# defineProperty(_:descriptor:) Defines a property on the JavaScript object value or modifies a property’s definition. ``` func defineProperty(_ property: Any!, descriptor: Any!) ``` ## Parameters `property` The name of the property to define or modify. `descriptor` A JavaScript object whose keys and values define the property’s behavior. ## Discussion Calling this method is equivalent to using the `Object.defineProperty` method in JavaScript. The `descriptor` parameter has the same format required by that JavaScript method; for convenience when calling from Objective-C or Swift, you can also construct it as a dictionary with the keys listed in [Property Descriptor Keys](/documentation/JavaScriptCore/property-descriptor-keys). The descriptor determines the behavior of the JavaScript property, and must fit one of three cases: - Data Descriptor: Contains one or both of the keys `value` and `writable`, and optionally also contains the keys `enumerable` or `configurable`. Cannot contain the keys `get` or `set`. Use a data descriptor to create or modify the attributes of a data property on an object (replacing any existing accessor property). - Accessor Descriptor: Contains one or both of the keys `get` or `set`, and optionally also contains the keys `enumerable` or `configurable`. Cannot contain the keys `value` and `writable`. Use an accessor descriptor to create or modify the attributes of an accessor property on an object (replacing any existing data property). - Generic Descriptor: Contains one or both of the keys `enumerable` or `configurable`, and cannot contain any other keys. Use a genetic descriptor to modify the attributes of an existing data or accessor property, or to create a new data property. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)