ECMAScript 2015 (ES6)

In 2015, JavaScript introduced an important new keyword: const .

It has become a common practice to declare arrays using const :

Example

const cars = ["Saab", "Volvo", "BMW"];
Try it Yourself »

Cannot be Reassigned

An array declared with const cannot be reassigned:

Example

const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"];    // ERROR Try it Yourself »

Arrays are Not Constants

The keyword const is a little misleading.

It does NOT define a constant array. It defines a constant reference to an array.

Because of this, we can still change the elements of a constant array.

Elements Can be Reassigned

You can change the elements of a constant array:

Example

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

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

// You can add an element:
cars.push("Audi");
Try it Yourself »

Browser Support

The const keyword is not supported in Internet Explorer 10 or earlier.

The following table defines the first browser versions with full support for the const keyword:

Assigned when Declared

JavaScript const variables must be assigned a value when they are declared:

Meaning: An array declared with const must be initialized when it is declared.

Using const without initializing the array is a syntax error:

Example

This will not work:

const cars;
cars = ["Saab", "Volvo", "BMW"];

Arrays declared with var can be initialized at any time.

You can even use the array before it is declared:

Example

This is OK:

cars = ["Saab", "Volvo", "BMW"];
var cars; Try it Yourself »

Const Block Scope

An array declared with const has Block Scope .

An array declared in a block is not the same as an array declared outside the block:

Example

const cars = ["Saab", "Volvo", "BMW"];
// Here cars[0] is "Saab"
const cars = ["Toyota", "Volvo", "BMW"];
// Here cars[0] is "Toyota"
// Here cars[0] is "Saab" Try it Yourself »

An array declared with var does not have block scope:

Example

var cars = ["Saab", "Volvo", "BMW"];
// Here cars[0] is "Saab"
var cars = ["Toyota", "Volvo", "BMW"];
// Here cars[0] is "Toyota"
// Here cars[0] is "Toyota" Try it Yourself »

You can learn more about Block Scope in the chapter: JavaScript Scope .

Redeclaring Arrays

Redeclaring an array declared with var is allowed anywhere in a program:

Example

var cars = ["Volvo", "BMW"];   // Allowed
var cars = ["Toyota", "BMW"];  // Allowed
cars = ["Volvo", "Saab"];      // Allowed

Redeclaring or reassigning an array to const , in the same scope, or in the same block, is not allowed:

Example

var cars = ["Volvo", "BMW"];     // Allowed
const cars = ["Volvo", "BMW"];   // Not allowed
var cars = ["Volvo", "BMW"];   // Allowed
const cars = ["Volvo", "BMW"]; // Not allowed

Redeclaring or reassigning an existing const array, in the same scope, or in the same block, is not allowed:

Example

const cars = ["Volvo", "BMW"];   // Allowed
const cars = ["Volvo", "BMW"];   // Not allowed
var cars = ["Volvo", "BMW"];     // Not allowed
cars = ["Volvo", "BMW"];         // Not allowed
const cars = ["Volvo", "BMW"]; // Allowed
const cars = ["Volvo", "BMW"]; // Not allowed
var cars = ["Volvo", "BMW"];   // Not allowed
cars = ["Volvo", "BMW"];       // Not allowed

Redeclaring an array with const , in another scope, or in another block, is allowed:

Example

const cars = ["Volvo", "BMW"];   // Allowed
const cars = ["Volvo", "BMW"]; // Allowed
const cars = ["Volvo", "BMW"]; // Allowed

Complete Array Reference

For a complete Array reference, go to our:

Complete JavaScript Array Reference .

The reference contains descriptions and examples of all Array properties and methods.

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
[email protected]

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
[email protected]

W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning.
Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness
of all content. While using W3Schools, you agree to have read and accepted our terms of use , cookie and privacy policy .

W3Schools is Powered by W3.CSS .