JavaScript Array Const

ECMAScript 2015 (ES6)

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

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


const cars = ["Saab""Volvo""BMW"];


Cannot be Reassigned

An array declared with const cannot be reassigned:


const cars = ["Saab""Volvo""BMW"];
cars = [
"Toyota""Volvo""Audi"];    // ERROR


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:


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

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

// You can add an element:


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:


Chrome 49

IE 11 / Edge

Firefox 36

Safari 10

Opera 36

Mar, 2016

Oct, 2013

Feb, 2015

Sep, 2016

Mar, 2016

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:


This will not work:

const cars;
cars = [

Arrays declared with var can be initialized at any time.

You can even use the array before it is declared:


This is OK:

cars = ["Saab""Volvo""BMW"];
var cars;


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:


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"


An array declared with var does not have block scope:


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"


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:


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:


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