How many JavaScript developers are there? A lot. What are your odds that a given candidate is a good programmer? Not great.
Identifying with a single language is rather myopic, in my view. Languages evolve, new ones arise, and old ones get discarded. It’s true that you can make a killing as a COBOL developer—I had a professor once who said that he took a COBOL job and made a good amount of money from it. Thing is, as soon as that job was over, he took COBOL off his résumé.
I used to identify as a developer in a particular language. But hey, I’m still under 30 and I’ve moved away from that, so that’s good.