Beginning Progressive Web App Development

1. Introduction to Progressive Web Apps

What is a Progressive Web App
A browser-based web application built from a set of technologies that bring fast, always-available, and native mobile app experience to users.

Components that make a PWA

  • Offline Support
  • Notifications
  • Home screen

Service Workers vs Web Workers
Web Workers are simple scripts that you can run in the background for resource-intensive processes. It lacks the capability of Service workers to intercept HTTP requests. Service Workers run on the same Worker thread.

2. Tools to Measure Progressive Web Apps

Helpful links for testing your PWA’s

3. Service Workers

Technical features of service workers

  • Runs in the background
  • Does not have access to the DOM
  • Runs on its own thread and not on the UI thread
  • Uses promises heavily
  • Captures HTTP requests

Basic Service Worker registration

Add this script tag in your HTML file
<script src="/js/main.js"></script>
(() => {
 if ('serviceWorker' in navigator) {
   window.addEventListener('load', () => {
       .then((registration) => {
         // registration is of type ServiceWorkerRegistration
         console.log('registered', registration);
       }, (err) => {
 } else {
   alert('No service worker support in this browser');
/* /js/service-worker.js */

self.addEventListener('install', (event) => {
 // forces the waiting service worker to become active.

 // event is an instance of InstallEvent
 console.log('service worker installed', event);
self.addEventListener('activate', (event) => {
 // event is an instance of ExtendableEvent
 console.log('service worker activated', event);

Why is the placement of your service worker file important?
The location where you put your service worker determines its scope. In the example above, the service worker file is placed inside the js folder. This means that your worker will have access to every HTTP request that has /js/* in its URL. It’s scope then is everything in and below the js folder.

4. Caching and Offline Functionality with Service Workers

npm helper packages for caching and offline functionality

5. Background Sync for Offline Apps with Service Workers

npm libraries to make IndexedDB queries easier

6. Adding your App to the Home Screen with Web App Manifest

Analytics tracking for users who installed your site on their home screen
Hook into the window.addEventListener('beforeinstallprompt', ()=>{} to run custom code you need to gain insight when users install your site on their home screen

7. Notifications

New code constructs


8. App Shell Architecture and Loading Performance

async and defer in the script tag Async and Defer

Both allow you to download the HTML resource beforehand. Preload is for the current page while Prefetch is for the next page and to let the browser decide when the download begins. Large images, fonts, videos, and huge scripts are good candidates for these attributes.

 <meta charset="utf-8">
 <link rel="preload" href="style.css" as="style">
 <link rel="preload" href="main.js" as="script">
 <link rel="stylesheet" href="style.css">
 <script src="main.js"></script>

9. Exploring HTTP/2 and Server Push

What HTTP/2 improves on from HTTP/1.1

Http/1.1 Http/2
Head-of-line Blocking Yes Sends multiple requests in a single TCP connection (Pipelining) File Requests are returned based on the order they are sent. Subsequent requests are block until the previous one is completed. Single bundled file is preferred to save extra HTTP trips No Multiplexing support - single bidrectional connection for HTTP Requests Multiplexing is a newer version of SPDY Several smaller files are preferred to invalidate cache faster
Compressed Headers No Yes HPACK Header Compression reduces redundant Request and Response Headers data
Server Push No Yes - Server push allows you to send HTTP resources even before the browser requests it. It isn't something you write as a server-side piece of code but rather a feature your hosting provider offers for you to configure.

How do I get access to the HTTP/2 Features?
Access to these features are offered out-of-the-box by hosting companies. For local development, the npm package spdy will aid in creating an HTTP/2 server. Head-of-line blocking and compressed headers help to speed up your page’s load times. Server push can do the same but there are considerations to make like how much you should push and how to time it correctly. As such, measuring performance improvements should be the main driver when adopting server push.

10. Turning a Real App into a PWA

Web App Manifest Generator . You can use Chrome’s Dev Tools > Application > Manifest to view the configuration

11. PWAs From the Start

Javascript libraries that support PWA capabilities out-of-the-box

  • React - to use runtime caching, npm run eject to configure it manually
  • Preact - to use runtime caching, npm install --save-dev preact-cli-sw-precache
  • Vuejs - provides a pwa template
  • Angular - ng new project --service-worker=true. Least performant out of all the mentioned frameworks

12. Leveling Up Your PWA


  • Google’s Workbox beta at the time of writing, a collection of libraries for building pwa’s