Writing a utility function signature and moving on is a solid interview tip. It's one of the tips I give during an interview if the candidate isn't sure how to do something. I tell them to think at a higher level, create a function signature, and leave the body blank.
There's a difference between what you do infrequently at work, and what comes up during an interview. It's all about time during an interview. I would never punish somebody for not knowing the advanced stuff, or even forgetting the basics, it happens. However, every slip costs valuable time. I find that knowing this type of info is what is helping candidates make it through interviews -- since they spend all of their time thinking at a high-level on the important parts of the problem.
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