registerServiceWorker.js

// 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();
        });
    }
}