if (!Array.prototype.forEach) {

    Array.prototype.forEach = function (fn, thisObj) {

        var scope = thisObj || window;

        for (var i = 0, j = this.length; i < j; ++i) {

            fn.call(scope, this[i], i, this);

        }

    };

}

if (!Array.prototype.map) {

    Array.prototype.map = function (fn, thisObj) {

        var scope = thisObj || window;

        var res = [];//区别在于这里，forEach不会生成新的数组

        for (var i = 0, j = this.length; i < j; ++i) {

         res[i] = fn.call(scope, this[i], i, this);

        }

    return res;

    };

}

if (!Array.prototype.filter) {

    Array.prototype.filter = function (fn, thisObj) {

        var scope = thisObj || window;

        var a = [];

        for (var i = 0, j = this.length; i < j; ++i) {

            if (!fn.call(scope, this[i], i, this)) {

                continue;

            }

            a.push(this[i]);

        }

        return a;

    };

}

function logArrayElements(element, index, array) {

    console.log("a[" + index + "] = " + element);

}

[2, 5, 9].forEach(logArrayElements);

// a[0] = 2

// a[1] = 5

// a[2] = 9

var map = Array.prototype.map

var a = map.call("Hello World", function(x) { return x.charCodeAt(0); })

// a now equals [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]

var numbers = [1, 4, 9];

var roots = numbers.map(Math.sqrt);

/* roots is now [1, 2, 3], numbers is still [1, 4, 9] */

function isBigEnough(element, index, array) {

  return (element >= 10);

}

var filtered = [12, 5, 8, 130, 44].filter(isBigEnough);

// filtered is [12, 130, 44]