var声明的变量会挂载在window上，而let，const不会

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

var声明变量存在声明提升，其他声明方式不提升

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;

let和const声明形成块级作用域

if(1){
    var a = 100;
    let  b = 10;
}

console.log(a); // 100
console.log(b)  // 报错：b is not defined  ===> 找不到b这个变量</pre>

if(1){ var a = 100;

    const  c = 1;
}
 console.log(a); // 100
 console.log(c)  // 报错：c is not defined  ===> 找不到c这个变量</pre>

同一作用域下let和const不能声明相同变量，var可以

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

var a = 10;
console.log(a); // 10

let a = 100;
let a = 10;

//  控制台报错：Identifier 'a' has already been declared  ===> 标识符a已经被声明了。