This snippet shows to to notify user when updates are available using a service worker

notify-user-of-update.js

//Offline Web Applications course on Udacity : Grow with Google Application Course 
//code is placed in the 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 no controller found - latest version is already loaded
        if (!navigator.serviceWorker.controller) return;

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

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

        //otherwise, listen for new installing workers arriving.
        // If one arrives, track its progress.
        reg.addEventListener('updateFound', function () {
            indexController._trackInstalling(reg.installing);
        });

    });
};