HTML CSS JAVASCRIPT MySQL PHP JQUERY LIQUID

JavaScript Array Const

❮ Previous Next ❯

ECMAScript 2015 (ES6)

In 2015, JavaScript added a significant new term: const.

Using const is now a usual way to create arrays.

Cannot be Reassigned

An array created using the const keyword can't be changed once it's defined.

Arrays are Not Constants

The keyword const is a little misleading.

This doesn't create a fixed array. Instead, it establishes a permanent link to an array.

Due to this, it's still possible to modify the elements of an unchanging array.

Elements Can be Reassigned

You can change the elements of a constant array:

Assigned when Declared

In JavaScript, when you use the const keyword to create a variable, you have to give it a value right at the moment when you declare it.

Explanation: When you create an array using const you need to set its initial values right when you declare it.

Using const without initializing the array is a syntax error:

Example

This will not work:

const cars;
cars = ["Saab", "Volvo", "BMW"];

Arrays created using the var keyword can be given values whenever you want.

You can even use the array before it is declared:

Const Block Scope

An array created with the const keyword has a scope that is limited to the block of code it's defined in.

An array declared in a block is not the same as an array declared outside the block:

An array created using the var keyword does not have block scope.

Find additional information on Block Scope in the chapter titled JavaScript Scope.

Redeclaring Arrays

You can use the same array name anywhere in your program if you initially declared it with var.

Example

var cars = ["Volvo", "BMW"];   // Allowed
 var cars = ["Toyota", "BMW"];  // Allowed
 cars = ["Volvo", "Saab"];      // Allowed

You cannot use the same name for an array when you've already declared it with const in the same part of your code.

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
 }

Reusing an array using const in a different part of the code or within a new section is acceptable.

Example

const cars = ["Volvo", "BMW"];   // Allowed
 {
  const cars = ["Volvo", "BMW"]; // Allowed
 }
{
  const cars = ["Volvo", "BMW"]; // Allowed
 }
❮ Previous Next ❯