Join MDN and developers like you at Mozilla's View Source conference, 12-14 September in Berlin, Germany. Learn more at

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




The object which should be checked.

Return value

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


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().


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

// ...but that can be changed.
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


In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES6, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return false.

// TypeError: 1 is not an object (ES5 code)

// false                         (ES6 code)


Specification Status Comment
ECMAScript 5.1 (ECMA-262)
The definition of 'Object.isExtensible' in that specification.
Standard Initial definition. Implemented in JavaScript 1.8.5.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Object.isExtensible' in that specification.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Object.isExtensible' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 6 4.0 (2.0) 9 12 5.1
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? ? ? ? ?

See also

Document Tags and Contributors

 Last updated by: eduardoboucas,