[PWA] Deal with caches in PWA

const CACHE_NAME = 'v1';
self.addEventListener('install', function (event) {
    // waitUntil take a promise
    event.waitUntil(
        caches.open(CACHE_NAME).then(function (cache) {
            return cache.addAll([
                '/',
                'js/main.js',
                'css/main.css',
                'imgs/icon.png',
                'https://fonts.gstatic.com/s/roboto/v15/2UX7WLTfW3W8TclTUvlFyQ.woff',
                'https://fonts.gstatic.com/s/roboto/v15/d-6IYplOFocCacKzxwXSOD8E0i7KZn-EPnyo3HZu7kw.woff'
            ]);
        })
    );
});

self.addEventListener('activate', function (event) {
    // waitUntil take a promise
    event.waitUntil(
        // caches.keys, return all the cache names
        caches.keys().then(function (cacheNames) {
            return Promise.all(
                // only get cache for this application
                cacheNames.filter(function(name) {
                    return name !== CACHE_NAME && name.startsWith('witter-');
                }).map(function(cacheName) {
                    // delete each cache by name
                    return caches.delete(cacheName);
                })
            );
        })
    );
});