JavaScript Event Loop Explained: How Single-Threaded JS Handles Async Tasks

JavaScript runs on a single thread with one call stack, meaning it can only execute one task at a time. To avoid blocking the main thread during slow operations like API calls or timers, JavaScript relies on an event loop — an infinite loop that checks whether the call stack is empty and pushes queued tasks onto it. Crucially, the event loop is not part of the JavaScript engine itself; it is provided by the host environment, such as the browser or Node.js via its libuv library. This architecture explains behaviors like why setTimeout with a zero-millisecond delay does not execute immediately, and why Promises resolve before setTimeout callbacks. Understanding the distinction between the JS engine and its runtime environment is key to grasping how JavaScript achieves non-blocking, asynchronous behavior despite being single-threaded.