Array.isArray()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The Array.isArray() static method determines whether the passed value is an Array.

Try it

console.log(Array.isArray([1, 3, 5]));
// Expected output: true

console.log(Array.isArray("[]"));
// Expected output: false

console.log(Array.isArray(new Array(5)));
// Expected output: true

console.log(Array.isArray(new Int16Array([15, 33])));
// Expected output: false

Syntax

Array.isArray(value)

Parameters

value

The value to be checked.

Return value

true if value is an Array; otherwise, false. false is always returned if value is a TypedArray instance.

Description

Array.isArray() checks if the passed value is an Array. It performs a branded check, similar to the in operator, for a private field initialized by the Array() constructor.

It is a more robust alternative to instanceof Array because it avoids false positives and false negatives:

Array.isArray() rejects values that aren't actual Array instances, even if they have Array.prototype in their prototype chain — instanceof Array would accept these as it does check the prototype chain.
Array.isArray() accepts Array objects constructed in another realm — instanceof Array returns false for these because the identity of the Array constructor is different across realms.

See the article "Determining with absolute accuracy whether or not a JavaScript object is an array" for more details.

Examples

Using Array.isArray()

js
// all following calls return true
Array.isArray([]);
Array.isArray([1]);
Array.isArray(new Array());
Array.isArray(new Array("a", "b", "c", "d"));
Array.isArray(new Array(3));
// Little known fact: Array.prototype itself is an array:
Array.isArray(Array.prototype);

// all following calls return false
Array.isArray();
Array.isArray({});
Array.isArray(null);
Array.isArray(undefined);
Array.isArray(17);
Array.isArray("Array");
Array.isArray(true);
Array.isArray(false);
Array.isArray(new Uint8Array(32));
// This is not an array, because it was not created using the
// array literal syntax or the Array constructor
Array.isArray({ __proto__: Array.prototype });

instanceof vs. Array.isArray()

When checking for Array instance, Array.isArray() is preferred over instanceof because it works across realms.

js
const iframe = document.createElement("iframe");
document.body.appendChild(iframe);
const xArray = window.frames[window.frames.length - 1].Array;
const arr = new xArray(1, 2, 3); // [1, 2, 3]

// Correctly checking for Array
Array.isArray(arr); // true
// The prototype of arr is xArray.prototype, which is a
// different object from Array.prototype
arr instanceof Array; // false