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"];

 

Cannot be Reassigned

An array declared with const cannot be reassigned:

Example

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:

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");

 

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;

 

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"

 

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"

 

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"];