Nested properties with javascript

[Apte, Dhanashree (Noblestar)]

Hi,
 
In my jsp, i am using nested properties of a form. 
 
RoleForm has UserForm which has agencies, agencyName and agencyId within it.
The agencies and divisions need to be displayed in dropdown boxes.
 
I got this to display correctly on my jsp using: 
 
<html:select property="userForm.agencyId" onchange="fillSelect(this.form)">

<html:option key="select.generic" value="" /> 

<html:optionsCollection property="userForm.agencies" value="agencyId"
label="agencyName" />

</html:select>

However, in the javascript for the onChange of this dropdown, i need a
handle to this dropdown.
the fillSelect function needs to do:
function fillSelect(el)

{

       var theSelect = el.agencyId;  // <== How do i get the dropdown handle
here?

       var num = theSelect.options[theSelect.selectedIndex].value;

                          :

                          : // more procesing

}

With the above, i get the error: theSelect has no properties.

If i put in var theSelect = el.userForm.agencyId, i get the error:
el.userForm has no properties.
 
What would be the correct way to access the dropdown box?
 
Any help appreciated.
Thanks,
Dhanashree.