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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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.
const obj = {};
const obj2 = Object.preventExtensions(obj);
obj === obj2; // true

// Objects are extensible by default.
const 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.
const 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:

const 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, return it.

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

// 1                             (ES2015 code)


ECMAScript Language Specification
# sec-object.preventextensions

Browser compatibility

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See also