Java script ile tabloda tıklanan column adını almak

<table id="tabela">
    <tr>
        <td></td>
        <td>A</td>
        <td>B</td>
        <td>C</td>
        <td>D</td>
    </tr>
    <tr>
        <td>001</td>
        <td>M</td>
        <td>N</td>
        <td>O</td>
        <td>P</td>
    </tr>
    <tr>
        <td>002</td>
        <td>R</td>
        <td>S</td>
        <td>T</td>
        <td>U</td>
    </tr>
</table>​

<script>var table = document.getElementById("tabela");
var rows = table.rows;
for (var i = 1; i < rows.length; i++) {
    rows[i].onclick = (function (e) {
        var rowid = (this.cells[0].innerHTML);
        var j = 0;
        var td = e.target;
        while( (td = td.previousElementSibling) != null ) 
            j++;
        alert(rows[0].cells[j].innerHTML);
    });
}</script>​