自定义reduce

  Array.prototype.reduce = function (callback, initValue) {
   // 判断callback 是不是函数
    if (typeof callback !== "function") {
      throw new TypeError("callback is no function");
    }
    let arr = this;
    let isInitValue= typeof initValue !== "undefined"
    // 是否有初始值、
    let base = isInitValue ? initValue : arr[0];
    // 如果有初始值：数组下标0，没有则是1
    let index = isInitValue ? 0 : 1;
    let len = arr.length;
    while (index < len) {
      base = callback(base, arr[index++], index - 1, this);
    }
    return base;
  };

  let list = [{ name: "嘻嘻" }];
  [{ name: "小红" }, { name: "小明" }].reduce((pre, next, index, arr) => {
    pre.push(next);
    return pre;
  });
  [1, 2, 3, 4, 5].reduce((pre, next, index) => {
    return pre + next;
  });