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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.


// 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' };


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)


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

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support6 Yes49125.1
ES2015 behavior for non-object argument44 ?3511319
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes Yes Yes
ES2015 behavior for non-object argument Yes Yes ?35 Yes9 Yes

See also

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