Understanding app.js and router.js in a Node.js Application
When building a Node.js application, especially with frameworks like Express, it's important to organize your project efficiently. Two critical components of this structure are the app.js file and the router system. These components help manage the flow of requests and improve the maintainability of the codebase.

In this article, we’ll explore the roles of app.js and router.js, how they work together, and why they are important in building scalable and modular applications.

1. What is app.js?

The app.js file is the entry point of your Node.js application. It initializes the Express application, sets up middleware, and defines how requests are handled by routing them to different parts of the application.

const express = require('express');
const app = express();
const { Member, EventTicket, DiscountCode } = require("./models");
const { comparePassword } = require('./helpers/bcryptjs');
const { signToken, verifyToken } = require('./helpers/jwt');

app.use(express.urlencoded({ extended: true }));
app.use(express.json());

// Root Endpoint
app.get('/', (req, res) => {
  res.send('Welcome');
});

2. What is router.js?

The router.js file defines the application's routes and connects them to their respective controllers or handlers. This keeps the routing logic clean and modular, separating the core server logic (app.js) from the individual route definitions.
**
Key Responsibilities of router.js:**
Define the endpoints (routes) of the application.
Delegate the request handling to controllers or handler functions.
Support different HTTP methods such as GET, POST, PUT, DELETE, etc.
Enable modular routing by grouping related routes.

Example of router.js:

// AUTH 
router.post('/register', UserController.register);
router.post('/login', UserController.login);

module.exports = router;

Route Delegation: app.js directs incoming requests to the appropriate route using app.use(). The routes are defined in router.js and specify how the requests are handled.

Scalability: As your application grows, more routes and features will be added. Instead of placing all routes in app.js, they can be split into multiple files and grouped logically (e.g., userRouter.js, postRouter.js), improving maintainability and scalability.

4. Benefits of Using app.js and router.js
Separation of Concerns: Keeping route definitions separate from application logic results in cleaner and more maintainable code.
Easier Testing: With clear separation, it's easier to test routes independently without worrying about the rest of the application.
Modular Routing: Group related routes together, which is especially helpful as the application grows.

Conclusion
Both app.js and router.js are critical parts of a well-structured Node.js and Express application. app.js handles the initialization, middleware setup, and server configuration, while router.js defines the routing structure, connecting each endpoint to its corresponding controller or handler. This organization promotes cleaner, modular, and maintainable code that scales well as your application grows.