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).

Try it

const object1 = {};

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

Reflect.preventExtensions(object1);

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

const object2 = Object.seal({});

console.log(Reflect.isExtensible(object2));
// Expected output: false

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 [[IsExtensible]] 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

Specifications

Browser compatibility

See also

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