/*
 * Copyright (c) 2020, Stephan Unverwerth <s.unverwerth@serenityos.org>
 * Copyright (c) 2020-2023, Linus Groh <linusg@serenityos.org>
 * Copyright (c) 2023-2025, Andreas Kling <andreas@ladybird.org>
 * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Debug.h>
#include <AK/Function.h>
#include <LibGC/DeferGC.h>
#include <LibJS/Bytecode/Debug.h>
#include <LibJS/Runtime/AbstractOperations.h>
#include <LibJS/Runtime/Array.h>
#include <LibJS/Runtime/AsyncFunctionDriverWrapper.h>
#include <LibJS/Runtime/AsyncGenerator.h>
#include <LibJS/Runtime/ECMAScriptFunctionObject.h>
#include <LibJS/Runtime/Error.h>
#include <LibJS/Runtime/ExecutionContext.h>
#include <LibJS/Runtime/FunctionEnvironment.h>
#include <LibJS/Runtime/GeneratorObject.h>
#include <LibJS/Runtime/GlobalEnvironment.h>
#include <LibJS/Runtime/GlobalObject.h>
#include <LibJS/Runtime/NativeFunction.h>
#include <LibJS/Runtime/PromiseCapability.h>
#include <LibJS/Runtime/PromiseConstructor.h>
#include <LibJS/Runtime/VM.h>
#include <LibJS/Runtime/Value.h>
#include <LibJS/Runtime/ValueInlines.h>
#include <LibJS/RustIntegration.h>

namespace JS {

GC_DEFINE_ALLOCATOR(ECMAScriptFunctionObject);

static GC::Ref<Object> prototype_for_function_kind(Realm& realm, FunctionKind kind)
{
    switch (kind) {
    case FunctionKind::Normal:
        return realm.intrinsics().function_prototype();
    case FunctionKind::Generator:
        return realm.intrinsics().generator_function_prototype();
    case FunctionKind::Async:
        return realm.intrinsics().async_function_prototype();
    case FunctionKind::AsyncGenerator:
        return realm.intrinsics().async_generator_function_prototype();
    }
    VERIFY_NOT_REACHED();
}

GC::Ref<ECMAScriptFunctionObject> ECMAScriptFunctionObject::create_from_function_data(
    GC::Ref<Realm> realm,
    GC::Ref<SharedFunctionInstanceData> shared_data,
    GC::Ptr<Environment> parent_environment,
    GC::Ptr<PrivateEnvironment> private_environment,
    Object& prototype)
{
    return realm->create<ECMAScriptFunctionObject>(
        *shared_data,
        parent_environment,
        private_environment,
        prototype);
}

GC::Ref<ECMAScriptFunctionObject> ECMAScriptFunctionObject::create_from_function_data(
    GC::Ref<Realm> realm,
    GC::Ref<SharedFunctionInstanceData> shared_data,
    GC::Ptr<Environment> parent_environment,
    GC::Ptr<PrivateEnvironment> private_environment)
{
    auto prototype = prototype_for_function_kind(*realm, shared_data->m_kind);
    return create_from_function_data(realm, shared_data, parent_environment, private_environment, *prototype);
}

ECMAScriptFunctionObject::ECMAScriptFunctionObject(
    GC::Ref<SharedFunctionInstanceData> shared_data,
    Environment* parent_environment,
    PrivateEnvironment* private_environment,
    Object& prototype)
    : FunctionObject(prototype)
    , m_shared_data(shared_data)
    , m_environment(parent_environment)
    , m_private_environment(private_environment)
{
    set_is_ecmascript_function_object();
    if (!is_arrow_function() && kind() == FunctionKind::Normal) {
        auto normal_function_shape = realm()->intrinsics().normal_function_shape();
        if (normal_function_shape->prototype() == &prototype)
            unsafe_set_shape(normal_function_shape);
        else
            unsafe_set_shape(*normal_function_shape->create_prototype_transition(&prototype));
    }

    // 15. Set F.[[ScriptOrModule]] to GetActiveScriptOrModule().
    m_script_or_module = vm().get_active_script_or_module();
}

void ECMAScriptFunctionObject::initialize(Realm& realm)
{
    auto& vm = this->vm();
    Base::initialize(realm);
    // Note: The ordering of these properties must be: length, name, prototype which is the order
    //       they are defined in the spec: https://tc39.es/ecma262/#sec-function-instances .
    //       This is observable through something like: https://tc39.es/ecma262/#sec-ordinaryownpropertykeys
    //       which must give the properties in chronological order which in this case is the order they
    //       are defined in the spec.

    m_name_string = PrimitiveString::create(vm, name());

    if (!is_arrow_function() && kind() == FunctionKind::Normal) {
        put_direct(realm.intrinsics().normal_function_length_offset(), Value(function_length()));
        put_direct(realm.intrinsics().normal_function_name_offset(), m_name_string);
        m_may_need_lazy_prototype_instantiation = true;
    } else {
        PropertyDescriptor length_descriptor { .value = Value(function_length()), .writable = false, .enumerable = false, .configurable = true };
        MUST(define_property_or_throw(vm.names.length, length_descriptor));
        PropertyDescriptor name_descriptor { .value = m_name_string, .writable = false, .enumerable = false, .configurable = true };
        MUST(define_property_or_throw(vm.names.name, name_descriptor));

        if (!is_arrow_function()) {
            Object* prototype = nullptr;
            switch (kind()) {
            case FunctionKind::Normal:
                VERIFY_NOT_REACHED();
                break;
            case FunctionKind::Generator:
                // prototype is "g1.prototype" in figure-2 (https://tc39.es/ecma262/img/figure-2.png)
                prototype = Object::create_prototype(realm, realm.intrinsics().generator_function_prototype_prototype());
                break;
            case FunctionKind::Async:
                break;
            case FunctionKind::AsyncGenerator:
                prototype = Object::create_prototype(realm, realm.intrinsics().async_generator_function_prototype_prototype());
                break;
            }
            // 27.7.4 AsyncFunction Instances, https://tc39.es/ecma262/#sec-async-function-instances
            // AsyncFunction instances do not have a prototype property as they are not constructible.
            if (kind() != FunctionKind::Async)
                define_direct_property(vm.names.prototype, prototype, Attribute::Writable);
        }
    }
}

void ECMAScriptFunctionObject::get_stack_frame_info(size_t& registers_and_locals_count, ReadonlySpan<Value>& constants, size_t& argument_count)
{
    auto executable = shared_data().m_executable;
    if (!executable) {
        auto rust_executable = RustIntegration::compile_function(vm(), *m_shared_data, false);
        VERIFY(rust_executable);
        m_shared_data->set_executable(rust_executable);
        executable = rust_executable;
        executable->name = name();
        if (Bytecode::g_dump_bytecode)
            executable->dump();
        m_shared_data->clear_compile_inputs();
    }
    registers_and_locals_count = executable->registers_and_locals_count;
    constants = executable->constants;
    argument_count = max(argument_count, static_cast<size_t>(formal_parameter_count()));
}

// 10.2.1 [[Call]] ( thisArgument, argumentsList ), https://tc39.es/ecma262/#sec-ecmascript-function-objects-call-thisargument-argumentslist
FLATTEN ThrowCompletionOr<Value> ECMAScriptFunctionObject::internal_call(ExecutionContext& callee_context, Value this_argument)
{
    auto& vm = this->vm();

    ASSERT(bytecode_executable());

    // 1. Let callerContext be the running execution context.
    // NOTE: No-op, kept by the VM in its execution context stack.

    // 2. Let calleeContext be PrepareForOrdinaryCall(F, undefined).
    prepare_for_ordinary_call(vm, callee_context, nullptr);

    // 3. Assert: calleeContext is now the running execution context.
    ASSERT(&vm.running_execution_context() == &callee_context);

    // 4. If F.[[IsClassConstructor]] is true, then
    if (is_class_constructor()) [[unlikely]] {
        // a. Let error be a newly created TypeError object.
        // b. NOTE: error is created in calleeContext with F's associated Realm Record.
        auto throw_completion = vm.throw_completion<TypeError>(ErrorType::ClassConstructorWithoutNew, name());

        // c. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
        vm.pop_execution_context();

        // d. Return ThrowCompletion(error).
        return throw_completion;
    }

    // 5. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
    if (uses_this())
        ordinary_call_bind_this(vm, callee_context, this_argument);

    // 6. Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
    auto result = ordinary_call_evaluate_body(vm, callee_context);

    // 7. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
    vm.pop_execution_context();

    // 8. If result.[[Type]] is return, return result.[[Value]].
    // 9. Assert: result is a throw completion.
    // 10. Return ? result.
    return result;
}

// 10.2.2 [[Construct]] ( argumentsList, newTarget ), https://tc39.es/ecma262/#sec-ecmascript-function-objects-construct-argumentslist-newtarget
FLATTEN ThrowCompletionOr<GC::Ref<Object>> ECMAScriptFunctionObject::internal_construct(ExecutionContext& callee_context, FunctionObject& new_target)
{
    auto& vm = this->vm();

    ASSERT(bytecode_executable());

    // 1. Let callerContext be the running execution context.
    // NOTE: No-op, kept by the VM in its execution context stack.

    // 2. Let kind be F.[[ConstructorKind]].
    auto kind = constructor_kind();

    GC::Ptr<Object> this_argument;

    // 3. If kind is base, then
    if (kind == ConstructorKind::Base) {
        // a. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget, "%Object.prototype%").
        this_argument = TRY(ordinary_create_from_constructor<Object>(vm, *realm(), new_target, &Intrinsics::object_prototype, ConstructWithPrototypeTag::Tag));
    }

    // 4. Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
    prepare_for_ordinary_call(vm, callee_context, &new_target);

    // 5. Assert: calleeContext is now the running execution context.
    ASSERT(&vm.running_execution_context() == &callee_context);

    // 6. If kind is base, then
    if (kind == ConstructorKind::Base) {
        // a. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
        if (uses_this())
            ordinary_call_bind_this(vm, callee_context, this_argument);

        // b. Let initializeResult be Completion(InitializeInstanceElements(thisArgument, F)).
        auto initialize_result = this_argument->initialize_instance_elements(*this);

        // c. If initializeResult is an abrupt completion, then
        if (initialize_result.is_throw_completion()) {
            // i. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
            vm.pop_execution_context();

            // ii. Return ? initializeResult.
            return initialize_result.throw_completion();
        }
    }

    // 7. Let constructorEnv be the LexicalEnvironment of calleeContext.
    auto constructor_env = callee_context.lexical_environment;

    // 8. Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
    auto result = ordinary_call_evaluate_body(vm, callee_context);

    // 9. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
    vm.pop_execution_context();

    // 10. If result is a throw completion, then
    if (result.is_error()) {
        // a. Return ? result.
        return result.release_error();
    }

    // 11. Assert: result is a return completion.
    // NOTE: We already checked !is_error() above.

    // 12. If Type(result.[[Value]]) is Object, return result.[[Value]].
    if (result.value().is_object())
        return GC::Ref<Object> { const_cast<Object&>(result.value().as_object()) };

    // 13. If kind is base, return thisArgument.
    if (kind == ConstructorKind::Base)
        return *this_argument;

    // 14. If result.[[Value]] is not undefined, throw a TypeError exception.
    if (!result.value().is_undefined())
        return vm.throw_completion<TypeError>(ErrorType::DerivedConstructorReturningInvalidValue);

    // 15. Let thisBinding be ? constructorEnv.GetThisBinding().
    auto this_binding = TRY(constructor_env->get_this_binding(vm));

    // 16. Assert: Type(thisBinding) is Object.
    ASSERT(this_binding.is_object());

    // 17. Return thisBinding.
    return this_binding.as_object();
}

void ECMAScriptFunctionObject::visit_edges(Visitor& visitor)
{
    Base::visit_edges(visitor);
    visitor.visit(m_environment);
    visitor.visit(m_private_environment);
    visitor.visit(m_home_object);
    visitor.visit(m_name_string);
    visitor.visit(m_shared_data);

    if (m_class_data) {
        for (auto& field : m_class_data->fields) {
            visitor.visit(field.initializer);
            if (auto* property_key_ptr = field.name.get_pointer<PropertyKey>(); property_key_ptr && property_key_ptr->is_symbol())
                visitor.visit(property_key_ptr->as_symbol());
        }

        for (auto& private_element : m_class_data->private_methods)
            visitor.visit(private_element.value);
    }

    visitor.visit(m_script_or_module);
}

// 10.2.7 MakeMethod ( F, homeObject ), https://tc39.es/ecma262/#sec-makemethod
void ECMAScriptFunctionObject::make_method(Object& home_object)
{
    // 1. Set F.[[HomeObject]] to homeObject.
    m_home_object = &home_object;
    m_is_method = true;
    m_may_need_lazy_prototype_instantiation = false;

    // 2. Return unused.
}

// 10.2.1.1 PrepareForOrdinaryCall ( F, newTarget ), https://tc39.es/ecma262/#sec-prepareforordinarycall
void ECMAScriptFunctionObject::prepare_for_ordinary_call(VM& vm, ExecutionContext& callee_context, Object* new_target)
{
    // 1. Let callerContext be the running execution context.
    // 2. Let calleeContext be a new ECMAScript code execution context.

    // 3. Set the Function of calleeContext to F.
    callee_context.function = this;

    // 4. Let calleeRealm be F.[[Realm]].
    // 5. Set the Realm of calleeContext to calleeRealm.
    callee_context.realm = realm();

    // 6. Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
    callee_context.script_or_module = m_script_or_module;

    if (function_environment_needed()) {
        // 7. Let localEnv be NewFunctionEnvironment(F, newTarget).
        auto local_environment = new_function_environment(*this, new_target);
        auto function_environment_bindings_count = shared_data().m_function_environment_bindings_count;
        local_environment->set_environment_shape_cache(shared_data().m_function_environment_shape, function_environment_bindings_count);
        local_environment->ensure_capacity(function_environment_bindings_count);

        // 8. Set the LexicalEnvironment of calleeContext to localEnv.
        callee_context.lexical_environment = local_environment;

        // 9. Set the VariableEnvironment of calleeContext to localEnv.
        callee_context.variable_environment = local_environment;
    } else {
        callee_context.lexical_environment = environment();
        callee_context.variable_environment = environment();
    }

    // 10. Set the PrivateEnvironment of calleeContext to F.[[PrivateEnvironment]].
    callee_context.private_environment = m_private_environment;

    // 11. If callerContext is not already suspended, suspend callerContext.
    // 12. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.

    // NOTE: We don't check for stack overflow here. The bytecode interpreter will do it anyway
    //       when entering the function we're about to call.
    vm.push_execution_context(callee_context);

    // 13. NOTE: Any exception objects produced after this point are associated with calleeRealm.
    // 14. Return calleeContext.
    // NOTE: See the comment after step 2 above about how contexts are allocated on the C++ stack.
}

// 10.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument ), https://tc39.es/ecma262/#sec-ordinarycallbindthis
void ECMAScriptFunctionObject::ordinary_call_bind_this(VM& vm, ExecutionContext& callee_context, Value this_argument)
{
    // 1. Let thisMode be F.[[ThisMode]].
    // If thisMode is lexical, return unused.
    if (this_mode() == ThisMode::Lexical)
        return;

    // 3. Let calleeRealm be F.[[Realm]].
    auto callee_realm = realm();

    // 4. Let localEnv be the LexicalEnvironment of calleeContext.
    auto local_env = callee_context.lexical_environment;

    Value this_value;

    // 5. If thisMode is strict, let thisValue be thisArgument.
    if (this_mode() == ThisMode::Strict) {
        this_value = this_argument;
    }
    // 6. Else,
    else {
        // a. If thisArgument is undefined or null, then
        if (this_argument.is_nullish()) {
            // i. Let globalEnv be calleeRealm.[[GlobalEnv]].
            // ii. Assert: globalEnv is a global Environment Record.
            auto& global_env = callee_realm->global_environment();

            // iii. Let thisValue be globalEnv.[[GlobalThisValue]].
            this_value = &global_env.global_this_value();
        }
        // b. Else,
        else {
            // i. Let thisValue be ! ToObject(thisArgument).
            this_value = MUST(this_argument.to_object(vm));

            // ii. NOTE: ToObject produces wrapper objects using calleeRealm.
            ASSERT(vm.current_realm() == callee_realm);
        }
    }

    // 7. Assert: localEnv is a function Environment Record.
    // 8. Assert: The next step never returns an abrupt completion because localEnv.[[ThisBindingStatus]] is not initialized.
    // 9. Perform ! localEnv.BindThisValue(thisValue).
    callee_context.this_value = this_value;
    if (function_environment_needed())
        MUST(as<FunctionEnvironment>(*local_env).bind_this_value(vm, this_value));

    // 10. Return unused.
}

// 27.7.5.1 AsyncFunctionStart ( promiseCapability, asyncFunctionBody ), https://tc39.es/ecma262/#sec-async-functions-abstract-operations-async-function-start
template<typename T>
void async_function_start(VM& vm, PromiseCapability const& promise_capability, T const& async_function_body)
{
    // 1. Let runningContext be the running execution context.
    auto& running_context = vm.running_execution_context();

    // 2. Let asyncContext be a copy of runningContext.
    auto async_context = running_context.copy();

    // 3. NOTE: Copying the execution state is required for AsyncBlockStart to resume its execution. It is ill-defined to resume a currently executing context.

    // 4. Perform AsyncBlockStart(promiseCapability, asyncFunctionBody, asyncContext).
    async_block_start(vm, async_function_body, promise_capability, *async_context);

    // 5. Return unused.
}

// 27.7.5.2 AsyncBlockStart ( promiseCapability, asyncBody, asyncContext ), https://tc39.es/ecma262/#sec-asyncblockstart
template<typename T>
void async_block_start(VM& vm, T const& async_body, PromiseCapability const& promise_capability, ExecutionContext& async_context)
{
    auto& realm = *vm.current_realm();

    // 1. Let runningContext be the running execution context.
    auto& running_context = vm.running_execution_context();

    // 2. Let closure be a new Abstract Closure with no parameters that captures promiseCapability and asyncBody and performs the following steps when called:
    auto closure = NativeFunction::create(realm, {}, [&async_body, &promise_capability](auto& vm) -> ThrowCompletionOr<Value> {
        Completion result;

        // a. Let acAsyncContext be the running execution context.

        // b. If asyncBody is a Parse Node, then
        //    i. Let result be Completion(Evaluation of asyncBody).
        // c. Else,
        //    i. Assert: asyncBody is an Abstract Closure with no parameters.
        //    ii. Let result be asyncBody().
        result = async_body.function()();
        // d. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
        // e. Remove acAsyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
        vm.pop_execution_context();

        // f. If result is a normal completion, then
        if (result.type() == Completion::Type::Normal) {
            // i. Perform ! Call(promiseCapability.[[Resolve]], undefined, « undefined »).
            MUST(call(vm, *promise_capability.resolve(), js_undefined(), js_undefined()));
        }
        // g. Else if result is a return completion, then
        else if (result.type() == Completion::Type::Return) {
            // i. Perform ! Call(promiseCapability.[[Resolve]], undefined, « result.[[Value]] »).
            MUST(call(vm, *promise_capability.resolve(), js_undefined(), result.value()));
        }
        // h. Else,
        else {
            // i. Assert: result is a throw completion.
            VERIFY(result.type() == Completion::Type::Throw);

            // ii. Perform ! Call(promiseCapability.[[Reject]], undefined, « result.[[Value]] »).
            MUST(call(vm, *promise_capability.reject(), js_undefined(), result.value()));
        }
        // i. Return unused.
        // NOTE: We don't support returning an empty/optional/unused value here.
        return js_undefined();
    });

    // 3. Set the code evaluation state of asyncContext such that when evaluation is resumed for that execution context, closure will be called with no arguments.
    // 4. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
    auto push_result = vm.push_execution_context(async_context, {});
    if (push_result.is_error())
        return;

    // 5. Resume the suspended evaluation of asyncContext. Let result be the value returned by the resumed computation.
    auto result = call(vm, *closure, *async_context.this_value);

    // 6. Assert: When we return here, asyncContext has already been removed from the execution context stack and runningContext is the currently running execution context.
    VERIFY(&vm.running_execution_context() == &running_context);

    // 7. Assert: result is a normal completion with a value of unused. The possible sources of this value are Await or, if the async function doesn't await anything, step 2.i above.
    VERIFY(result.has_value() && result.value().is_undefined());

    // 8. Return unused.
}

template void async_block_start(VM&, GC::Function<Completion()> const& async_body, PromiseCapability const&, ExecutionContext&);
template void async_function_start(VM&, PromiseCapability const&, GC::Function<Completion()> const& async_function_body);

// 10.2.1.4 OrdinaryCallEvaluateBody ( F, argumentsList ), https://tc39.es/ecma262/#sec-ordinarycallevaluatebody
// 15.8.4 Runtime Semantics: EvaluateAsyncFunctionBody, https://tc39.es/ecma262/#sec-runtime-semantics-evaluatefunctionbody
ThrowCompletionOr<Value> ECMAScriptFunctionObject::ordinary_call_evaluate_body(VM& vm, ExecutionContext& context)
{
    auto result = TRY(vm.run_executable(context, *bytecode_executable(), {}));

    // NOTE: Running the bytecode should eventually return a completion.
    // Until it does, we assume "return" and include the undefined fallback from the call site.
    if (kind() == FunctionKind::Normal)
        return result;

    if (kind() == FunctionKind::AsyncGenerator)
        return AsyncGenerator::create(*context.realm, GC::Ref { *this }, context.copy());

    auto generator_object = GeneratorObject::create(*context.realm, GC::Ref { *this }, context.copy());

    // NOTE: Async functions are entirely transformed to generator functions, and wrapped in a custom driver that returns a promise.
    if (kind() == FunctionKind::Async)
        return AsyncFunctionDriverWrapper::create(*context.realm, generator_object);

    ASSERT(kind() == FunctionKind::Generator);
    return generator_object;
}

void ECMAScriptFunctionObject::set_name(Utf16FlyString const& name)
{
    auto& vm = this->vm();
    m_name = name;
    m_name_string = PrimitiveString::create(vm, name);
    PropertyDescriptor descriptor { .value = m_name_string, .writable = false, .enumerable = false, .configurable = true };
    MUST(define_property_or_throw(vm.names.name, descriptor));
}

void ECMAScriptFunctionObject::set_inferred_name(Variant<PropertyKey, PrivateName> const& name, Optional<StringView> const& prefix)
{
    auto function_name = make_function_name(name, prefix);
    set_name(Utf16FlyString(function_name->utf16_string()));
}

ECMAScriptFunctionObject::ClassData& ECMAScriptFunctionObject::ensure_class_data() const
{
    if (!m_class_data)
        m_class_data = make<ClassData>();
    return *m_class_data;
}

Utf16String ECMAScriptFunctionObject::name_for_call_stack() const
{
    return m_name_string->utf16_string();
}

bool ECMAScriptFunctionObject::supports_legacy_caller_or_arguments() const
{
    // https://tc39.es/ecma262/#sec-forbidden-extensions
    //
    // ECMAScript function objects defined using syntactic constructors in strict mode code must not be created with own
    // properties named *"caller"* or *"arguments"*. Such own properties also must not be created for function objects
    // defined using an |ArrowFunction|, |MethodDefinition|, |GeneratorDeclaration|, |GeneratorExpression|,
    // |AsyncGeneratorDeclaration|, |AsyncGeneratorExpression|, |ClassDeclaration|, |ClassExpression|,
    // |AsyncFunctionDeclaration|, |AsyncFunctionExpression|, or |AsyncArrowFunction| regardless of whether the definition
    // is contained in strict mode code.
    // Built-in functions, strict functions created using the Function constructor, generator functions created using
    // the Generator constructor, async functions created using the AsyncFunction constructor, and functions created
    // using the `bind` method also must not be created with such own properties.
    return kind() == FunctionKind::Normal
        && !is_arrow_function()
        && !is_class_constructor()
        && !m_is_method
        && !is_strict_mode();
}

Value ECMAScriptFunctionObject::legacy_caller() const
{
    ECMAScriptFunctionObject* caller = nullptr;
    bool found_this_function = false;

    vm().for_each_execution_context_top_to_bottom([&](ExecutionContext const& context) {
        if (!found_this_function) {
            if (context.function.ptr() == this)
                found_this_function = true;
            return true;
        }

        if (!context.function)
            return true;

        caller = as_if<ECMAScriptFunctionObject>(*context.function);
        return false;
    });

    // https://tc39.es/ecma262/#sec-forbidden-extensions
    //
    // If an implementation extends any function object with an own property named *"caller"* the value of that property,
    // as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function object. If it is an accessor
    // property, the function that is the value of the property's [[Get]] attribute must never return a strict function
    // when called.
    if (!caller || !caller->supports_legacy_caller_or_arguments())
        return js_null();

    return caller;
}

Value ECMAScriptFunctionObject::legacy_arguments() const
{
    ExecutionContext const* active_context = nullptr;

    vm().for_each_execution_context_top_to_bottom([&](ExecutionContext const& context) {
        if (context.function.ptr() != this)
            return true;

        active_context = &context;
        return false;
    });

    if (!active_context)
        return js_null();

    auto arguments = active_context->arguments_span();
    auto passed_arguments = ReadonlySpan<Value> { arguments.data(), active_context->passed_argument_count };
    auto* arguments_object = create_unmapped_arguments_object(vm(), passed_arguments);
    if (has_simple_parameter_list())
        arguments_object->define_direct_property(
            vm().names.callee, this, Attribute::Writable | Attribute::Configurable);
    return arguments_object;
}

ThrowCompletionOr<Optional<PropertyDescriptor>> ECMAScriptFunctionObject::internal_get_own_property(PropertyKey const& property_key) const
{
    if (supports_legacy_caller_or_arguments()) {
        auto descriptor = TRY(Base::internal_get_own_property(property_key));
        if (descriptor.has_value())
            return descriptor;

        if (property_key == vm().names.caller) {
            return PropertyDescriptor {
                .value = legacy_caller(),
                .writable = false,
                .enumerable = false,
                .configurable = false,
            };
        }
        if (property_key == vm().names.arguments) {
            return PropertyDescriptor {
                .value = legacy_arguments(),
                .writable = false,
                .enumerable = false,
                .configurable = false,
            };
        }
    }

    if (m_may_need_lazy_prototype_instantiation && property_key == vm().names.prototype) {
        auto& realm = *this->realm();
        auto metadata = shape().lookup(property_key);
        if (!metadata.has_value()) {
            auto prototype = Object::create_with_premade_shape(realm.intrinsics().normal_function_prototype_shape());
            prototype->put_direct(realm.intrinsics().normal_function_prototype_constructor_offset(), this);
            const_cast<ECMAScriptFunctionObject*>(this)->define_direct_property(vm().names.prototype, prototype, Attribute::Writable);
        }
        m_may_need_lazy_prototype_instantiation = false;
    }

    return Base::internal_get_own_property(property_key);
}

ThrowCompletionOr<GC::RootVector<Value>> ECMAScriptFunctionObject::internal_own_property_keys() const
{
    if (m_may_need_lazy_prototype_instantiation)
        (void)TRY(internal_get_own_property(vm().names.prototype));

    auto keys = TRY(Base::internal_own_property_keys());
    if (!supports_legacy_caller_or_arguments())
        return keys;

    auto& vm = this->vm();
    auto insertion_index = keys.size();
    for (size_t i = 0; i < keys.size(); ++i) {
        if (keys[i].is_string() && keys[i].as_string().utf16_string() == vm.names.name.as_string().to_utf16_string()) {
            insertion_index = i + 1;
            break;
        }
    }

    if (!TRY(Base::internal_get_own_property(vm.names.arguments)).has_value())
        keys.insert(insertion_index++, PrimitiveString::create(vm, vm.names.arguments.as_string()));
    if (!TRY(Base::internal_get_own_property(vm.names.caller)).has_value())
        keys.insert(insertion_index, PrimitiveString::create(vm, vm.names.caller.as_string()));

    return keys;
}

}
