Object.preventExtensions()

The Object.preventExtensions() static method prevents new properties from ever being added to an object (i.e. prevents future extensions to the object). It also prevents the object's prototype from being re-assigned.

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, in strict mode, throwing a TypeError.

Unlike Object.seal() and Object.freeze(), Object.preventExtensions() invokes an intrinsic JavaScript behavior and cannot be replaced with a composition of several other operations. It also has its Reflect counterpart (which only exists for intrinsic operations), Reflect.preventExtensions().

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

// Objects are extensible by default.
const empty = {};
Object.isExtensible(empty); // true

// They can be made un-extensible
Object.preventExtensions(empty);
Object.isExtensible(empty); // false

// Object.defineProperty throws when adding
// a new property to a non-extensible object.
const 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:

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

Non-object argument

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 returned as-is without any errors, since primitives are already, by definition, immutable.

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

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