3.声明

一.var和let

 let 

代码块内有效

不能重复定义

静态语言的感觉

二.const

let的不可变版 

三.解构

1.数组解构

变量

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

函数参数

function fun([a,b]:[number,string]) {
    console.log(a.toString());
    console.log(b);
}

部分解构

let list = [1, 2, 3, 4, 5];
let [a, ...list1] = list;
//list1  [2,3,4,5]

忽略剩余

let [first] = [1, 2, 3, 4];
console.log(first); // outputs 1

跳过

let [, second, , fourth] = [1, 2, 3, 4];

2.对象解构

用{}表示对象

 o = { a: "abc", b: 3, c: true };
let { a, b } = o;

剩余变量放到一个对象里

let o = { a: "abc", b: 3, c: true };
let { a, ...other } = o;

重命名

let { a: newName1, b: newName2 } = o;
//相当于
let newName1 = o.a;

默认值

function fun(a = 10) {
    console.log(a);
}

四.展开

赋值给一个数组

let first = [1, 2];
let second = [3, 4];
let bothPlus = [0, ...first, ...second, 5];

赋值对象

let defaults = { food: "spicy", price: "$$", ambiance: "noisy" };
let search = { ...defaults, food: "rich" };