[kerryoco]

it has nothing to do with server side business logic. It's simply that javascripts package manager, npm, was widely adopted to server / node needs. And then the notorious "node_modules" folder, which was before handled by server compilation, became a part of clientside dev too because it had so many goodies in it, so now the client was de facto having to do the same build as the server, so, by "server side code", I think it's safe to say what they mean is "node modules".