How to Declare and Initialize an Array in JavaScript

JavaScript allows declaring an Array in several ways. Let's consider two most common ways: the array constructor and the literal notation.

The new Array() Constructor

The <kbd class="highlighted">Array()</kbd> constructor creates Array objects. You can declare an array with the "new" keyword to instantiate the array in memory. Here’s how you can declare new <kbd class="highlighted">Array()</kbd> constructor:

• let x = new Array(); - an empty array
• let x = new Array(10,20,30); - three elements in the array: 10,20,30
• let x = new Array(10); - ten empty elements in array: ,,,,,,,,,
• let x = new Array('10'); - an array with 1 element: ‘10’

Let's see an example where the array has five elements:

Javascript Array constructor

The new keyword only complicates the code. It can also produce some unexpected results.

The Literal Notation

Instead of <kbd class="highlighted">new Array()</kbd> , you can use square brackets []. Using square brackets is called the "array literal notation":

• let x = []; - an empty array
• let x = [10]; - initialized array
• let x = [10,20,30]; - three elements in the array: 10,20,30
• let x = ["10", "20", "30"]; - declares the same: ‘10’,’20’,’30’

Javascript array use square brackets

Line breaks and spaces are not important. The declaration can span multiple lines like in this example:

Javascript array declaration can span multiple lines

The Difference Between Array() and []

Using Array literal notation if you put a number in the square brackets it will return the number while using new <kbd class="highlighted">Array()</kbd> if you pass a number to the constructor, you will get an array of that length.

you call the <kbd class="highlighted">Array()</kbd> constructor with two or more arguments, the arguments will create the array elements. If you only invoke one argument, the argument initializes the length of the new array; the new array's elements are not initialized.