var、let和const的区别

1.var声明的变量会挂载在window上

var a = 10;

console.log(a,window.a)    //10  10

let b = 10;

console.log(b,window.b)   //10   undefined

let c = 10; 

console.log(c,window.c)   //10   undefined

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2.var声明的变量存在变量提升

console.log(a)

var a = 10;

console.log(a) 

//undefined

//10

其执行过程相当于

var a

console.log(a)

a = 10;

console.log(a)

console.log(a);

let a = 10; 

Uncaught ReferenceError: Cannot access 'a' before initialization

初始化前无法访问a

const和let一样

3、let和const声明的变量会形成作用域

 if (true) {

            var a = 10

            let b = 20

        }

console.log(a)   //10

console.log(b)    //  b is not defined

const和let一样

4、同一作用域下let和const不能声明重名变量

vara = 100;

console.log(a);   // 100

var a = 10;

console.log(a);   // 10

let a = 100;

let a = 10;

//Identifier 'a' has already been declared

//标识符“a”已经被声明

const和let一样

5、const一经声明必须赋值

const a

//Missing initializer in const declaration

// const声明中缺少初始化程序

6、变量一经const声明不能修改

const a = 10;

a = 20;

console.log(a)

// Assignment to constant variable.

//分配给常数变量。

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但是const声明的数组、对象可以修改

const a = [ ]

a[0] = 20

const b = { }

b.name = "张三"

console.log(a)   //     [20]

console.log(b.name)    // 张三