What are the differences between var let const in JavaScript

1. Repeated declaration

var supports repeated declaration, but let and const do not support repeated declaration.

1.1 var

var a = 1;
var a = 2;
console.log(a);

Output:

2

1.2 let

let b = 3;
let b = 4;
console.log(b);

Output:

Uncaught SyntaxError: Identifier 'b' has already been declared

1.3 const

const c = 5;
const c = 6;
console.log(c);

Output:

Uncaught SyntaxError: Identifier 'c' has already been declared

2. Variable promotion

var supports variable promotion, but only promotes the declaration and not the value. let and const do not support variable promotion.

2.1 var

a=2;
console.log(a);
var a = 1;

Output:

2

2.2 let

a=2;
console.log(a);
let a = 1;

Output:

Uncaught ReferenceError: Cannot access 'a' before initialization at index.html:28

2.3 const

a=2;
console.log(a);
const a = 1;

Output:

Uncaught ReferenceError: Cannot access 'a' before initialization at index.html:28

3. Temporary dead zone

There is no temporary dead zone for var, but there is a temporary dead zone for let and const.
As long as let and const exist in the scope, the variables or constants they declare are automatically "bound" to this area and are no longer affected by the external scope.

3.1 var

var a = 1;
function fun() {
    console.log(a);
    var a = 2;
}
fun();

Output:

undefined

3.2 let

let a = 1;
function fun() {
    console.log(a);
    let a = 2;
}
fun();

Output:

Uncaught ReferenceError: Cannot access 'a' before initialization

3.3 conset

let a = 1;
function fun() {
    console.log(a);
    const a = 2;
}
fun();

Output:

Uncaught ReferenceError: Cannot access 'a' before initialization

4. Properties and methods of the window object

In the global scope, variables declared by var and functions declared by function will automatically become properties and methods of the window object.

var a = 1;
function add() { };
console.log(window.a === a);
console.log(window.add === add);

Output:

true
true

5. Block Scope

var does not have block-level scope, but let and const do.
Use var to define the variable i in the for loop:

for (var i = 0; i < 3; i++) {
    // console.log(i);
}
console.log(i);

Output:

3

Use let to define the variable i in the for loop:

for (let i = 0; i < 3; i++) {
    // console.log(i);
}
console.log(i);

Output:

Uncaught ReferenceError: i is not defined

This concludes this article about the differences between the usage of var, let, and const in JavaScript. For more information on the usage of JavaScript var, please search previous articles on 123WORDPRESS.COM or continue to browse the following related articles. I hope you will support 123WORDPRESS.COM in the future!