Question 1

What is the Node.js runtime, and how does it differ from browser JavaScript?

Accepted Answer

Node.js is a V8-based JS runtime plus libuv for async I/O, with OS APIs (fs, net, process) instead of DOM/window. It shares the language and much of ECMAScript with browsers but exposes different globals and modules. Single-threaded JS drives concurrent I/O via the event loop. Understanding the runtime boundary (JS vs native addons vs OS) is foundational for performance and debugging interviews.

Question 2

Explain the roles of V8 and libuv in Node.

Accepted Answer

V8 executes JavaScript and manages the heap/GC; libuv provides the event loop, thread pool, and cross-platform async I/O. CPU-bound JS runs on V8's main thread; many file/DNS/crypto ops offload to libuv's thread pool. Native bindings bridge both. Misattributing slowness to 'Node is slow' vs 'blocked event loop' vs 'saturated UV pool' is a common senior trap.

Question 3

What does process expose that matters in production services?

Accepted Answer

Env, args, PID, signals, memoryUsage, exit codes, and stdio — the process control surface. Use env for config, handle SIGTERM for graceful shutdown, and avoid sync work in signal handlers. process.exit abruptly skips cleanup — prefer orderly shutdown. Interviewers often ask how you drain connections on deploy.

Question 4

When would you choose Node vs a threaded runtime for a backend?

Accepted Answer

Node shines at many concurrent I/O-bound connections; CPU-heavy or hard-realtime workloads often favor other runtimes or offload workers. Node can still do CPU work via workers/native, but the default concurrency model is cooperative async. Match the runtime to workload shape, team skills, and ecosystem — not hype.

Question 5

What are Node's module systems today?

Accepted Answer

CommonJS (require/module.exports) and ES modules (import/export), with dual-package hazards. Package.json type, .mjs/.cjs, and conditional exports control resolution. Mixing CJS/ESM incorrectly causes 'ERR_REQUIRE_ESM' and dual instances of singleton modules — a frequent production bug.

Question 6

Lead: how do you standardize Node runtime versions across a large org?

Accepted Answer

Pin engines, use .nvmrc/Volta, enforce in CI, container base images, and LTS policy with upgrade windows. Document ABI/native addon constraints. Stage major upgrades behind canaries. Treat runtime bumps as security work, not optional chore. Align local, CI, and prod deliberately.

Runtime Node.js Interview Questions

Other Node.js interview topics