IMO it should be somebody who can build a Linux kernel module or a single page web app. They can do explicit memory management in C, or query a DB via ORM. But the way it's used is usually just "can write both server and client JavaScript."
A developer that writes Javascript and sometimes runs it in Node as a server process or as a desktop app in Electron, but actually doesn’t know the /full/ stack at all.
I always assume someone who can build a small production web app from DB Schemas to the front end, including authentication and payments but nothing more specialist than that. Able to jump in anywhere along that stack and be dangerous enough. Probably abstracted via frameworks, so knowing one in-depth front-end and back-end.