Node.js Event Loop Phases
As a Fresher engineer, explain Node.js Event Loop Phases. What problem does it solve and how would you describe it in an interview?
Answers use simple, clear English.
Quick interview answer
At Fresher depth: start with the problem, then mechanism, then a short example. Node runs libuv phases: timers → pending → idle/prepare → poll → check (setImmediate) → close.
Detailed answer
At Fresher depth: start with the problem, then mechanism, then a short example. Node runs libuv phases: timers → pending → idle/prepare → poll → check (setImmediate) → close. process.nextTick and Promise microtasks run between phases and can starve I/O if abused. Core: Node runs libuv phases: timers → pending → idle/prepare → poll → check (setImmediate) → close. process.nextTick and Promise microtasks run between phases and can starve I/O if abused. Real-time example: API server: heavy sync JSON.parse in request handler blocks the poll phase and delays all concurrent connections. Pros: Single-threaded model simplifies reasoning; non-blocking I/O scales connection count. Cons: CPU-bound work blocks the loop; microtask flooding delays timers. Common mistakes: Assuming setTimeout(0) runs before setImmediate always (order varies at startup). Best practices: Offload CPU work to worker_threads; never block the event loop in hot paths. Audience level: Fresher.
Full explanation
Node runs libuv phases: timers → pending → idle/prepare → poll → check (setImmediate) → close. process.nextTick and Promise microtasks run between phases and can starve I/O if abused.
Real example & use case
API server: heavy sync JSON.parse in request handler blocks the poll phase and delays all concurrent connections.
Pros & cons
Pros: Single-threaded model simplifies reasoning; non-blocking I/O scales connection count. Cons: CPU-bound work blocks the loop; microtask flooding delays timers.
Common mistakes
Assuming setTimeout(0) runs before setImmediate always (order varies at startup).
Best practices
Offload CPU work to worker_threads; never block the event loop in hot paths.
Follow-up questions
- How would you test Node.js Event Loop Phases?
- What metrics prove Node.js Event Loop Phases is healthy in prod?
- How does Node.js Event Loop Phases change at 10× traffic?