code: function(btn, options, settings, callback) {
              // argument mapping
              if (typeof settings === 'function') {
                  // (btn, options, callback)
                  callback = settings;
                  settings = {};
              }
  
              settings = $.extend({}, Garnish.SelectMenu.defaults, settings);
  
              this.base(btn, options, settings, callback);
  
              this.selected = -1;
          }
docs: Constructor