Is Node.js single-threaded? If so, how does it handle concurrency?

Question

Accepted Answer

Yes, Node.js runs JavaScript on a single main thread , but it handles concurrency efficiently through its event loop and non-blocking I/O model. Event loop — continuously picks up completed I/O events and runs their callbacks on the main thread. Non-blocking I/O — I/O operations (network, DB, files) are delegated to the OS or libuv, so the main thread never waits. libuv thread pool — handles blocking operations like filesystem, DNS, and crypto in background threads (default size: 4). Reactor Pattern — Node reacts to events instead of blocking on them, allowing thousands of concurrent connections on one thread. Key terms: event loop, non-blocking I/O, libuv, thread pool, worker threads, Reactor Pattern. This is one of the most frequently asked Node.js questions. Clarify that the JavaScript execution is single-threaded but libuv uses a thread pool internally. Mentioning worker threads and the cluster module as solutions for CPU-bound work shows deeper understanding.