ES6

ECMAScript就是JavaScript中的语法规范

ES6新增用法

let:

let特点:

1.没有变量提升，具有块级作用域

for (let i = 0; i < 5; i++) {

// var num = 4;

let num = 3;

console.log(num)

}

console.log(num);//num is not defined

function fnb() {

console.log(num);

let num = -1;//Cannot access 'num' before initialization 不能变量提升

} fnb();

2.不同重复同一个变量

for (let i = 0; i < 5; i++) {

var num = 4;

let num = 3;

console.log(num)//Identifier 'num' has already been declared

}

3.函数内不能用let重新声明函数的参数

function fn(name){

let name1 = '李四';

var name = name;

console.log(name1) //Identifier 'name' has already

}

fn('张三')

const

1、const声明变量（不能重复定义）

var num = 1;

const num = 2;

console.log(num)// Identifier 'num' has already been declared

2、变量不能提升

function fn1() {

console.log(num)

const num = 12;

} fn1();//Cannot access 'num' before initialization

3、具有块级作用域

for (var i = 0; i < 5; i++) {

const num = 10;

}

console.log(num)//num is not defined

4、const 是声明常量的,后面不能是变量，必须定义一个值

const num1 = "undefined";

const num2 = undefined;

const num3 = true;

console.log(num1, num2, num3)

5、常量不可修改

const num = 10;

num = 20;

console.log(num); //Assignment to constant variable

解构赋值

ES6允许按照一定模式，从数组和对象中提取值，对变量进行赋值，这被称为解构

数组的解构赋值

　完全解构

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

console.log(a, c, b)

不完全解构

let [a, b, c] = [1, 2]

console.log(a, b, c)//1,2,undefined

嵌套

let [a, b, [c, d]] = ["a", "b", ["C", "d"]]

console.log(a, b, c, d)

let [a = 1, b, c, d] = [10, 2, 3, 4]//可以给初始值(默认值)，1被10给覆盖了

console.log(a, b, c, d)//10,2,3,4

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

console.log(a, b, c)//1,2,3

交换两个变量的值

x = 1;

y = 2;

let [a, b] = [y, x]

console.log(a, b)

对象解构赋值(左右格式一样)

let { a, b, c } = { "a": 1, "b": 2, "c": 3 }

console.log(a, b, c)//1,2,3

var { a } = { "b": a }

console.log(a)//undefined

var { "b": a } = { "b": 2 }

console.log(a)//2

var { "a": b } = { "b": 2 }

console.log(b) //undefined

允许默认值

var { a, b = 2 } = { "a": 1, "b": 3 }