Object.isExtensible()

Summary

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.

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 = {};
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);

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

Based on Kangax's compat table.

See also

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