code: function (el, cls) {
          cls = getString(cls);
          if (!cls) return;
          if (Array.isArray(cls)) {
              cls.forEach(function(c) {
                  dom.addClass(el, c);
              });
          } else if (el.classList) {
              el.classList.add(cls);
          } else {
              if (!hasClass(el, cls)) {
                  if (el.classList) {
                      el.classList.add(cls);
                  } else {
                      el.className += ' ' + cls;
                  }
              }
          }
      }
docs: optimize if we have classList