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

Try it

const object = {};

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

Reflect.preventExtensions(object);

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

Syntax

Reflect.preventExtensions(target)

Parameters

target

The target object on which to prevent extensions.

Return value

A Boolean indicating whether or not the target was successfully set to prevent extensions.

Exceptions

TypeError

Thrown if target is not an object.

Description

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

• Reflect.preventExtensions() throws a TypeError if the target is not an object, while Object.preventExtensions() always returns non-object targets as-is.
• Reflect.preventExtensions() returns a 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 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 TypeError. With 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

Browser compatibility

See also

• Polyfill of Reflect.preventExtensions in core-js
• Reflect
• Object.preventExtensions()
• handler.preventExtensions()