Set focus after last character textarea

I have tried many solutions from this discussion but only 1 of them is working for me. Here is the codes :
<script type="text/javascript">$("textarea#messageposting").focus(function () { var val = this.value; var $this = $(this); $this.val(""); setTimeout(function () { //1 millisecond $this.val(val); }, 1);});$(function() { $("textarea#messageposting").focus();});</script><textarea><?php echo $my_note_db; ?></textarea>