Thank you. It was really hard to figure out which tools even could do this so I knew which one I ought to deeply "RTFM" for, which was really frustrating to me because it seemed like the most obvious default behavior for any JS build/bundle tool, which should be what it did if you just typed "some-build-tool build" or "bundle" or whatever, from your project root with nothing but a package.json containing a "main" entry.

I agree that this is really not a good experience. I prefer js projects that don't rely on build tools at all for this reason, but most devs, frameworks and libraries don't support such a use-case.

When I have to use build-tools I usually go for esbuild.