Difference between var, let and const keywords in JavaScript

15 Dec, 2021
In JavaScript, users can declare a variable using 3 keywords that are var, let, and const. In this article, we will see the differences between the var, let, and const keywords. We will discuss the scope and other required concepts about each keyword.
var keyword in JavaScript: The var is the oldest keyword to declare a variable in JavaScript. 
Scope: Global scoped or function scoped. The scope of the var keyword is the global or function scope. It means variables defined outside the function can be accessed globally, and variables defined inside a particular function can be accessed within the function. 
Example 1: Variable ‘a’ is declared globally. So, the scope of the variable ‘a’ is global, and it can be accessible everywhere in the program. The output shown is in the console.
Javascript
<script>    var a = 10        function f(){            console.log(a)        }    f();    console.log(a);</script>
Output: 
10
10
Example 2: The variable ‘a’ is declared inside the function. If the user tries to access it outside the function, it will display the error. Users can declare the 2 variables with the same name using the var keyword. Also, the user can reassign the value into the var variable. The output shown in the console.
Javascript
<script>    function f() {         // It can be accessible any        // where within this function        var a = 10;        console.log(a)    }    f();     // A cannot be accessible    // outside of function    console.log(a);</script>
Output:
10
ReferenceError: a is not defined
Example 3: User can re-declare variable using var and user can update var variable. The output is shown in the console.
Javascript
<script>     var a = 10       // User can re-declare    // variable using var    var a = 8     // User can update var variable    a = 7</script>
Output:
7
Example 4: If users use the var variable before the declaration, it initializes with the undefined value. The output is shown in the console.
Javascript
<script>    console.log(a);    var a = 10;<script>
Output:
undefined
let keyword in JavaScript: The let keyword is an improved version of the var keyword. 
Scope: block scoped: The scope of a let variable is only block scoped. It can’t be accessible outside the particular block ({block}). Let’s see the below example.
Example 1: The output is shown in the console.
Javascript
<script>    let a = 10;    function f() {        let b = 9        console.log(b);        console.log(a);    }    f();</script>
Output:
9
10
Example 2: The code returns an error because we are accessing the let variable outside the function block. The output is shown in the console.
Javascript
<script>    let a = 10;    function f() {        if (true) {            let b = 9             // It prints 9            console.log(b);        }         // It gives error as it        // defined in if block        console.log(b);    }    f()     // It prints 10    console.log(a)</script>
Output:
9
ReferenceError: b is not defined
Example 3: Users cannot re-declare the variable defined with the let keyword but can update it.
Javascript
<script>     let a = 10     // It is not allowed    let a = 10     // It is allowed    a = 10</script>
Output:
Uncaught SyntaxError: Identifier 'a' has already been declared
Example 4: Users can declare the variable with the same name in different blocks using the let keyword.
Javascript
<script>  let a = 10  if (true) {    let a=9    console.log(a) // It prints 9  }  console.log(a) // It prints 10</script>
Output:
9 
10
Example 5: If users use the let variable before the declaration, it does not initialize with undefined just like a var variable and return an error.
Javascript
<script>    console.log(a);    let a = 10;</script>
Output:
Uncaught ReferenceError: Cannot access 'a' before initialization
const keyword in JavaScript: The const keyword has all the properties that are the same as the let keyword, except the user cannot update it.
Scope: block scoped: When users declare a const variable, they need to initialize it, otherwise, it returns an error. The user cannot update the const variable once it is declared. 
Example 1: We are changing the value of the const variable so that it returns an error. The output is shown in the console.
Javascript
<script>    const a = 10;    function f() {        a = 9        console.log(a)    }    f();</script>
Output:
a=9
TypeError:Assignment to constant variable.
Example 2: Users cannot change the properties of the const object, but they can change the value of properties of the const object.
Javascript
<script>    const a = {        prop1: 10,        prop2: 9    }         // It is allowed    a.prop1 = 3     // It is not allowed    a = {        b: 10,        prop2: 9    }</script>
Output:
Uncaught SyntaxError:Unexpected identifier
Differences between var, let, and const
Note: Sometimes, users face the problem while working with the var variable as they change the value of it in the particular block. So, users should use the let and const keyword to declare a variable in JavaScript. 
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