Source: registerServiceWorker.js

/**
 * @file
 *
 * Summary.
 * <p>In production, we register a service worker to serve assets from local cache.</p>
 *
 * This lets the app load faster on subsequent visits in production, and gives
 * it offline capabilities. <br>
 * However, it also means that developers (and users)
 * will only see deployed updates on the "N+1" visit to a page, <br>
 * since previously cached resources are updated in the background.
 *
 * <p>To learn more about the benefits of this model, read about {@link https://create-react-app.dev/docs/making-a-progressive-web-app/ PWA}.<br>
 * This link also includes instructions on opting out of this behavior.</p>
 * @see <a href="https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API">Service Workers, MDN</a>
 * @see <a href="https://developers.google.com/web/fundamentals/primers/service-workers/">Service Workers, Google Web Fundamentals</a>
 * @see <a href="https://www.smashingmagazine.com/2016/02/making-a-service-worker/">Server worker: A case study, Smashing Magazine</a>
 * @see <a href="https://css-tricks.com/serviceworker-for-offline/">Simple offline site using service worker, CSS Tricks</a>
 * @see <a href="https://github.com/facebook/create-react-app/blob/main/packages/cra-template/template/README.md">Getting Started with Create React App</a>
 * @see <a href="https://www.npmjs.com/package/register-service-worker">npm</a>
 */
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();
    });
  }
}