let声明变量

// var 声明的变量没有局部作用域
// let 声明的变量  有局部作用域
{
    var a = 1;
    let b = 2;
}
console.log(a)
console.log(b)  //b is not defined


// var 可以声明多次
// let 只能声明一次
var m = 1;
var m = 2;
let n = 10;
let n = 20;  //'n' has already been declared
console.log(m)
console.log(n)