Limit/Show Number of Characters Remaining in Multiline Textbox or Textarea - JS/JavaScript

<asp:TextBox ID="txtComments"  TextMode="MultiLine"  Columns="45" Rows="6"  runat="server"  onkeyup="CharactersCount(this,500,'lblcounter');"  ></asp:TextBox><br />Character Left: <span id="lblcounter">500</span>

<script language="javascript">
 function CharactersCount(txtCtrl, CharLength, charLeft) {
var chars = txtCtrl.value.length;
document.getElementById(charLeft).innerHTML = CharLength - chars;
if (chars > CharLength) {
txtCtrl.value = txtCtrl.value.substring(0, CharLength);
}}
</script>