All browsers have JS byte code caching, but in my experience building web apps it doesn't make a huge difference in the real world. A user will hit a site, down

All browsers have JS byte code caching, but in my experience building web apps it doesn't make a huge difference in the real world. A user will hit a site, download and compile the JS bundle, and then that tab will be kept open for a day or two. They might close the tab and reopen it the following day. In any modern app that uses continuous deployment that means they'll certainly be downloading a new client bundle, and a lot of the time they'll also also get a new vendor bundle (client bundle is the app code, vendor bundle is the NPM packages). I have no doubt code could be split to optimize for fewest changes but JS tooling is complicated enough already so I doubt many sites would bother.