When hiring front-end developers, there's a lot of people out there who fall into the "web designer guy who learned some coding along the way." They aren't engineers--and while they likely could study and learn, they really haven't. To me this is a really simple screening question; an engineer who understands closures, scope, and variable references will immediately say, "yeah, I see what's happening here..." and answer correctly.

Having conducted likely close to a hundred UI Engineer interviews, definitely over a hundred phone screens, and seen what feels like thousands of resumes (probably hyperbole on that last one), I can assure you that many candidates I've seen couldn't answer this question.

It could have been the first question of a series. This would probably make a pretty good screen question, at least.