变量的解构总结

es变量赋值的新写法

let a=1;
let b=2;
let c=3;
let d=4;

lat [a,b,c]=[1,2,3]

不完全解构、

let [x,y]=[1,2,3] // x=1; y=2; 

报错的几种情况：

let [foo]=1;
let [foo]=false;
let [foo] = NaN;
let [foo] = undefined;
let [foo] = null;
let [foo] = {}

默认值

let [x=1, y=x] = []// 1,1
let [x=1, y=x]=[2]  // 2, 2
let [x=1, y=x]=[1,2] //1,2 
let [x=y, y=1] = []; // error

对象解构，对象解构没有秩序

let {foo, bar} = {foo:"aaa", bar:"bbb"}  // aaa, bbb

let obj={first: 'hello', second:'word'}
let {first:1, second:2} = obj;  //  first: hello; second: world

//嵌套解构
let obj = {
    p:[
        'hello',
        {
        y:'world'
        }
    ]
}

let {p:[x, {y}]} = obj;

字符串解构

[a,b,c,d,e,f] = 'hello'  

函数解构

[[1,2]. [3,4]].map(([a,b])=> a+b); // [3, 7]

用途

1. 值交换

   let x=1;
   let y=2;
   [x,y]=[y,x]
   
   

2. 从函数中返回多个值

   function example(){
       return {1,2,3}
   }
   let [a,b,c]=example(0;)
   

3. 提取json

   let jsonData = {
       id:42,
       status:"OK",
       data:[876, 5309],
   }
   
   let {id, status, data, number} = jsonData;
   

4. map的遍历

   var map=new Map()
   map.set('fist','hello');
   map.set('second','world');
   for(let [key, value] of map){
       console.log(key, value);
   }
   
   // 只遍历key
   for(let [key] of map){
       console.log(key);
   }
   
   // 只遍历value
   for(let [, value] of value){
       console.log(value);
   }