Uncaught TypeError: Cannot read property 'length' of undefined

<table border="1" width="50%" id="table">
        <tr>
          <th>学号</th>
          <th>姓名</th>
          <th>操作</th>
        </tr>
        <tr>
          <td>xh001</td>
          <td>阮雌</td>
          <td><a href="javascript:;" onclick="delete(this)">删除</a></td>   <!--在删除按钮上添加点击事件  -->
        </tr>
        <tr>
           <td>xh002</td>
           <td>刘赫</td>
           <td><a href="javascript:;" onclick="delete(this)" >删除</a></td>   <!--在删除按钮上添加点击事件  -->
        </tr>
    </table>
    <input type="button" value="添加一行" onclick="add()" />   <!--在添加按钮上添加点击事件  -->
<script>
        window.onload = function(){
          var tr=document.getElementsByTagName("tr");
          var td=document.getElementsByTagName("td");
         for(var i=0;i<tr.length;i++){
              var n=1;
                n++;
             var k=0;
             while(k<cells.length){
                 k++;
             }
             let cells=tr[i+n].cells;
//            tr[i+n].cells.innerHTML+="xh00";
            console.log(tr[i+n].cells.length)
         }
     }

报错：没找到length元素，判断下cells是否为underfined或者为空，如是则不遍历，就不会js报错了。
正确改为：
window.onload = function(){

         var tr=document.getElementsByTagName("tr");
          var td=document.getElementsByTagName("td");
         for(var i=0;i<tr.length;i++){
              var n=1;
                n++;
             var k=0;
             while(k<tr[i+n].cells.length){
                 k++;
             }
             let cells=tr[i+n].cells;
//            tr[i+n].cells.innerHTML+="xh00";
            console.log(tr[i+n].cells.length)
         }
     }
</script>