Reflect.preventExtensions()

{{JSRef}} 

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()")}} 

const object1 = {};

console.log(Reflect.isExtensible(object1));
// Expected output: true

Reflect.preventExtensions(object1);

console.log(Reflect.isExtensible(object1));
// Expected output: false

Syntax

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()