//声明变量
        let a;
        let b,c,d;
        let e = 100;
        let f = 521, g = 'iloveyou', h = [];

        //1. 变量不能重复声明
        let star = '罗志祥';
        let star = '小猪';

        //2. 块儿级作用域  全局, 函数, eval 读取不到girl变量,因为超出作用域了
        // if else while for 
        {
            let girl = '周扬青';
        }
        console.log(girl);

        //3. 不存在变量提升 会报错
        console.log(song);
        let song = '恋爱达人'; 

        //4. 不影响作用域链
        {
            let school = '尚硅谷';
            function fn(){
                console.log(school);
            }
            fn();
        }