Next, we use the ServiceWorkerContainer.register() function to register the service worker for this site, which is just a JavaScript file residing inside our app (note this is the file's URL relative to the origin, not the JS file that references it.)

The Service worker will now control pages, but only those opened after the register() is successful. i.e. a document starts life with or without a Service worker and maintains that for its lifetime. So documents will have to be reloaded to actually be controlled.