ECMAScript 2015 (ES6)
In 2015, JavaScript introduced the const keyword, which has become a common practice for declaring arrays.
Example
const fruits = ["Apple", "Banana", "Cherry"];
Cannot be Reassigned
An array declared with const cannot be reassigned to a different array.
Example
const fruits = ["Apple", "Banana", "Cherry"];fruits = ["Orange", "Mango", "Pineapple"]; // ERROR
Arrays are Not Constants
The const keyword defines a constant reference to an array, not a constant array itself. This means you can change the elements of the array, but you cannot reassign the array variable to a new array.
Elements Can be Reassigned
You can change the elements of a constant array.
Example
// Creating a constant arrayconst fruits = ["Apple", "Banana", "Cherry"];
// Changing an elementfruits[0] = "Orange";// Adding a new elementfruits.push("Pineapple");
Browser Support
The const keyword is not supported in Internet Explorer 10 or earlier.
The following table shows the first browser versions with full support for the const keyword:
Browser
Version
Chrome
49 (Mar 2016)
IE / Edge
11 (Oct 2013)
Firefox
36 (Feb 2015)
Safari
10 (Sep 2016)
Opera
36 (Mar 2016)
Assigned when Declared
JavaScript const variables must be assigned a value when they are declared. This means an array declared with const must be initialized during declaration.
Example
This will not work:
const fruits;fruits =
["Apple", "Banana", "Cherry"]; // ERROR
Arrays declared with var can be initialized at any time and even used before they are declared.
Example
This is OK:
fruits = ["Apple", "Banana", "Cherry"];var fruits;
Const Block Scope
An array declared with const has block scope. This means an array declared inside a block is different from an array declared outside the block.
Example
const fruits = ["Apple", "Banana", "Cherry"];// Here fruits[0] is "Apple"{ const fruits = ["Orange", "Mango", "Pineapple"]; // Here fruits[0] is "Orange"}// Here fruits[0] is "Apple"
An array declared with var does not have block scope.
Example
var fruits = ["Apple", "Banana", "Cherry"];// Here fruits[0] is "Apple"{ var fruits = ["Orange", "Mango", "Pineapple"]; // Here fruits[0] is "Orange"}// Here fruits[0] is "Orange"
Redeclaring Arrays
Redeclaring an array declared with var is allowed anywhere in a program.
Example
var fruits = ["Banana", "Cherry"]; // Allowedvar fruits = ["Orange", "Mango"]; // Allowedfruits = ["Apple", "Pineapple"]; // Allowed
Redeclaring or reassigning an array declared with const in the same scope or block is not allowed.
Example
var fruits = ["Banana", "Cherry"]; // Allowedconst fruits = ["Orange", "Mango"]; // Not allowed{ var fruits = ["Apple", "Pineapple"]; // Allowed const fruits = ["Peach", "Grape"]; // Not allowed}
Redeclaring or reassigning an existing const array in the same scope or block is not allowed.
Example
const fruits = ["Banana", "Cherry"]; // Allowedconst fruits = ["Orange", "Mango"]; // Not allowedvar fruits = ["Apple", "Pineapple"];
// Not allowedfruits = ["Peach", "Grape"]; // Not allowed{ const fruits = ["Banana", "Cherry"]; // Allowed const fruits = ["Orange", "Mango"]; // Not allowed var fruits = ["Apple", "Pineapple"]; // Not allowed fruits = ["Peach", "Grape"]; // Not allowed}
Redeclaring an array with const in another scope or block is allowed.
Example
javascriptCopy codeconst fruits = ["Banana", "Cherry"];
// Allowed{ const fruits = ["Orange", "Mango"]; // Allowed}{ const fruits = ["Apple", "Pineapple"]; // Allowed}
