The first example of a junior developer making an application. They are full of pride, having taught themselves one technology stack to produce one application. Unfortunately she leaves out the approach. With assumptions, this junior developer is looking for validation of their achievement from a senior developer working at the same company. They are not peers, they are not friends.
If the first thing out of the senior developers mouth was a straight faced "is it responsive" with no sign of sarcasm then they don't want to be your friend, mentor, and only care about the number of bugs you create or fix. Great, you've learned as a junior developer that this senior developer is not on your side, and you should seek validation else where. From other junior developers perhaps, or ask another senior developer for feedback while first informing them you put a lot of work into it and what kind of feedback you want.
The senior developer taught themselves that same stack, and a dozen other stacks. They did it for free, it doesn't matter that you did, you've made 60 pages of code they've wrote several libraries of congress's worth of bugs. Your success doesn't mean anything to them because there is nothing special in it, and they are not your friend. There are other junior developers on the team that also taught themselves a stack, maybe two. You won't find admiration for your achievements from within the group. If you want a mentor then approach the senior developers with that, say you want to learn more, want to shadow them, peer program perhaps. However don't expect it from them, no one is entitled to it so you're asking them to put in care and effort. Even asking someone to review your project has a cost, it takes time, caring, and effort to do it right.
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The interviewer asking a question like that was attempting to engage you in a critical problem solving thought process. You don't know, that doesn't matter, how would you find out, what steps would you take, why would there be alternatives.
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The feedback the junior developer received isn't skeptical it is dismissive. Being dismissive isn't nice. It isn't snobbery, you get to be senior developer by being a better programmer, by being better at just that stack, by being there first, by luck or by time.
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If you worked at a Michelin star restaurant as a sous chef, and on the side you worked for a month perfecting a veal dish, then presented it to your head chef. You bet your ass they would be critical of it. If you presented it to your friend, they would shower you in praise and enjoy eating it with you.
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On hiring, yes there are problems, there are cliques, existing developers might feel threatened, but those don't just apply to junior developers.
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All that being said I love a developer who takes the initiative to teach themselves a stack on their own time. That is awesome, keep doing it, don't stop, find other sources of camaraderie and mentorship.