masking account number

 <script type="text/javascript">

  $('#acnumber').text(function(_, str) {

    return str = str.replace(/\d(?=\d{4})/g, "X");

  });

</script>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="ccnumber">1234567891234567</div>