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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

Browser compatibilityUpdate compatibility data on GitHub

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

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible