Array.from simple polyfill

gistfile1.js

function arrayFrom(arrayLike, mapFn, thisArg) {
    var arr = [];
    if (typeof mapFn != 'function') {
        arr.push.apply(arr, arrayLike);
        return arr;
    }
    for (var i = 0, len = arrayLike.length; i < len; i++) {
        arr[i] = mapFn.call(thisArg, arrayLike[i], i);
    }
    return arr;
}

console.log(arrayFrom([1,,2])); //[ 1, undefined, 2 ]
console.log(arrayFrom([1,,2], function(val) {return val * val;})); //[ 1, NaN, 4 ]