ES6 基础实践

let 和 const

let 和 var 的区别

var a = 1;
window.a;    // 输出：1
let b = 2;
window.b;    // 输出：undefined

{
    let a = 1;
    // let a = 2;
    var b = 2;
}

console.log(a);
console.log(b);

let a = 1;
{
    console.log(a);
    let a = 2;
}

let i = "a";

for(let i = 0; i < 10; i++){
    // let i = "b";
    console.log(i);
}

// 猜猜是否报错，以及输出什么

const 常量

    const a = [1,2,3];
    const b = a;
    const c = [1,2,3];

模板字符串

普通字符串

    `我是一个好人`

多行字符串

    console.log(`我还是可以
                换行的字符串`);

let a = `1
2
3
4`;

a.length //猜猜a的长度是多少？

let b = "1\n2\n3\n4";

a === b //猜猜是否相等？

插入变量

    let name = "张三";
    let age = 12;
    function say(name){
        return "你好，很高兴认识你，" + name;
    }
    
    console.log(`我是${name}，我的性别是：${ "男" }，我今年 ${ age * 2 }岁，我想说： ${ say("李四") }`);

延伸思考

let a = "abc";
let b = new String("abc");
a == b?
a === b?

解构赋值

数组解构

let a = 1, b = 2, c = 3;

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

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

let [a = 1] = [2];

let [a = 1] = [null];

let [a = 1] = [undefined];

let [a] = [null];

let [a] = [];

对象解构

let {a, b} = {a: 1, b: 2 };

let {a : newA, b: newB} = {a: 1, b: 2 };

let {a = 1} = {};

let {a = 1} = {a:2};

1、先两句一起执行：
    let a = 1;
    {a} = {a: 2}

2、分开一条一条执行
    let a = 1;
    
    {a} = {a: 2}

3、反过来执行
    {a} = {a: 2}
    let a = 1;
    
    a ?
    window.a ?
    上面的a 和 window.a 各输出什么？

let a = 1;
({a} = {a: 2})
如果这样的两句一起执行呢？

数组和对象解构？

用途

• 交换变量的值

    let a = 1;
    let b = 2;
    [a, b] = [b, a];

• 从函数返回多个值

    function example() {
      return [1, 2, 3];
    }
    let [a, b, c] = example();

    function example() {
      return {
        a: 1,
        b: 2
      };
    }
    let {a, b} = example();

• 函数参数的定义

    function example([a, b, c]) {
         console.log(a, b, c); 
    }
    
    example([1, 2, 3]);

• 提取JSON数据

    let myDataList = {
      name: "张三",
      sex: "男",
      age: "24"
    };
    
    let { name, sex, age} = myDataList;

• 函数参数的默认值

    let throwMyErr = (errName) => {
        throw new Error(`请传入参数 ${errName}`);
    }
    
    let example = (name = throwMyErr("name")) => {
        console.log(name);
    }

箭头函数

    let example = x => x * 2;

    let myObj = {
        funA : function(){
            setTimeout(function(){
                console.log(this);
            }, 500);
        }
    }
    
    let myObj = {
        funA : function(){
            setTimeout(() => {
                console.log(this);
            }, 500);
        }
    }

class和extends

    function SayHellowES5 (){
        this.name = "SARS";
    }
    
    SayHellowES5.prototype.say = function () {
        console.log(this.name);
    };

    class SayHellow {
      constructor() {
        this.name = "SARS";
      }
      say(){
        console.log(this.name);
      }
    }
    
    typeof SayHellow
    SayHellow === SayHellow.prototype.constructor

    var a = new SayHellow();
    a === SayHellow ?
    a.constructor === SayHellow ?

    var b = new SayHellow();
    a.__proto__ === b.__proto__ ?
    原型是否相等？

    b.__proto__.say2 = function(){
        alert("say2");
    }
    
    a.say2() ?
    
    思考 this 到底指的哪？原型呢？同样下面的继承是否也一样？
    （确认下节课内容）

    class SayBey extends SayHellow{
      sayBey(){
        console.log("beybey!");
      }
    }

    class toBey extends SayHellow{
      sayBey(){
        console.log(`${this.name} beybey!`);
      }
    }

    class toBey2 extends SayHellow{
      constructor() {
        this.sex = "女";
      }
      sayBey(){
        console.log(`${this.sex} beybey!`);
      }
    }
    //想想为什么报错？

模块化 export import

导出模块

    export let myLetA = 1;
    export let myLetB = 2;
    export let example = () => {
        alert("SARS");
    }

    let myLetA = 1;
    let myLetB = 2;
    let example = () => {
        alert("SARS");
    }
    export {myLetA, myLetB, example as name};

    // 正确
    var a = 1;
    export default a;
    
    // 错误
    export default var a = 1;

导入模块

    import {myLetA, myLetB, example as name} from ‘./main‘;
    import SARS from ‘./main‘;