Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-register-on-demand.js

if('serviceWorker' in navigator) {
  document.querySelector('.hero').addEventListener('animationend', function () {
    // service worker is registered only when the animation ends
    navigator.serviceWorker.register('/serviceworker.js');
  });
}
// ---
App.init({
  // config
  complete: function() {
    // service worker is registered only when the app is initiated
    if('serviceWorker' in navigator) {
      navigator.serviceWorker.register('/serviceworker.js');
    }
  }
});