Node.js Interview Questions 2026, 35 Deep Q&A for Backend Roles

Node.js powers a huge share of India's API and real-time backends, and candidates report it is one of the most common backend interview tracks in 2026. Interviewers at Flipkart, Razorpay, Swiggy, and funded startups go past "what is Node" into the event loop, streams, clustering, and production failure modes. This guide compiles 35 questions from candidate-reported rounds and public preparation resources, each with code and the exact trade-off being tested.

Note: for a focused deep dive on the event loop specifically, see the companion guide linked below. This page covers the full backend surface.

Related: Node.js Event Loop Interview Questions 2026 | Express JS Interview Questions 2026 | REST API Interview Questions 2026 | MongoDB Interview Questions 2026

Fundamentals (Q1 to Q12)

Q1. What is Node.js and how does it differ from a browser runtime?

Q2. Is Node.js single-threaded? Explain.

Q3. What is non-blocking I/O and why does it matter?

Q4. What are the differences between require and import?

Q5. What is the difference between process.nextTick and setImmediate?

Q6. How do you handle asynchronous code in Node?

const data = await fs.promises.readFile('f.txt', 'utf8');

Q7. What is callback hell and how do you avoid it?

Q8. What are streams and why use them?

fs.createReadStream('big.csv').pipe(transform).pipe(res);

Q9. What is backpressure in streams?

Q10. What is the Buffer class?

Q11. What is the difference between exec, spawn, and fork?

Q12. How does the module caching work?

Concurrency and Performance (Q13 to Q24)

Q13. How do you use the cluster module to scale Node?

const cluster = require('cluster');
const os = require('os');
if (cluster.isPrimary) {
  os.cpus().forEach(() => cluster.fork());
} else {
  require('./server');
}

Each worker is a separate process with its own event loop, sharing the listening port. This uses all CPU cores, since one Node process uses one core for JS.

Q14. When do you use worker threads vs clustering?

Q15. How do you handle a CPU-intensive task without blocking the event loop?

Q16. What causes an event loop block and how do you detect it?

Q17. How do you handle memory leaks in Node?

Q18. What is the difference between setTimeout(fn, 0) and setImmediate(fn)?

Q19. How do you implement graceful shutdown?

process.on('SIGTERM', async () => {
  server.close();           // stop accepting new connections
  await db.disconnect();    // drain resources
  process.exit(0);
});

Graceful shutdown prevents dropped requests during deploys.

Q20. What is the purpose of streaming a response?

Q21. How do you cache in a Node service?

Q22. What is connection pooling and why does it matter?

Q23. How do you profile a slow Node endpoint?

Q24. What is the difference between vertical and horizontal scaling for Node?

Error Handling, Security, and Production (Q25 to Q35)

Q25. How do you handle errors in async/await?

try {
  const data = await risky();
} catch (err) {
  logger.error(err);
  throw new AppError('failed', { cause: err });
}

Always handle rejections; an unhandled rejection can crash the process in modern Node.

Q26. What is an uncaughtException and how should you treat it?

Q27. How do you prevent common Node security issues?

Q28. What is the difference between authentication and authorization in a Node API?

Q29. How do JWTs work in a Node backend?

Q30. How do you handle environment configuration?

Q31. What is rate limiting and how do you implement it?

Q32. How do you structure logging in production?

Q33. Scenario: an API works locally but times out under load. Walk through diagnosis.

Q34. How do you ensure a Node service is observable?

Q35. What testing layers do you use for a Node backend?

Node.js Mock Test, 2026 Edition

5 original questions calibrated to the 2026 Node batch by Aditya Sharma, from candidate-reported patterns.

Question 1

What handles blocking I/O in Node?

a) The single JS thread b) libuv thread pool plus OS async I/O c) the GPU d) web workers

Solution: JS is single-threaded; libuv and the OS handle I/O off-thread. Answer: (b)

Question 2

To use all CPU cores for request handling you use:

a) bigger instance b) the cluster module c) async/await d) streams

Solution: Cluster forks one process per core sharing the port. Answer: (b)

Question 3

Backpressure is automatically handled by:

a) require b) pipe c) Buffer d) JSON.parse

Solution: pipe pauses the source when the destination is full. Answer: (b)

Question 4

After an uncaughtException you should:

a) ignore it b) log, clean up, and exit c) restart the loop d) retry forever

Solution: The process state is undefined; exit and let the supervisor restart. Answer: (b)

Question 5

For CPU-bound work that must not block requests, use:

a) setTimeout b) worker threads c) more middleware d) bigger Buffer

Solution: Worker threads run CPU work off the main thread. Answer: (b)

FAQ, Node.js Interview Questions

Q: How deep is the event loop tested? Very. Candidates report it is the most probed Node topic; see the dedicated event loop guide for puzzle-level depth.

Q: Do I need TypeScript for Node roles in 2026? Increasingly yes at product companies. Confirm the stack on the official company careers page when invited.

Q: Is Express still expected? Often, alongside newer frameworks. See the Express guide linked below.

Q: What single mistake fails candidates? Blocking the event loop with synchronous work, per candidate-reported feedback.

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