ES6 之reduce的高级技巧

reduce() 方法接收一个函数作为累加器，数组中的每个值（从左到右）开始缩减，最终计算为一个值。reduce() 方法接受四个参数：初始值（或者上一次回调函数的返回值），当前元素值，当前索引，调用 reduce() 的数组。

reduce() 的几个强大用法：

数组求和

var total = [ 0, 1, 2, 3 ].reduce(( acc, cur ) => {

　　return acc + cur

}, 0);

console.log(total) // 6

二维数组转为一维数组

var array = [[1, 2], [3, 4], [5, 6]].reduce(( acc, cur ) => {

　　return acc.concat(cur)

}, []);

console.log(array) // [ 0, 1, 3, 4, 5, 6 ]

 

计算数组中每个元素出现的次数

1. 方法一

let names = ['tom', 'jim', 'jack', 'tom', 'jack'];

const countNames = names.reduce((allNames, name) => {

　　if (name in allNames) {

　　allNames[name] ++;

　　} else {

　　allNames[name] = 1;

　　}

　　return allNames;

}, {});

console.log(countNames) // { tom: 2, jim: 1, jack: 2 }

 

2. 方法二

const arraySum = (arr, val) => arr.reduce((acc, cur) => {

　　return cur == val ? acc + 1 : acc + 0

}, 0);

let arr = [ 0, 1, 3, 0, 2, 0, 2, 3 ]

console.log(arraySum(arr, 0)) // 数组arr中 0 元素出现的次数为3

 

数组去重

1.方法一

let arr = [1, 2, 1, 2, 3, 5, 4, 5, 3, 4, 4, 4, 4];

let result = arr.sort().reduce((init, current) => {

if (init.length === 0 || init[init.length - 1] !== current) {

　　init.push(current); } return init;

}, []);

console.log(result); //[1,2,3,4,5]

 

2.方法二

 

// console.log(...new Set([1,2,3,4,5,2,4,1]))

const dedupe = (array) => {

　　　　return Array.from(new Set(array));

  }

console.log(dedupe([1, 1, 2, 3])) //[1,2,3]

 

let person = [
{id: 0, name: "小明"},
{id: 1, name: "小张"},
{id: 2, name: "小李"},
{id: 3, name: "小孙"},
{id: 1, name: "小周"},
{id: 2, name: "小陈"},
{id: 1, name: "小陈"},
{id: 2, name: "小陈"},
];

let obj = {};

person = person.reduce((cur,next) => {
obj[next.name] ? "" : obj[next.name] = true && cur.push(next);
return cur;
},[]) //设置cur默认类型为数组，并且初始值为空的数组
console.log(person);

 

3.方法三

var arr = [1, 2, 3, 3, 4, 4, 5, 5, 6, 1, 9, 3, 25, 4];

 

function deRepeat() {

    var newArr = [];

    var obj = {};

    var index = 0;

    var l = arr.length;

    for (var i = 0; i < l; i++) {

        if (obj[arr[i]] == undefined) {

            obj[arr[i]] = 1;

            newArr[index++] = arr[i];

        } else if (obj[arr[i]] == 1)

            continue;

    }

    return newArr;

 

}

var newArr2 = deRepeat(arr);

alert(newArr2); //输出1,2,3,4,5,6,9,25