Node.js is a popular choice for backend development due to its non-blocking I/O, performance, and JavaScript ubiquity. If you're preparing for a Node.js backend developer interview, here’s a list of commonly asked questions and answers to boost your confidence and help you ace technical rounds.

✅ 1. What is Node.js?

Answer:
Node.js is a runtime environment built on Chrome's V8 JavaScript engine. It allows JavaScript to run outside the browser, primarily used for building scalable backend applications using an event-driven, non-blocking I/O model.

✅ 2. How does the Event Loop work in Node.js?

Answer:
The Event Loop is a single-threaded loop that handles asynchronous operations by delegating tasks to the system kernel or background threads. Phases include:

  • Timers
  • Pending callbacks
  • Idle, prepare
  • Poll
  • Check
  • Close callbacks

This model helps Node.js handle thousands of concurrent connections.

✅ 3. Difference between process.nextTick(), setImmediate(), and setTimeout()?

Function Description
process.nextTick() Executes after current operation, before event loop continues
setImmediate() Executes on the check phase of the event loop
setTimeout() Executes after the specified delay in milliseconds

✅ 4. What is middleware in Express.js?

Answer:
Middleware functions in Express.js have access to req, res, and next(). They are used to:

  • Execute code
  • Modify request/response objects
  • End the request-response cycle
  • Call the next middleware

Example:

app.use((req, res, next) => {
  console.log('Request URL:', req.url);
  next();
});

✅ 5. How do you handle errors globally in Express?

Answer:
You define an error-handling middleware:

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: err.message });
});

Call next(err) inside other middlewares or routes to forward the error.

✅ 6. How is asynchronous code handled in Node.js?

Answer:
Using:

  • Callbacks: Traditional style
  • Promises: Cleaner handling with .then().catch()
  • Async/Await: Syntax sugar over Promises

Example with async/await:

async function getData() {
  try {
    const data = await fetchData();
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

✅ 7. How does Node.js handle multiple requests if it’s single-threaded?

Answer:
Node.js uses an event loop and asynchronous callbacks to manage concurrent requests. Long-running operations like file I/O are offloaded to worker threads or system resources, so the main thread remains non-blocked.

✅ 8. What are Streams in Node.js?

Answer:
Streams are used to read or write data piece-by-piece. Types:

  • Readable
  • Writable
  • Duplex
  • Transform

Example:

const fs = require('fs');
const stream = fs.createReadStream('file.txt');
stream.on('data', chunk => console.log(chunk));

✅ 9. What is the purpose of cluster module?

Answer:
Node.js apps run on a single thread. The cluster module allows you to fork multiple child processes (workers) sharing the same server port, thus leveraging multi-core CPUs.

✅ 10. How do you secure a Node.js API?

Answer:

  • Use HTTPS
  • Validate inputs (e.g., with Joi)
  • Use Helmet for securing HTTP headers
  • Implement Rate Limiting
  • Use JWT tokens
  • Avoid exposing sensitive data

✅ 11. How does Node.js handle memory leaks?

Answer:
Memory leaks are usually caused by:

  • Unreleased timers
  • Global variable overuse
  • Closures retaining memory
  • Event listener accumulation

Use tools like:

  • --inspect with Chrome DevTools
  • heapdump
  • clinic.js

✅ 12. What is the difference between require and import?

Feature require() (CommonJS) import (ESM)
Synchronous Yes No
Static analysis No Yes (at compile time)
Dynamic loading Difficult Easier with import()

✅ 13. What is the difference between res.send() and res.json()?

Answer:

  • res.send() can send a string, object, or buffer.
  • res.json() automatically sets the content-type to application/json and converts object to JSON.

✅ 14. How does JWT authentication work in Node.js?

Answer:

  • User logs in with credentials.
  • Server generates a JWT with user info, signed with a secret.
  • Client stores JWT and includes it in Authorization headers.
  • Server validates the JWT on each request.

✅ 15. What is the role of package.json?

Answer:
package.json holds metadata for the project, including:

  • Project name, version
  • Scripts
  • Dependencies
  • Entry file

✅ 16. What is the difference between npm and npx?

  • npm installs packages.
  • npx runs binaries from packages without installing globally.

Example:

npx create-react-app my-app

✅ 17. How do you use environment variables in Node.js?

Answer:
Using dotenv package:

require('dotenv').config();
console.log(process.env.PORT);

.env file:

PORT=5000

✅ 18. How do you structure a large Node.js application?

Answer:

src/
├── controllers/
├── routes/
├── services/
├── models/
├── middlewares/
├── utils/
├── config/
server.js

Follow MVC, isolate business logic, and keep code testable.

✅ 19. What are some popular libraries used in Node.js backend?

  • Express.js – Web framework
  • Mongoose – ODM for MongoDB
  • Sequelize – ORM for SQL
  • Joi / Zod – Data validation
  • Winston / Morgan – Logging
  • jsonwebtoken – JWT auth

✅ 20. How do you test Node.js applications?

Answer:
Use tools like:

  • Jest or Mocha for unit testing
  • Supertest for API testing
  • Sinon for mocks/spies

Example with Jest:

test('adds 1 + 2 to equal 3', () => {
  expect(1 + 2).toBe(3);
});

✍️ Final Thoughts

Practicing these Q&As and building a real-world project (like a blog, chat app, or task manager API) will help solidify your knowledge. Node.js interviews often mix theory with hands-on scenarios, so be ready to write code, optimize performance, and architect clean APIs.