Interview Preparation: 150+ Questions & Answers

Core Architectural Questions

Q1: Can you explain how the Node.js event loop works?

Answer: The Node.js event loop is powered by libuv and consists of six distinct phases: Timers (setTimeout, setInterval), Pending callbacks, Idle/Prepare, Poll (retrieving new I/O events), Check (setImmediate), and Close callbacks. Between every single phase and before returning to the event loop, Node drains the microtask queue—executing process.nextTick callbacks first, followed by resolved Promise microtasks.

Q2: How do you prevent event loop blocking in Node.js?

Answer: Heavy computations—such as cryptographic key generation, image resizing, or large JSON parsing—will block the single main JavaScript thread. To prevent this, CPU-bound tasks should be offloaded to Worker Threads (worker_threads), delegated to external background workers via job queues like BullMQ, or broken down into chunks using asynchronous iteration.

Q3: What is the difference between process.nextTick and setImmediate?

Answer: Despite its name, process.nextTick fires immediately after the currently executing operation finishes, before the event loop advances to the next phase or microtask. setImmediate schedules a callback to run during the Check phase of the event loop, after the Poll phase has processed pending I/O events.

Q4: How do you handle database transactions under high concurrency?

Answer: We use atomic database sessions with optimistic locking. In MongoDB or PostgreSQL, transactions verify record versions during updates. If a concurrent write is detected, the transaction aborts and retries with exponential backoff, preventing lost updates and phantom reads.

Q5: How do you gracefully shut down a Node.js web server?

Answer: When receiving a SIGTERM signal from Docker or Kubernetes, the server stops accepting new connections by calling server.close(). It sets a timeout window (e.g. 15 seconds) to allow in-flight HTTP requests to complete, flushes write streams and log buffers, closes database and Redis connection pools, and exits with code 0.