Object.isExtensible

Summary

Determines if an object is extensible (whether it can have new properties added to it).

Method of Object
Implemented in	JavaScript 1.8.5
ECMAScript Edition	ECMAScript 5th Edition

Syntax

Object.isExtensible(obj)

Parameters

obj

The object which should be checked.

Description

Objects are extensible by default: they can have new properties added to them, and (in engines that support __proto__ Deprecated their __proto__ property) can be modified. An object can be marked as non-extensible using Object.preventExtensions, Object.seal, or Object.freeze.

Examples

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

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

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

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

Browser compatibility

Based on Kangax's compat table.

See also

• Object.preventExtensions
• Object.seal
• Object.isSealed
• Object.freeze
• Object.isFrozen