JavaScript Interview questions everyone gets wrong

In this video I’m going over MORE JavaScript interview questions that you might encounter in your software developer interviews. This video will feature questions that really test your understanding of common pitfalls and weird parts of JavaScript, such as “this” keyword and scope chain.
Click here for a playlist of all of my JavaScript Interview and tutorial questions: bit.ly/3Gr54ce
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[@artemsapegin]

The correct answer to all of these: it won’t pass code review.

[@josem3933]

Have an interview tomorrow and have been binge watching all your videos

[@catherinelijs]

Thank you! I hope you got it!

[@LucianoClassicalGuitar]

How did it go lol

[@martapfahl940]

Do they really still ask questions including var ? I literally NEVER use var

[@6-dpegasus425]

Same I only ever use let or const

[@Latidoz_music]

Yea

[@triggerticker]

It's crazy to me that after 4 years of uni, now that I am lookinf for a new job, I am looking at this vids to prepare for interview. Most concepts aren't really used at all in real life and thus things get forgotten. Makes you question the real use of these questions xD Anyways thanks for the great vid

[@catherinelijs]

Thank you and you got this! Were you able to find a job in the past 2 months?

[@meetkass]

Binging all of these videos now that I am expecting some tech interviews. This is also great insight! I wish I was looking these up BEFORE when I was giving interviews. Thanks!

[@keifer7813]

The more I watch these vids, the more I cringe at my prior confidence in JavaScript

[@srirampaga2249]

At 3:27, the settimeout example, it prints 5, 5 times. I have a question, how can it print 5 when the for Condition is false since i=5, but condition says i

[@nycsuperagent]

2:49 it’s because setTimeout runs after the for loop is completed. At that point “i” is 5. In the desired implementation “i” still goes to 5 and satisfies the for loop’s condition which then prints out the values of i WHILE i is less than 5

[@costcomembership4617]

How common are questions regarding var vs let and arrow funcs vs normal funcs? I started learning JS self taught around June of 2022 so almost everything I’ve learned and built has been with the latest syntax. Would you recommend learning more about pre-es6 topics solely to prep for interviews?

[@catherinelijs]

Your main focus should be ES6 and modern JS and frameworks. That’s where we’re all headed. For older tech companies you can look at pre-es6 topics but that’s a lower priority imo

[@MehdiGlz]

Hey Costco Membership, I know this is an old comment, but for anyone out there: ime this is important to know for interviews. Last year I did a ton of interviews and these questions came up more than once, both at small companies with old stacks and big international consulting companies. So it's good to study up before interviews

[@alexandersnider734]

These are definitely good academic things to know to understand some of Javascript's history and design philosophy, but any company actually asking me this many questions about my understanding of "this" and "var" would make my really wary of moving forward. This would indicate to me that first, the company might still be using var in practice which for all intents and purposes is almost always the worst option to use for instantiation, and that second the company might be using JS for OOP, which would indicate to me a big mistake, as JS really shines now with a functional focus and there are much better languages to use for OOP. I will concede though its important to understand the context of the arrow function vs a functional declaration. I would say much more important and likely questions to get in a modern JS interview, relate to especially closures, first-class functions, and might even touch on typing with TS.

[@TuanNguyen-ed9rb]

why don't you think that the company is using let in their code, and they want to find a candidate that know the difference between var and let so they don't screw their code by using var everywhere?

[@Gio-m]

Wow, super straightforward thanks Catherine!

[@catherinelijs]

Thanks!

[@TheEletricboy]

The var keyword has so little uses nowadays that its only function is to test interviewees really. Let and Const are much more reliable when reading and executing code. There's still uses obviously but it's niche that I can almost guarantee you could go 5y without even seeing a var keyword at work (or more even).

[@KAZVorpal]

That's why you don't commit the obscenity of putting the curly braces on the next line. They go on the same line as the statement. Always.

[@madimakes]

canadian confirmed, lol. also...on the first one, i've been doing python the past 2 months and i'm going to be ruined when i get back to JS...now to watch the rest of this video

[@TokyoXtreme]

Me in 2024: "Did var ever exist in JavaScript? I've never seen it in any codebase."

[@lunchboxUFX]

in the last question, b++ of inner() is incrementing var b = 2 of outer(), correct? (This is a closure, yes?)

[@petrkavulok9753]

Hi Harold, it is not. in this case it appears as the line 5 is ignored completely and the console logs only line 6 in an unchanged form. To get a better understanding, look up JavaScript Hoisting. Cheers

[@myPrzeslaw]

@@petrkavulok9753 Well, line 5 isn't ignored, it assigns NaN value to local b var (as NaN is valid result of incrementing undefined in js). But that value isn't logged and is overwritten in line 6 by the value 3.

[@myPrzeslaw]

No. It would be a closure if line 6 did't existed (or existed without 'var' keyword). Once you declare var x anywhere in the function (even in its last line) THE WHOLE SCOPE OF FUNCTION don't have access to outer scope var x at ANY LINE (even BEFORE initialization).

[@anuj1994k]

I like it somuch.... kindly do more vdo based questions on javascript function

[@oghenefejiroprinceimene6090]

This was really nice

[@haidermansoor4760]

I specifically search for your videos lol. they have helped me previously in my interviews alot. Thanks alot Chatherine 🙌

[@catherinelijs]

Thank you! I have lots more content ready so I'm really hoping it will be helpful!

[@Phearzmeh1]

Thanks i never really understood arrow functions but this helps

[@catherinelijs]

Happy to help!

[@cabbage-dev]

Some of these questions appear to be made before linters existed and its a wonder they keep being asked

[@aten488]

Thank you for this video. Much needed :) .

[@stewart6395]

Really love these old school questions ❤ Everyone should know this

[@TamPham-oe2xc]

You better than my teacher 😂

[@FelipeBasilio-s3o]

Awesome video!

[@catherinelijs]

Thanks Felipe!

[@margaret8114]

So helpful!

[@catherinelijs]

Thanks so much!

[@crowtek2503]

Thank you for the tips 😊

[@MengLinMaker]

I prefer to use "const" if possible, then "let" and avoid "var" due to the strange behaviours it causes

[@myPrzeslaw]

And it's great and modern approach but if company you applying to also maintains some pre ES6 code it is valid approach to examine candidates it that regard.

[@thisisYirou]

The second example, console.log(x) would actually return 21 instead of undefined. Yes variable x is hoisted, but code runs from line 1 and below. So if girl() is invoked, that means at that point x is initialized with the value of 21. (if you try in your browser)

[@tunacant9106]

Nah, it would be undefined, because inside the girl function, x is undefined.

[@vivekcse1]

@@tunacant9106 On Mac, it gave me 21. Let me try it on PC.

[@vivekcse1]

On windows also it's giving me 21. So this girl's premise is false. This is like a closure. But the higher order function being non-existent, global scope is taking over.

[@agmonblynkos]

​@@vivekcse1the real reason it logs 21 is because var is also adding the variable to the window, which is where the console.log takes the result from. Try with let or const, it will log undefined

[@myPrzeslaw]

It's not. JS hoisting is performed before running the code. It means interpreter firstly moves all the local vars declarations to the top of the scope (but leaves initializations in its original line). The only way var x still has its value from higher scope it has to not be declared ANYWHERE inside girl-scope.

[@islamfahmy9784]

Thank you so much

[@ohyr8948]

Can I Wait for your INTERESTING front-end web devlopment course?

[@maksworldwebboy1119]

The answer for the last question is Nan ;)

[@myPrzeslaw]

No, NaN indeed is the result of incrementation in line 5 but it is not console logged. Console logs the result of assignment from line 6.

[@adrianibarra9589]

LOL i have 3 years of experience and the first one got me haha

[@downanddirtytruth]

Well. I'd have failed... I haven't invested much time into var. It's throwing me off quite badly.

[@moneyDev1111]

const happy = new Year('🌟')

[@catherinelijs]

Same to you! All the best in 2023

[@lqueryvg]

1. Don't use `var`. Ever. 2. If an interviewer obsesses about code behaviour when using `var`, it's a red flag; don't take the job.

[@k-yo]

Honestly, these kinds of trick questions are really dumb. It tests a subset of edge-cases you wont usually stumble upon on real projects. I hate this.

[@computeraidedyami]

You are like a god

[@gourabsarker9552]

Mam how much do you earn as a software engineer? Plz reply. Thanks a lot.

[@anasouardini]

If only JS and browsers didn't suck, interviewers wouldn't have the chance to ask us these useless stupid questions. In the question about `setTimeout`, it's not because it's asyncronous but because browsers copy references instead of values.

[@myPrzeslaw]

I don't think working as a JS dev is anyhow mandatory. But if you intend to do, it's pretty important to understand how do closures work.

[@ziadomar5057]

Thank you and you are so beautiful

[@betatester03]

Javascript is such an obnoxious language.

[@nittamonisudeepchand5325]

😍

[@yusufdur3240]

I'll puke soonly because of this industries nonsenses. I failed the interview before I see this. They didn't even test my algorithm knowledge. Instead they prefered to ask these type of questions and stricly wait the answers.

[@CaliburPANDAs]

Welcome back Catherine :) do you has twitter?

[@catherinelijs]

Thanks! I’m setting up new social media accounts and will post the links on this channel soon

[@dbarless]

always. add. semicolons.

[@КатяСаввова]

always; add; semicolons;

[@josecastroesq]

This video is gold. 🪙 Thank you for your content!

[@catherinelijs]

Thank you!