The way I've done this in the past is to check for whether or not a "callback" query parameter was provided in the URL, and if so, use the function name provided to perform the callback. If not provided, send back the object itself.
case "api" :: "events" :: "person" :: _ Get req => {
val jsonObj = json_person(...)
S.param("callback") match {
case Full(callbackName) => JSFunc(callbackName, jsonObj).cmd
case _ => JsonResponse(jsonObj)
}
}