js获取对象为null的解决方法

    success:function(data){ 
        var arr = data.split('_'); 
        var numArr = arr[0].split(","); 
        var numStr = ""; 
        if(numArr.length==undefined || numArr.length==0){ 
            $("#phonenolist").html('对不起,没有查询到相应的号码!'); 
        }else{ 
            for(var i=0;i<numArr.length;i++){ 
                numStr += "<li><a id='n_"+i+"' href=/"javascript:choose_mobile(" 
                    + numArr[i].toString()+",n_"+i+");/" class='buy'>" 
                    + numArr[i].toString().substring(0, 3) 
                    + "<span class='org'> " 
                    + numArr[i].toString().substring(3, 7) + " " 
                    + numArr[i].toString().substring(7, 11) 
                    + "</span></a></li>"; 
                } 
                $("#phonenolist").html(numStr); 
            } 
            index = arr[1]; 
            total = arr[2]; 
             $("#curr").html('  第'+index+'页,共'+total+'页 '); 
        } 
     function choose_mobile(num,id){ 
        alert(num+" "+id); 
        $("#mobile").val(num); 
        $("#xhmobile").html("<h3><b>"+num+"</b></h3>"); 
        $("#"+id).addClass("buy").siblings().removeClass("buy"); 
    } 

    numStr += "<li><a id='n_"+i+"' href=/"javascript:choose_mobile('" 
        + numArr[i].toString()+"','n_"+i+"');/" class='buy'>"