JavaScript 自己动手撸源码之 reduce

//判断初始值是存在
 //reduce
 Array.prototype.myReduce = function(fn, firstVal) {
   let val = firstVal || this[0]
   let startIndex = firstVal ? 0 : 1
   for(let i = startIndex; i < this.length; i++) {
     val = fn(val, this[i], i, this)
  }
   return val
 }
 
 //reduceRight
 Array.prototype.myReduce = function(fn, firstVal) {
   let val = firstVal || this[this.length - 1]
   let startIndex = firstVal ? (this.length - 1) : (this.length - 2)
   for(let i = startIndex; i < this.length; i++) {
     val = fn(val, this[i], i, this)
  }
   return val
 }