手写reduce

        let arr = [1,2,3]
        Array.prototype.myReduce = function(callback,initValue){
            let arr = this
            for(let i=0;i<arr.length;i++){
                initValue = callback(initValue,arr[i],i,arr)
            }
            return initValue
        }
        let sum1 = arr.myReduce((result,item,index)=>{
            return result +=item
        },0)
        //sum1 -> 6