Progressive Web App

Progressive Web Apps (PWAs)

Overview

A Progressive Web App is a web application that uses modern browser capabilities to deliver a fast, reliable, and installable experience — even on unreliable networks. The three required pillars are:

1. HTTPS — Required in production for service workers to register (localhost is exempt for development).
2. Web App Manifest (manifest.json) — Makes the app installable and defines its appearance on device home screens.
3. Service Worker (sw.js) — A background script that intercepts network requests, manages caches, and enables offline functionality.

When to Use This Skill

• Use when the user wants their web app to work offline or on unreliable networks.
• Use when building a mobile-first web project where users should be able to install the app to their home screen.
• Use when the user asks about caching strategies, service workers, or improving web app performance and resilience.
• Use when the user mentions Workbox, web app manifests, background sync, or push notifications for the web.
• Use when the user asks "can my website be installed like an app?" or "how do I make my site work offline?" — even if they don't use the word PWA.

Deliverables Checklist

Every PWA implementation must include these files at minimum:

• index.html — Links manifest, registers service worker
• manifest.json — Full app metadata and icon set
• sw.js — Service worker with install, activate, and fetch handlers
• app.js — Main app logic with SW registration and install prompt handling
• offline.html — Fallback page shown when navigation fails offline (required — missing file will cause install to fail)

Step 1: Web App Manifest (manifest.json)

Defines how the app appears when installed. Must be linked from <head> via <link rel="manifest">.

{
  "name": "My Awesome PWA",
  "short_name": "MyPWA",
  "description": "A fast, offline-capable Progressive Web App.",
  "start_url": "/",
  "scope": "/",
  "display": "standalone",
  "orientation": "portrait-primary",
  "background_color": "#ffffff",
  "theme_color": "#0055ff",
  "icons": [
    {
      "src": "/assets/icons/icon-192x192.png",
      "sizes": "192x192",
      "type": "image/png",
      "purpose": "any maskable"
    },
    {
      "src": "/assets/icons/icon-512x512.png",
      "sizes": "512x512",
      "type": "image/png",
      "purpose": "any maskable"
    }
  ],
  "screenshots": [
    {
      "src": "/assets/screenshots/desktop.png",
      "sizes": "1280x720",
      "type": "image/png",
      "form_factor": "wide"
    }
  ]
}

Key fields:

• display: standalone hides browser UI; minimal-ui shows minimal controls; browser is standard tab.
• purpose: "maskable" on icons enables adaptive icons on Android (safe zone matters — keep content in center 80%).
• screenshots is optional but required for Chrome's enhanced install dialog on desktop.

Step 2: HTML Shell (index.html)

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>My Awesome PWA</title>

  <!-- PWA manifest -->
  <link rel="manifest" href="/manifest.json">

  <!-- Theme color for browser chrome -->
  <meta name="theme-color" content="#0055ff">

  <!-- iOS-specific (Safari doesn't fully use manifest) -->
  <meta name="apple-mobile-web-app-capable" content="yes">
  <meta name="apple-mobile-web-app-status-bar-style" content="default">
  <meta name="apple-mobile-web-app-title" content="MyPWA">
  <link rel="apple-touch-icon" href="/assets/icons/icon-192x192.png">

  <link rel="stylesheet" href="/styles.css">
</head>
<body>
  <div id="app">
    <header><h1>My PWA</h1></header>
    <main id="content">Loading...</main>
    <!-- Optional: install button, hidden by default -->
    <button id="install-btn" hidden>Install App</button>
  </div>
  <script src="/app.js"></script>
</body>
</html>

Step 3: Service Worker Registration & Install Prompt (app.js)

// ─── Service Worker Registration ───────────────────────────────────────────
if ('serviceWorker' in navigator) {
  window.addEventListener('load', async () => {
    try {
      const registration = await navigator.serviceWorker.register('/sw.js');
      console.log('[App] SW registered, scope:', registration.scope);
    } catch (err) {
      console.error('[App] SW registration failed:', err);
    }
  });
}

// ─── Install Prompt (Add to Home Screen) ───────────────────────────────────
let deferredPrompt;
const installBtn = document.getElementById('install-btn'); // may be null if omitted

// Capture the browser's install prompt — it fires before the browser's own UI
window.addEventListener('beforeinstallprompt', (e) => {
  e.preventDefault(); // Stop automatic mini-infobar on mobile
  deferredPrompt = e;
  if (installBtn) installBtn.hidden = false; // Show your custom install button
});

if (installBtn) {
  installBtn.addEventListener('click', async () => {
    if (!deferredPrompt) return;
    deferredPrompt.prompt();
    const { outcome } = await deferredPrompt.userChoice;
    console.log('[App] Install outcome:', outcome);
    deferredPrompt = null;
    installBtn.hidden = true;
  });
}
// Fires when the app is installed (via browser or your button)
window.addEventListener('appinstalled', () => {
  console.log('[App] PWA installed successfully');
  installBtn.hidden = true;
});

Step 4: Service Worker (sw.js)

Cache Versioning (critical — always increment on deploy)

const CACHE_VERSION = 'v1';
const STATIC_CACHE = `static-${CACHE_VERSION}`;
const DYNAMIC_CACHE = `dynamic-${CACHE_VERSION}`;

// Files to pre-cache during install (the "App Shell")
const APP_SHELL = [
  '/',
  '/index.html',
  '/styles.css',
  '/app.js',
  '/assets/icons/icon-192x192.png',
  '/offline.html', // Fallback page shown when network is unavailable
];

Install — Pre-cache the App Shell

self.addEventListener('install', (event) => {
  console.log('[SW] Installing...');
  event.waitUntil(
    caches.open(STATIC_CACHE).then((cache) => {
      console.log('[SW] Pre-caching app shell');
      return cache.addAll(APP_SHELL);
    })
  );
  // Activate immediately without waiting for old SW to die
  self.skipWaiting();
});

Activate — Clean Up Old Caches

self.addEventListener('activate', (event) => {
  console.log('[SW] Activating...');
  event.waitUntil(
    caches.keys().then((cacheNames) => {
      return Promise.all(
        cacheNames
          .filter((name) => name !== STATIC_CACHE && name !== DYNAMIC_CACHE)
          .map((name) => {
            console.log('[SW] Deleting old cache:', name);
            return caches.delete(name);
          })
      );
    })
  );
  // Take control of all pages immediately
  self.clients.claim();
});

Fetch — Caching Strategies

Choose the right strategy per resource type:

self.addEventListener('fetch', (event) => {
  const { request } = event;
  const url = new URL(request.url);

  // Only handle GET requests from our own origin
  if (request.method !== 'GET' || url.origin !== location.origin) return;

  // Strategy A: Cache-First (for static assets — fast, tolerates stale)
  if (url.pathname.match(/\.(css|js|png|jpg|svg|woff2)$/)) {
    event.respondWith(cacheFirst(request));
    return;
  }

  // Strategy B: Network-First (for HTML pages — fresh, falls back to cache)
  if (request.headers.get('Accept')?.includes('text/html')) {
    event.respondWith(networkFirst(request));
    return;
  }

  // Strategy C: Stale-While-Revalidate (for API data — fast and eventually fresh)
  if (url.pathname.startsWith('/api/')) {
    event.respondWith(staleWhileRevalidate(request));
    return;
  }
});

// ─── Strategy Implementations ──────────────────────────────────────────────

async function cacheFirst(request) {
  const cached = await caches.match(request);
  if (cached) return cached;
  try {
    const response = await fetch(request);
    const cache = await caches.open(STATIC_CACHE);
    cache.put(request, response.clone());
    return response;
  } catch {
    // Nothing useful to fall back to for assets
    return new Response('Asset unavailable offline', { status: 503 });
  }
}

async function networkFirst(request) {
  try {
    const response = await fetch(request);
    const cache = await caches.open(DYNAMIC_CACHE);
    cache.put(request, response.clone());
    return response;
  } catch {
    const cached = await caches.match(request);
    return cached || caches.match('/offline.html');
  }
}

async function staleWhileRevalidate(request) {
  const cache = await caches.open(DYNAMIC_CACHE);
  const cached = await cache.match(request);
  const fetchPromise = fetch(request).then((response) => {
    cache.put(request, response.clone());
    return response;
  });
  return cached || fetchPromise;
}

Edge Cases & Platform Notes

iOS / Safari Quirks

• Safari supports manifests and service workers but does not support beforeinstallprompt — users must install via the Share → "Add to Home Screen" menu manually.
• Use the apple-mobile-web-app-* meta tags (shown in index.html above) for proper iOS integration.
• Safari may clear service worker caches after ~7 days of inactivity (Intelligent Tracking Prevention).

HTTPS Requirement

• Service workers only register on https:// origins. http://localhost is the only exception for development.
• Use a tool like mkcert or ngrok if you need HTTPS locally with a custom hostname.

Cache-Busting on Deploy

• Always increment CACHE_VERSION in sw.js when deploying new assets. This ensures activate clears old caches and users get fresh files.
• A common pattern is to inject the version automatically via your build tool (e.g., Vite, Webpack).

Opaque Responses (cross-origin requests)

• Requests to external origins (e.g., CDN fonts, third-party APIs) return "opaque" responses that cannot be inspected. Cache them with caution — a failed opaque response still gets a 200 status.
• Prefer staleWhileRevalidate for cross-origin resources, or use a library like Workbox which handles this safely.

Workbox (Optional: Production Shortcut)

For production apps, consider Workbox (Google's PWA library) instead of hand-rolling strategies. It handles edge cases, cache expiry, and versioning automatically.

// With Workbox (via CDN for simplicity — use npm + bundler in production)
importScripts('https://storage.googleapis.com/workbox-cdn/releases/7.0.0/workbox-sw.js');

const { registerRoute } = workbox.routing;
const { CacheFirst, NetworkFirst, StaleWhileRevalidate } = workbox.strategies;
const { precacheAndRoute } = workbox.precaching;

precacheAndRoute(self.__WB_MANIFEST || []); // Injected by build plugin

registerRoute(({ request }) => request.destination === 'image', new CacheFirst());
registerRoute(({ request }) => request.mode === 'navigate', new NetworkFirst());
registerRoute(({ request }) => request.destination === 'script', new StaleWhileRevalidate());

Checklist Before Shipping

• Site is served over HTTPS
• manifest.json has name, short_name, start_url, display, icons (192 + 512)
• Icons have purpose: "any maskable"
• sw.js registers without errors in DevTools → Application → Service Workers
• App shell loads from cache when network is throttled to "Offline" in DevTools
• offline.html fallback is cached and served when navigation fails offline
• Lighthouse PWA audit passes (Chrome DevTools → Lighthouse tab)
• Tested on iOS Safari (manual install flow) and Android Chrome (install prompt)