✅ 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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✅ Backend Basics Interview Questions – Part 4 (REST API) 🚀💻

📍 1. What is a REST API?
A: REST (Representational State Transfer) APIs allow clients to interact with server resources using HTTP methods like GET, POST, PUT, DELETE.

📍 2. Difference Between REST & SOAP
⦁ REST is an architectural style using HTTP, lightweight, and supports JSON/XML for stateless communication.
⦁ SOAP is a strict protocol, heavier, XML-only, stateful, with built-in standards for enterprise reliability and security.

📍 3. HTTP Methods
⦁ GET → Read data
⦁ POST → Create data
⦁ PUT → Update data (full replacement)
⦁ DELETE → Delete data
⦁ PATCH → Partial update

📍 4. Example – Creating a GET Route

app.get('/users', (req, res) => {
  res.send(users); // Return all users
});

📍 5. Example – POST Route

app.post('/users', (req, res) => {
  const newUser = req.body;
  users.push(newUser);
  res.status(201).send(newUser);
});

📍 6. Route Parameters & Query Parameters

app.get('/users/:id', (req, res) => {
  res.send(users.find(u => u.id == req.params.id));
});
app.get('/search', (req, res) => {
  res.send(users.filter(u => u.name.includes(req.query.name)));
});

📍 7. Status Codes
⦁ 200 → OK
⦁ 201 → Created
⦁ 400 → Bad Request
⦁ 404 → Not Found
⦁ 500 → Server Error

📍 8. Best Practices
⦁ Validate request data
⦁ Handle errors properly
⦁ Use consistent endpoint naming (e.g., /api/v1/users)
⦁ Keep routes modular using express.Router()

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✅ Backend Basics Interview Questions – Part 5 (Error Handling) 🚀💻

📍 1. What is Error Handling?
A: Error handling is the process of catching, logging, and responding to runtime errors or exceptions during request processing to prevent server crashes and provide clear feedback to clients—like validation fails or DB timeouts. It keeps your app robust and user-friendly.

📍 2. Built-in Error Handling
⦁ Express auto-catches sync errors in routes, but async needs extra care.
⦁ Example:

app.get('/error', (req, res) => {
  throw new Error('Something went wrong!');
});

This triggers the default handler or your custom one—logs to console by default.

📍 3. Custom Error-handling Middleware
⦁ Define with 4 params (err, req, res, next) and place it last in your middleware chain.
⦁ Example:

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ 
    success: false, 
    message: err.message || 'Internal Server Error' 
  });
});

In dev mode, include stack traces; hide them in prod for security.

📍 4. Try-Catch in Async Functions
⦁ Wrap async code in try-catch and pass errors to next() for middleware handling.
⦁ Example:

app.get('/async', async (req, res, next) => {
  try {
    const data = await getData(); // Assume this might fail
    res.json(data);
  } catch (err) {
    next(err); // Passes to error middleware
  }
});

Pro tip: Use async-error wrappers like express-async-errors for cleaner code without manual next().

📍 5. Sending Proper Status Codes
⦁ 400 → Bad Request (invalid input)
⦁ 401 → Unauthorized (auth failed)
⦁ 403 → Forbidden (no access)
⦁ 404 → Not Found (resource missing)
⦁ 500 → Internal Server Error (server-side issue)
Always pair with descriptive messages, but keep sensitive details out.

📍 6. Error Logging
⦁ Use console.error() for quick logs or libraries like Winston/Morgan for structured logging (e.g., to files or services like Sentry).
⦁ Track errors with timestamps, user IDs, and request paths for easier debugging in production.

📍 7. Best Practices
⦁ Keep messages user-friendly (e.g., "Invalid email" vs. raw stack).
⦁ Never expose stack traces in prod—use env checks.
⦁ Centralize with global middleware; validate inputs early to avoid errors.
⦁ Test with tools like Postman to simulate failures.

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