Top 50 Node.js Interview Questions Complete Summary and Quick Reference

Top 50 Node.js Interview Questions Complete Summary

This is a complete quick-reference for the most common Node.js interview questions.

Event Loop (10 Questions)

1. What is the event loop? Mechanism for non-blocking I/O in single-threaded Node.js
2. What are the phases? Timers, Poll, Check, Close (microtasks between phases)
3. setTimeout vs setImmediate vs nextTick? nextTick > Promise > setTimeout/setImmediate
4. Why is Node.js single-threaded? Reduces context switching, uses libuv thread pool for I/O
5. What is process.nextTick? Fires after current operation, before next phase (highest priority)
6. What is the call stack? LIFO stack where V8 executes JavaScript functions
7. What is the callback queue? FIFO queue of callbacks waiting to be executed
8. What is libuv? C library that provides the event loop and thread pool
9. What is the thread pool? 4 threads (default) for heavy I/O operations
10. What is event loop starvation? nextTick or heavy sync code blocks the event loop

Modules (8 Questions)

11. require vs import? CommonJS (sync) vs ES Modules (async)
12. How does require work? Resolve, Load, Wrap, Evaluate, Cache
13. What is module caching? Same instance returned on repeated requires
14. Circular dependencies? Returns partial exports; avoid by refactoring
15. module.exports vs exports? Always use module.exports
16. What are core modules? http, fs, path, crypto, events, stream
17. What is __dirname? Absolute path of the current module's directory
18. What is package.json? Project manifest with dependencies and scripts

Async Programming (8 Questions)

19. What is a callback? Function executed after an async operation
20. What is callback hell? Deeply nested callbacks; fix with Promises/async-await
21. What is a Promise? Object representing a future value (pending/fulfilled/rejected)
22. What is async/await? Syntactic sugar over Promises (synchronous-looking async code)
23. Promise.all vs allSettled vs race? All (fail-fast), allSettled (all results), race (first)
24. How to handle async errors? try-catch with async/await
25. How to run parallel? Promise.all([fetchA(), fetchB(), fetchC()])
26. How to limit concurrency? Batch processing with Promise.all per batch

Express (7 Questions)

27. What is middleware? Function in request-response cycle with next()
28. app.use vs app.get? All methods vs specific method
29. Types of middleware? Application, Router, Built-in, Error-handling, Third-party
30. Error handling in Express? 4-param middleware (err, req, res, next)
31. What is asyncHandler? Wrapper that catches async errors and passes to next()
32. What is Express Router? Modular route organization
33. What is req.body? Parsed request body (requires express.json() middleware)

MongoDB/Mongoose (7 Questions)

34. MongoDB vs Mongoose? Database vs ODM with schemas and validation
35. Schema vs Model? Structure definition vs constructor for documents
36. What are indexes? Sorted structures that speed up queries
37. What is ref and populate? Reference + replace ObjectId with document
38. What is aggregation? Pipeline of stages ($match, $group, $sort, $lookup)
39. find vs findOne? Array vs single document
40. What are hooks? pre/post middleware (hash password in pre-save)

Security (5 Questions)

41. How to implement auth? JWT in HTTP-only cookies + auth middleware
42. What is NoSQL injection? Malicious operators in queries; prevent with express-mongo-sanitize
43. How to store passwords? bcrypt with salt and cost factor
44. What security headers? HSTS, X-Frame-Options, X-Content-Type-Options, CSP (use helmet)
45. How to rate limit? express-rate-limit middleware

Performance (5 Questions)

46. How to scale? Cluster mode, load balancing, caching, CDN
47. What are streams? Process data in chunks (memory-efficient for large data)
48. How to handle CPU-intensive tasks? worker_threads, child_process, or queues
49. How to detect memory leaks? Monitor process.memoryUsage() periodically
50. How to optimize queries? Indexes, projection, lean(), pagination, caching

The Takeaway

These 50 questions cover the most important Node.js interview topics: event loop (phases, microtasks, single-thread), modules (require/import, caching, circular deps), async programming (callbacks, Promises, async/await, Promise.all), Express (middleware, error handling, Router), MongoDB/Mongoose (schemas, indexes, populate, aggregation), security (JWT, bcrypt, injection, headers, rate limiting), and performance (clustering, streams, worker threads, memory, query optimization). Master these to ace any Node.js interview.