let var const

1. var 可声明前置

a = 3
var a
var a = 4

2. let不可声明前置

a = 3 //报错
let a

3. let不可重复声明

let a = 3
let a = 4  //报错
var a = 5  //报错
#### 4. 存在块级作用域
for(let i =0; i<3; i++){
  console.log(i)
}
console.log(i) //报错

IIFE的替换
(function(){
  var a = 1
}())

{
  let a = 1
}

暂时性死区（TDZ）：在let声明变量之前都是该变量的死区，在死区内该变量不可使用

5. const 声明的常量不可改变

const a = 1
a = 2 //报错

const obj = {a: 1}
obj.a = 2   //没问题
obj = {a: 2}  //报错

6. 适用于let的同样适用于const

解构赋值

1. 数组的解构

let [a,b,c] = [1,2,3]
console.log(a, b, c) 

let [a, [b], c] = [2, [3], 4]
a //2
b //3
c //4

let [a] = 1 //报错

2. 默认值

let [a, b = 2] = [3]
a // 3
b // 2

let [a, b = 2] = [3, 4]
a //3
b //4
数组对应对值有没有？如果没有（数组对没有指undefined）就使用默认值，如果有就使用对应值

let [a=2, b=3] = [undefined, null]
a //2
b //null
let [a=1, b=a] = [2]
a //2
b //2

3. 对象的解构赋值

前置知识

let [name, age] = ['hunger', 3]
let p1 = {name, age}
//等同于
let p2 = {name: name, age: age}
解构范例

let {name, age} = {name: 'jirengu', age: 4}
name //‘jirengu’
age //4
以上代码等同于

let name
let age
({name: name, age: age} = {name: 'jirengu', age: 4})

4. 默认值

let {x, y=5} = {x: 1}
x //1
y //5

5. 函数解构

function add([x=1, y=2]){
  return x+y
}
add() //Error
add([2]) //4
add([3,4]) //7

function sum({x, y}={x:0, y:0}, {a=1, b=1}){
    return [x+a, y+b]
}
sum({x:1, y:2}, {a:2}) //[3, 3]

6. 作用

let [x, y] = [1, 2];
[x, y] = [y, x]
x //2
y // 1
function ajax({url, type=‘GET’}){
}
ajax({url: ‘http://localhost:3000/getData’})