First, when you're a [language] programmer, every problem is solved with [language]. If there was one language that was perfect for everything, we'd just name it "programming" and be done with this whole debate.
Secondly, and more importantly, I've never worked on a website or app where only one language or technology was used -- and that's completely ignoring the importance of skills like design, usability, product management, databases, etc.
One-language-programmers tend to have a "that's not my job" approach to anything that isn't their language, meaning code from them often isn't wired up or well designed or whatever.
(Naturally, there are some amazing one-language-programmers out there, however my experience tends to be the opposite.)
If you feel that's the case it's probably worth asking for a title change.
In our consulting company everyone is polyglot and we tend to assemble teams based on the technologies requested by the customer on the RFPs.
It is quite natural that the technology stack completely changes between projects.
Having said that, javascript is pretty ubiquitous now and it only seems to be growing. I wonder what the odds are of it being replaced, given how slow the specifications evolve. Is there any reason to think it won't be here to stay for a long time?
I agree that being programmer we should not _think_ that we are .Net or PHP or Javascript or XYZ Programmer, rather we should have confidence in our developerness with "loose coupling".
Being interviewer I would personally judge if the person is master in particular technology, if he/she is I would conclude that, he/she could code anything using any technology and it has been proven in my case.