1、 reduce定义和用法

语法：

• prev （上一次调用回调返回的值，或者是提供的初始值（initialValue））
• cur （数组中当前被处理的元素）
• index （当前元素在数组中的索引)
• arr （调用的数组）
• init （传递给函数的初始值）

2、reduce浏览器支持情况

3、reduce累加

带初始值

var arr = [1,2,3,4]var sum = arr.reduce((pre, item) => {    return pre + item}, 10)console.log(sum) // 20

不带初始值

var arr = [1,2,3,4]var sum = arr.reduce((pre, item) => {    return pre + item},)console.log(sum) // 10

4、reduce数组去重

var arr = [1,2,3,3,2,1,4]arr.reduce((acc, cur) => {  if (!(acc.includes(cur))) {    acc.push(cur)  }  return acc}, [])// [1, 2, 3, 4]

5、reduce求数组项最大值

var arr = [1, 2, 3, 4];arr.reduce((prev, cur) => {    return Math.max(prev,cur);});//4

6、reduce将二维数组转为一维数组

var arr = [[1,2], [3,4], [5,6]]arr.reduce((acc, cur) => {  return acc.concat(cur)}, [])// [1,2,3,4,5,6]

7、reduce对象里的属性求和

var arr = [    {subject: 'Math', score: 90},    {subject: 'Chinese', score: 90},    {subject: 'English', score: 100}]arr.reduce((pre, cur) => {    return cur.score + pre}, 0)//280

8、reduce计算数组中每个元素出现的个数

var arr = [1, 2,3,3,2,1,2,1]arr.reduce((acc, cur) => {  if (!(cur in acc)) {    acc[cur] = 1  } else {    acc[cur] += 1  }  return acc}, {})//{1: 3, 2: 3, 3: 2}

9、reduce按属性给数组分类

var arr = [    {subject: 'Math', score: 90},    {subject: 'Chinese', score: 90},    {subject: 'English', score: 100},    {subject: 'Math', score: 80},    {subject: 'Chinese', score: 95}];arr.reduce((acc, cur) => {  if (!acc[cur.type]) {    acc[cur.type] = [];  }  acc[cur.type].push(cur)  return acc}, {})

10、reduce实现map

var arr = [1, 2, 3, 4]Array.prototype.reduceMap = function(callback) {  return this.reduce((acc, cur, index, array) => {    const item = callback(cur, index, array)    acc.push(item)    return acc  }, [])}arr.reduceMap((item, index) => {  return item + index})// [1, 3, 5, 7]

11、reduce实现forEach

var arr = [1, 2, 3, 4]Array.prototype.reduceForEach = function(callback) {  this.reduce((acc, cur, index, array) => {    callback(cur, index, array)  }, [])}
arr.reduceForEach((item, index, array) => {  console.log(item, index)})// 1234// 0123

12、reduce实现filter

var arr = [1, 2, 3, 4]Array.prototype.reduceFilter = function (callback) {   return this.reduce((acc, cur, index, array) => {    if (callback(cur, index, array)) {      acc.push(cur)    }    return acc  }, [])}arr.reduceFilter(item => item % 2 == 0) // 过滤出偶数项。// [2, 4]

13、reduce实现find

var arr = [1, 2, 3, 4]var obj = [{ a: 1 }, { a: 2 }, { a: 3 }, { a: 4 }]Array.prototype.reduceFind = function (callback) {  return this.reduce((acc, cur, index, array) => {    if (callback(cur, index, array)) {      if (acc instanceof Array && acc.length == 0) {        acc = cur      }    }        if ((index == array.length - 1) && acc instanceof Array && acc.length == 0) {      acc = undefined    }    return acc  }, [])}arr.reduceFind(item => item % 2 == 0) // 2obj.reduceFind(item => item.a % 2 == 0) // {a: 2}obj.reduceFind(item => item.a % 9 == 0) // undefined

碰到数组复杂操作的时候，就是reduce大显身手的时候。深入研究reduce的用法，对开发大有裨益。

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