es6 let和const

if(true){
            var a=1;
            let b=2;
        }
        console.log("a:"+a);//a:1
        console.log("b:"+b);//Uncaught ReferenceError: b is not defined

let a=10;
        const b=20;
        console.log(a);
        console.log(b);
        a=100;
        b=200;//Uncaught TypeError: Assignment to constant variable.

const a;//Uncaught SyntaxError: Missing initializer in const declaration

let user={name:"starof",age:25};
        const LOVE_YOU=user;
        console.log(user);
        /*LOVE_YOU=1;//报错*/
        user.age=18;
        console.log(user);