复制
1Array.prototype.map = function(callback, thisArg) {
2  if (this == undefined) {
3    throw new TypeError('this is null or not defined');
4  }
5  if (typeof callback !== 'function') {
6    throw new TypeError(callback + ' is not a function');
7  }
8  const res = [];
9  // 同理
10  const O = Object(this);
11  const len = O.length >>> 0;
12  for (let i = 0; i < len; i++) {
13    if (i in O) {
14      // 调用回调函数并传入新数组
15      res[i] = callback.call(thisArg, O[i], i, this);
16    }
17  }
18  return res;
19}