1、const

声明常量，一旦声明必须立马赋值，否则报错

const PI = 3.14

const PI;

//报错：Uncaught SyntaxError: Missing initializer in const declaration

常量赋值以后，不能改变，否则同样报错

const PI = 3.14 ;

PI = 3;

//报错：Uncaught TypeError: Assignment to constant variable.

2、let

声明的变量只在let命令所在的代码块内有效

for(let i=0 ;i<3;i++){

    console.log(i)

}

console.log(i)

//报错：Uncaught ReferenceError: i is not defined