Object.isExtensible()

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

Syntax

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 (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().

Examples

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

// ...but that can be changed.
Object.preventExtensions(empty);
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

Notes

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.

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

Object.isExtensible(1);
// false                         (ES6 code)

Specifications

Browser compatibility

See also

• Object.preventExtensions()
• Object.seal()
• Object.isSealed()
• Object.freeze()
• Object.isFrozen()
• Reflect.isExtensible()