[TigerC10]

You're looking for substring.

<script type="text/javascript" src="http://www.mydomain.com/external.php?type=js"></script>
            <script type="text/javascript">
            <!--
                for (var i = 0; i < threads.length; i++)
                {
                    document.write('<li><a href="http://www.mydomain.com/showthread.php?t=' + threads[i]['threadid'] + '" title="' + threads[i]['title'] + '">' + threads[i]['title'].substr(0, 20) + '</a></li>');
                }
            //-->
            </script>

But that won't look very pretty (normally people expect things that are cut off to end with ellipsis), I would recommend splitting it up like so...

<script type="text/javascript" src="http://www.mydomain.com/external.php?type=js"></script>
            <script type="text/javascript">
            <!--
                for (var i = 0; i < threads.length; i++)
                {
                    var url = '<li><a href="http://www.mydomain.com/showthread.php?t=' + threads[i]['threadid'] + '" title="' + threads[i]['title'] + '">';

                    if(threads[i]['title'].length > 20) {
                             url += threads[i]['title'].substr(0, 20) + '...';
                    }
                    else {
                             url += threads[i]['title'];
                    }
                    url += '</a></li>';
                    document.write(url);
                }
            //-->
            </script>

That way you get the ellipsis at the end of thread titles that are cut off.