es6，es7,es8语法总结 =====>> (不定期更新)

ES6

1. var let const

let,const具有块级作用域，不具有变量提升

const 用于不能被重新赋值的变量

2. 箭头函数

我们经常要给回调函数给一个父级的this

常用办法就是 var self = this 定义一个变量接住他

使用 箭头函数，this 将不会受到影响,可以直接用this调用父级的this

3. 字符串

includes:
    const string = 'food';

    const substring = 'foo';

    console.log(string.includes(substring));
    返回的是布尔值。

string.repeat(str,count)

如果 string.length < count 即插入str到count == string.length为止

4. 模板字符串

  const name = 'Tiger';
    const age = 13;
    console.log(`My cat is named ${name} and is ${age} years old.`);

5.解构

结构数组：

    let [a, b, c, d] = [1, 2, 3, 4];

    console.log(a);

    console.log(b);

结构对象：

    var luke = { occupation: 'jedi', father: 'anakin' };

    var occupation = luke.occupation;

    var father = luke.father;

    -------------------------------------------------------------

    let luke = { occupation: 'jedi', father: 'anakin' };

    let {occupation, father} = luke;

    console.log(occupation);

    console.log(father);

6.模块

暴露对象：

    function sumThree(a, b, c) {

        return a + b + c;

        }

    export { sumThree };

引入：

    import { sumThree } from 'math/addition';

7.参数

es6支持设置默认值：

function addTwoNumbers(x=0, y=0) {

    return x + y;

}

8.rest参数

处理不定数目参数：

    function logArguments(...args) {

        for (let arg of args) {

            console.log(arg);

        }

    }

9.展开操作

可以展示数组：

    Math.max(...[-1, 100, 9001, -32]);

    let cities = ['San Francisco', 'Los Angeles'];

    let places = ['Miami', ...cities, 'Chicago']; // ['Miami', 'San Francisco', 'Los Angeles', 'Chicago']

10.类

创造类:

    class Person {

        constructor(name, age, gender) {

            this.name  = name;

            this.age    = age;

            this.gender = gender;

        }

        incrementAge() {

        this.age += 1;

        }

    }

11.Maps

可以理解成键值对

    let map = new Map();

    map.set('name', 'david');

    map.get('name');

    map.has('name');

12.Promises

远离回调地狱，可以转换成垂直代码

    func1(value1)

    .then(func2)

    .then(func3)

    .then(func4)

    .then(func5, value5 => {

    });

13.Generators

用同步的代码风格来写异步代码

function* genFunc() {

    // (A)

    console.log('First');

    yield; //(B)

    console.log('Second'); //(C)

}

ES7

1. includes

代码:

    let array = ['1','2','3']

    if(array.includes('2')){

        console.log('有')

    }

2. 指数操作符

2**3 == 8 

ES8

1. object.entries()

代码:

    let obj = {a: 1, b: 2, c: 3};

    Object.entries(obj).forEach(([key, value]) =>{

        console.log(key + ": " + value); // 输出a: 1, b: 2, c: 3

    })

2.Async Await

异步看起来和同步写法一样

代码:

    async fetchData(query) =>{

        try {

            const response = await axios.get(`/q?query=${query}`);

            const data = response.data;

            return data;

        }

        catch (error) {

            console.log(error)

        }

    }

    fetchData(query).then(data => {

        this.props.processfetchedData(data)

    })