target

The target object on which to prevent extensions.

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

TypeError

Thrown if target is not an object.

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.

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

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

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