let

let 用来声明变量

一、语法

let a;
let b,c,d;
let a = 1;
let f = 2, g = 'hello', h = [];

二、变量不能重复声明

let start = '猪八戒';
let start = '呆子';

报错

报错：Uncaught SyntaxError: Identifier 'start' has already been declared

三、块级作用域

if (true) {
    let a = 1;
}
console.log(a);

报错

报错：Uncaught ReferenceError: a is not defined

四、不存在变量提升

console.log(song);
let song = 1;

报错

报错：Uncaught ReferenceError:Cannot access 'song' before initialization

五、不影响作用域链

{
    let school = "北京大学"
    function fn(){
        console.log(school);
    }
    fn()
}

控制台输出：

：北京大学