# JSObjectHasInstanceCallback The callback type for checking whether an object is an instance of a particular type. ``` typealias JSObjectHasInstanceCallback = (JSContextRef?, JSObjectRef?, JSValueRef?, UnsafeMutablePointer?) -> Bool ``` ## Parameters `ctx` The execution context to use. `constructor` The [`JSObjectRef`](/documentation/JavaScriptCore/JSObjectRef) that is the target of the `instanceof` expression. `possibleInstance` The [`JSValueRef`](/documentation/JavaScriptCore/JSValueRef) to test to determine if it’s an instance of `constructor`. `exception` A pointer to a [`JSValueRef`](/documentation/JavaScriptCore/JSValueRef) to return an exception in, if any. ## Return Value if `possibleInstance` is an instance of `constructor` according to the JavaScript `instanceof` expression; otherwise, . ## Discussion If you name your function `HasInstance`, you declare it like this: ```c bool HasInstance(JSContextRef ctx, JSObjectRef constructor, JSValueRef possibleInstance, JSValueRef* exception); ``` If the JavaScript expression `someValue instanceof myObject` invokes your callback, it sets `constructor` to `myObject`, and `possibleInstance` to `someValue`. If this callback is `NULL`, `instanceof` expressions that target your object return . Standard JavaScript practice calls for objects that implement the [`callAsConstructor`](/documentation/JavaScriptCore/JSClassDefinition/callAsConstructor) callback to implement the [`hasInstance`](/documentation/JavaScriptCore/JSClassDefinition/hasInstance) callback, as well. --- 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)