Don't see how you could consider the question they posed to be trivia for someone who knows javascript? Abuse of truthy evaluation is very rampant. All the JS codebases I've worked on are filled with `if (!myArray.length)` and etc. I would be very sad if anyone with a year of JS experience couldn't get this problem.
By trivia, I mean it can be thoroughly explained in a paragraph or two.
Separately, if it's the kind of thing that can be learned in less than a year of experience, it's not helpful for determining whether someone should be hired as a senior engineer.
Precision is different from recall.
You're asking trivia. I'd rather ask questions that can't be studied for, only practiced through years of experience.
I also sometimes hire people who don't know JavaScript, yet.