Node.js Interview Questions

Node.js has revolutionized backend development by enabling JavaScript to run outside the browser, offering developers a unified language across the full stack. Its event-driven, non-blocking architecture makes it ideal for building scalable network applications, real-time systems, and microservices. As demand for Node.js expertise continues to grow, mastering its core concepts and ecosystem is essential for backend developers, technical writers, and educators alike.
Whether you’re preparing for a job interview, curating content for learners, or simply deepening your understanding, this guide offers a structured approach to Node.js interview questions—complete with clear answers and practical insights. From foundational topics like the event loop and npm to advanced concepts such as clustering, worker threads, and performance optimization, each question is designed to test both theoretical knowledge and real-world application.
Node.js interviews often go beyond syntax and APIs—they assess how well you understand asynchronous programming, error handling, middleware architecture, and system design. Employers look for developers who can write clean, maintainable code, explain trade-offs, and make informed decisions about libraries, deployment strategies, and security practices. That’s why this guide doesn’t just list questions—it provides context, examples, and concise explanations that reinforce learning and retention.
For technical educators and content creators, these questions can serve as scaffolding for tutorials, blog series, or classroom discussions. Each answer is crafted to be modular and adaptable—whether you’re teaching beginners about Express.js routing or diving into advanced topics like stream manipulation and process management.
By the end of this guide, you’ll not only be interview-ready but also equipped to explain Node.js concepts with clarity and confidence. Whether you’re building chat apps, RESTful APIs, or real-time dashboards, the principles covered here will help you write better code and teach others to do the same.

Beginner-Level Questions & Answers

1. What is Node.js and how is it different from JavaScript in the browser?
   Node.js is a runtime environment that allows you to run JavaScript on the server side. It uses the V8 engine (same as Chrome) but adds APIs for file system access, networking, and more—things browsers don’t expose.
2. Key features of Node.js
   • Asynchronous & Event-driven: Uses non-blocking I/O.
   • Single-threaded: Efficient for I/O-heavy tasks.
   • Fast execution: Powered by the V8 engine.
   • Cross-platform: Runs on Windows, Linux, macOS.
3. What is npm and how does it work?
   npm (Node Package Manager) manages packages/modules. It reads to install dependencies via: npm install
4. What is the event loop in Node.js?
   The event loop is the mechanism that handles asynchronous operations. It listens for events and executes callbacks in phases like timers, I/O, and close events.
5. Why is Node.js single-threaded, and how does it handle concurrency?
   Node.js uses a single thread for JavaScript execution but delegates I/O tasks to the system via libuv. This allows it to handle thousands of concurrent connections efficiently.

Intermediate-Level Questions & Answers

6. Difference between require() and import
   • require() is CommonJS (used in older Node.js).
   • import is ES Module syntax (modern, supports export). To use import, set “type”: “module” in package.json.

7. Handling errors in async code

• Async/Await:
• Promises: .catch() handles errors.

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

8. What are streams in Node.js?

Streams handle data chunk-by-chunk:

• Readable: fs.createReadStream()
• Writable: fs.createWriteStream()
• Duplex: Both readable and writable.
• Transform: Modify data (e.g., compression).

9. How does middleware work in Express.js?

Middleware functions intercept requests:

app.use((req, res, next) => { console.log(‘Request received’); next(); });

They can modify req, res, or end the response.

10. Role of the process object

• process.env: Access environment variables.
• process.argv: Command-line arguments.
• process.exit(): Exit the app.
• process.on('exit'): Handle cleanup.

Advanced-Level Questions & Answers

11. Child processes and worker threads

• child_process: Spawn external processes (e.g., shell commands).
• worker_threads: Run JS in parallel threads for CPU-heavy tasks.

const { Worker } = require(‘worker_threads’);

12. Common design patterns

• MVC: Model-View-Controller for structure.
• Singleton: Shared instance (e.g., DB connection).
• Factory: Create objects dynamically.
• Observer: EventEmitter for pub-sub.

13. Securing Node.js apps

• Validate inputs (e.g., Joi, express-validator).
• Use HTTPS.
• Sanitize data to prevent XSS/SQLi.
• Use helmet for HTTP headers.
• Authenticate with JWT or OAuth.

14. What is clustering and why use it?

Clustering allows Node.js to fork multiple processes (workers) to utilize multi-core CPUs:

const cluster = require(‘cluster’);

Each worker handles requests independently.

15. Performance optimization tips

• Use caching (Redis, memory).
• Avoid blocking code.
• Use async/await or Promises.
• Profile with clinic.js, node --inspect.
• Load balance with Nginx or PM2.