Section 1: Core Node.js Concepts (20+ Questions)

1. What is the event loop in Node.js?
The event loop is the mechanism that allows Node.js to perform non-blocking I/O operations by offloading operations to the system kernel. It continuously checks the call stack and callback queue and processes tasks asynchronously.

2. Explain the difference between process.nextTick(), setImmediate(), and setTimeout().

• process.nextTick(): Executes a callback after the current operation.

• setImmediate(): Executes after I/O events.

• setTimeout(): Executes after a specified delay.

process.nextTick(() => console.log('nextTick'));
setImmediate(() => console.log('setImmediate'));
setTimeout(() => console.log('setTimeout'), 0);

3. What are streams in Node.js?
Streams are abstract interfaces for working with streaming data in Node.js. Types include:

• Readable

• Writable

• Duplex

• Transform

4. How does Node.js handle child processes?
Using child_process module: spawn, exec, fork, execFile.

const { exec } = require('child_process');
exec('ls -la', (err, stdout, stderr) => {
  if (err) throw err;
  console.log(stdout);
});

5. What is the purpose of the cluster module?
To utilize multi-core systems by spawning worker processes that share the same server port.

6. What is the difference between require and import in Node.js?

• require is CommonJS.

• import is ES6 Module. Use "type": "module" in package.json to enable.

7. What is a memory leak and how do you prevent it in Node.js?
Unintended memory retention. Use tools like heapdump, v8-profiler, and monitor global variables.

8. Explain the Buffer class in Node.js.
Buffer handles binary data.

const buf = Buffer.from('Hello');
console.log(buf.toString()); // Hello

9. How does Node.js handle errors?
Through error-first callbacks, try/catch in async/await, or .catch() with promises.

10. What are the different types of timers in Node.js?

• setTimeout()

• setInterval()

• setImmediate()

11. What is the purpose of global in Node.js?
It's the global namespace object, similar to window in browsers.

12. Explain the role of V8 in Node.js.
V8 is the JavaScript engine by Google. Node.js uses it to compile JS to machine code.

13. How do you manage asynchronous operations in Node.js?
Using callbacks, promises, or async/await.

14. What is middleware in context of Node.js?
Functions that execute during the request-response cycle. In Express, used to modify req, res, or finish the cycle.

15. How to handle uncaught exceptions in Node.js?

process.on('uncaughtException', (err) => {
  console.error('Caught exception:', err);
});

16. What is the REPL in Node.js?
Read-Eval-Print Loop: interactive shell to test JS/Node expressions.

17. Explain the difference between synchronous and asynchronous methods.
Synchronous blocks the event loop. Asynchronous allows other code to run.

18. What is a callback hell and how to avoid it?
Nested callbacks make code hard to read. Avoid with promises/async-await.

19. What are worker threads?
Allow running JS in parallel on multiple threads.

const { Worker } = require('worker_threads');

20. What is the difference between fork() and spawn()?

• spawn() runs a command.

• fork() creates a new Node.js process and enables communication via IPC.

21. What tools can you use to debug Node.js applications?
node --inspect, Chrome DevTools, ndb, console.log, VS Code debugger.

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Section 2: Express.js Core Concepts (20+ Questions)

1. What is Express.js?
A fast, unopinionated, minimalist web framework for Node.js that simplifies routing, middleware, and server management.

2. How do you set up a basic Express server?

const express = require('express');
const app = express();

app.get('/', (req, res) => res.send('Hello World'));

app.listen(3000, () => console.log('Server running'));

3. What are middleware functions in Express?
Functions that execute during the lifecycle of a request to the server. They have access to req, res, and next().

4. What is the difference between application-level and router-level middleware?

• Application-level applies to entire app.

• Router-level applies to specific router instance.

5. What is the use of next() in Express?
Passes control to the next middleware function.

6. How do you handle 404 errors in Express?
Define middleware at the end:

app.use((req, res) => res.status(404).send('Not Found'));

7. How do you handle errors globally in Express?
Error-handling middleware has 4 arguments:

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

8. How do you parse incoming JSON requests in Express?
Use built-in middleware:

app.use(express.json());

9. What are route parameters in Express?
Variables in URL:

app.get('/user/:id', (req, res) => res.send(req.params.id));

10. What is the purpose of express.Router()?
Modular way to organize routes in different files.

11. How can you serve static files with Express?

app.use(express.static('public'));

12. How do you implement CORS in Express?
Use cors middleware:

const cors = require('cors');
app.use(cors());

13. How to handle file uploads in Express?
Use multer middleware:

const multer = require('multer');
const upload = multer({ dest: 'uploads/' });
app.post('/upload', upload.single('file'), (req, res) => res.send('Uploaded'));

14. How do you protect routes in Express?
Add middleware to check authentication before proceeding.

15. What is a query parameter and how is it accessed?
Parameters in the URL after ?. Accessed via req.query.

app.get('/search', (req, res) => res.send(req.query.q));

16. How do you redirect a request in Express?

res.redirect('/new-url');

17. What are the HTTP methods supported by Express?
GET, POST, PUT, DELETE, PATCH, OPTIONS, HEAD

18. How to implement custom middleware in Express?

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

19. How do you chain route handlers in Express?

app.get('/example', middleware1, middleware2, (req, res) => res.send('Done'));

20. How to use async/await in Express routes?
Wrap async functions in try/catch:

app.get('/', async (req, res) => {
  try {
    const data = await someAsyncCall();
    res.send(data);
  } catch (err) {
    res.status(500).send(err.message);
  }
});

21. How do you handle nested routes using Express Router?

const router = express.Router();
router.get('/profile', (req, res) => res.send('Profile'));
app.use('/user', router); // /user/profile