Sort Table Javascript

HTML

<table id="caltbl" border="1">
    <thead onclick="sortTable()">
        <tr>
            <th>
                Date
            </th>
            <th>
                hours
            </th>
            <th>
                Module
            </th>
            
        </tr>
    </thead>
     <tbody>
        <tr>
            <td class="sortnr">759</td>
        <td></td>
            <td></td>
        </tr>
   <tr>
      <td class="sortnr">1</td>
      <td></td>
       <td></td>
   </tr>
   <tr>
      <td class="sortnr">3</td>
      <td></td>
       <td></td>
   </tr>
   <tr>
      <td class="sortnr">2</td>
      <td></td>
       <td></td>
   </tr>
   </tbody>
</table>

JavaScript

function sortTable(){
    var tbl = document.getElementById("caltbl").tBodies[0];
    var store = [];
    for(var i=0, len=tbl.rows.length;i<len; i++){
        var row = tbl.rows[i];
        var sortnr = parseFloat(row.cells[0].textContent || row.cells[0].innerText);
        if(!isNaN(sortnr)) store.push([sortnr, row]);
    }
    store.sort(function(x,y){
        return x[0] - y[0];
    });
    for(var i=0, len=store.length; i<len; i++){
        tbl.appendChild(store[i][1]);
    }
    store = null;
}
sortTable();