text: // 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]' ||
      // 127.0.0.1/8 is considered localhost for IPv4.
      window.location.hostname.match(
          /^127(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$/
      )
  );
  
  // noinspection JSUnusedGlobalSymbols
  export function register(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, 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, config) {
      // Check if the service worker can be found. If it can't reload the page.
      fetch(swUrl)
          .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();
          });
      }
  }
  