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

```

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### 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);