module.exports = function (xs, fn) {

var res = [];
for (var i = 0; i < xs.length; i++) {
    var x = fn(xs[i], i);
    if (isArray(x)) res.push.apply(res, x);
    else res.push(x);
}
return res;

};

var isArray = Array.isArray || function (xs) {

return Object.prototype.toString.call(xs) === '[object Array]';

};