The interview was asked about the difference between var, let, and const in ES6. It is enough to read this article

var、let、const

1. Variables declared by var will be mounted on the window, while variables declared by let and const will not:

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

2. There is variable promotion for var declaration variables, but there is no variable promotion for let and const

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;

3. Let and const declarations form block scope

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

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

4. Let and const cannot declare variables with the same name in the same scope, but var can

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。

5. Temporary dead zone

var a = 100;

if(1){
    
    
    a = 10;
    //在当前块作用域中存在a使用let/const声明的情况下，给a赋值10时，只会在当前作用域找变量a，
    // 而这时，还未到声明时候，所以控制台Error:a is not defined
    let a = 1;
}

Six, const

/*
* 　　1、一旦声明必须赋值,不能使用null占位。
*
* 　　2、声明后不能再修改
*
* 　　3、如果声明的是复合类型数据，可以修改其属性
*
* */

const a = 100; 

const list = [];
list[0] = 10;
console.log(list);　　// [10]

const obj = {
    
    a:100};
obj.name = 'apple';
obj.a = 10000;
console.log(obj);　　// {a:10000,name:'apple'}