Active State For Navigation Bar In Website

I'm quite new to web design and Javascript in particular. How to resolve a problem with the navigation bar in a website I am currently working on. The navigation bar has 3 main categories and there is an 'active' state so the page currently viewed is shown as grey underline text in the navigation bar. The thing is, I also have sub-categories that are not included in the navigation bar but I'd like to make it so the sub-category page currently being viewed makes the main category to which it is attached showing as 'active' in the navigation bar.

This is the structure of my site:
index.html
contact.thml
illustration
index.html
illustrationproject1.html
illustrationproject2.html
illustrationproject3.html
photography
index.html
photography1.html
photography2.html

So for example, if I'm in illustrationproject1.html, the Illustration tab in the navigation bar should be on the active state. This is the code I have so far:
Navigation Bar:
[CODE]
<ul id="nav">
<li class="nav_item"><a href="illustration/index.html">Illustration</a></li>
<li class="nav_item"><a href="photography/index.html">Photography</a></li>
<li class="nav_item"><a href="design/index.html">Design</a></li>
<li class="nav_item"><a href="stories/index.html">Stories</a></li>
<li class="nav_item"><a href="profile.html" title="Profile">Profile</a></li>
<li class="nav_item"><a href="contact.html" title="Contact">Contact</a></li>
<li class="nav_item"><a href="links.html" title="Links">Links</a></li>
</ul>
[ICODE]

This is the Javscript for the active state:
[CODE]
function setActive() {
aObj = document.getElementById('nav').getElementsByTagName('a');
for(i=0;i<aObj.length;i++) {
if(document.location.href.indexOf(aObj[i].href)>=0) {
aObj[i].className='active';
}}}
[ICODE]

So this code is working but I'm just not sure how to apply it to the subcategories as well. I can see what I should do, write some "if" statement but I don't enough of Javascript yet for this...