Add service worker activation event listener

sw.js

@@ -16,4 +16,20 @@ self.addEventListener('fetch', event => {
             return response || fetch(event.request);
         })
     );
+});
+
+self.addEventListener('activate', event => {
+    event.waitUntil(
+        caches.keys().then(cacheNames => {
+            return Promise.all(
+                cacheNames.filter(cacheName => {
+                    // Return true if you want to remove this cache,
+                    // but remember that caches are shared across
+                    // the whole origin
+                }).map(cacheName => {
+                    return caches.delete(cacheName);
+                })
+            );
+        })
+    );
 });