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Indexed collections

This chapter introduces collections of data which are ordered by an index value. This includes arrays and array-like constructs such as Array objects and TypedArray objects.

An array is an ordered list of values that you refer to with a name and an index.

For example, consider an array called emp, which contains employees' names indexed by their numerical employee number. So emp[0] would be employee number zero, emp[1] employee number one, and so on.

JavaScript does not have an explicit array data type. However, you can use the predefined Array object and its methods to work with arrays in your applications. The Array object has methods for manipulating arrays in various ways, such as joining, reversing, and sorting them. It has a property for determining the array length and other properties for use with regular expressions.

We will be focusing on arrays in this article, but many of the same concepts apply to typed arrays as well, since arrays and typed arrays share many similar methods. For more information on typed arrays, see the typed array guide.

Creating an array

The following statements create equivalent arrays:

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const arr1 = new Array(element0, element1, /* …, */ elementN);
const arr2 = Array(element0, element1, /* …, */ elementN);
const arr3 = [element0, element1, /* …, */ elementN];

element0, element1, …, elementN is a list of values for the array's elements. When these values are specified, the array is initialized with them as the array's elements. The array's length property is set to the number of arguments.

The bracket syntax is called an "array literal" or "array initializer." It's shorter than other forms of array creation, and so is generally preferred. See Array literals for details.

To create an array with non-zero length, but without any items, either of the following can be used:

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// This...
const arr1 = new Array(arrayLength);

// … results in the same array as this
const arr2 = Array(arrayLength);

// This has exactly the same effect
const arr3 = [];
arr3.length = arrayLength;

Note: In the above code, arrayLength must be a Number. Otherwise, an array with a single element (the provided value) will be created. Calling arr.length will return arrayLength, but the array doesn't contain any elements. A for...in loop will not find any property on the array.

In addition to a newly defined variable as shown above, arrays can also be assigned as a property of a new or an existing object:

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