My solution to clean image cache, promises :)

_cleanImgCache.js

...

IndexController.prototype._cleanImageCache = function() {
  return this._dbPromise.then(function(db) {
    if (!db) return;

    // TODO: open the 'wittr' object store, get all the messages,
    // gather all the photo urls.
    var store = db.transaction('wittrs').objectStore('wittrs');
    return store.getAll().then((messages) => {
        if (!messages) return;
        // get url from each message, retun urls
        var urls = messages.map(message => {
            if (!message.photo) return;
            return "http://localhost:8888" + message.photo;
        }).filter(url => {
            return url;
        });
        return urls;
    }).then( photoUrls => {
        if (!photoUrls) return;
        // Open the 'wittr-content-imgs' cache, and delete any entry
        // that you no longer need.
        return caches.open('wittr-content-imgs').then(cache => {
            return cache.keys().then(requests => {
                return requests.forEach(request => {
                    if (!photoUrls.includes(request.url)){
                        return cache.delete(request);
                    }
                    return;
                });
            });
        });
    });
  });
};
...