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
原文：http://www.cnblogs.com/donna-z/p/7285626.html