Does appending myjsfile.js?2342343 always force the browser to get the new .js file? Does appending myjsfile.js?2342343 always force the browser to get the new .js file?

It depends on the caching headers you send back with it. Without any Cache-Control or Expires headers, some browsers (e.g. IE) will treat the URL as always-stale, but this behavior is not guaranteed. On the other hand, if there are valid caching directives, browsers will respect them.