IMO, it's because they're abstract enough to be completely opaque to someone who doesn't understand the concept of them, but ridiculously simple once you do. A

IMO, it's because they're abstract enough to be completely opaque to someone who doesn't understand the concept of them, but ridiculously simple once you do. Also, if you're doing a lot of front-end/javascript work you're bound to have run into them, even if you don't realize what they actually are at the time. For me it was just one of those "a-ha" moments where you realize there's an easier/better way to do something. And if the interviewee doesn't know about them before the interview, hopefully s/he has learned something from the interview (I learn a lot from interviews just from looking up all the stuff I didn't know).