Thank you so much for this!! arc-js looks great. I'm able to define and run Arc macros with it.
This changes everything. With arc-js you can start writing client-side apps in the browser without dealing with Javascript. You can add libraries in arc-js that do that (are there any already?) The big advantage is you bypass HTML: you can make a browser and a server send sexps back and forth.
There I was thinking there was no hope with an Arc in Javascript and sure enough Arc is quietly thriving.