ES6：var，let，const的区别

let

1. let不能在同一作用域内重复声明变量

let a = 10;
let a = 100;
//Uncaught SyntaxError: Identifier 'a' has already been declared
var a = 10;
var a = 100;
a	//100

2. let声明的变量属于块级作用域

{
	let a = 10;
	var b = 100;
}
a	//a is not defined
b	//undefined

3. let声明的变量不存在变量提升

console.log(a);	//报错
console.log(b); //10
let a = 10;
var b = 10;

const

1. const声明常量，声明时一定要赋初值

const A;	//Missing initializer in const declaration
const A = 10;

2. const声明的常量不能修改

const A = 10;
A = 100;	//Assignment to constant variable.

3. const声明的变量属于块级作用域

{
	const A = 10;
}
A	//A is not defined