Clients.openWindow()

self.clients.openWindow(url).then(function(windowClient) {
  // Do something with your WindowClient
});

// Send notification to OS if applicable
if (self.Notification.permission === 'granted') {
  const notificationObject = {
    body: 'Click here to view your messages.',
    data: { url: self.location.origin + '/some/path' },
    // data: { url: 'http://example.com' },
  };
  self.registration.showNotification('You\'ve got messages!', notificationObject);
}

// Notification click event listener
self.addEventListener('notificationclick', e => {
  // Close the notification popout
  e.notification.close();
  // Get all the Window clients
  e.waitUntil(clients.matchAll({ type: 'window' }).then(clientsArr => {
    // If a Window tab matching the targeted URL already exists, focus that;
    const hadWindowToFocus = clientsArr.some(windowClient => windowClient.url === e.notification.data.url ? (windowClient.focus(), true) : false);
    // Otherwise, open a new tab to the applicable URL and focus it.
    if (!hadWindowToFocus) clients.openWindow(e.notification.data.url).then(windowClient => windowClient ? windowClient.focus() : null);
  }));
});