The way it works for Debian packaging is that they usually have their own copy of the project's source code (what they call
upstream). So the packaging process does start from the actual, original source code repo of the upstream project being packaged. This code is kept in a "
upstream" branch, to which Debian-specific files and patches are added, usually in a different branch named "
debian". For new versions, the "
upstream" branch is updated with new code from the project.
All of which, if you ask me, is the correct way of doing any kind of packaging. Following that, IMO the same should be done for JavaScript libraries: the packaging should be done by cloning the project repo and adding whatever packager-specific files in there.
Notice in your link how in the upper part it says: [ Source: jq ], where "jq" is a link to the source description. In that page, the right hand section will now contain a link to the Debian copy repository where packaging is done:
https://salsa.debian.org/debian/jq
You can clone and explore the branches of that repo.
(Maybe you are a Debian maintainer, in any case I'm writing this for whoever passes by and is curious about how I think JS or whatever else should be packaged if done properly)