My experience having written all of the above is that JS is significantly more fragmented, and also has to deal with a frontend-backend severed line. Rails, Fl

My experience having written all of the above is that JS is significantly more fragmented, and also has to deal with a frontend-backend severed line. Rails, Flask, Django (and PHP, purely server side JS that generate HTML with templates like express) are basically all "backend serving frontend and templating". Front end JS is more of a "frontend + backend with frontend serving backend" in the form of server side rendering being more akin to running a browser, loading the frontend, and then serialising that to a page. This is the complete opposite of the above. It's kind of a different mindset, but at the same time you frequently see flask serving an API, and JS consuming that API from the browser.