理解reduce方法，手写一个reduce方法
代码比白话更容易解释了…
Array.prototype.myReduce = function (f, value=0) {
for (let i = 0; i < this.length; i++) {
value = f(this[i], value)
}
return value
}
let data = [4,8,15,16,23,42]
let add = function (a,b){
return a+b
}
let sum = data.myReduce(add)
console.log(sum ,'----')
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