| 12345678910111213 | 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]';};
 |