owncast/webroot/js/notification/registerWeb.js

export async function registerWebPushNotifications(vapidPublicKey) {
  const registration = await navigator.serviceWorker.ready;
  let subscription = await registration.pushManager.getSubscription();

  if (!subscription) {
    subscription = await registration.pushManager.subscribe({
      userVisibleOnly: true,
      applicationServerKey: urlBase64ToUint8Array(vapidPublicKey),
    });
  }

  return JSON.stringify(subscription);
}

export function isPushNotificationSupported() {
  return 'serviceWorker' in navigator && 'PushManager' in window;
}

function urlBase64ToUint8Array(base64String) {
  var padding = '='.repeat((4 - (base64String.length % 4)) % 4);
  var base64 = (base64String + padding).replace(/\-/g, '+').replace(/_/g, '/');

  var rawData = window.atob(base64);
  var outputArray = new Uint8Array(rawData.length);

  for (var i = 0; i < rawData.length; ++i) {
    outputArray[i] = rawData.charCodeAt(i);
  }
  return outputArray;
}