求助一段js

当checkbox1被选中后，表格的第二行才出现，不选中则不显示，并且在选中的状态下，表单提交判断url文本域不能为空。

 1<form action="" method="POST">
 2<table border="1" cellpadding="0" cellspacing="0" width="100%">
 3<tr>
 4<td colspan="2" width="100%"><input name="checkbox1" type="checkbox" value="ON"/></td>
 5</tr>
 6<tr>
 7<td width="50%">url</td>
 8<td width="50%"><input name="url" size="20" type="text"/></td>
 9</tr>
10</table>
11</form>

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 1<script>   
 2function showRow(){   
 3tRow = document.getElementById('abc');   
 4if(!tRow) return;   
 5if(tRow.style.display=='none') tRow.style.display="";   
 6}   
 7function hideRow(){   
 8tRow = document.getElementById('abc');   
 9if(!tRow) return;   
10if(tRow.style.display!='none') tRow.style.display="none";   
11}   
12</script>

 1<form action="" method="POST">
 2<table border="1" cellpadding="0" cellspacing="0" width="100%">
 3<tr>
 4<td colspan="2" width="100%"><input checked="" name="checkbox1" onclick="if(this.checked){showRow();}else{hideRow();}" type="checkbox" value="ON"/></td>
 5</tr>
 6<tr id="abc" style="display:none">
 7<td width="50%">url</td>
 8<td width="50%"><input name="url" size="20" type="text"/></td>
 9</tr>
10</table>
11<input name="submit" onclick="if(this.form.checkbox1.checked &amp;&amp; this.form.url.value==''){alert('must fill URL...')}" type="submit" value="submit"/>
12</form>