> Consider the poor quality of the existing js modes for 26.x, any idea why this depends on a future version of Emacs instead of current versions?

You might as well look at the thread js2-mode was revoked[0] and ergoemacs' article[1].

> Alternatively, could somebody convince the author of the railwaycat port (Emacs with better macOS integration) to get a version 27 out soon?

Er... I would like to point out that it's not 'railwaycat' port but more of Mitsuharu Yamamoto's port[2], and AFAIK he has claimed that he has no interest in maintaing an emacs27 version.

[0] https://lists.gnu.org/archive/html/emacs-devel/2009-08/msg00... [1] http://ergoemacs.org/misc/emacs_javascript_mode_war.html [2] https://bitbucket.org/mituharu/emacs-mac

Disregarding the history behind js2-mode, if it has been decided that Emacs 27 will feature a much improved js-mode, I was just puzzled that it wasn't put into 26.x as a package already.

I couldn't remember the name of Yamamoto, which is why I said the author of the railwaycat port, where the latter is mostly packaging if I understand it correctly. But Yamamoto is doing most of the heavy lifting yes, and I'm thankful for it.

No. He builds off of stable releases because otherwise something could change in the underlying implementation that breaks emacs-mac. It would mean more maintenance on his part.

IIRC the creator also releases his port as a patch. You can always try to patch emacs 27 with it.