Routes and Controllers in Express.js

Synopsis:

  We will learn how to structure our Express.js applications in a clean and maintainable way, separating route definitions from business logic using controllers.

• 1. Understanding Routes in Express.js:

  A route is a combination of an HTTP request method (GET, POST, PUT, DELETE, etc.) and a path pattern, which defines the endpoints for client requests.

  The basic syntax for defining a route is: app.METHOD(PATH, HANDLER)

  • app: An instance of the Express application.
  • METHOD: A lowercase HTTP request method (get, post, put, delete, etc.).
  • PATH: The path on the server. It can be a string, a string pattern, or a regular expression.
  • HANDLER: A callback function (or a series of callback functions) that executes when the route matches. This function receives the request (req) and response (res) objects, and the next function.

  As your application grows, having all route handling functions directly in app.js (or the main file) can become messy and difficult to maintain. This is where controllers come in handy.

• 2. Introduction to Controllers:

  A controller is a function that encapsulates the business logic that executes when a route is reached. In the MVC (Model-View-Controller) design pattern, the controller is responsible for receiving user input, performing the necessary logic (interacting with the model), and preparing the data for the view. In the context of a RESTful API, the controller prepares the data for the HTTP response.

  Benefits of using controllers:

  • Separation of Concerns: Routes handle routing, controllers handle logic.
  • Modularity: More organized code in separate files.
  • Reusability: A controller can be used by multiple routes if its logic is generic.
  • Maintainability: Facilitates debugging and adding new functionalities.

• 3. Implementing Routes and Controllers:

  Let's structure a simple example for user management.

  Project Structure:

  Step 1: Create the Controller file (controllers/users_controller.js)

  Here we define the functions that will contain the logic for each operation.

  Step 2: Create the Routes file (routes/users_routes.js)

  Here we define the routes and link them to the controller functions. We use express.Router() to create a routing module.

  Step 3: Use the Router in the main file (app.js)

  In our main application file, we import the user router and "mount" it to a base path.

  Now, when a client makes a GET request to /api/v1/users, Express uses the userRoutes router to handle it, which in turn invokes the getAllUsers function of the controller.

• 4. Additional Considerations:
  • Input Validation: It is good practice to validate user input data in the controller or a specific middleware before processing it.
  • Error Handling: Controllers should be responsible for catching errors and passing them to Express's global error handling middleware (using next(error)).
  • Complex Business Logic: For large applications, you might consider a "services" layer between controllers and database models to encapsulate more complex business logic.
  • Chained Routes: You can chain HTTP methods on a single route to handle different verbs at the same URL: