service-worker.js

var CACHE_NAME = 'my-web-app-version-v1';

// 1 - Install
// Here we wait for an InstallEvent to fire, then tell the cache which URLs it should retrieve and cache
self.addEventListener('install', event => {
    event.waitUntil(
         caches.open(CACHE_NAME)
         .then(cache => {
            return cache.addAll([
                './',
                './index.php'
            ]);
         })
    );
});

// 2 - Activate
// Here we wait for an ActivateEvent to fire, then we remove any old cache. That is to say, any cache stored 
// that does not have the same key as the one we defined in CACHE_NAME
self.addEventListener('activate', event => {
    event.waitUntil(
        caches.keys().then(cacheNames => {
            return Promise.all(
                cacheNames.map(cacheName => {
                    if (cacheName !== CACHE_NAME) 
                    return caches.delete(cacheName);
                })
            );
        })
    );
});

// 3 - Fetch
// Anytime our website requests a new resource we first check the cache if we have anything available already 
// and if so, just use that. If not, we retrieve the resource and put it in cache for the next time we may need it.
self.addEventListener('fetch', event => {
    event.respondWith(
        caches.match(event.request)
        .then(response => {
            if (response) return response;

            return fetch(event.request)
            .then(response => {
                if (!response || response.status !== 200 || response.type !== 'basic') return response;

                var responseToCache = response.clone();

                caches.open(CACHE_NAME).then(cache => { 
                    cache.put(event.request, responseToCache) 
                });
            });
        })
    )
});

// 4 - Message
// Here we wait for a MessageEvent to fire, if the message contains skipWaiting we should execute that method.
self.addEventListener('message', event => {
    if (event.data === 'skipWaiting') return skipWaiting();
});