reduce

Array.prototype.myReduce = function(fn,initValue){
    if(this.length == 0){
        throw new Error('Reduce of empty array with no initial value')
    }
    let i,total;
    if(initValue){
        total = initValue
        i = 0;
    }else{
        total = this[0]
        i = 1;
    }
    for(;i < this.length;i++){
        let item = this[i]
        total = fn(total,item,i,this)
    }
    return total
}

有初始值，下表从0开始；无初始值，下标从1开始。