Node.js Event Loop: Why Is It So Fast?

This article uses the analogy of a coffee shop waiter to explain the core mechanism of Node.js for efficiently handling concurrent requests—the event loop. Despite being single-threaded, Node.js can process a large number of concurrent requests efficiently, with the key lying in the collaboration between non-blocking I/O and the event loop: when executing asynchronous operations (such as file reading and network requests), Node.js delegates the task to the underlying libuv library and immediately responds to other requests. Once the operation is completed, the callback function is placed into the task queue. The event loop is the core scheduler, processing tasks in fixed phases: starting with timer callbacks (Timers), system callbacks (Pending Callbacks), followed by the crucial Poll phase to wait for I/O events, and then handling immediate callbacks (Check) and close callbacks (Close Callbacks). It ensures the ordered execution of asynchronous tasks through the call stack, task queues, and phase-based processing. The efficient design stems from three points: non-blocking I/O avoids CPU waiting, callback scheduling is executed in an ordered manner across phases, and the combination of single-threaded execution with asynchronous concurrency achieves high throughput. Understanding the scheduling logic of the event loop helps developers write more efficient Node.js code.