# src/serviceWorker.js

    // 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 http://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}$/
        )
    );

    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 http://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 http://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();
        });
      }
    }