Separating the Express app and server in Node.js applications is a recommended practice for several reasons, primarily focusing on modularity, ease of testing, scalability, and maintainability.
Modularity
Separating the Express app from the server configuration allows developers to organize the application more cleanly. The app module (app.js) can focus solely on the application logic, such as routes, middleware, and controllers, while the server module (server.js) handles the network aspects, like setting up the server and listening on a port. This separation of concerns makes the codebase easier to understand and manage[1][2][3][5][6][7][10].
Ease of Testing
By decoupling the app from the server, it becomes much easier to write and execute tests. You can test the application logic in isolation without needing to start up the server. This not only speeds up the testing process but also improves test coverage and reliability. Testing the app without actual network operations means faster execution and more focused tests[1][2][3][5][6][7][10].
Scalability
With separated app and server files, scaling the application becomes more straightforward. You can reuse the app logic across different server instances or even different server configurations without duplication of code. This is particularly useful in microservices architectures where different services might share similar application logic but require different server configurations[1][2][3][5][6][7][10].
Maintainability
Maintaining an application with a clear separation between the app logic and server setup is easier. Changes to the network configuration or to the core application logic can be made independently of each other, reducing the risk of introducing bugs across unrelated parts of the application. This separation also allows different teams to work on the application logic and server configuration independently, streamlining development processes[1][2][3][5][6][7][10].
