Add service worker

pwa/service-worker.js

@@ -0,0 +1,34 @@
+'use strict';
+
+var cacheVersion = 1;
+var currentCache = {
+  offline: 'offline-cache' + cacheVersion
+};
+const offlineUrl = 'offline';
+
+this.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(currentCache.offline).then(function (cache) {
+      return cache.addAll([
+        offlineUrl
+      ]);
+    })
+  );
+});
+
+this.addEventListener('fetch', event => {
+  if (event.request.mode === 'navigate' || (event.request.method === 'GET' && event.request.headers.get('accept').includes('text/html'))) {
+    event.respondWith(
+      fetch(event.request.url).catch(error => {
+        return caches.match(offlineUrl);
+      })
+    );
+  }
+  else {
+    event.respondWith(caches.match(event.request)
+      .then(function (response) {
+        return response || fetch(event.request);
+      })
+    );
+  }
+});