Object.isExtensible()

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

Try it

Syntax

js

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

Examples

Using Object.isExtensible

js

// 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 TypeError. In ES2015, it will return false without any errors if a non-object argument is passed, since primitives are, by definition, immutable.

js

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

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

Specifications

Specification
ECMAScript Language Specification
# sec-object.isextensible

Browser compatibility

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See also