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 ES2015, 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                         (ES2015 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 Latest Draft (ECMA-262)
The definition of 'Object.isExtensible' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support6 Yes49125.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support Yes Yes Yes Yes Yes Yes Yes

See also

Document Tags and Contributors

 Last updated by: fscholz,