docs: def serialize(self, obj):
          """Serialize an object for sending to the front-end."""
          if hasattr(obj, '_jsid'):
              return {'immutable': False, 'value': obj._jsid}
          else:
              obj_json = {'immutable': True}
              try:
                  json.dumps(obj)
                  obj_json['value'] = obj
              except:
                  pass
              if callable(obj):
                  guid = str(uuid.uuid4())
                  callback_registry[guid] = obj
                  obj_json['callback'] = guid
              return obj_json