js手动实现 Reduce 方法

Array.prototype.myReduce = function (fn, initValue) {
  if (initValue === undefined && !this.length) {
     throw new Error('myReduce of empty array with no initial value');
   }
  // let result = initValue === undefined ? this[0] : initValue;不要这样写
  let result = initValue ? initValue : this[0];
   for (let i = initValue ? 0 : 1; i < this.length; i++) {
     result = fn(result, this[i], i, this);
   }
   return result;
};