Thank you for the fast reply.
Having those external repositories should be a problem because:
1) epel, uses the repo "https://rpm.nodesource.com/pub_8.x/el/$releasever/$basearch" and the other ones will fail. works as expected
2) fedora-27-i386 --> has a nodejs repo, still fails and disables it. It should have used "https://rpm.nodesource.com/pub_8.x/fc/$releasever/$basearch" repo does not work as expected. this is the particular case where the repo should have been downloaded but fails like it was offline
3) fedora-28 and rawhide --> does not have a repo, but hits on the fc/27/ repo
I can, indeed, configure repo for fedora epel 7 and fedora 26 and use fedora own's version of npm/nodejs for fedora 27 up to rawhide. Nevertheless, I'm taking your suggestion and have two rpm specs for different chroots. Thank you.