Javascript Array 的 map 与 forEach

map 方法和 forEach 方法是数组的两个常用方法；都是对数组的所有元素进行callback处理； map会返回一个新数组, 但是forEach 的返回值是undefined。

map的语法是：

var new_array = arr.map(function callback(currentValue[, index[, array]]) {
 // Return element for new_array }[, 
thisArg])

forEach的语法是：

arr.forEach(callback[, thisArg]);

map的实现

Array.prototype.myMap = function (callback, thisArg) {

    // thisArg 执行函数时使用的的this

    // 3.如果callback不是函数,则抛出TypeError异常.
    if (Object.prototype.toString.call(callback) != "[object Function]") { // typeof callback == function 
        throw new TypeError(callback + " is not a function");
    }

    if (this == null) {
        throw new TypeError(" this is null or not defined");
    }

    // 1. 将O赋值为调用map方法的数组.
    var O = Object(this);

    // 2.将len赋值为数组O的长度.
    var len = O.length >>> 0;


    // 4. 如果参数thisArg有值,则将T赋值为thisArg;否则T为undefined.
    var T 
    if (thisArg) {
        T = thisArg; // 执行callback函数时使用的this值
    }

    // 5. 创建新数组A,长度为原数组O长度len
    var A = new Array(len);

    // 6. 将k赋值为0
    var k = 0;

    // 7. 当 k < len 时,执行循环. 尽量不用for循环
    while (k < len) {

        var kValue, mappedValue;

        //遍历O,k为原数组索引
        if (k in O) {

            //kValue为索引k对应的值.
            kValue = O[k];

            // 执行callback,this指向T,参数有三个.分别是kValue:值,k:索引,O:原数组.
            mappedValue = callback.call(T, kValue, k, O);

            // 返回值添加到新数组A中.
            A[k] = mappedValue;
        }
        // k自增1
        k++;
    }

    // 8. 返回新数组A
    return A;
};   

myMap的测试用例

var array1 = [1, 4, 9, 16];

// pass a function to map
const map1 = array1.myMap(x => x * 2);

console.log(map1);
// expected output: Array [2, 8, 18, 32]

forEach的实现

Array.prototype.myForEach = function (callback, thisArg) {

    var T, k;

    if (this == null) {
        throw new TypeError(' this is null or not defined');
    }

    // 如果callback 不是函数 抛出异常
    if (typeof callback !== "function") {
        throw new TypeError(callback + ' is not a function');
    }

    // 1. Let O be the result of calling toObject() passing the
    // |this| value as the argument.
    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get() internal
    // method of O with the argument "length".
    // 3. Let len be toUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If isCallable(callback) is false, throw a TypeError exception. 
    // See: http://es5.github.com/#x9.11
    

    // 5. If thisArg was supplied, let T be thisArg; else let
    // T be undefined.
    //  既可以 thisArg存在就赋值 或者arguments.length > 1 或者 arguments.length >= 2
    if (arguments.length > 1) { 
        T = thisArg;
    }

    // 6. Let k be 0
    k = 0;

    // 7. Repeat, while k < len
    while (k < len) {

        var kValue;

        // a. Let Pk be ToString(k).
        //    This is implicit for LHS operands of the in operator
        // b. Let kPresent be the result of calling the HasProperty
        //    internal method of O with argument Pk.
        //    This step can be combined with c
        // c. If kPresent is true, then
        if (k in O) {

            // i. Let kValue be the result of calling the Get internal
            // method of O with argument Pk.
            kValue = O[k];

            // ii. Call the Call internal method of callback with T as
            // the this value and argument list containing kValue, k, and O.
            callback.call(T, kValue, k, O);
        }
        // d. Increase k by 1.
        k++;
    }
    // 8. return undefined
};

if (!Array.prototype.reduce) {
    Object.defineProperty(Array.prototype, 'reduce', {
        value: function (callback /*, initialValue*/) {
            if (this === null) {
                throw new TypeError('Array.prototype.reduce ' +
                    'called on null or undefined');
            }
            if (typeof callback !== 'function') {
                throw new TypeError(callback +
                    ' is not a function');
            }

            // 1. Let O be ? ToObject(this value).
            var o = Object(this);

            // 2. Let len be ? ToLength(? Get(O, "length")).
            var len = o.length >>> 0;

            // Steps 3, 4, 5, 6, 7      
            var k = 0;
            var value;

            if (arguments.length >= 2) {
                value = arguments[1];
            } else {
                while (k < len && !(k in o)) {
                    k++;
                }

                // 3. If len is 0 and initialValue is not present,
                //    throw a TypeError exception.
                if (k >= len) {
                    throw new TypeError('Reduce of empty array ' +
                        'with no initial value');
                }
                value = o[k++];
            }

            // 8. Repeat, while k < len
            while (k < len) {
                // a. Let Pk be ! ToString(k).
                // b. Let kPresent be ? HasProperty(O, Pk).
                // c. If kPresent is true, then
                //    i.  Let kValue be ? Get(O, Pk).
                //    ii. Let accumulator be ? Call(
                //          callbackfn, undefined,
                //          « accumulator, kValue, k, O »).
                if (k in o) {
                    value = callback(value, o[k], k, o);
                }

                // d. Increase k by 1.      
                k++;
            }

            // 9. Return accumulator.
            return value;
        }
    });
}

myForEach的测试用例：

var array1 = ['a', 'b', 'c'];

array1.myForEach(function(element) {
  console.log(element);
});

// expected output: "a"
// expected output: "b"
// expected output: "c"