TOPIC OVERVIEW

nodejs/Event Loop

Understand how Node.js handles thousands of tasks without creating a thread for every request.

20 QuestionsAdvancedHigh Priority ~15 min read

Node.js can handle many operations at the same time even though JavaScript code normally runs on a single main thread. The event loop keeps checking for work and decides when callbacks, timers, I/O operations, and other asynchronous tasks should be processed.

The tricky part is that not every asynchronous operation behaves the same way. Node.js uses different event loop phases and also interacts with the operating system and its underlying libraries to handle tasks such as file operations, networking, and timers. Understanding this is essential for explaining Node.js performance in interviews.

What you'll walk away knowing

How the Node.js event loop works

What happens during the different event loop phases

How asynchronous I/O avoids blocking JavaScript execution

How microtasks, timers, and callbacks are scheduled

1Explain the event loop.Intermediate 2How does the browser's event loop differ from Node.js's event loop?Intermediate 3What is the difference between the microtask queue and the macrotask (callback) queue?Intermediate 4How would you profile and detect event loop lag in a production Node.js service? What tools and metrics do you use? (e.g., perf_hooks, Clinic.js, event_loop_utilization)Advanced 5How do you design a CPU-intensive task (e.g., image processing, ML inference) in a Node.js app without blocking the event loop?Intermediate 6How does the event loop relate to memory leaks? Can retained closures in the callback queue prevent garbage collection?Intermediate 7What is 'starvation' in the context of the event loop? Give a real-world example.Intermediate 8How does the event loop handle long-running synchronous code? What is 'blocking the event loop' and how do you avoid it in production?Intermediate 9How does Promise.then() differ from setTimeout() in terms of execution order?Intermediate 10How do streams and backpressure in Node.js relate to the event loop? Why does improper stream handling block or overload the loop?Intermediate 11What could be the disadvantages of process.nextTick()Intermediate 12Explain process.next.tick()Intermediate 13What is queueMicrotask()? How does it differ from Promise.resolve().then()?Intermediate 14Why use process.nextTick()?Intermediate 15What strategies do you use to keep the event loop healthy at scale? (offloading, chunking, job queues like BullMQ, worker threads)Advanced

All Topics

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File Operations

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Performance Optimisation

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