sw.js

var CACHE_NAME = "progress-bar-cache-v1";
var urlsToCache = ["/progress-bar/", "/progress-bar/index.html"];

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

self.addEventListener("fetch", function (event) {
	event.respondWith(
		caches.match(event.request).then(function (response) {
			if (response) {
				return response;
			}

			return fetch(event.request).then(function (response) {
				if (
					!response ||
					response.status !== 200 ||
					response.type !== "basic" ||
					!(event.request.url.indexOf("http") === 0)
				) {
					return response;
				}

				var responseToCache = response.clone();

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

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