How Node.js Handles Thousands of Requests on a Single Thread: The Event Loop

Node.js is single-threaded yet handles massive concurrency through a mechanism called the event loop, which offloads slow operations like file reads and network calls to the system and retrieves results asynchronously. Each iteration of the event loop cycles through ordered phases — timers, I/O polling, setImmediate callbacks, and close callbacks — before moving on. Between every phase, microtasks such as resolved Promises and process.nextTick() callbacks are drained first, meaning they execute before the loop advances to its next stage. This explains why a resolved Promise logs before a zero-delay setTimeout in execution order. Developers are advised to avoid blocking the event loop with heavy CPU tasks, and instead use worker threads, the cluster module, or external queues for compute-intensive work.