I'll try to turn the table on you. Pick some particular technology that you don't hold in your head. three.js, Angular, Bootstrap, canvas, WebRT - whatever. I'm sure among those we'll find some details you don't have in your head right now and you will look up some APIs/details. Now I'll structure my interview around those topics which I happen to hold in my head since we use them all the time.

I am not a front-end web developer at all. I play with some front-end stuff occasionally. But if I had to work in that area I'm sure I could pick it up very quickly and become very effective. I think you need to differentiate between things people can read up on a day and catch up and things that are more fundamental to how they think and learn.