基于ajax后台返回的数据为空前台显示出现undefined的解决方法

$(document).ready(function () {  $("#rankTable tbody").html(""); var id = 1;  $.ajax({    url:"bookInfo/bookInfo_rank.action",    type:"POST",    data:{},    dataType:"json",    success:function (data) {         $(data).each(function(i,item)     {     var index = id++;     var bookId = item.bookId;     var bookName = item.bookName;     var author = item.author;     //判断是否为空出现undefined；     var translator = (item.translator==undefined ?"":item.translator);     var ISBN = item.ISBN;     var press = item.press;     var page = item.page;     var bookCase = item.bookCase;     var totalNum = item.totalNum;     var stock = item.stock;     var borrowNum = item.borrowNum;     var typeName = item.typeName;     var str = '<tr> ' + '<td>' + index +      '</td>' + '<td>' + bookId +      '</td>' + '<td>' +bookName +      '</td>' + '<td>' + author+     '</td>' +'<td>'+translator+     '</td>' +'<td>'+ISBN+     '</td>' +'<td>'+press+     '</td>' +'<td>'+page+     '</td>' +'<td>'+bookCase+     '</td>' +'<td>'+totalNum+     '</td>' +'<td>'+stock+     '</td>' +'<td>'+borrowNum+     '</td>' +'<td>'+typeName+     '</td></tr>';          $("#rankTable tbody").append(str);     str = "";          })        },   error:function(XMLHttpRequest, textStatus, errorThrown){   alert("系统异常，请稍后重试！");   alert(XMLHttpRequest.status);      alert(XMLHttpRequest.readyState);      alert(textStatus);   }   })});