Reflect.isExtensible()

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

{{InteractiveExample("JavaScript Demo: Reflect.isExtensible()", "taller")}}

```js interactive-example 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

```js-nolint
Reflect.isExtensible(target)

Parameters#

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

Return value#

A {{jsxref("Boolean")}} indicating whether or not the target is extensible.

Exceptions#

• {{jsxref("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 {{jsxref("Object.isExtensible()")}} is how non-object targets are handled. Reflect.isExtensible() throws a {{jsxref("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 {{jsxref("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 {{jsxref("TypeError")}}. With {{jsxref("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#

{{Specifications}}

Browser compatibility#

{{Compat}}

See also#

• Polyfill of Reflect.isExtensible in core-js
• {{jsxref("Reflect")}}
• {{jsxref("Object.isExtensible()")}}
• handler.isExtensible()