var与let、const的区别

var a = 100;
console.log(a,window.a);    // 100 100

let b = 10;
console.log(b,window.b);    // 10 undefined

const c = 1;
console.log(c,window.c);    // 1 undefined

console.log(a); // undefined  ===>  a已声明还没赋值，默认得到undefined值
var a = 100;
console.log(b); // 报错：b is not defined  ===> 找不到b这个变量
let b = 10;
console.log(c); // 报错：c is not defined  ===> 找不到c这个变量
const c = 10;