此处let与var区别

Question

var a = 2;

(function IIFE(global){
    var a =3
    console.log(a)//3
    console.log(global.a)//2
})(window);
console.log(a);//2

let a = 2;

(function IIFE(global){
    var a =3
    console.log(a)//3
    console.log(global.a)//undefine   为什么变成undefined
})(window);
console.log(a);//2

Answer 1

let const即便在全局环境中定义变量也不会绑定到window对象上
可以找本基础的书看看

Answer 2

顶层对象的属性