如何用JS获取TR中TD的某个标签的值。然后根据大小，交换TR的位置

 <table id="tb"><tr>
    <td>   <h6 id="id0" name="name"> 123</h6> </td>
</tr>
<tr>
    <td>   <h6 id="id1" name="name"> 456</h6> </td>
</tr>
    <tr>
        <td>   <h6 id="id2" name="name"> 789</h6> </td>
    </tr>
    <tr>
        <td>   <h6 id="id3" name="name"> 100</h6> </td>
    </tr>
</table>
<script>
    var tb=document.getElementById('tb'),rows = tb.rows, trs = [];
    for (i in rows) trs[i] = rows[i];
    console.log(trs);
    trs.sort(function (a, b) {
        //降序，升序是a在前面
        return parseInt(b.cells[0].getElementsByTagName('h6')[0].innerHTML.replace(/\s/g, ''))
            -  parseInt(a.cells[0].getElementsByTagName('h6')[0].innerHTML.replace(/\s/g, ''))
    });
    for (i in trs) tb.appendChild(trs[i]);
</script>