<script language="JavaScript">

function formCheck(formobj)
{
  //1) Enter name of mandatory fields
  var fieldRequired = Array("FirstName", "LastName");
  //2) Enter field description to appear in the dialog box
  var fieldDescription = Array("First Name", "Last Name");
  //3) Enter dialog message
  var alertMsg = "Please complete the following fields:\n";
  var l_Msg = alertMsg.length;
  for (var i = 0; i < fieldRequired.length; i++)
  {
    var obj = formobj.elements[fieldRequired[i]];
    if (obj)
    {
      switch(obj.type)
      {
        case "select-one":
          if (obj.selectedIndex == -1 || obj.options[obj.selectedIndex].text == "")
          {
            alertMsg += " - " + fieldDescription[i] + "\n";
          }
          break;
        case "select-multiple":
          if (obj.selectedIndex == -1)
          {
            alertMsg += " - " + fieldDescription[i] + "\n";
          }
          break;
        case "text":
        case "textarea":
          if (obj.value == "" || obj.value == null)
          {
            alertMsg += " - " + fieldDescription[i] + "\n";
          }
          break;
        default:
          if (obj.value == "" || obj.value == null)
          {
            alertMsg += " - " + fieldDescription[i] + "\n";
          }
      }
    }
  }
  if (alertMsg.length == l_Msg)
  {
    return true;
  }
  else
  {
    alert(alertMsg);
    return false;
  }
}

</script>

<form name="formcheck" onsubmit="return formCheck(this);">
First Name: <input type=text name="FirstName" size="25"><br>
Last Name: <input type=text name="LastName" size="25"><br>
<input type=submit value="Submit Form">
</form>