your script will not work, try<script>
function selectall()
{
if (document.all.mid == null)
{
alert("no checkbox");
return;
} var length = document.all.mid.length;
if (length == null )
{
document.del.mid.checked=document.del.checkall.checked ;
}
for (var i = 0; i < length; i++)
{
document.del.mid[i].checked=document.del.checkall.checked;
}
}
</script>
<form action="try.asp" method="post" name="del">
<input name="mid" type="checkbox" value="a">
<input name="mid" type="checkbox" value="b">
<input name="checkall" type="checkbox" onClick="selectall()">
<input name="submit" type="submit">
</form>
function selectall()
{
if (document.all.mid == null)
{
alert("no checkbox");
return;
} var length = document.all.mid.length;
if (length == null )
{
document.del.mid.checked=document.del.checkall.checked ;
}
for (var i = 0; i < length; i++)
{
document.del.mid[i].checked=document.del.checkall.checked;
}
}
</script>
<form action="try.asp" method="post" name="del">
<input name="mid" type="checkbox" value="a">
<input name="mid" type="checkbox" value="b">
<input name="checkall" type="checkbox" onClick="selectall()">
<input name="submit" type="submit">
</form>
function selectall()
{
var o=document.getElementsByName("mid");
for (var i = 0; i < o.length; i++)
{
o[i].checked=document.del.checkall.checked;
}
}
</script>
<form action="try.asp" method="post" name="del">
<input name="mid" type="checkbox" value="a">
<input name="mid" type="checkbox" value="b">
<input name="checkall" type="checkbox" onClick="selectall()">
<input name="submit" type="submit">
</form>