No doubt there are situations where hugely optimized code is necessary. An interview isn't one of them unless you're hiring someone to write exactly that code. Most companies aren't.
knowing the difference, and consciously deciding that it doesn't matter for a given task is often a strong positive signal. hopefully a good interviewer will not actually ask you this question if it doesnt matter for the given task.
If they mentioned the fact that React 18 batches updates in concurrent mode which would greatly improve the responsiveness of a dashboard app that's updating lots of small components frequently, or if they suggested the graphs might be better written using GLSL shaders to move the rendering to the GPU so JS only needs to update a uniform array, then I'd pay attention. Potential new ways to approach a problem or changes to architecture to avoid the problem entirely rather than micro-optimizations are much more interesting signals about someone's deeper knowledge of building things.
The question you're asking involves 10k * 3 * 30 = 900,000 data points. If a candidate wants to simply loop over all of them multiple times a second, I'd say that's a much bigger problem.
unless the task is to animate them in a rAF loop (which runs at ~60Hz), right? the task matters. in some cases the question is stupid & irrelevant, and in the other case it means getting the job done, or not.
the difference between the brain-dead way and the fast way is literally 30x (53ms vs 1.8ms on my machine): https://jsfiddle.net/3ho6f5k1/
For an array of 5-10 items the performance hit of foreach is worth it for readability. Maybe an interesting discussion could occur about when to sacrifice readability for performance but you won’t get that by grilling people about minutiae.
As always it depends, maybe that component is being called 500 times on the page making it actually 5,000 invocations.
I'd FULLY expect a candidate to understand the difference and would ask in a whiteboard session. There is a lot of bloated slow javascript libraries out there and they almost always stem from the fact that most devs don't realize how much overhead function calls introduce.
For the interview, there is no discussion necessary, the correct answer is simply:
"I used a foreach for readability, because performance doesn't matter in this case because X."
or
"I used a for because performance matters in this case because Y"
End of story.
100% agree. my point is more about OP's example being used to prove that dumb questions get asked. if it's being asked by a good interviewer, it's usually relevant.
100% trivia.
