var let const 的区别

1.var声明变量会挂载在widow上，而let和const声明的变量不会

var a = 10;
console.log(window.a);//10
let b = 11;
console.log(window.b);//undefined
const c = 12;
console.log(window.c);//undefined

2.var声明的变量存在变量提升，let和const不存在变量提升

console.log(a);//undefined
var a = 1;

console.log(b);//报错
let b = 1;

console.log(c);//报错
const c = 1;

3.let和const声明形成块作用域

if(1){
	var a = 1;
	let b = 2;
	const c = 3;
}
console.log(a);//1
console.log(b);//报错
console.log(c);//报错

4.同一作用域下let和const不能声明同名变量，var可以声明同名变量

var a = 10;
	console.log(a);//10
	var a = 100;
	console.log(a);//100

	const b = 11;
	console.log(b);//报错
	const b = 22;
	console.log(b);//报错

	let c = 33;
	console.log(c);//报错
	let c = 44;
	console.log(c);//报错

5.const一旦声明变量必须赋值，不能使用NULL站位，声明之后不能再修改，如果声明的是复合类型数据，可以修改其属性