ServiceWorkerContainer.getRegistration()

The getRegistration() method of the ServiceWorkerContainer interface gets a ServiceWorkerRegistration object whose scope URL matches the provided document URL.  The method returns a Promise that resolves to a ServiceWorkerRegistration or undefined.

serviceWorkerContainer.getRegistration(scope).then(function(serviceWorkerRegistration) { ... });

scope Optional

A unique identifier for a service worker registration — the scope URL of the registration object you want to return. This is usually a relative URL.

A Promise that resolves to a ServiceWorkerRegistration object or undefined.

navigator.serviceWorker.getRegistration('/app').then(function(registration) {
  if(registration){
    document.querySelector('#status').textContent = 'ServiceWorkerRegistration found.';
  }
});