code: function() {
      const ui = assign( {}, this.ui );
      let key;
      let element;
  
      for ( key in ui ) {
        if ( ui.hasOwnProperty( key ) ) {
          element = this.element.querySelectorAll( ui[key] );
          if ( element.length === 1 ) {
            ui[key] = element[0];
          } else if ( element.length > 1 ) {
            ui[key] = element;
          } else {
            ui[key] = null;
          }
        }
      }
      this.ui = ui;
  
      return ui;
    }
docs: TODO Fix complexity issue /* eslint-disable complexity 
  Function used to set the cached DOM elements.
  
  @returns {Object} Hash of event names and cached elements.