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.

This method makes the [[prototype]] of the target immutable; any [[prototype]] re-assignment will throw a TypeError. This behavior is specific to the internal [[prototype]] property, other properties of the target object will remain mutable.

There is no way to make an object extensible again once it has been made non-extensible.


Using Object.preventExtensions

// 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';
  // throws a TypeError
  nonExtensible.newProperty = 'FAIL';

A non-extensible object's prototype is immutable:

var fixed = Object.preventExtensions({});
// throws a 'TypeError'.
fixed.__proto__ = { oh: 'hai' };

Non-object coercion

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES2015, 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                             (ES2015 code)


ECMAScript (ECMA-262)
The definition of 'Object.preventExtensions' in that specification.

Browser compatibility

ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
preventExtensionsChrome Full support 6Edge Full support 12Firefox Full support 4IE Full support 9Opera Full support 12Safari Full support 5.1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 12Safari iOS Full support 6Samsung Internet Android Full support 1.0nodejs Full support Yes
ES2015 behavior for non-object argumentChrome Full support 44Edge Full support 12Firefox Full support 35IE Full support 11Opera Full support 31Safari Full support 9WebView Android Full support 44Chrome Android Full support 44Firefox Android Full support 35Opera Android Full support 32Safari iOS Full support 9Samsung Internet Android Full support 4.0nodejs Full support Yes


Full support
Full support

See also