关于let , var , const 的使用与区别

let

let 声明变量

let a;
let b,c,d;
let e=100;
let f=200, g=300;

let不允许重复声明

let a = 2;
let a = 3;
console.log(a);//Identifier 'a' has already been declared

let块儿级作用域

if(){ } else {} for(){ } while(){ } { } 都是代码块，函数不算

    	{
            let c = 4;
            console.log(c);//4
        }
        console.log(c);// c is not defined

let 不存在变量提升

console.log(a); //Cannot access 'a' before initialization
let a = 'hahaha';

var

var声明变量

		var a;
        var b,c,d;
        var e=100;
        var f=200, g=300;

var 可以声明多次

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

var声明的变量没有局部作用域

	  {
            var b = 3;
            console.log(b);//3
        }
        console.log(b);//3

var声明的变量可以进行提升

console.log(a); //undefined
let a = 'hahaha';

const 关键字用来声明常量

1) 一旦声明必须赋值，否则报错

const b;
console.log(b);//Missing initializer in const declaration(const声明中缺少初始值)

2) 不允许重复声明

const c= 4;
const c= 5;
console.log(c)//Identifier 'c' has already been declared(标识符“ c”已被声明)

3) 值不允许修改

const a =5;
a=6;
console.log(a);//TypeError: Assignment to constant variable.

4) 块儿级作用域(和let一样)

{
    const A = 100;
    console.log(A); //100
}
    console.log(A);// A is not defined

注：

常量的名称一般为 『大写』 （小写也不错）

关于数组和对象的元素的修改（只改变地址里的数据正常显示，修改常量值会报错）

       const TEAM = ['UZI','MLXG','LETME','meiko'];
        TEAM.push('xiye');
        TEAM[0] = '小狗';
        console.log(TEAM)  // ["小狗", "MLXG", "LETME", "meiko", "xiye"]

        const MIDDLE = {
            name: 'Faker'
        };
        console.log(MIDDLE)  //{name: "瓜皮"}
        MIDDLE = 200; // Assignment to constant variable.

图片