Node.js mock interviews
Node.js interviews start with "it is single-threaded and non-blocking" and spend the rest of the round finding out whether you know what that means in practice: what blocks the loop, where the async errors go, and what your API does when a thousand requests arrive at once.
Rounds with Sia from ₹99 · Live interviews with a working engineer from ₹299 · Pay per session
What a Node.js round covers
The event loop, for real
What runs on the main thread and what libuv hands off, why a tight loop freezes every request, and the order of nextTick, promises, timers and setImmediate.
Async code and errors
Callbacks to promises to async/await, what happens to an error nobody awaits, and running work in parallel without losing track of failures.
APIs with Express
Middleware order, validation, authentication with sessions or JWTs, and status codes that mean what they say. Expect to defend a route you have written.
Streams, scale and production
Streams and backpressure for large data, worker threads against the cluster module, connection pooling, and what you would log to debug a slow endpoint.
Node.js interview questions you might be asked
Not a list to memorise. Each is the kind of opening a real round uses, with what a strong answer covers — the follow-ups depend on what you say.
An endpoint that resizes images makes every other request slow. Why, and what do you change?
CPU work blocks the event loop, so nothing else runs. Move it to a worker thread or a queue — and say why making it async alone does not help.
This script prints its logs in an order you did not expect. Walk me through why.
The queue order: synchronous code, then nextTick, then promise callbacks, then timers and setImmediate depending on the phase. Predicting it before running it is the answer.
You need to send a 2 GB file to the client. How do you avoid running out of memory?
Stream it instead of reading it into memory, and let backpressure slow the read when the client is slow. pipeline() handles errors on both sides.
Your service crashed with an "unhandled rejection". How did that happen, and how do you prevent it?
A promise rejected with no catch, often a forgotten await. Await or handle every promise, and have a last-resort handler that logs and exits cleanly.
The API runs on one process and traffic has tripled. What are your options?
Run several processes with the cluster module or a process manager behind a load balancer, keep the app stateless, and check the database pool can take it.
Answer questions like these in a scored round and see exactly where your answer fell short.
What the interviewer is listening for
The signal is whether you can say what the event loop is doing during your code. Candidates who explain why something blocks, rather than only that it does, consistently score above those who know the API surface.
Where candidates lose marks
- Calling Node.js "multi-threaded" or "single-threaded" without saying what that covers.
- Wrapping every handler in try/catch but leaving promise rejections unhandled.
- Doing CPU-heavy work on the main thread and blaming the database.
Two ways to interview
A round with Sia, on your own
One question at a time with real follow-ups on what you actually said. Choose three, five or eight questions. Scored the moment you finish, with a written breakdown across technical depth, problem solving and communication.
A live interview with an engineer
Forty minutes over Zoom with someone who runs Node.js interviews for a living. They score five criteria and write up what to fix, and you keep a numbered certificate for the session.
Choose Node.js as your stack in the backend or full-stack track.
Questions about Node.js interviews
Which track should I pick for a Node.js round?
Node.js can be chosen as your stack in the backend and full-stack tracks. Backend for APIs, data and async behaviour; full-stack if you build the frontend too.
Is a Node.js round right for a fresher?
Yes. At Beginner difficulty the round stays on JavaScript basics, async code and a simple API, rather than scaling and production concerns.
Do I need to know Express or NestJS?
If a framework is on your resume, expect questions on it — Express most often. Otherwise the round stays on core Node.js and how you would build an API with it.
How is a Node.js round scored?
On the same five criteria as every round — technical depth, problem solving, communication, behavioural and code quality — with a written breakdown.
Read before your round
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Not sure which to book? The FAQ covers how sessions are scored, and candidate stories show what people did with the feedback.