Node.js Interview Questions and Answers for Backend and Full Stack Roles

Our full stack developers at Creuto write Node.js APIs for real products, so we care about fundamentals: async behaviour, clean API design and safety. These are the questions worth practising.

The runtime

1. What is Node.js?

A JavaScript runtime built on the V8 engine that runs JavaScript on the server, with non-blocking input and output suited to many concurrent connections.

2. Explain the event loop.

Node runs JavaScript on one main thread. Slow operations such as network or file access are handed off, and their callbacks are queued and run when the main thread is free. The Node.js event loop guide explains the phases.

3. What blocks the event loop?

Heavy synchronous work such as large JSON parsing, complex calculations or synchronous file calls. Move it to worker threads, a queue or another service.

4. Callbacks, promises and async/await?

Three ways to handle async results. Promises avoid nested callbacks; async/await makes promise code read like synchronous code. Always handle rejections.

Building APIs

5. How do you structure an Express API?

Separate routes, controllers, services and data access, validate input at the edge, and centralise error handling in middleware.

6. What is middleware?

A function that runs during a request, for logging, authentication, parsing or validation, before passing control on.

7. How do you handle errors?

Throw or pass errors to a central handler, return consistent error responses, log details on the server and never leak stack traces to users.

Data and authentication

8. SQL or NoSQL?

Relational databases such as PostgreSQL suit structured data and transactions; document databases suit flexible data. Many business apps start with PostgreSQL.

9. How do you avoid N+1 queries?

Fetch related data in one query with joins or batched lookups instead of querying inside a loop.

10. How does JWT authentication work?

The server issues a signed token after login; the client sends it with each request; the server verifies the signature. Keep tokens short-lived and store refresh tokens securely.

Security

11. How do you secure a Node.js API?

Validate and sanitise input, use parameterised queries, hash passwords with a slow algorithm, set security headers, rate-limit sensitive routes and keep secrets in environment variables. The OWASP Top 10 lists the common risks.

Scaling and reliability

12. How do you scale a Node.js app?

Run several instances behind a load balancer, keep servers stateless, cache frequent reads and move slow jobs to queues.

13. Why use a queue?

To handle slow or unreliable work, such as sending emails or processing files, outside the request so users get fast responses and failed jobs can retry.

Testing

14. How do you test an API?

Unit tests for logic, integration tests for routes with a test database, and a few end-to-end checks for critical flows.

How to prepare

Build one small, complete API and deploy it. Pair this list with our React interview questions if you are applying for full stack roles. For architecture questions, our guide on choosing a tech stack shows how teams think about trade-offs.

Working at Creuto

At Creuto we build APIs and web apps with Node.js and hire remotely across India. See the full stack developer role and all open roles.