index.js

module.exports = function (xs, f) {
    if (xs.map) return xs.map(f);
    var res = [];
    for (var i = 0; i < xs.length; i++) {
        var x = xs[i];
        if (hasOwn.call(xs, i)) res.push(f(x, i, xs));
    }
    return res;
};

var hasOwn = Object.prototype.hasOwnProperty;