without a unique key in the js name/path, or any server action to enable it? If so I would like to read about this particular development - can you point me to an article on how they're doing it?

Yes, Chrome uses V8 which has Isolates (also used by some FaaS platforms like Cloudflare Workers), and adds more optimizations on top like disk-based caching to share across processes. The script is keyed from a hash of its contents.

https://v8.dev/blog/code-caching-for-devs

Thanks! I guess will have to see if FF and Safari support same thing. Perhaps in another year can remove cache busting from builds.

They do, it's linked in the blog:

https://blog.mozilla.org/javascript/2017/12/12/javascript-st...

https://bugs.webkit.org/show_bug.cgi?id=192782

Also did you mean http caching? Not sure why would want to remove that. It's still important for the browser to get the latest script content before the bytecode caching happens.

How can the download step be skipped then if you are using the hash of the content as a key??

That's what HTTP caching is. Browsers use headers and heuristics.