text: // In production, we register a service worker to serve assets from local cache.
  
  // 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 the "N+1" visit to a page, since previously
  // cached resources are updated in the background.
  
  // To learn more about the benefits of this model, read https://goo.gl/KwvDNy.
  // This link also includes instructions on opting out of this behavior.
  
  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 default function register() {
      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);
          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/facebookincubator/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. Lets check if a service worker still exists or not.
                  checkValidServiceWorker(swUrl);
  
                  // 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://goo.gl/SC7cgQ"
                      );
                  });
              } else {
                  // Is not local host. Just register service worker
                  registerValidSW(swUrl);
              }
          });
      }
  }
  
  function registerValidSW(swUrl) {
      navigator.serviceWorker
          .register(swUrl)
          .then(registration => {
              registration.onupdatefound = () => {
                  const installingWorker = registration.installing;
                  installingWorker.onstatechange = () => {
                      if (installingWorker.state === "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 your web app.
                              console.log("New content is available; please refresh.");
                          } 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.");
                          }
                      }
                  };
              };
          })
          .catch(error => {
              console.error("Error during service worker registration:", error);
          });
  }
  
  function checkValidServiceWorker(swUrl) {
      // 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.
              if (response.status === 404 || response.headers.get("content-type").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);
              }
          })
          .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();
          });
      }
  }
  