题解 | #Array.reduce#

Array.prototype._reduce = function(fn, prev){
            if(typeof fn !== 'function') return;
            for(let i = 0; i < this.length; i++){
                if(prev == undefined){
                    prev = fn(this[i], this[i+1]);
                    i++;
                }
                else{
                    prev = fn(prev, this[i])
                }
            }
            return prev;
        }