Dealing with javascript functions as parameters and properties

I am working on a Jsinterop wrapper for the GSAP javascript library, and I am quite new to GWT, so I have come across a few problems I am not sure how to solve. 

My main issue revolves around trying to wrap javascript functions into Java objects. 

For example, if I have a native javascript function that has a function as one of its parameters, how can I go about wrapping this function

in java? 

And if a javascript class has a function has one of its properties, how can I go about wrapping this property?

I've had some success using the @JsFunction annotation along with functional interfaces, but I am faced with the problem

where I have to declare return types, parameters, and paramater types in the java interface, when I want to be able to supply

any arbitrary function, just like I would in native javascript. Is there any way to mimic this functionality? 

Thanks for your help!

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