What the heck is the event loop anyway? | Philip Roberts | JSConf EU

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/speakers/philip...
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.

[@marios2liquid]

This talk is the perfect example of "If you can't explain it simply, you don't understand it well enough". Well done Philip.

[@MehdiRaash]

That's quit simple!

[@JenJHayden]

agreed

[@gopalgplus]

Agreed!

[@pastuh]

CS50 teacher explains very well too.. Sometimes i'm worried he will forget to breath..

[@seongamkim4834]

That sounds wonderful.

[@syedmuhammadibtisam441]

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!!!

[@simpleffective186]

Not nearly at the same level though

[@apoorvasa1026]

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 🙂

[@davidlee588]

amazing

[@mohamedGado3]

nice!

[@darthvader1368]

Did you get the job though?

[@zikoelitbook1422]

Nice !

[@damonesswu8254]

congratulations

[@sumanyusoniwal5194]

9 years later, and this is still pure gold.

[@AndreOliveira-vq7iv]

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."

[@mementomori8856]

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??

[@mementomori8856]

never mind ...

[@khaledelnagar4135]

@@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!

[@luuuzeta]

@@khaledelnagar4135 This makes no sense

[@kaelon9170]

@@luuuzeta 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.

[@luciferonetwothree2030]

watching this in 2019, and it is still the best source to learn JS event loop.

[@bennyrussell4966]

couldn't agree more, just rewatched it there for a refresh

[@castelocl]

watching this in 2020

[@petrd6173]

@@castelocl and still relevant!

[@user-xb8bk1hd8s]

still the best in 2020

[@yuanmin6496]

@@user-xb8bk1hd8s I agree

[@R3fuge]

8 years ago and this is still my favorite explanation of the event loop. Brilliant communication.

[@sunmustbedestroyed]

I like this guy. He's so humble and explains things with such clarity - an for a universal audience. That's no easy feat.

[@ChrisBrooksbank]

John totally agree, great presentation

[@hoorayimhelping3978]

+John Yeah! I just saw he's from &yet, they seem like a really thoughtful bunch

[@raigorstonehoof6477]

This was hardly for an universal audience

[@thothtrismegistus929]

@@bvrulez why man?

[@Daniel_WR_Hart]

"Fucking Gilfoyle"

[@thedevcyclist]

Best event-loop explanation ever .... !!!

[@johannbauer2863]

What about Jake Archibalds explanation?

[@domaincontroller]

00:53 how does javascript actually work ? 02:46 V8, setTimeout 04:03 the call tack 07:18 blocking, what happens when things are slow 10:35 aynchronous callbacks, setTimout 11:13 aynchronous callbacks, the call stack 11:56 concurrency 12:50 stack, webapis, eventloop, task queue, console

[@FoysalAhmed-gd9sp]

00:53 how does javascript actually work ? 02:46 V8, setTimeout 04:03 the call stack 07:18 blocking, what happens when things are slow 10:35 aynchronous callbacks, setTimout 11:13 aynchronous callbacks, the call stack 11:56 concurrency 12:50 stack, webapis, eventloop, task queue, console

[@andriciandu]

For videos like this, youtube should implement a multi-thumbs-up system.

[@RockDavid]

They already do, how else you think Fake news is so "Popular" these days

[@coldblackice]

@@RockDavid I think you misunderstood what he was meaning.

[@djkater]

the gareth bale of js!

[@quanghoang8031]

so great!!

[@BudyKIr]

was thinking that he looks like Gareth

[@thelolladorfking2416]

Exactly !!

[@SushilYadav7]

🤣

[@radoncreepgaming2604]

😂😂

[@nadiar.syaripul]

this video changed my life,.. great

[@babas_babas]

+Nadiar AS Same, his show is really clear, I learned so much thanks to him :p

[@jonnieve2483]

Yeah, awesome presentation

[@lakshithaweerakkody5326]

Truly is. Mine too. Simply Brilliant Video

[@aishahale5504]

You need to know about Tony Alecia

[@Gigusx]

lol.

[@c3eb4]

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.

[@BURN-ADDiCT]

"I did not do a computer science degree, so these words... they're words" I relate so so deeply with that

[@jonsanders2069]

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!

[@element3663]

This gave me a breakthrough moment in realizing how async JS actually works. Really good talk.

[@Hariharan-sh6bo]

Graduated in 2015, worked in JS alone for 4+ years, discovered this only today! Thank you

[@capeeshewebdesignanddevelo681]

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!

[@manojmj5479]

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!

[@ManasGupta_mag]

This guy just cleared the stack for my callback queue of understanding javascript to execute. Great explanation.

[@Nadrioc]

@@xxicenturyfuck1195 He mentions he used Keynote, which is the powerpoint for Apple software. There a ton of tutorials online showing how to do animations

[@agusmoles]

2019 and this is still awesome. Great explanation!

[@navaneethagastya]

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!! :)

[@hraynaud]

This is by far one of the best presentations on any programming concept I've ever seen. Absolutely brilliant. Thank you!!

[@rameshwari]

Wow... amazing lecture. The way he is explaining is great. I wish a whole JS course should be taught by him.

[@maximpekarsky7266]

This was my introduction to great conference talks, and, I really believe, one of the experiences that turned me from hobbyist to developer.

[@sushilkumartechy]

thanks @Philip Roberts first time I completely understand event loop :). If you have some other video on js please share.

[@aishahale5504]

Javascript Understanding the weird parts

[@NishadAhsan]

@@aishahale5504 Different author.

[@aishahale5504]

@@NishadAhsan Yes, try it.

[@AshokDey]

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.

[@Martinit0]

Puh, hey I'm your callback. Why did you queue me with 5 years delay?

[@mdmobasshir595]

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.

[@chunk1978]

This is by far the most informative and accessible talks about asynchronous functionality in JavaScript. Thanks, Philip, for showing us the light.

[@sethfeingold]

This will no doubt remain a fantastic presentation well into 2020 and beyond. Thank you, Philip!

[@carlellis9647]

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.

[@inordirection_]

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

[@msbecks3806]

Honestly, your explanation is down-to-earth for the understanding of all. Great work!

[@DecaSpace]

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.

[@johnnychan6755]

I have been trying very hard understanding this whole JavaScript event-loop, callback, and asynchronous concepts for WEEKS (and failed), despite tons of google searches, article readings and tutorials. I think I finally "got it" after watching this video. So thank you SO MUCH for the talk Philip!!! (and thanks for sharing this JSConf!). I sense "hope"... in understanding and using JavaScripts :)

[@ravabat5841]

It's true!

[@BlueTreeCode]

Honestly one of the best videos I've come across. Everything is explained so clearly. He Is excellent at what he does and I hope to hear more from him!

[@dushyantsabharwal6706]

Its not just the content of the video that is amazing but also the slides are done so nicely and as a speaker i know making a simpler presentation on a technical topic is very hard. Kudos Philip!

[@JoseSotoCanHelp]

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!!

[@okikiolaosunronbi8766]

This has to be one of the best explanations out there about the event loop.

[@Middollo]

I find myself coming back to this talk as i progress in my career as a JS developer. I think this is my third time watching it, it gets better every time. Well done.

[@bharghavak]

Some explanations are so good that you understand the concept crystal clear. It really sticks with you, thank you.

[@nosajghoul]

@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.

[@timeslowingdown]

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 :)

[@lighterinthestorm]

@@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

[@timeslowingdown]

@@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

[@kotekutalia]

That's why Incognito Mode exists

[@theshermantanker7043]

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

[@TheHolyReality]

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)

[@ihateslowcars]

I just made the connection with what you're saying! It finally makes sense!

[@johnb1391]

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.

[@raulnoheagoodness]

Javascript: The Good Parts by Doug Crockford explains closures very well. Also function-orientation. Other core JS lang features, but not the event loop.

[@chriscorrigan14]

@@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?

[@boliussa]

@@chriscorrigan14 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 "?

[@kikevanegazz325]

There are really no words to say how great this talk was. Amazingly clear, fun, and straight to the point.

[@braunbaerhh]

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!

[@idrisawedazubair2562]

I've literally read many articles trying to understand this very well, but now I think I do. His explanation is amazing.

[@DanielRamBeats]

Really awesome talk, I knew conceptually what was happening in the event ques prior to watching this, but the visualizations clarify it so well.

[@sahilattri51]

brilliance! Absolutely stunning. Thank you

[@sredna3714]

So helpful! Great video. I've been learning computer science for years and you still explained all of this better than any professor I've had. Major kuddos to you :)

[@abhinavgupta1846]

By far the best explanation of the JS event loop I ever came across. Must watch for every JS lover out there!

[@ClaymorePT]

From someone who does not have a degree in computer science, I have to say, this is one hell of a good explanation! I only which that the teachers that I had during my CS degree, explained things as good as this guy. Really awesome!

[@malakies999]

wish

[@alejandrocr9293]

wish

[@schizo5189]

wish

[@prashantchauhan1360]

One of the best js videos till today. I wonder it was uploaded 8 years ago. If i would have watched then I would be that much AOT (Ahead of time).

[@aminukano_]

I just pause while watching to recheck the speaker's name, wondering that we still have such great speakers. This is by far the best tech conference talk I ever watched in my life!!

[@joshdellay3603]

Talk about boss mode, great work by Philip! The loop tool he created is amazing for helping to visualize the runtime, event queue/loop and web api! I can only imagine the time that went into creating and researching how to build it and you can see by his facial expression how much of a challenge it must have been hahah!

[@guilhermeferreirabr]

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!

[@arinspanner]

I love this talk, he explains asynchronous code so it's totally intuitive and clear. Thanks Philip!

[@dynamic75]

Thank you for breaking down these concepts into more digestible pieces, still trying to comprehend all of the details but I get the overall idea. Great job!

[@shalizi1280]

This is absolutely a marvelous explanation of event loops within javascript. It doesn't get better than this, thank you.

[@yeshiqing8929]

Do you know what the ppt is made by? I like the ppt style(animation most). but i think the microsoft ppt is so heavy

[@4Y0P]

Ye Shiqing its made in keynote

[@Josematube]

Great explanation.

[@fhc9916]

Probably one of the best talks I've seen in recent times.

[@msshwetap7095]

This explanation changes my life every time I watch it. Thank you!

[@MrLiudongtony]

Watched it at the end of year 2020 and it is still one of the best in its kind to explain the event loop.

[@kevinbuffington4008]

I randomly watched this video a week or two before I went on a job interview five years ago. The event loop came up in some advanced JS discussion and I'm convinced it's a big part of why I got the job. Got another interview coming up and I figured I should probably revisit this just to make sure I still remember it. Still just as informative as I recall.

[@gerryramosftw]

holy crap this talk was from 6 years ago, still the best video I've watched on explaining javascript event loop in 2020

[@syedbaryalay5849]

worth every second, watching it the second time.

[@mohammedelhadididi4612]

This lecture is just a masterpiece

[@deepakkj]

No one can explain better than this guy. So far the best demo I have ever seen on jsconf videos

[@parveznawaz99]

Philip describes the event loop mechanism in a perfect way. Lots of love and respect.

[@tak68tak]

the best video ever seen. it's still awesome in 2019!

[@prashantchauhan6591]

watching this in 2022 and still one of the best resource to learn JS event loop.

[@kandysman86]

This was an awesome talk, and i love the tool this dude built. He has changed the way i look at writting my JS.

[@timzhang8751]

watching this is 2020, and it is still the best source to learn JS event loop.

[@CodeSynergy]

oh, that was in 2014 and I thought it is the latest talk, amazing!

[@4ndrs_dev]

I love this talk so much. Can't help but keep coming to watch again every few months.

[@rimoros.1020]

Didn't realize this was 6 and a half years ago, still the best explanation and the only explanation I need!

[@gnclmorais]

Excellent presentation! 👏👏👏

[@hasanmirarabshahi1956]

Still the best explanation of what event loop is, with great examples. Thank you Philip

[@ighsight]

Brilliant. No question that this talk opened up new doors for this young man.

[@user-sl6zf3te7t]

Рассказ, понятней чем этот мне ни разу не встречались. Спасибо большое за видео и за перевод

[@rahulraut1569]

This video has helped me cracking interviews!! Thanks Man!

[@justadev____7232]

Mind blown!!!!! The lightbulb just turned on. THANK YOU!!!!!!

[@Silenoid]

This is better than anything any professor have ever done to explain anything to me in university.

[@juanmanueltoni2506]

Amazing,!! the best and simplest explanation I've seen, thanks

[@mbasitbilal]

best explanation ever ! literally

[@theyoutubeliva]

honestly this is the best convo about js runtime ive ever listened. thank you phil!

[@mryildiz702]

After 5 days It will be 2022, and this is still the best introduction to the event-loop ever. Thank you :)

[@manualautomaton]

I should never had started learning JS w/o seeing this. #Roberts4President

[@teamworkucla]

Well explained! Thank you.

[@user-lu7dm2fg2x]

This video was an absolute gem - 26 minutes well spent! Thankyou so much for this.

[@AlayDhagia]

I knew the concept of event loop but man your explanation is simply amazing. Thanks a lot for putting this up !!

[@rozoneri]

thank you, excellent presentation.

[@kwstasl2]

Great explanation! Just one note: at 23:35 , lines 12 and 13, I think it should instead be array.forEach(function (i) { setTimeout(cb, 0, i); or else the array element will not be passed to the callback and console.log() will log them as 'undefined'.

[@hoangminhvu2425]

+kwstasl2 Thanks!

[@kwstasl2]

***** You're right

[@muratcan__22]

Yeah, thanks

[@mdirshath4174]

Man, I wish I got a teacher like him. First time I understood how JS works. Legend.

[@tiennguyen-pz6ie]

sometime i still rewatch this, still a good thing after working with js for over 2 years

[@oksanaserhiivna2634]

Fantastic explanation!!!

[@BernardoLeon]

The timeout is executed in the WebAPI right? What does that means? WebAPI executes in another thread? or why the code running in the Web API (section?) does not block the unique Thread JS has or does JS have other threads in the background?

[@BernardoLeon]

Federico Rodriguez The most upvoted answer here will throw some light. qr.ae/ROfg32

[@polyglotdev]

+Bernardo Leon Yup there is a dedicated thread for the event loop and a bunch of threads in a thread pool to handle the callbacks. The thread pool is transparent and is not meant to be accessible from any JS constructs. However as of lately we have Web Worker thread pool progammatically accessible to JS to offload long running CPU and I/O intensive work to a different background thread pool.

[@BernardoLeon]

Thank you *****

[@TheBohrabohrafamily]

Trying to answer all your questions The timeout is executed in the WebAPI right? - Correct What does that means? - It means the *timeout* code is running as part of a different part of the Browser process (Chrome, Firefox). It is *NOT* running within the Javascript section WebAPI executes in another thread? - It runs in a different area of the browser process and it able to run concurrently with the code that is running on the Javascript side or why the code running in the Web API (section?) does not block the unique Thread JS has or does JS have other threads in the background? - The browser is able to run multiple things (the WebAPI, the event loop, the rendering engine and the Javascript processor to list a few) concurrent. But the JS processor does *NOT* have other threads in the background, in its limited view it can only do one thing at a time which is defined as what's one the top of the call stack

[@philtrem]

​@@TheBohrabohrafamily Thanks, I was suspecting that the "it runs in another thread" explanation was incorrect. n.b: The distinction between concurrency and parallelism is an important one.

[@cronus663]

I remember I watched it back in 2016, and now in 2022, I came back to check if it was as good as I remember, and... yes, it's definitively is. There's a real good stuff here, congrats Philip!

[@sfgmote]

came here redirected from async npm package docs... how come i've been doing things without knowing about this? Really useful, Thx

[@channel-ug9gt]

i have a question : what is the priority of the dom events ? such as button clicks ? is it higher priority than rendering ? lower priority ? higher priority than events triggered by setTimeout ?