files/en-us/web/javascript/reference/global_objects/object/isextensible/index.md

metadata

title: Object.isExtensible()
short-title: isExtensible()
slug: Web/JavaScript/Reference/Global_Objects/Object/isExtensible
page-type: javascript-static-method
browser-compat: javascript.builtins.Object.isExtensible
sidebar: jsref

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

{{InteractiveExample("JavaScript Demo: Object.isExtensible()")}}

const object = {};

console.log(Object.isExtensible(object));
// Expected output: true

Object.preventExtensions(object);

console.log(Object.isExtensible(object));
// Expected output: false

Syntax

Object.isExtensible(obj)

Parameters

• obj
  • : The object which should be checked.

Return value

A {{jsxref("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 their [[Prototype]] can be re-assigned. An object can be marked as non-extensible using one of {{jsxref("Object.preventExtensions()")}}, {{jsxref("Object.seal()")}}, {{jsxref("Object.freeze()")}}, or {{jsxref("Reflect.preventExtensions()")}}.

Examples

Using Object.isExtensible

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

// They can be made un-extensible
Object.preventExtensions(empty);
Object.isExtensible(empty); // false

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

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

Non-object argument

In ES5, if the argument to this method is not an object (a primitive), then it will cause a {{jsxref("TypeError")}}. In ES2015, it will return false without any errors if a non-object argument is passed, since primitives are, by definition, immutable.

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

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

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{jsxref("Object.preventExtensions()")}}
• {{jsxref("Object.seal()")}}
• {{jsxref("Object.isSealed()")}}
• {{jsxref("Object.freeze()")}}
• {{jsxref("Object.isFrozen()")}}
• {{jsxref("Reflect.isExtensible()")}}