getSubscription()

// We need the service worker registration to check for a subscription
navigator.serviceWorker.ready.then((serviceWorkerRegistration) => {
  // Do we already have a push message subscription?
  serviceWorkerRegistration.pushManager
    .getSubscription()
    .then((subscription) => {
      // Enable any UI which subscribes / unsubscribes from
      // push messages.
      const pushButton = document.querySelector(".js-push-button");
      pushButton.disabled = false;

      if (!subscription) {
        // We aren't subscribed to push, so set UI
        // to allow the user to enable push
        return;
      }

      // Keep your server in sync with the latest subscriptionId
      sendSubscriptionToServer(subscription);

      showCurlCommand(subscription);

      // Set your UI to show they have subscribed for
      // push messages
      pushButton.textContent = "Disable Push Messages";
      isPushEnabled = true;
    })
    .catch((err) => {
      console.error(`Error during getSubscription(): ${err}`);
    });
});