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JavaScript•Working with Data

JavaScript Array Const

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JavaScript Array Const

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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");

Assigned when Declared 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"];   // 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.

RegExp Quantifiers

JavaScript RegExp Patterns

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Learn/JavaScript/Working with Data

JavaScript•Working with Data

JavaScript Array Const

Concept visual