Var, Let, and Const

We all have used var, let, and const variables many times in JavaScript but sometimes we all get confused about which is the best variable to use for a particular program. I am writing this blog to understand let, var, and const variables for helping someone to clear their basics regarding the same.

Var

It is the oldest keyword to declare a variable in JavaScript. The scope of it is global or function scope. Variables defined outside the function can be accessed globally and variables defined inside a particular function can be accessed within the function.
It can be updated and re-declared into the scope.

Example -

    function f() {
        var a = 7;
        console.log(a)
    }
    f();
    console.log(a);

Output-

7
ReferenceError: a is not defined

Let

It is an improved version of the var keyword. The scope of a let keyword is only block scoped. It cannot be accessible outside the particular block.
It can be updated but cannot be re-declared into the scope.

Example 1-

    let a =7;
    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 7
    console.log(a)

Output-

9
ReferenceError: b is not defined
7

Example 2-

let a = 7
if (true) {
    let a=9
    console.log(a) // It prints 9
}
console.log(a) // It prints 7

Output-

9
7

Const

It has all the properties same as the let keyword, except the user cannot update it. It cannot be updated or re-declared into the scope.

Example-

    const a = {
        prop1: 7,
        prop2: 9
    }

    // It is allowed
    a.prop1 = 3

    // It is not allowed
    a = {
        b: 7,
        prop2: 9
    }