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)