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Object.isExtensible()

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 Object.isExtensible() static method determines if an object is extensible (whether it can have new properties added to it).

Try it

const object = {};

console.log(Object.isExtensible(object));
// Expected output: true

Object.preventExtensions(object);

console.log(Object.isExtensible(object));
// Expected output: false

Syntax

Object.isExtensible(obj)

Parameters

obj: The object which should be checked.

Return value

A Boolean indicating whether or not the given object is extensible.

Description

Objects are extensible by default: they can have new properties added to them, and their [[Prototype]] can be re-assigned. An object can be marked as non-extensible using one of Object.preventExtensions(), Object.seal(), Object.freeze(), or Reflect.preventExtensions().

Examples

Using Object.isExtensible

// New objects are extensible.
const empty = {};
Object.isExtensible(empty); // true

// They can be made un-extensible
Object.preventExtensions(empty);
Object.isExtensible(empty); // false

// Sealed objects are by definition non-extensible.
const sealed = Object.seal({});
Object.isExtensible(sealed); // false

// Frozen objects are also by definition non-extensible.
const frozen = Object.freeze({});
Object.isExtensible(frozen); // false

Non-object argument

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES2015, it will return false without any errors if a non-object argument is passed, since primitives are, by definition, immutable.

Object.isExtensible(1);
// TypeError: 1 is not an object (ES5 code)

Object.isExtensible(1);
// false                         (ES2015 code)

Specifications

Specification
ECMAScript® 2026 Language Specification> # sec-object.isextensible>

Browser compatibility

	Desktop					Mobile							Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	WebView on iOS	Bun	Deno	Node.js
`isExtensible`	6	12	4	12	5.1	18	4	12	5	1.0	4.4	5	1.0.0	1.0	0.10.0