JavaScript Const Keyword

Updated: May 31st, 2022, 03:56:31 IST

Published: May 31st, 2022

The const keyword was added in ES6 and is used to define a new variable in JavaScript. The var keyword was often used to declare a JavaScript variable, earlier phase before ES6. When you declare a variable and don't want to modify its value for the duration of the program, you can use the const keyword.

The const keyword was added in ES6 (2015).
Variables using the const keyword cannot be redeclared.
Const-defined variables cannot be reassigned.
Variables defined with const have Block Scope.

A const variable cannot be reassigned.

const PI = 3.141592653589793;
PI = 3.14;      // This will give an error
PI = PI + 10;   // This will also give an error

When declaring const variables in JavaScript, they must be given a value (must be assigned).

// Declaring and Assigning a value together
const PI = 3.14159265359;

 // 1. Declaring
const PI;
// 2. Assigning a value
PI = 3.14159265359;

When should you use JavaScript const?

As a general guideline, unless you know the value will change, always declare a variable using const.

When declaring: use const.

A new Array
A new Object
A new Function
A new RegExp

Example: Constant Arrays

You can change the elements of a constant array:

 // You can create a constant array:
const cars = ["Audi", "Bentley", "BMW"];

// You can change an element:
cars[0] = "Toyota";

// You can add an element:
cars.push("Maruti Swift");

But you can NOT reassign the array:

 const cars = ["Audi", "Bentley", "BMW"];

cars = ["Toyota", "Volvo", "Audi"];    // ERROR will be generated.

Block Scope for const keyword

Declaring a variable with const is similar to let when it comes to Block Scope.

The n declared in the block, in this example, is not the same as the n declared outside the block.

const n = 10;
// Here n is 10

{
const n = 2;
// Here n is 2
}

// Here n is 10

Const Hoisting

Variables defined with const are also hoisted to the top, but not initialized.

Meaning: Using a const variable before results in a ReferenceError:

 // const Not allowed to use like this
alert (myName);
const myName = "Sagar InspireMe";

// Correct Method to do is:
// First declare then assign value and then use it as a block scope.
const myName = "Sagar InspireMe";
alert (myName);