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What the heck is the event loop anyway? | Philip Roberts | JSConf EU
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- Published on Feb 12, 2026
- JavaScript programmers like to use words like, “event-loop”, “non-blocking”, “callback”, “asynchronous”, “single-threaded” and “concurrency”.
We say things like “don’t block the event loop”, “make sure your code runs at 60 frames-per-second”, “well of course, it won’t work, that function is an asynchronous callback!”
If you’re anything like me, you nod and agree, as if it’s all obvious, even though you don’t actually know what the words mean; and yet, finding good explanations of how JavaScript actually works isn’t all that easy, so let’s learn!
With some handy visualisations, and fun hacks, let’s get an intuitive understanding of what happens when JavaScript runs.
Transcript: 2014.jsconf.eu/...
License: For reuse of this video under a more permissive license please get in touch with us. The speakers retain the copyright for their performances.
9 years later, and this is still pure gold.
yeah, but now there are microTask queue too, which one is prioritized and execute the callbacks before task queue
@vitvitvitvitvitvitvitvit we also have WebWorker(s) too
@vitvitvitvitvitvitvitvit you got answer to this??
yes i agree
10 years from my time!
He understood it in 18 months, for me it took 26 minutes, that is how much he helped me, really appreciate it. Time is all you have. Thank you man!!!
Not nearly at the same level though
Not at the same lvl of depth though
This talk is the perfect example of "If you can't explain it simply, you don't understand it well enough". Well done Philip.
That's quit simple!
agreed
Agreed!
CS50 teacher explains very well too.. Sometimes i'm worried he will forget to breath..
That sounds wonderful.
"I did not do a computer science degree, so these words... they're words"
I relate so so deeply with that
Today I had interview and was asked about execution context and i explained him based on this video. my interviewer was so impressed with the my answer. he said "This is the best explanation i have heard so far". made my day 🙂
amazing
nice!
Did you get the job though?
Nice !
congratulations
13:45
"The event loop job is to look at the stack and look at the task queue. If the stack is empty, it takes the first thing on the queue and pushed it on to the stack."
what if there's multiple tasks in the queue ... and they'll get done in some order ... doesn't that make it a sync-function of it's world??
never mind ...
@mementomori8856 it's async for you as a developer, but at the end, there has to be a queue for that poor single thread... serverless is serverless for you but at the end there has to be a server to run your code!
@khaledelnagar4135 This makes no sense
@viridianite It does make sense, but only if you have some basic working knowledge or better on how multithreading works, and the fact that JS is still a single-threaded language despite supporting asynchronous code.
it is 2026 and I still come back to this video to relearn about event loop lol
I like this guy. He's so humble and explains things with such clarity - an for a universal audience. That's no easy feat.
John totally agree, great presentation
+John Yeah! I just saw he's from &yet, they seem like a really thoughtful bunch
This was hardly for an universal audience
@bvrulez why man?
"Fucking Gilfoyle"
8 years ago and this is still my favorite explanation of the event loop. Brilliant communication.
watching this in 2019, and it is still the best source to learn JS event loop.
couldn't agree more, just rewatched it there for a refresh
@castilloexperience and still relevant!
still the best in 2020
@shevdi-22 I agree
@castilloexperience same here )
Came here from the Odin Project. This video is gold
Damn is there few of us under this video. I guess not that many keep going for so long.
I came from TOP too
TOP here as well
TOP here, second time. I have reached the backend part so this is the 2nd time I am asked to watch it 😅
You know this is the best video when content creators link this video and call it as the "best explanation for event loops".
Graduated in 2015, worked in JS alone for 4+ years, discovered this only today! Thank you
Best event-loop explanation ever .... !!!
What about Jake Archibalds explanation?
@johannbauer2863 yeah, that one helped me to learn about the missing Microtasks queue of Promises/Await since they came later in ES!!
Even after 11 years, it’s still amazing. Thank you, Philip Roberts!
the gareth bale of js!
so great!!
was thinking that he looks like Gareth
Exactly !!
🤣
😂😂
10 years later, and this conference is still gold for developers!
this video changed my life,.. great
+Nadiar AS Same, his show is really clear, I learned so much thanks to him :p
Yeah, awesome presentation
Truly is. Mine too. Simply Brilliant Video
You need to know about Tony Alecia
lol.
This gave me a breakthrough moment in realizing how async JS actually works. Really good talk.
i can't even express how much i appreciate this video. i watch it every couple of months as a refresher. and encourage my team to do the same.
Here I am 11 years later and this is still useful af!
For videos like this, youtube should implement a multi-thumbs-up system.
They already do, how else you think Fake news is so "Popular" these days
@RockDavid I think you misunderstood what he was meaning.
@coldblackice I think you misunderstood what he was meaning.
@RockDavid ?????????
those last examples were really mind blowing
This guy just cleared the stack for my callback queue of understanding javascript to execute.
Great explanation.
@xxicenturyfuck1195 He mentions he used Keynote, which is the powerpoint for Apple software. There a ton of tutorials online showing how to do animations
For anyone interested: the audio snippet at the beginning of the video is Ellen Allien - Sensucht. I recognized that track right away.
Funny how many "senior" JS devs think that they are clever AF because they interview you with this kind of questions but, in here, he managed to explain so clearly these concepts that destroy all the "mystery" of these guys intelligence ...
What an awesome explanation! Simple and sharp!
i come back to this everytime i start interviewing again
2 mins into the video i thought let me skip this but then i watched it fully and now i am glad that i did
Philip nailed it. 10 years later and it still explains well
I watch in 2024, April 1st, sleepless night...
I just love keep coming back to this video time and again
I've returned to this video several times to engrain the concept of the event loop in my head. It's so clearly explained and so useful. Thanks so much for making it!
Philip describes the event loop mechanism in a perfect way. Lots of love and respect.
Genius talk, seriously. Not hiding behind many fancy technicalities and being able to still convey high-level concepts and make them understandable is surely not an easy skill. Props!
Still coming back to the video when I have an interview scheduled around the corner
This is by far one of the best presentations on any programming concept I've ever seen. Absolutely brilliant. Thank you!!
This is the best example of event loop. You have made it so simple to understand.
This is better than anything any professor have ever done to explain anything to me in university.
Ive been a dev for years and this explanation was by far the best and clearest out there. fire. thank you!
This has to be one of the best explanations out there about the event loop.
8 years down the lane for this video, current in year 2023. Buffering through many videos on event loop still find this one the best. The energy of this man...can feel it on screen as well. Wish i could attend his session once in my life in person.
7 years later some things have changed. Now we have service workers in a seperate thread. And we have await/async functions. But yes, this talk makes the things clearer to understand. thank you!
In 2022, this video was recommended to me, and I feel fortunate.
This was my introduction to great conference talks, and, I really believe, one of the experiences that turned me from hobbyist to developer.
This is pureee diamond talk at conf!!! Really cool, how deep are those insights are! Happy Coding
thanks @Philip Roberts first time I completely understand event loop :). If you have some other video on js please share.
Javascript Understanding the weird parts
@aishahale5504 Different author.
@NishadAhsan Yes, try it.
8 years have passed, but this explanation seems to me the best on youtube
Great explanation.
One of the greatest talk on event loops that I came across
Wow... amazing lecture. The way he is explaining is great. I wish a whole JS course should be taught by him.
watching this in 2022 and still one of the best resource to learn JS event loop.
Interesting how no textbooks mention this stuff wich in my opinion is crucial in understanding core javascript and especially closures (especially when every example on closures out there contains *for* loop with setTimeout and never explains or even mentions event loop and why does *for* loop first finish its iterations and then invokes setTimeout callbacks)
I just made the connection with what you're saying! It finally makes sense!
What he means is that a very common practice question given to novices to see if they understand closures is the following:
const arr = [0,1,2,3];
for (var i = 0; i < arr.length; i++) {
setTimeout(function() {
console.log('index ' + i + ', value: ' + arr[i]);
}, 3000);
}
//Prints 'index 4, value: undefined'
The issue here is that because var is function scoped and not block scoped, i will break out of the loop when it hits the value 4 (as i will now equal arr.length, breaking the test of the for loop). As index 4 is out-of-bounds, it returns undefined as the value for arr[4]. Closures via something like the let keyword mitigate this problem, however. So the test in question is to see whether or not the novice understands the issues of closure with var versus the new block-level variable definers: let and const.
Javascript: The Good Parts by Doug Crockford explains closures very well. Also function-orientation. Other core JS lang features, but not the event loop.
@johnb1391 Or maybe a less granular way to explain it is: once the for loop is done setting up the setTimeout callbacks, it is finished, and the variable is at its final value of 4. Meanwhile the callbacks run for 3 seconds each, and they are still active - when they print the value of i, it is always 4 (unexpected). You can create closures around the timeout function value to keep the value of i as it was when the callback was created, by either passing it to a function outside of the loop to create the callback, or just making that setTimeout function an IIFE - immediately invoking it creates the closure while the i is still at its iterative value.
Well I guess that was more words lol. Is there any simple way to explain closures?
@stormwarrow thanks that's a great explanation,, can you include some code examples for "You can create closures around the timeout function value to keep the value of i as it was when the callback was created, by either passing it to a function outside of the loop to create the callback, or just making that setTimeout function an IIFE "?
Some explanations are so good that you understand the concept crystal clear. It really sticks with you, thank you.
oh, that was in 2014 and I thought it is the latest talk, amazing!
I love this talk so much. Can't help but keep coming to watch again every few months.
I've been watching some interviews and mocks preparing for my own, and people nebulously reference 'the event loop' and single-threadedness very often, like "How does X work?" "ahh, the event loop", but more in a buzzword way than as an explanation, so thank you for this video. It was super helpful
Even 8 years later this is an incredible talk.
This is by far the most informative and accessible talks about asynchronous functionality in JavaScript. Thanks, Philip, for showing us the light.
There are really no words to say how great this talk was. Amazingly clear, fun, and straight to the point.
2019 and this is still awesome. Great explanation!
love how the logo covers bits of code when i'm trying to understand
This is one of the best lectures on JavaScript that I've seen anywhere. Phillip does a great job of using visualization to explain some of the more complicated aspects of JavaScript programming while making it look easy. That's truly commendable.
this video is pure gold. thank you philip for creating the tool and this video.
Really, Amazing talk, bro! :)
Once I came up across a scenario where calling a function, say "func()" did not work rather, setTimeout(func, 0) worked!
Now, I understood the reason completely!! :)
I haven't heard such a good explanation in a long time.
The intro music tho.... I turned off the lights and got my glowstick game on. He's amazing. It was an excellent video. My ADHD did not kicked in ever while watching it. 10/10. Will buy again.
My word this is Gold. Thank You Roberts.
This video is life-changing! Thank you for this, Philip! It takes great understanding to explain complex things in simple ways. And thank you for not gatekeeping - we need more people to come into tech and videos like this make sure that even noobs understand and can work with seemingly hard concepts!
Mind blown!!!!! The lightbulb just turned on. THANK YOU!!!!!!
It's the first time I'm giving a comment in 7 years. This guy did such a great presentation. It was fantastic. Such a complex topic broken down into small lucid chunks ! Great work.
Puh, hey I'm your callback. Why did you queue me with 5 years delay?
This is called pure skill. Awesome man !
When I watched this video, I knew it would prove useful for my work. Less than a month later, it happened. Thank you for the great presentation!!
god I watched this video a year ago and now finally understood how it works. that loop simulation really helped.
worth every second, watching it the second time.
I loved every second of this presentation!
This will no doubt remain a fantastic presentation well into 2020 and beyond. Thank you, Philip!
sometime i still rewatch this, still a good thing after working with js for over 2 years
Omg when did Gareth Bale started to code, this is amazing
I've literally read many articles trying to understand this very well, but now I think I do. His explanation is amazing.
This lecture is just a masterpiece
10yrs ago?!!!! Genius! 🔥
brilliance! Absolutely stunning. Thank you
Second time watching this after 9 months, nice refresher. Good stuff.
@19:18 you can see his history. I found it reassuring that a guy with this level of knowledge still has to look up the syntax for Date(). :-) Great dissection of the event loop.
Good example of why code interviews that just test how well someone has memorized various language apis/functions are an ineffective way to determine the candidate's knowledge :)
@timeslowingdown None of those tests actually check if you have those things memorized. All those tests are there to see how you look for the best available solution
@lighterinthestorm Finding the solution to a single function with input and output is very different than writing an entire application or maintaining it, so I beg to differ
That's why Incognito Mode exists
Remembering the correct parameters for a function in a library is not a prerequisite for being a good developer, in the same way that needing a calculator in a math exam doesn't mean you're cheating. If you like calculating or memorising sure go ahead, but if you're lazy it's fine
watching this in 2022 and it is still the best source to learn JS event loop
came here redirected from async npm package docs... how come i've been doing things without knowing about this? Really useful, Thx
This is how to do a video tutorial in technical topics. Splendid animations that make you part of the "journey"
Amazing,!! the best and simplest explanation I've seen, thanks
Soooo basically I watch this video before every interview I have 😅 Ty!
This video has helped me cracking interviews!! Thanks Man!
Man, I wish I got a teacher like him. First time I understood how JS works. Legend.
Excellent presentation! 👏👏👏
this is still the most appropriate resource to understand Event loop :)
Well explained! Thank you.
2024 and the video still helps my interview! really appreciate
Рассказ, понятней чем этот мне ни разу не встречались. Спасибо большое за видео и за перевод
Great video - first time learning this
Fantastic explanation!!!
honestly this is the best convo about js runtime ive ever listened. thank you phil!
best explanation ever ! literally