Questions like this don't tell much about a candidate's ability to design, write, and test maintainable code, however they do give information on a candidate's specific area of knowledge.

These questions are useful if you are looking for an experienced X programmer who can immediately start being productive in an X code base. While the original problem is definitely a trick question, I don't believe that any of the individual pieces of knowledge required are merely "bits of trivia" for an experienced Javascript developer.

It's the interviewer's responsibility to know what areas of knowledge are required for a role, and how important it is that a candidate is familiar with them. Memorizing the intricacies of "==" vs "===" may not be important, but I'd expect an experienced Javascript developer to know something about it.

What about ability to test? Design? Function as a team member?

Those skills are much harder to test for so most companies ignore them during the interview process and instead focus on attributes which can be tested for, and accept that the process is not 100% reliable.