JS控制table中tr位置互换

<script type="text/javascript">
    function doit(f, t) {
        $("#tbl tr:nth-child(" + f + ")").insertAfter($("#tbl tr:nth-child(" + t + ")"));
    }
</script>
<table id="tbl">
    <tr><td>1</td></tr>
    <tr><td>2</td></tr>
    <tr><td>3</td></tr>
    <tr><td>4</td></tr>
</table>
<input type="button" onclick="doit(1, 3);" value="click">