Reflect.preventExtensions()

The Reflect.preventExtensions() static method is like {{jsxref("Object.preventExtensions()")}}. It prevents new properties from ever being added to an object (i.e., prevents future extensions to the object).

{{InteractiveExample("JavaScript Demo: Reflect.preventExtensions()")}}

```js interactive-example const object = {};

console.log(Reflect.isExtensible(object)); // Expected output: true

Reflect.preventExtensions(object);

console.log(Reflect.isExtensible(object)); // Expected output: false

## Syntax

```js-nolint
Reflect.preventExtensions(target)

Parameters#

• target
• : The target object on which to prevent extensions.

Return value#

A {{jsxref("Boolean")}} indicating whether or not the target was successfully set to prevent extensions.

Exceptions#

• {{jsxref("TypeError")}}
• : Thrown if target is not an object.

Description#

Reflect.preventExtensions() provides the reflective semantic of preventing extensions of an object. The differences with {{jsxref("Object.preventExtensions()")}} are:

• Reflect.preventExtensions() throws a {{jsxref("TypeError")}} if the target is not an object, while Object.preventExtensions() always returns non-object targets as-is.
• Reflect.preventExtensions() returns a {{jsxref("Boolean")}} indicating whether or not the target was successfully set to prevent extensions, while Object.preventExtensions() returns the target object.

Reflect.preventExtensions() invokes the [[PreventExtensions]] object internal method of target.

Examples#

Using Reflect.preventExtensions()#

See also {{jsxref("Object.preventExtensions()")}}.

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

// … but that can be changed.
Reflect.preventExtensions(empty);
Reflect.isExtensible(empty); // false

Difference with Object.preventExtensions()#

If the target argument to this method is not an object (a primitive), then it will cause a {{jsxref("TypeError")}}. With {{jsxref("Object.preventExtensions()")}}, a non-object target will be returned as-is without any errors.

Reflect.preventExtensions(1);
// TypeError: 1 is not an object

Object.preventExtensions(1);
// 1

Specifications#

{{Specifications}}

Browser compatibility#

{{Compat}}

See also#

• Polyfill of Reflect.preventExtensions in core-js
• {{jsxref("Reflect")}}
• {{jsxref("Object.preventExtensions()")}}
• handler.preventExtensions()