character remaining count

I want to display character remaining count when a user start typing. lblcommentscount is displayed 512 when txtbox is empty. How can I prevent this? Instead it should be displayed only when a user start typing.

<script type="text/javascript">
  
     function countCharacters(textbox, label, maxcount) {
        var count = parseInt(document.getElementById(textbox).value.length);
        document.getElementById(label).innerHTML = maxcount - count;
    }


 

 txtcomments.Attributes.Add("onKeyUp", String.Format("countCharacters('{0}','{1}', 512) ", txtcomments.ClientID, lblcommentscount.ClientID))

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