The Object.preventExtensions()
method prevents new properties from ever being added to an object (i.e. prevents future extensions to the object).
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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
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.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' };
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.
Object.preventExtensions(1); // TypeError: 1 is not an object (ES5 code) Object.preventExtensions(1); // 1 (ES2015 code)
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Object.preventExtensions' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
preventExtensions | Chrome Full support 6 | Edge Full support 12 | Firefox Full support 4 | IE Full support 9 | Opera Full support 12 | Safari Full support 5.1 | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 12 | Safari iOS Full support 6 | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
ES2015 behavior for non-object argument | Chrome Full support 44 | Edge Full support 12 | Firefox Full support 35 | IE Full support 11 | Opera Full support 31 | Safari Full support 9 | WebView Android Full support 44 | Chrome Android Full support 44 | Firefox Android Full support 35 | Opera Android Full support 32 | Safari iOS Full support 9 | Samsung Internet Android Full support 4.0 | nodejs Full support Yes |
Legend
- Full support
- Full support