The active property of the ServiceWorkerRegistration interface returns a service worker whose ServiceWorker.state is activating or activated. This property is initially set to null.
An active worker controls a Client if the client's URL falls within the scope of the registration (the scope option set when ServiceWorkerContainer.register is first called.)
Note: This feature is available in Web Workers.
Note: Once an active worker is activating, neither a runtime script error nor a force termination of the active worker prevents the active worker from getting activated.