Fork from http://javascript.info/string#internals-unicode

<!DOCTYPE html>
<html>

<body>
  <script>
    'use strict';

    function digital_root(n) {
      
      var res = 0;
      console.log("ВХОД: "+ n);
      
      for(let i = 0;i< +String(n).length;i++){
          res += +(String(n)[i]);
      }
      
      if(+String(res).length>1) {
        digital_root(+res);
      } else {
        console.log("Res : "+ res);
        return res;
      }
  }
  
  alert(digital_root(99999));
  </script>
</body>

</html>