Demystifying JavaScript’s Event Loop

Understanding Callbacks, Promises, and Async Await

Photo by Kier in Sight Archives on Unsplash

As an asynchronous language, JavaScript uses an event loop to handle non-blocking operations. Understanding how the event loop manages callbacks, promises, async functions, and other asynchronous actions demystifies async programming.

In this article, we’ll explain the what, why, and how of JavaScript’s magical event loop — crucial for unlocking asynchronous superpowers and writing cleaner async code.

What is the Event Loop?

The event loop continually checks a queue for pending callbacks and runs them one by one when the call stack is clear.

This event loop allows non-blocking asynchronous JavaScript execution. Rather than waiting for each operation to complete before moving forward, async tasks get queued to execute later after existing stack frames complete.

For example:

console.log("Hi");
setTimeout(() => {
 console.log("Callback!");
}, 0);
console.log("Bye");
// Logs: 
// Hi 
// Bye
// Callback!

Even with a 0ms delay, setTimeout callback gets postponed until after the sync code runs per event loop behavior.

Why Use an Event Loop?