Count Number of letter in Textarea or Textbox using Javascript

Kindly write anything in Textbox:

Characters left

Code:

Javascript Function:

<script language="javascript">
function textCounter(field,cntfield,maxlimit) {
if (field.value.length > maxlimit) // if too long...trim it!
field.value = field.value.substring(0, maxlimit);
// otherwise, update 'characters left' counter
else
cntfield.value = maxlimit - field.value.length;
}
</script>

HTML
<form name="frm">
<textarea name="comments" id="comments" cols="30" rows="4" wrap="physical" onKeyDown="textCounter(document.frm.comments,document.frm.remLen1,100)"onKeyUp="textCounter(document.frm.comments,document.frm.remLen1,100)" tabindex="14"> </textarea>
<br />
<input readonly type="text" name="remLen1" style="border:0; width:30px;" size="3" maxlength="3" value="100"> Characters left
</form>