Separation of Concerns

Building database-driven web applications requires a lot of moving parts: the server, views, routes, data access code, etc. For your application to be maintainable, it's very important to adhere to the Single Responsibility Principle by putting each of these responsibilities in their own modules (files).

Model-View-Controller (MVC) is one of the most common separated presentation patterns. These patterns describe how to structure code according to the Single Responsibility Principle.

Model is the data representation we apply to our templates. This is the second parameter we've been passing to templates with res.render('view', model). We've also called it a view model because this model is built specifically for a view.

View is the visual representation part of the code - our templates. We've built these using HTML and Handlebars.

Controller is the code that coordinates the interaction of the data (the model) and the view. We've been doing this in our routes.js file, but that often causes our routes to do too much. By creating a separate controller.js we can reduce the responsibility of our routes to get the view model from the controller, and apply it to the view.