This snippet shows how to notify the user of a new version of the site (a new service worker version)

notify-user-of-update.js

// Offline Web Applications course on Udacity https://classroom.udacity.com/courses/ud899

//This code is to be placed in a controller file, not in the service worker itself
IndexController.prototype._registerServiceWorker = function() {
  if (!navigator.serviceWorker) return; //Feature detection

  var indexController = this;

  navigator.serviceWorker.register('/sw.js').then(function(reg) {
    // If there's no controller, this page wasn't loaded via a service worker, so they're looking at the latest version.
    // In that case, exit early
    if (!navigator.serviceWorker.controller) return;

    // If there's an updated worker already waiting, call
    // indexController._updateReady()
    if (reg.waiting) {
      indexController._updateReady();
      return;
    }

    // If there's an updated worker installing, track its progress. If it becomes "installed", call
    // indexController._updateReady()
    if (reg.installing) {
      indexController._trackInstalling(reg.installing);
      return;
    }

    // TODO: otherwise, listen for new installing workers arriving.
    // If one arrives, track its progress.
    // If it becomes "installed", call
    // indexController._updateReady()
    reg.addEventListener('updatefound', function() {
      indexController._trackInstalling(reg.installing);
    });
  });
};

//A function to make it easy to let the user know there is an update ready
IndexController.prototype._updateReady = function() {
  //Toasts view is some external plugin. This is where you will use whatever plugin, message or alert() that you want to notify the user of the update
  var toast = this._toastsView.show("New version available", {
    buttons: ['Refresh']
  });
};

//A function the track the installation status of a worker
IndexController.prototype._trackInstalling = function(worker) {
  var indexController = this;
  
  //The worker will fire a statechange event when moving between 'installing', 'installed', 'active' etc
  worker.addEventListener('statechange', function() {
    //If the worker is now installed, let the user know that there is an update ready
    if (worker.state == 'installed') {
      indexController._updateReady();
    }
  });
};