Architecture: Routes & Controllers

Master the flow of data in Node.js. Learn to modularize your Express applications using the Router and MVC patterns for professional-grade code.

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> Welcome to the Server Side. We are going to trace the lifecycle of an HTTP Request in an Express application.

const app = express();
app.listen(3000, () => console.log('Server Ready'));

Routes & HTTP Verbs

A route creates a map between a specific URL URL and a function that handles the request. Express uses methods corresponding to HTTP verbs:

  • app.get() - Retrieve data
  • app.post() - Create data
  • app.put() - Update data (replace)
  • app.delete() - Remove data

Endpoint Analysis

Which HTTP verb is best suited for creating a new user resource?

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MVC Architect

Successfully separate routes from controller logic.

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Router Ranger

Correctly configure an Express Router instance.

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Handler Hero

Write a perfect request handler function.

Code Lab: The Router

Mission: Define the Product Endpoint

Construct an Express route that listens for GET requests on '/products' and sends back a response.

Debugger Output:

> Awaiting code input...

Logic: Request Lifecycle

Challenge: Execution Order

Drag the components of the Express request lifecycle into the correct chronological order.

router.use('/users', userRoutes)
controller.getAllUsers(req, res)
Incoming Request: GET /users
res.json(data)

Syntax: Module Config

Challenge: Configure the Router

Fill in the missing keywords to initialize a modular Express router file.

const = require('express');
const router = express.();
// ... define routes ...
module. = router;

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Code Review

Submit your "Product Controller" for a security audit by peers.

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Scalable Architecture: Routes & Controllers

When building a simple Express.js server, it's tempting to put all your logic inside `app.js`. However, as an application grows, this leads to a "Monolithic File" anti-pattern—thousands of lines of code that are impossible to maintain or test.

The Separation of Concerns (SoC)

The gold standard in backend development is the MVC (Model-View-Controller) pattern. In the context of a JSON API, we typically focus on MC (Models and Controllers), letting the Frontend handle the View.

❌ Monolithic (Bad)

// app.js
app.get('/users', (req, res) => {
  // Database logic here
  // Validation logic here
  // Response logic here
  // ... 50 lines later ...
});

Hard to read, hard to test, hard to reuse.

✔️ Modular (Good)

// routes/users.js
router.get('/', userController.getAll);

// controllers/users.js
exports.getAll = (req, res) => {
  // Logic isolated here
};

Clean, testable, and strictly defined responsibilities.

The Role of the Controller

Controllers act as the traffic cops of your application. They shouldn't necessarily contain deep business logic (that belongs in Services or Models), but they *must* handle the HTTP layer:

  • Parsing Request data (body, params, query).
  • Invoking the correct Business Logic.
  • Formatting the Response (Status codes, JSON structure).
  • Handling Errors passed to `next()`.
Pro Tip: Keep your controllers "skinny". If a controller function is longer than 20 lines, you probably need to extract logic into a separate Service or Utility helper.

Node/Express Glossary

Router
A mini-application within Express capable of performing middleware and routing functions. Used to modularize the main application.
Controller
A function responsible for accepting the request, orchestrating the necessary operations, and sending the response back to the client.
Handler Function
The callback function `(req, res, next) => ` that executes when a route matches a request.
Middleware
Functions that execute during the request-response cycle, having access to `req`, `res`, and `next`.
MVC Pattern
Model-View-Controller. An architectural pattern that separates an application into three main logical components.
REST
Representational State Transfer. A set of constraints for creating web services, using standard HTTP methods (GET, POST, PUT, DELETE).

Technical Verification

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Backend Team

Specialists in Node.js architecture and Scalable Systems.

Reviewed for accuracy against Node 20 LTS and Express 4.x/5.x standards.

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