js的var let const 解构赋值 去重 promise执行顺序

//es6
//var let const
//1.声明提升
console.log(a);//undefined
var a = 1;

//2.变量覆盖
var a = 2;
var a = 3;
console.log(a);//3

//3.没有块级作用域
function fn() {
    for (var index = 0; index < 3; index++) {
        console.log(index);//0 1 2
    }
    console.log(index);//3
}
fn()

//const
//1.const声明之后必须赋值
const ABC;//报错
//2.定义的值不能被修改
const ABCD = 'abcd';
ABCD = 'aaaa';//报错
//3.支持let的其他属性

//解构赋值
let a = 1;
let b = 2;
[a, b] = [b, a ]
console.log(a, b);//2, 1

//去重
let arr = [1, 1, 2, 2, 3, 3]
let res = [...new Set(arr)]
console.log(res);//[1, 2, 3]

//构造函数同步执行
const pormise = new Promise((resolve, reject)=> {
    console.log(1);
    resolve();
    console.log(2);
})
//.then异步执行
pormise.then(()=>{
    console.log(3);
})
console.log(4);
//1 2 4 3