Function get_callback

fn get_callback(
    cx: JSContext,
    prototype: HandleObject<'_>,
    name: &CStr,
) -> Fallible<Option<Rc<Function<DomTypeHolder>>>>

https://html.spec.whatwg.org/multipage/#dom-customelementregistry-define Step 14.4: Get callbackValue for all callbackName in lifecycleCallbacks.