求助一段js

Question

<script>
function showRow(){
  tRow = document.getElementById('abc');
  if(!tRow) return;
  if(tRow.style.display=='none') tRow.style.display="";
}
function hideRow(){
  tRow = document.getElementById('abc');
  if(!tRow) return;
  if(tRow.style.display!='none') tRow.style.display="none";
}
</script>
<form method="POST" action="">
  <table border="1" cellpadding="0" cellspacing="0" width="100%" >
    <tr>
      <td width="100%" colspan="2"><input type="checkbox" name="checkbox1" value="ON" checked onclick="if(this.checked){showRow();}else{hideRow();}"></td>
    </tr>
    <tr id=abc style="display:none">
      <td width="50%">url</td>
      <td width="50%"><input type="text" name="url" size="20"></td>
    </tr>
  </table>
<input type=submit name=submit value=submit onclick="if(this.form.checkbox1.checked && this.form.url.value==''){alert('must fill URL...')}">
</form>