Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-trim-cache-function-event.js

const trimCache = (cacheName, maxItems) => {
  caches.open(cacheName).then(cache => {
    cache.keys().then(keys => {
      if(keys.length > maxItems)
          cache.delete(keys[0]).then(trimCache(cacheName, maxItems));
    });
  });
};

self.addEventListener('message', event => {
  if(event.data.command == 'trimCache') {
    trimCache(otherCacheName, 50);
    trimCache(imagesCacheName, 25);
  }
});