ES6 基础实践

摘要：基础实践和和的区别输出输出猜猜是否报错，以及输出什么常量模板字符串普通字符串我是一个好人多行字符串我还是可以换行的字符串猜猜的长度是多少猜猜是否相等插入变量张三你好，很高兴认识你，我是，我的性别是男，我今年岁，我想说李四延伸思考解构赋值数组

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

    `我是一个好人`

    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
2
3
4";

a === b //猜猜是否相等？

    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") }`);

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

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] = [];

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？

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);
    }

    let example = x => x * 2;

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

    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？

    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;

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