Node.js Developer Interview Questions UK: 8 Real Questions and What Interviewers Want

Most Node.js Developer interviews in the UK follow a pattern, and knowing it beats guessing. Below are eight questions from our bank of 12, each with a note on what the interviewer is really after.

Eight Node.js Developer interview questions, and what the interviewer is thinking

1. Explain how the Node.js event loop works and what blocks it.

Why they ask: The event loop is the core concept in Node and the most commonly asked topic.

2. How do you handle asynchronous code in Node.js, and how do you deal with errors in async functions?

Why they ask: Async handling is daily work and errors here cause crashes in production.

3. How would you structure a production-ready REST API in Node.js, for example with Express or NestJS?

Why they ask: They want to see maintainable architecture and awareness of production concerns.

4. How do you secure a Node.js application against common vulnerabilities?

Why they ask: Security knowledge is a standard expectation of backend developers handling user data.

5. What are streams in Node.js, and when would you use them?

Why they ask: Streams show whether you can handle large data without exhausting memory.

6. Your Node.js API is responding slowly under load and CPU is at 100 per cent. How would you diagnose it?

Why they ask: They test practical production debugging skills.

7. Tell me about a time you had to add features to a Node.js codebase with poor structure and no tests. How did you manage it?

Why they ask: They test pragmatism and risk management when working with legacy code.

8. What do you enjoy about being a Node.js developer, and where do you think the platform is heading?

Why they ask: They want to gauge genuine enthusiasm and awareness of the ecosystem.

A sample answer to question 1

Explain how the Node.js event loop works and what blocks it.

Node runs JavaScript on a single thread, but handles concurrency by delegating I/O to the operating system or to libuv's thread pool, then processing callbacks when work completes. The event loop cycles through phases: timers for setTimeout and setInterval, pending callbacks, poll for new I/O events, check for setImmediate callbacks, and close callbacks. Between phases, process.nextTick callbacks and promise microtasks are drained, which means they can starve I/O if abused. Anything CPU-heavy on the main thread blocks the loop, so no other requests are served; examples are large JSON.parse calls, synchronous fs methods, complex regular expressions with catastrophic backtracking, and tight loops. I avoid this by using asynchronous APIs, streaming large data, and moving CPU-bound work to worker threads or a separate service. I monitor event loop lag with tools such as clinic.js or perf_hooks to catch problems early.

Common mistake to avoid: Describing Node as multithreaded; ignoring nextTick and promise ordering

Want the rest?

Full model answers, key points and common mistakes for all 12 questions are on the Node.js Developer interview questions page. The first five are free, no sign-up needed.