javascript ile tablodaki tıklanan hücre değerini almak

<style>
td {
    border:solid 1px black;
    cursor:pointer;
}
</style>
<table>
    <tr>
        <td>1</td>
        <td>2</td>
        <td>3</td>
        <td>4</td>
    </tr> <tr>
        <td>6</td>
        <td>7</td>
        <td>8</td>
        <td>9</td>
    </tr>
</table>
var cells = document.getElementsByTagName('td');
for(var i = 0; i <= cells.length; i++){
    cells[i].addEventListener('click', clickHandler);
}

function clickHandler()
{
    alert(this.textContent);
}