手写 reduce

/**
 * reduce方法
 * @param {*} fu 回调函数
 * @param {*} initVal 初始值
 */
Array.prototype.myReduce = function(fu, initVal){
    let prev = initVal == undefined ? this[0] : initVal;
    let i = initVal == undefined ? 1 : 0;
    for(; i< this.length; i++){
        prev = fu(prev, this[i], i, this)
    };
    return prev
}

var arr = [1,2,3]
var result = arr.myReduce((prev, curr, index, arr)=>{
    return prev + curr
}, 999)

// result ⇒ 1005