1.     let arr = [1, 32, 41, 2, 1312, 41, 1, 321]
    2.     // map方法是返回原来的数组
    3.     Array.prototype._map = function (fn) {
    4.       if (typeof fn !== "function") {
    5.         throw new TypeError('Not Function')
    6.       }
    7.       let res = []
    8.       for (let i of this) {
    9.         fn(i) && res.push(fn(i))
    10.       }
    11.       return res
    12.     }
    13.     console.log(arr._map(x => x * 2));