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70 JavaScript Interview Questions

Mark Abeto on January 03, 2020

Hi Guys Good Day and a Happy New Year 🎆🎆🎆! This is a long one, so bear with me for a second or an hour. In every answer for every questi...

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Sebastian Vargr • Jan 3 '20 • Edited

It's funny how interviews often focus on memorization these kind of things, which are the easiest things to look up when you need them.

Half of these things i encountered and used without' having to know the right name for what i was doing, so whats the value?

Very rarely do i get asked about my thoughts on MVC, component orchestration, function vs Class based programming, what over-engineering is, interesting problems i have solved and how, or just maybe how i handle differences of opinion in PR's.

Comparatively speaking, having those kinds of discussions seem much more relevant than whether or not i have memorized all the latest programmer pop culture and random terms one might use once every blue moon.

That being said.

This is a nice list to memorize if you wanna play the default interview game.
Which i might in the future, so thanks! :)

Mark Abeto • Jan 3 '20

I feel you man but most of the time memorizing or knowing this things are not about answering an Interview problem or problems but solving a problem you are having with JavaScript and btw Glad you like it!!!

Patrik Kiss • Jan 3 '20 • Edited

Great article, well done! 👏

If I was asked any of these at a job interview, I couldn't answer a single one 👍

Mark Abeto • Jan 3 '20

Thanks Kiss 😁

Alek Vila • May 1 '20

Regarding Question 24, I want to really understand "this" so I tested each example in the Console. I found a different result than what you shared for this line...

myFavoriteObj.guessThis();

You wrote that it logs the window object; however, I'm seeing that in normal mode it logs nothing and in strict mode it throws an error. Maybe it is working differently on your browser? I'm using Chrome 81 on Mac.

By the way, I am enjoying studying this post. Thank you!

Mark Abeto • May 2 '20

Thanks, for finding my mistake. I edited it just now. No problem man, glad you like it.

Alek Vila • May 2 '20

Believe it or not, I'm still going through your list and taking notes. This is a very cool post.

By the way, I've cleared my browser cache the graphic is the same. Maybe it's cached on the server side?

Mark Abeto • May 2 '20

I've changed the object.

It looks like this now.

 const myFavoriteObj = {
     guessThis(){
        function getThis(){
          console.log(this);
        }
        getThis();
     },
     name: 'Marko Polo',
     thisIsAnnoying(callback){
       callback();
     }
   };

Thanks

Alek Vila • May 2 '20

Ah, I see the change now. You removed .name from where it used to say console.log(this.name); Thank you!

Mark Abeto • May 2 '20

Your welcome. Thanks, again for finding my mistake.

Kuncheria Kuruvilla • Oct 15 '20 • Edited

Hey , even now i think there is a problem.
Shouldn't it be
this.name = "Ford Ranger";

rather than
var name = "Ford Ranger";

for question number 24 ?

Mehdi Vasigh • Jan 3 '20

Nice work! Most of these are pretty good. If you're a hiring manager reading this though, I'd caution against asking questions like "is using the + or unary plus operator the fastest way in converting a string to a number?" because it really doesn't matter and you shouldn't make hiring decisions based on one's knowledge of that.

Bartosz Cytrowski • Jan 6 '20

Great set of questions :) I want to clarify one thing though: "What does the && operator do?" - It does not return last "truthy" or "falsy" value. It returns the last value it has to calculate to determine the truthiness of the whole expression, eg. 0 || 0 || 0 - will return 0 and 1 && 1 && 1 - will return 1. So the statement in the article is not 100% accurate :) Can't wait to see the update ;)

Raashish Aggarwal • Dec 6 '22

G.O.A.T

Alecc • Jun 9 '20

Mark, you did a grate job. I am finding this list very useful. In a way, it even helped me to make the list of questions for our small organisation(I have even sent it to some of our HRs))) So, believe it or not ..It is used now in practical way in the eastern EU))))If being completely serious, I would recommend more Typescript questions. I know this is the JavaScript Int. Q. I really thing < that using of typescript is growing tremendously. Anyway , great Job, Mark. Thank you.

Mark Abeto • Jun 9 '20 • Edited

Hi Alecc, I'm really glad that you like it. That's a nice suggestion. I've been using typescript for over two years now and it's really great it makes your JavaScript code more readable and maintainable. I'll make some questions in the future. Thanks and God bless.

Alecc • Jun 9 '20

Yes,m Mark, that is amazing how fast the Typescript "cases" grow. I mean , two years ago I have the project, where I had one TS file and I do not even remember why. But now. My team just received a source , There were no JS file extension. TS, TSx). I am, actually, just starting with Typescript.

bgauryy • Apr 11 '20

Great !

Personally, I think that good interviews should be focused on data flows and performance optimization, and asking about certain API's should be only for choosing the right question as an interviewer.
Also, interviewers should understand the difference between someone who is not knowing something, to someone who is not capable of learning something new.

Good luck for everyone :)

kotik-spb • Apr 9 '20

Hi! Many thanks for the article, it's really very useful)))
And one thing in Question 31:
const result = strs.reduce((acc, currentStr) => acc + str, "");
I suppose you mean "currentStr" instead of "str" ;)

Mark Abeto • Apr 9 '20

I edited it just now. Thanks.

DrOutback • Jan 4 '20

“let x = 5;

x = (x++ , x = addFive(x), x *= 2, x -= 5, x += 10);

function addFive(num) {
return num + 5;
}
If you log the value of x it would be 27. First, we increment the value of x it would be 6, then we invoke the function incrementBy5(6) and pass the 6 as a parameter”

Do you mean addFive(6)?

I’m new to js, so I’m probably wrong.

Mark Abeto • Jan 4 '20

Thanks for telling me this little problem 😁

DrOutback • Jan 4 '20

Thanks for writing the article. I’m enjoying it.

Also,

reduce.apply(obj1, [1, 2, 3, 4, 5]); // returns 15
reduce.apply(obj2, 1, 2, 3, 4, 5); // returns 15

Did you mean reduce.call obj2, 1, 2, 3, 4, 5); ?

Mark Abeto • Jan 4 '20

Thanks again 😁

Udit Kaushik • Jul 31 '20

Great article. Very informative. For the question 34, 35 and 36. The implementation might not be quite accurate. You might wanna re-check. For example, to map(), we just pass the callback function. While in your implementation above, you are passing both array and callback to the map(). The array will be available as this inside the map(). Please do correct me if i am wrong.

Mohammed Asker • Jan 3 '20

I think this is the longest read time I ever see on the internet. Thank you for the article. I'd bookmark for future references.

Habdul Hazeez • Jan 3 '20 • Edited

Thanks guys for reading this post.

Thank you for the effort.

EDIT: In Question 5 you wrote:

Image if we have an HTML structure like this.

Did you intend to use the word Imagine instead of Image?

Amir.H Ebrahimi • Jul 22 '20

actually Implement the Array.prototype.map is not too precise for what really wanted to be implemented.

[,1,2,3,].map(console.log) 
//  [empty, undefined, undefined, undefined]

map([,1,2,3,], console.log) 
// [undefined, undefined, undefined, undefined]

Kairat Talantbekov • Aug 4 '20

Great content! My name is Kairat, I'm a freelance self tough js developer. I'm planning to get a job as a Front end developer instead of freelancing.

Question: This article is very helpful for my theoretical knowledge. I'm just curious will I use these terms in my daily job? Seems like preparation for the exam and I will not use this knowledge in work. Or use very rarely.

Alecc • Sep 3 '21

Thank you a lot. This article helped me once. You have a good style! What about react hooks? especially custom ones? I would like to read one written by You. thank you. And as we discussed already the typescript for react. Because, I think - TS and TSX are in great demand now.

Phan Huy Hoàng • Jan 3 '20 • Edited

I think there're a mistake here...!

function myFunc() {
  let a = b;
  a = b = 0;
}
myFunc();

Isn't this invalid syntax...?
It should be:

let a, b;

Mark Abeto • Jan 3 '20

Thanks man, I totally miss that one, hahaha

Shin, SJ • Jan 6 '20

Great article!
Can I translate it into Korean and post it on my blog?
I'll clarify the source.

Mark Abeto • Jan 6 '20

No Problem man and Thanks 😃!

YogeshDharya • Mar 16 '23

Hey Mark ! Thank u so much for the wonderful blog ! I wanted to share that as of 17th March 2023 00:10 am IST my comment is the 100th one on this blog . Keep enlightening us with your knowledge like this in the future .

overFlow • Jul 24 '23

This a beast. Thank you.
The longest. I did not expect the answers at the bottom after the questions.You put in a lot of work here. bigUps.
I am gonna go through every aspect of and concept of the questions you put up. Thank you.
This should help me rank up my abilities.

Maxim • Jan 4 '20

Question #26

var li = document.querySelector('.list-group > li');

Probably here should be querySelectorAll instead of querySelector🙂

Mark Abeto • Jan 4 '20

Thanks 😁, my bad.

prashanth • Feb 12 '22 • Edited

Hoisting

let and const are hoisted in a different scope but they are in the temporal dead zone. That's why they throw the error 'let variable cannot be accessed before declaration'

The DotNET Weekly • Jan 3 '20

Nice list, my favorite one is what is hoisting :)

Mark Abeto • Jan 3 '20

Thanks, yea Hoisting is pretty fun to learn

Comment deleted

Mark Abeto • Jan 4 '20

Thanks man! Glad you like it 😁

Daniel • Jan 5 '20 • Edited

Well done, it is helpful

Mark Abeto • Jan 5 '20

Thanks 😁

Jean-Sébastien Goupil • Jan 7 '20

I like this article, quite the refresher.
However, I think people forget quite a bit about the method bind that can do partially applied function! And it's not mentioned here.

Carlos Augusto de Medeir Filho • Jan 7 '20

Thanks!

Jeff Davidson • Jan 7 '20 • Edited

Thanks Mark - great cheat sheet! Btw, what does your dog think of the article?

Ankit Kanojia • Jan 3 '20

Great article. Great article !!!!

Mark Abeto • Jan 3 '20

Thanks 😁😁😁

A. Gharibi • Jan 3 '20

Awesome, well done mate!

Mark Abeto • Jan 3 '20

Thanks mate! 😁

𝐍𝐚𝐭𝐚𝐥𝐢𝐞 𝐝𝐞 𝐖𝐞𝐞𝐫𝐝 • Jan 3 '20

I need to brush up on my JS so this will be super helpful for me! Thank you!

Mark Abeto • Jan 3 '20

Glad you like it 😁

JP • Jan 4 '20

Very detailed answers

Mark Abeto • Jan 4 '20

Thanks man!

Pavel Potapkin • Jan 4 '20

Thank you for the great article. It must be mistake in 1 question when you provide example of casting null and undefined to Boolean, you cast only null in both cases.

Mark Abeto • Jan 4 '20

Thanks man for pointing that one out.

Mohamed ELIDRISSI • Jan 3 '20

This is a gem, thanks for writing this!

Carlos Zinato • Jan 3 '20

Awesome article! I think that there is an error on the 3rd example of the Set object comments. It should say K and Z instead of "a". Look like a copy paste from the 4th example

Mark Abeto • Jan 3 '20

Thanks man for pointing that one out hahaha

mmutinda • Jan 4 '20

This is great stuff

Mark Abeto • Jan 4 '20

Thanks 😃!

Paul Facklam • Jan 3 '20

Thank you for the time and the passion you have spent for this article. It is awesome!

Mark Abeto • Jan 3 '20 • Edited

Thanks man, I just put all the things that I learned for the past two years, still I feel dumb with JavaScript

Hozefa • Jan 3 '20

Great article. Well written 👍

Mark Abeto • Jan 3 '20

Thanks

Matze Falk • Jan 5 '20

Great work!

Mark Abeto • Jan 5 '20

Thanks! Glad you like it 😁

Thomas • Feb 21 '20 • Edited

Great article!!

Can I translate this awesome article into a Japanese version to share if you let me do?

Thank you!

Mark Abeto • Feb 21 '20

No problem man! And Thanks.

Islam Sayed • Feb 5 '20

Great list! Thank you for the efforts and time you took to collect them.💙💙

Osman YILMAZ • Feb 19 '20

Thank you :)

Freddie J • May 1 '20

hi sir salamat sa post, timing nag hahanap pa naman ako ng work ngayon, bale unang web dev job ko

capscode • Jun 6 '21

what an amazing post...

Mark Abeto • Jan 3 '20

No problem man, and btw Thanks 😁

chmiguel • May 8 '21

Did you guys notice how the closures concept is working in the "Implement a memoization helper function" question?
BTW, nice set of questions.

Sunil Kumar Singh • Feb 23 '20

Thanks Mark for this beautiful compilation!

Mitesh Kamat • Aug 11 '20

Biggest takeaway for me is the memoization technique. This has surely prompted me to make most of this feature. Thanks buddy for sharing this !!!

Abhishek Sharma • Jun 17 '20

This was so helpful! Thank you so much!

thuanpv1 • May 29 '20

for the case 59:
let obj = { prop: undefined }
the third method will not work with this object.

Mark Abeto • May 29 '20

Hi, yes It will not work because you're setting the value of that property as 'undefined ' value.

Giannis Saperas • Nov 17 '20

Good job!!! 👏

Ganesh Singh • Nov 24 '20

Bro thanks for this post it really helped to gain knowledge in Javascript :)

J.D Nicholls • Jan 7 '21

Excellent article, thanks for sharing! <3

Ankur Goyal • Jul 29 '21

I have read from youtube : namaste js and mdn docs too. Let and const are hoisted, but they console to error(which is different thing). Imp thing is : let and const are HOISTED too