The Key to Async Magic: Mastering JavaScript’s Event Loop

A deep dive into the execution context that makes async possible

Photo by Tine Ivanič on Unsplash

The event loop is at the heart of JavaScript’s asynchronous programming model. It allows JavaScript to perform non-blocking I/O operations while keeping the main thread responsive.

Understanding how the event loop works under the hood unlocks the power of callbacks, promises, and async/await. It’s key to writing optimal asynchronous code and debugging some of the trickiest bugs.

In this article, we’ll learn how the event loop lies at the foundation of async JavaScript.

What is the JavaScript Event Loop?

The event loop continually checks a queue for tasks and executes them. These tasks include DOM events, async I/O, and timers.

Here’s how it works:

While queue has tasks:
   Execute next task

The queue is filled by calling functions like setTimeout or issuing network requests. The event loop allows JavaScript to handle asynchronous actions without blocking.

For example:

console.log('Hi') 

setTimeout(() => {
  console.log('Callback!')  
}, 1000)

console.log('Bye')