Array.isArray()
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
js
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 property 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 actualArrayinstances, even if they haveArray.prototypetheir prototype chain —instanceof Arraywould accept these as it does check the prototype chain.Array.isArray()acceptsArrayobjects constructed in another realm —instanceof Arrayreturnsfalsefor these because the identity of theArrayconstructor 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