One of the problems with the js community is the idea that it is a good idea to name just about anything that is js-ish in a file with a .js extension. I love Dan Abramov, but I think a large part of the blame for this can be placed on his feet for encouraging the use of js extensions for jsx. This sent a lot of people down a dark path. File extensions serve a purpose. Even more so if you don’t have a standardized header line disambiguating the contents of the file. When you rely on tribal knowledge and sophisticated build tools that rely on strange assumptions or the requirement to parse whole files with different syntaxes just to know what you are looking at you have a problem.
If the c++ community had a tendency to name their java files with cpp, would you be so forgiving?
To be honest, I don't run into much JS code that doesn't have proper extensions, but that's just been my experience for the past 6 or whatever years I've been using Node.js. If I use JSX, I use .jsx as an extension. Likewise for TS and TSX. And CoffeeScript in years past. I can't recall coming across any NPM packages that couldn't run in Node. So I can't really relate. Maybe it's confusing that there's different flavors of JS though with the same file extension: browser-flavored JS and Node-flavored JS.
I wish I could say the same, but nearly half of the teams I've worked with on web development go exclusively with .js because it's been pushed by prominent members of the React community for "ergonomics"[0] (which here is just a fancy way of saying devs can't be bothered to invest the two seconds it takes to make file extensions accurate when switching to a superset language)
The trend of unreliable file extensions has lots of other nasty side effects on IDEs, build tools, and browsers (e.g. [1])
I worked in Facebook briefly, and the reason why Dan (who also is/was at Facebook) might be suggesting this is because this is the practice at Facebook: all "JS" files use the same build system and file extension. Which is to say, there's no JS, or TS, or even CommonJS APIs. It's all and always Facebook-flavored Flow (with the occasional caveat).
And perhaps this is why it didn't seem obvious for Dan (I'm speculating) that anyone would want to use multiple file extensions for things, but when you're outside of Facebook's ecosystem and you have to set up your own build tools and deal with all the pain it comes with, you realize stuff like this makes no sense. Because ya know, maybe targeting by file extension in your build system would make it easier to use the right transpiler (e.g. that's what Parcel does). But that's just been my perspective being in both worlds.
If the c++ community had a tendency to name their java files with cpp, would you be so forgiving?