Senior level JavaScript I would imagine it to be entirely about frameworks and ES6 shims a.k.a. things you don't actually need. I recommend using as many buzz words as possible.
Though it sounds a bit jaded— can confirm. I've been asked to re-implement `bind` before.
I gave it a shot but ultimately shrugged and explained that I'd rarely ever extend a native object and would rather use a community-supported polyfill. Nevermind that in my current field we haven't had to support ES3-capable browsers for ages.
I'd certainly be better off knowing, but rarely would I need to know on the spot. There is more beneficial knowledge to have up front.