docs: function _callback(changed_keys) {
        if (intersection(keys_to_watch_for_changes, changed_keys).length) {
          // only update the state if a key we care about has changed
          var state_update_obj = {};
          keys_to_watch_for_changes.forEach(function (k) {
            return state_update_obj[k] = store._store[k];
          });
          this.setState(state_update_obj);
  
          // if some other custom callback is required by the component
          // call that function as well
          if (additonal_callback) {
            additonal_callback(changed_keys);
          }
        }
      }