claim() method of the
Clients interface allows an active service worker to set itself as the
controller for all clients within its
scope. This triggers a "
controllerchange" event on
navigator.serviceWorker in any clients that become controlled by this service worker.
When a service worker is initially registered, pages won't use it until they next load. The
claim() method causes those pages to be controlled immediately. Be aware that this results in your service worker controlling pages that loaded regularly over the network, or possibly via a different service worker.