JavaScript const

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JavaScript const

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JavaScript Array Const ❮ Previous Next ❯ 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.

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