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diff --git a/docs/_book/gitbook/gitbook-plugin-offline/service-worker-registration.js b/docs/_book/gitbook/gitbook-plugin-offline/service-worker-registration.js
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+/**
+ * Copyright 2015 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/* eslint-env browser */
+'use strict';
+
+if ('serviceWorker' in navigator) {
+  // Delay registration until after the page has loaded, to ensure that our
+  // precaching requests don't degrade the first visit experience.
+  // See https://developers.google.com/web/fundamentals/instant-and-offline/service-worker/registration
+  window.addEventListener('load', function() {
+    // Your service-worker.js *must* be located at the top-level directory relative to your site.
+    // It won't be able to control pages unless it's located at the same level or higher than them.
+    // *Don't* register service worker file in, e.g., a scripts/ sub-directory!
+    // See https://github.com/slightlyoff/ServiceWorker/issues/468
+    navigator.serviceWorker.register('service-worker.js').then(function(reg) {
+      // updatefound is fired if service-worker.js changes.
+      reg.onupdatefound = function() {
+        // The updatefound event implies that reg.installing is set; see
+        // https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-container-updatefound-event
+        var installingWorker = reg.installing;
+
+        installingWorker.onstatechange = function() {
+          switch (installingWorker.state) {
+            case 'installed':
+              if (navigator.serviceWorker.controller) {
+                // At this point, the old content will have been purged and the fresh content will
+                // have been added to the cache.
+                // It's the perfect time to display a "New content is available; please refresh."
+                // message in the page's interface.
+                console.log('New or updated content is available.');
+              } else {
+                // At this point, everything has been precached.
+                // It's the perfect time to display a "Content is cached for offline use." message.
+                console.log('Content is now available offline!');
+              }
+              break;
+
+            case 'redundant':
+              console.error('The installing service worker became redundant.');
+              break;
+          }
+        };
+      };
+    }).catch(function(e) {
+      console.error('Error during service worker registration:', e);
+    });
+  });
+}