Web Development on Telegram: “✅ Backend Basics Interview Questions

✅ Backend Basics Interview Questions – Part 3 (Express.js Middleware) 🚀🧠

📍 1. What is Middleware in Express.js?
A: Middleware are functions that execute during the request-response cycle. They can modify req/res, execute code, or end the request.

📍 2. Types of Middleware
⦁ Application-level: Applied to all routes or specific routes (using app.use())
⦁ Router-level: Applied to router instances
⦁ Built-in: e.g., express.json(), express.static()
⦁ Third-party: e.g., cors, morgan, helmet

📍 3. Example – Logging Middleware

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // Pass to next middleware/route
});

📍 4. Built-in Middleware

app.use(express.json()); // Parses JSON body
app.use(express.urlencoded({ extended: true })); // Parses URL-encoded body
app.use(express.static('public')); // Serves static files

📍 5. Router-level Middleware

const router = express.Router();
router.use((req, res, next) => {
  console.log('Router Middleware Active');
  next();
});
app.use('/api', router);

📍 6. Error-handling Middleware

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

📍 7. Chaining Middleware

app.get('/profile', authMiddleware, logMiddleware, (req, res) => {
  res.send('User Profile');
});

💡 Pro Tip: Middleware order matters—always place error-handling middleware last.

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