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"];

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Cannot be Reassigned

An array declared with const cannot be reassigned:

Example

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

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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");

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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

If you expect that any of your users runs Internet Explorer 10 or earlier, you should avoid using const.

It will produce a syntax error, and the code will not run.

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Assigned When Declared

JavaScript const variables must be assigned a value when they are declared:

Meaning: An arrays 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;

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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"

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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"

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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
}

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Previous Next

JavaScript Array Const

ECMAScript 2015 (ES6)

Example

Cannot be Reassigned

Example

Arrays are Not Constants

Elements Can be Reassigned

Example

Browser Support

Assigned When Declared

Example

Example

Const Block Scope

Example

Redeclaring Arrays

Example

Example

Example

Example

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