Array.isArray()
The Array.isArray()
static method determines whether the passed value is an Array
.
Try it
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 does not check the value's prototype chain, nor does it rely on the Array
constructor it is attached to. It returns true
for any value that was created using the array literal syntax or the Array
constructor. This makes it safe to use with cross-realm objects, where the identity of the Array
constructor is different and would therefore cause instanceof Array
to fail.
See the article "Determining with absolute accuracy whether or not a JavaScript object is an array" for more details.
Array.isArray()
also rejects objects with Array.prototype
in its prototype chain but aren't actual arrays, which instanceof Array
would accept.
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
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.isarray |
Browser compatibility
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