The Object.preventExtensions() method prevents new properties from ever being added to an object (i.e. prevents future extensions to the object).




The object which should be made non-extensible.

Return value

The object being made non-extensible.


An object is extensible if new properties can be added to it. Object.preventExtensions() marks an object as no longer extensible, so that it will never have properties beyond the ones it had at the time it was marked as non-extensible. Note that the properties of a non-extensible object, in general, may still be deleted. Attempting to add new properties to a non-extensible object will fail, either silently or by throwing a TypeError (most commonly, but not exclusively, when in strict mode).

Object.preventExtensions() only prevents addition of own properties. Properties can still be added to the object prototype. However, calling Object.preventExtensions() on an object will also prevent extensions on its __proto__ property.

If there is a way to turn an extensible object to a non-extensible one, there is no way to do the opposite in ECMAScript 5.


// Object.preventExtensions returns the object being made non-extensible.
var obj = {};
var obj2 = Object.preventExtensions(obj);
obj === obj2; // true

// Objects are extensible by default.
var empty = {};
Object.isExtensible(empty); // === true

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

// Object.defineProperty throws when adding a new property to a non-extensible object.
var nonExtensible = { removable: true };
Object.defineProperty(nonExtensible, 'new', { value: 8675309 }); // throws a TypeError

// In strict mode, attempting to add new properties to a non-extensible object throws a TypeError.
function fail() {
  'use strict';
  nonExtensible.newProperty = 'FAIL'; // throws a TypeError

// EXTENSION (only works in engines supporting __proto__
// (which is deprecated. Use Object.getPrototypeOf instead)):
// A non-extensible object's prototype is immutable.
var fixed = Object.preventExtensions({});
fixed.__proto__ = { oh: 'hai' }; // throws a TypeError


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 it.

// TypeError: 1 is not an object (ES5 code)

// 1                             (ES6 code)


Specification Status Comment
ECMAScript 5.1 (ECMA-262)
The definition of 'Object.preventExtensions' in that specification.
Standard Initial definition. Implemented in JavaScript 1.8.5.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Object.preventExtensions' in that specification.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Object.preventExtensions' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 6 4.0 (2.0) 9 12 5.1
ES6 behavior for non-object argument 44 35.0 (35.0) 11 31 ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? ? ? ? ?
ES6 behavior for non-object argument ? ? 35.0 (35.0) ? ? ?

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

ECMAScript5 JavaScript JavaScript 1.8.5 Method Object