✨♻️ JavaScript Visualized: Event Loop

If you're here in 2024 (or later), here's an updated blog post!](https://lydiahallie.com/blog/event-loop)

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Oh boi the event loop. It’s one of those things that every JavaScript developer has to deal with in one way or another, but it can be a bit confusing to understand at first. I’m a visual learner so I thought I’d try to help you by explaining it in a visual way through low-res gifs because it's 2019 and gifs are somehow still pixelated and blurry.

But first, what is the event loop and why should you care?

JavaScript is single-threaded: only one task can run at a time. Usually that’s no big deal, but now imagine you’re running a task which takes 30 seconds.. Ya.. During that task we’re waiting for 30 seconds before anything else can happen (JavaScript runs on the browser’s main thread by default, so the entire UI is stuck) 😬 It’s 2019, no one wants a slow, unresponsive website.

Luckily, the browser gives us some features that the JavaScript engine itself doesn’t provide: a Web API. This includes the DOM API, setTimeout, HTTP requests, and so on. This can help us create some async, non-blocking behavior 🚀

When we invoke a function, it gets added to something called the call stack. The call stack is part of the JS engine, this isn’t browser specific. It’s a stack, meaning that it’s first in, last out (think of a pile of pancakes). When a function returns a value, it gets popped off the stack 👋

The respond function returns a setTimeout function. The setTimeout is provided to us by the Web API: it lets us delay tasks without blocking the main thread. The callback function that we passed to the setTimeout function, the arrow function () => { return 'Hey' } gets added to the Web API. In the meantime, the setTimeout function and the respond function get popped off the stack, they both returned their values!

In the Web API, a timer runs for as long as the second argument we passed to it, 1000ms. The callback doesn’t immediately get added to the call stack, instead it’s passed to something called the queue.

This can be a confusing part: it doesn't mean that the callback function gets added to the callstack(thus returns a value) after 1000ms! It simply gets added to the queue after 1000ms. But it’s a queue, the function has got to wait for its turn!

Now this is the part we’ve all been waiting for… Time for the event loop to do its only task: connecting the queue with the call stack! If the call stack is empty, so if all previously invoked functions have returned their values and have been popped off the stack, the first item in the queue gets added to the call stack. In this case, no other functions were invoked, meaning that the call stack was empty by the time the callback function was the first item in the queue.

The callback is added to the call stack, gets invoked, and returns a value, and gets popped off the stack.

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Reading an article is fun, but you'll only get entirely comfortable with this by actually working with it over and over. Try to figure out what gets logged to the console if we run the following:

const foo = () => console.log("First");
const bar = () => setTimeout(() => console.log("Second"), 500);
const baz = () => console.log("Third");

bar();
foo();
baz();

Got it? Let's quickly take a look at what's happening when we're running this code in a browser:

1. We invoke bar. bar returns a setTimeout function.
2. The callback we passed to setTimeout gets added to the Web API, the setTimeout function and bar get popped off the callstack.
3. The timer runs, in the meantime foo gets invoked and logs First. foo returns (undefined),baz gets invoked, and the callback gets added to the queue.
4. baz logs Third. The event loop sees the callstack is empty after baz returned, after which the callback gets added to the call stack.
5. The callback logs Second.

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Hope that this makes you feel a bit more comfortable with the event loop! Don't worry if it still seems confusing, the most important thing is to understand where certain errors/behavior can come from in order to Google the right terms efficiently and end up on the correct Stack Overflow page 💪🏼 Feel free to reach out to me if you have any questions!

Comments

[Logan]

Hi, Lydia Hallie, it is a great article.

Can I translate this article into Chinese to help more Chinese developers(With original author and link, of course).

[Lydia Hallie]

Absolutely!

[Diego Palacios Lepore]

Hi, Lydia!

This is such a great article! 😃

I was looking a clear post about this subject and yours is super comprehensive and well explained (btw, I loved the gifs). I'm planning to talk to my team about the event loop soon (in spanish).

I'm sure you might have heard about this tool that helps you play with this cycle and see in real time how it goes:

latentflip.com/loupe

[Shelby Anne]

woah that tool is awesome!

[Muhammad Ayaz]

The tool is amazing, if we have 2 aysnc functions, the web api section will only show the latest one

[Sherlock]

Amazing tooling！！！

[Miodrag Dzamic]

I tried to copy paste some code into tool and it does not save after clicking Save+Run button. Does someone else has same issue? Thanks!

[Eugene Karataev]

Thanks for the great article and animations!
It's interesting why "event loop" question is so common on interviews, if it's job is just to transport code blocks from the queue to the call stack. I think better question would be to ask to describe how the JS mechanism works as a whole thing with call stack, web api, queue and event loop.

[Rafael Beckel]

Interesting article! What did you use to animate the gifs?

[Lydia Hallie]

As a true professional I use keynote and screen record it lol

[squidbe]

Professional enough!

[thebilson]

I'm also a visual learner, so this was very helpful. I'm still early on with JavaScript and it's challenging to understand it all; are there any books out there for us visual learner that you might recommend?

[Anoop Sivadas]

Try the book written by a John duckett

[Oscar Calderon]

Hi Lydia. Thanks for taking the time of generating these animations in order to explain it in a very simple way. Still, I have doubts regarding the Call Stack. Is it the same as the main thread in which JS runs? I mean, I understood that, although JS is single threaded, but for the asynchronous logic that gets executed, NodeJS would spawn threads for it while processing the main logic in the single thread. Thanks in advance :)

[Manav Misra]

This is a great ❓, but probably gets a bit more 'low-level' than we need to just understand the behavior of our JS code with relation to the event loop.
However, my understanding is that the JS Engine/Runtime Environment consists of only the stack and the heap. The stack is what is 🏃🏽‍♂️on that single thread. Meanwhile, that message queue is part of the asynchronous browser environment. So, JS's single thread runs through call stack on its single thread and then checks that mesage queue to see what else needs to be done on its thread when it has the chance.

[avneeshroks]

I think in case of NodeJs, it's just the c++ Api's Insted of web/browser's Api.

[SebasQuiroga]

Hi @avneeshroks , I have recently cloned and run the NodeJS code and effectively C/C++ are dominant.
Remember that NodeJS is on top of the V8 engine (the one used in the Chrome browser) that Google opensourced and it is natively written in C++, running in the browser and running in a server are two different environments with different purposes and indeed different APIs. NodeJS is literally running on the same engine than Chrome, but for NodeJS it is not needed to have APIs such as those for the DOM, Window, etc as Chrome needs.

[Jude]

I'm not a JS developer but every now and then I have to dig into it. Over the past few years I've read lots of random things about the event loop but this is the first time I've had a clear picture of what's going on in the web browser (via the web api). Thanks!

[Rogério Munhoz (Roz)]

Awesome article! This has helped me a lot!

I was thinking about translating this series into Brazillian Portuguese. Do you happen to know if anyone has done it yet? Would you mind me doing it (ofc with links to the original series)

[João Paulo]

I had the same ideia. I really want to share with the team i'm part of so did you translate it or not? You found someone that did the translation of this article?

[Stanley Sathler]

Thank you so much for the illustrations, Lydia! Very helpful, indeed!

One question: what happens when the main thread is blocked - i.e. the UI freezes? Would the whole interaction within that viewport be blocked - e.g. cursor wouldn't change after hovering a link?

I think I've never faced this situation before, so I'm curious how to identify such situation. Is there any way to simulate a huge processing in order to freeze my UI, just to see that?

Thanks in advance!

[bru]

Hi, you can try this:

        function wait5seconds() {
            const plus5seconds = new Date().getTime() + 5000;
            while ( plus5seconds > new Date().getTime() ) {}

            console.log( 'wait5seconds end' );
        }


        function clickHandler() {
            console.log( 'click event' );
        }

        document.addEventListener( 'click',
            clickHandler ); // al comunicarme con la API del DOM estoy saliendo de JS 

        wait5seconds();
        console.log( 'end script execution' );

Execute this script, and press a lot of clicks, the clicks events will appear at the end ( because the dom events are push into the queue.

I think that what you need to take into consideration is when you are performing a task that it might take some time, that might block the UI.

[GaurangDhorda]

In this particular case de-bouncing is useful when we click on button..

[Pankaj Kumar]

Just write a for loop which is printing millions numbers. Call this function on any click function. You will see the effect.

[Nick Chen]

Hi Lydia, I love the series. Just wondering what tool did you use to make those animations? I'm also writing some tech stuff but I can't find a perfect virtualization tool to illustrate my ideas, but your animations really match what I was looking for.