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();
 });
 }
}