Iris/mopidy_iris/static/service-worker.js

const blacklist = [
  'jamesbarnsley.co.nz',
  'following/contains',
  'followers/contains',
  'me/tracks',
  'me/albums',
  'me/following',
  'refresh_spotify_token'
];
function inBlacklist(url) {
  for (const item of blacklist) {
    if (url.indexOf(item) >= 0) {
      return true;
    }
  }
  return false;
}

self.addEventListener('activate', function(event) {
  event.waitUntil(
    caches.keys().then((cacheNames) => {
      return Promise.all(cacheNames.map((cacheName) => caches.delete(cacheName)));
    })
  );
});

self.addEventListener('fetch', (event) => {
  const { request } = event;

  event.respondWith(

    // Opens Cache objects that start with 'font'.
    caches.open('iris').then((cache) => {
      return cache.match(request)
        .then((response) => {
          if (response) {
            return response;
          }

          // Not cached, so we make the request, return that and also save response in cache
          return fetch(request)
            .then((liveResponse) => {
              const isBlacklisted = inBlacklist(request.url);

              // Only cache successful GET requests
              if (
                !isBlacklisted
                && request.method === 'GET'
                && liveResponse.status >= 200
                && liveResponse.status < 400
              ) {
                // Fixes Edge browser "'chrome-extension' is unsupported" issue
                // See https://stackoverflow.com/questions/49157622/service-worker-typeerror-when-opening-chrome-extension
                if (!/^https?:$/i.test(new URL(request.url).protocol)) return;

                cache.put(request, liveResponse.clone());
              } else {
                console.info(`Not caching ${isBlacklisted ? '(blacklisted) ' : ''}${request.method} ${request.url}`);
              }

              return liveResponse;
            });

        // Exceptions from match() or fetch()
        }).catch((error) => {
          throw error;
        });
    }),
  );
});