Techniques for making a website more modular?

So I've written a basic web server in Node.js. It seems pretty inefficient to have to kill the server and re-run node server.js each time I want to add a new feature to my site.

What are the techniques for making a website more modular so I don't have to do this? How is this done in a larger scale, say, at very large website? I would like to be able to work on a small piece of the site without restarting the entire server, as it seems like overkill.