gistfile1.js

    if ('serviceWorker' in navigator && location.protocol !== 'file:') {
      window.addEventListener('load', function(){
        navigator.serviceWorker.register('/sw.js')
          .then(function(reg) { 
            reg.addEventListener('updatefound', function() {
              const toast = document.createElement('div');
              toast.innerHTML = '<button>Update available</button>';

              document.body.appendChild(toast);
            })
           })
          .catch(function(err) { console.log('service worker error', err) });
      });
    }