Difference between var, let and const keyword in JavaScript

 In JavaScript, We can declare a variable using below keywords:

1. var
2. let
3. const

Below are the difference between these keywords:

Scope:

The scope of the "var" keyword is the global scope. Means variables defined outside the function can be accessed globally.

<script>

    var a = 20;

    function fun() {

        if (true) { 

            var b = 30 // declared in "if" block but can be accessible outside of this "if" block.

        }

        console.log(b);

        console.log(a);

    }

    fun();

</script>

Output: 

30

20

The scope of a "let" variable is only block scoped. It can’t be accessible outside the particular block. Have a look at below example:

<script>

    let a = 20;

    function fun() {

        if (true) { 

            let b = 30

        }

        console.log(b);

        console.log(a);

    }

    fun();

</script>

Output: 

ReferenceError: b is not defined

20

Re-declaration

User can re-declare variable using "var" and user can update variable value. Have a look at below example:

<script> 

    // declare the variable.

    var a = 20

   

    // Re-declare the variable

    var a = 30 // No error with this re-declaration.

 

    // Update the variable value

    a = 40

    

    console.log(a);

</script> 

Output: 40

Users cannot re-declare the variable defined with the "let" keyword but can update it. Have a look at below example:

<script> 

    // declare the variable.

    let a = 20

   

    // Re-declare the variable

    let a = 30 // It is not allowed. Uncaught Syntax Error: Identifier 'a' has already been declared.

 

    // Update the variable value

    a = 40 // It is allowed

    

    console.log(a);

</script> 

Output: 

Uncaught Syntax Error: Identifier 'a' has already been declared.

40

"const" keyword:

"const" keyword has all the properties that are the same as the "let" keyword, except the user cannot update it's value.

<script>

    const a = 20;

    function fun() {

        a = 30  // It is not allowed. Type Error: Assignment to constant variable.

        console.log(a);

    }

    fun();

</script>

Output: 

Type Error: Assignment to constant variable.

20

Users cannot change the properties of an object declared using "const" keyword, but user can change the value of properties of a object declared using "const" keyword.

<script>

    const address = {

        city: 'Jaipur',

        state: 'Rajasthan'

    }

     

    // It is allowed

    address.city  = 'Udaipur';

 

    // It is not allowed

    address = {

        city: 'Udaipur',

        state: 'Rajasthan'

    }

</script>