Tutorials References Exercises Videos Menu
Create Website Get Certified Upgrade

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Events JS Strings JS String Methods JS String Search JS String Templates JS Numbers JS BigInt JS Number Methods JS Number Properties JS Arrays JS Array Methods JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS If Else JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Iterables JS Sets JS Maps JS Typeof JS Type Conversion JS Bitwise JS RegExp JS Precedence JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS Modules JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS 2019 JS 2020 JS 2021 JS IE / Edge JS History

JS Objects

Object Definitions Object Properties Object Methods Object Display Object Accessors Object Constructors Object Prototypes Object Iterables Object Sets Object Maps Object Reference

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Bind Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON Server JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor JS Exercises JS Quiz JS Certificate

JS References

JavaScript Objects HTML DOM Objects


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:

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:

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:

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.