Web/JavaScript/Reference/Global objects/Object/isExtensible

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

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 (in engines that support __proto__) their __proto__ property can be modified. An object can be marked as non-extensible using Object.preventExtensions(), Object.seal(), or Object.freeze().

Examples

Using Object.isExtensible

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

// ...but that can be changed.
Object.preventExtensions(empty);
Object.isExtensible(empty); // === false

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

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

Non-object coercion

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES2015, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return false.

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

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

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'Object.isExtensible' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`isExtensible`	Chrome Full support 6	Edge Full support 12	Firefox Full support 4	IE Full support 9	Opera Full support 12	Safari Full support 5.1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 12	Safari iOS Full support 6	Samsung Internet Android Full support 1.0	nodejs Full support Yes

Legend

Full support: Full support