# JSObjectCallAsConstructorCallback The callback type for using an object as a constructor. ``` typealias JSObjectCallAsConstructorCallback = (JSContextRef?, JSObjectRef?, Int, UnsafePointer?, UnsafeMutablePointer?) -> JSObjectRef? ``` ## Parameters `ctx` The execution context to use. `constructor` A [`JSObjectRef`](/documentation/JavaScriptCore/JSObjectRef) that is the constructor to call. `argumentCount` An integer count of the number of arguments in `arguments`. `arguments` A [`JSValueRef`](/documentation/JavaScriptCore/JSValueRef) array of the arguments to pass to the function. `exception` A pointer to a [`JSValueRef`](/documentation/JavaScriptCore/JSValueRef) to return an exception in, if any. ## Return Value A [`JSObjectRef`](/documentation/JavaScriptCore/JSObjectRef) that is the constructor’s return value. ## Discussion If you name your function `CallAsConstructor`, you declare it like this: ```c JSObjectRef CallAsConstructor(JSContextRef ctx, JSObjectRef constructor, size_t argumentCount, const JSValueRef arguments[], JSValueRef* exception); ``` If the JavaScript expression `new myConstructor()` invokes your callback, it sets `constructor` to `myConstructor`. If this callback is `NULL`, using your object as a constructor in a `new` expression throws an exception. --- 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)