Reflect.isExtensible()

The Reflect.isExtensible() static method is like Object.isExtensible. It determines if an object is extensible (whether it can have new properties added to it).

Syntax

Reflect.isExtensible(target)

Parameters

target
- : The target object which to check if it is extensible.

Return value

A Boolean indicating whether or not the target is extensible.

Exceptions

TypeError
- : Thrown if target is not an object.

Description

Reflect.isExtensible() provides the reflective semantic of checking if an object is extensible. The only difference with Object.isExtensible is how non-object targets are handled. Reflect.isExtensible() throws a TypeError if the target is not an object, while Object.isExtensible() always returns false for non-object targets.

Reflect.isExtensible() invokes the <span class="selflink">IsExtensible</span> object internal method of target.

Examples

Using Reflect.isExtensible()

See also Object.isExtensible.

// New objects are extensible.
const empty = {};
Reflect.isExtensible(empty); // true

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

// Sealed objects are by definition non-extensible.
const sealed = Object.seal({});
Reflect.isExtensible(sealed); // false

// Frozen objects are also by definition non-extensible.
const frozen = Object.freeze({});
Reflect.isExtensible(frozen); // false

Difference with Object.isExtensible()

If the target argument to this method is not an object (a primitive), then it will cause a TypeError. With Object.isExtensible, a non-object target will return false without any errors.

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

Object.isExtensible(1);
// false