实现Array的map

Array.prototype.map = function(callback, thisArg) {  
    if (this == null) {
        throw new TypeError("this is null or not defined");
    }  
    if (Object.prototype.toString.call(callback) != "[object Function]") {
        throw new TypeError(callback + " is not a function");
    }
    // 将O赋值为调用map方法的数组. 防止callback方法中修改原数组.
    var originArr = Object(this);
    // 将len赋值为数组O的长度. 如果length未定义就取0.
    var len = originArr.length >>> 0;
    var copyArg;
    if (thisArg) {
        copyArg = thisArg;
    }
    var result = new Array(len);
    var k = 0;
    while(k < len) {
        var kValue, mappedValue;
        // 遍历O,k为原数组索引
        if (k in originArr) {
            // kValue为索引k对应的值.
            kValue = originArr[k];
            // 执行callback,this指向copyArg.
            // originArr.map((kValue, k, originArr) => {});
            mappedValue = callback.call(copyArg, kValue, k, originArr);
            // 返回值添加到新数组A中.
            result[k] = mappedValue;
        }
        k++;
    }
    return result;
};