Can I use Jquery to insert a closing </tr> tag and an opening <tr> tag inside a dynamic table?

I'm trying to use the code below to dynamically add closing tag followed by opening so that i creates a new row every three cells. Almost working, DOM inspector shows a TR node, problem is, something happens tr isn't closing the tr tag. I'm new to Jquery, is there anything I'm doing wrong with this code?

         <script src="http://ajax.aspnetcdn.com/ajax/jquery/jquery-1.5.1.min.js" type="text/javascript"></script>
        <script type="text/javascript">
        $(document).ready(function(){
        $('td:nth-child(3n)').after('</tr><tr>');
        });
        </script>

        <table id="mytable" width="266" border="1" cellspacing="10" cellpadding="10">
        <tbody>
          <tr>
        <?php
        function somelongassfunction(){
            return 'Hello';
            }
        function have_products($a){
            return $a<=20;
            }
        $x=0;
        while (have_products($x)) {
            echo '<td>' . somelongassfunction() . '</td>';
            $x++;
        //------------------------------------- 
        /*if (fmod($x,3) == 0) {
                        echo '</tr><tr>';
                        continue;
                        }*/
        //--------------------------------------                    
        if ($x==20){
            break;
            }   
        }   
        ?>
        </tr>
        </tbody>
        &开发者_StackOverflow中文版lt;/table>    

You can't work on a DOM selection as if it was an HTML document. A DOM document is a hierarchy of nodes, not of tags. The tags in your HTML are parsed into a DOM document by the browser. You can't then add a single bit of HTML and then expect it to be parsed back into a DOM structure.

Instead, you'll need to do the wrapping in jQuery. This is a viable approach -- it may not be the most efficient.

$('td').each(function(idx) {
    if (idx % 3) {
        return;
    } else {
        $(this).nextAll(':lt(2)').andSelf().wrapAll('<tr/>');
    }
}).parent().unwrap();

jsFiddle

Another approach is to detach all the td elements, and then split them into groups of 3 using splice:

var td = $('tr td').detach();
$('tr').remove();
while(td.length>0){ 
 $(td.splice(0,3)).wrapAll('<tr />').parent().appendTo('tbody');   
}

example: http://jsfiddle.net/niklasvh/C6unV/

As lonesomeday said, you can't treat a DOM like a bit of HTML. Besides, TR closing tags are optional so when you insert some markup with an opening TR, the browser will close it. You don't need a closing tag.