PKCS1_PSS_PADDING`. The special value `crypto.constants.RSA_PSS_SALTLEN_DIGEST` sets the salt length to the digest size, `crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN` (default) sets it to the maximum permissible value.

- `context` `<ArrayBuffer>` | `<Buffer>` | `<TypedArray>` | `<DataView>` For Ed255193 (using Ed25519ctx from RFC 8032), Ed448, ML-DSA, and SLH-DSA, this option specifies the optional context to differentiate signatures generated for different purposes with the same key.

The `signature` argument is the previously calculated signature for the `data`.

Because public keys can be derived from private keys, a private key or a public key may be passed for `key`.

If the `callback` function is provided this function uses libuv's threadpool.

#### `crypto.webcrypto`#

Added in: v15.0.0

Type: `<Crypto>` An implementation of the Web Crypto API standard.

See the Web Crypto API documentation for details.

### Notes#

#### Using strings as inputs to cryptographic APIs#

For historical reasons, many cryptographic APIs provided by Node.js accept strings as inputs where the underlying cryptographic algorithm works on byte sequences. These instances include plaintexts, ciphertexts, symmetric keys, initialization vectors, passphrases, salts, authentication tags, and additional authenticated data.

When passing strings to cryptographic APIs, consider the following factors.

- Not all byte sequences are valid UTF-8 strings. Therefore, when a byte sequence of length `n` is derived from a string, its entropy is generally lower than the entropy of a random or pseudorandom `n` byte sequence. For example, no UTF-8 string will result in the byte sequence `c0 af`. Secret keys should almost exclusively be random or pseudorandom byte sequences.

- Similarly, when converting random or pseudorandom byte sequences to UTF-8 strings, subsequences that do not represent valid code points may be replaced by the Unicode replacement character (`U+FFFD`). The byte representation of the resulting Unicode string may, therefore, not be equal to the byte sequence that the string was created from.

      const original = [0xc0, 0xaf];
      const bytesAsString = Buffer.from(original).toString('utf8');
      const stringAsBytes = Buffer.from(bytesAsString, 'utf8');
      console.log(stringAsBytes);
      // Prints '<Buffer ef bf bd ef bf bd>'.
      js

  The outputs of ciphers, hash functions, signature algorithms, and key derivation functions are pseudorandom byte sequences and should not be used as Unicode strings.

- When strings are obtained from user input, some Unicode characters can be represented in multiple equivalent ways that result in different byte sequences. For example, when passing a user passphrase to a key derivation function, such as PBKDF2 or scrypt, the result of the key derivation function depends on whether the string uses composed or decomposed characters. Node.js does not normalize character representations. Developers should consider using `String.prototype.normalize()` on user inputs before passing them to cryptographic APIs.

#### Legacy streams API (prior to Node.js 0.10)#

The Crypto module was added to Node.js before there was the concept of a unified Stream API, and before there were `Buffer` objects for handling binary data. As such, many `crypto` classes have methods not typically found on other Node.js classes that implement the streams API (e.g. `update()`, `final()`, or `digest()`). Also, many methods accepted and returned `'latin1'` encoded strings by default rather than `Buffer`s. This default was changed in Node.js 0.9.3 to use `Buffer` objects by default instead.

#### Support for weak or compromised algorithms#

The `node:crypto` module still supports some algorithms which are already compromised and are not recommended for use. The API also allows the use of ciphers and hashes with a small key size that are too weak for safe use.

Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.

Based on the recommendations of NIST SP 800-131A:

- MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
- The key used with RSA, DSA, and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
- The DH groups of `modp1`, `modp2` and `modp5` have a key size smaller than 2048 bits and are not recommended.

See the reference for other recommendations and details.

Some algorithms that have known weaknesses and are of little relevance in practice are only available through the legacy provider, which is not enabled by default.

#### CCM mode#

CCM is one of the supported AEAD algorithms. Applications which use this mode must adhere to certain restrictions when using the cipher API:

- The authentication tag length must be specified during cipher creation by setting the `authTagLength` option and must be one of 4, 6, 8, 10, 12, 14 or 16 bytes.
- The length of the initialization vector (nonce) `N` must be between 7 and 13 bytes (`7 ≤ N ≤ 13`).
- The length of the plaintext is limited to `2 ** (8 * (15 - N))` bytes.
- When decrypting, the authentication tag must be set via `setAuthTag()` before calling `update()`. Otherwise, decryption will fail and `final()` will throw an error in compliance with section 2.6 of RFC 3610.
- Using stream methods such as `write(data)`, `end(data)` or `pipe()` in CCM mode might fail as CCM cannot handle more than one chunk of data per instance.
- When passing additional authenticated data (AAD), the length of the actual message in bytes must be passed to `setAAD()` via the `plaintextLength` option. Many crypto libraries include the authentication tag in the ciphertext, which means that they produce ciphertexts of the length `plaintextLength + authTagLength`. Node.js does not include the authentication tag, so the ciphertext length is always `plaintextLength`. This is not necessary if no AAD is used.
- As CCM processes the whole message at once, `update()` must be called exactly once.
- Even though calling `update()` is sufficient to encrypt/decrypt the message, applications must call `final()` to compute or verify the authentication tag.

    import { Buffer } from 'node:buffer';
    const {
      createCipheriv,
      createDecipheriv,
      randomBytes,
    } = await import('node:crypto');

    const key = 'keykeykeykeykeykeykeykey';
    const nonce = randomBytes(12);

    const aad = Buffer.from('0123456789', 'hex');

    const cipher = createCipheriv('aes-192-ccm', key, nonce, {
      authTagLength: 16,
    });
    const plaintext = 'Hello world';
    cipher.setAAD(aad, {
      plaintextLength: Buffer.byteLength(plaintext),
    });
    const ciphertext = cipher.update(plaintext, 'utf8');
    cipher.final();
    const tag = cipher.getAuthTag();

    // Now transmit { ciphertext, nonce, tag }.

    const decipher = createDecipheriv('aes-192-ccm', key, nonce, {
      authTagLength: 16,
    });
    decipher.setAuthTag(tag);
    decipher.setAAD(aad, {
      plaintextLength: ciphertext.length,
    });
    const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');

    try {
      decipher.final();
    } catch (err) {
      throw new Error('Authentication failed!', { cause: err });
    }

    console.log(receivedPlaintext);
    const { Buffer } = require('node:buffer');
    const {
      createCipheriv,
      createDecipheriv,
      randomBytes,
    } = require('node:crypto');

    const key = 'keykeykeykeykeykeykeykey';
    const nonce = randomBytes(12);

    const aad = Buffer.from('0123456789', 'hex');

    const cipher = createCipheriv('aes-192-ccm', key, nonce, {
      authTagLength: 16,
    });
    const plaintext = 'Hello world';
    cipher.setAAD(aad, {
      plaintextLength: Buffer.byteLength(plaintext),
    });
    const ciphertext = cipher.update(plaintext, 'utf8');
    cipher.final();
    const tag = cipher.getAuthTag();

    // Now transmit { ciphertext, nonce, tag }.

    const decipher = createDecipheriv('aes-192-ccm', key, nonce, {
      authTagLength: 16,
    });
    decipher.setAuthTag(tag);
    decipher.setAAD(aad, {
      plaintextLength: ciphertext.length,
    });
    const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');

    try {
      decipher.final();
    } catch (err) {
      throw new Error('Authentication failed!', { cause: err });
    }

    console.log(receivedPlaintext);
    javascript

#### FIPS mode#

When using OpenSSL 3, Node.js supports FIPS 140-2 when used with an appropriate OpenSSL 3 provider, such as the FIPS provider from OpenSSL 3 which can be installed by following the instructions in OpenSSL's FIPS README file.

For FIPS support in Node.js you will need:

- A correctly installed OpenSSL 3 FIPS provider.
- An OpenSSL 3 FIPS module configuration file.
- An OpenSSL 3 configuration file that references the FIPS module configuration file.

Node.js will need to be configured with an OpenSSL configuration file that points to the FIPS provider. An example configuration file looks like this:

    nodejs_conf = nodejs_init

    .include /<absolute path>/fipsmodule.cnf

    [nodejs_init]
    providers = provider_sect

    [provider_sect]
    default = default_sect
    # The fips section name should match the section name inside the
    # included fipsmodule.cnf.
    fips = fips_sect

    [default_sect]
    activate = 1
    text

where `fipsmodule.cnf` is the FIPS module configuration file generated from the FIPS provider installation step:

    openssl fipsinstall
    bash

Set the `OPENSSL_CONF` environment variable to point to your configuration file and `OPENSSL_MODULES` to the location of the FIPS provider dynamic library. e.g.

    export OPENSSL_CONF=/<path to configuration file>/nodejs.cnf
    export OPENSSL_MODULES=/<path to openssl lib>/ossl-modules
    bash

FIPS mode can then be enabled in Node.js either by:

- Starting Node.js with `--enable-fips` or `--force-fips` command line flags.
- Programmatically calling `crypto.setFips(true)`.

Optionally FIPS mode can be enabled in Node.js via the OpenSSL configuration file. e.g.

    nodejs_conf = nodejs_init

    .include /<absolute path>/fipsmodule.cnf

    [nodejs_init]
    providers = provider_sect
    alg_section = algorithm_sect

    [provider_sect]
    default = default_sect
    # The fips section name should match the section name inside the
    # included fipsmodule.cnf.
    fips = fips_sect

    [default_sect]
    activate = 1

    [algorithm_sect]
    default_properties = fips=yes
    text

### Crypto constants#

The following constants exported by `crypto.constants` apply to various uses of the `node:crypto`, `node:tls`, and `node:https` modules and are generally specific to OpenSSL.

#### OpenSSL options#

See the list of SSL OP Flags for details.

Constant | Description
--- | ---
`SSL_OP_ALL` | Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_options.html for detail.
`SSL_OP_ALLOW_NO_DHE_KEX` | Instructs OpenSSL to allow a non-[EC]DHE-based key exchange mode for TLS v1.3
`SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION` | Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_options.html.
`SSL_OP_CIPHER_SERVER_PREFERENCE` | Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_options.html.
`SSL_OP_CISCO_ANYCONNECT` | Instructs OpenSSL to use Cisco's version identifier of DTLS_BAD_VER.
`SSL_OP_COOKIE_EXCHANGE` | Instructs OpenSSL to turn on cookie exchange.
`SSL_OP_CRYPTOPRO_TLSEXT_BUG` | Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft.
`SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS` | Instructs OpenSSL to disable an SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d.
`SSL_OP_LEGACY_SERVER_CONNECT` | Allows initial connection to servers that do not support RI.
`SSL_OP_NO_COMPRESSION` | Instructs OpenSSL to disable support for SSL/TLS compression.
`SSL_OP_NO_ENCRYPT_THEN_MAC` | Instructs OpenSSL to disable encrypt-then-MAC.
`SSL_OP_NO_QUERY_MTU` |
`SSL_OP_NO_RENEGOTIATION` | Instructs OpenSSL to disable renegotiation.
`SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION` | Instructs OpenSSL to always start a new session when performing renegotiation.
`SSL_OP_NO_SSLv2` | Instructs OpenSSL to turn off SSL v2
`SSL_OP_NO_SSLv3` | Instructs OpenSSL to turn off SSL v3
`SSL_OP_NO_TICKET` | Instructs OpenSSL to disable use of RFC4507bis tickets.
`SSL_OP_NO_TLSv1` | Instructs OpenSSL to turn off TLS v1
`SSL_OP_NO_TLSv1_1` | Instructs OpenSSL to turn off TLS v1.1
`SSL_OP_NO_TLSv1_2` | Instructs OpenSSL to turn off TLS v1.2
`SSL_OP_NO_TLSv1_3` | Instructs OpenSSL to turn off TLS v1.3
`SSL_OP_PRIORITIZE_CHACHA` | Instructs OpenSSL server to prioritize ChaCha20-Poly1305 when the client does. This option has no effect if `SSL_OP_CIPHER_SERVER_PREFERENCE` is not enabled.
`SSL_OP_TLS_ROLLBACK_BUG` | Instructs OpenSSL to disable version rollback attack detection.

#### OpenSSL engine constants#

Constant | Description
--- | ---
`ENGINE_METHOD_RSA` | Limit engine usage to RSA
`ENGINE_METHOD_DSA` | Limit engine usage to DSA
`ENGINE_METHOD_DH` | Limit engine usage to DH
`ENGINE_METHOD_RAND` | Limit engine usage to RAND
`ENGINE_METHOD_EC` | Limit engine usage to EC
`ENGINE_METHOD_CIPHERS` | Limit engine usage to CIPHERS
`ENGINE_METHOD_DIGESTS` | Limit engine usage to DIGESTS
`ENGINE_METHOD_PKEY_METHS` | Limit engine usage to PKEY_METHS
`ENGINE_METHOD_PKEY_ASN1_METHS` | Limit engine usage to PKEY_ASN1_METHS
`ENGINE_METHOD_ALL` |
`ENGINE_METHOD_NONE` |

#### Other OpenSSL constants#

Constant | Description
--- | ---
`DH_CHECK_P_NOT_SAFE_PRIME` |
`DH_CHECK_P_NOT_PRIME` |
`DH_UNABLE_TO_CHECK_GENERATOR` |
`DH_NOT_SUITABLE_GENERATOR` |
`RSA_PKCS1_PADDING` |
`RSA_SSLV23_PADDING` |
`RSA_NO_PADDING` |
`RSA_PKCS1_OAEP_PADDING` |
`RSA_X931_PADDING` |
`RSA_PKCS1_PSS_PADDING` |
`RSA_PSS_SALTLEN_DIGEST` | Sets the salt length for `RSA_PKCS1_PSS_PADDING` to the digest size when signing or verifying.
`RSA_PSS_SALTLEN_MAX_SIGN` | Sets the salt length for `RSA_PKCS1_PSS_PADDING` to the maximum permissible value when signing data.
`RSA_PSS_SALTLEN_AUTO` | Causes the salt length for `RSA_PKCS1_PSS_PADDING` to be determined automatically when verifying a signature.
`POINT_CONVERSION_COMPRESSED` |
`POINT_CONVERSION_UNCOMPRESSED` |
`POINT_CONVERSION_HYBRID` |

#### Node.js crypto constants#

Constant | Description
--- | ---
`defaultCoreCipherList` | Specifies the built-in default cipher list used by Node.js.
`defaultCipherList` | Specifies the active default cipher list used by the current Node.js process.

## Footnotes

1. Requires OpenSSL >= 3.5 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13 ↩14 ↩15 ↩16 ↩17 ↩18 ↩19 ↩20 ↩21 ↩22 ↩23 ↩24

2. Requires OpenSSL >= 3.0 ↩ ↩2

3. Requires OpenSSL >= 3.2 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8

## Debugger#

Stability: 2 - Stable

Node.js includes a command-line debugging utility. The Node.js debugger client is not a full-featured debugger, but simple stepping and inspection are possible.

The debugger supports two modes of operation: interactive mode and non-interactive probe mode.

### Interactive mode#

    $ node inspect [--port=<port>] [<node-option> ...] [<script> [<script-args>] | <host>:<port> | -p <pid>]
    console

To use it, start Node.js with the `inspect` argument followed by the path to the script to debug.

    $ node inspect myscript.js
    < Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8
    < For help, see: https://nodejs.org/learn/getting-started/debugging
    <
    connecting to 127.0.0.1:9229 ... ok
    < Debugger attached.
    <
     ok
    Break on start in myscript.js:2
      1 // myscript.js
    > 2 global.x = 5;
      3 setTimeout(() => {
      4   debugger;
    debug>
    console

The debugger automatically breaks on the first executable line. To instead run until the first breakpoint (specified by a `debugger` statement), set the `NODE_INSPECT_RESUME_ON_START` environment variable to `1`.

    $ cat myscript.js
    // myscript.js
    global.x = 5;
    setTimeout(() => {
      debugger;
      console.log('world');
    }, 1000);
    console.log('hello');
    $ NODE_INSPECT_RESUME_ON_START=1 node inspect myscript.js
    < Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2
    < For help, see: https://nodejs.org/learn/getting-started/debugging
    <
    connecting to 127.0.0.1:9229 ... ok
    < Debugger attached.
    <
    < hello
    <
    break in myscript.js:4
      2 global.x = 5;
      3 setTimeout(() => {
    > 4   debugger;
      5   console.log('world');
      6 }, 1000);
    debug> next
    break in myscript.js:5
      3 setTimeout(() => {
      4   debugger;
    > 5   console.log('world');
      6 }, 1000);
      7 console.log('hello');
    debug> repl
    Press Ctrl+C to leave debug repl
    > x
    5
    > 2 + 2
    4
    debug> next
    < world
    <
    break in myscript.js:6
      4   debugger;
      5   console.log('world');
    > 6 }, 1000);
      7 console.log('hello');
      8
    debug> .exit
    $
    console

The `repl` command allows code to be evaluated remotely. The `next` command steps to the next line. Type `help` to see what other commands are available.

Pressing `enter` without typing a command will repeat the previous debugger command.

#### Watchers#

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type `watch('my_expression')`. The command `watchers` will print the active watchers. To remove a watcher, type `unwatch('my_expression')`.

### Command reference#

#### Stepping#

- `cont`, `c`: Continue execution
- `next`, `n`: Step next
- `step`, `s`: Step in
- `out`, `o`: Step out
- `pause`: Pause running code (like pause button in Developer Tools)

##### Breakpoints#

- `setBreakpoint()`, `sb()`: Set breakpoint on current line
- `setBreakpoint(line)`, `sb(line)`: Set breakpoint on specific line
- `setBreakpoint('fn()')`, `sb(...)`: Set breakpoint on a first statement in function's body
- `setBreakpoint('script.js', 1)`, `sb(...)`: Set breakpoint on first line of `script.js`
- `setBreakpoint('script.js', 1, 'num < 4')`, `sb(...)`: Set conditional breakpoint on first line of `script.js` that only breaks when `num < 4` evaluates to `true`
- `clearBreakpoint('script.js', 1)`, `cb(...)`: Clear breakpoint in `script.js` on line 1

It is also possible to set a breakpoint in a file (module) that is not loaded yet:

    $ node inspect main.js
    < Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9
    < For help, see: https://nodejs.org/learn/getting-started/debugging
    <
    connecting to 127.0.0.1:9229 ... ok
    < Debugger attached.
    <
    Break on start in main.js:1
    > 1 const mod = require('./mod.js');
      2 mod.hello();
      3 mod.hello();
    debug> setBreakpoint('mod.js', 22)
    Warning: script 'mod.js' was not loaded yet.
    debug> c
    break in mod.js:22
     20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
     21
    >22 exports.hello = function() {
     23   return 'hello from module';
     24 };
    debug>
    console

It is also possible to set a conditional breakpoint that only breaks when a given expression evaluates to `true`:

    $ node inspect main.js
    < Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35
    < For help, see: https://nodejs.org/learn/getting-started/debugging
    <
    connecting to 127.0.0.1:9229 ... ok
    < Debugger attached.
    Break on start in main.js:7
      5 }
      6
    > 7 addOne(10);
      8 addOne(-1);
      9
    debug> setBreakpoint('main.js', 4, 'num < 0')
      1 'use strict';
      2
      3 function addOne(num) {
    > 4   return num + 1;
      5 }
      6
      7 addOne(10);
      8 addOne(-1);
      9
    debug> cont
    break in main.js:4
      2
      3 function addOne(num) {
    > 4   return num + 1;
      5 }
      6
    debug> exec('num')
    -1
    debug>
    console

##### Information#

- `backtrace`, `bt`: Print backtrace of current execution frame
- `list(5)`: List scripts source code with 5 line context (5 lines before and after)
- `watch(expr)`: Add expression to watch list
- `unwatch(expr)`: Remove expression from watch list
- `unwatch(index)`: Remove expression at specific index from watch list
- `watchers`: List all watchers and their values (automatically listed on each breakpoint)
- `repl`: Open debugger's repl for evaluation in debugging script's context
- `exec expr`, `p expr`: Execute an expression in debugging script's context and print its value
- `profile`: Start CPU profiling session
- `profileEnd`: Stop current CPU profiling session
- `profiles`: List all completed CPU profiling sessions
- `profiles[n].save(filepath = 'node.cpuprofile')`: Save CPU profiling session to disk as JSON
- `takeHeapSnapshot(filepath = 'node.heapsnapshot')`: Take a heap snapshot and save to disk as JSON

##### Execution control#

- `run`: Run script (automatically runs on debugger's start)
- `restart`: Restart script
- `kill`: Kill script

##### Various#

- `scripts`: List all loaded scripts
- `version`: Display V8's version

### Probe mode#

Added in: v26.1.0
History

Version | Changes
--- | ---
v26.6.0 | Add per-probe `--cond <expr>` option to only record a hit when the condition is truthy at the probe location.
v26.4.0 | Add per-probe `--max-hit <n>` option to limit evaluated hits and finish with a `completed` terminal event as soon as any probe reaches its limit.
v26.3.0 | Add `probe_failure` terminal `error` event for inspector-side mid-session failures, and `error.details` for additional context on per-hit and terminal errors.
v26.2.0 | JSON report schema bumped to v2. Probe `target` is now `{ suffix, line, column? }` instead of an array. Each "hit" event carries a `location` field reporting the actual evaluation location. When the probe does not specify a column, it binds to the first executable column instead of defaulting to 1.

Stability: 1 - Experimental

`node inspect` supports a non-interactive probe mode for inspecting runtime values in an application via the flag `--probe`.

Currently, probe mode only supports launching a new process from the entry point script specified on the command line.

The probe mode sets one or more source breakpoints, evaluates specified expressions whenever the execution reaches a breakpoint, and prints one final report of all the evaluated expressions when the session ends (either on normal completion, error, or timeout). This allows developers to perform printf-style debugging without having to modify the application code and clean up afterwards. It also supports structured JSON output for tool use.

    $ node inspect --probe <file>:<line>[:<col>] --expr <expr> [--cond <expr>] [--max-hit <n>]
                  [--probe <file>:<line>[:<col>] --expr <expr> [--cond <expr>] [--max-hit <n>] ...]
                  [--json] [--preview] [--timeout=<ms>] [--port=<port>]
                  [--] [<node-option> ...] <script> [<script-args> ...]
    console

- `--probe <file>:<line>[:<col>]`: Source location of the probe. When execution reaches the location, the provided expressions are evaluated and printed in the output. `<file>` matches the URL suffix of the script to probe. `<line>` and `<col>` numbers are 1-based. When `<col>` is omitted, the probe binds to the first executable column on the line.
- `--expr <expr>`: JavaScript expression to evaluate whenever execution reaches the location specified by the preceding `--probe`. Must immediately follow the `--probe` it belongs to.
- `--cond <expr>`: An optional condition for the probe location. The probe only records a hit when `<expr>` is truthy at the location. A condition that throws is treated as false.
- `--max-hit <n>`: An optional per-probe limit on the number of times the probe can be hit. When not specified, there's no hit limit. When any probe reaches its hit limit, the probing process will detach and report the results. The process being probed will continue to run. If any other probe is never reached by the time the session ends, it will be reported as a missed probe.
- `--timeout=<ms>`: A global wall-clock deadline for the entire probe session. The default is `30000`. This can be used to probe a long-running application that can be terminated externally.
- `--json`: If used, prints a structured JSON report instead of the default text report.
- `--preview`: If used, non-primitive values will include CDP property previews for object-like JSON probe values.
- `--port=<port>`: Selects the local inspector port where the probing session will listen. Defaults to `0`, which requests a random port.
- `--` is optional unless the child needs its own Node.js flags.

Additional rules about the composition of the options:

- `--probe <file>:<line>[:<col>]` and `--expr <expr>` are strict pairs. Each `--probe` must be followed immediately by exactly one `--expr`.
- `--cond <expr>` and `--max-hit <n>` are optional modifiers written after the `--probe`/`--expr` pair they apply to, each at most once per pair. They may not appear before the first `--probe` or between a `--probe` and its matching `--expr`.
- `--max-hit` scopes to the `--probe`/`--expr` pair it follows, so pairs sharing a location may set different limits. `--cond` scopes to the whole location, probes sharing a location must share one condition (or none).
- `--timeout`, `--json`, `--preview`, and `--port` are global probe options for the whole probe session. They may appear before or between probe pairs, but not between a `--probe` and its matching `--expr`.
- If additional Node.js execution arguments need to be passed to the child script, `--` must be used to separate the probe options from the Node.js options for the child script.

Example:

    $ node inspect --probe app.js:10 --expr "user"
                   --probe src/utils.js:5:15 --expr "config.options"
                   --json --preview -- --no-warnings app.js --arg-for-app=foo
    console

#### Probe output format#

When the probe session ends, the probing process prints a final report of all the probe hits and results.

Consider this script:

    // cli.js
    let maxRSS = 0;
    for (let i = 0; i < 2; i++) {
      const { rss } = process.memoryUsage();
      maxRSS = Math.max(maxRSS, rss);
    }
    js

Without `--json`, by default the output is printed in a human-readable text format:

    $ node inspect --probe cli.js:5 --expr 'rss' cli.js
    Hit 1 at file:///path/to/cli.js:5:3
      rss = 54935552
    Hit 2 at file:///path/to/cli.js:5:3
      rss = 55083008
    Completed
    console

The original `<file>:<line>[:<col>]` passed to `--probe` may be resolved to a different location to ensure it's pausable, or it can match multiple loaded scripts, so the actual evaluation location helps disambiguate the results.

Primitive results are printed directly, while objects and arrays use Chrome DevTools Protocol preview data when available. Other non-primitive values fall back to the Chrome DevTools Protocol `description` string. Expression failures are recorded as `[error] ...` lines and do not fail the overall session. If richer text formatting is needed, wrap the expression in `JSON.stringify(...)` or `util.inspect(...)`.

When `--json` is used, the output shape looks like this:

    $ node inspect --json --probe cli.js:5 --expr 'rss' cli.js
    {"v":2,"probes":[{"expr":"rss","target":{"suffix":"cli.js","line":5}}],"results":[{"probe":0,"event":"hit","hit":1,"location":{"url":"file:///path/to/cli.js","line":5,"column":3},"result":{"type":"number","value":55443456,"description":"55443456"}},{"probe":0,"event":"hit","hit":2,"location":{"url":"file:///path/to/cli.js","line":5,"column":3},"result":{"type":"number","value":55574528,"description":"55574528"}},{"event":"completed"}]}
    console

    {
      "v": 2, // Probe JSON schema version.
      "probes": [
        {
          "expr": "rss", // The expression paired with --probe.
          "target": {
            // The user's probe specification. `suffix` is the raw <file> passed
            // to --probe and is matched as a path-separator-anchored suffix
            // against every loaded script's URL. `column` is present only if the
            // user supplied `:col`. The actual evaluation location may differ
            // from the target and will be reported in each hit's `location` field.
            "suffix": "cli.js",
            "line": 5
          }
          // `condition` is present only when the probe was given a --cond expression.
          // `maxHit` is present only when the probe was given a --max-hit limit.
        }
      ],
      "results": [
        {
          "probe": 0, // Index into probes[].
          "event": "hit", // Hit events are recorded in observation order.
          "hit": 1, // 1-based hit count for this probe.
          "location": {
            // The actual location where the execution is paused to evaluate
            // the expression of the probe. This may differ from the probe's
            // target due to pausability adjustments or multiple matches.
            "url": "file:///path/to/cli.js",
            "line": 5,
            "column": 3
          },
          "result": {
            "type": "number",
            "value": 55443456,
            "description": "55443456"
          }
          // If the probe expression throws, fails, or never completes, the entry
          // carries an `error` field instead of `result` with the shape
          // `{ message: string, details?: object }`. The `message` and `details`
          // content is informational only and may change between releases.
        },
        {
          "probe": 0,
          "event": "hit",
          "hit": 2,
          "location": { "url": "file:///path/to/cli.js", "line": 5, "column": 3 },
          "result": {
            "type": "number",
            "value": 55574528,
            "description": "55574528"
          }
        },
        {
          "event": "completed"
          // The final entry is always a terminal event, for example:
          // 1. { "event": "completed" }
          // 2. { "event": "miss", "pending": [0, 1] }
          // 3. {
          //      "event": "timeout",
          //      "pending": [0],
          //      "error": {
          //       "code": "probe_timeout",
          //       "message": "Timed out after 30000ms waiting for probes: app.js:10"
          //      }
          //    }
          // 4. {
          //      "event": "error",
          //      "pending": [0],
          //      "error": {
          //       "code": "probe_target_exit",
          //       "exitCode": 1,
          //       "stderr": "Error: boom",
          //       "message": "Target exited with code 1 before probes: app.js:10"
          //      }
          //    }
          // 5. {
          //      "event": "error",
          //      "pending": [1],
          //      "error": {
          //       "code": "probe_failure",
          //       "probe": 0,
          //       "stderr": "...",
          //       "message": "Target process exited during probe evaluation before probes: app.js:12. If the failure repeats, review the probe expression.",
          //       "details": { "lastCdpMethod": "Debugger.evaluateOnCallFrame" }
          //      }
          //    }
        }
      ]
    }
    json

#### Output and exit codes#

Probe mode only prints the final probe report to stdout, and otherwise silences stdout/stderr from the child process. When the probing session ends, the probing process typically exits with code `0` and prints a final report to stdout. If the child process exits with a non-zero code before the probe session ends, or the probe session cannot complete for another reason, the final report records a terminal `error` event.

When `error.code` is `'probe_failure'` or `'probe_timeout'`, the probing process exits with a non-zero code, indicating recorded hits may be incomplete. In this case, `error.message` will contain recovery hints, and `error.probe`, when present, is an index into the report's `probes` array that identifies the possible culprit probe on a best-effort basis to help guide debugging.

Invalid arguments and fatal launch or connect failures may cause the probing process to exit with a non-zero code and print an error message to stderr without a final probe report.

#### Probing multiple expressions at the same execution point#

When multiple `--probe`/`--expr` pairs share the same `--probe`, the expressions will be evaluated on the same pause in the order they appear on the command line.

For each location, there can only be at most one `--cond` (or none). Multiple `--probe`/`--expr` pairs with conflicting conditions at the same location will be rejected at launch time.

    // app.js
    const x = { x: 42 };       // line 2
    const y = { y: 35 };       // line 3
    const z = { ...x, ...y };  // line 4
    js

    $ node inspect --probe app.js:4 --expr 'x' --probe app.js:4 --expr 'y' -- app.js
    console

Prints

    Hit 1 at file:///path/to/app.js:4:1
      x = {x: 42}
    Hit 1 at file:///path/to/app.js:4:1
      y = {y: 35}
    Completed
    text

    $ node inspect --probe app.js:4 --expr 'x' --probe app.js:4 --expr 'y' --json --preview -- app.js
    console

Prints

    {"v":2,"probes":[{"expr":"x","target":{"suffix":"app.js","line":4}},{"expr":"y","target":{"suffix":"app.js","line":4}}],"results":[{"probe":0,"event":"hit","hit":1,"location":{"url":"file:///path/to/app.js","line":4,"column":1},"result":{"type":"object","description":"Object","preview":{"type":"object","description":"Object","overflow":false,"properties":[{"name":"x","type":"number","value":"42"}]}}},{"probe":1,"event":"hit","hit":1,"location":{"url":"file:///path/to/app.js","line":4,"column":1},"result":{"type":"object","description":"Object","preview":{"type":"object","description":"Object","overflow":false,"properties":[{"name":"y","type":"number","value":"35"}]}}},{"event":"completed"}]}
    json

#### Selecting the probe location#

The expressions are evaluated in the lexical scope of the probe location when execution reaches it. Avoid probing a variable declared by `let` or `const` at its declaration site, as this leads to a `ReferenceError` caused by accessing the variable in its temporal dead zone (TDZ).

    // app.js
    const x = 42;        // line 2
    console.log(x);      // line 3
    js

    $ node inspect --probe app.js:1 --expr 'x' app.js
    Hit 1 at file:///path/to/app.js:1:1
      [error] x = ReferenceError: Cannot access 'x' from debugger
      ...
    Completed
    console

Instead, probe at a location where the variable is already initialized:

    $ node inspect --probe app.js:3 --expr 'x' app.js
    Hit 1 at file:///path/to/app.js:3:1
      x = 42
    Completed
    console

The `<file>` argument is matched as a path suffix of every loaded script URL, anchored on a path separator. Passing only a basename matches every loaded script with that basename, similar to how native debuggers typically match breakpoints, while passing a partial path narrows the match. Given:

    project/
      - src/utils.js
      - lib/utils.js
    text

`--probe utils.js:10` binds to both files and produces one hit per match. Each hit carries its own `location` field identifying where the expression was actually executed, so consumers can attribute the result to one of the two files accurately. To disambiguate at bind time, specify a fuller path that only matches the intended file:

    $ node inspect --probe src/utils.js:10 --expr 'x' main.js   # matches only src/utils.js
    console

#### Probe examples#

##### Probing a variable conditionally#

    // app.js
    let total = 0;
    for (let i = 0; i < 10; i++) {
      total += i;  // line 4
    }
    js

    $ out/Release/node inspect --probe app.js:4 --expr 'total' \
                               --cond 'i % 3 === 0' app.js
    console

    Hit 1 at file:///path/to/app.js:3:3
      total = 0
    Hit 2 at file:///path/to/app.js:3:3
      total = 3
    Hit 3 at file:///path/to/app.js:3:3
      total = 15
    Hit 4 at file:///path/to/app.js:3:3
      total = 36
    Completed
    text

### Advanced usage#

#### V8 inspector integration for Node.js#

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol.

V8 Inspector can be enabled by passing the `--inspect` flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. `--inspect=9222` will accept DevTools connections on port 9222.

Using the `--inspect` flag will execute the code immediately before debugger is connected. This means that the code will start running before you can start debugging, which might not be ideal if you want to debug from the very beginning.

In such cases, you have two alternatives:

1. `--inspect-wait` flag: This flag will wait for debugger to be attached before executing the code. This allows you to start debugging right from the beginning of the execution.
2. `--inspect-brk` flag: Unlike `--inspect`, this flag will break on the first line of the code as soon as debugger is attached. This is useful when you want to debug the code step by step from the very beginning, without any code execution prior to debugging.

So, when deciding between `--inspect`, `--inspect-wait`, and `--inspect-brk`, consider whether you want the code to start executing immediately, wait for debugger to be attached before execution, or break on the first line for step-by-step debugging.

    $ node --inspect index.js
    Debugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
    For help, see: https://nodejs.org/learn/getting-started/debugging
    console

(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)

## Deprecated APIs#

Node.js APIs might be deprecated for any of the following reasons:

- Use of the API is unsafe.
- An improved alternative API is available.
- Breaking changes to the API are expected in a future major release.

Node.js uses four kinds of deprecations:

- Documentation-only
- Application (non-`node_modules` code only)
- Runtime (all code)
- End-of-Life

A Documentation-only deprecation is one that is expressed only within the Node.js API docs. These generate no side-effects while running Node.js. Some Documentation-only deprecations trigger a runtime warning when launched with `--pending-deprecation` flag (or its alternative, `NODE_PENDING_DEPRECATION=1` environment variable), similarly to Runtime deprecations below. Documentation-only deprecations that support that flag are explicitly labeled as such in the list of Deprecated APIs.

An Application deprecation for only non-`node_modules` code will, by default, generate a process warning that will be printed to `stderr` the first time the deprecated API is used in code that's not loaded from `node_modules`. When the `--throw-deprecation` command-line flag is used, a Runtime deprecation will cause an error to be thrown. When `--pending-deprecation` is used, warnings will also be emitted for code loaded from `node_modules`.

A runtime deprecation for all code is similar to the runtime deprecation for non-`node_modules` code, except that it also emits a warning for code loaded from `node_modules`.

An End-of-Life deprecation is used when functionality is or will soon be removed from Node.js.

### Revoking deprecations#

Occasionally, the deprecation of an API might be reversed. In such situations, this document will be updated with information relevant to the decision. However, the deprecation identifier will not be modified.

### List of deprecated APIs#

#### DEP0001: `http.OutgoingMessage.prototype.flush`#

History

Version | Changes
--- | ---
v14.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v1.6.0 | Runtime deprecation.

Type: End-of-Life

`OutgoingMessage.prototype.flush()` has been removed. Use `OutgoingMessage.prototype.flushHeaders()` instead.

#### DEP0002: `require('_linklist')`#

History

Version | Changes
--- | ---
v8.0.0 | End-of-Life.
v6.12.0 | A deprecation code has been assigned.
v5.0.0 | Runtime deprecation.

Type: End-of-Life

The `_linklist` module is deprecated. Please use a userland alternative.

#### DEP0003: `_writableState.buffer`#

History

Version | Changes
--- | ---
v14.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.15 | Runtime deprecation.

Type: End-of-Life

The `_writableState.buffer` has been removed. Use `_writableState.getBuffer()` instead.

#### DEP0004: `CryptoStream.prototype.readyState`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.4.0 | Documentation-only deprecation.

Type: End-of-Life

The `CryptoStream.prototype.readyState` property was removed.

#### DEP0005: `Buffer()` constructor#

History

Version | Changes
--- | ---
v10.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.

Type: Application (non-`node_modules` code only)

The `Buffer()` function and `new Buffer()` constructor are deprecated due to API usability issues that can lead to accidental security issues.

As an alternative, use one of the following methods of constructing `Buffer` objects:

- `Buffer.alloc(size[, fill[, encoding]])`: Create a `Buffer` with initialized memory.
- `Buffer.allocUnsafe(size)`: Create a `Buffer` with uninitialized memory.
- `Buffer.allocUnsafeSlow(size)`: Create a `Buffer` with uninitialized memory.
- `Buffer.from(array)`: Create a `Buffer` with a copy of `array`
- `Buffer.from(arrayBuffer[, byteOffset[, length]])` - Create a `Buffer` that wraps the given `arrayBuffer`.
- `Buffer.from(buffer)`: Create a `Buffer` that copies `buffer`.
- `Buffer.from(string[, encoding])`: Create a `Buffer` that copies `string`.

Without `--pending-deprecation`, runtime warnings occur only for code not in `node_modules`. This means there will not be deprecation warnings for `Buffer()` usage in dependencies. With `--pending-deprecation`, a runtime warning results no matter where the `Buffer()` usage occurs.

#### DEP0006: `child_process` `options.customFds`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.14 | Runtime deprecation.
v0.5.10 | Documentation-only deprecation.

Type: End-of-Life

Within the `child_process` module's `spawn()`, `fork()`, and `exec()` methods, the `options.customFds` option is deprecated. The `options.stdio` option should be used instead.

#### DEP0007: Replace `cluster` `worker.suicide` with `worker.exitedAfterDisconnect`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v7.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.

Type: End-of-Life

In an earlier version of the Node.js `cluster`, a boolean property with the name `suicide` was added to the `Worker` object. The intent of this property was to provide an indication of how and why the `Worker` instance exited. In Node.js 6.0.0, the old property was deprecated and replaced with a new `worker.exitedAfterDisconnect` property. The old property name did not precisely describe the actual semantics and was unnecessarily emotion-laden.

#### DEP0008: `require('node:constants')`#

History

Version | Changes
--- | ---
v6.12.0 | A deprecation code has been assigned.
v6.3.0 | Documentation-only deprecation.

Type: Documentation-only

The `node:constants` module is deprecated. When requiring access to constants relevant to specific Node.js builtin modules, developers should instead refer to the `constants` property exposed by the relevant module. For instance, `require('node:fs').constants` and `require('node:os').constants`.

#### DEP0009: `crypto.pbkdf2` without digest#

History

Version | Changes
--- | ---
v14.0.0 | End-of-Life (for `digest === null`).
v11.0.0 | Runtime deprecation (for `digest === null`).
v8.0.0 | End-of-Life (for `digest === undefined`).
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Runtime deprecation (for `digest === undefined`).

Type: End-of-Life

Use of the `crypto.pbkdf2()` API without specifying a digest was deprecated in Node.js 6.0 because the method defaulted to using the non-recommended `'SHA1'` digest. Previously, a deprecation warning was printed. Starting in Node.js 8.0.0, calling `crypto.pbkdf2()` or `crypto.pbkdf2Sync()` with `digest` set to `undefined` will throw a `TypeError`.

Beginning in Node.js 11.0.0, calling these functions with `digest` set to `null` would print a deprecation warning to align with the behavior when `digest` is `undefined`.

Now, however, passing either `undefined` or `null` will throw a `TypeError`.

#### DEP0010: `crypto.createCredentials`#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.13 | Runtime deprecation.

Type: End-of-Life

The `crypto.createCredentials()` API was removed. Please use `tls.createSecureContext()` instead.

#### DEP0011: `crypto.Credentials`#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.13 | Runtime deprecation.

Type: End-of-Life

The `crypto.Credentials` class was removed. Please use `tls.SecureContext` instead.

#### DEP0012: `Domain.dispose`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.7 | Runtime deprecation.

Type: End-of-Life

`Domain.dispose()` has been removed. Recover from failed I/O actions explicitly via error event handlers set on the domain instead.

#### DEP0013: `fs` asynchronous function without callback#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v7.0.0 | Runtime deprecation.

Type: End-of-Life

Calling an asynchronous function without a callback throws a `TypeError` in Node.js 10.0.0 onwards. See https://github.com/nodejs/node/pull/12562.

#### DEP0014: `fs.read` legacy String interface#

History

Version | Changes
--- | ---
v8.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v6.0.0 | Runtime deprecation.
v0.1.96 | Documentation-only deprecation.

Type: End-of-Life

The `fs.read()` legacy `String` interface is deprecated. Use the `Buffer` API as mentioned in the documentation instead.

#### DEP0015: `fs.readSync` legacy String interface#

History

Version | Changes
--- | ---
v8.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v6.0.0 | Runtime deprecation.
v0.1.96 | Documentation-only deprecation.

Type: End-of-Life

The `fs.readSync()` legacy `String` interface is deprecated. Use the `Buffer` API as mentioned in the documentation instead.

#### DEP0016: `GLOBAL`/`root`#

History

Version | Changes
--- | ---
v14.0.0 | End-of-Life.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Runtime deprecation.

Type: End-of-Life

The `GLOBAL` and `root` aliases for the `global` property were deprecated in Node.js 6.0.0 and have since been removed.

#### DEP0017: `Intl.v8BreakIterator`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v7.0.0 | Runtime deprecation.

Type: End-of-Life

`Intl.v8BreakIterator` was a non-standard extension and has been removed. See `Intl.Segmenter`.

#### DEP0018: Unhandled promise rejections#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v7.0.0 | Runtime deprecation.

Type: End-of-Life

Unhandled promise rejections are deprecated. By default, promise rejections that are not handled terminate the Node.js process with a non-zero exit code. To change the way Node.js treats unhandled rejections, use the `--unhandled-rejections` command-line option.

#### DEP0019: `require('.')` resolved outside directory#

History

Version | Changes
--- | ---
v12.0.0 | Removed functionality.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v1.8.1 | Runtime deprecation.

Type: End-of-Life

In certain cases, `require('.')` could resolve outside the package directory. This behavior has been removed.

#### DEP0020: `Server.connections`#

History

Version | Changes
--- | ---
v15.0.0 | Server.connections has been removed.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.9.7 | Runtime deprecation.

Type: End-of-Life

The `Server.connections` property was deprecated in Node.js 0.9.7 and has been removed. Please use the `Server.getConnections()` method instead.

#### DEP0021: `Server.listenFD`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.7.12 | Runtime deprecation.

Type: End-of-Life

The `Server.listenFD()` method was deprecated and removed. Please use `Server.listen({fd: <number>})` instead.

#### DEP0022: `os.tmpDir()`#

History

Version | Changes
--- | ---
v14.0.0 | End-of-Life.
v7.0.0 | Runtime deprecation.

Type: End-of-Life

The `os.tmpDir()` API was deprecated in Node.js 7.0.0 and has since been removed. Please use `os.tmpdir()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/tmpDir-to-tmpdir
    bash

#### DEP0023: `os.getNetworkInterfaces()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.6.0 | Runtime deprecation.

Type: End-of-Life

The `os.getNetworkInterfaces()` method is deprecated. Please use the `os.networkInterfaces()` method instead.

#### DEP0024: `REPLServer.prototype.convertToContext()`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v7.0.0 | Runtime deprecation.

Type: End-of-Life

The `REPLServer.prototype.convertToContext()` API has been removed.

#### DEP0025: `require('node:sys')`#

History

Version | Changes
--- | ---
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v1.0.0 | Runtime deprecation.

Type: Runtime

The `node:sys` module is deprecated. Please use the `util` module instead.

#### DEP0026: `util.print()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.3 | Runtime deprecation.

Type: End-of-Life

`util.print()` has been removed. Please use `console.log()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-print-to-console-log
    bash

#### DEP0027: `util.puts()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.3 | Runtime deprecation.

Type: End-of-Life

`util.puts()` has been removed. Please use `console.log()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-print-to-console-log
    bash

#### DEP0028: `util.debug()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.3 | Runtime deprecation.

Type: End-of-Life

`util.debug()` has been removed. Please use `console.error()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-print-to-console-log
    bash

#### DEP0029: `util.error()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.3 | Runtime deprecation.

Type: End-of-Life

`util.error()` has been removed. Please use `console.error()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-print-to-console-log
    bash

#### DEP0030: `SlowBuffer`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v24.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `SlowBuffer` class has been removed. Please use `Buffer.allocUnsafeSlow(size)` instead.

An automated migration is available (source).

    npx codemod@latest @nodejs/slow-buffer-to-buffer-alloc-unsafe-slow
    bash

#### DEP0031: `ecdh.setPublicKey()`#

History

Version | Changes
--- | ---
v25.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v5.2.0 | Documentation-only deprecation.

Type: Runtime

The `ecdh.setPublicKey()` method is now deprecated as its inclusion in the API is not useful.

#### DEP0032: `node:domain` module#

History

Version | Changes
--- | ---
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v1.4.2 | Documentation-only deprecation.

Type: Documentation-only

The `domain` module is deprecated and should not be used.

#### DEP0033: `EventEmitter.listenerCount()`#

History

Version | Changes
--- | ---
v25.4.0, v24.14.0 | Deprecation revoked.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v3.2.0 | Documentation-only deprecation.

Type: Revoked

The `events.listenerCount(emitter, eventName)` API was deprecated, as it provided identical functionality to `emitter.listenerCount(eventName)`. The deprecation was revoked because this function has been repurposed to also accept `<EventTarget>` arguments.

#### DEP0034: `fs.exists(path, callback)`#

History

Version | Changes
--- | ---
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v1.0.0 | Documentation-only deprecation.

Type: Documentation-only

The `fs.exists(path, callback)` API is deprecated. Please use `fs.stat()` or `fs.access()` instead.

#### DEP0035: `fs.lchmod(path, mode, callback)`#

History

Version | Changes
--- | ---
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.4.7 | Documentation-only deprecation.

Type: Documentation-only

The `fs.lchmod(path, mode, callback)` API is deprecated.

#### DEP0036: `fs.lchmodSync(path, mode)`#

History

Version | Changes
--- | ---
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.4.7 | Documentation-only deprecation.

Type: Documentation-only

The `fs.lchmodSync(path, mode)` API is deprecated.

#### DEP0037: `fs.lchown(path, uid, gid, callback)`#

History

Version | Changes
--- | ---
v10.6.0 | Deprecation revoked.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.4.7 | Documentation-only deprecation.

Type: Deprecation revoked

The `fs.lchown(path, uid, gid, callback)` API was deprecated. The deprecation was revoked because the requisite supporting APIs were added in libuv.

#### DEP0038: `fs.lchownSync(path, uid, gid)`#

History

Version | Changes
--- | ---
v10.6.0 | Deprecation revoked.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.4.7 | Documentation-only deprecation.

Type: Deprecation revoked

The `fs.lchownSync(path, uid, gid)` API was deprecated. The deprecation was revoked because the requisite supporting APIs were added in libuv.

#### DEP0039: `require.extensions`#

History

Version | Changes
--- | ---
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.10.6 | Documentation-only deprecation.

Type: Documentation-only

The `require.extensions` property is deprecated.

#### DEP0040: `node:punycode` module#

History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | Application deprecation.
v21.0.0 | Runtime deprecation.
v16.6.0 | Added support for `--pending-deprecation`.
v7.0.0 | Documentation-only deprecation.

Type: Application (non-`node_modules` code only)

The `punycode` module is deprecated. Please use a userland alternative instead.

#### DEP0041: `NODE_REPL_HISTORY_FILE` environment variable#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v3.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `NODE_REPL_HISTORY_FILE` environment variable was removed. Please use `NODE_REPL_HISTORY` instead.

#### DEP0042: `tls.CryptoStream`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v0.11.3 | Documentation-only deprecation.

Type: End-of-Life

The `tls.CryptoStream` class was removed. Please use `tls.TLSSocket` instead.

#### DEP0043: `tls.SecurePair`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v8.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.
v0.11.15 | Deprecation revoked.
v0.11.3 | Runtime deprecation.

Type: End-of-Life

The `tls.SecurePair` class is deprecated. Please use `tls.TLSSocket` instead.

#### DEP0044: `util.isArray()`#

History

Version | Changes
--- | ---
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: Runtime

The `util.isArray()` API is deprecated. Please use `Array.isArray()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0045: `util.isBoolean()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isBoolean()` API has been removed. Please use `typeof arg === 'boolean'` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0046: `util.isBuffer()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isBuffer()` API has been removed. Please use `Buffer.isBuffer()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0047: `util.isDate()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isDate()` API has been removed. Please use `arg instanceof Date` instead.

Also for stronger approaches, consider using: `Date.prototype.toString.call(arg) === '[object Date]' && !isNaN(arg)`. This can also be used in a `try/catch` block to handle invalid date objects.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0048: `util.isError()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isError()` API has been removed. Please use `Error.isError(arg)`.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0049: `util.isFunction()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isFunction()` API has been removed. Please use `typeof arg === 'function'` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0050: `util.isNull()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isNull()` API has been removed. Please use `arg === null` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0051: `util.isNullOrUndefined()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isNullOrUndefined()` API has been removed. Please use `arg === null || arg === undefined` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0052: `util.isNumber()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isNumber()` API has been removed. Please use `typeof arg === 'number'` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0053: `util.isObject()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isObject()` API has been removed. Please use `arg && typeof arg === 'object'` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0054: `util.isPrimitive()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isPrimitive()` API has been removed. Please use `Object(arg) !== arg` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0055: `util.isRegExp()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isRegExp()` API has been removed. Please use `arg instanceof RegExp` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0056: `util.isString()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isString()` API has been removed. Please use `typeof arg === 'string'` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0057: `util.isSymbol()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isSymbol()` API has been removed. Please use `typeof arg === 'symbol'` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0058: `util.isUndefined()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0, v4.8.6 | A deprecation code has been assigned.
v4.0.0, v3.3.1 | Documentation-only deprecation.

Type: End-of-Life

The `util.isUndefined()` API has been removed. Please use `arg === undefined` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-is
    bash

#### DEP0059: `util.log()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life deprecation.
v22.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `util.log()` API has been removed because it's an unmaintained legacy API that was exposed to user land by accident. Instead, consider the following alternatives based on your specific needs:

- Third-Party Logging Libraries

- Use `console.log(new Date().toLocaleString(), message)`

By adopting one of these alternatives, you can transition away from `util.log()` and choose a logging strategy that aligns with the specific requirements and complexity of your application.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-log-to-console-log
    bash

#### DEP0060: `util._extend()`#

History

Version | Changes
--- | ---
v22.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.

Type: Runtime

The `util._extend()` API is deprecated because it's an unmaintained legacy API that was exposed to user land by accident. Please use `target = Object.assign(target, source)` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/util-extend-to-object-assign
    bash

#### DEP0061: `fs.SyncWriteStream`#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v8.0.0 | Runtime deprecation.
v7.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `fs.SyncWriteStream` class was never intended to be a publicly accessible API and has been removed. No alternative API is available. Please use a userland alternative.

#### DEP0062: `node --debug`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v8.0.0 | Runtime deprecation.

Type: End-of-Life

`--debug` activates the legacy V8 debugger interface, which was removed as of V8 5.8. It is replaced by Inspector which is activated with `--inspect` instead.

#### DEP0063: `ServerResponse.prototype.writeHeader()`#

History

Version | Changes
--- | ---
v26.0.0 | End-of-Life.
v25.0.0 | Runtime deprecation.
v8.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `node:http` module `ServerResponse.prototype.writeHeader()` API is deprecated. Please use `ServerResponse.prototype.writeHead()` instead.

The `ServerResponse.prototype.writeHeader()` method was never documented as an officially supported API.

#### DEP0064: `tls.createSecurePair()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v8.0.0 | Runtime deprecation.
v6.12.0 | A deprecation code has been assigned.
v6.0.0 | Documentation-only deprecation.
v0.11.15 | Deprecation revoked.
v0.11.3 | Runtime deprecation.

Type: End-of-Life

The `tls.createSecurePair()` API was deprecated in documentation in Node.js 0.11.3. Users should use `tls.Socket` instead.

An automated migration is available (source):

    npx codemod @nodejs/tls-create-secure-pair-to-tls-socket
    bash

#### DEP0065: `repl.REPL_MODE_MAGIC` and `NODE_REPL_MODE=magic`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v8.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `node:repl` module's `REPL_MODE_MAGIC` constant, used for `replMode` option, has been removed. Its behavior has been functionally identical to that of `REPL_MODE_SLOPPY` since Node.js 6.0.0, when V8 5.0 was imported. Please use `REPL_MODE_SLOPPY` instead.

The `NODE_REPL_MODE` environment variable is used to set the underlying `replMode` of an interactive `node` session. Its value, `magic`, is also removed. Please use `sloppy` instead.

#### DEP0066: `OutgoingMessage.prototype._headers, OutgoingMessage.prototype._headerNames`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v12.0.0 | Runtime deprecation.
v8.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `node:http` module `OutgoingMessage.prototype._headers` and `OutgoingMessage.prototype._headerNames` properties are deprecated. Use one of the public methods (e.g. `OutgoingMessage.prototype.getHeader()`, `OutgoingMessage.prototype.getHeaders()`, `OutgoingMessage.prototype.getHeaderNames()`, `OutgoingMessage.prototype.getRawHeaderNames()`, `OutgoingMessage.prototype.hasHeader()`, `OutgoingMessage.prototype.removeHeader()`, `OutgoingMessage.prototype.setHeader()`) for working with outgoing headers.

The `OutgoingMessage.prototype._headers` and `OutgoingMessage.prototype._headerNames` properties were never documented as officially supported properties.

An automated migration is available (source):

    npx codemod@latest @nodejs/http-outgoingmessage-headers
    bash

#### DEP0067: `OutgoingMessage.prototype._renderHeaders`#

History

Version | Changes
--- | ---
v8.0.0 | Documentation-only deprecation.

Type: Documentation-only

The `node:http` module `OutgoingMessage.prototype._renderHeaders()` API is deprecated.

The `OutgoingMessage.prototype._renderHeaders` property was never documented as an officially supported API.

#### DEP0068: `node debug`#

History

Version | Changes
--- | ---
v15.0.0 | The legacy `node debug` command was removed.
v8.0.0 | Runtime deprecation.

Type: End-of-Life

`node debug` corresponds to the legacy CLI debugger which has been replaced with a V8-inspector based CLI debugger available through `node inspect`.

#### DEP0069: `vm.runInDebugContext(string)`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.
v8.0.0 | Documentation-only deprecation.

Type: End-of-Life

DebugContext has been removed in V8 and is not available in Node.js 10+.

DebugContext was an experimental API.

#### DEP0070: `async_hooks.currentId()`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v8.2.0 | Runtime deprecation.

Type: End-of-Life

`async_hooks.currentId()` was renamed to `async_hooks.executionAsyncId()` for clarity.

This change was made while `async_hooks` was an experimental API.

#### DEP0071: `async_hooks.triggerId()`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v8.2.0 | Runtime deprecation.

Type: End-of-Life

`async_hooks.triggerId()` was renamed to `async_hooks.triggerAsyncId()` for clarity.

This change was made while `async_hooks` was an experimental API.

#### DEP0072: `async_hooks.AsyncResource.triggerId()`#

History

Version | Changes
--- | ---
v9.0.0 | End-of-Life.
v8.2.0 | Runtime deprecation.

Type: End-of-Life

`async_hooks.AsyncResource.triggerId()` was renamed to `async_hooks.AsyncResource.triggerAsyncId()` for clarity.

This change was made while `async_hooks` was an experimental API.

#### DEP0073: Several internal properties of `net.Server`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

Accessing several internal, undocumented properties of `net.Server` instances with inappropriate names is deprecated.

As the original API was undocumented and not generally useful for non-internal code, no replacement API is provided.

#### DEP0074: `REPLServer.bufferedCommand`#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

The `REPLServer.bufferedCommand` property was deprecated in favor of `REPLServer.clearBufferedCommand()`.

#### DEP0075: `REPLServer.parseREPLKeyword()`#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

`REPLServer.parseREPLKeyword()` was removed from userland visibility.

#### DEP0076: `tls.parseCertString()`#

History

Version | Changes
--- | ---
v18.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.
v8.6.0 | Documentation-only deprecation.

Type: End-of-Life

`tls.parseCertString()` was a trivial parsing helper that was made public by mistake. While it was supposed to parse certificate subject and issuer strings, it never handled multi-value Relative Distinguished Names correctly.

Earlier versions of this document suggested using `querystring.parse()` as an alternative to `tls.parseCertString()`. However, `querystring.parse()` also does not handle all certificate subjects correctly and should not be used.

#### DEP0077: `Module._debug()`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

`Module._debug()` has been removed.

The `Module._debug()` function was never documented as an officially supported API.

#### DEP0078: `REPLServer.turnOffEditorMode()`#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

`REPLServer.turnOffEditorMode()` was removed from userland visibility.

#### DEP0079: Custom inspection function on objects via `.inspect()`#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.
v8.7.0 | Documentation-only deprecation.

Type: End-of-Life

Using a property named `inspect` on an object to specify a custom inspection function for `util.inspect()` is deprecated. Use `util.inspect.custom` instead. For backward compatibility with Node.js prior to version 6.4.0, both can be specified.

#### DEP0080: `path._makeLong()`#

History

Version | Changes
--- | ---
v9.0.0 | Documentation-only deprecation.

Type: Documentation-only

The internal `path._makeLong()` was not intended for public use. However, userland modules have found it useful. The internal API is deprecated and replaced with an identical, public `path.toNamespacedPath()` method.

#### DEP0081: `fs.truncate()` using a file descriptor#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

`fs.truncate()` `fs.truncateSync()` usage with a file descriptor is deprecated. Please use `fs.ftruncate()` or `fs.ftruncateSync()` to work with file descriptors.

An automated migration is available (source):

    npx codemod@latest @nodejs/fs-truncate-fd-deprecation
    bash

#### DEP0082: `REPLServer.prototype.memory()`#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v9.0.0 | Runtime deprecation.

Type: End-of-Life

`REPLServer.prototype.memory()` is only necessary for the internal mechanics of the `REPLServer` itself. Do not use this function.

#### DEP0083: Disabling ECDH by setting `ecdhCurve` to `false`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v9.2.0 | Runtime deprecation.

Type: End-of-Life

The `ecdhCurve` option to `tls.createSecureContext()` and `tls.TLSSocket` could be set to `false` to disable ECDH entirely on the server only. This mode was deprecated in preparation for migrating to OpenSSL 1.1.0 and consistency with the client and is now unsupported. Use the `ciphers` parameter instead.

#### DEP0084: requiring bundled internal dependencies#

History

Version | Changes
--- | ---
v12.0.0 | This functionality has been removed.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

Since Node.js versions 4.4.0 and 5.2.0, several modules only intended for internal usage were mistakenly exposed to user code through `require()`. These modules were:

- `v8/tools/codemap`
- `v8/tools/consarray`
- `v8/tools/csvparser`
- `v8/tools/logreader`
- `v8/tools/profile_view`
- `v8/tools/profile`
- `v8/tools/SourceMap`
- `v8/tools/splaytree`
- `v8/tools/tickprocessor-driver`
- `v8/tools/tickprocessor`
- `node-inspect/lib/_inspect` (from 7.6.0)
- `node-inspect/lib/internal/inspect_client` (from 7.6.0)
- `node-inspect/lib/internal/inspect_repl` (from 7.6.0)

The `v8/*` modules do not have any exports, and if not imported in a specific order would in fact throw errors. As such there are virtually no legitimate use cases for importing them through `require()`.

On the other hand, `node-inspect` can be installed locally through a package manager, as it is published on the npm registry under the same name. No source code modification is necessary if that is done.

#### DEP0085: AsyncHooks sensitive API#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v9.4.0, v8.10.0 | Runtime deprecation.

Type: End-of-Life

The AsyncHooks sensitive API was never documented and had various minor issues. Use the `AsyncResource` API instead. See https://github.com/nodejs/node/issues/15572.

#### DEP0086: Remove `runInAsyncIdScope`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.
v9.4.0, v8.10.0 | Runtime deprecation.

Type: End-of-Life

`runInAsyncIdScope` doesn't emit the `'before'` or `'after'` event and can thus cause a lot of issues. See https://github.com/nodejs/node/issues/14328.

#### DEP0089: `require('node:assert')`#

History

Version | Changes
--- | ---
v12.8.0 | Deprecation revoked.
v9.9.0, v8.13.0 | Documentation-only deprecation.

Type: Deprecation revoked

Importing assert directly was not recommended as the exposed functions use loose equality checks. The deprecation was revoked because use of the `node:assert` module is not discouraged, and the deprecation caused developer confusion.

#### DEP0090: Invalid GCM authentication tag lengths#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

Node.js used to support all GCM authentication tag lengths which are accepted by OpenSSL when calling `decipher.setAuthTag()`. Beginning with Node.js v11.0.0, only authentication tag lengths of 128, 120, 112, 104, 96, 64, and 32 bits are allowed. Authentication tags of other lengths are invalid per NIST SP 800-38D.

#### DEP0091: `crypto.DEFAULT_ENCODING`#

History

Version | Changes
--- | ---
v20.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

The `crypto.DEFAULT_ENCODING` property only existed for compatibility with Node.js releases prior to versions 0.9.3 and has been removed.

#### DEP0092: Top-level `this` bound to `module.exports`#

History

Version | Changes
--- | ---
v10.0.0 | Documentation-only deprecation.

Type: Documentation-only

Assigning properties to the top-level `this` as an alternative to `module.exports` is deprecated. Developers should use `exports` or `module.exports` instead.

#### DEP0093: `crypto.fips` is deprecated and replaced#

History

Version | Changes
--- | ---
v23.0.0 | Runtime deprecation.
v10.0.0 | Documentation-only deprecation.

Type: Runtime

The `crypto.fips` property is deprecated. Please use `crypto.setFips()` and `crypto.getFips()` instead.

An automated migration is available (source).

    npx codemod@latest @nodejs/crypto-fips-to-getFips
    bash

#### DEP0094: Using `assert.fail()` with more than one argument#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

Using `assert.fail()` with more than one argument is deprecated. Use `assert.fail()` with only one argument or use a different `node:assert` module method.

#### DEP0095: `timers.enroll()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

`timers.enroll()` has been removed. Please use the publicly documented `setTimeout()` or `setInterval()` instead.

An automated migration is available (source).

    npx codemod @nodejs/timers-deprecations
    bash

#### DEP0096: `timers.unenroll()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

`timers.unenroll()` has been removed. Please use the publicly documented `clearTimeout()` or `clearInterval()` instead.

An automated migration is available (source).

    npx codemod @nodejs/timers-deprecations
    bash

#### DEP0097: `MakeCallback` with `domain` property#

History

Version | Changes
--- | ---
v10.0.0 | Runtime deprecation.

Type: Runtime

Users of `MakeCallback` that add the `domain` property to carry context, should start using the `async_context` variant of `MakeCallback` or `CallbackScope`, or the high-level `AsyncResource` class.

#### DEP0098: AsyncHooks embedder `AsyncResource.emitBefore` and `AsyncResource.emitAfter` APIs#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v10.0.0, v9.6.0, v8.12.0 | Runtime deprecation.

Type: End-of-Life

The embedded API provided by AsyncHooks exposes `.emitBefore()` and `.emitAfter()` methods which are very easy to use incorrectly which can lead to unrecoverable errors.

Use `asyncResource.runInAsyncScope()` API instead which provides a much safer, and more convenient, alternative. See https://github.com/nodejs/node/pull/18513.

#### DEP0099: Async context-unaware `node::MakeCallback` C++ APIs#

History

Version | Changes
--- | ---
v10.0.0 | Compile-time deprecation.

Type: Compile-time

Certain versions of `node::MakeCallback` APIs available to native addons are deprecated. Please use the versions of the API that accept an `async_context` parameter.

#### DEP0100: `process.assert()`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.
v0.3.7 | Documentation-only deprecation.

Type: End-of-Life

`process.assert()` is deprecated. Please use the `assert` module instead.

This was never a documented feature.

An automated migration is available (source).

    npx codemod@latest @nodejs/process-assert-to-node-assert
    bash

#### DEP0101: `--with-lttng`#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.

Type: End-of-Life

The `--with-lttng` compile-time option has been removed.

#### DEP0102: Using `noAssert` in `Buffer#(read|write)` operations#

History

Version | Changes
--- | ---
v10.0.0 | End-of-Life.

Type: End-of-Life

Using the `noAssert` argument has no functionality anymore. All input is verified regardless of the value of `noAssert`. Skipping the verification could lead to hard-to-find errors and crashes.

#### DEP0103: `process.binding('util').is[...]` typechecks#

History

Version | Changes
--- | ---
v10.9.0 | Superseded by DEP0111.
v10.0.0 | Documentation-only deprecation.

Type: Documentation-only (supports `--pending-deprecation`)

Using `process.binding()` in general should be avoided. The type checking methods in particular can be replaced by using `util.types`.

This deprecation has been superseded by the deprecation of the `process.binding()` API (DEP0111).

#### DEP0104: `process.env` string coercion#

History

Version | Changes
--- | ---
v10.0.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

When assigning a non-string property to `process.env`, the assigned value is implicitly converted to a string. This behavior is deprecated if the assigned value is not a string, boolean, or number. In the future, such assignment might result in a thrown error. Please convert the property to a string before assigning it to `process.env`.

#### DEP0105: `decipher.finaltol`#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

`decipher.finaltol()` has never been documented and was an alias for `decipher.final()`. This API has been removed, and it is recommended to use `decipher.final()` instead.

#### DEP0106: `crypto.createCipher` and `crypto.createDecipher`#

History

Version | Changes
--- | ---
v22.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.
v10.0.0 | Documentation-only deprecation.

Type: End-of-Life

`crypto.createCipher()` and `crypto.createDecipher()` have been removed as they use a weak key derivation function (MD5 with no salt) and static initialization vectors. It is recommended to derive a key using `crypto.pbkdf2()` or `crypto.scrypt()` with random salts and to use `crypto.createCipheriv()` and `crypto.createDecipheriv()` to obtain the `Cipheriv` and `Decipheriv` objects respectively.

An automated migration is available (source):

    npx codemod @nodejs/crypto-createcipheriv-migration
    bash

#### DEP0107: `tls.convertNPNProtocols()`#

History

Version | Changes
--- | ---
v11.0.0 | End-of-Life.
v10.0.0 | Runtime deprecation.

Type: End-of-Life

This was an undocumented helper function not intended for use outside Node.js core and obsoleted by the removal of NPN (Next Protocol Negotiation) support.

#### DEP0108: `zlib.bytesRead`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.
v10.0.0 | Documentation-only deprecation.

Type: End-of-Life

Deprecated alias for `zlib.bytesWritten`. This original name was chosen because it also made sense to interpret the value as the number of bytes read by the engine, but is inconsistent with other streams in Node.js that expose values under these names.

An automated migration is available (source):

    npx codemod@latest @nodejs/zlib-bytesread-to-byteswritten
    bash

#### DEP0109: `http`, `https`, and `tls` support for invalid URLs#

History

Version | Changes
--- | ---
v16.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

Some previously supported (but strictly invalid) URLs were accepted through the `http.request()`, `http.get()`, `https.request()`, `https.get()`, and `tls.checkServerIdentity()` APIs because those were accepted by the legacy `url.parse()` API. The mentioned APIs now use the WHATWG URL parser that requires strictly valid URLs. Passing an invalid URL is deprecated and support will be removed in the future.

#### DEP0110: `vm.Script` cached data#

History

Version | Changes
--- | ---
v10.6.0 | Documentation-only deprecation.

Type: Documentation-only

The `produceCachedData` option is deprecated. Use `script.createCachedData()` instead.

#### DEP0111: `process.binding()`#

History

Version | Changes
--- | ---
v11.12.0 | Added support for `--pending-deprecation`.
v10.9.0 | Documentation-only deprecation.

Type: Documentation-only (supports `--pending-deprecation`)

`process.binding()` is for use by Node.js internal code only.

While `process.binding()` has not reached End-of-Life status in general, it is unavailable when the permission model is enabled.

#### DEP0112: `dgram` private APIs#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

The `node:dgram` module previously contained several APIs that were never meant to accessed outside of Node.js core: `Socket.prototype._handle`, `Socket.prototype._receiving`, `Socket.prototype._bindState`, `Socket.prototype._queue`, `Socket.prototype._reuseAddr`, `Socket.prototype._healthCheck()`, `Socket.prototype._stopReceiving()`, and `dgram._createSocketHandle()`. These have been removed.

#### DEP0113: `Cipher.setAuthTag()`, `Decipher.getAuthTag()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

`Cipher.setAuthTag()` and `Decipher.getAuthTag()` are no longer available. They were never documented and would throw when called.

#### DEP0114: `crypto._toBuf()`#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

The `crypto._toBuf()` function was not designed to be used by modules outside of Node.js core and was removed.

#### DEP0115: `crypto.prng()`, `crypto.pseudoRandomBytes()`, `crypto.rng()`#

History

Version | Changes
--- | ---
v11.0.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

In recent versions of Node.js, there is no difference between `crypto.randomBytes()` and `crypto.pseudoRandomBytes()`. The latter is deprecated along with the undocumented aliases `crypto.prng()` and `crypto.rng()` in favor of `crypto.randomBytes()` and might be removed in a future release.

#### DEP0116: Legacy URL API#

History

Version | Changes
--- | ---
v24.0.0 | DEP0169 covers also `url.format()` and `url.resolve()`.
v19.0.0, v18.13.0 | DEP0169 deprecates `url.parse()` again.
v15.13.0, v14.17.0 | Deprecation revoked. Status changed to "Legacy".
v11.0.0 | Documentation-only deprecation.

Type: Deprecation revoked

The legacy URL API is deprecated. This includes `url.format()`, `url.parse()`, `url.resolve()`, and the legacy `urlObject`. Please use the WHATWG URL API instead.

An automated migration is available (source).

    npx codemod@latest @nodejs/node-url-to-whatwg-url
    bash

#### DEP0117: Native crypto handles#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

Previous versions of Node.js exposed handles to internal native objects through the `_handle` property of the `Cipher`, `Decipher`, `DiffieHellman`, `DiffieHellmanGroup`, `ECDH`, `Hash`, `Hmac`, `Sign`, and `Verify` classes. The `_handle` property has been removed because improper use of the native object can lead to crashing the application.

#### DEP0118: `dns.lookup()` support for a falsy host name#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

Previous versions of Node.js supported `dns.lookup()` with a falsy host name like `dns.lookup(false)` due to backward compatibility. This has been removed.

#### DEP0119: `process.binding('uv').errname()` private API#

History

Version | Changes
--- | ---
v11.0.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

`process.binding('uv').errname()` is deprecated. Please use `util.getSystemErrorName()` instead.

#### DEP0120: Windows Performance Counter support#

History

Version | Changes
--- | ---
v12.0.0 | End-of-Life.
v11.0.0 | Runtime deprecation.

Type: End-of-Life

Windows Performance Counter support has been removed from Node.js. The undocumented `COUNTER_NET_SERVER_CONNECTION()`, `COUNTER_NET_SERVER_CONNECTION_CLOSE()`, `COUNTER_HTTP_SERVER_REQUEST()`, `COUNTER_HTTP_SERVER_RESPONSE()`, `COUNTER_HTTP_CLIENT_REQUEST()`, and `COUNTER_HTTP_CLIENT_RESPONSE()` functions have been deprecated.

#### DEP0121: `net._setSimultaneousAccepts()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v12.0.0 | Runtime deprecation.

Type: End-of-Life

The undocumented `net._setSimultaneousAccepts()` function was originally intended for debugging and performance tuning when using the `node:child_process` and `node:cluster` modules on Windows. The function is not generally useful and is being removed. See discussion here: https://github.com/nodejs/node/issues/18391

#### DEP0122: `tls` `Server.prototype.setOptions()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v12.0.0 | Runtime deprecation.

Type: End-of-Life

Please use `Server.prototype.setSecureContext()` instead.

#### DEP0123: setting the TLS ServerName to an IP address#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v12.0.0 | Runtime deprecation.

Type: End-of-Life

Setting the TLS ServerName to an IP address is not permitted by RFC 6066.

#### DEP0124: using `REPLServer.rli`#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v12.0.0 | Runtime deprecation.

Type: End-of-Life

This property is a reference to the instance itself.

#### DEP0125: `require('node:_stream_wrap')`#

History

Version | Changes
--- | ---
v26.0.0 | End-of-Life.
v12.0.0 | Runtime deprecation.

Type: End-of-Life

The `node:_stream_wrap` module is deprecated.

#### DEP0126: `timers.active()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v11.14.0 | Runtime deprecation.

Type: End-of-Life

The previously undocumented `timers.active()` has been removed. Please use the publicly documented `timeout.refresh()` instead. If re-referencing the timeout is necessary, `timeout.ref()` can be used with no performance impact since Node.js 10.

An automated migration is available (source).

    npx codemod @nodejs/timers-deprecations
    bash

#### DEP0127: `timers._unrefActive()`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v11.14.0 | Runtime deprecation.

Type: End-of-Life

The previously undocumented and "private" `timers._unrefActive()` has been removed. Please use the publicly documented `timeout.refresh()` instead. If unreferencing the timeout is necessary, `timeout.unref()` can be used with no performance impact since Node.js 10.

An automated migration is available (source).

    npx codemod @nodejs/timers-deprecations
    bash

#### DEP0128: modules with an invalid `main` entry and an `index.js` file#

History

Version | Changes
--- | ---
v16.0.0 | Runtime deprecation.
v12.0.0 | Documentation-only.

Type: Runtime

Modules that have an invalid `main` entry (e.g., `./does-not-exist.js`) and also have an `index.js` file in the top level directory will resolve the `index.js` file. That is deprecated and is going to throw an error in future Node.js versions.

#### DEP0129: `ChildProcess._channel`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v13.0.0 | Runtime deprecation.
v11.14.0 | Documentation-only.

Type: End-of-Life

The `_channel` property of child process objects returned by `spawn()` and similar functions is not intended for public use. Use `ChildProcess.channel` instead.

#### DEP0130: `Module.createRequireFromPath()`#

History

Version | Changes
--- | ---
v16.0.0 | End-of-life.
v13.0.0 | Runtime deprecation.
v12.2.0 | Documentation-only.

Type: End-of-Life

Use `module.createRequire()` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/create-require-from-path
    bash

#### DEP0131: Legacy HTTP parser#

History

Version | Changes
--- | ---
v13.0.0 | This feature has been removed.
v12.22.0 | Runtime deprecation.
v12.3.0 | Documentation-only.

Type: End-of-Life

The legacy HTTP parser, used by default in versions of Node.js prior to 12.0.0, is deprecated and has been removed in v13.0.0. Prior to v13.0.0, the `--http-parser=legacy` command-line flag could be used to revert to using the legacy parser.

#### DEP0132: `worker.terminate()` with callback#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v12.5.0 | Runtime deprecation.

Type: End-of-Life

Passing a callback to `worker.terminate()` is deprecated. Use the returned `Promise` instead, or a listener to the worker's `'exit'` event.

#### DEP0133: `http` `connection`#

History

Version | Changes
--- | ---
v12.12.0 | Documentation-only deprecation.

Type: Documentation-only

Prefer `response.socket` over `response.connection` and `request.socket` over `request.connection`.

#### DEP0134: `process._tickCallback`#

History

Version | Changes
--- | ---
v12.12.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

The `process._tickCallback` property was never documented as an officially supported API.

#### DEP0135: `WriteStream.open()` and `ReadStream.open()` are internal#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v13.0.0 | Runtime deprecation.

Type: End-of-Life

`WriteStream.open()` and `ReadStream.open()` are undocumented internal APIs that do not make sense to use in userland. File streams should always be opened through their corresponding factory methods `fs.createWriteStream()` and `fs.createReadStream()`) or by passing a file descriptor in options.

#### DEP0136: `http` `finished`#

History

Version | Changes
--- | ---
v13.4.0, v12.16.0 | Documentation-only deprecation.

Type: Documentation-only

`response.finished` indicates whether `response.end()` has been called, not whether `'finish'` has been emitted and the underlying data is flushed.

Use `response.writableFinished` or `response.writableEnded` accordingly instead to avoid the ambiguity.

To maintain existing behavior `response.finished` should be replaced with `response.writableEnded`.

#### DEP0137: Closing fs.FileHandle on garbage collection#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v14.0.0 | Runtime deprecation.

Type: End-of-Life

Allowing a `fs.FileHandle` object to be closed on garbage collection used to be allowed, but now throws an error.

Please ensure that all `fs.FileHandle` objects are explicitly closed using `FileHandle.prototype.close()` when the `fs.FileHandle` is no longer needed:

    const fsPromises = require('node:fs').promises;
    async function openAndClose() {
      let filehandle;
      try {
        filehandle = await fsPromises.open('thefile.txt', 'r');
      } finally {
        if (filehandle !== undefined)
          await filehandle.close();
      }
    }
    js

#### DEP0138: `process.mainModule`#

History

Version | Changes
--- | ---
v14.0.0 | Documentation-only deprecation.

Type: Documentation-only

`process.mainModule` is a CommonJS-only feature while `process` global object is shared with non-CommonJS environment. Its use within ECMAScript modules is unsupported.

It is deprecated in favor of `require.main`, because it serves the same purpose and is only available on CommonJS environment.

An automated migration is available (source):

    npx codemod@latest @nodejs/process-main-module
    bash

#### DEP0139: `process.umask()` with no arguments#

History

Version | Changes
--- | ---
v14.0.0, v12.19.0 | Documentation-only deprecation.

Type: Documentation-only

Calling `process.umask()` with no argument causes the process-wide umask to be written twice. This introduces a race condition between threads, and is a potential security vulnerability. There is no safe, cross-platform alternative API.

#### DEP0140: Use `request.destroy()` instead of `request.abort()`#

History

Version | Changes
--- | ---
v14.1.0, v13.14.0 | Documentation-only deprecation.

Type: Documentation-only

Use `request.destroy()` instead of `request.abort()`.

#### DEP0141: `repl.inputStream` and `repl.outputStream`#

History

Version | Changes
--- | ---
v14.3.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

The `node:repl` module exported the input and output stream twice. Use `.input` instead of `.inputStream` and `.output` instead of `.outputStream`.

#### DEP0142: `repl._builtinLibs`#

History

Version | Changes
--- | ---
v14.3.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

The `node:repl` module exports a `_builtinLibs` property that contains an array of built-in modules. It was incomplete so far and instead it's better to rely upon `require('node:module').builtinModules`.

An automated migration is available (source):

    npx codemod@latest @nodejs/repl-builtin-modules
    bash

#### DEP0143: `Transform._transformState`#

History

Version | Changes
--- | ---
v15.0.0 | End-of-Life.
v14.5.0 | Runtime deprecation.

Type: End-of-Life

`Transform._transformState` will be removed in future versions where it is no longer required due to simplification of the implementation.

#### DEP0144: `module.parent`#

History

Version | Changes
--- | ---
v14.6.0, v12.19.0 | Documentation-only deprecation.

Type: Documentation-only (supports `--pending-deprecation`)

A CommonJS module can access the first module that required it using `module.parent`. This feature is deprecated because it does not work consistently in the presence of ECMAScript modules and because it gives an inaccurate representation of the CommonJS module graph.

Some modules use it to check if they are the entry point of the current process. Instead, it is recommended to compare `require.main` and `module`:

    if (require.main === module) {
      // Code section that will run only if current file is the entry point.
    }
    js

When looking for the CommonJS modules that have required the current one, `require.cache` and `module.children` can be used:

    const moduleParents = Object.values(require.cache)
      .filter((m) => m.children.includes(module));
    js

#### DEP0145: `socket.bufferSize`#

History

Version | Changes
--- | ---
v14.6.0 | Documentation-only deprecation.

Type: Documentation-only

`socket.bufferSize` is just an alias for `writable.writableLength`.

#### DEP0146: `new crypto.Certificate()`#

History

Version | Changes
--- | ---
v14.9.0 | Documentation-only deprecation.

Type: Documentation-only

The `crypto.Certificate()` constructor is deprecated. Use static methods of `crypto.Certificate()` instead.

#### DEP0147: `fs.rmdir(path, { recursive: true })`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v16.0.0 | Runtime deprecation.
v15.0.0 | Runtime deprecation for permissive behavior.
v14.14.0 | Documentation-only deprecation.

Type: End-of-Life

The `fs.rmdir`, `fs.rmdirSync`, and `fs.promises.rmdir` methods used to support a `recursive` option. That option has been removed.

Use `fs.rm(path, { recursive: true, force: true })`, `fs.rmSync(path, { recursive: true, force: true })` or `fs.promises.rm(path, { recursive: true, force: true })` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/rmdir
    bash

#### DEP0148: Folder mappings in `"exports"` (trailing `"/"`)#

History

Version | Changes
--- | ---
v17.0.0 | End-of-Life.
v16.0.0 | Runtime deprecation.
v15.1.0 | Runtime deprecation for self-referencing imports.
v14.13.0 | Documentation-only deprecation.

Type: End-of-Life

Using a trailing `"/"` to define subpath folder mappings in the subpath exports or subpath imports fields is no longer supported. Use subpath patterns instead.

#### DEP0149: `http.IncomingMessage#connection`#

History

Version | Changes
--- | ---
v16.0.0 | Documentation-only deprecation.

Type: Documentation-only

Prefer `message.socket` over `message.connection`.

#### DEP0150: Changing the value of `process.config`#

History

Version | Changes
--- | ---
v19.0.0 | End-of-Life.
v16.0.0 | Runtime deprecation.

Type: End-of-Life

The `process.config` property provides access to Node.js compile-time settings. However, the property is mutable and therefore subject to tampering. The ability to change the value will be removed in a future version of Node.js.

#### DEP0151: Main index lookup and extension searching#

History

Version | Changes
--- | ---
v16.0.0 | Runtime deprecation.
v15.8.0, v14.18.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Runtime

Previously, `index.js` and extension searching lookups would apply to `import 'pkg'` main entry point resolution, even when resolving ES modules.

With this deprecation, all ES module main entry point resolutions require an explicit `"exports"` or `"main"` entry with the exact file extension.

#### DEP0152: Extension PerformanceEntry properties#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v16.0.0 | Runtime deprecation.

Type: End-of-Life

The `'gc'`, `'http2'`, and `'http'` `<PerformanceEntry>` object types used to have additional properties assigned to them that provide additional information. These properties are now available within the standard `detail` property of the `PerformanceEntry` object. The deprecated accessors have been removed.

#### DEP0153: `dns.lookup` and `dnsPromises.lookup` options type coercion#

History

Version | Changes
--- | ---
v18.0.0 | End-of-Life.
v17.0.0 | Runtime deprecation.
v16.8.0 | Documentation-only deprecation.

Type: End-of-Life

Using a non-nullish non-integer value for `family` option, a non-nullish non-number value for `hints` option, a non-nullish non-boolean value for `all` option, or a non-nullish non-boolean value for `verbatim` option in `dns.lookup()` and `dnsPromises.lookup()` throws an `ERR_INVALID_ARG_TYPE` error.

#### DEP0154: RSA-PSS generate key pair options#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v20.0.0 | Runtime deprecation.
v16.10.0 | Documentation-only deprecation.

Type: End-of-Life

Use `'hashAlgorithm'` instead of `'hash'`, and `'mgf1HashAlgorithm'` instead of `'mgf1Hash'`.

An automated migration is available (source):

    npx codemod@latest @nodejs/crypto-rsa-pss-update
    bash

#### DEP0155: Trailing slashes in pattern specifier resolutions#

History

Version | Changes
--- | ---
v17.0.0 | Runtime deprecation.
v16.10.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Runtime

The remapping of specifiers ending in `"/"` like `import 'pkg/x/'` is deprecated for package `"exports"` and `"imports"` pattern resolutions.

#### DEP0156: `.aborted` property and `'abort'`, `'aborted'` event in `http`#

History

Version | Changes
--- | ---
v17.0.0, v16.12.0 | Documentation-only deprecation.

Type: Documentation-only

Move to `<Stream>` API instead, as the `http.ClientRequest`, `http.ServerResponse`, and `http.IncomingMessage` are all stream-based. Check `stream.destroyed` instead of the `.aborted` property, and listen for `'close'` instead of `'abort'`, `'aborted'` event.

The `.aborted` property and `'abort'` event are only useful for detecting `.abort()` calls. For closing a request early, use the Stream `.destroy([error])` then check the `.destroyed` property and `'close'` event should have the same effect. The receiving end should also check the `readable.readableEnded` value on `http.IncomingMessage` to get whether it was an aborted or graceful destroy.

#### DEP0157: Thenable support in streams#

History

Version | Changes
--- | ---
v18.0.0 | End-of-life.
v17.2.0, v16.14.0 | Documentation-only deprecation.

Type: End-of-Life

An undocumented feature of Node.js streams was to support thenables in implementation methods. This is now deprecated, use callbacks instead and avoid use of async function for streams implementation methods.

This feature caused users to encounter unexpected problems where the user implements the function in callback style but uses e.g. an async method which would cause an error since mixing promise and callback semantics is not valid.

    const w = new Writable({
      async final(callback) {
        await someOp();
        callback();
      },
    });
    js

#### DEP0158: `buffer.slice(start, end)`#

History

Version | Changes
--- | ---
v17.5.0, v16.15.0 | Documentation-only deprecation.

Type: Documentation-only

This method was deprecated because it is not compatible with `Uint8Array.prototype.slice()`, which is a superclass of `Buffer`.

Use `buffer.subarray` which does the same thing instead.

#### DEP0159: `ERR_INVALID_CALLBACK`#

History

Version | Changes
--- | ---
v18.0.0 | End-of-Life.

Type: End-of-Life

This error code was removed due to adding more confusion to the errors used for value type validation.

An automated migration is available (source):

    npx codemod @nodejs/err-invalid-callback
    bash

#### DEP0160: `process.on('multipleResolves', handler)`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v18.0.0 | Runtime deprecation.
v17.6.0, v16.15.0 | Documentation-only deprecation.

Type: End-of-Life

This event was deprecated and removed because it did not work with V8 promise combinators which diminished its usefulness.

#### DEP0161: `process._getActiveRequests()` and `process._getActiveHandles()`#

History

Version | Changes
--- | ---
v17.6.0, v16.15.0 | Documentation-only deprecation.

Type: Documentation-only

The `process._getActiveHandles()` and `process._getActiveRequests()` functions are not intended for public use and can be removed in future releases.

Use `process.getActiveResourcesInfo()` to get a list of types of active resources and not the actual references.

#### DEP0162: `fs.write()`, `fs.writeFileSync()` coercion to string#

History

Version | Changes
--- | ---
v19.0.0 | End-of-Life.
v18.0.0 | Runtime deprecation.
v17.8.0, v16.15.0 | Documentation-only deprecation.

Type: End-of-Life

Implicit coercion of objects with own `toString` property, passed as second parameter in `fs.write()`, `fs.writeFile()`, `fs.appendFile()`, `fs.writeFileSync()`, and `fs.appendFileSync()` is deprecated. Convert them to primitive strings.

#### DEP0163: `channel.subscribe(onMessage)`, `channel.unsubscribe(onMessage)`#

History

Version | Changes
--- | ---
v24.8.0, v22.20.0 | Deprecation revoked.
v18.7.0, v16.17.0 | Documentation-only deprecation.

Type: Deprecation revoked

These methods were deprecated because their use could leave the channel object vulnerable to being garbage-collected if not strongly referenced by the user. The deprecation was revoked because channel objects are now resistant to garbage collection when the channel has active subscribers.

#### DEP0164: `process.exit(code)`, `process.exitCode` coercion to integer#

History

Version | Changes
--- | ---
v20.0.0 | End-of-Life.
v19.0.0 | Runtime deprecation.
v18.10.0, v16.18.0 | Documentation-only deprecation of `process.exitCode` integer coercion.
v18.7.0, v16.17.0 | Documentation-only deprecation of `process.exit(code)` integer coercion.

Type: End-of-Life

Values other than `undefined`, `null`, integer numbers, and integer strings (e.g., `'1'`) are deprecated as value for the `code` parameter in `process.exit()` and as value to assign to `process.exitCode`.

#### DEP0165: `--trace-atomics-wait`#

History

Version | Changes
--- | ---
v23.0.0 | End-of-Life.
v22.0.0 | Runtime deprecation.
v18.8.0, v16.18.0 | Documentation-only deprecation.

Type: End-of-Life

The `--trace-atomics-wait` flag has been removed because it uses the V8 hook `SetAtomicsWaitCallback`, that will be removed in a future V8 release.

#### DEP0166: Double slashes in imports and exports targets#

History

Version | Changes
--- | ---
v19.0.0 | Runtime deprecation.
v18.10.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Runtime

Package imports and exports targets mapping into paths including a double slash (of "/" or "\") are deprecated and will fail with a resolution validation error in a future release. This same deprecation also applies to pattern matches starting or ending in a slash.

#### DEP0167: Weak `DiffieHellmanGroup` instances (`modp1`, `modp2`, `modp5`)#

History

Version | Changes
--- | ---
v18.10.0, v16.18.0 | Documentation-only deprecation.

Type: Documentation-only

The well-known MODP groups `modp1`, `modp2`, and `modp5` are deprecated because they are not secure against practical attacks. See RFC 8247 Section 2.4 for details.

These groups might be removed in future versions of Node.js. Applications that rely on these groups should evaluate using stronger MODP groups instead.

#### DEP0168: Unhandled exception in Node-API callbacks#

History

Version | Changes
--- | ---
v18.3.0, v16.17.0 | Runtime deprecation.

Type: Runtime

The implicit suppression of uncaught exceptions in Node-API callbacks is now deprecated.

Set the flag `--force-node-api-uncaught-exceptions-policy` to force Node.js to emit an `'uncaughtException'` event if the exception is not handled in Node-API callbacks.

#### DEP0169: Insecure url.parse()#

History

Version | Changes
--- | ---
v24.0.0 | Application deprecation.
v19.9.0, v18.17.0 | Added support for `--pending-deprecation`.
v19.0.0, v18.13.0 | Documentation-only deprecation.

Type: Application (non-`node_modules` code only)

`url.parse()` behavior is not standardized and prone to errors that have security implications. Use the WHATWG URL API instead. CVEs are not issued for `url.parse()` vulnerabilities.

Calling `url.format(urlString)` or `url.resolve()` invokes `url.parse()` internally, and is therefore also covered by this deprecation.

#### DEP0170: Invalid port when using `url.parse()`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v20.0.0 | Runtime deprecation.
v19.2.0, v18.13.0 | Documentation-only deprecation.

Type: End-of-Life

`url.parse()` used to accept URLs with ports that are not numbers. This behavior might result in host name spoofing with unexpected input. These URLs will throw an error (which the WHATWG URL API also does).

#### DEP0171: Setters for `http.IncomingMessage` headers and trailers#

History

Version | Changes
--- | ---
v19.3.0, v18.13.0 | Documentation-only deprecation.

Type: Documentation-only

In a future version of Node.js, `message.headers`, `message.headersDistinct`, `message.trailers`, and `message.trailersDistinct` will be read-only.

#### DEP0172: The `asyncResource` property of `AsyncResource` bound functions#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v20.0.0 | Runtime-deprecation.

Type: End-of-Life

Older versions of Node.js would add the `asyncResource` when a function is bound to an `AsyncResource`. It no longer does.

#### DEP0173: the `assert.CallTracker` class#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v20.1.0 | Runtime deprecation.

Type: End-of-Life

The `assert.CallTracker` API has been removed.

#### DEP0174: calling `promisify` on a function that returns a `Promise`#

History

Version | Changes
--- | ---
v21.0.0 | Runtime deprecation.
v20.8.0 | Documentation-only deprecation.

Type: Runtime

Calling `util.promisify` on a function that returns a `Promise` will ignore the result of said promise, which can lead to unhandled promise rejections.

#### DEP0175: `util.toUSVString`#

History

Version | Changes
--- | ---
v20.8.0 | Documentation-only deprecation.

Type: Documentation-only

The `util.toUSVString()` API is deprecated. Please use `String.prototype.toWellFormed` instead.

#### DEP0176: `fs.F_OK`, `fs.R_OK`, `fs.W_OK`, `fs.X_OK`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v24.0.0 | Runtime deprecation.
v20.8.0 | Documentation-only deprecation.

Type: End-of-Life

`F_OK`, `R_OK`, `W_OK` and `X_OK` getters exposed directly on `node:fs` were removed. Get them from `fs.constants` or `fs.promises.constants` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/fs-access-mode-constants
    bash

#### DEP0177: `util.types.isWebAssemblyCompiledModule`#

History

Version | Changes
--- | ---
v21.7.0, v20.12.0 | End-of-Life.
v21.3.0, v20.11.0 | A deprecation code has been assigned.
v14.0.0 | Documentation-only deprecation.

Type: End-of-Life

The `util.types.isWebAssemblyCompiledModule` API has been removed. Please use `value instanceof WebAssembly.Module` instead.

#### DEP0178: `dirent.path`#

History

Version | Changes
--- | ---
v24.0.0 | End-of-Life.
v23.0.0 | Runtime deprecation.
v21.5.0, v20.12.0, v18.20.0 | Documentation-only deprecation.

Type: End-of-Life

The `dirent.path` property has been removed due to its lack of consistency across release lines. Please use `dirent.parentPath` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/dirent-path-to-parent-path
    bash

#### DEP0179: `Hash` constructor#

History

Version | Changes
--- | ---
v22.0.0 | Runtime deprecation.
v21.5.0, v20.12.0 | Documentation-only deprecation.

Type: Runtime

Calling `Hash` class directly with `Hash()` or `new Hash()` is deprecated due to being internals, not intended for public use. Please use the `crypto.createHash()` method to create Hash instances.

#### DEP0180: `fs.Stats` constructor#

History

Version | Changes
--- | ---
v22.0.0 | Runtime deprecation.
v20.13.0 | Documentation-only deprecation.

Type: Runtime

Calling `fs.Stats` class directly with `Stats()` or `new Stats()` is deprecated due to being internals, not intended for public use.

#### DEP0181: `Hmac` constructor#

History

Version | Changes
--- | ---
v22.0.0 | Runtime deprecation.
v20.13.0 | Documentation-only deprecation.

Type: Runtime

Calling `Hmac` class directly with `Hmac()` or `new Hmac()` is deprecated due to being internals, not intended for public use. Please use the `crypto.createHmac()` method to create Hmac instances.

#### DEP0182: Short GCM authentication tags without explicit `authTagLength`#

History

Version | Changes
--- | ---
v26.0.0 | End-of-Life.
v23.0.0 | Runtime deprecation.
v20.13.0 | Documentation-only deprecation.

Type: End-of-Life

For ciphers in GCM mode, the `decipher.setAuthTag()` function used to accept authentication tags of any valid length (see also DEP0090). This exception has been removed to better align with recommendations per NIST SP 800-38D, and applications that intend to use authentication tags that are shorter than the default authentication tag length (i.e., shorter than 16 bytes for AES-GCM) must explicitly set the `authTagLength` option of the `crypto.createDecipheriv()` function to the appropriate length.

#### DEP0183: OpenSSL engine-based APIs#

History

Version | Changes
--- | ---
v22.4.0, v20.16.0 | Documentation-only deprecation.

Type: Documentation-only

OpenSSL 3 has deprecated support for custom engines with a recommendation to switch to its new provider model. The `clientCertEngine` option for `https.request()`, `tls.createSecureContext()`, and `tls.createServer()`; the `privateKeyEngine` and `privateKeyIdentifier` for `tls.createSecureContext()`; and `crypto.setEngine()` all depend on this functionality from OpenSSL.

#### DEP0184: Instantiating `node:zlib` classes without `new`#

History

Version | Changes
--- | ---
v24.0.0 | Runtime deprecation.
v22.9.0, v20.18.0 | Documentation-only deprecation.

Type: Runtime

Instantiating classes without the `new` qualifier exported by the `node:zlib` module is deprecated. It is recommended to use the `new` qualifier instead. This applies to all Zlib classes, such as `Deflate`, `DeflateRaw`, `Gunzip`, `Inflate`, `InflateRaw`, `Unzip`, and `Zlib`.

#### DEP0185: Instantiating `node:repl` classes without `new`#

History

Version | Changes
--- | ---
v25.0.0 | End-of-Life.
v24.0.0 | Runtime deprecation.
v22.9.0, v20.18.0 | Documentation-only deprecation.

Type: End-of-Life

Instantiating classes without the `new` qualifier exported by the `node:repl` module is deprecated. The `new` qualifier must be used instead. This applies to all REPL classes, including `REPLServer` and `Recoverable`.

An automated migration is available (source):

    npx codemod@latest @nodejs/repl-classes-with-new
    bash

#### DEP0187: Passing invalid argument types to `fs.existsSync`#

History

Version | Changes
--- | ---
v24.0.0 | Runtime deprecation.
v23.4.0, v22.13.0, v20.19.3 | Documentation-only.

Type: Runtime

Passing non-supported argument types is deprecated and, instead of returning `false`, will throw an error in a future version.

#### DEP0188: `process.features.ipv6` and `process.features.uv`#

History

Version | Changes
--- | ---
v23.4.0, v22.13.0 | Documentation-only deprecation.

Type: Documentation-only

These properties are unconditionally `true`. Any checks based on these properties are redundant.

#### DEP0189: `process.features.tls_*`#

History

Version | Changes
--- | ---
v23.4.0, v22.13.0 | Documentation-only deprecation.

Type: Documentation-only

`process.features.tls_alpn`, `process.features.tls_ocsp`, and `process.features.tls_sni` are deprecated, as their values are guaranteed to be identical to that of `process.features.tls`.

#### DEP0190: Passing `args` to `node:child_process` `execFile`/`spawn` with `shell` option#

History

Version | Changes
--- | ---
v24.0.0 | Runtime deprecation.
v23.11.0, v22.15.0 | Documentation-only deprecation.

Type: Runtime

When an `args` array is passed to `child_process.execFile` or `child_process.spawn` with the option `{ shell: true }` or `{ shell: '/path/to/shell' }`, the values are not escaped, only space-separated, which can lead to shell injection.

#### DEP0191: `repl.builtinModules`#

History

Version | Changes
--- | ---
v24.0.0, v22.16.0 | Documentation-only deprecation with `--pending-deprecation` support.

Type: Documentation-only (supports `--pending-deprecation`)

The `node:repl` module exports a `builtinModules` property that contains an array of built-in modules. This was incomplete and matched the already deprecated `repl._builtinLibs` (DEP0142) instead it's better to rely upon `require('node:module').builtinModules`.

An automated migration is available (source):

    npx codemod@latest @nodejs/repl-builtin-modules
    bash

#### DEP0192: `require('node:_tls_common')` and `require('node:_tls_wrap')`#

History

Version | Changes
--- | ---
v24.2.0, v22.17.0, v20.19.6 | Runtime deprecation.

Type: Runtime

The `node:_tls_common` and `node:_tls_wrap` modules are deprecated as they should be considered an internal nodejs implementation rather than a public facing API, use `node:tls` instead.

#### DEP0193: `require('node:_stream_*')`#

History

Version | Changes
--- | ---
v26.0.0 | End-of-Life.
v24.2.0, v22.17.0, v20.19.6 | Runtime deprecation.

Type: End-of-Life

The `node:_stream_duplex`, `node:_stream_passthrough`, `node:_stream_readable`, `node:_stream_transform`, `node:_stream_wrap` and `node:_stream_writable` modules are deprecated as they should be considered an internal nodejs implementation rather than a public facing API, use `node:stream` instead.

#### DEP0194: HTTP/2 priority signaling#

History

Version | Changes
--- | ---
v24.2.0 | End-of-Life.
v24.2.0, v22.17.0, v20.19.6 | Documentation-only deprecation.

Type: End-of-Life

The support for priority signaling has been removed following its deprecation in the RFC 9113.

An automated migration is available (source):

    npx codemod@latest @nodejs/http2-priority-signaling
    bash

#### DEP0195: Instantiating `node:http` classes without `new`#

History

Version | Changes
--- | ---
v24.2.0, v22.17.0 | Documentation-only deprecation.

Type: Documentation-only

Instantiating classes without the `new` qualifier exported by the `node:http` module is deprecated. It is recommended to use the `new` qualifier instead. This applies to all http classes, such as `OutgoingMessage`, `IncomingMessage`, `ServerResponse` and `ClientRequest`.

An automated migration is available (source):

    npx codemod@latest @nodejs/http-classes-with-new
    bash

#### DEP0196: Calling `node:child_process` functions with `options.shell` as an empty string#

History

Version | Changes
--- | ---
v24.2.0, v22.17.0 | Documentation-only deprecation.

Type: Documentation-only

Calling the process-spawning functions with `{ shell: '' }` is almost certainly unintentional, and can cause aberrant behavior.

To make `child_process.execFile` or `child_process.spawn` invoke the default shell, use `{ shell: true }`. If the intention is not to invoke a shell (default behavior), either omit the `shell` option, or set it to `false` or a nullish value.

To make `child_process.exec` invoke the default shell, either omit the `shell` option, or set it to a nullish value. If the intention is not to invoke a shell, use `child_process.execFile` instead.

#### DEP0197: `util.types.isNativeError()`#

History

Version | Changes
--- | ---
v24.2.0 | Documentation-only deprecation.

Type: Documentation-only

The `util.types.isNativeError` API is deprecated. Please use `Error.isError` instead.

An automated migration is available (source):

    npx codemod@latest @nodejs/types-is-native-error
    bash

#### DEP0198: Creating SHAKE-128 and SHAKE-256 digests without an explicit `options.outputLength`#

History

Version | Changes
--- | ---
v25.0.0 | Runtime deprecation.
v24.4.0, v22.18.0, v20.19.5 | Documentation-only deprecation with support for `--pending-deprecation`.

Type: Runtime

Creating SHAKE-128 and SHAKE-256 digests without an explicit `options.outputLength` is deprecated.

#### DEP0199: `require('node:_http_*')`#

History

Version | Changes
--- | ---
v24.6.0, v22.19.0 | Documentation-only deprecation.

Type: Documentation-only

The `node:_http_agent`, `node:_http_client`, `node:_http_common`, `node:_http_incoming`, `node:_http_outgoing` and `node:_http_server` modules are deprecated as they should be considered an internal nodejs implementation rather than a public facing API, use `node:http` instead.

#### DEP0200: Closing fs.Dir on garbage collection#

History

Version | Changes
--- | ---
v24.9.0 | Documentation-only deprecation.

Type: Documentation-only

Allowing a `fs.Dir` object to be closed on garbage collection is deprecated. In the future, doing so might result in a thrown error that will terminate the process.

Please ensure that all `fs.Dir` objects are explicitly closed using `Dir.prototype.close()` or `using` keyword:

    import { opendir } from 'node:fs/promises';

    {
      await using dir = await opendir('/async/disposable/directory');
    } // Closed by dir[Symbol.asyncDispose]()

    {
      using dir = await opendir('/sync/disposable/directory');
    } // Closed by dir[Symbol.dispose]()

    {
      const dir = await opendir('/unconditionally/iterated/directory');
      for await (const entry of dir) {
        // process an entry
      } // Closed by iterator
    }

    {
      let dir;
      try {
        dir = await opendir('/legacy/closeable/directory');
      } finally {
        await dir?.close();
      }
    }
    mjs

#### DEP0201: Passing `options.type` to `Duplex.toWeb()`#

History

Version | Changes
--- | ---
v26.0.0 | Runtime deprecation.
v25.7.0 | Documentation-only deprecation.

Type: Runtime

Passing the `type` option to `Duplex.toWeb()` is deprecated. To specify the type of the readable half of the constructed readable-writable pair, use the `readableType` option instead.

#### DEP0202: `Http1IncomingMessage` and `Http1ServerResponse` options of HTTP/2 servers#

History

Version | Changes
--- | ---
v25.7.0 | Documentation-only deprecation.

Type: Documentation-only

The `Http1IncomingMessage` and `Http1ServerResponse` options of `http2.createServer()` and `http2.createSecureServer()` are deprecated. Use `http1Options.IncomingMessage` and `http1Options.ServerResponse` instead.

    // Deprecated
    const server = http2.createSecureServer({
      allowHTTP1: true,
      Http1IncomingMessage: MyIncomingMessage,
      Http1ServerResponse: MyServerResponse,
    });
    cjs

    // Use this instead
    const server = http2.createSecureServer({
      allowHTTP1: true,
      http1Options: {
        IncomingMessage: MyIncomingMessage,
        ServerResponse: MyServerResponse,
      },
    });
    cjs

#### DEP0203: Passing `CryptoKey` to `node:crypto` APIs#

History

Version | Changes
--- | ---
v26.0.0 | Runtime deprecation.
v25.9.0 | Documentation-only deprecation.

Type: Runtime

Passing a `CryptoKey` to `node:crypto` functions is deprecated and will throw an error in a future version. This includes `crypto.createPublicKey()`, `crypto.createPrivateKey()`, `crypto.sign()`, `crypto.verify()`, `crypto.publicEncrypt()`, `crypto.publicDecrypt()`, `crypto.privateEncrypt()`, `crypto.privateDecrypt()`, `Sign.prototype.sign()`, `Verify.prototype.verify()`, `crypto.createHmac()`, `crypto.createCipheriv()`, `crypto.createDecipheriv()`, `crypto.encapsulate()`, and `crypto.decapsulate()`.

#### DEP0204: `KeyObject.from()` with non-extractable `CryptoKey`#

History

Version | Changes
--- | ---
v26.0.0 | Runtime deprecation.
v25.9.0 | Documentation-only deprecation.

Type: Runtime

Passing a non-extractable `CryptoKey` to `KeyObject.from()` is deprecated and will throw an error in a future version.

#### DEP0205: `module.register()`#

History

Version | Changes
--- | ---
v26.0.0 | Runtime deprecation.
v25.9.0 | Documentation-only deprecation.

Type: Runtime

`module.register()` is deprecated. Use `module.registerHooks()` instead.

The `module.register()` API provides off-thread async hooks for customizing ES modules; the `module.registerHooks()` API provides similar hooks that are synchronous, in-thread, and work for all types of modules. Supporting async hooks has proven to be complex, involving worker threads orchestration, and there are issues that have proven unresolvable. See caveats of asynchronous customization hooks. Please migrate to `module.registerHooks()` as soon as possible as `module.register()` will be removed in a future version of Node.js.

#### DEP0206: Calling `digest()` on an already-finalized `Hmac` instance#

History

Version | Changes
--- | ---
v26.2.0 | Documentation-only deprecation.

Type: Documentation-only

Calling `hmac.digest()` more than once returns an empty buffer instead of throwing an error. This behavior is inconsistent with `hash.digest()` and may lead to subtle bugs. Calling `hmac.digest()` on a finalized `Hmac` instance will throw an error in a future version.

## Diagnostic report#

History

Version | Changes
--- | ---
v23.3.0, v22.13.0 | Added `--report-exclude-env` option for excluding environment variables from report generation.
v22.0.0, v20.13.0 | Added `--report-exclude-network` option for excluding networking operations that can slow down report generation in some cases.

Stability: 2 - Stable

Delivers a JSON-formatted diagnostic summary, written to a file.

The report is intended for development, test, and production use, to capture and preserve information for problem determination. It includes JavaScript and native stack traces, heap statistics, platform information, resource usage etc. With the report option enabled, diagnostic reports can be triggered on unhandled exceptions, fatal errors and user signals, in addition to triggering programmatically through API calls.

A complete example report that was generated on an uncaught exception is provided below for reference.

    {
      "header": {
        "reportVersion": 5,
        "event": "exception",
        "trigger": "Exception",
        "filename": "report.20181221.005011.8974.0.001.json",
        "dumpEventTime": "2018-12-21T00:50:11Z",
        "dumpEventTimeStamp": "1545371411331",
        "processId": 8974,
        "cwd": "/home/nodeuser/project/node",
        "commandLine": [
          "/home/nodeuser/project/node/out/Release/node",
          "--report-uncaught-exception",
          "/home/nodeuser/project/node/test/report/test-exception.js",
          "child"
        ],
        "nodejsVersion": "v12.0.0-pre",
        "glibcVersionRuntime": "2.17",
        "glibcVersionCompiler": "2.17",
        "wordSize": "64 bit",
        "arch": "x64",
        "platform": "linux",
        "componentVersions": {
          "node": "12.0.0-pre",
          "v8": "7.1.302.28-node.5",
          "uv": "1.24.1",
          "zlib": "1.2.11",
          "ares": "1.15.0",
          "modules": "68",
          "nghttp2": "1.34.0",
          "napi": "3",
          "llhttp": "1.0.1",
          "openssl": "1.1.0j"
        },
        "release": {
          "name": "node"
        },
        "osName": "Linux",
        "osRelease": "3.10.0-862.el7.x86_64",
        "osVersion": "#1 SMP Wed Mar 21 18:14:51 EDT 2018",
        "osMachine": "x86_64",
        "cpus": [
          {
            "model": "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz",
            "speed": 2700,
            "user": 88902660,
            "nice": 0,
            "sys": 50902570,
            "idle": 241732220,
            "irq": 0
          },
          {
            "model": "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz",
            "speed": 2700,
            "user": 88902660,
            "nice": 0,
            "sys": 50902570,
            "idle": 241732220,
            "irq": 0
          }
        ],
        "networkInterfaces": [
          {
            "name": "en0",
            "internal": false,
            "mac": "13:10:de:ad:be:ef",
            "address": "10.0.0.37",
            "netmask": "255.255.255.0",
            "family": "IPv4"
          }
        ],
        "host": "test_machine"
      },
      "javascriptStack": {
        "message": "Error: *** test-exception.js: throwing uncaught Error",
        "stack": [
          "at myException (/home/nodeuser/project/node/test/report/test-exception.js:9:11)",
          "at Object.<anonymous> (/home/nodeuser/project/node/test/report/test-exception.js:12:3)",
          "at Module._compile (internal/modules/cjs/loader.js:718:30)",
          "at Object.Module._extensions..js (internal/modules/cjs/loader.js:729:10)",
          "at Module.load (internal/modules/cjs/loader.js:617:32)",
          "at tryModuleLoad (internal/modules/cjs/loader.js:560:12)",
          "at Function.Module._load (internal/modules/cjs/loader.js:552:3)",
          "at Function.Module.runMain (internal/modules/cjs/loader.js:771:12)",
          "at executeUserCode (internal/bootstrap/node.js:332:15)"
        ]
      },
      "nativeStack": [
        {
          "pc": "0x000055b57f07a9ef",
          "symbol": "report::GetNodeReport(v8::Isolate*, node::Environment*, char const*, char const*, v8::Local<v8::String>, std::ostream&) [./node]"
        },
        {
          "pc": "0x000055b57f07cf03",
          "symbol": "report::GetReport(v8::FunctionCallbackInfo<v8::Value> const&) [./node]"
        },
        {
          "pc": "0x000055b57f1bccfd",
          "symbol": " [./node]"
        },
        {
          "pc": "0x000055b57f1be048",
          "symbol": "v8::internal::Builtin_HandleApiCall(int, v8::internal::Object**, v8::internal::Isolate*) [./node]"
        },
        {
          "pc": "0x000055b57feeda0e",
          "symbol": " [./node]"
        }
      ],
      "javascriptHeap": {
        "totalMemory": 5660672,
        "executableMemory": 524288,
        "totalCommittedMemory": 5488640,
        "availableMemory": 4341379928,
        "totalGlobalHandlesMemory": 8192,
        "usedGlobalHandlesMemory": 3136,
        "usedMemory": 4816432,
        "memoryLimit": 4345298944,
        "mallocedMemory": 254128,
        "externalMemory": 315644,
        "peakMallocedMemory": 98752,
        "nativeContextCount": 1,
        "detachedContextCount": 0,
        "doesZapGarbage": 0,
        "heapSpaces": {
          "read_only_space": {
            "memorySize": 524288,
            "committedMemory": 39208,
            "capacity": 515584,
            "used": 30504,
            "available": 485080
          },
          "new_space": {
            "memorySize": 2097152,
            "committedMemory": 2019312,
            "capacity": 1031168,
            "used": 985496,
            "available": 45672
          },
          "old_space": {
            "memorySize": 2273280,
            "committedMemory": 1769008,
            "capacity": 1974640,
            "used": 1725488,
            "available": 249152
          },
          "code_space": {
            "memorySize": 696320,
            "committedMemory": 184896,
            "capacity": 152128,
            "used": 152128,
            "available": 0
          },
          "map_space": {
            "memorySize": 536576,
            "committedMemory": 344928,
            "capacity": 327520,
            "used": 327520,
            "available": 0
          },
          "large_object_space": {
            "memorySize": 0,
            "committedMemory": 0,
            "capacity": 1520590336,
            "used": 0,
            "available": 1520590336
          },
          "new_large_object_space": {
            "memorySize": 0,
            "committedMemory": 0,
            "capacity": 0,
            "used": 0,
            "available": 0
          }
        }
      },
      "resourceUsage": {
        "rss": "35766272",
        "free_memory": "1598337024",
        "total_memory": "17179869184",
        "available_memory": "1598337024",
        "maxRss": "36624662528",
        "constrained_memory": "36624662528",
        "userCpuSeconds": 0.040072,
        "kernelCpuSeconds": 0.016029,
        "cpuConsumptionPercent": 5.6101,
        "userCpuConsumptionPercent": 4.0072,
        "kernelCpuConsumptionPercent": 1.6029,
        "pageFaults": {
          "IORequired": 0,
          "IONotRequired": 4610
        },
        "fsActivity": {
          "reads": 0,
          "writes": 0
        }
      },
      "uvthreadResourceUsage": {
        "userCpuSeconds": 0.039843,
        "kernelCpuSeconds": 0.015937,
        "cpuConsumptionPercent": 5.578,
        "userCpuConsumptionPercent": 3.9843,
        "kernelCpuConsumptionPercent": 1.5937,
        "fsActivity": {
          "reads": 0,
          "writes": 0
        }
      },
      "libuv": [
        {
          "type": "async",
          "is_active": true,
          "is_referenced": false,
          "address": "0x0000000102910900",
          "details": ""
        },
        {
          "type": "timer",
          "is_active": false,
          "is_referenced": false,
          "address": "0x00007fff5fbfeab0",
          "repeat": 0,
          "firesInMsFromNow": 94403548320796,
          "expired": true
        },
        {
          "type": "check",
          "is_active": true,
          "is_referenced": false,
          "address": "0x00007fff5fbfeb48"
        },
        {
          "type": "idle",
          "is_active": false,
          "is_referenced": true,
          "address": "0x00007fff5fbfebc0"
        },
        {
          "type": "prepare",
          "is_active": false,
          "is_referenced": false,
          "address": "0x00007fff5fbfec38"
        },
        {
          "type": "check",
          "is_active": false,
          "is_referenced": false,
          "address": "0x00007fff5fbfecb0"
        },
        {
          "type": "async",
          "is_active": true,
          "is_referenced": false,
          "address": "0x000000010188f2e0"
        },
        {
          "type": "tty",
          "is_active": false,
          "is_referenced": true,
          "address": "0x000055b581db0e18",
          "width": 204,
          "height": 55,
          "fd": 17,
          "writeQueueSize": 0,
          "readable": true,
          "writable": true
        },
        {
          "type": "signal",
          "is_active": true,
          "is_referenced": false,
          "address": "0x000055b581d80010",
          "signum": 28,
          "signal": "SIGWINCH"
        },
        {
          "type": "tty",
          "is_active": true,
          "is_referenced": true,
          "address": "0x000055b581df59f8",
          "width": 204,
          "height": 55,
          "fd": 19,
          "writeQueueSize": 0,
          "readable": true,
          "writable": true
        },
        {
          "type": "loop",
          "is_active": true,
          "address": "0x000055fc7b2cb180",
          "loopIdleTimeSeconds": 22644.8
        },
        {
          "type": "tcp",
          "is_active": true,
          "is_referenced": true,
          "address": "0x000055e70fcb85d8",
          "localEndpoint": {
            "host": "localhost",
            "ip4": "127.0.0.1",
            "port": 48986
          },
          "remoteEndpoint": {
            "host": "localhost",
            "ip4": "127.0.0.1",
            "port": 38573
          },
          "sendBufferSize": 2626560,
          "recvBufferSize": 131072,
          "fd": 24,
          "writeQueueSize": 0,
          "readable": true,
          "writable": true
        }
      ],
      "workers": [],
      "environmentVariables": {
        "REMOTEHOST": "REMOVED",
        "MANPATH": "/opt/rh/devtoolset-3/root/usr/share/man:",
        "XDG_SESSION_ID": "66126",
        "HOSTNAME": "test_machine",
        "HOST": "test_machine",
        "TERM": "xterm-256color",
        "SHELL": "/bin/csh",
        "SSH_CLIENT": "REMOVED",
        "PERL5LIB": "/opt/rh/devtoolset-3/root//usr/lib64/perl5/vendor_perl:/opt/rh/devtoolset-3/root/usr/lib/perl5:/opt/rh/devtoolset-3/root//usr/share/perl5/vendor_perl",
        "OLDPWD": "/home/nodeuser/project/node/src",
        "JAVACONFDIRS": "/opt/rh/devtoolset-3/root/etc/java:/etc/java",
        "SSH_TTY": "/dev/pts/0",
        "PCP_DIR": "/opt/rh/devtoolset-3/root",
        "GROUP": "normaluser",
        "USER": "nodeuser",
        "LD_LIBRARY_PATH": "/opt/rh/devtoolset-3/root/usr/lib64:/opt/rh/devtoolset-3/root/usr/lib",
        "HOSTTYPE": "x86_64-linux",
        "XDG_CONFIG_DIRS": "/opt/rh/devtoolset-3/root/etc/xdg:/etc/xdg",
        "MAIL": "/var/spool/mail/nodeuser",
        "PATH": "/home/nodeuser/project/node:/opt/rh/devtoolset-3/root/usr/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin",
        "PWD": "/home/nodeuser/project/node",
        "LANG": "en_US.UTF-8",
        "PS1": "\\u@\\h : \\[\\e[31m\\]\\w\\[\\e[m\\] >  ",
        "SHLVL": "2",
        "HOME": "/home/nodeuser",
        "OSTYPE": "linux",
        "VENDOR": "unknown",
        "PYTHONPATH": "/opt/rh/devtoolset-3/root/usr/lib64/python2.7/site-packages:/opt/rh/devtoolset-3/root/usr/lib/python2.7/site-packages",
        "MACHTYPE": "x86_64",
        "LOGNAME": "nodeuser",
        "XDG_DATA_DIRS": "/opt/rh/devtoolset-3/root/usr/share:/usr/local/share:/usr/share",
        "LESSOPEN": "||/usr/bin/lesspipe.sh %s",
        "INFOPATH": "/opt/rh/devtoolset-3/root/usr/share/info",
        "XDG_RUNTIME_DIR": "/run/user/50141",
        "_": "./node"
      },
      "userLimits": {
        "core_file_size_blocks": {
          "soft": "",
          "hard": "unlimited"
        },
        "data_seg_size_bytes": {
          "soft": "unlimited",
          "hard": "unlimited"
        },
        "file_size_blocks": {
          "soft": "unlimited",
          "hard": "unlimited"
        },
        "max_locked_memory_bytes": {
          "soft": "unlimited",
          "hard": 65536
        },
        "max_memory_size_bytes": {
          "soft": "unlimited",
          "hard": "unlimited"
        },
        "open_files": {
          "soft": "unlimited",
          "hard": 4096
        },
        "stack_size_bytes": {
          "soft": "unlimited",
          "hard": "unlimited"
        },
        "cpu_time_seconds": {
          "soft": "unlimited",
          "hard": "unlimited"
        },
        "max_user_processes": {
          "soft": "unlimited",
          "hard": 4127290
        },
        "virtual_memory_bytes": {
          "soft": "unlimited",
          "hard": "unlimited"
        }
      },
      "sharedObjects": [
        "/lib64/libdl.so.2",
        "/lib64/librt.so.1",
        "/lib64/libstdc++.so.6",
        "/lib64/libm.so.6",
        "/lib64/libgcc_s.so.1",
        "/lib64/libpthread.so.0",
        "/lib64/libc.so.6",
        "/lib64/ld-linux-x86-64.so.2"
      ]
    }
    json

### Usage#

    node --report-uncaught-exception --report-on-signal \
    --report-on-fatalerror app.js
    bash

- `--report-uncaught-exception` Enables report to be generated on un-caught exceptions. Useful when inspecting JavaScript stack in conjunction with native stack and other runtime environment data.

- `--report-on-signal` Enables report to be generated upon receiving the specified (or predefined) signal to the running Node.js process. (See below on how to modify the signal that triggers the report.) Default signal is `SIGUSR2`. Useful when a report needs to be triggered from another program. Application monitors may leverage this feature to collect report at regular intervals and plot rich set of internal runtime data to their views.

Signal based report generation is not supported in Windows.

Under normal circumstances, there is no need to modify the report triggering signal. However, if `SIGUSR2` is already used for other purposes, then this flag helps to change the signal for report generation and preserve the original meaning of `SIGUSR2` for the said purposes.

- `--report-on-fatalerror` Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime, such as out of memory) that leads to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.

- `--report-compact` Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

- `--report-directory` Location at which the report will be generated.

- `--report-filename` Name of the file to which the report will be written.

- `--report-signal` Sets or resets the signal for report generation (not supported on Windows). Default signal is `SIGUSR2`.

- `--report-exclude-network` Exclude `header.networkInterfaces` and disable the reverse DNS queries in `libuv.*.(remote|local)Endpoint.host` from the diagnostic report. By default this is not set and the network interfaces are included.

- `--report-exclude-env` Exclude `environmentVariables` from the diagnostic report. By default this is not set and the environment variables are included.

A report can also be triggered via an API call from a JavaScript application:

    process.report.writeReport();
    js

This function takes an optional additional argument `filename`, which is the name of a file into which the report is written.

    process.report.writeReport('./foo.json');
    js

This function takes an optional additional argument `err` which is an `Error` object that will be used as the context for the JavaScript stack printed in the report. When using report to handle errors in a callback or an exception handler, this allows the report to include the location of the original error as well as where it was handled.

    try {
      process.chdir('/non-existent-path');
    } catch (err) {
      process.report.writeReport(err);
    }
    // Any other code
    js

If both filename and error object are passed to `writeReport()` the error object must be the second parameter.

    try {
      process.chdir('/non-existent-path');
    } catch (err) {
      process.report.writeReport(filename, err);
    }
    // Any other code
    js

The content of the diagnostic report can be returned as a JavaScript Object via an API call from a JavaScript application:

    const report = process.report.getReport();
    console.log(typeof report === 'object'); // true

    // Similar to process.report.writeReport() output
    console.log(JSON.stringify(report, null, 2));
    js

This function takes an optional additional argument `err`, which is an `Error` object that will be used as the context for the JavaScript stack printed in the report.

    const report = process.report.getReport(new Error('custom error'));
    console.log(typeof report === 'object'); // true
    js

The API versions are useful when inspecting the runtime state from within the application, in expectation of self-adjusting the resource consumption, load balancing, monitoring etc.

The content of the report consists of a header section containing the event type, date, time, PID, and Node.js version, sections containing JavaScript and native stack traces, a section containing V8 heap information, a section containing `libuv` handle information, and an OS platform information section showing CPU and memory usage and system limits. An example report can be triggered using the Node.js REPL:

    $ node
    > process.report.writeReport();
    Writing Node.js report to file: report.20181126.091102.8480.0.001.json
    Node.js report completed
    >
    console

When a report is written, start and end messages are issued to stderr and the filename of the report is returned to the caller. The default filename includes the date, time, PID, and a sequence number. The sequence number helps in associating the report dump with the runtime state if generated multiple times for the same Node.js process.

### Report Version#

Diagnostic report has an associated single-digit version number (`report.header.reportVersion`), uniquely representing the report format. The version number is bumped when new key is added or removed, or the data type of a value is changed. Report version definitions are consistent across LTS releases.

#### Version history#

##### Version 5#

History

Version | Changes
--- | ---
v23.5.0, v22.13.0 | Fix typos in the memory limit units.

Replace the keys `data_seg_size_kbytes`, `max_memory_size_kbytes`, and `virtual_memory_kbytes` with `data_seg_size_bytes`, `max_memory_size_bytes`, and `virtual_memory_bytes` respectively in the `userLimits` section, as these values are given in bytes.

    {
      "userLimits": {
        // Skip some keys ...
        "data_seg_size_bytes": { // replacing data_seg_size_kbytes
          "soft": "unlimited",
          "hard": "unlimited"
        },
        // ...
        "max_memory_size_bytes": { // replacing max_memory_size_kbytes
          "soft": "unlimited",
          "hard": "unlimited"
        },
        // ...
        "virtual_memory_bytes": { // replacing virtual_memory_kbytes
          "soft": "unlimited",
          "hard": "unlimited"
        }
      }
    }
    json

##### Version 4#

History

Version | Changes
--- | ---
v23.3.0, v22.13.0 | Added `--report-exclude-env` option for excluding environment variables from report generation.

New fields `ipv4` and `ipv6` are added to `tcp` and `udp` libuv handles endpoints. Examples:

    {
      "libuv": [
        {
          "type": "tcp",
          "is_active": true,
          "is_referenced": true,
          "address": "0x000055e70fcb85d8",
          "localEndpoint": {
            "host": "localhost",
            "ip4": "127.0.0.1", // new key
            "port": 48986
          },
          "remoteEndpoint": {
            "host": "localhost",
            "ip4": "127.0.0.1", // new key
            "port": 38573
          },
          "sendBufferSize": 2626560,
          "recvBufferSize": 131072,
          "fd": 24,
          "writeQueueSize": 0,
          "readable": true,
          "writable": true
        },
        {
          "type": "tcp",
          "is_active": true,
          "is_referenced": true,
          "address": "0x000055e70fcd68c8",
          "localEndpoint": {
            "host": "ip6-localhost",
            "ip6": "::1", // new key
            "port": 52266
          },
          "remoteEndpoint": {
            "host": "ip6-localhost",
            "ip6": "::1", // new key
            "port": 38573
          },
          "sendBufferSize": 2626560,
          "recvBufferSize": 131072,
          "fd": 25,
          "writeQueueSize": 0,
          "readable": false,
          "writable": false
        }
      ]
    }
    json

##### Version 3#

History

Version | Changes
--- | ---
v19.1.0, v18.13.0 | Add more memory info.

The following memory usage keys are added to the `resourceUsage` section.

    {
      "resourceUsage": {
        "rss": "35766272",
        "free_memory": "1598337024",
        "total_memory": "17179869184",
        "available_memory": "1598337024",
        "constrained_memory": "36624662528"
      }
    }
    json

##### Version 2#

History

Version | Changes
--- | ---
v13.9.0, v12.16.2 | Workers are now included in the report.

Added `Worker` support. Refer to Interaction with workers section for more details.

##### Version 1#

This is the first version of the diagnostic report.

### Configuration#

Additional runtime configuration of report generation is available via the following properties of `process.report`:

`reportOnFatalError` triggers diagnostic reporting on fatal errors when `true`. Defaults to `false`.

`reportOnSignal` triggers diagnostic reporting on signal when `true`. This is not supported on Windows. Defaults to `false`.

`reportOnUncaughtException` triggers diagnostic reporting on uncaught exception when `true`. Defaults to `false`.

`signal` specifies the POSIX signal identifier that will be used to intercept external triggers for report generation. Defaults to `'SIGUSR2'`.

`filename` specifies the name of the output file in the file system. Special meaning is attached to `stdout` and `stderr`. Usage of these will result in report being written to the associated standard streams. In cases where standard streams are used, the value in `directory` is ignored. URLs are not supported. Defaults to a composite filename that contains timestamp, PID, and sequence number.

`directory` specifies the file system directory where the report will be written. URLs are not supported. Defaults to the current working directory of the Node.js process.

`excludeNetwork` excludes `header.networkInterfaces` from the diagnostic report.

    // Trigger report only on uncaught exceptions.
    process.report.reportOnFatalError = false;
    process.report.reportOnSignal = false;
    process.report.reportOnUncaughtException = true;

    // Trigger report for both internal errors as well as external signal.
    process.report.reportOnFatalError = true;
    process.report.reportOnSignal = true;
    process.report.reportOnUncaughtException = false;

    // Change the default signal to 'SIGQUIT' and enable it.
    process.report.reportOnFatalError = false;
    process.report.reportOnUncaughtException = false;
    process.report.reportOnSignal = true;
    process.report.signal = 'SIGQUIT';

    // Disable network interfaces reporting
    process.report.excludeNetwork = true;
    js

Configuration on module initialization is also available via environment variables:

    NODE_OPTIONS="--report-uncaught-exception \
      --report-on-fatalerror --report-on-signal \
      --report-signal=SIGUSR2  --report-filename=./report.json \
      --report-directory=/home/nodeuser"
    bash

Specific API documentation can be found under `process API documentation` section.

### Interaction with workers#

History

Version | Changes
--- | ---
v13.9.0, v12.16.2 | Workers are now included in the report.

`Worker` threads can create reports in the same way that the main thread does.

Reports will include information on any Workers that are children of the current thread as part of the `workers` section, with each Worker generating a report in the standard report format.

The thread which is generating the report will wait for the reports from Worker threads to finish. However, the latency for this will usually be low, as both running JavaScript and the event loop are interrupted to generate the report.

## Diagnostics Channel#

Source Code: lib/diagnostics_channel.js Added in: v15.1.0, v14.17.0
History

Version | Changes
--- | ---
v19.2.0, v18.13.0 | diagnostics_channel is now Stable.

Stability: 2 - Stable

The `node:diagnostics_channel` module provides an API to create named channels to report arbitrary message data for diagnostics purposes.

It can be accessed using:

    import diagnostics_channel from 'node:diagnostics_channel';
    const diagnostics_channel = require('node:diagnostics_channel');
    javascript

It is intended that a module writer wanting to report diagnostics messages will create one or many top-level channels to report messages through. Channels may also be acquired at runtime but it is not encouraged due to the additional overhead of doing so. Channels may be exported for convenience, but as long as the name is known it can be acquired anywhere.

If you intend for your module to produce diagnostics data for others to consume it is recommended that you include documentation of what named channels are used along with the shape of the message data. Channel names should generally include the module name to avoid collisions with data from other modules.

### Public API#

#### Overview#

Following is a simple overview of the public API.

    import diagnostics_channel from 'node:diagnostics_channel';

    // Get a reusable channel object
    const channel = diagnostics_channel.channel('my-channel');

    function onMessage(message, name) {
      // Received data
    }

    // Subscribe to the channel
    diagnostics_channel.subscribe('my-channel', onMessage);

    // Check if the channel has an active subscriber
    if (channel.hasSubscribers) {
      // Publish data to the channel
      channel.publish({
        some: 'data',
      });
    }

    // Unsubscribe from the channel
    diagnostics_channel.unsubscribe('my-channel', onMessage);
    const diagnostics_channel = require('node:diagnostics_channel');

    // Get a reusable channel object
    const channel = diagnostics_channel.channel('my-channel');

    function onMessage(message, name) {
      // Received data
    }

    // Subscribe to the channel
    diagnostics_channel.subscribe('my-channel', onMessage);

    // Check if the channel has an active subscriber
    if (channel.hasSubscribers) {
      // Publish data to the channel
      channel.publish({
        some: 'data',
      });
    }

    // Unsubscribe from the channel
    diagnostics_channel.unsubscribe('my-channel', onMessage);
    javascript

##### `diagnostics_channel.hasSubscribers(name)`#

Added in: v15.1.0, v14.17.0

- `name` `<string>` | `<symbol>` The channel name
- Returns: `<boolean>` If there are active subscribers

Check if there are active subscribers to the named channel. This is helpful if the message you want to send might be expensive to prepare.

This API is optional but helpful when trying to publish messages from very performance-sensitive code.

    import diagnostics_channel from 'node:diagnostics_channel';

    if (diagnostics_channel.hasSubscribers('my-channel')) {
      // There are subscribers, prepare and publish message
    }
    const diagnostics_channel = require('node:diagnostics_channel');

    if (diagnostics_channel.hasSubscribers('my-channel')) {
      // There are subscribers, prepare and publish message
    }
    javascript

##### `diagnostics_channel.channel(name)`#

Added in: v15.1.0, v14.17.0

- `name` `<string>` | `<symbol>` The channel name
- Returns: `<Channel>` The named channel object

This is the primary entry-point for anyone wanting to publish to a named channel. It produces a channel object which is optimized to reduce overhead at publish time as much as possible.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channel = diagnostics_channel.channel('my-channel');
    const diagnostics_channel = require('node:diagnostics_channel');

    const channel = diagnostics_channel.channel('my-channel');
    javascript

##### `diagnostics_channel.subscribe(name, onMessage)`#

Added in: v18.7.0, v16.17.0

- `name` `<string>` | `<symbol>` The channel name
- `onMessage` `<Function>` The handler to receive channel messages
  - `message` `<any>` The message data
  - `name` `<string>` | `<symbol>` The name of the channel

Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an `'uncaughtException'`.

    import diagnostics_channel from 'node:diagnostics_channel';

    diagnostics_channel.subscribe('my-channel', (message, name) => {
      // Received data
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    diagnostics_channel.subscribe('my-channel', (message, name) => {
      // Received data
    });
    javascript

##### `diagnostics_channel.unsubscribe(name, onMessage)`#

Added in: v18.7.0, v16.17.0

- `name` `<string>` | `<symbol>` The channel name
- `onMessage` `<Function>` The previous subscribed handler to remove
- Returns: `<boolean>` `true` if the handler was found, `false` otherwise.

Remove a message handler previously registered to this channel with `diagnostics_channel.subscribe(name, onMessage)`.

    import diagnostics_channel from 'node:diagnostics_channel';

    function onMessage(message, name) {
      // Received data
    }

    diagnostics_channel.subscribe('my-channel', onMessage);

    diagnostics_channel.unsubscribe('my-channel', onMessage);
    const diagnostics_channel = require('node:diagnostics_channel');

    function onMessage(message, name) {
      // Received data
    }

    diagnostics_channel.subscribe('my-channel', onMessage);

    diagnostics_channel.unsubscribe('my-channel', onMessage);
    javascript

##### `diagnostics_channel.tracingChannel(nameOrChannels)`#

Added in: v19.9.0, v18.19.0

Stability: 1 - Experimental

- `nameOrChannels` `<string>` | `<TracingChannel>` Channel name or object containing all the TracingChannel Channels
- Returns: `<TracingChannel>` Collection of channels to trace with

Creates a `TracingChannel` wrapper for the given TracingChannel Channels. If a name is given, the corresponding tracing channels will be created in the form of `tracing:${name}:${eventType}` where `eventType` corresponds to the types of TracingChannel Channels.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channelsByName = diagnostics_channel.tracingChannel('my-channel');

    // or...

    const channelsByCollection = diagnostics_channel.tracingChannel({
      start: diagnostics_channel.channel('tracing:my-channel:start'),
      end: diagnostics_channel.channel('tracing:my-channel:end'),
      asyncStart: diagnostics_channel.channel('tracing:my-channel:asyncStart'),
      asyncEnd: diagnostics_channel.channel('tracing:my-channel:asyncEnd'),
      error: diagnostics_channel.channel('tracing:my-channel:error'),
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channelsByName = diagnostics_channel.tracingChannel('my-channel');

    // or...

    const channelsByCollection = diagnostics_channel.tracingChannel({
      start: diagnostics_channel.channel('tracing:my-channel:start'),
      end: diagnostics_channel.channel('tracing:my-channel:end'),
      asyncStart: diagnostics_channel.channel('tracing:my-channel:asyncStart'),
      asyncEnd: diagnostics_channel.channel('tracing:my-channel:asyncEnd'),
      error: diagnostics_channel.channel('tracing:my-channel:error'),
    });
    javascript

##### `diagnostics_channel.boundedChannel(nameOrChannels)`#

Added in: v26.1.0

Stability: 1 - Experimental

- `nameOrChannels` `<string>` | `<BoundedChannel>` Channel name or object containing all the BoundedChannel Channels
- Returns: `<BoundedChannel>` Collection of channels to trace with

Creates a `BoundedChannel` wrapper for the given channels. If a name is given, the corresponding channels will be created in the form of `tracing:${name}:${eventType}` where `eventType` is `start` or `end`.

A `BoundedChannel` is a simplified version of `TracingChannel` that only traces synchronous operations. It only has `start` and `end` events, without `asyncStart`, `asyncEnd`, or `error` events, making it suitable for tracing operations that don't involve asynchronous continuations or error handling.

    import { boundedChannel, channel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    // or...

    const wc2 = boundedChannel({
      start: channel('tracing:my-operation:start'),
      end: channel('tracing:my-operation:end'),
    });
    const { boundedChannel, channel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    // or...

    const wc2 = boundedChannel({
      start: channel('tracing:my-operation:start'),
      end: channel('tracing:my-operation:end'),
    });
    javascript

#### Class: `Channel`#

Added in: v15.1.0, v14.17.0

The class `Channel` represents an individual named channel within the data pipeline. It is used to track subscribers and to publish messages when there are subscribers present. It exists as a separate object to avoid channel lookups at publish time, enabling very fast publish speeds and allowing for heavy use while incurring very minimal cost. Channels are created with `diagnostics_channel.channel(name)`, constructing a channel directly with `new Channel(name)` is not supported.

##### `channel.hasSubscribers`#

Added in: v15.1.0, v14.17.0

- Returns: `<boolean>` If there are active subscribers

Check if there are active subscribers to this channel. This is helpful if the message you want to send might be expensive to prepare.

This API is optional but helpful when trying to publish messages from very performance-sensitive code.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channel = diagnostics_channel.channel('my-channel');

    if (channel.hasSubscribers) {
      // There are subscribers, prepare and publish message
    }
    const diagnostics_channel = require('node:diagnostics_channel');

    const channel = diagnostics_channel.channel('my-channel');

    if (channel.hasSubscribers) {
      // There are subscribers, prepare and publish message
    }
    javascript

##### `channel.publish(message)`#

Added in: v15.1.0, v14.17.0

- `message` `<any>` The message to send to the channel subscribers

Publish a message to any subscribers to the channel. This will trigger message handlers synchronously so they will execute within the same context.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channel = diagnostics_channel.channel('my-channel');

    channel.publish({
      some: 'message',
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channel = diagnostics_channel.channel('my-channel');

    channel.publish({
      some: 'message',
    });
    javascript

##### `channel.subscribe(onMessage)`#

Added in: v15.1.0, v14.17.0
History

Version | Changes
--- | ---
v24.8.0, v22.20.0 | Deprecation revoked.
v18.7.0, v16.17.0 | Documentation-only deprecation.

- `onMessage` `<Function>` The handler to receive channel messages
  - `message` `<any>` The message data
  - `name` `<string>` | `<symbol>` The name of the channel

Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an `'uncaughtException'`.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channel = diagnostics_channel.channel('my-channel');

    channel.subscribe((message, name) => {
      // Received data
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channel = diagnostics_channel.channel('my-channel');

    channel.subscribe((message, name) => {
      // Received data
    });
    javascript

##### `channel.unsubscribe(onMessage)`#

Added in: v15.1.0, v14.17.0
History

Version | Changes
--- | ---
v24.8.0, v22.20.0 | Deprecation revoked.
v18.7.0, v16.17.0 | Documentation-only deprecation.
v17.1.0, v16.14.0, v14.19.0 | Added return value. Added to channels without subscribers.

- `onMessage` `<Function>` The previous subscribed handler to remove
- Returns: `<boolean>` `true` if the handler was found, `false` otherwise.

Remove a message handler previously registered to this channel with `channel.subscribe(onMessage)`.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channel = diagnostics_channel.channel('my-channel');

    function onMessage(message, name) {
      // Received data
    }

    channel.subscribe(onMessage);

    channel.unsubscribe(onMessage);
    const diagnostics_channel = require('node:diagnostics_channel');

    const channel = diagnostics_channel.channel('my-channel');

    function onMessage(message, name) {
      // Received data
    }

    channel.subscribe(onMessage);

    channel.unsubscribe(onMessage);
    javascript

##### `channel.bindStore(store[, transform])`#

Added in: v19.9.0, v18.19.0

Stability: 1 - Experimental

- `store` `<AsyncLocalStorage>` The store to which to bind the context data
- `transform` `<Function>` Transform context data before setting the store context

When `channel.runStores(context, ...)` is called, the given context data will be applied to any store bound to the channel. If the store has already been bound the previous `transform` function will be replaced with the new one. The `transform` function may be omitted to set the given context data as the context directly.

    import diagnostics_channel from 'node:diagnostics_channel';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const store = new AsyncLocalStorage();

    const channel = diagnostics_channel.channel('my-channel');

    channel.bindStore(store, (data) => {
      return { data };
    });
    const diagnostics_channel = require('node:diagnostics_channel');
    const { AsyncLocalStorage } = require('node:async_hooks');

    const store = new AsyncLocalStorage();

    const channel = diagnostics_channel.channel('my-channel');

    channel.bindStore(store, (data) => {
      return { data };
    });
    javascript

##### `channel.unbindStore(store)`#

Added in: v19.9.0, v18.19.0

Stability: 1 - Experimental

- `store` `<AsyncLocalStorage>` The store to unbind from the channel.
- Returns: `<boolean>` `true` if the store was found, `false` otherwise.

Remove a message handler previously registered to this channel with `channel.bindStore(store)`.

    import diagnostics_channel from 'node:diagnostics_channel';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const store = new AsyncLocalStorage();

    const channel = diagnostics_channel.channel('my-channel');

    channel.bindStore(store);
    channel.unbindStore(store);
    const diagnostics_channel = require('node:diagnostics_channel');
    const { AsyncLocalStorage } = require('node:async_hooks');

    const store = new AsyncLocalStorage();

    const channel = diagnostics_channel.channel('my-channel');

    channel.bindStore(store);
    channel.unbindStore(store);
    javascript

##### `channel.runStores(context, fn[, thisArg[, ...args]])`#

Added in: v19.9.0, v18.19.0

Stability: 1 - Experimental

- `context` `<any>` Message to send to subscribers and bind to stores
- `fn` `<Function>` Handler to run within the entered storage context
- `thisArg` `<any>` The receiver to be used for the function call.
- `...args` `<any>` Optional arguments to pass to the function.

Applies the given data to any AsyncLocalStorage instances bound to the channel for the duration of the given function, then publishes to the channel within the scope of that data is applied to the stores.

If a transform function was given to `channel.bindStore(store)` it will be applied to transform the message data before it becomes the context value for the store. The prior storage context is accessible from within the transform function in cases where context linking is required.

The context applied to the store should be accessible in any async code which continues from execution which began during the given function, however there are some situations in which context loss may occur.

    import diagnostics_channel from 'node:diagnostics_channel';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const store = new AsyncLocalStorage();

    const channel = diagnostics_channel.channel('my-channel');

    channel.bindStore(store, (message) => {
      const parent = store.getStore();
      return new Span(message, parent);
    });
    channel.runStores({ some: 'message' }, () => {
      store.getStore(); // Span({ some: 'message' })
    });
    const diagnostics_channel = require('node:diagnostics_channel');
    const { AsyncLocalStorage } = require('node:async_hooks');

    const store = new AsyncLocalStorage();

    const channel = diagnostics_channel.channel('my-channel');

    channel.bindStore(store, (message) => {
      const parent = store.getStore();
      return new Span(message, parent);
    });
    channel.runStores({ some: 'message' }, () => {
      store.getStore(); // Span({ some: 'message' })
    });
    javascript

##### `channel.withStoreScope(data)`#

Added in: v26.1.0

Stability: 1 - Experimental

- `data` `<any>` Message to bind to stores
- Returns: `<RunStoresScope>` Disposable scope object

Creates a disposable scope that binds the given data to any AsyncLocalStorage instances bound to the channel and publishes it to subscribers. The scope automatically restores the previous storage contexts when disposed.

This method enables the use of JavaScript's explicit resource management (`using` syntax with `Symbol.dispose`) to manage store contexts without closure wrapping.

    import { channel } from 'node:diagnostics_channel';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const store = new AsyncLocalStorage();
    const ch = channel('my-channel');

    ch.bindStore(store, (message) => {
      return { ...message, timestamp: Date.now() };
    });

    {
      using scope = ch.withStoreScope({ request: 'data' });
      // Store is entered, data is published
      console.log(store.getStore()); // { request: 'data', timestamp: ... }
    }
    // Store is automatically restored on scope exit
    const { channel } = require('node:diagnostics_channel');
    const { AsyncLocalStorage } = require('node:async_hooks');

    const store = new AsyncLocalStorage();
    const ch = channel('my-channel');

    ch.bindStore(store, (message) => {
      return { ...message, timestamp: Date.now() };
    });

    {
      using scope = ch.withStoreScope({ request: 'data' });
      // Store is entered, data is published
      console.log(store.getStore()); // { request: 'data', timestamp: ... }
    }
    // Store is automatically restored on scope exit
    javascript

#### Class: `RunStoresScope`#

Added in: v26.1.0

Stability: 1 - Experimental

The class `RunStoresScope` represents a disposable scope created by `channel.withStoreScope(data)`. It manages the lifecycle of store contexts and ensures they are properly restored when the scope exits.

The scope must be used with the `using` syntax to ensure proper disposal.

#### Class: `TracingChannel`#

Added in: v19.9.0, v18.19.0

Stability: 1 - Experimental

The class `TracingChannel` is a collection of TracingChannel Channels which together express a single traceable action. It is used to formalize and simplify the process of producing events for tracing application flow. `diagnostics_channel.tracingChannel()` is used to construct a `TracingChannel`. As with `Channel` it is recommended to create and reuse a single `TracingChannel` at the top-level of the file rather than creating them dynamically.

##### `tracingChannel.subscribe(subscribers)`#

Added in: v19.9.0, v18.19.0

- `subscribers` `<Object>` Set of TracingChannel Channels subscribers
  - `start` `<Function>` The `start` event subscriber
  - `end` `<Function>` The `end` event subscriber
  - `asyncStart` `<Function>` The `asyncStart` event subscriber
  - `asyncEnd` `<Function>` The `asyncEnd` event subscriber
  - `error` `<Function>` The `error` event subscriber

Helper to subscribe a collection of functions to the corresponding channels. This is the same as calling `channel.subscribe(onMessage)` on each channel individually.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.subscribe({
      start(message) {
        // Handle start message
      },
      end(message) {
        // Handle end message
      },
      asyncStart(message) {
        // Handle asyncStart message
      },
      asyncEnd(message) {
        // Handle asyncEnd message
      },
      error(message) {
        // Handle error message
      },
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.subscribe({
      start(message) {
        // Handle start message
      },
      end(message) {
        // Handle end message
      },
      asyncStart(message) {
        // Handle asyncStart message
      },
      asyncEnd(message) {
        // Handle asyncEnd message
      },
      error(message) {
        // Handle error message
      },
    });
    javascript

##### `tracingChannel.unsubscribe(subscribers)`#

Added in: v19.9.0, v18.19.0

- `subscribers` `<Object>` Set of TracingChannel Channels subscribers
  - `start` `<Function>` The `start` event subscriber
  - `end` `<Function>` The `end` event subscriber
  - `asyncStart` `<Function>` The `asyncStart` event subscriber
  - `asyncEnd` `<Function>` The `asyncEnd` event subscriber
  - `error` `<Function>` The `error` event subscriber
- Returns: `<boolean>` `true` if all handlers were successfully unsubscribed, and `false` otherwise.

Helper to unsubscribe a collection of functions from the corresponding channels. This is the same as calling `channel.unsubscribe(onMessage)` on each channel individually.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.unsubscribe({
      start(message) {
        // Handle start message
      },
      end(message) {
        // Handle end message
      },
      asyncStart(message) {
        // Handle asyncStart message
      },
      asyncEnd(message) {
        // Handle asyncEnd message
      },
      error(message) {
        // Handle error message
      },
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.unsubscribe({
      start(message) {
        // Handle start message
      },
      end(message) {
        // Handle end message
      },
      asyncStart(message) {
        // Handle asyncStart message
      },
      asyncEnd(message) {
        // Handle asyncEnd message
      },
      error(message) {
        // Handle error message
      },
    });
    javascript

##### `tracingChannel.traceSync(fn[, context[, thisArg[, ...args]]])`#

Added in: v19.9.0, v18.19.0

- `fn` `<Function>` Function to wrap a trace around
- `context` `<Object>` Shared object to correlate events through
- `thisArg` `<any>` The receiver to be used for the function call
- `...args` `<any>` Optional arguments to pass to the function
- Returns: `<any>` The return value of the given function

Trace a synchronous function call. This will always produce a `start` event and `end` event around the execution and may produce an `error` event if the given function throws an error. This will run the given function using `channel.runStores(context, ...)` on the `start` channel which ensures all events should have any bound stores set to match this trace context.

To ensure only correct trace graphs are formed, events will only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins will not receive future events from that trace, only future traces will be seen.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.traceSync(() => {
      // Do something
    }, {
      some: 'thing',
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.traceSync(() => {
      // Do something
    }, {
      some: 'thing',
    });
    javascript

##### `tracingChannel.tracePromise(fn[, context[, thisArg[, ...args]]])`#

Added in: v19.9.0, v18.19.0
History

Version | Changes
--- | ---
v26.5.0 | Non-native-Promise thenables are now returned as-is, preserving their original type and methods.
v26.0.0 | Non-thenables will be returned with a warning.

- `fn` `<Function>` Function to wrap a trace around
- `context` `<Object>` Shared object to correlate trace events through
- `thisArg` `<any>` The receiver to be used for the function call
- `...args` `<any>` Optional arguments to pass to the function
- Returns: `<any>` The return value of the given function. If the return value is a Promise or thenable, tracing events will be published when it settles. If the return value is not a Promise or thenable, it is returned as-is and a warning is emitted.

Trace an asynchronous function call which returns a `<Promise>` or thenable object. This will always produce a `start` event and `end` event around the synchronous portion of the function execution, and will produce an `asyncStart` event and `asyncEnd` event when the returned promise is resolved or rejected. It may also produce an `error` event if the given function throws an error or the returned promise is rejected. This will run the given function using `channel.runStores(context, ...)` on the `start` channel which ensures all events should have any bound stores set to match this trace context.

If the value returned by `fn` is not a Promise or thenable, then it will be returned with a warning, and no `asyncStart` or `asyncEnd` events will be produced.

To ensure only correct trace graphs are formed, events will only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins will not receive future events from that trace, only future traces will be seen.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.tracePromise(async () => {
      // Do something
    }, {
      some: 'thing',
    });
    const diagnostics_channel = require('node:diagnostics_channel');

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.tracePromise(async () => {
      // Do something
    }, {
      some: 'thing',
    });
    javascript

##### `tracingChannel.traceCallback(fn[, position[, context[, thisArg[, ...args]]]])`#

Added in: v19.9.0, v18.19.0

- `fn` `<Function>` callback using function to wrap a trace around
- `position` `<number>` Zero-indexed argument position of expected callback (defaults to last argument if `undefined` is passed)
- `context` `<Object>` Shared object to correlate trace events through (defaults to `{}` if `undefined` is passed)
- `thisArg` `<any>` The receiver to be used for the function call
- `...args` `<any>` arguments to pass to the function (must include the callback)
- Returns: `<any>` The return value of the given function

Trace a callback-receiving function call. The callback is expected to follow the error as first arg convention typically used. This will always produce a `start` event and `end` event around the synchronous portion of the function execution, and will produce a `asyncStart` event and `asyncEnd` event around the callback execution. It may also produce an `error` event if the given function throws or the first argument passed to the callback is set. This will run the given function using `channel.runStores(context, ...)` on the `start` channel which ensures all events should have any bound stores set to match this trace context.

To ensure only correct trace graphs are formed, events will only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins will not receive future events from that trace, only future traces will be seen.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.traceCallback((arg1, callback) => {
      // Do something
      callback(null, 'result');
    }, 1, {
      some: 'thing',
    }, thisArg, arg1, callback);
    const diagnostics_channel = require('node:diagnostics_channel');

    const channels = diagnostics_channel.tracingChannel('my-channel');

    channels.traceCallback((arg1, callback) => {
      // Do something
      callback(null, 'result');
    }, 1, {
      some: 'thing',
    }, thisArg, arg1, callback);
    javascript

The callback will also be run with `channel.runStores(context, ...)` which enables context loss recovery in some cases.

    import diagnostics_channel from 'node:diagnostics_channel';
    import { AsyncLocalStorage } from 'node:async_hooks';

    const channels = diagnostics_channel.tracingChannel('my-channel');
    const myStore = new AsyncLocalStorage();

    // The start channel sets the initial store data to something
    // and stores that store data value on the trace context object
    channels.start.bindStore(myStore, (data) => {
      const span = new Span(data);
      data.span = span;
      return span;
    });

    // Then asyncStart can restore from that data it stored previously
    channels.asyncStart.bindStore(myStore, (data) => {
      return data.span;
    });
    const diagnostics_channel = require('node:diagnostics_channel');
    const { AsyncLocalStorage } = require('node:async_hooks');

    const channels = diagnostics_channel.tracingChannel('my-channel');
    const myStore = new AsyncLocalStorage();

    // The start channel sets the initial store data to something
    // and stores that store data value on the trace context object
    channels.start.bindStore(myStore, (data) => {
      const span = new Span(data);
      data.span = span;
      return span;
    });

    // Then asyncStart can restore from that data it stored previously
    channels.asyncStart.bindStore(myStore, (data) => {
      return data.span;
    });
    javascript

##### `tracingChannel.hasSubscribers`#

Added in: v22.0.0, v20.13.0

- Returns: `<boolean>` `true` if any of the individual channels has a subscriber, `false` if not.

This is a helper method available on a `TracingChannel` instance to check if any of the TracingChannel Channels have subscribers. A `true` is returned if any of them have at least one subscriber, a `false` is returned otherwise.

    import diagnostics_channel from 'node:diagnostics_channel';

    const channels = diagnostics_channel.tracingChannel('my-channel');

    if (channels.hasSubscribers) {
      // Do something
    }
    const diagnostics_channel = require('node:diagnostics_channel');

    const channels = diagnostics_channel.tracingChannel('my-channel');

    if (channels.hasSubscribers) {
      // Do something
    }
    javascript

#### Class: `BoundedChannel`#

Added in: v26.1.0

Stability: 1 - Experimental

The class `BoundedChannel` is a simplified version of `TracingChannel` that only traces synchronous operations. It consists of two channels (`start` and `end`) instead of five, omitting the `asyncStart`, `asyncEnd`, and `error` events. This makes it suitable for tracing operations that don't involve asynchronous continuations or error handling.

Like `TracingChannel`, it is recommended to create and reuse a single `BoundedChannel` at the top-level of the file rather than creating them dynamically.

##### `boundedChannel.hasSubscribers`#

Added in: v26.1.0

- Returns: `<boolean>` `true` if any of the individual channels has a subscriber, `false` if not.

Check if any of the `start` or `end` channels have subscribers.

    import { boundedChannel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    if (wc.hasSubscribers) {
      // There are subscribers, perform traced operation
    }
    const { boundedChannel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    if (wc.hasSubscribers) {
      // There are subscribers, perform traced operation
    }
    javascript

##### `boundedChannel.subscribe(handlers)`#

Added in: v26.1.0

- `handlers` `<Object>` Set of channel subscribers
  - `start` `<Function>` The start event subscriber
  - `end` `<Function>` The end event subscriber

Subscribe to the bounded channel events. This is equivalent to calling `channel.subscribe(onMessage)` on each channel individually.

    import { boundedChannel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    wc.subscribe({
      start(message) {
        // Handle start
      },
      end(message) {
        // Handle end
      },
    });
    const { boundedChannel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    wc.subscribe({
      start(message) {
        // Handle start
      },
      end(message) {
        // Handle end
      },
    });
    javascript

##### `boundedChannel.unsubscribe(handlers)`#

Added in: v26.1.0

- `handlers` `<Object>` Set of channel subscribers
  - `start` `<Function>` The start event subscriber
  - `end` `<Function>` The end event subscriber
- Returns: `<boolean>` `true` if all handlers were successfully unsubscribed, `false` otherwise.

Unsubscribe from the bounded channel events. This is equivalent to calling `channel.unsubscribe(onMessage)` on each channel individually.

    import { boundedChannel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    const handlers = {
      start(message) {},
      end(message) {},
    };

    wc.subscribe(handlers);
    wc.unsubscribe(handlers);
    const { boundedChannel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    const handlers = {
      start(message) {},
      end(message) {},
    };

    wc.subscribe(handlers);
    wc.unsubscribe(handlers);
    javascript

##### `boundedChannel.run(context, fn[, thisArg[, ...args]])`#

Added in: v26.1.0

- `context` `<Object>` Shared object to correlate events through
- `fn` `<Function>` Function to wrap a trace around
- `thisArg` `<any>` The receiver to be used for the function call
- `...args` `<any>` Optional arguments to pass to the function
- Returns: `<any>` The return value of the given function

Trace a synchronous function call. This will produce a `start` event and `end` event around the execution. This runs the given function using `channel.runStores(context, ...)` on the `start` channel which ensures all events have any bound stores set to match this trace context.

    import { boundedChannel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    const result = wc.run({ operationId: '123' }, () => {
      // Perform operation
      return 42;
    });
    const { boundedChannel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    const result = wc.run({ operationId: '123' }, () => {
      // Perform operation
      return 42;
    });
    javascript

##### `boundedChannel.withScope([context])`#

Added in: v26.1.0

- `context` `<Object>` Shared object to correlate events through
- Returns: `<BoundedChannelScope>` Disposable scope object

Create a disposable scope for tracing a synchronous operation using JavaScript's explicit resource management (`using` syntax). The scope automatically publishes `start` and `end` events, enters bound stores, and handles cleanup when disposed.

    import { boundedChannel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    const context = { operationId: '123' };
    {
      using scope = wc.withScope(context);
      // Stores are entered, start event is published

      // Perform work and set result on context
      context.result = 42;
    }
    // End event is published, stores are restored automatically
    const { boundedChannel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    const context = { operationId: '123' };
    {
      using scope = wc.withScope(context);
      // Stores are entered, start event is published

      // Perform work and set result on context
      context.result = 42;
    }
    // End event is published, stores are restored automatically
    javascript

#### Class: `BoundedChannelScope`#

Added in: v26.1.0

Stability: 1 - Experimental

The class `BoundedChannelScope` represents a disposable scope created by `boundedChannel.withScope(context)`. It manages the lifecycle of a traced operation, automatically publishing events and managing store contexts.

The scope must be used with the `using` syntax to ensure proper disposal.

    import { boundedChannel } from 'node:diagnostics_channel';

    const wc = boundedChannel('my-operation');

    const context = {};
    {
      using scope = wc.withScope(context);
      // Start event is published, stores are entered
      context.result = performOperation();
      // End event is automatically published at end of block
    }
    const { boundedChannel } = require('node:diagnostics_channel');

    const wc = boundedChannel('my-operation');

    const context = {};
    {
      using scope = wc.withScope(context);
      // Start event is published, stores are entered
      context.result = performOperation();
      // End event is automatically published at end of block
    }
    javascript

#### BoundedChannel Channels#

A `BoundedChannel` consists of two diagnostics channels representing the lifecycle of a scope created with the `using` syntax:

- `tracing:${name}:start` - Published when the `using` statement executes (scope creation)
- `tracing:${name}:end` - Published when exiting the block (scope disposal)

When using the `using` syntax with [`boundedChannel.withScope([context])`][], the `start` event is published immediately when the statement executes, and the `end` event is automatically published when disposal occurs at the end of the block. All events share the same context object, which can be extended with additional properties like `result` during scope execution.

#### TracingChannel Channels#

A TracingChannel is a collection of several diagnostics_channels representing specific points in the execution lifecycle of a single traceable action. The behavior is split into five diagnostics_channels consisting of `start`, `end`, `asyncStart`, `asyncEnd`, and `error`. A single traceable action will share the same event object between all events, this can be helpful for managing correlation through a weakmap.

These event objects will be extended with `result` or `error` values when the task "completes". In the case of a synchronous task the `result` will be the return value and the `error` will be anything thrown from the function. With callback-based async functions the `result` will be the second argument of the callback while the `error` will either be a thrown error visible in the `end` event or the first callback argument in either of the `asyncStart` or `asyncEnd` events.

To ensure only correct trace graphs are formed, events should only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins should not receive future events from that trace, only future traces will be seen.

Tracing channels should follow a naming pattern of:

- `tracing:module.class.method:start` or `tracing:module.function:start`
- `tracing:module.class.method:end` or `tracing:module.function:end`
- `tracing:module.class.method:asyncStart` or `tracing:module.function:asyncStart`
- `tracing:module.class.method:asyncEnd` or `tracing:module.function:asyncEnd`
- `tracing:module.class.method:error` or `tracing:module.function:error`

##### `start(event)`#

- Name: `tracing:${name}:start`

The `start` event represents the point at which a function is called. At this point the event data may contain function arguments or anything else available at the very start of the execution of the function.

##### `end(event)`#

- Name: `tracing:${name}:end`

The `end` event represents the point at which a function call returns a value. In the case of an async function this is when the promise returned not when the function itself makes a return statement internally. At this point, if the traced function was synchronous the `result` field will be set to the return value of the function. Alternatively, the `error` field may be present to represent any thrown errors.

It is recommended to listen specifically to the `error` event to track errors as it may be possible for a traceable action to produce multiple errors. For example, an async task which fails may be started internally before the sync part of the task then throws an error.

##### `asyncStart(event)`#

- Name: `tracing:${name}:asyncStart`

The `asyncStart` event represents the callback or continuation of a traceable function being reached. At this point things like callback arguments may be available, or anything else expressing the "result" of the action.

For callbacks-based functions, the first argument of the callback will be assigned to the `error` field, if not `undefined` or `null`, and the second argument will be assigned to the `result` field.

For promises, the argument to the `resolve` path will be assigned to `result` or the argument to the `reject` path will be assign to `error`.

It is recommended to listen specifically to the `error` event to track errors as it may be possible for a traceable action to produce multiple errors. For example, an async task which fails may be started internally before the sync part of the task then throws an error.

##### `asyncEnd(event)`#

- Name: `tracing:${name}:asyncEnd`

The `asyncEnd` event represents the callback of an asynchronous function returning. It's not likely event data will change after the `asyncStart` event, however it may be useful to see the point where the callback completes.

##### `error(event)`#

- Name: `tracing:${name}:error`

The `error` event represents any error produced by the traceable function either synchronously or asynchronously. If an error is thrown in the synchronous portion of the traced function the error will be assigned to the `error` field of the event and the `error` event will be triggered. If an error is received asynchronously through a callback or promise rejection it will also be assigned to the `error` field of the event and trigger the `error` event.

It is possible for a single traceable function call to produce errors multiple times so this should be considered when consuming this event. For example, if another async task is triggered internally which fails and then the sync part of the function then throws and error two `error` events will be emitted, one for the sync error and one for the async error.

#### Built-in Channels#

##### Console#

Stability: 1 - Experimental

###### Event: `'console.log'`#

- `args` `<any>`[]

Emitted when `console.log()` is called. Receives and array of the arguments passed to `console.log()`.

###### Event: `'console.info'`#

- `args` `<any>`[]

Emitted when `console.info()` is called. Receives and array of the arguments passed to `console.info()`.

###### Event: `'console.debug'`#

- `args` `<any>`[]

Emitted when `console.debug()` is called. Receives and array of the arguments passed to `console.debug()`.

###### Event: `'console.warn'`#

- `args` `<any>`[]

Emitted when `console.warn()` is called. Receives and array of the arguments passed to `console.warn()`.

###### Event: `'console.error'`#

- `args` `<any>`[]

Emitted when `console.error()` is called. Receives and array of the arguments passed to `console.error()`.

##### HTTP#

Stability: 1 - Experimental

###### Event: `'http.client.request.created'`#

- `request` `<http.ClientRequest>`

Emitted when client creates a request object. Unlike `http.client.request.start`, this event is emitted before the request has been sent.

###### Event: `'http.client.request.start'`#

- `request` `<http.ClientRequest>`

Emitted when client starts a request.

###### Event: `'http.client.request.error'`#

- `request` `<http.ClientRequest>`
- `error` `<Error>`

Emitted when an error occurs during a client request.

###### Event: `'http.client.response.finish'`#

- `request` `<http.ClientRequest>`
- `response` `<http.IncomingMessage>`

Emitted when client receives a response.

###### Event: `'http.server.request.start'`#

- `request` `<http.IncomingMessage>`
- `response` `<http.ServerResponse>`
- `socket` `<net.Socket>`
- `server` `<http.Server>`

Emitted when server receives a request.

###### Event: `'http.server.response.created'`#

- `request` `<http.IncomingMessage>`
- `response` `<http.ServerResponse>`

Emitted when server creates a response. The event is emitted before the response is sent.

###### Event: `'http.server.response.finish'`#

- `request` `<http.IncomingMessage>`
- `response` `<http.ServerResponse>`
- `socket` `<net.Socket>`
- `server` `<http.Server>`

Emitted when server sends a response.

##### HTTP/2#

Stability: 1 - Experimental

###### Event: `'http2.client.stream.created'`#

- `stream` `<ClientHttp2Stream>`
- `headers` `<HTTP/2 Headers Object>`

Emitted when a stream is created on the client.

###### Event: `'http2.client.stream.start'`#

- `stream` `<ClientHttp2Stream>`
- `headers` `<HTTP/2 Headers Object>`

Emitted when a stream is started on the client.

###### Event: `'http2.client.stream.error'`#

- `stream` `<ClientHttp2Stream>`
- `error` `<Error>`

Emitted when an error occurs during the processing of a stream on the client.

###### Event: `'http2.client.stream.finish'`#

- `stream` `<ClientHttp2Stream>`
- `headers` `<HTTP/2 Headers Object>`
- `flags` `<number>`

Emitted when a stream is received on the client.

###### Event: `'http2.client.stream.bodyChunkSent'`#

- `stream` `<ClientHttp2Stream>`
- `writev` `<boolean>`
- `data` `<Buffer>` | `<string>` | `<Buffer>`[] | `<Object>`[]
  - `chunk` `<Buffer>` | `<string>`
  - `encoding` `<string>`
- `encoding` `<string>`

Emitted when a chunk of the client stream body is being sent.

###### Event: `'http2.client.stream.bodySent'`#

- `stream` `<ClientHttp2Stream>`

Emitted after the client stream body has been fully sent.

###### Event: `'http2.client.stream.close'`#

- `stream` `<ClientHttp2Stream>`

Emitted when a stream is closed on the client. The HTTP/2 error code used when closing the stream can be retrieved using the `stream.rstCode` property.

###### Event: `'http2.server.stream.created'`#

- `stream` `<ServerHttp2Stream>`
- `headers` `<HTTP/2 Headers Object>`

Emitted when a stream is created on the server.

###### Event: `'http2.server.stream.start'`#

- `stream` `<ServerHttp2Stream>`
- `headers` `<HTTP/2 Headers Object>`

Emitted when a stream is started on the server.

###### Event: `'http2.server.stream.error'`#

- `stream` `<ServerHttp2Stream>`
- `error` `<Error>`

Emitted when an error occurs during the processing of a stream on the server.

###### Event: `'http2.server.stream.finish'`#

- `stream` `<ServerHttp2Stream>`
- `headers` `<HTTP/2 Headers Object>`
- `flags` `<number>`

Emitted when a stream is sent on the server.

###### Event: `'http2.server.stream.close'`#

- `stream` `<ServerHttp2Stream>`

Emitted when a stream is closed on the server. The HTTP/2 error code used when closing the stream can be retrieved using the `stream.rstCode` property.

##### Modules#

Stability: 1 - Experimental

###### Event: `'tracing:module.require:start'`#

- `event` `<Object>` containing the following properties
  - `id` Argument passed to `require()`. Module name.
  - `parentFilename` Name of the module that attempted to require(id).

Emitted when `require()` is executed. See `start` event.

###### Event: `'tracing:module.require:end'`#

- `event` `<Object>` containing the following properties
  - `id` Argument passed to `require()`. Module name.
  - `parentFilename` Name of the module that attempted to require(id).

Emitted when a `require()` call returns. See `end` event.

###### Event: `'tracing:module.require:error'`#

- `event` `<Object>` containing the following properties
  - `id` Argument passed to `require()`. Module name.
  - `parentFilename` Name of the module that attempted to require(id).
- `error` `<Error>`

Emitted when a `require()` throws an error. See `error` event.

###### Event: `'tracing:module.import:asyncStart'`#

- `event` `<Object>` containing the following properties
  - `id` Argument passed to `import()`. Module name.
  - `parentURL` URL object of the module that attempted to import(id).

Emitted when `import()` is invoked. See `asyncStart` event.

###### Event: `'tracing:module.import:asyncEnd'`#

- `event` `<Object>` containing the following properties
  - `id` Argument passed to `import()`. Module name.
  - `parentURL` URL object of the module that attempted to import(id).

Emitted when `import()` has completed. See `asyncEnd` event.

###### Event: `'tracing:module.import:error'`#

- `event` `<Object>` containing the following properties
  - `id` Argument passed to `import()`. Module name.
  - `parentURL` URL object of the module that attempted to import(id).
- `error` `<Error>`

Emitted when a `import()` throws an error. See `error` event.

##### NET#

Stability: 1 - Experimental

###### Event: `'net.client.socket'`#

- `socket` `<net.Socket>` | `<tls.TLSSocket>`

Emitted when a new TCP or pipe client socket connection is created.

###### Event: `'net.server.socket'`#

- `socket` `<net.Socket>`

Emitted when a new TCP or pipe connection is received.

###### Event: `'tracing:net.server.listen:asyncStart'`#

- `server` `<net.Server>`
- `options` `<Object>`

Emitted when `net.Server.listen()` is invoked, before the port or pipe is actually setup.

###### Event: `'tracing:net.server.listen:asyncEnd'`#

- `server` `<net.Server>`

Emitted when `net.Server.listen()` has completed and thus the server is ready to accept connection.

###### Event: `'tracing:net.server.listen:error'`#

- `server` `<net.Server>`
- `error` `<Error>`

Emitted when `net.Server.listen()` is returning an error.

##### UDP#

Stability: 1 - Experimental

###### Event: `'udp.socket'`#

- `socket` `<dgram.Socket>`

Emitted when a new UDP socket is created.

##### Process#

Added in: v16.18.0

Stability: 1 - Experimental

###### Event: `'child_process'`#

- `process` `<ChildProcess>`

Emitted when a new process is created.

`tracing:child_process.spawn:start`

- `process` `<ChildProcess>`
- `options` `<Object>`

Emitted when `child_process.spawn()` is invoked, before the process is actually spawned.

`tracing:child_process.spawn:end`

- `process` `<ChildProcess>`

Emitted when `child_process.spawn()` has completed successfully and the process has been created.

`tracing:child_process.spawn:error`

- `process` `<ChildProcess>`
- `error` `<Error>`

Emitted when `child_process.spawn()` encounters an error.

###### Event: `'process.execve'`#

- `execPath` `<string>`
- `args` `<string>`[]
- `env` `<string>`[]

Emitted when `process.execve()` is invoked.

##### Web Locks#

Added in: v25.9.0

Stability: 1 - Experimental

These channels are emitted for each `locks.request()` call. See `worker_threads.locks` for details on Web Locks.

###### Event: `'locks.request.start'`#

- `name` `<string>` The name of the requested lock resource.
- `mode` `<string>` The lock mode: `'exclusive'` or `'shared'`.

Emitted when a lock request is initiated, before the lock is granted.

###### Event: `'locks.request.grant'`#

- `name` `<string>` The name of the requested lock resource.
- `mode` `<string>` The lock mode: `'exclusive'` or `'shared'`.

Emitted when a lock is successfully granted and the callback is about to run.

###### Event: `'locks.request.miss'`#

- `name` `<string>` The name of the requested lock resource.
- `mode` `<string>` The lock mode: `'exclusive'` or `'shared'`.

Emitted when `ifAvailable` is `true` and the lock is not immediately available, and the request callback is invoked with `null` instead of a `Lock` object.

###### Event: `'locks.request.end'`#

- `name` `<string>` The name of the requested lock resource.
- `mode` `<string>` The lock mode: `'exclusive'` or `'shared'`.
- `steal` `<boolean>` Whether the request uses steal semantics.
- `ifAvailable` `<boolean>` Whether the request uses ifAvailable semantics.
- `error` `<Error>` | `<undefined>` The error thrown by the callback, if any.

Emitted when a lock request has finished, whether the callback succeeded, threw an error, or the lock was stolen.

##### Worker Thread#

Added in: v16.18.0

Stability: 1 - Experimental

###### Event: `'worker_threads'`#

- `worker` `<Worker>`

Emitted when a new thread is created.

## DNS#

Source Code: lib/dns.js

Stability: 2 - Stable

The `node:dns` module enables name resolution. For example, use it to look up IP addresses of host names.

Although named for the Domain Name System (DNS), it does not always use the DNS protocol for lookups. `dns.lookup()` uses the operating system facilities to perform name resolution. It may not need to perform any network communication. To perform name resolution the way other applications on the same system do, use `dns.lookup()`.

    import dns from 'node:dns';

    dns.lookup('example.org', (err, address, family) => {
      console.log('address: %j family: IPv%s', address, family);
    });
    // address: "2606:2800:21f:cb07:6820:80da:af6b:8b2c" family: IPv6
    const dns = require('node:dns');

    dns.lookup('example.org', (err, address, family) => {
      console.log('address: %j family: IPv%s', address, family);
    });
    // address: "2606:2800:21f:cb07:6820:80da:af6b:8b2c" family: IPv6
    javascript

All other functions in the `node:dns` module connect to an actual DNS server to perform name resolution. They will always use the network to perform DNS queries. These functions do not use the same set of configuration files used by `dns.lookup()` (e.g. `/etc/hosts`). Use these functions to always perform DNS queries, bypassing other name-resolution facilities.

    import dns from 'node:dns';

    dns.resolve4('archive.org', (err, addresses) => {
      if (err) throw err;

      console.log(`addresses: ${JSON.stringify(addresses)}`);

      addresses.forEach((a) => {
        dns.reverse(a, (err, hostnames) => {
          if (err) {
            throw err;
          }
          console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
        });
      });
    });
    const dns = require('node:dns');

    dns.resolve4('archive.org', (err, addresses) => {
      if (err) throw err;

      console.log(`addresses: ${JSON.stringify(addresses)}`);

      addresses.forEach((a) => {
        dns.reverse(a, (err, hostnames) => {
          if (err) {
            throw err;
          }
          console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
        });
      });
    });
    javascript

See the Implementation considerations section for more information.

### Class: `dns.Resolver`#

Added in: v8.3.0

An independent resolver for DNS requests.

Creating a new resolver uses the default server settings. Setting the servers used for a resolver using `resolver.setServers()` does not affect other resolvers:

    import { Resolver } from 'node:dns';
    const resolver = new Resolver();
    resolver.setServers(['4.4.4.4']);

    // This request will use the server at 4.4.4.4, independent of global settings.
    resolver.resolve4('example.org', (err, addresses) => {
      // ...
    });
    const { Resolver } = require('node:dns');
    const resolver = new Resolver();
    resolver.setServers(['4.4.4.4']);

    // This request will use the server at 4.4.4.4, independent of global settings.
    resolver.resolve4('example.org', (err, addresses) => {
      // ...
    });
    javascript

The following methods from the `node:dns` module are available:

- `resolver.getServers()`
- `resolver.resolve()`
- `resolver.resolve4()`
- `resolver.resolve6()`
- `resolver.resolveAny()`
- `resolver.resolveCaa()`
- `resolver.resolveCname()`
- `resolver.resolveMx()`
- `resolver.resolveNaptr()`
- `resolver.resolveNs()`
- `resolver.resolvePtr()`
- `resolver.resolveSoa()`
- `resolver.resolveSrv()`
- `resolver.resolveTlsa()`
- `resolver.resolveTxt()`
- `resolver.reverse()`
- `resolver.setServers()`

#### `Resolver([options])`#

Added in: v8.3.0
History

Version | Changes
--- | ---
v16.7.0, v14.18.0 | The `options` object now accepts a `tries` option.
v12.18.3 | The constructor now accepts an `options` object. The single supported option is `timeout`.

Create a new resolver.

- `options` `<Object>`
  - `timeout` `<integer>` Query timeout in milliseconds, or `-1` to use the default timeout.
  - `tries` `<integer>` The number of tries the resolver will try contacting each name server before giving up. Default: `4`
  - `maxTimeout` `<integer>` The max retry timeout, in milliseconds. Default: `0`, disabled.

#### `resolver.cancel()`#

Added in: v8.3.0

Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code `ECANCELLED`.

#### `resolver.setLocalAddress([ipv4][, ipv6])`#

Added in: v15.1.0, v14.17.0

- `ipv4` `<string>` A string representation of an IPv4 address. Default: `'0.0.0.0'`
- `ipv6` `<string>` A string representation of an IPv6 address. Default: `'::0'`

The resolver instance will send its requests from the specified IP address. This allows programs to specify outbound interfaces when used on multi-homed systems.

If a v4 or v6 address is not specified, it is set to the default and the operating system will choose a local address automatically.

The resolver will use the v4 local address when making requests to IPv4 DNS servers, and the v6 local address when making requests to IPv6 DNS servers. The `rrtype` of resolution requests has no impact on the local address used.

### `dns.getServers()`#

Added in: v0.11.3

- Returns: `<string>`[]

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

    [
      "8.8.8.8",
      "2001:4860:4860::8888",
      "8.8.8.8:1053",
      "[2001:4860:4860::8888]:1053",
    ]
    json

### `dns.lookup(hostname[, options], callback)`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `verbatim` option is now deprecated in favor of the new `order` option.
v18.4.0 | For compatibility with `node:net`, when passing an option object the `family` option can be the string `'IPv4'` or the string `'IPv6'`.
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v17.0.0 | The `verbatim` options defaults to `true` now.
v8.5.0 | The `verbatim` option is supported now.
v1.2.0 | The `all` option is supported now.

- `hostname` `<string>`
- `options` `<integer>` | `<Object>`
  - `family` `<integer>` | `<string>` The record family. Must be `4`, `6`, or `0`. For backward compatibility reasons,`'IPv4'` and `'IPv6'` are interpreted as `4` and `6` respectively. The value `0` indicates that either an IPv4 or IPv6 address is returned. If the value `0` is used with `{ all: true }` (see below), either one of or both IPv4 and IPv6 addresses are returned, depending on the system's DNS resolver. Default: `0`.
  - `hints` `<number>` One or more supported `getaddrinfo` flags. Multiple flags may be passed by bitwise `OR`ing their values.
  - `all` `<boolean>` When `true`, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default: `false`.
  - `order` `<string>` When `verbatim`, the resolved addresses are returned unsorted. When `ipv4first`, the resolved addresses are sorted by placing IPv4 addresses before IPv6 addresses. When `ipv6first`, the resolved addresses are sorted by placing IPv6 addresses before IPv4 addresses. Default: `verbatim` (addresses are not reordered). Default value is configurable using `dns.setDefaultResultOrder()` or `--dns-result-order`.
  - `verbatim` `<boolean>` When `true`, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. When `false`, IPv4 addresses are placed before IPv6 addresses. This option will be deprecated in favor of `order`. When both are specified, `order` has higher precedence. New code should only use `order`. Default: `true` (addresses are not reordered). Default value is configurable using `dns.setDefaultResultOrder()` or `--dns-result-order`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `address` `<string>` A string representation of an IPv4 or IPv6 address. Not provided when `options.all` is `true`.
  - `family` `<integer>` `4` or `6`, denoting the family of `address`, or `0` if the address is not an IPv4 or IPv6 address. `0` is a likely indicator of a bug in the name resolution service used by the operating system. Not provided when `options.all` is `true`.
  - `addresses` `<Object>`[] An array of address objects when `options.all` is `true`. Each object has the following properties:
    - `address` `<string>` A string representation of an IPv4 or IPv6 address.
    - `family` `<integer>` `4` or `6`, denoting the family of `address`.

Resolves a host name (e.g. `'nodejs.org'`) into the first found A (IPv4) or AAAA (IPv6) record. All `option` properties are optional. If `options` is an integer, then it must be `4` or `6` – if `options` is not provided, then either IPv4 or IPv6 addresses, or both, are returned if found.

With the `all` option set to `true`, the arguments for `callback` change to `(err, addresses)`, with `addresses` being an array of objects with the properties `address` and `family`.

On error, `err` is an `Error` object, where `err.code` is the error code. Keep in mind that `err.code` will be set to `'ENOTFOUND'` not only when the host name does not exist but also when the lookup fails in other ways such as no available file descriptors.

`dns.lookup()` does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using `dns.lookup()`.

Example usage:

    import dns from 'node:dns';
    const options = {
      family: 6,
      hints: dns.ADDRCONFIG | dns.V4MAPPED,
    };
    dns.lookup('example.org', options, (err, address, family) =>
      console.log('address: %j family: IPv%s', address, family));
    // address: "2606:2800:21f:cb07:6820:80da:af6b:8b2c" family: IPv6

    // When options.all is true, the result will be an Array.
    options.all = true;
    dns.lookup('example.org', options, (err, addresses) =>
      console.log('addresses: %j', addresses));
    // addresses: [{"address":"2606:2800:21f:cb07:6820:80da:af6b:8b2c","family":6}]
    const dns = require('node:dns');
    const options = {
      family: 6,
      hints: dns.ADDRCONFIG | dns.V4MAPPED,
    };
    dns.lookup('example.org', options, (err, address, family) =>
      console.log('address: %j family: IPv%s', address, family));
    // address: "2606:2800:21f:cb07:6820:80da:af6b:8b2c" family: IPv6

    // When options.all is true, the result will be an Array.
    options.all = true;
    dns.lookup('example.org', options, (err, addresses) =>
      console.log('addresses: %j', addresses));
    // addresses: [{"address":"2606:2800:21f:cb07:6820:80da:af6b:8b2c","family":6}]
    javascript

If this method is invoked as its `util.promisify()`ed version, and `all` is not set to `true`, it returns a `Promise` for an `Object` with `address` and `family` properties.

#### Supported getaddrinfo flags#

History

Version | Changes
--- | ---
v13.13.0, v12.17.0 | Added support for the `dns.ALL` flag.

The following flags can be passed as hints to `dns.lookup()`.

- `dns.ADDRCONFIG`: Limits returned address types to the types of non-loopback addresses configured on the system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured.
- `dns.V4MAPPED`: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. It is not supported on some operating systems (e.g. FreeBSD 10.1).
- `dns.ALL`: If `dns.V4MAPPED` is specified, return resolved IPv6 addresses as well as IPv4 mapped IPv6 addresses.

### `dns.lookupService(address, port, callback)`#

Added in: v0.11.14
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `address` `<string>`
- `port` `<number>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `hostname` `<string>` e.g. `example.com`
  - `service` `<string>` e.g. `http`

Resolves the given `address` and `port` into a host name and service using the operating system's underlying `getnameinfo` implementation.

If `address` is not a valid IP address, a `TypeError` will be thrown. The `port` will be coerced to a number. If it is not a legal port, a `TypeError` will be thrown.

On an error, `err` is an `Error` object, where `err.code` is the error code.

    import dns from 'node:dns';
    dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
      console.log(hostname, service);
      // Prints: localhost ssh
    });
    const dns = require('node:dns');
    dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
      console.log(hostname, service);
      // Prints: localhost ssh
    });
    javascript

If this method is invoked as its `util.promisify()`ed version, it returns a `Promise` for an `Object` with `hostname` and `service` properties.

### `dns.resolve(hostname[, rrtype], callback)`#

Added in: v0.1.27
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>` Host name to resolve.
- `rrtype` `<string>` Resource record type. Default: `'A'`.
- `callback` `<Function>`
  - `err` `<Error>`
  - `records` `<string>`[] | `<Object>`[] | `<Object>`

Uses the DNS protocol to resolve a host name (e.g. `'nodejs.org'`) into an array of the resource records. The `callback` function has arguments `(err, records)`. When successful, `records` will be an array of resource records. The type and structure of individual results varies based on `rrtype`:

`rrtype` | `records` contains | Result type | Shorthand method
--- | --- | --- | ---
`'A'` | IPv4 addresses (default) | `<string>` | `dns.resolve4()`
`'AAAA'` | IPv6 addresses | `<string>` | `dns.resolve6()`
`'ANY'` | any records | `<Object>` | `dns.resolveAny()`
`'CAA'` | CA authorization records | `<Object>` | `dns.resolveCaa()`
`'CNAME'` | canonical name records | `<string>` | `dns.resolveCname()`
`'MX'` | mail exchange records | `<Object>` | `dns.resolveMx()`
`'NAPTR'` | name authority pointer records | `<Object>` | `dns.resolveNaptr()`
`'NS'` | name server records | `<string>` | `dns.resolveNs()`
`'PTR'` | pointer records | `<string>` | `dns.resolvePtr()`
`'SOA'` | start of authority records | `<Object>` | `dns.resolveSoa()`
`'SRV'` | service records | `<Object>` | `dns.resolveSrv()`
`'TLSA'` | certificate associations | `<Object>` | `dns.resolveTlsa()`
`'TXT'` | text records | `<string>`[] | `dns.resolveTxt()`

On error, `err` is an `Error` object, where `err.code` is one of the DNS error codes.

### `dns.resolve4(hostname[, options], callback)`#

Added in: v0.1.16
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v7.2.0 | This method now supports passing `options`, specifically `options.ttl`.

- `hostname` `<string>` Host name to resolve.
- `options` `<Object>`
  - `ttl` `<boolean>` Retrieves the Time-To-Live value (TTL) of each record. When `true`, the callback receives an array of `{ address: '1.2.3.4', ttl: 60 }` objects rather than an array of strings, with the TTL expressed in seconds.
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<string>`[] | `<Object>`[]

Uses the DNS protocol to resolve an IPv4 addresses (`A` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of IPv4 addresses (e.g. `['74.125.79.104', '74.125.79.105', '74.125.79.106']`).

### `dns.resolve6(hostname[, options], callback)`#

Added in: v0.1.16
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.
v7.2.0 | This method now supports passing `options`, specifically `options.ttl`.

- `hostname` `<string>` Host name to resolve.
- `options` `<Object>`
  - `ttl` `<boolean>` Retrieve the Time-To-Live value (TTL) of each record. When `true`, the callback receives an array of `{ address: '0:1:2:3:4:5:6:7', ttl: 60 }` objects rather than an array of strings, with the TTL expressed in seconds.
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<string>`[] | `<Object>`[]

Uses the DNS protocol to resolve IPv6 addresses (`AAAA` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of IPv6 addresses.

### `dns.resolveAny(hostname, callback)`#

History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `ret` `<Object>`[]

Uses the DNS protocol to resolve all records (also known as `ANY` or `*` query). The `ret` argument passed to the `callback` function will be an array containing various types of records. Each object has a property `type` that indicates the type of the current record. And depending on the `type`, additional properties will be present on the object:

Type | Properties
--- | ---
`'A'` | `address`/`ttl`
`'AAAA'` | `address`/`ttl`
`'CAA'` | Refer to `dns.resolveCaa()`
`'CNAME'` | `value`
`'MX'` | Refer to `dns.resolveMx()`
`'NAPTR'` | Refer to `dns.resolveNaptr()`
`'NS'` | `value`
`'PTR'` | `value`
`'SOA'` | Refer to `dns.resolveSoa()`
`'SRV'` | Refer to `dns.resolveSrv()`
`'TLSA'` | Refer to `dns.resolveTlsa()`
`'TXT'` | This type of record contains an array property called `entries` which refers to `dns.resolveTxt()`, e.g. `{ entries: ['...'], type: 'TXT' }`

Here is an example of the `ret` object passed to the callback:

    [ { type: 'A', address: '127.0.0.1', ttl: 299 },
      { type: 'CNAME', value: 'example.com' },
      { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
      { type: 'NS', value: 'ns1.example.com' },
      { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
      { type: 'SOA',
        nsname: 'ns1.example.com',
        hostmaster: 'admin.example.com',
        serial: 156696742,
        refresh: 900,
        retry: 900,
        expire: 1800,
        minttl: 60 } ];
    js

DNS server operators may choose not to respond to `ANY` queries. It may be better to call individual methods like `dns.resolve4()`, `dns.resolveMx()`, and so on. For more details, see RFC 8482.

### `dns.resolveCname(hostname, callback)`#

Added in: v0.3.2
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<string>`[]

Uses the DNS protocol to resolve `CNAME` records for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of canonical name records available for the `hostname` (e.g. `['bar.example.com']`).

### `dns.resolveCaa(hostname, callback)`#

Added in: v15.0.0, v14.17.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `records` `<Object>`[]

Uses the DNS protocol to resolve `CAA` records for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of certification authority authorization records available for the `hostname` (e.g. `[{critical: 0, iodef: 'mailto:pki@example.com'}, {critical: 128, issue: 'pki.example.com'}]`).

### `dns.resolveMx(hostname, callback)`#

Added in: v0.1.27
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<Object>`[]

Uses the DNS protocol to resolve mail exchange records (`MX` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of objects containing both a `priority` and `exchange` property (e.g. `[{priority: 10, exchange: 'mx.example.com'}, ...]`).

### `dns.resolveNaptr(hostname, callback)`#

Added in: v0.9.12
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<Object>`[]

Uses the DNS protocol to resolve regular expression-based records (`NAPTR` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of objects with the following properties:

- `flags`
- `service`
- `regexp`
- `replacement`
- `order`
- `preference`

    ({
      flags: 's',
      service: 'SIP+D2U',
      regexp: '',
      replacement: '_sip._udp.example.com',
      order: 30,
      preference: 100,
    });
    js

### `dns.resolveNs(hostname, callback)`#

Added in: v0.1.90
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<string>`[]

Uses the DNS protocol to resolve name server records (`NS` records) for the `hostname`. The `addresses` argument passed to the `callback` function will contain an array of name server records available for `hostname` (e.g. `['ns1.example.com', 'ns2.example.com']`).

### `dns.resolvePtr(hostname, callback)`#

Added in: v6.0.0
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<string>`[]

Uses the DNS protocol to resolve pointer records (`PTR` records) for the `hostname`. The `addresses` argument passed to the `callback` function will be an array of strings containing the reply records.

### `dns.resolveSoa(hostname, callback)`#

Added in: v0.11.10
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `address` `<Object>`

Uses the DNS protocol to resolve a start of authority record (`SOA` record) for the `hostname`. The `address` argument passed to the `callback` function will be an object with the following properties:

- `nsname`
- `hostmaster`
- `serial`
- `refresh`
- `retry`
- `expire`
- `minttl`

    ({
      nsname: 'ns.example.com',
      hostmaster: 'root.example.com',
      serial: 2013101809,
      refresh: 10000,
      retry: 2400,
      expire: 604800,
      minttl: 3600,
    });
    js

### `dns.resolveSrv(hostname, callback)`#

Added in: v0.1.27
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `addresses` `<Object>`[]

Uses the DNS protocol to resolve service records (`SRV` records) for the `hostname`. The `addresses` argument passed to the `callback` function will be an array of objects with the following properties:

- `priority`
- `weight`
- `port`
- `name`

    ({
      priority: 10,
      weight: 5,
      port: 21223,
      name: 'service.example.com',
    });
    js

### `dns.resolveTlsa(hostname, callback)`#

Added in: v23.9.0, v22.15.0

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `records` `<Object>`[]

Uses the DNS protocol to resolve certificate associations (`TLSA` records) for the `hostname`. The `records` argument passed to the `callback` function is an array of objects with these properties:

- `certUsage`
- `selector`
- `match`
- `data`

    ({
      certUsage: 3,
      selector: 1,
      match: 1,
      data: [ArrayBuffer],
    });
    js

### `dns.resolveTxt(hostname, callback)`#

Added in: v0.1.27
History

Version | Changes
--- | ---
v18.0.0 | Passing an invalid callback to the `callback` argument now throws `ERR_INVALID_ARG_TYPE` instead of `ERR_INVALID_CALLBACK`.

- `hostname` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `records` `<string>`[]

Uses the DNS protocol to resolve text queries (`TXT` records) for the `hostname`. The `records` argument passed to the `callback` function is a two-dimensional array of the text records available for `hostname` (e.g. `[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]`). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

### `dns.reverse(ip, callback)`#

Added in: v0.1.16

- `ip` `<string>`
- `callback` `<Function>`
  - `err` `<Error>`
  - `hostnames` `<string>`[]

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.

On error, `err` is an `Error` object, where `err.code` is one of the DNS error codes.

### `dns.setDefaultResultOrder(order)`#

Added in: v16.4.0, v14.18.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `ipv6first` value is supported now.
v17.0.0 | Changed default value to `verbatim`.

- `order` `<string>` must be `'ipv4first'`, `'ipv6first'` or `'verbatim'`.

Set the default value of `order` in `dns.lookup()` and `dnsPromises.lookup()`. The value could be:

- `ipv4first`: sets default `order` to `ipv4first`.
- `ipv6first`: sets default `order` to `ipv6first`.
- `verbatim`: sets default `order` to `verbatim`.

The default is `verbatim` and `dns.setDefaultResultOrder()` have higher priority than `--dns-result-order`. When using worker threads, `dns.setDefaultResultOrder()` from the main thread won't affect the default dns orders in workers.

### `dns.getDefaultResultOrder()`#

Added in: v20.1.0, v18.17.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `ipv6first` value is supported now.

Get the default value for `order` in `dns.lookup()` and `dnsPromises.lookup()`. The value could be:

- `ipv4first`: for `order` defaulting to `ipv4first`.
- `ipv6first`: for `order` defaulting to `ipv6first`.
- `verbatim`: for `order` defaulting to `verbatim`.

### `dns.setServers(servers)`#

Added in: v0.11.3

- `servers` `<string>`[] array of RFC 5952 formatted addresses

Sets the IP address and port of servers to be used when performing DNS resolution. The `servers` argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

    dns.setServers([
      '8.8.8.8',
      '[2001:4860:4860::8888]',
      '8.8.8.8:1053',
      '[2001:4860:4860::8888]:1053',
    ]);
    js

An error will be thrown if an invalid address is provided.

The `dns.setServers()` method must not be called while a DNS query is in progress.

The `dns.setServers()` method affects only `dns.resolve()`, `dns.resolve*()` and `dns.reverse()` (and specifically not `dns.lookup()`).

This method works much like resolve.conf. That is, if attempting to resolve with the first server provided results in a `NOTFOUND` error, the `resolve()` method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

### DNS promises API#

Added in: v10.6.0
History

Version | Changes
--- | ---
v15.0.0 | Exposed as `require('dns/promises')`.
v11.14.0, v10.17.0 | This API is no longer experimental.

The `dns.promises` API provides an alternative set of asynchronous DNS methods that return `Promise` objects rather than using callbacks. The API is accessible via `require('node:dns').promises` or `require('node:dns/promises')`.

#### Class: `dnsPromises.Resolver`#

Added in: v10.6.0

An independent resolver for DNS requests.

Creating a new resolver uses the default server settings. Setting the servers used for a resolver using `resolver.setServers()` does not affect other resolvers:

    import { Resolver } from 'node:dns/promises';
    const resolver = new Resolver();
    resolver.setServers(['4.4.4.4']);

    // This request will use the server at 4.4.4.4, independent of global settings.
    const addresses = await resolver.resolve4('example.org');
    const { Resolver } = require('node:dns').promises;
    const resolver = new Resolver();
    resolver.setServers(['4.4.4.4']);

    // This request will use the server at 4.4.4.4, independent of global settings.
    resolver.resolve4('example.org').then((addresses) => {
      // ...
    });

    // Alternatively, the same code can be written using async-await style.
    (async function() {
      const addresses = await resolver.resolve4('example.org');
    })();
    javascript

The following methods from the `dnsPromises` API are available:

- `resolver.getServers()`
- `resolver.resolve()`
- `resolver.resolve4()`
- `resolver.resolve6()`
- `resolver.resolveAny()`
- `resolver.resolveCaa()`
- `resolver.resolveCname()`
- `resolver.resolveMx()`
- `resolver.resolveNaptr()`
- `resolver.resolveNs()`
- `resolver.resolvePtr()`
- `resolver.resolveSoa()`
- `resolver.resolveSrv()`
- `resolver.resolveTlsa()`
- `resolver.resolveTxt()`
- `resolver.reverse()`
- `resolver.setServers()`

#### `resolver.cancel()`#

Added in: v15.3.0, v14.17.0

Cancel all outstanding DNS queries made by this resolver. The corresponding promises will be rejected with an error with the code `ECANCELLED`.

#### `dnsPromises.getServers()`#

Added in: v10.6.0

- Returns: `<string>`[]

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

    [
      "8.8.8.8",
      "2001:4860:4860::8888",
      "8.8.8.8:1053",
      "[2001:4860:4860::8888]:1053"
    ]
    json

#### `dnsPromises.lookup(hostname[, options])`#

Added in: v10.6.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `verbatim` option is now deprecated in favor of the new `order` option.

- `hostname` `<string>`
- `options` `<integer>` | `<Object>`
  - `family` `<integer>` The record family. Must be `4`, `6`, or `0`. The value `0` indicates that either an IPv4 or IPv6 address is returned. If the value `0` is used with `{ all: true }` (see below), either one of or both IPv4 and IPv6 addresses are returned, depending on the system's DNS resolver. Default: `0`.
  - `hints` `<number>` One or more supported `getaddrinfo` flags. Multiple flags may be passed by bitwise `OR`ing their values.
  - `all` `<boolean>` When `true`, the `Promise` is resolved with all addresses in an array. Otherwise, returns a single address. Default: `false`.
  - `order` `<string>` When `verbatim`, the `Promise` is resolved with IPv4 and IPv6 addresses in the order the DNS resolver returned them. When `ipv4first`, IPv4 addresses are placed before IPv6 addresses. When `ipv6first`, IPv6 addresses are placed before IPv4 addresses. Default: `verbatim` (addresses are not reordered). Default value is configurable using `dns.setDefaultResultOrder()` or `--dns-result-order`. New code should use `{ order: 'verbatim' }`.
  - `verbatim` `<boolean>` When `true`, the `Promise` is resolved with IPv4 and IPv6 addresses in the order the DNS resolver returned them. When `false`, IPv4 addresses are placed before IPv6 addresses. This option will be deprecated in favor of `order`. When both are specified, `order` has higher precedence. New code should only use `order`. Default: `true` (addresses are not reordered). Default value is configurable using `dns.setDefaultResultOrder()` or `--dns-result-order`.

Resolves a host name (e.g. `'nodejs.org'`) into the first found A (IPv4) or AAAA (IPv6) record. All `option` properties are optional. If `options` is an integer, then it must be `4` or `6` – if `options` is not provided, then either IPv4 or IPv6 addresses, or both, are returned if found.

With the `all` option set to `true`, the `Promise` is resolved with `addresses` being an array of objects with the properties `address` and `family`.

On error, the `Promise` is rejected with an `Error` object, where `err.code` is the error code. Keep in mind that `err.code` will be set to `'ENOTFOUND'` not only when the host name does not exist but also when the lookup fails in other ways such as no available file descriptors.

`dnsPromises.lookup()` does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using `dnsPromises.lookup()`.

Example usage:

    import dns from 'node:dns';
    const dnsPromises = dns.promises;
    const options = {
      family: 6,
      hints: dns.ADDRCONFIG | dns.V4MAPPED,
    };

    await dnsPromises.lookup('example.org', options).then((result) => {
      console.log('address: %j family: IPv%s', result.address, result.family);
      // address: "2606:2800:21f:cb07:6820:80da:af6b:8b2c" family: IPv6
    });

    // When options.all is true, the result will be an Array.
    options.all = true;
    await dnsPromises.lookup('example.org', options).then((result) => {
      console.log('addresses: %j', result);
      // addresses: [{"address":"2606:2800:21f:cb07:6820:80da:af6b:8b2c","family":6}]
    });
    const dns = require('node:dns');
    const dnsPromises = dns.promises;
    const options = {
      family: 6,
      hints: dns.ADDRCONFIG | dns.V4MAPPED,
    };

    dnsPromises.lookup('example.org', options).then((result) => {
      console.log('address: %j family: IPv%s', result.address, result.family);
      // address: "2606:2800:21f:cb07:6820:80da:af6b:8b2c" family: IPv6
    });

    // When options.all is true, the result will be an Array.
    options.all = true;
    dnsPromises.lookup('example.org', options).then((result) => {
      console.log('addresses: %j', result);
      // addresses: [{"address":"2606:2800:21f:cb07:6820:80da:af6b:8b2c","family":6}]
    });
    javascript

#### `dnsPromises.lookupService(address, port)`#

Added in: v10.6.0

- `address` `<string>`
- `port` `<number>`

Resolves the given `address` and `port` into a host name and service using the operating system's underlying `getnameinfo` implementation.

If `address` is not a valid IP address, a `TypeError` will be thrown. The `port` will be coerced to a number. If it is not a legal port, a `TypeError` will be thrown.

On error, the `Promise` is rejected with an `Error` object, where `err.code` is the error code.

    import dnsPromises from 'node:dns/promises';
    const result = await dnsPromises.lookupService('127.0.0.1', 22);

    console.log(result.hostname, result.service); // Prints: localhost ssh
    const dnsPromises = require('node:dns').promises;
    dnsPromises.lookupService('127.0.0.1', 22).then((result) => {
      console.log(result.hostname, result.service);
      // Prints: localhost ssh
    });
    javascript

#### `dnsPromises.resolve(hostname[, rrtype])`#

Added in: v10.6.0

- `hostname` `<string>` Host name to resolve.
- `rrtype` `<string>` Resource record type. Default: `'A'`.

Uses the DNS protocol to resolve a host name (e.g. `'nodejs.org'`) into an array of the resource records. When successful, the `Promise` is resolved with an array of resource records. The type and structure of individual results vary based on `rrtype`:

`rrtype` | `records` contains | Result type | Shorthand method
--- | --- | --- | ---
`'A'` | IPv4 addresses (default) | `<string>` | `dnsPromises.resolve4()`
`'AAAA'` | IPv6 addresses | `<string>` | `dnsPromises.resolve6()`
`'ANY'` | any records | `<Object>` | `dnsPromises.resolveAny()`
`'CAA'` | CA authorization records | `<Object>` | `dnsPromises.resolveCaa()`
`'CNAME'` | canonical name records | `<string>` | `dnsPromises.resolveCname()`
`'MX'` | mail exchange records | `<Object>` | `dnsPromises.resolveMx()`
`'NAPTR'` | name authority pointer records | `<Object>` | `dnsPromises.resolveNaptr()`
`'NS'` | name server records | `<string>` | `dnsPromises.resolveNs()`
`'PTR'` | pointer records | `<string>` | `dnsPromises.resolvePtr()`
`'SOA'` | start of authority records | `<Object>` | `dnsPromises.resolveSoa()`
`'SRV'` | service records | `<Object>` | `dnsPromises.resolveSrv()`
`'TLSA'` | certificate associations | `<Object>` | `dnsPromises.resolveTlsa()`
`'TXT'` | text records | `<string>`[] | `dnsPromises.resolveTxt()`

On error, the `Promise` is rejected with an `Error` object, where `err.code` is one of the DNS error codes.

#### `dnsPromises.resolve4(hostname[, options])`#

Added in: v10.6.0

- `hostname` `<string>` Host name to resolve.
- `options` `<Object>`
  - `ttl` `<boolean>` Retrieve the Time-To-Live value (TTL) of each record. When `true`, the `Promise` is resolved with an array of `{ address: '1.2.3.4', ttl: 60 }` objects rather than an array of strings, with the TTL expressed in seconds.

Uses the DNS protocol to resolve IPv4 addresses (`A` records) for the `hostname`. On success, the `Promise` is resolved with an array of IPv4 addresses (e.g. `['74.125.79.104', '74.125.79.105', '74.125.79.106']`).

#### `dnsPromises.resolve6(hostname[, options])`#

Added in: v10.6.0

- `hostname` `<string>` Host name to resolve.
- `options` `<Object>`
  - `ttl` `<boolean>` Retrieve the Time-To-Live value (TTL) of each record. When `true`, the `Promise` is resolved with an array of `{ address: '0:1:2:3:4:5:6:7', ttl: 60 }` objects rather than an array of strings, with the TTL expressed in seconds.

Uses the DNS protocol to resolve IPv6 addresses (`AAAA` records) for the `hostname`. On success, the `Promise` is resolved with an array of IPv6 addresses.

#### `dnsPromises.resolveAny(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve all records (also known as `ANY` or `*` query). On success, the `Promise` is resolved with an array containing various types of records. Each object has a property `type` that indicates the type of the current record. And depending on the `type`, additional properties will be present on the object:

Type | Properties
--- | ---
`'A'` | `address`/`ttl`
`'AAAA'` | `address`/`ttl`
`'CAA'` | Refer to `dnsPromises.resolveCaa()`
`'CNAME'` | `value`
`'MX'` | Refer to `dnsPromises.resolveMx()`
`'NAPTR'` | Refer to `dnsPromises.resolveNaptr()`
`'NS'` | `value`
`'PTR'` | `value`
`'SOA'` | Refer to `dnsPromises.resolveSoa()`
`'SRV'` | Refer to `dnsPromises.resolveSrv()`
`'TLSA'` | Refer to `dnsPromises.resolveTlsa()`
`'TXT'` | This type of record contains an array property called `entries` which refers to `dnsPromises.resolveTxt()`, e.g. `{ entries: ['...'], type: 'TXT' }`

Here is an example of the result object:

    [ { type: 'A', address: '127.0.0.1', ttl: 299 },
      { type: 'CNAME', value: 'example.com' },
      { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
      { type: 'NS', value: 'ns1.example.com' },
      { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
      { type: 'SOA',
        nsname: 'ns1.example.com',
        hostmaster: 'admin.example.com',
        serial: 156696742,
        refresh: 900,
        retry: 900,
        expire: 1800,
        minttl: 60 } ];
    js

#### `dnsPromises.resolveCaa(hostname)`#

Added in: v15.0.0, v14.17.0

- `hostname` `<string>`

Uses the DNS protocol to resolve `CAA` records for the `hostname`. On success, the `Promise` is resolved with an array of objects containing available certification authority authorization records available for the `hostname` (e.g. `[{critical: 0, iodef: 'mailto:pki@example.com'},{critical: 128, issue: 'pki.example.com'}]`).

#### `dnsPromises.resolveCname(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve `CNAME` records for the `hostname`. On success, the `Promise` is resolved with an array of canonical name records available for the `hostname` (e.g. `['bar.example.com']`).

#### `dnsPromises.resolveMx(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve mail exchange records (`MX` records) for the `hostname`. On success, the `Promise` is resolved with an array of objects containing both a `priority` and `exchange` property (e.g. `[{priority: 10, exchange: 'mx.example.com'}, ...]`).

#### `dnsPromises.resolveNaptr(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve regular expression-based records (`NAPTR` records) for the `hostname`. On success, the `Promise` is resolved with an array of objects with the following properties:

- `flags`
- `service`
- `regexp`
- `replacement`
- `order`
- `preference`

    ({
      flags: 's',
      service: 'SIP+D2U',
      regexp: '',
      replacement: '_sip._udp.example.com',
      order: 30,
      preference: 100,
    });
    js

#### `dnsPromises.resolveNs(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve name server records (`NS` records) for the `hostname`. On success, the `Promise` is resolved with an array of name server records available for `hostname` (e.g. `['ns1.example.com', 'ns2.example.com']`).

#### `dnsPromises.resolvePtr(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve pointer records (`PTR` records) for the `hostname`. On success, the `Promise` is resolved with an array of strings containing the reply records.

#### `dnsPromises.resolveSoa(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve a start of authority record (`SOA` record) for the `hostname`. On success, the `Promise` is resolved with an object with the following properties:

- `nsname`
- `hostmaster`
- `serial`
- `refresh`
- `retry`
- `expire`
- `minttl`

    ({
      nsname: 'ns.example.com',
      hostmaster: 'root.example.com',
      serial: 2013101809,
      refresh: 10000,
      retry: 2400,
      expire: 604800,
      minttl: 3600,
    });
    js

#### `dnsPromises.resolveSrv(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve service records (`SRV` records) for the `hostname`. On success, the `Promise` is resolved with an array of objects with the following properties:

- `priority`
- `weight`
- `port`
- `name`

    ({
      priority: 10,
      weight: 5,
      port: 21223,
      name: 'service.example.com',
    });
    js

#### `dnsPromises.resolveTlsa(hostname)`#

Added in: v23.9.0, v22.15.0

- `hostname` `<string>`

Uses the DNS protocol to resolve certificate associations (`TLSA` records) for the `hostname`. On success, the `Promise` is resolved with an array of objects with these properties:

- `certUsage`
- `selector`
- `match`
- `data`

    ({
      certUsage: 3,
      selector: 1,
      match: 1,
      data: [ArrayBuffer],
    });
    js

#### `dnsPromises.resolveTxt(hostname)`#

Added in: v10.6.0

- `hostname` `<string>`

Uses the DNS protocol to resolve text queries (`TXT` records) for the `hostname`. On success, the `Promise` is resolved with a two-dimensional array of the text records available for `hostname` (e.g. `[ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]`). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

#### `dnsPromises.reverse(ip)`#

Added in: v10.6.0

- `ip` `<string>`

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.

On error, the `Promise` is rejected with an `Error` object, where `err.code` is one of the DNS error codes.

#### `dnsPromises.setDefaultResultOrder(order)`#

Added in: v16.4.0, v14.18.0
History

Version | Changes
--- | ---
v22.1.0, v20.13.0 | The `ipv6first` value is supported now.
v17.0.0 | Changed default value to `verbatim`.

- `order` `<string>` must be `'ipv4first'`, `'ipv6first'` or `'verbatim'`.

Set the default value of `order` in `dns.lookup()` and `dnsPromises.lookup()`. The value could be:

- `ipv4first`: sets default `order` to `ipv4first`.
- `ipv6first`: sets default `order` to `ipv6first`.
- `verbatim`: sets default `order` to `verbatim`.

The default is `verbatim` and `dnsPromises.setDefaultResultOrder()` have higher priority than `--dns-result-order`. When using worker threads, `dnsPromises.setDefaultResultOrder()` from the main thread won't affect the default dns orders in workers.

#### `dnsPromises.getDefaultResultOrder()`#

Added in: v20.1.0, v18.17.0

Get the value of `dnsOrder`.

#### `dnsPromises.setServers(servers)`#

Added in: v10.6.0

- `servers` `<string>`[] array of RFC 5952 formatted addresses

Sets the IP address and port of servers to be used when performing DNS resolution. The `servers` argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

    dnsPromises.setServers([
      '8.8.8.8',
      '[2001:4860:4860::8888]',
      '8.8.8.8:1053',
      '[2001:4860:4860::8888]:1053',
    ]);
    js

An error will be thrown if an invalid address is provided.

The `dnsPromises.setServers()` method must not be called while a DNS query is in progress.

This method works much like resolve.conf. That is, if attempting to resolve with the first server provided results in a `NOTFOUND` error, the `resolve()` method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

### Error codes#

Each DNS query can return one of the following error codes:

- `dns.NODATA`: DNS server returned an answer with no data.
- `dns.FORMERR`: DNS server claims query was misformatted.
- `dns.SERVFAIL`: DNS server returned general failure.
- `dns.NOTFOUND`: Domain name not found.
- `dns.NOTIMP`: DNS server does not implement the requested operation.
- `dns.REFUSED`: DNS server refused query.
- `dns.BADQUERY`: Misformatted DNS query.
- `dns.BADNAME`: Misformatted host name.
- `dns.BADFAMILY`: Unsupported address family.
- `dns.BADRESP`: Misformatted DNS reply.
- `dns.CONNREFUSED`: Could not contact DNS servers.
- `dns.TIMEOUT`: Timeout while contacting DNS servers.
- `dns.EOF`: End of file.
- `dns.FILE`: Error reading file.
- `dns.NOMEM`: Out of memory.
- `dns.DESTRUCTION`: Channel is being destroyed.
- `dns.BADSTR`: Misformatted string.
- `dns.BADFLAGS`: Illegal flags specified.
- `dns.NONAME`: Given host name is not numeric.
- `dns.BADHINTS`: Illegal hints flags specified.
- `dns.NOTINITIALIZED`: c-ares library initialization not yet performed.
- `dns.LOADIPHLPAPI`: Error loading `iphlpapi.dll`.
- `dns.ADDRGETNETWORKPARAMS`: Could not find `GetNetworkParams` function.
- `dns.CANCELLED`: DNS query cancelled.

The `dnsPromises` API also exports the above error codes, e.g., `dnsPromises.NODATA`.

### Implementation considerations#

Although `dns.lookup()` and the various `dns.resolve*()/dns.reverse()` functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs.

#### `dns.lookup()`#

Under the hood, `dns.lookup()` uses the same operating system facilities as most other programs. For instance, `dns.lookup()` will almost always resolve a given name the same way as the `ping` command. On most POSIX-like operating systems, the behavior of the `dns.lookup()` function can be modified by changing settings in `nsswitch.conf(5)` and/or `resolv.conf(5)`, but changing these files will change the behavior of all other programs running on the same operating system.

Though the call to `dns.lookup()` will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to `getaddrinfo(3)` that runs on libuv's threadpool. This can have surprising negative performance implications for some applications, see the `UV_THREADPOOL_SIZE` documentation for more information.

Various networking APIs will call `dns.lookup()` internally to resolve host names. If that is an issue, consider resolving the host name to an address using `dns.resolve()` and using the address instead of a host name. Also, some networking APIs (such as `socket.connect()` and `dgram.createSocket()`) allow the default resolver, `dns.lookup()`, to be replaced.

#### `dns.resolve()`, `dns.resolve*()`, and `dns.reverse()`#

These functions are implemented quite differently than `dns.lookup()`. They do not use `getaddrinfo(3)` and they always perform a DNS query on the network. This network communication is always done asynchronously and does not use libuv's threadpool.

As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that `dns.lookup()` can have.

They do not use the same set of configuration files that `dns.lookup()` uses. For instance, they do not use the configuration from `/etc/hosts`.

## Domain#

Source Code: lib/domain.js Deprecated in: v1.4.2
History

Version | Changes
--- | ---
v8.8.0 | Any `Promise`s created in VM contexts no longer have a `.domain` property. Their handlers are still executed in the proper domain, however, and `Promise`s created in the main context still possess a `.domain` property.
v8.0.0 | Handlers for `Promise`s are now invoked in the domain in which the first promise of a chain was created.

Stability: 0 - Deprecated

This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most developers should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.

Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an `'error'` event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the `process.on('uncaughtException')` handler, or causing the program to exit immediately with an error code.

### Warning: Don't ignore errors!#

Domain error handlers are not a substitute for closing down a process when an error occurs.

By the very nature of how `throw` works in JavaScript, there is almost never any way to safely "pick up where it left off", without leaking references, or creating some other sort of undefined brittle state.

The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, there may be many open connections, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.

The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.

In this way, `domain` usage goes hand-in-hand with the cluster module, since the primary process can fork a new worker when a worker encounters an error. For Node.js programs that scale to multiple machines, the terminating proxy or service registry can take note of the failure, and react accordingly.

For example, this is not a good idea:

    // XXX WARNING! BAD IDEA!

    const d = require('node:domain').create();
    d.on('error', (er) => {
      // The error won't crash the process, but what it does is worse!
      // Though we've prevented abrupt process restarting, we are leaking
      // a lot of resources if this ever happens.
      // This is no better than process.on('uncaughtException')!
      console.log(`error, but oh well ${er.message}`);
    });
    d.run(() => {
      require('node:http').createServer((req, res) => {
        handleRequest(req, res);
      }).listen(PORT);
    });
    js

By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.

    // Much better!

    const cluster = require('node:cluster');
    const PORT = +process.env.PORT || 1337;

    if (cluster.isPrimary) {
      // A more realistic scenario would have more than 2 workers,
      // and perhaps not put the primary and worker in the same file.
      //
      // It is also possible to get a bit fancier about logging, and
      // implement whatever custom logic is needed to prevent DoS
      // attacks and other bad behavior.
      //
      // See the options in the cluster documentation.
      //
      // The important thing is that the primary does very little,
      // increasing our resilience to unexpected errors.

      cluster.fork();
      cluster.fork();

      cluster.on('disconnect', (worker) => {
        console.error('disconnect!');
        cluster.fork();
      });

    } else {
      // the worker
      //
      // This is where we put our bugs!

      const domain = require('node:domain');

      // See the cluster documentation for more details about using
      // worker processes to serve requests. How it works, caveats, etc.

      const server = require('node:http').createServer((req, res) => {
        const d = domain.create();
        d.on('error', (er) => {
          console.error(`error ${er.stack}`);

          // We're in dangerous territory!
          // By definition, something unexpected occurred,
          // which we probably didn't want.
          // Anything can happen now! Be very careful!

          try {
            // Make sure we close down within 30 seconds
            const killtimer = setTimeout(() => {
              process.exit(1);
            }, 30000);
            // But don't keep the process open just for that!
            killtimer.unref();

            // Stop taking new requests.
            server.close();

            // Let the primary know we're dead. This will trigger a
            // 'disconnect' in the cluster primary, and then it will fork
            // a new worker.
            cluster.worker.disconnect();

            // Try to send an error to the request that triggered the problem
            res.statusCode = 500;
            res.setHeader('content-type', 'text/plain');
            res.end('Oops, there was a problem!\n');
          } catch (er2) {
            // Oh well, not much we can do at this point.
            console.error(`Error sending 500! ${er2.stack}`);
          }
        });

        // Because req and res were created before this domain existed,
        // we need to explicitly add them.
        // See the explanation of implicit vs explicit binding below.
        d.add(req);
        d.add(res);

        // Now run the handler function in the domain.
        d.run(() => {
          handleRequest(req, res);
        });
      });
      server.listen(PORT);
    }

    // This part is not important. Just an example routing thing.
    // Put fancy application logic here.
    function handleRequest(req, res) {
      switch (req.url) {
        case '/error':
          // We do some async stuff, and then...
          setTimeout(() => {
            // Whoops!
            flerb.bark();
          }, timeout);
          break;
        default:
          res.end('ok');
      }
    }
    js

### Additions to `Error` objects#

Any time an `Error` object is routed through a domain, a few extra fields are added to it.

- `error.domain` The domain that first handled the error.
- `error.domainEmitter` The event emitter that emitted an `'error'` event with the error object.
- `error.domainBound` The callback function which was bound to the domain, and passed an error as its first argument.
- `error.domainThrown` A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.

### Implicit binding#

If domains are in use, then all new `EventEmitter` objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation.

Additionally, callbacks passed to low-level event loop requests (such as to `fs.open()`, or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error.

In order to prevent excessive memory usage, `Domain` objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected.

To nest `Domain` objects as children of a parent `Domain` they must be explicitly added.

Implicit binding routes thrown errors and `'error'` events to the `Domain`'s `'error'` event, but does not register the `EventEmitter` on the `Domain`. Implicit binding only takes care of thrown errors and `'error'` events.

### Explicit binding#

Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.

For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.

That is possible via explicit binding.

    // Create a top-level domain for the server
    const domain = require('node:domain');
    const http = require('node:http');
    const serverDomain = domain.create();

    serverDomain.run(() => {
      // Server is created in the scope of serverDomain
      http.createServer((req, res) => {
        // Req and res are also created in the scope of serverDomain
        // however, we'd prefer to have a separate domain for each request.
        // create it first thing, and add req and res to it.
        const reqd = domain.create();
        reqd.add(req);
        reqd.add(res);
        reqd.on('error', (er) => {
          console.error('Error', er, req.url);
          try {
            res.writeHead(500);
            res.end('Error occurred, sorry.');
          } catch (er2) {
            console.error('Error sending 500', er2, req.url);
          }
        });
      }).listen(1337);
    });
    js

### `domain.create()`#

- Returns: `<Domain>`

### Class: `Domain`#

- Extends: `<EventEmitter>`

The `Domain` class encapsulates the functionality of routing errors and uncaught exceptions to the active `Domain` object.

To handle the errors that it catches, listen to its `'error'` event.

#### `domain.members`#

- Type: `<Array>`

An array of event emitters that have been explicitly added to the domain.

#### `domain.add(emitter)`#

History

Version | Changes
--- | ---
v9.3.0 | No longer accepts timer objects.

- `emitter` `<EventEmitter>` emitter to be added to the domain

Explicitly adds an emitter to the domain. If any event handlers called by the emitter throw an error, or if the emitter emits an `'error'` event, it will be routed to the domain's `'error'` event, just like with implicit binding.

If the `EventEmitter` was already bound to a domain, it is removed from that one, and bound to this one instead.

#### `domain.bind(callback)`#

- `callback` `<Function>` The callback function
- Returns: `<Function>` The bound function

The returned function will be a wrapper around the supplied callback function. When the returned function is called, any errors that are thrown will be routed to the domain's `'error'` event.

    const d = domain.create();

    function readSomeFile(filename, cb) {
      fs.readFile(filename, 'utf8', d.bind((er, data) => {
        // If this throws, it will also be passed to the domain.
        return cb(er, data ? JSON.parse(data) : null);
      }));
    }

    d.on('error', (er) => {
      // An error occurred somewhere. If we throw it now, it will crash the program
      // with the normal line number and stack message.
    });
    js

#### `domain.enter()`#

The `enter()` method is plumbing used by the `run()`, `bind()`, and `intercept()` methods to set the active domain. It sets `domain.active` and `process.domain` to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see `domain.exit()` for details on the domain stack). The call to `enter()` delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain.

Calling `enter()` changes only the active domain, and does not alter the domain itself. `enter()` and `exit()` can be called an arbitrary number of times on a single domain.

#### `domain.exit()`#

The `exit()` method exits the current domain, popping it off the domain stack. Any time execution is going to switch to the context of a different chain of asynchronous calls, it's important to ensure that the current domain is exited. The call to `exit()` delimits either the end of or an interruption to the chain of asynchronous calls and I/O operations bound to a domain.

If there are multiple, nested domains bound to the current execution context, `exit()` will exit any domains nested within this domain.

Calling `exit()` changes only the active domain, and does not alter the domain itself. `enter()` and `exit()` can be called an arbitrary number of times on a single domain.

#### `domain.intercept(callback)`#

- `callback` `<Function>` The callback function
- Returns: `<Function>` The intercepted function

This method is almost identical to `domain.bind(callback)`. However, in addition to catching thrown errors, it will also intercept `Error` objects sent as the first argument to the function.

In this way, the common `if (err) return callback(err);` pattern can be replaced with a single error handler in a single place.

    const d = domain.create();

    function readSomeFile(filename, cb) {
      fs.readFile(filename, 'utf8', d.intercept((data) => {
        // Note, the first argument is never passed to the
        // callback since it is assumed to be the 'Error' argument
        // and thus intercepted by the domain.

        // If this throws, it will also be passed to the domain
        // so the error-handling logic can be moved to the 'error'
        // event on the domain instead of being repeated throughout
        // the program.
        return cb(null, JSON.parse(data));
      }));
    }

    d.on('error', (er) => {
      // An error occurred somewhere. If we throw it now, it will crash the program
      // with the normal line number and stack message.
    });
    js

#### `domain.remove(emitter)`#

- `emitter` `<EventEmitter>` emitter to be removed from the domain

The opposite of `domain.add(emitter)`. Removes domain handling from the specified emitter.

#### `domain.run(fn[, ...args])`#

- `fn` `<Function>`
- `...args` `<any>`

Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and low-level requests that are created in that context. Optionally, arguments can be passed to the function.

This is the most basic way to use a domain.

    const domain = require('node:domain');
    const fs = require('node:fs');
    const d = domain.create();
    d.on('error', (er) => {
      console.error('Caught error!', er);
    });
    d.run(() => {
      process.nextTick(() => {
        setTimeout(() => { // Simulating some various async stuff
          fs.open('non-existent file', 'r', (er, fd) => {
            if (er) throw er;
            // proceed...
          });
        }, 100);
      });
    });
    js

In this example, the `d.on('error')` handler will be triggered, rather than crashing the program.

### Domains and promises#

As of Node.js 8.0.0, the handlers of promises are run inside the domain in which the call to `.then()` or `.catch()` itself was made:

    const d1 = domain.create();
    const d2 = domain.create();

    let p;
    d1.run(() => {
      p = Promise.resolve(42);
    });

    d2.run(() => {
      p.then((v) => {
        // running in d2
      });
    });
    js

A callback may be bound to a specific domain using `domain.bind(callback)`:

    const d1 = domain.create();
    const d2 = domain.create();

    let p;
    d1.run(() => {
      p = Promise.resolve(42);
    });

    d2.run(() => {
      p.then(p.domain.bind((v) => {
        // running in d1
      }));
    });
    js

Domains will not interfere with the error handling mechanisms for promises. In other words, no `'error'` event will be emitted for unhandled `Promise` rejections.

## Environment Variables#

Environment variables are variables associated to the environment the Node.js process runs in.

### CLI Environment Variables#

There is a set of environment variables that can be defined to customize the behavior of Node.js, for more details refer to the CLI Environment Variables documentation.

### `process.env`#

The basic API for interacting with environment variables is `process.env`, it consists of an object with pre-populated user environment variables that can be modified and expanded.

For more details refer to the `process.env` documentation.

### DotEnv#

Stability: 2 - Stable

Set of utilities for dealing with additional environment variables defined in `.env` files.

#### .env files#

`.env` files (also known as dotenv files) are files that define environment variables, which Node.js applications can then interact with (popularized by the dotenv package).

The following is an example of the content of a basic `.env` file:

    MY_VAR_A = "my variable A"
    MY_VAR_B = "my variable B"
    text

This type of file is used in various different programming languages and platforms but there is no formal specification for it, therefore Node.js defines its own specification described below.

A `.env` file is a file that contains key-value pairs, each pair is represented by a variable name followed by the equal sign (`=`) followed by a variable value.

The name of such files is usually `.env` or it starts with `.env` (like for example `.env.dev` where `dev` indicates a specific target environment). This is the recommended naming scheme but it is not mandatory and dotenv files can have any arbitrary file name.

##### Variable Names#

A valid variable name must contain only letters (uppercase or lowercase), digits and underscores (`_`) and it can't begin with a digit.

More specifically a valid variable name must match the following regular expression:

    ^[a-zA-Z_]+[a-zA-Z0-9_]*$
    text

The recommended convention is to use capital letters with underscores and digits when necessary, but any variable name respecting the above definition will work just fine.

For example, the following are some valid variable names: `MY_VAR`, `MY_VAR_1`, `my_var`, `my_var_1`, `myVar`, `My_Var123`, while these are instead not valid: `1_VAR`, `'my-var'`, `"my var"`, `VAR_#1`.

##### Variable Values#

Variable values are comprised by any arbitrary text, which can optionally be wrapped inside single (`'`) or double (`"`) quotes.

Quoted variables can span across multiple lines, while non quoted ones are restricted to a single line.

Noting that when parsed by Node.js all values are interpreted as text, meaning that any value will result in a JavaScript string inside Node.js. For example the following values: `0`, `true` and `{ "hello": "world" }` will result in the literal strings `'0'`, `'true'` and `'{ "hello": "world" }'` instead of the number zero, the boolean `true` and an object with the `hello` property respectively.

Examples of valid variables:

    MY_SIMPLE_VAR = a simple single line variable
    MY_EQUALS_VAR = "this variable contains an = sign!"
    MY_HASH_VAR = 'this variable contains a # symbol!'
    MY_MULTILINE_VAR = '
    this is a multiline variable containing
    two separate lines\nSorry, I meant three lines'
    text

##### Spacing#

Leading and trailing whitespace characters around variable keys and values are ignored unless they are enclosed within quotes.

For example:

       MY_VAR_A   =    my variable a
        MY_VAR_B   =    '   my variable b   '
    text

will be treated identically to:

    MY_VAR_A = my variable a
    MY_VAR_B = '   my variable b   '
    text

##### Comments#

Hash-tag (`#`) characters denote the beginning of a comment, meaning that the rest of the line will be completely ignored.

Hash-tags found within quotes are however treated as any other standard character.

For example:

    # This is a comment
    MY_VAR = my variable # This is also a comment
    MY_VAR_A = "# this is NOT a comment"
    text

##### `export` prefixes#

The `export` keyword can optionally be added in front of variable declarations, such keyword will be completely ignored by all processing done on the file.

This is useful so that the file can be sourced, without modifications, in shell terminals.

Example:

    export MY_VAR = my variable
    text

#### CLI Options#

`.env` files can be used to populate the `process.env` object via one the following CLI options:

- `--env-file=file`

- `--env-file-if-exists=file`

#### Programmatic APIs#

There following two functions allow you to directly interact with `.env` files:

- `process.loadEnvFile` loads an `.env` file and populates `process.env` with its variables

- `util.parseEnv` parses the raw content of an `.env` file and returns its value in an object

## Errors#

Applications running in Node.js will generally experience the following categories of errors:

- Standard JavaScript errors such as `<EvalError>`, `<SyntaxError>`, `<RangeError>`, `<ReferenceError>`, `<TypeError>`, and `<URIError>`.
- Standard `DOMException`s.
- System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist or attempting to send data over a closed socket.
- `AssertionError`s are a special class of error that can be triggered when Node.js detects an exceptional logic violation that should never occur. These are raised typically by the `node:assert` module.
- User-specified errors triggered by application code.

All JavaScript and system errors raised by Node.js inherit from, or are instances of, the standard JavaScript `<Error>` class and are guaranteed to provide at least the properties available on that class.

The `error.message` property of errors raised by Node.js may be changed in any versions. Use `error.code` to identify an error instead. For a `DOMException`, use `domException.name` to identify its type.

### Error propagation and interception#

Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of `Error` and the style of the API that is called.

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript `throw` mechanism. These are handled using the `try…catch` construct provided by the JavaScript language.

    // Throws with a ReferenceError because z is not defined.
    try {
      const m = 1;
      const n = m + z;
    } catch (err) {
      // Handle the error here.
    }
    js

Any use of the JavaScript `throw` mechanism will raise an exception that must be handled or the Node.js process will exit immediately.

With few exceptions, Synchronous APIs (any blocking method that does not return a `<Promise>` nor accept a `callback` function, such as `fs.readFileSync`), will use `throw` to report errors.

Errors that occur within Asynchronous APIs may be reported in multiple ways:

- Some asynchronous methods returns a `<Promise>`, you should always take into account that it might be rejected. See `--unhandled-rejections` flag for how the process will react to an unhandled promise rejection.

      const fs = require('node:fs/promises');

      (async () => {
        let data;
        try {
          data = await fs.readFile('a file that does not exist');
        } catch (err) {
          console.error('There was an error reading the file!', err);
          return;
        }
        // Otherwise handle the data
      })();
      js

- Most asynchronous methods that accept a `callback` function will accept an `Error` object passed as the first argument to that function. If that first argument is not `null` and is an instance of `Error`, then an error occurred that should be handled.

      const fs = require('node:fs');
      fs.readFile('a file that does not exist', (err, data) => {
        if (err) {
          console.error('There was an error reading the file!', err);
          return;
        }
        // Otherwise handle the data
      });
      js

- When an asynchronous method is called on an object that is an `EventEmitter`, errors can be routed to that object's `'error'` event.

      const net = require('node:net');
      const connection = net.connect('localhost');

      // Adding an 'error' event handler to a stream:
      connection.on('error', (err) => {
        // If the connection is reset by the server, or if it can't
        // connect at all, or on any sort of error encountered by
        // the connection, the error will be sent here.
        console.error(err);
      });

      connection.pipe(process.stdout);
      js

- A handful of typically asynchronous methods in the Node.js API may still use the `throw` mechanism to raise exceptions that must be handled using `try…catch`. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.

The use of the `'error'` event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail).

For all `EventEmitter` objects, if an `'error'` event handler is not provided, the error will be thrown, causing the Node.js process to report an uncaught exception and crash unless either: a handler has been registered for the `'uncaughtException'` event, or the deprecated `node:domain` module is used.

    const EventEmitter = require('node:events');
    const ee = new EventEmitter();

    setImmediate(() => {
      // This will crash the process because no 'error' event
      // handler has been added.
      ee.emit('error', new Error('This will crash'));
    });
    js

Errors generated in this way cannot be intercepted using `try…catch` as they are thrown after the calling code has already exited.

Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.

### Class: `Error`#

A generic JavaScript `<Error>` object that does not denote any specific circumstance of why the error occurred. `Error` objects capture a "stack trace" detailing the point in the code at which the `Error` was instantiated, and may provide a text description of the error.

All errors generated by Node.js, including all system and JavaScript errors, will either be instances of, or inherit from, the `Error` class.

#### `new Error(message[, options])`#

- `message` `<string>`
- `options` `<Object>`
  - `cause` `<any>` The error that caused the newly created error.

Creates a new `Error` object and sets the `error.message` property to the provided text message. If an object is passed as `message`, the text message is generated by calling `String(message)`. If the `cause` option is provided, it is assigned to the `error.cause` property. The `error.stack` property will represent the point in the code at which `new Error()` was called. Stack traces are dependent on V8's stack trace API. Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property `Error.stackTraceLimit`, whichever is smaller.

#### `Error.captureStackTrace(targetObject[, constructorOpt])`#

- `targetObject` `<Object>`
- `constructorOpt` `<Function>`

Creates a `.stack` property on `targetObject`, which when accessed returns a string representing the location in the code at which `Error.captureStackTrace()` was called.

    const myObject = {};
    Error.captureStackTrace(myObject);
    myObject.stack;  // Similar to `new Error().stack`
    js

The first line of the trace will be prefixed with `${myObject.name}: ${myObject.message}`.

The optional `constructorOpt` argument accepts a function. If given, all frames above `constructorOpt`, including `constructorOpt`, will be omitted from the generated stack trace.

The `constructorOpt` argument is useful for hiding implementation details of error generation from the user. For instance:

    function a() {
      b();
    }

    function b() {
      c();
    }

    function c() {
      // Create an error without stack trace to avoid calculating the stack trace twice.
      const { stackTraceLimit } = Error;
      Error.stackTraceLimit = 0;
      const error = new Error();
      Error.stackTraceLimit = stackTraceLimit;

      // Capture the stack trace above function b
      Error.captureStackTrace(error, b); // Neither function c, nor b is included in the stack trace
      throw error;
    }

    a();
    js

#### `Error.stackTraceLimit`#

- Type: `<number>`

The `Error.stackTraceLimit` property specifies the number of stack frames collected by a stack trace (whether generated by `new Error().stack` or `Error.captureStackTrace(obj)`).

The default value is `10` but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

#### `error.cause`#

Added in: v16.9.0

- Type: `<any>`

If present, the `error.cause` property is the underlying cause of the `Error`. It is used when catching an error and throwing a new one with a different message or code in order to still have access to the original error.

The `error.cause` property is typically set by calling `new Error(message, { cause })`. It is not set by the constructor if the `cause` option is not provided.

This property allows errors to be chained. When serializing `Error` objects, `util.inspect()` recursively serializes `error.cause` if it is set.

    const cause = new Error('The remote HTTP server responded with a 500 status');
    const symptom = new Error('The message failed to send', { cause });

    console.log(symptom);
    // Prints:
    //   Error: The message failed to send
    //       at REPL2:1:17
    //       at Script.runInThisContext (node:vm:130:12)
    //       ... 7 lines matching cause stack trace ...
    //       at [_line] [as _line] (node:internal/readline/interface:886:18) {
    //     [cause]: Error: The remote HTTP server responded with a 500 status
    //         at REPL1:1:15
    //         at Script.runInThisContext (node:vm:130:12)
    //         at REPLServer.defaultEval (node:repl:574:29)
    //         at bound (node:domain:426:15)
    //         at REPLServer.runBound [as eval] (node:domain:437:12)
    //         at REPLServer.onLine (node:repl:902:10)
    //         at REPLServer.emit (node:events:549:35)
    //         at REPLServer.emit (node:domain:482:12)
    //         at [_onLine] [as _onLine] (node:internal/readline/interface:425:12)
    //         at [_line] [as _line] (node:internal/readline/interface:886:18)
    js

#### `error.code`#

- Type: `<string>`

The `error.code` property is a string label that identifies the kind of error. `error.code` is the most stable way to identify an error. It will only change between major versions of Node.js. In contrast, `error.message` strings may change between any versions of Node.js. See Node.js error codes for details about specific codes.

#### `error.message`#

- Type: `<string>`

The `error.message` property is the string description of the error as set by calling `new Error(message)`. The `message` passed to the constructor will also appear in the first line of the stack trace of the `Error`, however changing this property after the `Error` object is created may not change the first line of the stack trace (for example, when `error.stack` is read before this property is changed).

    const err = new Error('The message');
    console.error(err.message);
    // Prints: The message
    js

#### `error.stack`#

- Type: `<string>`

The `error.stack` property is a string describing the point in the code at which the `Error` was instantiated.

    Error: Things keep happening!
       at /home/gbusey/file.js:525:2
       at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
       at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
       at increaseSynergy (/home/gbusey/actors.js:701:6)
    console

The first line is formatted as `<error class name>: <error message>`, and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses.

Frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called `cheetahify` which itself calls a JavaScript function, the frame representing the `cheetahify` call will not be present in the stack traces:

    const cheetahify = require('./native-binding.node');

    function makeFaster() {
      // `cheetahify()` *synchronously* calls speedy.
      cheetahify(function speedy() {
        throw new Error('oh no!');
      });
    }

    makeFaster();
    // will throw:
    //   /home/gbusey/file.js:6
    //       throw new Error('oh no!');
    //           ^
    //   Error: oh no!
    //       at speedy (/home/gbusey/file.js:6:11)
    //       at makeFaster (/home/gbusey/file.js:5:3)
    //       at Object.<anonymous> (/home/gbusey/file.js:10:1)
    //       at Module._compile (module.js:456:26)
    //       at Object.Module._extensions..js (module.js:474:10)
    //       at Module.load (module.js:356:32)
    //       at Function.Module._load (module.js:312:12)
    //       at Function.Module.runMain (module.js:497:10)
    //       at startup (node.js:119:16)
    //       at node.js:906:3
    js

The location information will be one of:

- `native`, if the frame represents a call internal to V8 (as in `[].forEach`).
- `plain-filename.js:line:column`, if the frame represents a call internal to Node.js.
- `/absolute/path/to/file.js:line:column`, if the frame represents a call in a user program (using CommonJS module system), or its dependencies.
- `<transport-protocol>:///url/to/module/file.mjs:line:column`, if the frame represents a call in a user program (using ES module system), or its dependencies.

The number of frames captured by the stack trace is bounded by the smaller of `Error.stackTraceLimit` or the number of available frames on the current event loop tick.

`error.stack` is a getter/setter for a hidden internal property which is only present on builtin `Error` objects (those for which `Error.isError` returns true). If `error` is not a builtin error object, then the `error.stack` getter will always return `undefined`, and the setter will do nothing. This can occur if the accessor is manually invoked with a `this` value that is not a builtin error object, such as a `<Proxy>`.

### Class: `AssertionError`#

- Extends: `<errors.Error>`

Indicates the failure of an assertion. For details, see `Class: assert.AssertionError`.

### Class: `RangeError`#

- Extends: `<errors.Error>`

Indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.

    require('node:net').connect(-1);
    // Throws "RangeError: "port" option should be >= 0 and < 65536: -1"
    js

Node.js will generate and throw `RangeError` instances immediately as a form of argument validation.

### Class: `ReferenceError`#

- Extends: `<errors.Error>`

Indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.

While client code may generate and propagate these errors, in practice, only V8 will do so.

    doesNotExist;
    // Throws ReferenceError, doesNotExist is not a variable in this program.
    js

Unless an application is dynamically generating and running code, `ReferenceError` instances indicate a bug in the code or its dependencies.

### Class: `SyntaxError`#

- Extends: `<errors.Error>`

Indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of `eval`, `Function`, `require`, or vm. These errors are almost always indicative of a broken program.

    try {
      require('node:vm').runInThisContext('binary ! isNotOk');
    } catch (err) {
      // 'err' will be a SyntaxError.
    }
    js

`SyntaxError` instances are unrecoverable in the context that created them – they may only be caught by other contexts.

### Class: `SystemError`#

- Extends: `<errors.Error>`

Node.js generates system errors when exceptions occur within its runtime environment. These usually occur when an application violates an operating system constraint. For example, a system error will occur if an application attempts to read a file that does not exist.

- `address` `<string>` If present, the address to which a network connection failed
- `code` `<string>` The string error code
- `dest` `<string>` If present, the file path destination when reporting a file system error
- `errno` `<number>` The system-provided error number
- `info` `<Object>` If present, extra details about the error condition
- `message` `<string>` A system-provided human-readable description of the error
- `path` `<string>` If present, the file path when reporting a file system error
- `port` `<number>` If present, the network connection port that is not available
- `syscall` `<string>` The name of the system call that triggered the error

#### `error.address`#

- Type: `<string>`

If present, `error.address` is a string describing the address to which a network connection failed.

#### `error.code`#

- Type: `<string>`

The `error.code` property is a string representing the error code.

#### `error.dest`#

- Type: `<string>`

If present, `error.dest` is the file path destination when reporting a file system error.

#### `error.errno`#

- Type: `<number>`

The `error.errno` property is a negative number which corresponds to the error code defined in `libuv Error handling`.

On Windows the error number provided by the system will be normalized by libuv.

To get the string representation of the error code, use `util.getSystemErrorName(error.errno)`.

#### `error.info`#

- Type: `<Object>`

If present, `error.info` is an object with details about the error condition.

#### `error.message`#

- Type: `<string>`

`error.message` is a system-provided human-readable description of the error.

#### `error.path`#

- Type: `<string>`

If present, `error.path` is a string containing a relevant invalid pathname.

#### `error.port`#

- Type: `<number>`

If present, `error.port` is the network connection port that is not available.

#### `error.syscall`#

- Type: `<string>`

The `error.syscall` property is a string describing the syscall that failed.

#### Common system errors#

This is a list of system errors commonly-encountered when writing a Node.js program. For a comprehensive list, see the `errno`(3) man page.

- `EACCES` (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.

- `EADDRINUSE` (Address already in use): An attempt to bind a server (`net`, `http`, or `https`) to a local address failed due to another server on the local system already occupying that address.

- `ECONNREFUSED` (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.

- `ECONNRESET` (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the `http` and `net` modules.

- `EEXIST` (File exists): An existing file was the target of an operation that required that the target not exist.

- `EISDIR` (Is a directory): An operation expected a file, but the given pathname was a directory.

- `EMFILE` (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, run `ulimit -n 2048` in the same shell that will run the Node.js process.

- `ENOENT` (No such file or directory): Commonly raised by `fs` operations to indicate that a component of the specified pathname does not exist. No entity (file or directory) could be found by the given path.

- `ENOTDIR` (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by `fs.readdir`.

- `ENOTEMPTY` (Directory not empty): A directory with entries was the target of an operation that requires an empty directory, usually `fs.unlink`.

- `ENOTFOUND` (DNS lookup failed): Indicates a DNS failure of either `EAI_NODATA` or `EAI_NONAME`. This is not a standard POSIX error.

- `EPERM` (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.

- `EPIPE` (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the `net` and `http` layers, indicative that the remote side of the stream being written to has been closed.

- `ETIMEDOUT` (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by `http` or `net`. Often a sign that a `socket.end()` was not properly called.

### Class: `TypeError`#

- Extends `<errors.Error>`

Indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be a `TypeError`.

    require('node:url').parse(() => { });
    // Throws TypeError, since it expected a string.
    js

Node.js will generate and throw `TypeError` instances immediately as a form of argument validation.

### Exceptions vs. errors#

A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a `throw` statement. While it is not required that these values are instances of `Error` or classes which inherit from `Error`, all exceptions thrown by Node.js or the JavaScript runtime will be instances of `Error`.

Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include `assert()` checks or `abort()` calls in the C++ layer.

### OpenSSL errors#

Errors originating in `crypto` or `tls` are of class `Error`, and in addition to the standard `.code` and `.message` properties, may have some additional OpenSSL-specific properties.

#### `error.opensslErrorStack`#

An array of errors that can give context to where in the OpenSSL library an error originates from.

#### `error.function`#

The OpenSSL function the error originates in.

#### `error.library`#

The OpenSSL library the error originates in.

#### `error.reason`#

A human-readable string describing the reason for the error.

### Node.js error codes#

#### `ABORT_ERR`#

Added in: v15.0.0

Used when an operation has been aborted (typically using an `AbortController`).

APIs not using `AbortSignal`s typically do not raise an error with this code.

This code does not use the regular `ERR_*` convention Node.js errors use in order to be compatible with the web platform's `AbortError`.

#### `ERR_ACCESS_DENIED`#

A special type of error that is triggered whenever Node.js tries to get access to a resource restricted by the Permission Model.

#### `ERR_AMBIGUOUS_ARGUMENT`#

A function argument is being used in a way that suggests that the function signature may be misunderstood. This is thrown by the `node:assert` module when the `message` parameter in `assert.throws(block, message)` matches the error message thrown by `block` because that usage suggests that the user believes `message` is the expected message rather than the message the `AssertionError` will display if `block` does not throw.

#### `ERR_ARG_NOT_ITERABLE`#

An iterable argument (i.e. a value that works with `for...of` loops) was required, but not provided to a Node.js API.

#### `ERR_ASSERTION`#

A special type of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the `node:assert` module.

#### `ERR_ASYNC_CALLBACK`#

An attempt was made to register something that is not a function as an `AsyncHooks` callback.

#### `ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED`#

An operation related to module loading is customized by an asynchronous loader hook that never settled the promise before the loader thread exits.

#### `ERR_ASYNC_TYPE`#

The type of an asynchronous resource was invalid. Users are also able to define their own types if using the public embedder API.

#### `ERR_BROTLI_COMPRESSION_FAILED`#

Data passed to a Brotli stream was not successfully compressed.

#### `ERR_BROTLI_INVALID_PARAM`#

An invalid parameter key was passed during construction of a Brotli stream.

#### `ERR_BUFFER_CONTEXT_NOT_AVAILABLE`#

An attempt was made to create a Node.js `Buffer` instance from addon or embedder code, while in a JS engine Context that is not associated with a Node.js instance. The data passed to the `Buffer` method will have been released by the time the method returns.

When encountering this error, a possible alternative to creating a `Buffer` instance is to create a normal `Uint8Array`, which only differs in the prototype of the resulting object. `Uint8Array`s are generally accepted in all Node.js core APIs where `Buffer`s are; they are available in all Contexts.

#### `ERR_BUFFER_OUT_OF_BOUNDS`#

An operation outside the bounds of a `Buffer` was attempted.

#### `ERR_BUFFER_TOO_LARGE`#

An attempt has been made to create a `Buffer` larger than the maximum allowed size.

#### `ERR_CANNOT_WATCH_SIGINT`#

Node.js was unable to watch for the `SIGINT` signal.

#### `ERR_CHILD_CLOSED_BEFORE_REPLY`#

A child process was closed before the parent received a reply.

#### `ERR_CHILD_PROCESS_IPC_REQUIRED`#

Used when a child process is being forked without specifying an IPC channel.

#### `ERR_CHILD_PROCESS_STDIO_MAXBUFFER`#

Used when the main process is trying to read data from the child process's STDERR/STDOUT, and the data's length is longer than the `maxBuffer` option.

#### `ERR_CLOSED_MESSAGE_PORT`#

Added in: v10.5.0
History

Version | Changes
--- | ---
v16.2.0, v14.17.1 | The error message was reintroduced.
v11.12.0 | The error message was removed.

There was an attempt to use a `MessagePort` instance in a closed state, usually after `.close()` has been called.

#### `ERR_CONSOLE_WRITABLE_STREAM`#

`Console` was instantiated without `stdout` stream, or `Console` has a non-writable `stdout` or `stderr` stream.

#### `ERR_CONSTRUCT_CALL_INVALID`#

Added in: v12.5.0

A class constructor was called that is not callable.

#### `ERR_CONSTRUCT_CALL_REQUIRED`#

A constructor for a class was called without `new`.

#### `ERR_CONTEXT_NOT_INITIALIZED`#

The vm context passed into the API is not yet initialized. This could happen when an error occurs (and is caught) during the creation of the context, for example, when the allocation fails or the maximum call stack size is reached when the context is created.

#### `ERR_CPU_PROFILE_ALREADY_STARTED`#

Added in: v24.8.0, v22.20.0

The CPU profile with the given name is already started.

#### `ERR_CPU_PROFILE_NOT_STARTED`#

Added in: v24.8.0, v22.20.0

The CPU profile with the given name is not started.

#### `ERR_CPU_PROFILE_TOO_MANY`#

Added in: v24.8.0, v22.20.0

There are too many CPU profiles being collected.

#### `ERR_CRYPTO_ARGON2_NOT_SUPPORTED`#

Argon2 is not supported by the current version of OpenSSL being used.

#### `ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED`#

An OpenSSL engine was requested (for example, through the `clientCertEngine` or `privateKeyEngine` TLS options) that is not supported by the version of OpenSSL being used, likely due to the compile-time flag `OPENSSL_NO_ENGINE`.

#### `ERR_CRYPTO_ECDH_INVALID_FORMAT`#

An invalid value for the `format` argument was passed to the `crypto.ECDH()` class `getPublicKey()` method.

#### `ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY`#

An invalid value for the `key` argument has been passed to the `crypto.ECDH()` class `computeSecret()` method. It means that the public key lies outside of the elliptic curve.

#### `ERR_CRYPTO_ENGINE_UNKNOWN`#

An invalid crypto engine identifier was passed to `require('node:crypto').setEngine()`.

#### `ERR_CRYPTO_FIPS_FORCED`#

The `--force-fips` command-line argument was used but there was an attempt to enable or disable FIPS mode in the `node:crypto` module.

#### `ERR_CRYPTO_FIPS_UNAVAILABLE`#

An attempt was made to enable or disable FIPS mode, but FIPS mode was not available.

#### `ERR_CRYPTO_HASH_FINALIZED`#

`hash.digest()` was called multiple times. The `hash.digest()` method must be called no more than one time per instance of a `Hash` object.

#### `ERR_CRYPTO_HASH_UPDATE_FAILED`#

`hash.update()` failed for any reason. This should rarely, if ever, happen.

#### `ERR_CRYPTO_INCOMPATIBLE_KEY`#

The given crypto keys are incompatible with the attempted operation.

#### `ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS`#

The selected public or private key encoding is incompatible with other options.

#### `ERR_CRYPTO_INITIALIZATION_FAILED`#

Added in: v15.0.0

Initialization of the crypto subsystem failed.

#### `ERR_CRYPTO_INVALID_AUTH_TAG`#

Added in: v15.0.0

An invalid authentication tag was provided.

#### `ERR_CRYPTO_INVALID_COUNTER`#

Added in: v15.0.0

An invalid counter was provided for a counter-mode cipher.

#### `ERR_CRYPTO_INVALID_CURVE`#

Added in: v15.0.0

An invalid elliptic-curve was provided.

#### `ERR_CRYPTO_INVALID_DIGEST`#

An invalid crypto digest algorithm was specified.

#### `ERR_CRYPTO_INVALID_IV`#

Added in: v15.0.0

An invalid initialization vector was provided.

#### `ERR_CRYPTO_INVALID_JWK`#

Added in: v15.0.0

An invalid JSON Web Key was provided.

#### `ERR_CRYPTO_INVALID_KEYLEN`#

Added in: v15.0.0

An invalid key length was provided.

#### `ERR_CRYPTO_INVALID_KEYPAIR`#

Added in: v15.0.0

An invalid key pair was provided.

#### `ERR_CRYPTO_INVALID_KEYTYPE`#

Added in: v15.0.0

An invalid key type was provided.

#### `ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE`#

The given crypto key object's type is invalid for the attempted operation.

#### `ERR_CRYPTO_INVALID_MESSAGELEN`#

Added in: v15.0.0

An invalid message length was provided.

#### `ERR_CRYPTO_INVALID_SCRYPT_PARAMS`#

Added in: v15.0.0

One or more `crypto.scrypt()` or `crypto.scryptSync()` parameters are outside their legal range.

#### `ERR_CRYPTO_INVALID_STATE`#

A crypto method was used on an object that was in an invalid state. For instance, calling `cipher.getAuthTag()` before calling `cipher.final()`.

#### `ERR_CRYPTO_INVALID_TAG_LENGTH`#

Added in: v15.0.0

An invalid authentication tag length was provided.

#### `ERR_CRYPTO_JOB_INIT_FAILED`#

Added in: v15.0.0

Initialization of an asynchronous crypto operation failed.

#### `ERR_CRYPTO_JWK_UNSUPPORTED_CURVE`#

Key's Elliptic Curve is not registered for use in the JSON Web Key Elliptic Curve Registry.

#### `ERR_CRYPTO_JWK_UNSUPPORTED_KEY_TYPE`#

Key's Asymmetric Key Type is not registered for use in the JSON Web Key Types Registry.

#### `ERR_CRYPTO_KEM_NOT_SUPPORTED`#

Added in: v24.7.0

Attempted to use KEM operations while Node.js was not compiled with OpenSSL with KEM support.

#### `ERR_CRYPTO_OPERATION_FAILED`#

Added in: v15.0.0

A crypto operation failed for an otherwise unspecified reason.

#### `ERR_CRYPTO_PBKDF2_ERROR`#

The PBKDF2 algorithm failed for unspecified reasons. OpenSSL does not provide more details and therefore neither does Node.js.

#### `ERR_CRYPTO_SCRYPT_NOT_SUPPORTED`#

Node.js was compiled without `scrypt` support. Not possible with the official release binaries but can happen with custom builds, including distro builds.

#### `ERR_CRYPTO_SIGN_KEY_REQUIRED`#

A signing `key` was not provided to the `sign.sign()` method.

#### `ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH`#

`crypto.timingSafeEqual()` was called with `Buffer`, `TypedArray`, or `DataView` arguments of different lengths.

#### `ERR_CRYPTO_UNKNOWN_CIPHER`#

An unknown cipher was specified.

#### `ERR_CRYPTO_UNKNOWN_DH_GROUP`#

An unknown Diffie-Hellman group name was given. See `crypto.getDiffieHellman()` for a list of valid group names.

#### `ERR_CRYPTO_UNSUPPORTED_OPERATION`#

Added in: v15.0.0, v14.18.0

An attempt to invoke an unsupported crypto operation was made.

#### `ERR_DEBUGGER_ERROR`#

Added in: v16.4.0, v14.17.4

An error occurred with the debugger.

#### `ERR_DEBUGGER_STARTUP_ERROR`#

Added in: v16.4.0, v14.17.4

The debugger timed out waiting for the required host/port to be free.

#### `ERR_DIR_CLOSED`#

The `fs.Dir` was previously closed.

#### `ERR_DIR_CONCURRENT_OPERATION`#

Added in: v14.3.0

A synchronous read or close call was attempted on an `fs.Dir` which has ongoing asynchronous operations.

#### `ERR_DLOPEN_DISABLED`#

Added in: v16.10.0, v14.19.0

Loading native addons has been disabled using `--no-addons`.

#### `ERR_DLOPEN_FAILED`#

Added in: v15.0.0

A call to `process.dlopen()` failed.

#### `ERR_DNS_SET_SERVERS_FAILED`#

`c-ares` failed to set the DNS server.

#### `ERR_DOMAIN_CALLBACK_NOT_AVAILABLE`#

The `node:domain` module was not usable since it could not establish the required error handling hooks, because `process.setUncaughtExceptionCaptureCallback()` had been called at an earlier point in time.

#### `ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE`#

`process.setUncaughtExceptionCaptureCallback()` could not be called because the `node:domain` module has been loaded at an earlier point in time.

The stack trace is extended to include the point in time at which the `node:domain` module had been loaded.

#### `ERR_DUPLICATE_STARTUP_SNAPSHOT_MAIN_FUNCTION`#

`v8.startupSnapshot.setDeserializeMainFunction()` could not be called because it had already been called before.

#### `ERR_ENCODING_INVALID_ENCODED_DATA`#

Data provided to `TextDecoder()` API was invalid according to the encoding provided.

#### `ERR_ENCODING_NOT_SUPPORTED`#

Encoding provided to `TextDecoder()` API was not one of the WHATWG Supported Encodings.

#### `ERR_EVAL_ESM_CANNOT_PRINT`#

`--print` cannot be used with ESM input.

#### `ERR_EVENT_RECURSION`#

Thrown when an attempt is made to recursively dispatch an event on `EventTarget`.

#### `ERR_EXECUTION_ENVIRONMENT_NOT_AVAILABLE`#

The JS execution context is not associated with a Node.js environment. This may occur when Node.js is used as an embedded library and some hooks for the JS engine are not set up properly.

#### `ERR_FALSY_VALUE_REJECTION`#

A `Promise` that was callbackified via `util.callbackify()` was rejected with a falsy value.

#### `ERR_FEATURE_UNAVAILABLE_ON_PLATFORM`#

Added in: v14.0.0

Used when a feature that is not available to the current platform which is running Node.js is used.

#### `ERR_FFI_CALL_FAILED`#

A low-level FFI call failed.

#### `ERR_FFI_INVALID_POINTER`#

An invalid pointer was passed to an FFI operation.

#### `ERR_FFI_LIBRARY_CLOSED`#

An operation was attempted on an FFI dynamic library after it was closed.

#### `ERR_FS_CP_DIR_TO_NON_DIR`#

Added in: v16.7.0

An attempt was made to copy a directory to a non-directory (file, symlink, etc.) using `fs.cp()`.

#### `ERR_FS_CP_EEXIST`#

Added in: v16.7.0

An attempt was made to copy over a file that already existed with `fs.cp()`, with the `force` and `errorOnExist` set to `true`.

#### `ERR_FS_CP_EINVAL`#

Added in: v16.7.0

When using `fs.cp()`, `src` or `dest` pointed to an invalid path.

#### `ERR_FS_CP_FIFO_PIPE`#

Added in: v16.7.0

An attempt was made to copy a named pipe with `fs.cp()`.

#### `ERR_FS_CP_NON_DIR_TO_DIR`#

Added in: v16.7.0

An attempt was made to copy a non-directory (file, symlink, etc.) to a directory using `fs.cp()`.

#### `ERR_FS_CP_SOCKET`#

Added in: v16.7.0

An attempt was made to copy to a socket with `fs.cp()`.

#### `ERR_FS_CP_SYMLINK_TO_SUBDIRECTORY`#

Added in: v16.7.0

When using `fs.cp()`, a symlink in `dest` pointed to a subdirectory of `src`.

#### `ERR_FS_CP_UNKNOWN`#

Added in: v16.7.0

An attempt was made to copy to an unknown file type with `fs.cp()`.

#### `ERR_FS_EISDIR`#

Path is a directory.

#### `ERR_FS_FILE_TOO_LARGE`#

An attempt was made to read a file larger than the supported 2 GiB limit for `fs.readFile()`. This is not a limitation of `Buffer`, but an internal I/O constraint. For handling larger files, consider using `fs.createReadStream()` to read the file in chunks.

#### `ERR_FS_WATCH_QUEUE_OVERFLOW`#

The number of file system events queued without being handled exceeded the size specified in `maxQueue` in `fs.watch()`.

#### `ERR_HTTP2_ALTSVC_INVALID_ORIGIN`#

HTTP/2 ALTSVC frames require a valid origin.

#### `ERR_HTTP2_ALTSVC_LENGTH`#

HTTP/2 ALTSVC frames are limited to a maximum of 16,382 payload bytes.

#### `ERR_HTTP2_CONNECT_AUTHORITY`#

For HTTP/2 requests using the `CONNECT` method, the `:authority` pseudo-header is required.

#### `ERR_HTTP2_CONNECT_PATH`#

For HTTP/2 requests using the `CONNECT` method, the `:path` pseudo-header is forbidden.

#### `ERR_HTTP2_CONNECT_SCHEME`#

For HTTP/2 requests using the `CONNECT` method, the `:scheme` pseudo-header is forbidden.

#### `ERR_HTTP2_ERROR`#

A non-specific HTTP/2 error has occurred.

#### `ERR_HTTP2_GOAWAY_SESSION`#

New HTTP/2 Streams may not be opened after the `Http2Session` has received a `GOAWAY` frame from the connected peer.

#### `ERR_HTTP2_HEADERS_AFTER_RESPOND`#

Additional headers were specified after an HTTP/2 response was initiated.

#### `ERR_HTTP2_HEADERS_SENT`#

An attempt was made to send multiple response headers.

#### `ERR_HTTP2_HEADER_SINGLE_VALUE`#

Multiple values were provided for an HTTP/2 header field that was required to have only a single value.

#### `ERR_HTTP2_INFO_STATUS_NOT_ALLOWED`#

Informational HTTP status codes (`1xx`) may not be set as the response status code on HTTP/2 responses.

#### `ERR_HTTP2_INVALID_CONNECTION_HEADERS`#

HTTP/1 connection specific headers are forbidden to be used in HTTP/2 requests and responses.

#### `ERR_HTTP2_INVALID_HEADER_VALUE`#

An invalid HTTP/2 header value was specified.

#### `ERR_HTTP2_INVALID_INFO_STATUS`#

An invalid HTTP informational status code has been specified. Informational status codes must be an integer between `100` and `199` (inclusive).

#### `ERR_HTTP2_INVALID_ORIGIN`#

HTTP/2 `ORIGIN` frames require a valid origin.

#### `ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH`#

Input `Buffer` and `Uint8Array` instances passed to the `http2.getUnpackedSettings()` API must have a length that is a multiple of six.

#### `ERR_HTTP2_INVALID_PSEUDOHEADER`#

Only valid HTTP/2 pseudoheaders (`:status`, `:path`, `:authority`, `:scheme`, and `:method`) may be used.

#### `ERR_HTTP2_INVALID_SESSION`#

An action was performed on an `Http2Session` object that had already been destroyed.

#### `ERR_HTTP2_INVALID_SETTING_VALUE`#

An invalid value has been specified for an HTTP/2 setting.

#### `ERR_HTTP2_INVALID_STREAM`#

An operation was performed on a stream that had already been destroyed.

#### `ERR_HTTP2_MAX_PENDING_SETTINGS_ACK`#

Whenever an HTTP/2 `SETTINGS` frame is sent to a connected peer, the peer is required to send an acknowledgment that it has received and applied the new `SETTINGS`. By default, a maximum number of unacknowledged `SETTINGS` frames may be sent at any given time. This error code is used when that limit has been reached.

#### `ERR_HTTP2_NESTED_PUSH`#

An attempt was made to initiate a new push stream from within a push stream. Nested push streams are not permitted.

#### `ERR_HTTP2_NO_MEM`#

Out of memory when using the `http2session.setLocalWindowSize(windowSize)` API.

#### `ERR_HTTP2_NO_SOCKET_MANIPULATION`#

An attempt was made to directly manipulate (read, write, pause, resume, etc.) a socket attached to an `Http2Session`.

#### `ERR_HTTP2_ORIGIN_LENGTH`#

HTTP/2 `ORIGIN` frames are limited to a length of 16382 bytes.

#### `ERR_HTTP2_OUT_OF_STREAMS`#

The number of streams created on a single HTTP/2 session reached the maximum limit.

#### `ERR_HTTP2_PAYLOAD_FORBIDDEN`#

A message payload was specified for an HTTP response code for which a payload is forbidden.

#### `ERR_HTTP2_PING_CANCEL`#

An HTTP/2 ping was canceled.

#### `ERR_HTTP2_PING_LENGTH`#

HTTP/2 ping payloads must be exactly 8 bytes in length.

#### `ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED`#

An HTTP/2 pseudo-header has been used inappropriately. Pseudo-headers are header key names that begin with the `:` prefix.

#### `ERR_HTTP2_PUSH_DISABLED`#

An attempt was made to create a push stream, which had been disabled by the client.

#### `ERR_HTTP2_SEND_FILE`#

An attempt was made to use the `Http2Stream.prototype.responseWithFile()` API to send a directory.

#### `ERR_HTTP2_SEND_FILE_NOSEEK`#

An attempt was made to use the `Http2Stream.prototype.responseWithFile()` API to send something other than a regular file, but `offset` or `length` options were provided.

#### `ERR_HTTP2_SESSION_ERROR`#

The `Http2Session` closed with a non-zero error code.

#### `ERR_HTTP2_SETTINGS_CANCEL`#

The `Http2Session` settings canceled.

#### `ERR_HTTP2_SOCKET_BOUND`#

An attempt was made to connect a `Http2Session` object to a `net.Socket` or `tls.TLSSocket` that had already been bound to another `Http2Session` object.

#### `ERR_HTTP2_SOCKET_UNBOUND`#

An attempt was made to use the `socket` property of an `Http2Session` that has already been closed.

#### `ERR_HTTP2_STATUS_101`#

Use of the `101` Informational status code is forbidden in HTTP/2.

#### `ERR_HTTP2_STATUS_INVALID`#

An invalid HTTP status code has been specified. Status codes must be an integer between `100` and `599` (inclusive).

#### `ERR_HTTP2_STREAM_CANCEL`#

An `Http2Stream` was destroyed before any data was transmitted to the connected peer.

#### `ERR_HTTP2_STREAM_ERROR`#

A non-zero error code was been specified in an `RST_STREAM` frame.

#### `ERR_HTTP2_STREAM_SELF_DEPENDENCY`#

When setting the priority for an HTTP/2 stream, the stream may be marked as a dependency for a parent stream. This error code is used when an attempt is made to mark a stream and dependent of itself.

#### `ERR_HTTP2_TOO_MANY_CUSTOM_SETTINGS`#

The number of supported custom settings (10) has been exceeded.

#### `ERR_HTTP2_TOO_MANY_INVALID_FRAMES`#

Added in: v15.14.0

The limit of acceptable invalid HTTP/2 protocol frames sent by the peer, as specified through the `maxSessionInvalidFrames` option, has been exceeded.

#### `ERR_HTTP2_TOO_MANY_ORIGINS`#

Added in: v26.3.1

The number of uniq origin sent by the server has exceeded the value defined in `options.maxOriginSetSize`.

#### `ERR_HTTP2_TRAILERS_ALREADY_SENT`#

Trailing headers have already been sent on the `Http2Stream`.

#### `ERR_HTTP2_TRAILERS_NOT_READY`#

The `http2stream.sendTrailers()` method cannot be called until after the `'wantTrailers'` event is emitted on an `Http2Stream` object. The `'wantTrailers'` event will only be emitted if the `waitForTrailers` option is set for the `Http2Stream`.

#### `ERR_HTTP2_UNSUPPORTED_PROTOCOL`#

`http2.connect()` was passed a URL that uses any protocol other than `http:` or `https:`.

#### `ERR_HTTP_BODY_NOT_ALLOWED`#

An error is thrown when writing to an HTTP response which does not allow contents.

#### `ERR_HTTP_CONTENT_LENGTH_MISMATCH`#

Response body size doesn't match with the specified content-length header value.

#### `ERR_HTTP_HEADERS_SENT`#

An attempt was made to add more headers after the headers had already been sent.

#### `ERR_HTTP_INVALID_HEADER_VALUE`#

An invalid HTTP header value was specified.

#### `ERR_HTTP_INVALID_STATUS_CODE`#

Status code was outside the regular status code range (100-999).

#### `ERR_HTTP_REQUEST_TIMEOUT`#

The client has not sent the entire request within the allowed time.

#### `ERR_HTTP_SOCKET_ASSIGNED`#

The given `ServerResponse` was already assigned a socket.

#### `ERR_HTTP_SOCKET_ENCODING`#

Changing the socket encoding is not allowed per RFC 7230 Section 3.

#### `ERR_HTTP_TRAILER_INVALID`#

The `Trailer` header was set even though the transfer encoding does not support that.

#### `ERR_ILLEGAL_CONSTRUCTOR`#

An attempt was made to construct an object using a non-public constructor.

#### `ERR_IMPORT_ATTRIBUTE_MISSING`#

Added in: v21.1.0

An import attribute is missing, preventing the specified module to be imported.

#### `ERR_IMPORT_ATTRIBUTE_TYPE_INCOMPATIBLE`#

Added in: v21.1.0

An import `type` attribute was provided, but the specified module is of a different type.

#### `ERR_IMPORT_ATTRIBUTE_UNSUPPORTED`#

Added in: v21.0.0, v20.10.0, v18.19.0

An import attribute is not supported by this version of Node.js.

#### `ERR_INCOMPATIBLE_OPTION_PAIR`#

An option pair is incompatible with each other and cannot be used at the same time.

#### `ERR_INPUT_TYPE_NOT_ALLOWED`#

The `--input-type` flag was used to attempt to execute a file. This flag can only be used with input via `--eval`, `--print`, or `STDIN`.

#### `ERR_INSPECTOR_ALREADY_ACTIVATED`#

While using the `node:inspector` module, an attempt was made to activate the inspector when it already started to listen on a port. Use `inspector.close()` before activating it on a different address.

#### `ERR_INSPECTOR_ALREADY_CONNECTED`#

While using the `node:inspector` module, an attempt was made to connect when the inspector was already connected.

#### `ERR_INSPECTOR_CLOSED`#

While using the `node:inspector` module, an attempt was made to use the inspector after the session had already closed.

#### `ERR_INSPECTOR_COMMAND`#

An error occurred while issuing a command via the `node:inspector` module.

#### `ERR_INSPECTOR_NOT_ACTIVE`#

The `inspector` is not active when `inspector.waitForDebugger()` is called.

#### `ERR_INSPECTOR_NOT_AVAILABLE`#

The `node:inspector` module is not available for use.

#### `ERR_INSPECTOR_NOT_CONNECTED`#

While using the `node:inspector` module, an attempt was made to use the inspector before it was connected.

#### `ERR_INSPECTOR_NOT_WORKER`#

An API was called on the main thread that can only be used from the worker thread.

#### `ERR_INTERNAL_ASSERTION`#

There was a bug in Node.js or incorrect usage of Node.js internals. To fix the error, open an issue at https://github.com/nodejs/node/issues.

#### `ERR_INVALID_ADDRESS`#

The provided address is not understood by the Node.js API.

#### `ERR_INVALID_ADDRESS_FAMILY`#

The provided address family is not understood by the Node.js API.

#### `ERR_INVALID_ARG_TYPE`#

An argument of the wrong type was passed to a Node.js API.

#### `ERR_INVALID_ARG_VALUE`#

An invalid or unsupported value was passed for a given argument.

#### `ERR_INVALID_ASYNC_ID`#

An invalid `asyncId` or `triggerAsyncId` was passed using `AsyncHooks`. An id less than -1 should never happen.

#### `ERR_INVALID_BUFFER_SIZE`#

A swap was performed on a `Buffer` but its size was not compatible with the operation.

#### `ERR_INVALID_CHAR`#

Invalid characters were detected in headers.

#### `ERR_INVALID_CURSOR_POS`#

A cursor on a given stream cannot be moved to a specified row without a specified column.

#### `ERR_INVALID_FD`#

A file descriptor ('fd') was not valid (e.g. it was a negative value).

#### `ERR_INVALID_FD_TYPE`#

A file descriptor ('fd') type was not valid.

#### `ERR_INVALID_FILE_URL_HOST`#

A Node.js API that consumes `file:` URLs (such as certain functions in the `fs` module) encountered a file URL with an incompatible host. This situation can only occur on Unix-like systems where only `localhost` or an empty host is supported.

#### `ERR_INVALID_FILE_URL_PATH`#

A Node.js API that consumes `file:` URLs (such as certain functions in the `fs` module) encountered a file URL with an incompatible path. The exact semantics for determining whether a path can be used is platform-dependent.

The thrown error object includes an `input` property that contains the URL object of the invalid `file:` URL.

#### `ERR_INVALID_HANDLE_TYPE`#

An attempt was made to send an unsupported "handle" over an IPC communication channel to a child process. See `subprocess.send()` and `process.send()` for more information.

#### `ERR_INVALID_HTTP_TOKEN`#

An invalid HTTP token was supplied.

#### `ERR_INVALID_IP_ADDRESS`#

An IP address is not valid.

#### `ERR_INVALID_MIME_SYNTAX`#

The syntax of a MIME is not valid.

#### `ERR_INVALID_MODULE`#

Added in: v15.0.0, v14.18.0

An attempt was made to load a module that does not exist or was otherwise not valid.

#### `ERR_INVALID_MODULE_SPECIFIER`#

The imported module string is an invalid URL, package name, or package subpath specifier.

#### `ERR_INVALID_OBJECT_DEFINE_PROPERTY`#

An error occurred while setting an invalid attribute on the property of an object.

#### `ERR_INVALID_PACKAGE_CONFIG`#

An invalid `package.json` file failed parsing.

#### `ERR_INVALID_PACKAGE_TARGET`#

The `package.json` `"exports"` field contains an invalid target mapping value for the attempted module resolution.

#### `ERR_INVALID_PROTOCOL`#

An invalid `options.protocol` was passed to `http.request()`.

#### `ERR_INVALID_REPL_EVAL_CONFIG`#

Both `breakEvalOnSigint` and `eval` options were set in the `REPL` config, which is not supported.

#### `ERR_INVALID_REPL_INPUT`#

The input may not be used in the `REPL`. The conditions under which this error is used are described in the `REPL` documentation.

#### `ERR_INVALID_RETURN_PROPERTY`#

Thrown in case a function option does not provide a valid value for one of its returned object properties on execution.

#### `ERR_INVALID_RETURN_PROPERTY_VALUE`#

Thrown in case a function option does not provide an expected value type for one of its returned object properties on execution.

#### `ERR_INVALID_RETURN_VALUE`#

Thrown in case a function option does not return an expected value type on execution, such as when a function is expected to return a promise.

#### `ERR_INVALID_STATE`#

Added in: v15.0.0

Indicates that an operation cannot be completed due to an invalid state. For instance, an object may have already been destroyed, or may be performing another operation.

#### `ERR_INVALID_SYNC_FORK_INPUT`#

A `Buffer`, `TypedArray`, `DataView`, or `string` was provided as stdio input to an asynchronous fork. See the documentation for the `child_process` module for more information.

#### `ERR_INVALID_THIS`#

A Node.js API function was called with an incompatible `this` value.

    const urlSearchParams = new URLSearchParams('foo=bar&baz=new');

    const buf = Buffer.alloc(1);
    urlSearchParams.has.call(buf, 'foo');
    // Throws a TypeError with code 'ERR_INVALID_THIS'
    js

#### `ERR_INVALID_TUPLE`#

An element in the `iterable` provided to the WHATWG `URLSearchParams` constructor did not represent a `[name, value]` tuple – that is, if an element is not iterable, or does not consist of exactly two elements.

#### `ERR_INVALID_TYPESCRIPT_SYNTAX`#

Added in: v23.0.0, v22.10.0
History

Version | Changes
--- | ---
v23.7.0, v22.14.0 | This error is no longer thrown on valid yet unsupported syntax.

The provided TypeScript syntax is not valid.

#### `ERR_INVALID_URI`#

An invalid URI was passed.

#### `ERR_INVALID_URL`#

An invalid URL was passed to the WHATWG `URL` constructor or the legacy `url.parse()` to be parsed. The thrown error object typically has an additional property `'input'` that contains the URL that failed to parse.

#### `ERR_INVALID_URL_PATTERN`#

An invalid URLPattern was passed to the WHATWG `URLPattern` constructor to be parsed.

#### `ERR_INVALID_URL_SCHEME`#

An attempt was made to use a URL of an incompatible scheme (protocol) for a specific purpose. It is only used in the WHATWG URL API support in the `fs` module (which only accepts URLs with `'file'` scheme), but may be used in other Node.js APIs as well in the future.

#### `ERR_IPC_CHANNEL_CLOSED`#

An attempt was made to use an IPC communication channel that was already closed.

#### `ERR_IPC_DISCONNECTED`#

An attempt was made to disconnect an IPC communication channel that was already disconnected. See the documentation for the `child_process` module for more information.

#### `ERR_IPC_ONE_PIPE`#

An attempt was made to create a child Node.js process using more than one IPC communication channel. See the documentation for the `child_process` module for more information.

#### `ERR_IPC_SYNC_FORK`#

An attempt was made to open an IPC communication channel with a synchronously forked Node.js process. See the documentation for the `child_process` module for more information.

#### `ERR_IP_BLOCKED`#

IP is blocked by `net.BlockList`.

#### `ERR_LOADER_CHAIN_INCOMPLETE`#

Added in: v18.6.0, v16.17.0

An ESM loader hook returned without calling `next()` and without explicitly signaling a short circuit.

#### `ERR_LOAD_SQLITE_EXTENSION`#

Added in: v23.5.0, v22.13.0

An error occurred while loading a SQLite extension.

#### `ERR_MEMORY_ALLOCATION_FAILED`#

An attempt was made to allocate memory (usually in the C++ layer) but it failed.

#### `ERR_MESSAGE_TARGET_CONTEXT_UNAVAILABLE`#

Added in: v14.5.0, v12.19.0

A message posted to a `MessagePort` could not be deserialized in the target vm `Context`. Not all Node.js objects can be successfully instantiated in any context at this time, and attempting to transfer them using `postMessage()` can fail on the receiving side in that case.

#### `ERR_METHOD_NOT_IMPLEMENTED`#

A method is required but not implemented.

#### `ERR_MISSING_ARGS`#

A required argument of a Node.js API was not passed. This is only used for strict compliance with the API specification (which in some cases may accept `func(undefined)` but not `func()`). In most native Node.js APIs, `func(undefined)` and `func()` are treated identically, and the `ERR_INVALID_ARG_TYPE` error code may be used instead.

#### `ERR_MISSING_OPTION`#

For APIs that accept options objects, some options might be mandatory. This code is thrown if a required option is missing.

#### `ERR_MISSING_PASSPHRASE`#

An attempt was made to read an encrypted key without specifying a passphrase.

#### `ERR_MISSING_PLATFORM_FOR_WORKER`#

The V8 platform used by this instance of Node.js does not support creating Workers. This is caused by lack of embedder support for Workers. In particular, this error will not occur with standard builds of Node.js.

#### `ERR_MODULE_LINK_MISMATCH`#

A module can not be linked because the same module requests in it are not resolved to the same module.

#### `ERR_MODULE_NOT_FOUND`#

A module file could not be resolved by the ECMAScript modules loader while attempting an `import` operation or when loading the program entry point.

#### `ERR_MULTIPLE_CALLBACK`#

A callback was called more than once.

A callback is almost always meant to only be called once as the query can either be fulfilled or rejected but not both at the same time. The latter would be possible by calling a callback more than once.

#### `ERR_NAPI_CONS_FUNCTION`#

While using `Node-API`, a constructor passed was not a function.

#### `ERR_NAPI_INVALID_DATAVIEW_ARGS`#

While calling `napi_create_dataview()`, a given `offset` was outside the bounds of the dataview or `offset + length` was larger than a length of given `buffer`.

#### `ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT`#

While calling `napi_create_typedarray()`, the provided `offset` was not a multiple of the element size.

#### `ERR_NAPI_INVALID_TYPEDARRAY_LENGTH`#

While calling `napi_create_typedarray()`, `(length * size_of_element) + byte_offset` was larger than the length of given `buffer`.

#### `ERR_NAPI_TSFN_CALL_JS`#

An error occurred while invoking the JavaScript portion of the thread-safe function.

#### `ERR_NAPI_TSFN_GET_UNDEFINED`#

An error occurred while attempting to retrieve the JavaScript `undefined` value.

#### `ERR_NON_CONTEXT_AWARE_DISABLED`#

A non-context-aware native addon was loaded in a process that disallows them.

#### `ERR_NOT_BUILDING_SNAPSHOT`#

An attempt was made to use operations that can only be used when building V8 startup snapshot even though Node.js isn't building one.

#### `ERR_NOT_IN_SINGLE_EXECUTABLE_APPLICATION`#

Added in: v21.7.0, v20.12.0

The operation cannot be performed when it's not in a single-executable application.

#### `ERR_NOT_SUPPORTED_IN_SNAPSHOT`#

An attempt was made to perform operations that are not supported when building a startup snapshot.

#### `ERR_NO_CRYPTO`#

An attempt was made to use crypto features while Node.js was not compiled with OpenSSL crypto support.

#### `ERR_NO_ICU`#

An attempt was made to use features that require ICU, but Node.js was not compiled with ICU support.

#### `ERR_NO_TEMPORAL`#

Added in: v26.2.0

An attempt was made to use features that require `Temporal`, but Node.js was not compiled with `Temporal` support or it has been disabled in the current environment (for example, when running with `--no-harmony-temporal`).

#### `ERR_NO_TYPESCRIPT`#

Added in: v23.0.0, v22.12.0

An attempt was made to use features that require Native TypeScript support, but Node.js was not compiled with TypeScript support.

#### `ERR_OPERATION_FAILED`#

Added in: v15.0.0

An operation failed. This is typically used to signal the general failure of an asynchronous operation.

#### `ERR_OPTIONS_BEFORE_BOOTSTRAPPING`#

Added in: v23.10.0, v22.16.0

An attempt was made to get options before the bootstrapping was completed.

#### `ERR_OUT_OF_RANGE`#

A given value is out of the accepted range.

#### `ERR_PACKAGE_IMPORT_NOT_DEFINED`#

The `package.json` `"imports"` field does not define the given internal package specifier mapping.

#### `ERR_PACKAGE_MAP_EXTERNAL_FILE`#

Added in: v26.4.0

A module attempted to resolve a bare specifier using the package map, but the importing file is not located within any package defined in the map.

    $ node --experimental-package-map=./package-map.json /tmp/script.js
    Error [ERR_PACKAGE_MAP_EXTERNAL_FILE]: Cannot resolve "dep-a" from "/tmp/script.js": file is not within any package defined in /path/to/package-map.json
    console

To fix this error, ensure the importing file is inside one of the package directories listed in the package map, or add a new package entry whose `url` covers the importing file.

#### `ERR_PACKAGE_MAP_INVALID`#

Added in: v26.4.0

The package map configuration file is invalid. This can occur when:

- The file does not exist at the specified path.
- The file contains invalid JSON.
- The file is missing the required `packages` object.
- A package entry is missing the required `url` field.
- Two package entries have the same `url` value.

    $ node --experimental-package-map=./missing.json app.js
    Error [ERR_PACKAGE_MAP_INVALID]: Invalid package map at "./missing.json": file not found
    console

#### `ERR_PACKAGE_MAP_KEY_NOT_FOUND`#

Added in: v26.4.0

A package's `dependencies` object in the package map references a package key that is not defined in the `packages` object.

    {
      "packages": {
        "app": {
          "url": "./app",
          "dependencies": {
            "foo": "nonexistent"
          }
        }
      }
    }
    json

In this example, `"nonexistent"` is referenced as a dependency target but not defined in `packages`, which will throw this error.

To fix this error, ensure all package keys referenced in `dependencies` values are defined in the `packages` object.

#### `ERR_PACKAGE_PATH_NOT_EXPORTED`#

The `package.json` `"exports"` field does not export the requested subpath. Because exports are encapsulated, private internal modules that are not exported cannot be imported through the package resolution, unless using an absolute URL.

#### `ERR_PARSE_ARGS_INVALID_OPTION_VALUE`#

Added in: v18.3.0, v16.17.0

When `strict` set to `true`, thrown by `util.parseArgs()` if a `<boolean>` value is provided for an option of type `<string>`, or if a `<string>` value is provided for an option of type `<boolean>`.

#### `ERR_PARSE_ARGS_UNEXPECTED_POSITIONAL`#

Added in: v18.3.0, v16.17.0

Thrown by `util.parseArgs()`, when a positional argument is provided and `allowPositionals` is set to `false`.

#### `ERR_PARSE_ARGS_UNKNOWN_OPTION`#

Added in: v18.3.0, v16.17.0

When `strict` set to `true`, thrown by `util.parseArgs()` if an argument is not configured in `options`.

#### `ERR_PERFORMANCE_INVALID_TIMESTAMP`#

An invalid timestamp value was provided for a performance mark or measure.

#### `ERR_PERFORMANCE_MEASURE_INVALID_OPTIONS`#

Invalid options were provided for a performance measure.

#### `ERR_PROTO_ACCESS`#

Accessing `Object.prototype.__proto__` has been forbidden using `--disable-proto=throw`. `Object.getPrototypeOf` and `Object.setPrototypeOf` should be used to get and set the prototype of an object.

#### `ERR_PROXY_INVALID_CONFIG`#

Failed to proxy a request because the proxy configuration is invalid.

#### `ERR_PROXY_TUNNEL`#

Failed to establish proxy tunnel when `NODE_USE_ENV_PROXY` or `--use-env-proxy` is enabled.

#### `ERR_QUIC_APPLICATION_ERROR`#

Added in: v23.4.0, v22.13.0

Stability: 1 - Experimental

A QUIC application error occurred.

#### `ERR_QUIC_CONNECTION_FAILED`#

Added in: v23.0.0, v22.10.0

Stability: 1 - Experimental

Establishing a QUIC connection failed.

#### `ERR_QUIC_ENDPOINT_CLOSED`#

Added in: v23.0.0, v22.10.0

Stability: 1 - Experimental

A QUIC Endpoint closed with an error.

#### `ERR_QUIC_OPEN_STREAM_FAILED`#

Added in: v23.0.0, v22.10.0

Stability: 1 - Experimental

Opening a QUIC stream failed.

#### `ERR_QUIC_STREAM_ABORTED`#

Added in: v26.2.0

Stability: 1 - Experimental

The Node.js error code for a `QuicError` thrown to abort a QUIC stream or session with an explicit application or transport error code.

#### `ERR_QUIC_STREAM_RESET`#

Added in: v26.2.0

Stability: 1 - Experimental

A QUIC stream was reset by the peer. The error includes the reset code provided by the peer.

#### `ERR_QUIC_TRANSPORT_ERROR`#

Added in: v23.4.0, v22.13.0

Stability: 1 - Experimental

A QUIC transport error occurred.

#### `ERR_QUIC_VERSION_NEGOTIATION_ERROR`#

Added in: v23.4.0, v22.13.0

Stability: 1 - Experimental

A QUIC session failed because version negotiation is required.

#### `ERR_REQUIRE_ASYNC_MODULE`#

History

Version | Changes
--- | ---
v26.5.0 | Added the `requireStack` and `topLevelAwaitLocations` properties.

When trying to `require()` an ES Module, the module turns out to be asynchronous. That is, it contains top-level await.

When uncaught, the flag `--experimental-print-required-tla` prints the locations of the top-level awaits in the graph to stderr.

This error has the following additional non-enumerable properties:

- `requireStack` `<string>`[] The chain of modules that led to the failing `require()`, starting with the module that required the asynchronous module.
- `topLevelAwaitLocations` `<Object>`[] The locations of the top-level awaits in the graph. Only populated when `--experimental-print-required-tla` is enabled. Each entry has the following properties:
  - `url` `<string>` The URL of the module containing the top-level await.
  - `line` `<number>` The 1-based line number of the top-level await.
  - `column` `<number>` The 1-based column number of the top-level await.
  - `sourceLine` `<string>` The source line containing the top-level await.

#### `ERR_REQUIRE_CYCLE_MODULE`#

When trying to `require()` an ES Module, a CommonJS to ESM or ESM to CommonJS edge participates in an immediate cycle. This is not allowed because ES Modules cannot be evaluated while they are already being evaluated.

To avoid the cycle, the `require()` call involved in a cycle should not happen at the top-level of either an ES Module (via `createRequire()`) or a CommonJS module, and should be done lazily in an inner function.

#### `ERR_REQUIRE_ESM`#

History

Version | Changes
--- | ---
v23.0.0, v22.12.0, v20.19.0 | require() now supports loading synchronous ES modules by default.

Stability: 0 - Deprecated

An attempt was made to `require()` an ES Module.

This error has been deprecated since `require()` now supports loading synchronous ES modules. When `require()` encounters an ES module that contains top-level `await`, it will throw `ERR_REQUIRE_ASYNC_MODULE` instead.

#### `ERR_REQUIRE_ESM_RACE_CONDITION`#

Added in: v26.1.0

Stability: 1 - Experimental.

An attempt was made to `require()` an ES Module while another `import()` call was already in progress to load it asynchronously.

#### `ERR_SCRIPT_EXECUTION_INTERRUPTED`#

Script execution was interrupted by `SIGINT` (For example, `Ctrl`+`C` was pressed.)

#### `ERR_SCRIPT_EXECUTION_TIMEOUT`#

Script execution timed out, possibly due to bugs in the script being executed.

#### `ERR_SERVER_ALREADY_LISTEN`#

The `server.listen()` method was called while a `net.Server` was already listening. This applies to all instances of `net.Server`, including HTTP, HTTPS, and HTTP/2 `Server` instances.

#### `ERR_SERVER_NOT_RUNNING`#

The `server.close()` method was called when a `net.Server` was not running. This applies to all instances of `net.Server`, including HTTP, HTTPS, and HTTP/2 `Server` instances.

#### `ERR_SINGLE_EXECUTABLE_APPLICATION_ASSET_NOT_FOUND`#

Added in: v21.7.0, v20.12.0

A key was passed to single executable application APIs to identify an asset, but no match could be found.

#### `ERR_SOCKET_ALREADY_BOUND`#

An attempt was made to bind a socket that has already been bound.

#### `ERR_SOCKET_BAD_BUFFER_SIZE`#

An invalid (negative) size was passed for either the `recvBufferSize` or `sendBufferSize` options in `dgram.createSocket()`.

#### `ERR_SOCKET_BAD_PORT`#

An API function expecting a port >= 0 and < 65536 received an invalid value.

#### `ERR_SOCKET_BAD_TYPE`#

An API function expecting a socket type (`udp4` or `udp6`) received an invalid value.

#### `ERR_SOCKET_BUFFER_SIZE`#

While using `dgram.createSocket()`, the size of the receive or send `Buffer` could not be determined.

#### `ERR_SOCKET_CLOSED`#

An attempt was made to operate on an already closed socket.

#### `ERR_SOCKET_CLOSED_BEFORE_CONNECTION`#

When calling `net.Socket.write()` on a connecting socket and the socket was closed before the connection was established.

#### `ERR_SOCKET_CONNECTION_TIMEOUT`#

The socket was unable to connect to any address returned by the DNS within the allowed timeout when using the family autoselection algorithm.

#### `ERR_SOCKET_DGRAM_IS_CONNECTED`#

A `dgram.connect()` call was made on an already connected socket.

#### `ERR_SOCKET_DGRAM_NOT_CONNECTED`#

A `dgram.disconnect()` or `dgram.remoteAddress()` call was made on a disconnected socket.

#### `ERR_SOCKET_DGRAM_NOT_RUNNING`#

A call was made and the UDP subsystem was not running.

#### `ERR_SOCKET_HANDLE_ADOPTED`#

An operation was attempted on a `BoundSocket` that had already been adopted by a `net.Server` or `net.Socket`. Once a bound socket is adopted, its `address()` and `close()` methods can no longer be used.

#### `ERR_SOURCE_MAP_CORRUPT`#

The source map could not be parsed because it does not exist, or is corrupt.

#### `ERR_SOURCE_MAP_MISSING_SOURCE`#

A file imported from a source map was not found.

#### `ERR_SOURCE_PHASE_NOT_DEFINED`#

Added in: v24.0.0

The provided module import does not provide a source phase imports representation for source phase import syntax `import source x from 'x'` or `import.source(x)`.

#### `ERR_SQLITE_ERROR`#

Added in: v22.5.0

An error was returned from SQLite.

#### `ERR_SRI_PARSE`#

A string was provided for a Subresource Integrity check, but was unable to be parsed. Check the format of integrity attributes by looking at the Subresource Integrity specification.

#### `ERR_STREAM_ALREADY_FINISHED`#

A stream method was called that cannot complete because the stream was finished.

#### `ERR_STREAM_CANNOT_PIPE`#

An attempt was made to call `stream.pipe()` on a `Writable` stream.

#### `ERR_STREAM_DESTROYED`#

A stream method was called that cannot complete because the stream was destroyed using `stream.destroy()`.

#### `ERR_STREAM_ITER_MISSING_FLAG`#

A stream/iter API was used without the `--experimental-stream-iter` CLI flag enabled.

#### `ERR_STREAM_NULL_VALUES`#

An attempt was made to call `stream.write()` with a `null` chunk.

#### `ERR_STREAM_PREMATURE_CLOSE`#

An error returned by `stream.finished()` and `stream.pipeline()`, when a stream or a pipeline ends non gracefully with no explicit error.

#### `ERR_STREAM_PUSH_AFTER_EOF`#

An attempt was made to call `stream.push()` after a `null`(EOF) had been pushed to the stream.

#### `ERR_STREAM_UNABLE_TO_PIPE`#

An attempt was made to pipe to a closed or destroyed stream in a pipeline.

#### `ERR_STREAM_UNSHIFT_AFTER_END_EVENT`#

An attempt was made to call `stream.unshift()` after the `'end'` event was emitted.

#### `ERR_STREAM_WRAP`#

Prevents an abort if a string decoder was set on the Socket or if the decoder is in `objectMode`.

    const Socket = require('node:net').Socket;
    const instance = new Socket();

    instance.setEncoding('utf8');
    js

#### `ERR_STREAM_WRITE_AFTER_END`#

An attempt was made to call `stream.write()` after `stream.end()` has been called.

#### `ERR_STRING_TOO_LONG`#

An attempt has been made to create a string longer than the maximum allowed length.

#### `ERR_SYNTHETIC`#

An artificial error object used to capture the call stack for diagnostic reports.

#### `ERR_SYSTEM_ERROR`#

An unspecified or non-specific system error has occurred within the Node.js process. The error object will have an `err.info` object property with additional details.

#### `ERR_TEST_FAILURE`#

This error represents a failed test. Additional information about the failure is available via the `cause` property. The `failureType` property specifies what the test was doing when the failure occurred.

#### `ERR_TLS_ALPN_CALLBACK_INVALID_RESULT`#

This error is thrown when an `ALPNCallback` returns a value that is not in the list of ALPN protocols offered by the client.

#### `ERR_TLS_ALPN_CALLBACK_WITH_PROTOCOLS`#

This error is thrown when creating a `TLSServer` if the TLS options include both `ALPNProtocols` and `ALPNCallback`. These options are mutually exclusive.

#### `ERR_TLS_CERT_ALTNAME_FORMAT`#

This error is thrown by `checkServerIdentity` if a user-supplied `subjectaltname` property violates encoding rules. Certificate objects produced by Node.js itself always comply with encoding rules and will never cause this error.

#### `ERR_TLS_CERT_ALTNAME_INVALID`#

While using TLS, the host name/IP of the peer did not match any of the `subjectAltNames` in its certificate.

#### `ERR_TLS_DH_PARAM_SIZE`#

While using TLS, the parameter offered for the Diffie-Hellman (`DH`) key-agreement protocol is too small. By default, the key length must be greater than or equal to 1024 bits to avoid vulnerabilities, even though it is strongly recommended to use 2048 bits or larger for stronger security.

#### `ERR_TLS_HANDSHAKE_TIMEOUT`#

A TLS/SSL handshake timed out. In this case, the server must also abort the connection.

#### `ERR_TLS_INVALID_CONTEXT`#

Added in: v13.3.0

The context must be a `SecureContext`.

#### `ERR_TLS_INVALID_PROTOCOL_METHOD`#

The specified `secureProtocol` method is invalid. It is either unknown, or disabled because it is insecure.

#### `ERR_TLS_INVALID_PROTOCOL_VERSION`#

Valid TLS protocol versions are `'TLSv1'`, `'TLSv1.1'`, or `'TLSv1.2'`.

#### `ERR_TLS_INVALID_STATE`#

Added in: v13.10.0, v12.17.0

The TLS socket must be connected and securely established. Ensure the 'secure' event is emitted before continuing.

#### `ERR_TLS_PROTOCOL_VERSION_CONFLICT`#

Attempting to set a TLS protocol `minVersion` or `maxVersion` conflicts with an attempt to set the `secureProtocol` explicitly. Use one mechanism or the other.

#### `ERR_TLS_PSK_SET_IDENTITY_HINT_FAILED`#

Failed to set PSK identity hint. Hint may be too long.

#### `ERR_TLS_RENEGOTIATION_DISABLED`#

An attempt was made to renegotiate TLS on a socket instance with renegotiation disabled.

#### `ERR_TLS_RENEGOTIATION_UNSUPPORTED`#

An attempt was made to renegotiate TLS, but the TLS implementation does not support caller-initiated renegotiation.

#### `ERR_TLS_REQUIRED_SERVER_NAME`#

While using TLS, the `server.addContext()` method was called without providing a host name in the first parameter.

#### `ERR_TLS_SESSION_ATTACK`#

An excessive amount of TLS renegotiations is detected, which is a potential vector for denial-of-service attacks.

#### `ERR_TLS_SNI_FROM_SERVER`#

An attempt was made to issue Server Name Indication from a TLS server-side socket, which is only valid from a client.

#### `ERR_TRACE_EVENTS_CATEGORY_REQUIRED`#

The `trace_events.createTracing()` method requires at least one trace event category.

#### `ERR_TRACE_EVENTS_UNAVAILABLE`#

The `node:trace_events` module could not be loaded because Node.js was compiled with the `--without-v8-platform` flag.

#### `ERR_TRAILING_JUNK_AFTER_STREAM_END`#

Trailing junk found after the end of the compressed stream. This error is thrown when extra, unexpected data is detected after the end of a compressed stream (for example, in zlib or gzip decompression).

#### `ERR_TRANSFORM_ALREADY_TRANSFORMING`#

A `Transform` stream finished while it was still transforming.

#### `ERR_TRANSFORM_WITH_LENGTH_0`#

A `Transform` stream finished with data still in the write buffer.

#### `ERR_TTY_INIT_FAILED`#

The initialization of a TTY failed due to a system error.

#### `ERR_UNAVAILABLE_DURING_EXIT`#

Function was called within a `process.on('exit')` handler that shouldn't be called within `process.on('exit')` handler.

#### `ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET`#

`process.setUncaughtExceptionCaptureCallback()` was called twice, without first resetting the callback to `null`.

This error is designed to prevent accidentally overwriting a callback registered from another module.

#### `ERR_UNESCAPED_CHARACTERS`#

A string that contained unescaped characters was received.

#### `ERR_UNHANDLED_ERROR`#

An unhandled error occurred (for instance, when an `'error'` event is emitted by an `EventEmitter` but an `'error'` handler is not registered).

#### `ERR_UNKNOWN_BUILTIN_MODULE`#

Used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself.

#### `ERR_UNKNOWN_CREDENTIAL`#

A Unix group or user identifier that does not exist was passed.

#### `ERR_UNKNOWN_ENCODING`#

An invalid or unknown encoding option was passed to an API.

#### `ERR_UNKNOWN_FILE_EXTENSION`#

An attempt was made to load a module with an unknown or unsupported file extension.

#### `ERR_UNKNOWN_MODULE_FORMAT`#

An attempt was made to load a module with an unknown or unsupported format.

#### `ERR_UNKNOWN_SIGNAL`#

An invalid or unknown process signal was passed to an API expecting a valid signal (such as `subprocess.kill()`).

#### `ERR_UNSUPPORTED_DIR_IMPORT`#

`import` a directory URL is unsupported. Instead, self-reference a package using its name and define a custom subpath in the `"exports"` field of the `package.json` file.

    import './'; // unsupported
    import './index.js'; // supported
    import 'package-name'; // supported
    mjs

#### `ERR_UNSUPPORTED_ESM_URL_SCHEME`#

`import` with URL schemes other than `file` and `data` is unsupported.

#### `ERR_UNSUPPORTED_NODE_MODULES_TYPE_STRIPPING`#

Added in: v22.6.0

Type stripping is not supported for files descendant of a `node_modules` directory.

#### `ERR_UNSUPPORTED_RESOLVE_REQUEST`#

An attempt was made to resolve an invalid module referrer. This can happen when importing or calling `import.meta.resolve()` with either:

- a bare specifier that is not a builtin module from a module whose URL scheme is not `file`.
- a relative URL from a module whose URL scheme is not a special scheme.

    try {
      // Trying to import the package 'bare-specifier' from a `data:` URL module:
      await import('data:text/javascript,import "bare-specifier"');
    } catch (e) {
      console.log(e.code); // ERR_UNSUPPORTED_RESOLVE_REQUEST
    }
    mjs

#### `ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX`#

Added in: v23.7.0, v22.14.0

The provided TypeScript syntax is unsupported. This could happen when using TypeScript syntax that requires transformation with type-stripping.

#### `ERR_USE_AFTER_CLOSE`#

An attempt was made to use something that was already closed.

#### `ERR_VALID_PERFORMANCE_ENTRY_TYPE`#

While using the Performance Timing API (`perf_hooks`), no valid performance entry types are found.

#### `ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING`#

A dynamic import callback was not specified.

#### `ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING_FLAG`#

A dynamic import callback was invoked without `--experimental-vm-modules`.

#### `ERR_VM_MODULE_ALREADY_LINKED`#

The module attempted to be linked is not eligible for linking, because of one of the following reasons:

- It has already been linked (`linkingStatus` is `'linked'`)
- It is being linked (`linkingStatus` is `'linking'`)
- Linking has failed for this module (`linkingStatus` is `'errored'`)

#### `ERR_VM_MODULE_CACHED_DATA_REJECTED`#

The `cachedData` option passed to a module constructor is invalid.

#### `ERR_VM_MODULE_CANNOT_CREATE_CACHED_DATA`#

Cached data cannot be created for modules which have already been evaluated.

#### `ERR_VM_MODULE_DIFFERENT_CONTEXT`#

The module being returned from the linker function is from a different context than the parent module. Linked modules must share the same context.

#### `ERR_VM_MODULE_LINK_FAILURE`#

The module was unable to be linked due to a failure.

#### `ERR_VM_MODULE_NOT_MODULE`#

The fulfilled value of a linking promise is not a `vm.Module` object.

#### `ERR_VM_MODULE_STATUS`#

The current module's status does not allow for this operation. The specific meaning of the error depends on the specific function.

#### `ERR_WASI_ALREADY_STARTED`#

The WASI instance has already started.

#### `ERR_WASI_NOT_STARTED`#

The WASI instance has not been started.

#### `ERR_WEBASSEMBLY_NOT_SUPPORTED`#

A feature requiring WebAssembly was used, but WebAssembly is not supported or has been disabled in the current environment (for example, when running with `--jitless`).

#### `ERR_WEBASSEMBLY_RESPONSE`#

Added in: v18.1.0

The `Response` that has been passed to `WebAssembly.compileStreaming` or to `WebAssembly.instantiateStreaming` is not a valid WebAssembly response.

#### `ERR_WORKER_HANDLE_NOT_TRANSFERABLE`#

An attempt was made to transfer a `net.Socket` or `net.Server` to another thread via a `worker_threads` `postMessage()` call while it was not in a transferable state, for example because it had already started reading or had buffered data.

#### `ERR_WORKER_INIT_FAILED`#

The `Worker` initialization failed.

#### `ERR_WORKER_INVALID_EXEC_ARGV`#

The `execArgv` option passed to the `Worker` constructor contains invalid flags.

#### `ERR_WORKER_MESSAGING_ERRORED`#

Added in: v22.5.0

Stability: 1.1 - Active development

The destination thread threw an error while processing a message sent via `postMessageToThread()`.

#### `ERR_WORKER_MESSAGING_FAILED`#

Added in: v22.5.0

Stability: 1.1 - Active development

The thread requested in `postMessageToThread()` is invalid or has no `workerMessage` listener.

#### `ERR_WORKER_MESSAGING_SAME_THREAD`#

Added in: v22.5.0

Stability: 1.1 - Active development

The thread id requested in `postMessageToThread()` is the current thread id.

#### `ERR_WORKER_MESSAGING_TIMEOUT`#

Added in: v22.5.0

Stability: 1.1 - Active development

Sending a message via `postMessageToThread()` timed out.

#### `ERR_WORKER_NOT_RUNNING`#

An operation failed because the `Worker` instance is not currently running.

#### `ERR_WORKER_OUT_OF_MEMORY`#

The `Worker` instance terminated because it reached its memory limit.

#### `ERR_WORKER_PATH`#

The path for the main script of a worker is neither an absolute path nor a relative path starting with `./` or `../`.

#### `ERR_WORKER_UNSERIALIZABLE_ERROR`#

All attempts at serializing an uncaught exception from a worker thread failed.

#### `ERR_WORKER_UNSUPPORTED_OPERATION`#

The requested functionality is not supported in worker threads.

#### `ERR_ZLIB_INITIALIZATION_FAILED`#

Creation of a `zlib` object failed due to incorrect configuration.

#### `ERR_ZSTD_INVALID_PARAM`#

An invalid parameter key was passed during construction of a Zstd stream.

#### `HPE_CHUNK_EXTENSIONS_OVERFLOW`#

Added in: v21.6.2, v20.11.1, v18.19.1

Too much data was received for a chunk extensions. In order to protect against malicious or malconfigured clients, if more than 16 KiB of data is received then an `Error` with this code will be emitted.

#### `HPE_HEADER_OVERFLOW`#

History

Version | Changes
--- | ---
v11.4.0, v10.15.0 | Max header size in `http_parser` was set to 8 KiB.

Too much HTTP header data was received. In order to protect against malicious or malconfigured clients, if more than `maxHeaderSize` of HTTP header data is received then HTTP parsing will abort without a request or response object being created, and an `Error` with this code will be emitted.

#### `HPE_UNEXPECTED_CONTENT_LENGTH`#

Server is sending both a `Content-Length` header and `Transfer-Encoding: chunked`.

`Transfer-Encoding: chunked` allows the server to maintain an HTTP persistent connection for dynamically generated content. In this case, the `Content-Length` HTTP header cannot be used.

Use `Content-Length` or `Transfer-Encoding: chunked`.

#### `MODULE_NOT_FOUND`#

History

Version | Changes
--- | ---
v12.0.0 | Added `requireStack` property.

A module file could not be resolved by the CommonJS modules loader while attempting a `require()` operation or when loading the program entry point.

### Legacy Node.js error codes#

Stability: 0 - Deprecated. These error codes are either inconsistent, or have been removed.

#### `ERR_CANNOT_TRANSFER_OBJECT`#

Added in: v10.5.0Removed in: v12.5.0

The value passed to `postMessage()` contained an object that is not supported for transferring.

#### `ERR_CPU_USAGE`#

Removed in: v15.0.0

The native call from `process.cpuUsage` could not be processed.

#### `ERR_CRYPTO_HASH_DIGEST_NO_UTF16`#

Added in: v9.0.0Removed in: v12.12.0

The UTF-16 encoding was used with `hash.digest()`. While the `hash.digest()` method does allow an `encoding` argument to be passed in, causing the method to return a string rather than a `Buffer`, the UTF-16 encoding (e.g. `ucs` or `utf16le`) is not supported.

#### `ERR_CRYPTO_SCRYPT_INVALID_PARAMETER`#

Removed in: v23.0.0

An incompatible combination of options was passed to `crypto.scrypt()` or `crypto.scryptSync()`. New versions of Node.js use the error code `ERR_INCOMPATIBLE_OPTION_PAIR` instead, which is consistent with other APIs.

#### `ERR_FS_INVALID_SYMLINK_TYPE`#

Removed in: v23.0.0

An invalid symlink type was passed to the `fs.symlink()` or `fs.symlinkSync()` methods.

#### `ERR_HTTP2_FRAME_ERROR`#

Added in: v9.0.0Removed in: v10.0.0

Used when a failure occurs sending an individual frame on the HTTP/2 session.

#### `ERR_HTTP2_HEADERS_OBJECT`#

Added in: v9.0.0Removed in: v10.0.0

Used when an HTTP/2 Headers Object is expected.

#### `ERR_HTTP2_HEADER_REQUIRED`#

Added in: v9.0.0Removed in: v10.0.0

Used when a required header is missing in an HTTP/2 message.

#### `ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND`#

Added in: v9.0.0Removed in: v10.0.0

HTTP/2 informational headers must only be sent prior to calling the `Http2Stream.prototype.respond()` method.

#### `ERR_HTTP2_STREAM_CLOSED`#

Added in: v9.0.0Removed in: v10.0.0

Used when an action has been performed on an HTTP/2 Stream that has already been closed.

#### `ERR_HTTP_INVALID_CHAR`#

Added in: v9.0.0Removed in: v10.0.0

Used when an invalid character is found in an HTTP response status message (reason phrase).

#### `ERR_IMPORT_ASSERTION_TYPE_FAILED`#

Added in: v17.1.0, v16.14.0Removed in: v21.1.0

An import assertion has failed, preventing the specified module to be imported.

#### `ERR_IMPORT_ASSERTION_TYPE_MISSING`#

Added in: v17.1.0, v16.14.0Removed in: v21.1.0

An import assertion is missing, preventing the specified module to be imported.

#### `ERR_IMPORT_ASSERTION_TYPE_UNSUPPORTED`#

Added in: v17.1.0, v16.14.0Removed in: v21.1.0

An import attribute is not supported by this version of Node.js.

#### `ERR_INDEX_OUT_OF_RANGE`#

Added in: v10.0.0Removed in: v11.0.0

A given index was out of the accepted range (e.g. negative offsets).

#### `ERR_INVALID_OPT_VALUE`#

Added in: v8.0.0Removed in: v15.0.0

An invalid or unexpected value was passed in an options object.

#### `ERR_INVALID_OPT_VALUE_ENCODING`#

Added in: v9.0.0Removed in: v15.0.0

An invalid or unknown file encoding was passed.

#### `ERR_INVALID_PERFORMANCE_MARK`#

Added in: v8.5.0Removed in: v16.7.0

While using the Performance Timing API (`perf_hooks`), a performance mark is invalid.

#### `ERR_INVALID_TRANSFER_OBJECT`#

Removed in: v21.0.0
History

Version | Changes
--- | ---
v21.0.0 | A `DOMException` is thrown instead.

An invalid transfer object was passed to `postMessage()`.

#### `ERR_MANIFEST_ASSERT_INTEGRITY`#

Removed in: v22.2.0

An attempt was made to load a resource, but the resource did not match the integrity defined by the policy manifest. See the documentation for policy manifests for more information.

#### `ERR_MANIFEST_DEPENDENCY_MISSING`#

Removed in: v22.2.0

An attempt was made to load a resource, but the resource was not listed as a dependency from the location that attempted to load it. See the documentation for policy manifests for more information.

#### `ERR_MANIFEST_INTEGRITY_MISMATCH`#

Removed in: v22.2.0

An attempt was made to load a policy manifest, but the manifest had multiple entries for a resource which did not match each other. Update the manifest entries to match in order to resolve this error. See the documentation for policy manifests for more information.

#### `ERR_MANIFEST_INVALID_RESOURCE_FIELD`#

Removed in: v22.2.0

A policy manifest resource had an invalid value for one of its fields. Update the manifest entry to match in order to resolve this error. See the documentation for policy manifests for more information.

#### `ERR_MANIFEST_INVALID_SPECIFIER`#

Removed in: v22.2.0

A policy manifest resource had an invalid value for one of its dependency mappings. Update the manifest entry to match to resolve this error. See the documentation for policy manifests for more information.

#### `ERR_MANIFEST_PARSE_POLICY`#

Removed in: v22.2.0

An attempt was made to load a policy manifest, but the manifest was unable to be parsed. See the documentation for policy manifests for more information.

#### `ERR_MANIFEST_TDZ`#

Removed in: v22.2.0

An attempt was made to read from a policy manifest, but the manifest initialization has not yet taken place. This is likely a bug in Node.js.

#### `ERR_MANIFEST_UNKNOWN_ONERROR`#

Removed in: v22.2.0

A policy manifest was loaded, but had an unknown value for its "onerror" behavior. See the documentation for policy manifests for more information.

#### `ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST`#

Removed in: v15.0.0

This error code was replaced by `ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST` in Node.js 15.0.0, because it is no longer accurate as other types of transferable objects also exist now.

#### `ERR_MISSING_TRANSFERABLE_IN_TRANSFER_LIST`#

Added in: v15.0.0Removed in: v21.0.0
History

Version | Changes
--- | ---
v21.0.0 | A `DOMException` is thrown instead.

An object that needs to be explicitly listed in the `transferList` argument is in the object passed to a `postMessage()` call, but is not provided in the `transferList` for that call. Usually, this is a `MessagePort`.

In Node.js versions prior to v15.0.0, the error code being used here was `ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST`. However, the set of transferable object types has been expanded to cover more types than `MessagePort`.

#### `ERR_NAPI_CONS_PROTOTYPE_OBJECT`#

Added in: v9.0.0Removed in: v10.0.0

Used by the `Node-API` when `Constructor.prototype` is not an object.

#### `ERR_NAPI_TSFN_START_IDLE_LOOP`#

Added in: v10.6.0, v8.16.0Removed in: v14.2.0, v12.17.0

On the main thread, values are removed from the queue associated with the thread-safe function in an idle loop. This error indicates that an error has occurred when attempting to start the loop.

#### `ERR_NAPI_TSFN_STOP_IDLE_LOOP`#

Added in: v10.6.0, v8.16.0Removed in: v14.2.0, v12.17.0

Once no more items are left in the queue, the idle loop must be suspended. This error indicates that the idle loop has failed to stop.

#### `ERR_NO_LONGER_SUPPORTED`#

A Node.js API was called in an unsupported manner, such as `Buffer.write(string, encoding, offset[, length])`.

#### `ERR_OUTOFMEMORY`#

Added in: v9.0.0Removed in: v10.0.0

Used generically to identify that an operation caused an out of memory condition.

#### `ERR_PARSE_HISTORY_DATA`#

Added in: v9.0.0Removed in: v10.0.0

The `node:repl` module was unable to parse data from the REPL history file.

#### `ERR_SOCKET_CANNOT_SEND`#

Added in: v9.0.0Removed in: v14.0.0

Data could not be sent on a socket.

#### `ERR_STDERR_CLOSE`#

Removed in: v10.12.0
History

Version | Changes
--- | ---
v10.12.0 | Rather than emitting an error, `process.stderr.end()` now only closes the stream side but not the underlying resource, making this error obsolete.

An attempt was made to close the `process.stderr` stream. By design, Node.js does not allow `stdout` or `stderr` streams to be closed by user code.

#### `ERR_STDOUT_CLOSE`#

Removed in: v10.12.0
History

Version | Changes
--- | ---
v10.12.0 | Rather than emitting an error, `process.stderr.end()` now only closes the stream side but not the underlying resource, making this error obsolete.

An attempt was made to close the `process.stdout` stream. By design, Node.js does not allow `stdout` or `stderr` streams to be closed by user code.

#### `ERR_STREAM_READ_NOT_IMPLEMENTED`#

Added in: v9.0.0Removed in: v10.0.0

Used when an attempt is made to use a readable stream that has not implemented `readable._read()`.

#### `ERR_TAP_LEXER_ERROR`#

An error representing a failing lexer state.

#### `ERR_TAP_PARSER_ERROR`#

An error representing a failing parser state. Additional information about the token causing the error is available via the `cause` property.

#### `ERR_TAP_VALIDATION_ERROR`#

This error represents a failed TAP validation.

#### `ERR_TLS_RENEGOTIATION_FAILED`#

Added in: v9.0.0Removed in: v10.0.0

Used when a TLS renegotiation request has failed in a non-specific way.

#### `ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER`#

Added in: v10.5.0Removed in: v14.0.0

A `SharedArrayBuffer` whose memory is not managed by the JavaScript engine or by Node.js was encountered during serialization. Such a `SharedArrayBuffer` cannot be serialized.

This can only happen when native addons create `SharedArrayBuffer`s in "externalized" mode, or put existing `SharedArrayBuffer` into externalized mode.

#### `ERR_UNKNOWN_STDIN_TYPE`#

Added in: v8.0.0Removed in: v11.7.0

An attempt was made to launch a Node.js process with an unknown `stdin` file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.

#### `ERR_UNKNOWN_STREAM_TYPE`#

Added in: v8.0.0Removed in: v11.7.0

An attempt was made to launch a Node.js process with an unknown `stdout` or `stderr` file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.

#### `ERR_V8BREAKITERATOR`#

The V8 `BreakIterator` API was used but the full ICU data set is not installed.

#### `ERR_VALUE_OUT_OF_RANGE`#

Added in: v9.0.0Removed in: v10.0.0

Used when a given value is out of the accepted range.

#### `ERR_VM_MODULE_LINKING_ERRORED`#

Added in: v10.0.0Removed in: v18.1.0, v16.17.0

The linker function returned a module for which linking has failed.

#### `ERR_VM_MODULE_NOT_LINKED`#

The module must be successfully linked before instantiation.

#### `ERR_WORKER_UNSUPPORTED_EXTENSION`#

Added in: v11.0.0Removed in: v16.9.0

The pathname used for the main script of a worker has an unknown file extension.

#### `ERR_ZLIB_BINDING_CLOSED`#

Added in: v9.0.0Removed in: v10.0.0

Used when an attempt is made to use a `zlib` object after it has already been closed.

### OpenSSL Error Codes#

#### Time Validity Errors#

##### `CERT_NOT_YET_VALID`#

The certificate is not yet valid: the notBefore date is after the current time.

##### `CERT_HAS_EXPIRED`#

The certificate has expired: the notAfter date is before the current time.

##### `CRL_NOT_YET_VALID`#

The certificate revocation list (CRL) has a future issue date.

##### `CRL_HAS_EXPIRED`#

The certificate revocation list (CRL) has expired.

##### `CERT_REVOKED`#

The certificate has been revoked; it is on a certificate revocation list (CRL).

#### Trust or Chain Related Errors#

##### `UNABLE_TO_GET_ISSUER_CERT`#

The issuer certificate of a looked up certificate could not be found. This normally means the list of trusted certificates is not complete.

##### `UNABLE_TO_GET_ISSUER_CERT_LOCALLY`#

The certificate’s issuer is not known. This is the case if the issuer is not included in the trusted certificate list.

##### `DEPTH_ZERO_SELF_SIGNED_CERT`#

The passed certificate is self-signed and the same certificate cannot be found in the list of trusted certificates.

##### `SELF_SIGNED_CERT_IN_CHAIN`#

The certificate’s issuer is not known. This is the case if the issuer is not included in the trusted certificate list.

##### `CERT_CHAIN_TOO_LONG`#

The certificate chain length is greater than the maximum depth.

##### `UNABLE_TO_GET_CRL`#

The CRL reference by the certificate could not be found.

##### `UNABLE_TO_VERIFY_LEAF_SIGNATURE`#

No signatures could be verified because the chain contains only one certificate and it is not self signed.

##### `CERT_UNTRUSTED`#

The root certificate authority (CA) is not marked as trusted for the specified purpose.

#### Basic Extension Errors#

##### `INVALID_CA`#

A CA certificate is invalid. Either it is not a CA or its extensions are not consistent with the supplied purpose.

##### `PATH_LENGTH_EXCEEDED`#

The basicConstraints pathlength parameter has been exceeded.

#### Name Related Errors#

##### `HOSTNAME_MISMATCH`#

Certificate does not match provided name.

#### Usage and Policy Errors#

##### `INVALID_PURPOSE`#

The supplied certificate cannot be used for the specified purpose.

##### `CERT_REJECTED`#

The root CA is marked to reject the specified purpose.

#### Formatting Errors#

##### `CERT_SIGNATURE_FAILURE`#

The signature of the certificate is invalid.

##### `CRL_SIGNATURE_FAILURE`#

The signature of the certificate revocation list (CRL) is invalid.

##### `ERROR_IN_CERT_NOT_BEFORE_FIELD`#

The certificate notBefore field contains an invalid time.

##### `ERROR_IN_CERT_NOT_AFTER_FIELD`#

The certificate notAfter field contains an invalid time.

##### `ERROR_IN_CRL_LAST_UPDATE_FIELD`#

The CRL lastUpdate field contains an invalid time.

##### `ERROR_IN_CRL_NEXT_UPDATE_FIELD`#

The CRL nextUpdate field contains an invalid time.

##### `UNABLE_TO_DECRYPT_CERT_SIGNATURE`#

The certificate signature could not be decrypted. This means that the actual signature value could not be determined rather than it not matching the expected value, this is only meaningful for RSA keys.

##### `UNABLE_TO_DECRYPT_CRL_SIGNATURE`#

The certificate revocation list (CRL) signature could not be decrypted: this means that the actual signature value could not be determined rather than it not matching the expected value.

##### `UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY`#

The public key in the certificate SubjectPublicKeyInfo could not be read.

#### Other OpenSSL Errors#

##### `OUT_OF_MEM`#

An error occurred trying to allocate memory. This should never happen.