JavaScript bắt lổi đăng nhập

<script language="JavaScript">
function kt()
{
if(f.us.value=="")
{
alert("Enter username please");
f.us.focus();
return false;
}
if(f.pa.value=="")
{
alert("Enter password please");
f.pa.focus();
return false;
}
return true;
}
</script>
<form name="f" method="post" action="dangnhap_xuly_ss.php" onSubmit="return kt();">
  <table width="38%" border="0" align="center">
    <tr>
      <td colspan="2"><div align="center"><strong><font color="#0000FF">Login</font></strong></div></td>
    </tr>
    <tr>
      <td width="19%">username</td>
      <td width="47%"><input name="us" type="text" id="us" placeholder="Enter username"></td>
    </tr>
    <tr>
      <td>Password</td>
      <td><input name="pa" type="password" id="pa" placeholder="Enter password"></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td><input type="submit" name="Submit" value="Submit">
        <input type="reset" name="Submit2" value="Reset"></td>
    </tr>
  </table>
</form>