Var const let the difference between the

A, var variables can be mounted on the window, and const, let not mounted on the window.

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

Two, var concept of variable lift, and const, let no concept of variable lift

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;

 

Three, there is no concept of block-level scope var, and const, let the concept of block-level acting

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

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

 

Four, var not scratch the dead, and const, let there staging the dead

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;
}

Five, let, const can not declare a variable of the same name in the same scope, and var is possible

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。

Six, const-related

When defining const variable time, if the value is the value of a variable, we can not re-assignment; if the value is a reference type, we can change its properties.

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'}