手写一个reduce函数

Array.prototype.myReduce = function (f, v) {
    let sum = v || this[0] || 0
    let curIndex = v ? 0 : 1
    for (let i = curIndex; i< this.length; i++) {
        sum = f(sum,this[i],i,this)
    }
    return sum;
}

let arr = [1, 2, 3]
let sum = arr.myReduce(function(s, c) {
   return s + c;
}, 0)
console.log(sum) // 6