hometown/app/javascript/mastodon/service_worker/entry.js

import './web_push_notifications';

function openSystemCache() {
  return caches.open('mastodon-system');
}

function openWebCache() {
  return caches.open('mastodon-web');
}

function fetchRoot() {
  return fetch('/', { credentials: 'include' });
}

// Cause a new version of a registered Service Worker to replace an existing one
// that is already installed, and replace the currently active worker on open pages.
self.addEventListener('install', function(event) {
  event.waitUntil(Promise.all([openWebCache(), fetchRoot()]).then(([cache, root]) => cache.put('/', root)));
});
self.addEventListener('activate', function(event) {
  event.waitUntil(self.clients.claim());
});
self.addEventListener('fetch', function(event) {
  const url = new URL(event.request.url);

  if (url.pathname.startsWith('/web/')) {
    const asyncResponse = fetchRoot();
    const asyncCache = openWebCache();

    event.respondWith(asyncResponse.then(async response => {
      if (response.ok) {
        const cache = await asyncCache;
        await cache.put('/', response);
        return response.clone();
      }

      throw null;
    }).catch(() => asyncCache.then(cache => cache.match('/'))));
  } else if (url.pathname === '/auth/sign_out') {
    const asyncResponse = fetch(event.request);
    const asyncCache = openWebCache();

    event.respondWith(asyncResponse.then(async response => {
      if (response.ok || response.type === 'opaqueredirect') {
        const cache = await asyncCache;
        await cache.delete('/');
      }

      return response;
    }));
  } else if (process.env.CDN_HOST ? url.host === process.env.CDN_HOST : url.pathname.startsWith('/system/')) {
    event.respondWith(openSystemCache().then(async cache => {
      const cached = await cache.match(event.request.url);

      if (cached === undefined) {
        const fetched = await fetch(event.request);

        if (fetched.ok) {
          await cache.put(event.request.url, fetched.clone());
        }

        return fetched;
      }

      return cached;
    }));
  }
});