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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"];

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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




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;

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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"

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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Reference: https://www.w3schools.com/js/js_array_const.asp