When it got to the variadic arguments one I had the same thought - it's important to know that it is possible, but less important to know the syntax for that in a particular language as you will I use it very rarely, and it can easily be looked up. There is a danger with this sort of questioning that it veers into esoteric corners of the language which are not a test of ability at the job, but a test of ability at language quizzes. The context questions I had more sympathy with, as it is a potential pitfall for js developers, but there are other ways to mitigate this, like not stripping object functions of their context in the first place (not sure why you would do that here?) - this kind of stuff is often handled by libraries, particularly the native shim.

Thankfully he does move on to problem solving later on, which strikes me as far more useful - an open-ended question will reveal lots about the candidate.