Cross-browser compatible code for requesting notification permission

permission.js

async function subscribeButtonHandler() {
  function permissionRequestCallback(result) {
    permissionStatus = result;
    subscribe();
  }
  async function subscribe() {
    if (subscriptionInFlight) return;
    subscriptionInFlight = true;
    if (permissionStatus === 'denied') {
      console.error('The user explicitly denied notification permission.');
      return;
    }
    const registration = await navigator.serviceWorker.getRegistration();
    console.info({registration});
    if (!registration.pushManager) {
      console.error('PushManager interface not found.');
      return;
    }
    const subscription = await registration.pushManager.subscribe({
      userVisibleOnly: true,
      applicationServerKey: urlB64ToUint8Array(VAPID_PUBLIC_KEY)
    });
    if (subscription.endpoint) {
      console.info('Successfully subscribed to push notifications!');
      console.info({subscription});
      unsubscribeButton.disabled = false;
    }
    fetch('/add-subscription', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json'
      },
      body: JSON.stringify(subscription)
    });
  }
  let permissionStatus;
  let subscriptionInFlight = false;
  try {
    Notification.requestPermission(permissionRequestCallback).then(result => {
      permissionStatus = result;
      subscribe();
    });
  } catch (error) {
    if (error instanceof TypeError) {
      console.info('Callback version of Notification.requestPermission() detected');
    }
  }
}