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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).

Syntax

Object.preventExtensions(obj)

Parameters

obj
The object which should be made non-extensible.

Return value

The object being made non-extensible.

Description

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.

Examples

// 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.preventExtensions(empty);
Object.isExtensible(empty); // === false

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

A non-extensible object's prototype is immutable:

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

Notes

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.

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

Object.preventExtensions(1);
// 1                             (ES2015 code)

Specifications

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

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support 6Edge Full support YesFirefox Full support 4IE Full support 9Opera Full support 12Safari Full support 5.1WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
ES2015 behavior for non-object argumentChrome Full support 44Edge ? Firefox Full support 35IE Full support 11Opera Full support 31Safari Full support 9WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 35Opera Android Full support YesSafari iOS Full support 9Samsung Internet Android Full support Yesnodejs Full support Yes

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown

See also

Tag del documento e collaboratori

Ultima modifica di: rwaldron,