---
title: Object.isExtensible()
short-title: isExtensible()
slug: Web/JavaScript/Reference/Global_Objects/Object/isExtensible
page-type: javascript-static-method
browser-compat: javascript.builtins.Object.isExtensible
sidebar: jsref
---

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

{{InteractiveExample("JavaScript Demo: Object.isExtensible()")}}

```js interactive-example
const object = {};

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

Object.preventExtensions(object);

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

## Syntax

```js-nolint
Object.isExtensible(obj)
```

### Parameters

- `obj`
  - : The object which should be checked.

### Return value

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

## Description

Objects are extensible by default: they can have new properties added to them, and their `[[Prototype]]` can be re-assigned. An object can be marked as non-extensible using one of {{jsxref("Object.preventExtensions()")}}, {{jsxref("Object.seal()")}}, {{jsxref("Object.freeze()")}}, or {{jsxref("Reflect.preventExtensions()")}}.

## Examples

### Using Object.isExtensible

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

// They can be made un-extensible
Object.preventExtensions(empty);
Object.isExtensible(empty); // false

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

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

### Non-object argument

In ES5, if the argument to this method is not an object (a primitive), then it will cause a {{jsxref("TypeError")}}. In ES2015, it will return `false` without any errors if a non-object argument is passed, since primitives are, by definition, immutable.

```js
Object.isExtensible(1);
// TypeError: 1 is not an object (ES5 code)

Object.isExtensible(1);
// false                         (ES2015 code)
```

## Specifications

{{Specifications}}

## Browser compatibility

{{Compat}}

## See also

- {{jsxref("Object.preventExtensions()")}}
- {{jsxref("Object.seal()")}}
- {{jsxref("Object.isSealed()")}}
- {{jsxref("Object.freeze()")}}
- {{jsxref("Object.isFrozen()")}}
- {{jsxref("Reflect.isExtensible()")}}