# registerServiceWorker.js

## Maintainability

81.41

## Lines of code

118

## Difficulty

22.20

## Estimated Errors

0.56

## Function weight

### By Complexity

### By SLOC

// 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(); }); } }