express() application?

For those people who did not understand Wiktor's explanation, this has to do with a concept you may not have run across yet: closures. Closures create an instance of a function with a closed environment for tracking variables rather than needing the global scope to track them. By defining const app = express(), we create an instance (a term you may have heard when talking about objects created using a Class constructor; this functions similarly) of the Express module with it's own unique environment. If we added const app2 = express() to our app.js file, we could create another server instance that tracked its own variables and would not interfere with the app instance.

This Express basics module goes on for a long time (it took me about a week with spaced repetition of previous lessons and in-depth examination of the express and pug documentation) so if you're really very curious about closures, you can hop over to the Treehouse closures lesson to get a quick idea. It's a short course, is next in line in Fullstack Javascript after Express Basics, and you don't need to have finished this course to understand it. Hope that helps for people who are slightly confused by all this new syntax!