Install Service Worker and cache new requests cumulatively (Only GET method)

service-worker.js

var CACHE_NAME = 'example-cache-v1';
var urlsToCache = [];

self.addEventListener('install', function(event) {
    // Perform install steps
    event.waitUntil(
        caches.open(CACHE_NAME)
            .then(function(cache) {
                console.log('Opened cache');
                return cache.addAll(urlsToCache);
            })
    );
});

self.addEventListener('fetch', function(event) {
    // Only cache GET method
    if(event.request.method === 'GET') {
        event.respondWith(
            caches.match(event.request)
                .then(function (response) {
                    // Cache hit - return response
                    if (response) {
                        return response;
                    }

                    // IMPORTANT: Clone the request. A request is a stream and
                    // can only be consumed once. Since we are consuming this
                    // once by cache and once by the browser for fetch, we need
                    // to clone the response.
                    var fetchRequest = event.request.clone();

                    return fetch(fetchRequest).then(
                        function (response) {
                            // Check if we received a valid response
                            if (!response || response.status !== 200 || response.type !== 'basic') {
                                return response;
                            }

                            // IMPORTANT: Clone the response. A response is a stream
                            // and because we want the browser to consume the response
                            // as well as the cache consuming the response, we need
                            // to clone it so we have two streams.
                            var responseToCache = response.clone();

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

                            return response;
                        }
                    );
                })
        );
    }
})