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During a very technical phone interview, I was asked:

Tell me something about JavaScript that I don't know.

The interviewer has a lot of proven experience in the technology - more than expected for the position I was applying to.

I couldn't think of anything, as I assumed he'd always know it. I replied by saying:

I know you have a lot of experience, I can't think of anything you wouldn't know.

What would the interviewer be trying to assess here? What would be an OK response?

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    The answer is "I do not know what you know" – Ed Heal Mar 10 '18 at 10:30
  • What was the interviewer's reaction to your response? – Masked Man Mar 10 '18 at 12:41
  • @MaskedMan - This was a while ago and over the phone so not very easy to tell. I believe he just "OK"d and moved on. To be fair it felt like he was challenging me in order to prove himself. He has his own blog where he posts about that specific technology. I knew this beforehand which might have affected my response. Or I'm just overthinking :) – Salomao Rodrigues Mar 10 '18 at 16:36
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    Maybe he asks that question to every candidate just to feel good about himself, either because everyone tells him what he already knows or gives him a cheesy response like you did. Either way, it is a stupid question. – Masked Man Mar 10 '18 at 16:39
  • next time learn how threading works in javascript. most people don't know. hell, I don't know. – bharal Mar 10 '18 at 18:14
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They're presumably trying to find out how much you know about the language.

Their thinking is probably that, if you know a lot about something, chances are you've picked up some obscure or uncommon bit of knowledge about it along the way.

"I can't think of anything" won't look good. I'd suggest, if nothing else, mentioning anything that many people might not know, even if you suspect or assume they already know what you're thinking of (which you can note, to avoid coming across as condescending):

Many people might not know X, Y or Z, but someone with a lot of experience would probably already know that.

If you don't know how much experience they have, you can also just opt for saying as much (or stick to the above response):

While I don't know what you specifically don't know, many people might not know X, Y or Z.

If you don't know anything obscure, you can probably use roughly the same response with any sort of "advanced" knowledge. Although you should be careful with that, in that they might consider it something everyone should know, so you could come across as ignorant. I might recommend adding:

But that's just more of an advanced concept rather than anything obscure.

Lacking either of the above, I'd recommend trying to throw something at least somewhat positive into your response:

I'm fairly [or "very", if justifiable] comfortable with the basic concepts in JavaScript, but I unfortunately can't think of any obscure factoids at the moment.

You may expect a response like the above to be followed up by them throwing a few concepts at you to see how comfortable you really are with the "basics" (or whether you might be familiar with some lesser known concepts).

Whether or not it's a good question is left up to opinion.

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    I voted this question up, but don't be fake with an interviewer by saying something along the lines of "but someone like yourself...". That is blatant pandering and not likely to score you any points. You don't know the interviewer, and maybe they know nothing about JavaScript, but they are recording your answers for someone else. – Stacy Mar 11 '18 at 4:04
  • @Stacy Why don't you post your own answer? – Masked Man Mar 11 '18 at 14:31
  • @Stacy I did mention that the interviewer had lots of proven experience, but agree that it's something we shouldn't do as a general rule. In a reply to a comment, I also mentioned he runs a blog where he posts about the technology in question. – Salomao Rodrigues Mar 13 '18 at 20:52
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It's a bit of a silly question really, how are you meant to know what they don't know? I suppose they're hoping you may have come across some obscure quirk of the language along your work experience to show some depth of knowledge.

In this case I would try to think of something that a person with only superficial experience wouldn't know, maybe like hoisting in Javascript:

x = 5;

var x;

(Is perfectly valid because the declaration of x gets hoisted above its usage).

  • Hoisting did come to my mind but it would have been more of an offense to him, as it's something you'd expect a Junior/Mid level to have heard of. I do understand the point though. – Salomao Rodrigues Mar 10 '18 at 10:50
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    I might recommend removing the JavaScript specifics, as a discussion about obscure JS features is not really within scope of this site. (There's always a JS chatroom, where such a discussion would presumably be acceptable) – Dukeling Mar 10 '18 at 11:11
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    @Dukeling I think it should be left, as an example to the querent of the sort of response the answerer thinks is a good idea. – the dark wanderer Mar 10 '18 at 18:06
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    @Dukeling, I also think the example should be left. Javascript is not a programming language I work with, but even I was able to understand that example and the silliness of knowing something obscure like that. – Stephan Branczyk Mar 10 '18 at 21:18
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It sounds like a good question to check your knowledge and attitude at the same time.

Let's look at possible answers:

  1. Yes, sure, I know about this bug xyz which was published on the abc website
  2. Yes, sure, I published this bug xyz on the abc website
  3. I recently discovered xyz, but probably you know about that already
  4. I don't know anything you don't know
  5. Yes, I know many things you wouldn't know
  6. I know you have a lot of experience, I can't think of anything you wouldn't know.

If you answer 1 you basically tell him he does not pay attention to known bugs, not a good answer. If you tell him 2 he will likely be impressed (at least if it was something unusual). 3 sounds good because we all "discover" from time to time strange things. But more knowledgeable people probably will know many of these things already. 4 sounds like you have very little knowledge because you can't even imagine there might be something he does not know. 5 sounds like you have an attitude problem. And 6, your answer sounds very reasonable to me. You know some special things (you have knowledge) but you also recognize that the guy you talk to is an expert.

Summary: I think your answer is a good answer. Maybe you don't get any "extra points" for it. But you also don't lose any evaluation points because of a bad attitude answer.

One more detail about answer option 3: I would be very careful with that. If you tell the guy something unusual that he also maybe just discovered he will likely be impressed. If you tell him something which maybe 50% of all programmers know then you show him that you are 50/50 experienced. If you tell him that you just discovered what most programmers know after a week then you better shouldn't have answered like that...

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There is no objectively correct answer to questions like these - often they are asked to see how you react to being asked for an answer you can't possibly provide.

When you interview, you should have some idea of how you want to present yourself to the company you're interviewing with, and then answer every question with that in mind. For example, did you write something like "fast learner" in your cover letter? So think about how someone who is a "fast learner" would answer the question. Usually learners ask a lot of questions, so you might ask for clarification. For example, "Do you mean something that only very experienced developers know, or just something weird or obscure?" If you want to show them that you are very analytical, you might say something along the lines of what was suggested in a comment: "I don't know how to answer that because I don't know what you don't know."

Always think about what specific impression you want the interviewer to have of you after the interview. It should be something honest (don't expect to get away with pretending you're something that you're not) and focused (you have many good qualities, focus on the one or two things that are important for this interview). Often we make interviews harder for ourselves than they need to be because we are trying to give the interviewer what they want instead of showing them our strengths.

The best answer to any question in an interview depends on what want the interviewer to remember about you.

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You can answer by turning your answer into a question. I'd do it like this:

Tell him that you expect interviews to a great extent consist of micro-experiments, trial runs at observing what you'd do in a typical work situation, and assessing you for it.

Since the requested activity involves teaching, say you'd conclude that, at work, you'd be expected to teach.

Now recap what your job description appeared to have been just until this moment, immediately prior this question was asked. Determine whether or not any elements of teaching appeared to be in it.

If teaching was already required, agree to the task, but say the subject selection filter "subject to be unknown to the interviewer" can't be resolved due to insufficient information. Say the lesson subject can't be chosen unless more information is provided. Ask what a subject may be, he can only name but not explain, and that you'll hopefully proceed to explain. If he says he's ignorant of many subjects he cannot name, mention subjects by name until he gets to hear one he does not know.

If teaching was not already required, ask whether this question means teaching is actually required (it might have been omitted earlier), or if it's a 'test' question, a 'character' question, 'reaction' question, exploratory, maybe seemingly irrelevant... what is its purpose? Ask him to explain the rationale behind the question, how it will be treated, and how anyone can benefit. Tell him you suspect the question may be part of a bigger problem, or context, and you'd like to know more about this context before you give this question a treatment. Tell him that you won't treat (meaning answer) this question without context because your first reaction would be to comprehend this motivation or context.

Appear to be logical, thorough, rational, constructive, but don't rush at constructing. Do not appear to actively participate in situations you don't have a good understanding of. If you're asked to participate, seek to further your understanding before you decide when you've got enough to engage.

  • I would expect anybody working for me to be able to teach, and it isnt (like many things) called out explicitly. – Jon Custer Mar 12 '18 at 19:10

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