复制
1Array.prototype.reduce = function(callback, initialValue) {
2  if (this == undefined) {
3    throw new TypeError('this is null or not defined');
4  }
5  if (typeof callback !== 'function') {
6    throw new TypeError(callbackfn + ' is not a function');
7  }
8  const O = Object(this);
9  const len = this.length >>> 0;
10  let accumulator = initialValue;
11  let k = 0;
12  // 如果第二个参数为undefined的情况下
13  // 则数组的第一个有效值作为累加器的初始值
14  if (accumulator === undefined) {
15    while (k < len && !(k in O)) {
16      k++;
17    }
18    // 如果超出数组界限还没有找到累加器的初始值，则TypeError
19    if (k >= len) {
20      throw new TypeError('Reduce of empty array with no initial value');
21    }
22    accumulator = O[k++];
23  }
24  while (k < len) {
25    if (k in O) {
26      accumulator = callback.call(undefined, accumulator, O[k], k, O);
27    }
28    k++;
29  }
30  return accumulator;
31}
32