Restore create-react-app serviceWorker file for later use

frontend/src/index.tsx

@@ -8,6 +8,7 @@ import * as ReactDOM from 'react-dom';
 
 import { App } from './app';
 import { AboutButton } from './components/about';
+import * as serviceWorker from './serviceWorker';
 
 function useTheme() {
     const [themeName, setThemeName] = useLocalStorage<'light' | 'dark'>('themeName', 'dark');
@@ -70,3 +71,19 @@ const Root = (_props: {}) => {
 };
 
 ReactDOM.render(<Root />, document.getElementById('root'));
+
+// If you want your app to work offline and load faster, you can change
+// unregister() to register() below. Note this comes with some pitfalls.
+// Learn more about service workers: https://bit.ly/CRA-PWA
+serviceWorker.unregister();
+
+// TODO: Try using a service worker to aid in reloading when needed.
+// let refreshing = false;
+// serviceWorker.register({
+//     onUpdate: () => {
+//         if (!refreshing) {
+//             refreshing = true;
+//             window.location.reload();
+//         }
+//     }
+// });

frontend/src/serviceWorker.ts

@@ -0,0 +1,139 @@
+// This optional code is used to register a service worker.
+// register() is not called by default.
+
+// This lets the app load faster on subsequent visits in production, and gives
+// it offline capabilities. However, it also means that developers (and users)
+// will only see deployed updates on subsequent visits to a page, after all the
+// existing tabs open on the page have been closed, since previously cached
+// resources are updated in the background.
+
+// To learn more about the benefits of this model and instructions on how to
+// opt-in, read https://bit.ly/CRA-PWA
+
+const isLocalhost = Boolean(
+    window.location.hostname === 'localhost' ||
+        // [::1] is the IPv6 localhost address.
+        window.location.hostname === '[::1]' ||
+        // 127.0.0.0/8 are considered localhost for IPv4.
+        window.location.hostname.match(/^127(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$/)
+);
+
+type Config = {
+    onSuccess?: (registration: ServiceWorkerRegistration) => void;
+    onUpdate?: (registration: ServiceWorkerRegistration) => void;
+};
+
+export function register(config?: Config) {
+    if (process.env.NODE_ENV === 'production' && 'serviceWorker' in navigator) {
+        // The URL constructor is available in all browsers that support SW.
+        const publicUrl = new URL(process.env.PUBLIC_URL, window.location.href);
+        if (publicUrl.origin !== window.location.origin) {
+            // Our service worker won't work if PUBLIC_URL is on a different origin
+            // from what our page is served on. This might happen if a CDN is used to
+            // serve assets; see https://github.com/facebook/create-react-app/issues/2374
+            return;
+        }
+
+        window.addEventListener('load', () => {
+            const swUrl = `${process.env.PUBLIC_URL}/service-worker.js`;
+
+            if (isLocalhost) {
+                // This is running on localhost. Let's check if a service worker still exists or not.
+                checkValidServiceWorker(swUrl, config);
+
+                // Add some additional logging to localhost, pointing developers to the
+                // service worker/PWA documentation.
+                navigator.serviceWorker.ready.then(() => {
+                    console.log(
+                        'This web app is being served cache-first by a service ' +
+                            'worker. To learn more, visit https://bit.ly/CRA-PWA'
+                    );
+                });
+            } else {
+                // Is not localhost. Just register service worker
+                registerValidSW(swUrl, config);
+            }
+        });
+    }
+}
+
+function registerValidSW(swUrl: string, config?: Config) {
+    navigator.serviceWorker
+        .register(swUrl)
+        .then((registration) => {
+            registration.onupdatefound = () => {
+                const installingWorker = registration.installing;
+                if (installingWorker === null) {
+                    return;
+                }
+                installingWorker.onstatechange = () => {
+                    if (installingWorker.state === 'installed') {
+                        if (navigator.serviceWorker.controller) {
+                            // At this point, the updated precached content has been fetched,
+                            // but the previous service worker will still serve the older
+                            // content until all client tabs are closed.
+                            console.log(
+                                'New content is available and will be used when all ' +
+                                    'tabs for this page are closed. See https://bit.ly/CRA-PWA.'
+                            );
+
+                            // Execute callback
+                            if (config && config.onUpdate) {
+                                config.onUpdate(registration);
+                            }
+                        } 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 cached for offline use.');
+
+                            // Execute callback
+                            if (config && config.onSuccess) {
+                                config.onSuccess(registration);
+                            }
+                        }
+                    }
+                };
+            };
+        })
+        .catch((error) => {
+            console.error('Error during service worker registration:', error);
+        });
+}
+
+function checkValidServiceWorker(swUrl: string, config?: Config) {
+    // Check if the service worker can be found. If it can't reload the page.
+    fetch(swUrl, {
+        headers: { 'Service-Worker': 'script' },
+    })
+        .then((response) => {
+            // Ensure service worker exists, and that we really are getting a JS file.
+            const contentType = response.headers.get('content-type');
+            if (response.status === 404 || (contentType !== null && contentType.indexOf('javascript') === -1)) {
+                // No service worker found. Probably a different app. Reload the page.
+                navigator.serviceWorker.ready.then((registration) => {
+                    registration.unregister().then(() => {
+                        window.location.reload();
+                    });
+                });
+            } else {
+                // Service worker found. Proceed as normal.
+                registerValidSW(swUrl, config);
+            }
+        })
+        .catch(() => {
+            console.log('No internet connection found. App is running in offline mode.');
+        });
+}
+
+export function unregister() {
+    if ('serviceWorker' in navigator) {
+        navigator.serviceWorker.ready
+            .then((registration) => {
+                registration.unregister();
+            })
+            .catch((error) => {
+                console.error(error.message);
+            });
+    }
+}