Check All Checkbox

I'm using the following code for a checkbox that when clicked, either
checks or unchecks a group of checkboxes on a form. The code works
fine, except when there is only one checkbox in the group in which case
the check all checkbox doesn't work at all.

The code working with three checkboxes in a group is as follows, but
remove two of the three and you will see the code stops working:

<script language="javascript" type="text/javascript">
function checkAll( control, cbGroupName )
{
var cbGroup = control.form.elements[cbGroupName], i = 0, cb;
while( cb = cbGroup[i++] )
{
cb.checked = control.checked;
}}
</script>

<form>
Check all
<input type="checkbox" this,'select[]' );" >
<br><br>
<input type="checkbox" name="select[]" value="234">
<input type="checkbox" name="select[]" value="235">
<input type="checkbox" name="select[]" value="236">
</form>