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);
 } 
 })
});