Well, just an actual HTTP proxy would work for that, properly tuned. But the JS world mostly doesn't bother, maybe due to long history as a browser-based lang

Well, just an actual HTTP proxy would work for that, properly tuned. But the JS world mostly doesn't bother, maybe due to long history as a browser-based language, where the webserver serves that function for the browser. The ones who do are focused on caching private npm servers like verdaccio.