1.let命令：

相当于var,声明的变量只在所在的代码块内有效，eg:

{

 let a = 1;

var b =2;

console.log(a)  //1

}

console.log(a) //not defined

console.log(b) //2

let变量在域解析的时候不好被提升，eg:

console.log(a)  //undefined

var a = 1;

console.log(b)  //not defined报错

let b = 2;

let变量只能

ES6