JavaScript Event Loop: How it Works and Why it Matters in 5 Minutes

One of the most important concepts to learn in JavaScript! The Event Loop is core to how JavaScript handles asynchronous code. In this video, we'll break down the call stack, event loop, and event queue to show you how JavaScript works under the hood.
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*** Resources ***
Original Article - dev.to/nodedoctors/an-animated-guide-to-nodejs-event-loop-3g62
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*** Timestamps ***
00:00 - Intro
00:50 - Advent of CSS and JavaScript Course Discounts
01:18 - JavaScript is Single Threaded
02:09 - JavaScript Call Stack
04:15 - Event Queue and Event Loop
06:20 - Wrap Up

[@thanhattran284]

Finally, after looking for n^1000 videos, forums... I found a person who can explain these terms in real simple and understandable way 😭really appreciate your work man.

[@Mo-Sam]

I loved it, thank you. It would be so great if you make a series like this , explaining advanced concepts under 5 minutes.

[@georgenonis5967]

Amazing video. Whenever I forget how it works I just jump over here and everything is clear again. Thank you mate!

[@felixurrutia4246]

Excellent explanation. This is exactly what I was looking for.

[@rick.d]

Understanding the LIFO/FIFO nature of the call stack vs the event stack is really cool, thank you.

[@JamesQQuick]

So glad that was helpful :)

[@goyii]

This is true but i think its important to not call event queue "stack" when the real data structure its a Queue. When you think about stacks and queues from real life everything makes sense

[@kasiad7291]

Perfect explanation, thank you!

[@ming3957]

Expected a 5 minute video but I got 2 whole extra minutes of James for free! What a deal

[@JamesQQuick]

Always here to please!

[@jean-pierrebanchereau8474]

Perfectly explained. Thank you

[@kostiantynkarzhanov9216]

Thank you! Very concise and simple explanation of not an easy concept! 💛

[@smachohalla]

Thanks Mr. Quick. This was neat magnificently explained

[@godswillumukoro8908]

Great explanation here James!!!

[@tinaxing1020]

This is amazing explanation !!! thank you!

[@praveenvarma1]

Super bro ... u explained a complex theory so simply with a beautiful animation.

[@user-ol6tq5hw7s]

Great video, keep going James! :)

[@dpetrik99]

Great and short tutorial! Highly recommended

[@koffworld7857]

Very clear explanation... great video!

[@elielberra2867]

Amazing explanation, it was very clear! Thanks :)

[@rayenselmen603]

Finally ! Amazing explanation Sir !

[@denniszenanywhere]

I like this overexplanation. Please do more. I am even watching tiktok videos that loop short coding lessons and they’re sinking in more.

[@indraappkey]

one of the clearest explanation of event loop, thanks James

[@JamesQQuick]

Yayyy!!

[@Walocial]

Awesome explanation, much appreciated. Thanks :)

[@barrypun9079]

this explanation was amazing

[@kacygilbert9434]

Thanks for this!!!

[@damianodamian7907]

Now I got it, thank you. That is a really great explanation James! You've got a new sub

[@JamesQQuick]

Yayyyyy!!! Glad it helped!

[@Rashomon69]

Brilliant. I think I finally understand. SUBBED!

[@ahlembs790]

i am really thankful for this video just before my interview, , feel having clear idea about what happen behind the scenes with node.js, please more advanced topics like this with such great explanantion

[@JamesQQuick]

Wow, that's so cool to hear. Hope it went well!

[@alexmiserandino1062]

fantastic video, thanks!

[@Yunraii]

Thank you very much. It was explained very well and I finally understood what an JS Even Loop is. You did a great job in explaining it!

[@JamesQQuick]

So glad to hear that!

[@umangkathuria7097]

Always come here for a quick refresher before I have to explain it to anyone :) That article is excellent in itself too! great content!

[@JamesQQuick]

Thanks for watching!

[@manerVlog]

Awsome sir now I understand watching many videos this is very simple to understand.

[@utkarshsinghalus3085]

wow great video , it simple thanks for the virtual presentation

[@user-wr4yl7tx3w]

really well explained.

[@RihaLV]

Great video, thank you!

[@juliorodriguez4946]

Thank you so much...

[@ctchen1988]

awesome...thank you so much.

[@asrnyagmur9802]

Clearly understand ❤ followed

[@scottweedo6707]

Thanks 🙌🏼

[@liamdev5145]

Woa truly appreciate it

[@hendodesktasks]

Great Explanation, Thanks a lot

[@JamesQQuick]

Glad it helped!

[@johnshepard3197]

Thanks for the quick refresher, man! Plus correcting my pronunciation of "lib-you-vee" library ;)

[@dorels6205]

Wow, really, really understandable!!!

[@satorugojo6]

thanks a lot man

[@Yan-vi6fd]

Amazing explanation!!

[@JamesQQuick]

So glad you found it helpful!

[@eduardocuadraneyra]

THNKAS A LOT i was havnig a hard time to get this idea but you have explained really simple, so Here you have a new student, thanks again and have a great day...and if your a reading this and your not the teacher , have a great day to smile, and remember you are important to.

[@afridperingaden846]

very good explanation

[@abdelrahman1669]

really impressive thanks man

[@JamesQQuick]

Glad you enjoyed it!

[@EmmanuelIbikunle]

Very good explanation. Thanks

[@JamesQQuick]

So glad it was helpful!

[@TheZeiZei]

Great explanation!

[@JamesQQuick]

Thank you. Hope it helped!

[@prinzadamz]

Thank you 👍👌

[@dilshodturabov]

thank you pls do not stop thank you

[@reeq5748]

U saved me man. Thank you.

[@JamesQQuick]

Yay!!!

[@ricardocnn]

Awesome tips!

[@JamesQQuick]

Thank you! Glad it was helpful!

[@marcelukeje3510]

I'm here with no background knowledge on JavaScript. I need to know this for an assignment and I find the video insanely easy to understand

[@shuvra03]

Best video on node js event loop in 5 minutes

[@JamesQQuick]

AYYYYY!!

[@iiinicky6224]

Thanks!

[@arjinpella6071]

thanks bro

[@Kanal5909]

For me the most tricky part is that you never really know when the function you call is going to be asynchronous or synchronous. For example if you read data from a file and then process it, for small files it will be synchronous and for the files with let's say 300k lines it will most likely be asynchronous (it also depends on what you do with these lines). It's obvious that all HTTP requests are asynchronous so this is quite easy to handle or predictable at least.

[@JamesQQuick]

Well, I think you can tell something is asynchronous based on two things: 1. it returns a promise or 2. if takes a callback function as a parameter

[@ghaithjr.9189]

note : libuv is written primarily in c , great explanation king

[@JamesQQuick]

Thanks!

[@SonAyoD]

Wow, I finally get it. Where does the heap come into this?

[@harishraj3616]

Clearly understand ❤️

[@JamesQQuick]

Yay!! Glad to hear that!

[@maxlev621]

Best explanation ever

[@JamesQQuick]

Yes! So glad to help!

[@mikkun_]

so if we have async/await, it will put the asynchronous to libuv, then event queue, and stack it to the call stack along with the other synchronous tasks?

[@neeravvinod2774]

great video

[@doniaelfouly4142]

Thanks

[@diegogutierrez7032]

Something to add there is that. This is the event loop explained from the NodeJS side. If you take a look at the event loop in the browser side. It's much mure complex due to UI taks, Render Tasks, Dom Tasks, Net tasks.

[@JamesQQuick]

I’m not sure that’s true? How is it different? It still uses the same offloading mechanism for async work right?

[@misterl8129]

Thanks for the video! And yes, I have a question. So we know that every async procedure will go to the event queue. And it will only passed to the call stack if this is empty. So... 1. What about if we have a large large project, with many sync procedures, the event loop will wait until ALL of them finish before pass it to the call stack and give the result? 2. How it work then the "await", if we know that we need to clear the call stack before pass the async functionality. How it work when we need to have some data before proceed with the other instructions? For example data mutation 1, data mutation 2, data mutation 3, await for something() data mutation 4 How it works when we need to have the "something" before go to the data mutation 4 Thank you!

[@doniaelfouly4142]

thanks

[@user-pm1gh3jd2r]

Hi, 5:15 Eventloop use to take the event from queue and pushes into the callstack only when the callstack is empty. Means, eventloop do not take the event from the queue when there is an event(atleast 1) in the callstack. Am I right? What if there are multiple events waiting in event queue. Do the eventloop dequeues all the events one by one from eventqueue when the call stack is empty or it will dequeue only one event?

[@ofeyofey]

He says the event loop is "first in last out" is that out from the place it came it in or other end?

[@ehtisham_hashmi]

where I can find more videos on js concepts?

[@mohmmadjesus6710]

Wonderful Info Sir! please make a series on vue js

[@JamesQQuick]

Glad you enjoyed it! I still haven’t really spent time with vue yet but I want to!

[@noorulamin2652]

Awesome! I have few questions 1. few days ago i was reading a medium article in which they use the term callback queue but here you mentioned event queue. 2 they called it Node APIs and you mentioned Libuv APIs. 3. They also did mention what's called MicroTask Queue, but here it was missing. 4. Does Event Loop handles only Async operations? 5. How does Event Loop handles setTimeout functions?

[@JamesQQuick]

So I believe "callback queue" would be the same as the "event queue". I would assume Node APIs are also referring to the Libuv APis as well. I have never heard of the MicroTask Queue, so I'm not sure where that fits in. For the event loop, yes, it is only looking for async operations that have finished running. setTimeout takes a callback function. So, when setTimeout is called, it sends it off to the underlying APIs to be handled. When the timeout is done, it then throws the event/callback onto the Event Queue. The Event Loop will find the event on the queue and pass it back to JavaScript call stack. hope that helps!

[@ivansandoval3757]

Same doubt as yours, I just watched a different video where the event loop components are mentioned as follows: Call stack, event loop, micro tasks queue, macro tasks queue and scheduled tasks queue.

[@And1997Ruz]

@@ivansandoval3757 You have probably watched the explanation of the browser event loop, whereas here it's the nodejs event loop They work in a different way. Not to mention the fact that browsers don't have the Libuv API

[@LeLeTesoro]

Hi there, I'm watching this in April - if I sign up on the website you linked, can I still see the 24 JS and CSS challenges you did for Advent?

[@JamesQQuick]

Yep!

[@andrewwall2730]

Just a minor correction, libuv is written in C, not C++

[@anushree3744]

what does it mean when we say that event loop is blocked and how does that happen?

[@hassanad94]

What about js workers? You can do multi threaded tasks with it.

[@mkrzyzowski]

What when call stack is empty and event queue too and some async task is running in libuv thread. How JS know to wait for libuv?

[@JamesQQuick]

Hmm, well JavaScript land just knows to constantly check the event queue when it's done running code. So as it constantly checks, eventually libuv will put something back on the queue for JavaScript to process.

[@shashwotbhattarai8731]

i don understand the part where the stack is empty and event loop pushes the callback to the stack. will the stack ever be empty? when does the stackbecome empty?

[@goyii]

The stack gets filled once javascript code its readed. Then it will execute functions from the call stack starting from the top, cleaning out functions once executed. In this particular case. The only function added to the call stack was a console.log. But the code also executed an asynchronous function. Wich is sent to the Lubuv Api to ne handled. Once this operation is completed is sent to the event queue. The event loop checks if the call stack is empty and if there is something inside the event queue. If that happens, it will move this callback from the quewe to the call stack. Execute the callback and clean the stack again.

[@krateskim4169]

great explanation

[@JamesQQuick]

Thank you! Did you already have a good idea of what the event loop is?

[@krateskim4169]

@@JamesQQuick nope not until today, learnt something new

[@JamesQQuick]

@@krateskim4169 Ah so glad it helped!!

[@alexanderwu]

Great video. One complaint, this is the node event loop which isn't exactly the same thing as JavaScript

[@devonchia5976]

so if there is an incoming synchronous code and a callback at the same given time, what is the precedence for the call stack?

[@AahadCyborg]

when synchronous code and a callback are both triggered at the same time, the synchronous code takes precedence over the callback function in the call stack. This means that the synchronous code will be executed first, and then the callback function will be added to the call stack and executed once the synchronous code has finished running.

[@notarealhandle123]

I've been developing in JavaScript for over 20 years now. But when I was interviewed recently for a new job, and asked about JavaScript event loop, I could not remember it for the life of me. This is mainly academical knowledge, not something that you need as an experienced developer. People who do interview are often dumb, and do not understand that.

[@user-ou9ky2cj4e]

For 5 mins it is fine, but the micro task loop concept is missing

[@mahedihassanshawon4821]

awesome

[@conallen3166]

🔥

[@krnkoravi]

First time hearing about LIBUV API. I thought async task directly goes from callstack to event queue. LIBUV only applicable in nodejs? Still not clear what exactly processing LIBUV does to async task before it goes to event queue

[@ghostkee5031]

you gonna confuse people between the built in javascript event loop and nodejs thread pool + event loop which libuv provide

[@Pareshbpatel]

A nice walkthrough the JS Event Loop. Thanks, James {2024-04-15}

[@nikman5541]

Okay this is Node platform with C++ wrapper. but what about browser where we dont have Node and still have async JS?

[@TheBlackmanIsGod]

Explain the “popping off the top” more

[@_naemi]

So this mean that "console.log()" doesnt go to event loop?

[@nikolozmacharashvili1212]

hey james i have a question. i have a recruiter promising me training and job placement. is this for real or is there a catch. i asked about info, they said when u get to the country than will talk, but keep on liking my projects on linkedin

[@nikolozmacharashvili1212]

also when u said, if the callback is an async code, the cycle repepats. officialy some async code went into the LIBUV API, now was that the callback or some different code ?

[@clarence9527]

In real node.js environment, this is more complex, the tasks are divided to microtask and task.

[@adhithyankrishna8220]

It kind a faking multi thread libuv api is like giving task ti subsystem to excute like os or kernal to excute the fetch operating libuv is waiting for result when it got its result it send to evet quew which means node is not creating thread but it use sub system thread which is also running on same cpu means is multithreaded

[@SiteMentor]

nice

[@moneyfr]

Getter and setter please and why using it

[@JamesQQuick]

You're interested in why people use getters and setters?

[@DavidLevy-qg7hd]

nice video but the title is a bit misleading, this is actually the node.js event loop as you say at the end of the video, not the javascript event loop as the title says.. JS natively does not use libuv, instead it uses the browser's built-in Web APIs to handle asynchronous tasks

[@vamshireddy8187]

❤

[@oncoding4520]

[@jakubdrewniak2668]

I have no idead why you added some external library (libuv) to this explanation. for me it makes it more complicated... i just wanted to refresh my knowledge, but for person who never grasped the idead of event loop- that's just not good approach.