Limitation on Textbox

Java script for Limitation 

     <script  type="text/javascript">
        function limitText(limitField, limitCount, limitNum) {
            if (limitField.value.length > limitNum) {
                limitField.value = limitField.value.substring(0, limitNum);
            } else {
                limitCount.value = limitNum - limitField.value.length;
            }
        }
</script>







 <asp:TextBox ID="txtdesc" runat="server" TextMode="MultiLine" onkeydown="limitText(this.form.txtdesc,this.form.countdown,200);" onkeyup="limitText(this.form.txtdesc,this.form.countdown,200);"></asp:TextBox><br>
<font size="1">(Maximum characters: 200)<br>
You have <input readonly type="text" name="countdown" size="3" value="200" style="width: 48px"> characters left.</font>