快速解决ajax传递为空但显示在页面上为undefined的问题

//这是修改后的ajax代码
 $("#ss").click(function(){
    var key = $("#firstname").val();
    if(key==''){

     alert("请输入内容查询！");
     return;
    }
    $.ajax({
    url:'${pageContext.request.contextPath}/door/searchBykey.do',
    type:'post',
    dataType:'json',
    data:{'key':key},
    success:function(data){

     $("table>tbody>tr").remove();
     for(var i=0;i<data.length;i++){
      var count = i+1;
      var doors = data[i].door;
      var str = doors.replace(/\#/g,"%23"); 
      $("table>tbody").append(
         '<tr><td>'+count+'</td>'+ 
         '<td>'+ (data[i].build==undefined"":data[i].build)+'</td>'+
         '<td>'+ (data[i].room==undefined"":data[i].room)+'</td>'+
         '<td>'+ (data[i].door==undefined"":data[i].door)+'</td>'+
         '<td>'+ (data[i].conSn==undefined"":data[i].conSn)+'</td>'+
         '<td style="width:131px;">'+
          '<button type="button" class="btn btn-success" data-toggle="modal" data-target="#myModal2" onclick="updatebutton('+str+')">'+"修改"+'</button>'+
         '<button type="button" class="btn btn-danger" onclick="delButton('+data[i].id+')">'+"删除"+'</button>'+
         '</td>'+ 
         '</tr>'
      );




     }


    },
    error:function(){
     alert("请求失败！");
    }
    });


   });

(data[i].room==undefined"":data[i].room)