So I didn't want a certain script to be cached by the serviceworker I'm working on. That's how it wasn't in the array of assets and urls to cache.
When I tested the worker I got all assets and pages I had in the array, as expected. But I also found https://url/undefined | OK on the list in the cache storage (in Chrome and Firefox).
When I navigated to the URL (https://url/undefined) I got the contents of the script not being on the list in the first place.
Does it make sense?