What does "Module has been externalized" actually mean?

[ZweiEuro]

I maintain a small module for a much larger project, i can't run tests with it with vite due to this kind of error:

Error: Module "npm:@wui/web-user-interface-node/TestMock" has been externalized for browser compatibility. Cannot access "npm:@wui/web-user-interface-node/TestMock.TestMock" in client code.  See https://vite.dev/guide/troubleshooting.html#module-externalized-for-browser-compatibility for more details.

Following the link, it just says to tell the maintainer. Well, since I am the maintainer I have no idea what to do next. I checked if my package.json or code has any significant differences to other libraries that can be importet just fine like jsdom or @testing-library/jest-dom/matchers but I don't see any significant difference. I've also tried all kinds of changes to package.json but the error persists. Even when I delete all the contents of the file in question, the error still persits, so I started assuming I am looking into the wrong direction.

Reverse engineering where the problem is caused leads nowhere as string searching the error brings no results.

What exact config problem is vitest upset with, here ? What does it expect vs what I am giving it that makes it think my library need polyfill? I am not using anything from the global space other than globalThis.window in order to link something back for custom hooks and other functionality, but that should be fine if i use react testing tools that allow that kind of behavior.

My best guess is that my tsconfig does something illegal that is unexpected by vitest, but without information on what it does incorrectly that is impossible to tell...

https://gitlab.zweieuro.at/wui/core/web-user-interface-node/-/tree/development is the library i wrote and am trying to make work.

Sidenote: I am using deno, but that only changes the imports to have a prefix.

TLDR:
What causes this error?
https://vite.dev/guide/troubleshooting.html#module-externalized-for-browser-compatibility

[sheremet-va]

This is not a Vitest error, this is a Vite error. Try importing your library using a simple Vite project.

[ZweiEuro]

I can't directly import it into a stackblitz since its hosted on my gitlab and the stackblitz afaik doesn't have a custom npmrc;
I am looking into vitejs and checking if i can find it in there and reverse engineer it from there

[ZweiEuro]

Okay, after forking the react-ts from vite and trying it there, at least it runs an arbitrary test and apparently, but that makes me question everything even more if apparently my package isn't the issue.

Nevermind, even a completly empty tsconfig.json on the starter template makes vitest run perfectly, so i got 0 clue