stale-serviceworker-notification.js

if (navigator.serviceWorker && navigator.serviceWorker.controller) {
  
  // Add an event to the Service Worker's lifecycle state change event
  navigator.serviceWorker.controller.onstatechange = event => {
    
    // When the worker becomes redundant, that means a new worker is
    // replacing it and the user is seeing stale content
    if (event.target.state === 'redundant') {
      
      // Replace this with some way to show a notification in your app
      showMessage(
        'A new version of the website is available. ' +
        'Click here or refresh the page to get it.',
        window.location.reload
      );
    }
  };
}