什么情况下会使用array.reduce函数

当业务需要从一个数组里求出某项的和的时候.

1.for遍历

var a = [1,2,3,4,5,6,7,8,9,10]

var resulte = 0;

for (let index = 0; index < a.length; index++) {
    resulte += a[index];
}

console.log(resulte); //55

又或者会去这样实现

var a = [1,10]

var resulte = 0;

a.forEach(i => resulte += i)

console.log(resulte); //55

其实js的数组内置函数早已经帮我们实现好了

var a = [1,10]

const INITVAL = 0;//默认从0开始累加,可要可不要

var resulte = a.reduce((prev,item) => prev + item,INITVAL)

console.log(resulte); //55

reduce使用教程

如何实现一个简单的reduce

var a = [1,INITVAL)

console.log(resulte); //55
```

[reduce使用教程](https://www.runoob.com/jsref/jsref-reduce.html)

### 如何实现一个简单的reduce

``` 
Array.prototype.myReduce = function (cb,initVal) {
    var resulte = initVal;
    var len = this.length;
    var i = 0;
    //空数组不执行
    if (len === 0) {
        return console.log('数组不能为空')
    }
    //initVal如果不为空,索引+1
    if (initVal === undefined) {
        resulte = this[i]
        i = 1
    } 
    for (i; i < len; i++) {
        var item = this[i];
        resulte = cb(resulte,item,i);
    }
    return resulte
}

var b = [1,6]
var persons = [
    {
        name: "张三",age: 18
    },{
        name: "李四",age: 36
    }
]

var agesum= persons.myReduce((a,c)=> a+c.age,0)
var r = b.myReduce(function (pre,index) {
    console.log(pre,index); //上一个值,遍历的当前项,索引
    return pre + item
},7)
console.log(r);
console.log(agesum);