Function call_setup 

pub fn call_setup<D, T, R>(
    cx: &mut JSContext,
    callback: &T,
    handling: ExceptionHandling,
    f: impl FnOnce(&mut JSContext) -> R,
) -> R
where
    D: DomTypes,
    T: CallbackContainer<D>,

A function wrapper that performs whatever setup we need to safely make a call.

https://webidl.spec.whatwg.org/#es-invoking-callback-functions