Closures Explained in 100 Seconds // Tricky JavaScript Interview Prep

[@theultimateshield5133]

click the f**king button man its his f**king dream😂😁😂

[@dfsgjlgsdklgjnmsidrg]

no i wont

[@anmolmaniyar6868]

Plot twist: it's an erog- Nvm

[@chiragsingla.]

clicked it from all my alts

[@baibhavvishal362]

Just clicked because u asked nicely 🤪

[@ihavetwofaces]

"If you ever find yourself faced with a question like this, the best thing you can do is talk through the code slowly and [demonstrate the knowledge you do have], so they know you're on the right track, even if you don't come to the right conclusion." Hands down the best tip for a technical interview of all time. I've assisted with coding interviews before, and the absolute worst thing you can do is shut up, say nothing, or say "I don't know." Even if you really don't know, show what you do know, ask questions, and try to apply what you have to the problem. That will earn you a tip or a hint or a rhetorical question, and that may lead you to the right answer. When I interviewed for my first job, I got two of three questions mostly wrong, but I was actively participating, asking questions, trying to work things through, etc., so I got the position.

[@ko-Daegu]

That doesn’t fully apply in todays world we’re competition is in all times high Talk thru your solution and get the right answer is now expected The curve to get an entry job got even steeper and it keeps getting this way

[@Jubinmail]

Good tip. But don't expect a call back.

[@jessh4016]

@@ko-Daegu I mean, of course being an expert in everything is the best. But if you don't know the full answer, talking through what you DO know is still infinitely better than just getting nervous and clamming up.

[@Marcosmapf]

About the question, it's also tricky that it prints 3 3 3 instead of 2 2 2 since the loop compares i to less than 3 instead of less or equal to 3, but it makes sense considering the i is incremented after the last loop (i++) and the last value evaluated is 3, which is false for the iteration but still a valid value for i

[@adeshas83]

It shouldn’t increment because the for condition has 2 rules that should prevent it from increments.

[@migueljara9399]

Thank you for clarifying that. I was trying to wrap my head around it.

[@ToddsDiscGolf]

Yes I spent a couple minutes staring at the problem on pause until I figured out why it printed 333 instead of 222. I’m glad I’m not the only one

[@TsoiIzAlive]

Tricky but not really tricky

[@UnfamiliarPlace]

Exactly -- if it stayed at 2 then i < 3 would remain true and the loop would go on infinitely -- the loop only ends because i has been incremented to 3. Understanding when the increment and check happen is key to understand loop conditions

[@jkjoker777]

the trick question at the end would still log 3 three times even if the setTimeout time was set to zero. setTime is an api that does not run in the current js runtime

[@Fireship]

Good point, I explain why that happens in the event loop in this video kzbin.info/www/bejne/rJ-WpaBmptqip6c

[@omaribbrahim]

I don’t think this is a trick question. I am new to Javascript and had to wreck my head to wrap it around this the first time it popped up in my code.

[@okyanusozsahin3439]

Thank you!

[@jkjoker777]

@@omaribbrahim that’s kinda the definition of a trick question ; non intuitive answer

[@dhkatz_]

Current JS runtime? That's some confusing wording. I think what you mean to say is that setTimeout will at minimum wait until the next tick of event loop plus whatever delay you set. Furthermore, if there are any other messages waiting to be processed by the loop (like other setTimeouts) then your callback must wait more. So a delay of 0 will just wait until the next event loop tick at minimum. Depending on what you're doing this could be far longer than 0 ms. The timeout is only the *minimum* guaranteed time.

[@KoyFoster]

3 years writing JS and I never took the time to look up the diff between let and var. Thank you very much.

[@dervaken3902]

nobody uses var nowadays lol

[@Kivencito]

@@dervaken3902 hi, i just started programming and i picked js for that. Would you please explain why is that? that nobody uses var? I use it quite often. Thanks!

[@kaoticgamer1404]

@@Kivencito from what I know, it's better to use 'let' because if you use 'var', it can be easily changed by any function because it is global, and that can lead to unexpected results. It's better to use 'let' because it encapsulates your data and limits the scope, so that makes it easier to know what is affecting your variable and to know the outcome. Also use 'const' if you know the variable won't change.

[@idc20627]

@@dervaken3902 should still know what it is, what if you're working on legacy code. Problem is, most people are in it for the money and not the knowledge and then you wonder why bob knows so much and you know just enough to get a job and stay at that junior role for life.

[@NoidoDev]

​@@dervaken3902 😮 Why would you have your functions be able to access every variable outside of it?

[@LoveThatSceneChannel]

One day I will watch beyond the 100 seconds, and then I'll get closure

[@MiguelQuinaRibeiro]

This comment made me rethink my life 😱.

[@xunxekri]

Shame it's not on Clojure

[@magne6049]

genius comment, multiple meanings, you are Christopher Nolan.

[@LoveThatSceneChannel]

@@magne6049 I see you 😂👍🏻

[@xrr-1]

Another way to log 0,1,2 with `var` is by forming a closure using an IIFE A good way to solve if the interviewer doesn't allow you to use `let`

[@jordizweden5201]

when you said "click the f*ckng button" you had me

[@pavanakumardr152]

you deserve more than 1 million subs 😊😊

[@devsauce]

Christian Bale === Closures. Got it Jeff.

[@naythaniel]

Christian Bale === actor Patrick Bateman === closures Christian Bale !== Patrick Bateman

[@PickleSurpriseVEVO]

But American psycho ends without closure

[@danieljoaquinsegoviacorona1734]

Whoever thought of closures is a psychopath for sure! lmao

[@deepkhamaru1154]

Well he did give closure to Batman! So... You're one hundred percent right!

[@CarlJohnson-iv7sn]

Adding sigma Patrick Bateman with closures (hard interview topic) is such a chad ohio enjoyer thing to do. I'm sure Jeff was mewing while he made this video.

[@TheresAStarman21]

Last time I was this early, fireship was not using firebase.

[@justingolden21]

Hi Earl

[@lars2299]

@@justingolden21 lol

[@GavHern]

wasnt his name "Angular Firebase" before tho?

[@TheresAStarman21]

fixed it.

[@deebakapadia]

@@GavHern Yep. Then Google bought that domain. Idk how that happened lol

[@kalvinpearce]

JavaScript is a fever dream 👀 Thanks for taking us further down the rabbit hole

[@spythere]

Just wait till you get to TypeScript...

[@ДмитроПрищепа-д3я]

@@spythere typescript is less of a fever dream though

[@spythere]

@@ДмитроПрищепа-д3я Yeah, it's like a obscure dream but with some good aspects (like Intelisense or types) :D

[@Krzysiekoy]

Some people in the comment suggested that you could still use var and log 0, 1, 2 by doing IIFE (immediately invoked function expression) and that is a common solution when you are forced to use var (for example, during an interview). I've been coming back to this video from time to time as my understanding of JS improved, and I have found another interesting solution with var while exerimenting: for (var i = 0; i < 3; i++) { function log(){ console.log(this); } setTimeout(log.bind(i), 100); } So, basically we create a bound log function, and we pass the value of i variable in the current iteration as "thisArg".

[@boiimcfacto2364]

I've said this before, I'll say it again - YOU'RE THE BEST when it comes to these kinds of tutorials.

[@cyrilogoh6171]

Finally understand how var and let work

[@skylark.kraken]

4:13 well, the event loop means that even if there was no delay it would still always run after i is 3. It's not a case of run it a million times and one won't be 3,3,3 due to timing, always 3,3,3. (also event loop in 100 seconds would help out a lot of people, understanding it solves a bunch of problems and I've seen people write in validations which would only fail if the event loop wasn't as it is)

[@hamdihadjkhlifa]

I think if you r a teacher,your class don't need hours To understand anything, best channel

[@gourabmukherjee2169]

This channel is my favourite. Best content + No bullshit = fireship ❤

[@muthukumar8009]

Probably the best explanation ever for closure

[@leoingson]

Yeah. But still I miss what it's actually good for. There is some form of protection for the inner function, but so what, why not just take arguments? Is it a workaround for incomplete language design of JS?

[@jb5631]

@@leoingson memory / performance efficiency

[@bilalchami1]

How convenient. Was working on a memory optimization issue today related to this topic, and here you are nailing it and explaining everything in ~5mins Thanks 🤘🏼

[@milindsharma7184]

My god !! In my 4 year experience it was first time I intentionally used closures. And the same day this video came out. Lucky day

[@ChaoticNeutralMatt]

Curious what you used it for

[@milindsharma7184]

@@ChaoticNeutralMatt In simple words, I was trying to avoid infinite dependency loop in react useCallback hook

[@spaaaaace8952]

What a legendary video. It took me around 10 minutes of pausing and rewinding just so I could absorb the information.

[@mayanxoni]

Man with this content, you deserve millions of subscribers!

[@cryptodive7175]

This is one of the best channels, short & to the point.

[@aspire.studios]

Even though I know enough js, I would love to watch an entire js series about every important concept just like this one.

[@shivamjhaa]

This is literally the best video explanation video on closures. I keep coming back to this. This explains why things we take for granted (for ex, debounce in lodash) works. Thanks a lot man.

[@Aly_Fahmy]

This is LITERALLY my favorite KZbin channel right now!!!

[@sujaigorai7752]

Whenever an execution context is created a new scope is created...and when a function is declared inside the execution context the function hold the reference to that scope....but if no function is declared inside the execution context a new scope still created but it's immediately garbage collected after the end of execution...it's very simple concept

[@surlysmiles]

Love getting that interview question right. Makes me feel like it's time to move past js

[@lirydasmaz9595]

I'm pretty sure you will go beyond 1 million subscribers... no doubt.

[@Arcwise]

Your presentations are truly awe-inspiring. Despite their short length, I‘m sure the more thought went into them to extract the bare essence for us to digest, with intentions to pique our interests and to motivate us to go in-depth on our own. The appreciation for your viewers valuable time is reflected though the content‘s succinctness and demonstrates self-awareness in this era of mind regurgitation incentivised by the algorithm.

[@TechdubberStudios]

believe it or not, I guessed that it will console log 3 for three times. But I didn't know that the behavior will change when using "let" instead of "var". There's always something new to learn, I guess.

[@arghyaguha6356]

This is prolly the best explanation of closures there is!

[@DH-qu8wh]

I finally get it! Thanks for always having concise explanations of complex concepts!

[@PabloGnesutta]

Another way of solving the for loop problem without using let, but taking advantage of closures: for (var i = 0; i < 5; i++) { const log = (i) => { console.log(i) } setTimeout(log(i), 100); } when passing the i parameter to the log function expression, it gets encapsulated and preserve it's value even after the timeout stops... the log function will encapsulate the sate within the for loop. of course in this case it is much better to use Let, since that avoids polluting the global space, but this example is an illustration of the power of closures

[@eyeofthetiger7]

I was literally just reading about Javascript closures this morning for an optimization problem at work and then you post this

[@chigozie_jesse]

I got asked this question in my interview today. I am happy i finally got some closure

[@Computeshorts]

Been trying to understand closures for ages and this video taught me them in 5min well worth it. Thanks.

[@danvilela]

Best closure explanation i have ever seen! Good job

[@samisbgh6253]

Styled-Components in 100s 🔥🔥

[@clebhosh]

i have never subscribed but watch your content. i just subscribed. you Sir are a brilliant mind

[@NFSprostreet98]

THIS IS THE BEST EXPLANATION FOR CLOSURE EVER! I WISHED THIS VIDEO WAS UPLOADED A FEW MONTHS AGO WHEN I WAS INTERVIEWING!!!! DAMNNNN

[@bader6409]

God bless you. I saw your vid on CRWD and now I look for you when I’m learning new JavaScript concepts from The Odin Project. You answer like how a perfect technical interview answer would be like and it’s helping me learn

[@suraj-ram7488]

Love the concise teaching style

[@bhaswarkashyap3069]

I got this exact question today in an interview and guess what I failed miserably :(. Thanks Fireship for enhancing my storage of knowledge :)

[@whysoasync4933]

This is the best channel on youtube!

[@currymagc]

The heap is also for reference types, like objects and arrays, even in pure functions, but I suppose for this video that would be...out of scope 🤪

[@theshermantanker7043]

Pffft

[@Paul-wy6tn]

yeah are primitives not stored in the stack in JS?

[@devanshsinghparmar]

He Deserve, trillions of subs

[@esc120]

And before 'let' was introduced, people had to use IIFE with 'i' as parameter to make it local.

[@fatgolem69]

700k Subs Let's Goooo Fireship

[@toxaq]

If you’re ever asked a question like that, tell them you don’t want to work for a company that doesn’t value people.

[@minercreepmc]

I learned closure yesterday and you release this vid. Awesome

[@wintorez]

Thank you! This video saved me in a job interview today!

[@miku4j]

These 100 second series really awaken my motivation. Thank you

[@namanagarwal690]

I don't how you do it but your content is amazing. You sometimes cover subtle details that anyone would overlook.

[@palachokrab5711]

Beautifull.. you worth 1 million subscribe

[@Pcoxproductions]

You'll get to a million its just a matter of time

[@Pcoxproductions]

When do you want to reach 1 million?

[@rylandbrough9616]

If you want to use a predefined variable without creating a closure, don’t forget about this: const add = (a, b = 3) => { return a + b; };

[@ChrisAthanas]

The best explanation of a tricky subject that I've ever seen

[@qzbnyv]

Jake Archibald and Surma do a great job of covering these subtleties of the scoping of for loops in one of their relatively recent HTTP 203 KZbin videos / podcasts. Strong recommend.

[@lelilimon]

2:02 it's 2024 and almost 3M subscribers. That equals to childhood dream fulfilled three times, congrats!

[@Krzysiekoy]

If we move the log function outside of the loop (to the top of the script), the results are the same 1. Why is the function log declared inside the loop? It this to add extra "trickiness" / "confusion" to the question? To make it more difficult? 2. What happens with the log function during every iteration? It is assigned to a const variable, so I would assume you could not reassign it? Do we create a brand new, independent function during each iteration? Also, trying to answer my own question (2) about the const. Would this be a correct way of thinking? (code is modified just for the purpose of understanding) const funcArray = []; for (let i = 0; i < 3; i++) { const log = () => console.log(i); funcArray.push(log); } const log1 = funcArray[0]; // this is an independent function that "remembers" variable i being 0; thanks to closure const log2 = funcArray[1]; // this is an independent function that "remembers" variable i being 1; thanks to closure const log3 = funcArray[2]; // this is an independent function that "remembers" variable i being 2; thanks to closure log1(); // logs 0 log2(); // logs 1 log3(); // logs 2 All if the i variables were recovered from the HEAP memory. Is the the right way of thinking about this?

[@Stoney_Eagle]

I just use let and const, saved me lots of headaches when I started learning.

[@robertrocker]

Okay, you convinced me. I clicked the f**king button

[@svs6666]

Happy to see you achieve your childhood dream of 1 million subs. Your explanations are so succinct.

[@mattunlu8916]

I like the way how you explain things in short time!

[@1J03B]

3:28 I think you missed the point in this explanation. It's not just that `i` is local to the for loop. If that were all, then the closures would still all reference the same variable and print 3, 3, 3. The point is that for loops re-declare a new `i` every iteration and copy the value from the previous iteration's `i`. That means each closure will capture its own separate variable (named `i`) that each ended with a different value.

[@ALXG]

Fireship you are awesome 👏 (as always)

[@talhaibnemahmud]

You deserve way more than a million subscribers ❤️

[@Youkakun]

1:58 This famous trick question isn't a problem for modern developers who always avoid var in favor of let. It's rather a bad signal from an interviewer still using old code.

[@wendellkenneddy8]

It's still a good closures knowledge testing

[@ChaoticNeutralMatt]

Depends on how you are writing it tbh

[@surge2284]

699k subs, Nice!

[@alexbhattaraicruze]

I recently gave an interview for a company. I was asked the same question. I wish I had watched this video before.

[@imanhosseini6908]

OrganicChemistryTutor of programming u great ser

[@lesterdelacruz5088]

Depends on how fast that code is processed. When the first timeout executes it's theoretically possible the for loop to still be executing.

[@vaibhavdesai8455]

Buy using Var also, we are able to print 0,1,2 but fr that we need to write another function, and put them inside in it, var is global variable, so they pointing to same memory location, that's the reason, so we just need to change that position, when everyone loop iterate, so for that we need closure, that definitely solve the problem.

[@13hdni37]

I love your 100 seconds videos, especially when they go beyond 😊. Keep up the good work!

[@listgaus]

Superb closure for my closure understanding. love your f**kin vid. +1 subscribed :D

[@JominJose]

This is the best video on closure

[@danielzaiser]

explaining your thought process is very good advice in general

[@seal0118]

impressive, very nice lets see paul allen's closure

[@fredhair]

If you want to learn more about stack / heap memory; learn C++. You don't get a garbage collector with C++ and you get to have full control over memory.. this is both a blessing and a curse; if you've ever had a memory leak somewhere then you know why it can be a curse. In general follow the good rules of memory allocation, RAII and use smart pointers.

[@SirDamatoIII]

Congrats again on making your childhood dream :-)

[@RobertWildling]

Aside from the fascinating way you can explain I am always amazed and amused by the images/videos to use! :-D

[@posivibez2094]

excellent explanation thank you! was so confused before

[@suyashlawand4268]

You are legend🔥❤

[@AnimusAgent]

good thing I work with Angular and typescript

[@MonsterlessonsAcademy]

Closures is the most asked questions in interviews. I had lots of interviews during last 10 years and literally from Junior to Senior on every single interview they asked me about closures. So I would recommend learn closures first before you go interview because they will ask it for sure.

[@stith_pragya]

Thank You So Much for this wonderful video............🙏🏻🙏🏻🙏🏻🙏🏻🙏🏻🙏🏻

[@melvinyit6859]

I know is a beginner question but can you do 100 second and beyond on js callback? Thanks for all your high quality video.

[@pratheeks]

2:09 I literally cried laughing

[@LuXxenatorX]

weirdo

[@evenaicantfigurethisout]

3:15 i really don't think the hoisting, albeit true, has anything to do with this difference in results between using var and let. using var only produces a single memory location for all 3 runs through the loop, overwriting the value there each time, while let produces as many location in memory as there are cycles through the loop, in this case, 3. with var, this single location is closed in which explains why all three occurrences of the log function closes in this same location in memory. whereas with let, each occurrence of the log function closes in a different location in memory, where each of the different i's were preserved as it was during that run through the loop.

[@im-singh]

This what I subscribe without thinking 👍

[@Sk8nRock]

If you ever find yourself faced with a question like this, the best thing you can do is to walk away and not waste your time with such interviews, because they might as well ask you what is the output of: "b" + "a" + +"a" + "a" and other "tricky" questions like this.

[@chektek]

"A closure is a function combined with its lexical environment." Just makes sense. As for hoisting just stop using var 😂

[@jkjoker777]

The var vs let for-loop issue has nothing to do with hoisting and everything to do with scoping. Not that simple with hoisting. Hoisting is a result of how the JS runtime processes a script, so hoisting, or reserving memory space for objects, will happen every time. Avoiding the pitfalls of hoisting by not using var is a solution to a not real problem. Hoisting is only a problem bcs JS will still attempt to run with bad code where other langs wouldn’t . The solution is to not use objects that haven’t been declared yet.

[@jkjoker777]

But yeah I’m being a nerd-ass, Totally agree with your insight that this one sentence definition of closures really does say it all

[@subhashgn1775]

Very well explained man.👌👏👏👏👏👏

[@IllIl]

That was quick! Thanks, man!

[@theidk9567]

Took me an hour to fully understand a 5 minute video, but I got it

[@MerthanMerter]

tbh 42.9% sub is pretty good. u deserve 100% tho, no doubt.

[@adnanamin3666]

"F***ing Button"! Why don't you guys subscribe?! I don't get it! 🙄 That outro fire animation was 🔥! Learned about "HOISTING" before, now this one made it stronger in knowledge. 😍

[@Nurutomo]

I didn't know this was called closure, i usually use this :)