//判斷初始值是存在
?//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
?}