Reduce the external files required for gem installation

assets/js/pwa/app.js

@@ -0,0 +1,9 @@
+---
+layout: compress
+permalink: '/app.js'
+---
+
+/* Registering Service Worker */
+if('serviceWorker' in navigator) {
+  navigator.serviceWorker.register('{{ "/sw.js" | relative_url }}');
+};

assets/js/pwa/sw.js

@@ -0,0 +1,87 @@
+---
+layout: compress
+permalink: '/sw.js'
+# PWA service worker
+---
+
+self.importScripts('{{ "/assets/js/data/swcache.js" | relative_url }}');
+
+const cacheName = 'chirpy-{{ "now" | date: "%Y%m%d.%H%M" }}';
+
+function verifyDomain(url) {
+  for (const domain of allowedDomains) {
+    const regex = RegExp(`^http(s)?:\/\/${domain}\/`);
+    if (regex.test(url)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+function isExcluded(url) {
+  for (const item of denyUrls) {
+    if (url === item) {
+      return true;
+    }
+  }
+  return false;
+}
+
+self.addEventListener('install', e => {
+  self.skipWaiting();
+  e.waitUntil(
+    caches.open(cacheName).then(cache => {
+      return cache.addAll(resource);
+    })
+  );
+});
+
+self.addEventListener('fetch', event => {
+  event.respondWith(
+    caches.match(event.request)
+      .then(response => {
+        if (response) {
+          return response;
+        }
+
+        return fetch(event.request)
+          .then(response => {
+            const url = event.request.url;
+
+            if (event.request.method !== 'GET' ||
+              !verifyDomain(url) ||
+              isExcluded(url)) {
+              return response;
+            }
+
+            /*
+              see: <https://developers.google.com/web/fundamentals/primers/service-workers#cache_and_return_requests>
+             */
+            let responseToCache = response.clone();
+
+            caches.open(cacheName)
+              .then(cache => {
+                /* console.log('[sw] Caching new resource: ' + event.request.url); */
+                cache.put(event.request, responseToCache);
+              });
+
+            return response;
+          });
+      })
+    );
+});
+
+self.addEventListener('activate', e => {
+  e.waitUntil(
+    caches.keys().then(keyList => {
+          return Promise.all(
+            keyList.map(key => {
+              if(key !== cacheName) {
+                return caches.delete(key);
+              }
+            })
+          );
+    })
+  );
+});