Service Worker

1) is a Javascript file run in background and assists in  offline web application.
2) is a cache

<script>

if ('serviceWorker' in navigator) {

navigator.serviceWorker.register('service-worker.js')

.then(() => console.log('service worker installed'))

.catch(err => console.error('Error', err));

}

</script>

service-worker.js

var CACHE_VERSION = '0.1.1'; //Update Version