IE、Firefox兼容Enter代替Tab的Javascript

document.onkeydown = function (evt)
              {
                var isie = (document.all) ? true : false ;
                var key;
                var srcobj;
                if (isie)
                {
                  key = event.keyCode;
                  srcobj = event.srcElement;
                  }
                else
                {
                  key = evt.which;
                  srcobj = evt.target;
                  }               
                  if (key == 13 && srcobj.type != ' button ' && srcobj.type != ' submit ' && srcobj.type != ' reset ' && srcobj.type != ' textarea ' && srcobj.type != '' )                 
                  {
                    if (isie)
                    {
                     event.keyCode = 9 ;
                    }
                    else
                    {                        
                      var el = getNextElement(evt.target);
                              if (el.type != ' hidden ' )
                                 el.focus();
                              else
                                 while (el.type == ' hidden ' )
                                    el = getNextElement(el);
                                 el.focus();
                              return false ;
                    }
                  }
               }                
                 function getNextElement (field) {
                    var form = field.form;
                    for ( var e = 0 ; e < form.elements.length; e ++ ) {
                      if (field == form.elements[e])
                          break ;
                    }
                    return form.elements[ ++ e % form.elements.length];
                  }