PWA Development Skill

Load with: base.md

Purpose: Build Progressive Web Apps that work offline, install like native apps, and deliver fast, reliable experiences across all devices.

---

Core PWA Requirements

┌─────────────────────────────────────────────────────────────────┐
│  THE THREE PILLARS OF PWA                                       │
│  ─────────────────────────────────────────────────────────────  │
│                                                                 │
│  1. HTTPS                                                       │
│     Required for service workers and security.                  │
│     localhost allowed for development.                          │
│                                                                 │
│  2. SERVICE WORKER                                              │
│     JavaScript that runs in background.                         │
│     Enables offline, caching, push notifications.               │
│                                                                 │
│  3. WEB APP MANIFEST                                            │
│     JSON file describing app metadata.                          │
│     Enables installation and app-like experience.               │
├─────────────────────────────────────────────────────────────────┤
│  INSTALLABILITY CRITERIA (Chrome)                               │
│  ─────────────────────────────────────────────────────────────  │
│  • HTTPS (or localhost)                                         │
│  • Service worker with fetch handler                            │
│  • Web app manifest with: name, icons (192px + 512px),          │
│    start_url, display: standalone/fullscreen/minimal-ui         │
└─────────────────────────────────────────────────────────────────┘

---

Web App Manifest

Required Fields

{
  "name": "My Progressive Web App",
  "short_name": "MyPWA",
  "description": "A description of what the app does",
  "start_url": "/",
  "display": "standalone",
  "background_color": "#ffffff",
  "theme_color": "#000000",
  "icons": [
    {
      "src": "/icons/icon-192.png",
      "sizes": "192x192",
      "type": "image/png"
    },
    {
      "src": "/icons/icon-512.png",
      "sizes": "512x512",
      "type": "image/png"
    },
    {
      "src": "/icons/icon-512-maskable.png",
      "sizes": "512x512",
      "type": "image/png",
      "purpose": "maskable"
    }
  ]
}

Enhanced Manifest (Full Features)

{
  "name": "My Progressive Web App",
  "short_name": "MyPWA",
  "description": "A full-featured PWA",
  "start_url": "/?source=pwa",
  "scope": "/",
  "display": "standalone",
  "orientation": "portrait-primary",
  "background_color": "#ffffff",
  "theme_color": "#3367D6",
  "dir": "ltr",
  "lang": "en",
  "categories": ["productivity", "utilities"],

  "icons": [
    { "src": "/icons/icon-72.png", "sizes": "72x72", "type": "image/png" },
    { "src": "/icons/icon-96.png", "sizes": "96x96", "type": "image/png" },
    { "src": "/icons/icon-128.png", "sizes": "128x128", "type": "image/png" },
    { "src": "/icons/icon-144.png", "sizes": "144x144", "type": "image/png" },
    { "src": "/icons/icon-152.png", "sizes": "152x152", "type": "image/png" },
    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
    { "src": "/icons/icon-384.png", "sizes": "384x384", "type": "image/png" },
    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png" },
    { "src": "/icons/icon-maskable.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
  ],

  "screenshots": [
    {
      "src": "/screenshots/desktop.png",
      "sizes": "1280x720",
      "type": "image/png",
      "form_factor": "wide"
    },
    {
      "src": "/screenshots/mobile.png",
      "sizes": "750x1334",
      "type": "image/png",
      "form_factor": "narrow"
    }
  ],

  "shortcuts": [
    {
      "name": "New Item",
      "short_name": "New",
      "description": "Create a new item",
      "url": "/new?source=shortcut",
      "icons": [{ "src": "/icons/shortcut-new.png", "sizes": "192x192" }]
    }
  ],

  "share_target": {
    "action": "/share",
    "method": "POST",
    "enctype": "multipart/form-data",
    "params": {
      "title": "title",
      "text": "text",
      "url": "url",
      "files": [{ "name": "files", "accept": ["image/*"] }]
    }
  },

  "protocol_handlers": [
    {
      "protocol": "web+myapp",
      "url": "/handle?url=%s"
    }
  ],

  "file_handlers": [
    {
      "action": "/open-file",
      "accept": {
        "text/plain": [".txt"]
      }
    }
  ]
}

Manifest Checklist

• name and short_name defined
• start_url set (use query param for analytics)
• display set to standalone or fullscreen
• Icons: 192x192 and 512x512 minimum
• Maskable icon included for Android adaptive icons
• theme_color matches app design
• background_color for splash screen
• Screenshots for richer install UI (optional)
• Shortcuts for quick actions (optional)

---

Service Worker Patterns

Basic Service Worker

// sw.js
const CACHE_NAME = 'app-cache-v1';
const STATIC_ASSETS = [
  '/',
  '/index.html',
  '/styles/main.css',
  '/scripts/app.js',
  '/offline.html'
];

// Install: Cache static assets
self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(CACHE_NAME)
      .then((cache) => cache.addAll(STATIC_ASSETS))
      .then(() => self.skipWaiting())
  );
});

// Activate: Clean old caches
self.addEventListener('activate', (event) => {
  event.waitUntil(
    caches.keys()
      .then((keys) => Promise.all(
        keys
          .filter((key) => key !== CACHE_NAME)
          .map((key) => caches.delete(key))
      ))
      .then(() => self.clients.claim())
  );
});

// Fetch: Serve from cache, fall back to network
self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.match(event.request)
      .then((cached) => cached || fetch(event.request))
      .catch(() => caches.match('/offline.html'))
  );
});

Registration

// main.js
if ('serviceWorker' in navigator) {
  window.addEventListener('load', async () => {
    try {
      const registration = await navigator.serviceWorker.register('/sw.js', {
        scope: '/'
      });
      console.log('SW registered:', registration.scope);
    } catch (error) {
      console.error('SW registration failed:', error);
    }
  });
}

---

Caching Strategies

Strategy Selection Guide

Strategy	Use Case	Description
Cache First	Static assets (CSS, JS, images)	Check cache, fall back to network
Network First	API responses, dynamic content	Try network, fall back to cache
Stale While Revalidate	Semi-static content (avatars, articles)	Serve cache immediately, update in background
Network Only	Non-cacheable requests (analytics)	Always use network
Cache Only	Offline-only assets	Only serve from cache

Cache First (Offline First)

// Best for: Static assets that rarely change
self.addEventListener('fetch', (event) => {
  if (event.request.destination === 'image' ||
      event.request.destination === 'style' ||
      event.request.destination === 'script') {
    event.respondWith(
      caches.match(event.request)
        .then((cached) => {
          if (cached) return cached;
          return fetch(event.request).then((response) => {
            const clone = response.clone();
            caches.open(CACHE_NAME).then((cache) => {
              cache.put(event.request, clone);
            });
            return response;
          });
        })
    );
  }
});

Network First (Fresh First)

// Best for: API data, frequently updated content
self.addEventListener('fetch', (event) => {
  if (event.request.url.includes('/api/')) {
    event.respondWith(
      fetch(event.request)
        .then((response) => {
          const clone = response.clone();
          caches.open(CACHE_NAME).then((cache) => {
            cache.put(event.request, clone);
          });
          return response;
        })
        .catch(() => caches.match(event.request))
    );
  }
});

Stale While Revalidate

// Best for: Content that's okay to be slightly outdated
self.addEventListener('fetch', (event) => {
  if (event.request.url.includes('/articles/')) {
    event.respondWith(
      caches.open(CACHE_NAME).then((cache) => {
        return cache.match(event.request).then((cached) => {
          const fetchPromise = fetch(event.request).then((response) => {
            cache.put(event.request, response.clone());
            return response;
          });
          return cached || fetchPromise;
        });
      })
    );
  }
});

---

Workbox (Recommended)

Why Workbox?

• Battle-tested caching strategies
• Precaching with revision management
• Background sync for offline forms
• Automatic cache cleanup
• TypeScript support

Installation

npm install workbox-webpack-plugin  # Webpack
npm install @vite-pwa/vite-plugin   # Vite

Workbox with Vite

// vite.config.js
import { VitePWA } from 'vite-plugin-pwa';

export default {
  plugins: [
    VitePWA({
      registerType: 'autoUpdate',
      includeAssets: ['favicon.ico', 'robots.txt', 'apple-touch-icon.png'],
      manifest: {
        name: 'My App',
        short_name: 'App',
        theme_color: '#ffffff',
        icons: [
          { src: 'pwa-192x192.png', sizes: '192x192', type: 'image/png' },
          { src: 'pwa-512x512.png', sizes: '512x512', type: 'image/png' }
        ]
      },
      workbox: {
        globPatterns: ['**/*.{js,css,html,ico,png,svg}'],
        runtimeCaching: [
          {
            urlPattern: /^https:\/\/api\.example\.com\/.*/i,
            handler: 'NetworkFirst',
            options: {
              cacheName: 'api-cache',
              expiration: {
                maxEntries: 100,
                maxAgeSeconds: 60 * 60 * 24 // 24 hours
              }
            }
          },
          {
            urlPattern: /\.(?:png|jpg|jpeg|svg|gif)$/,
            handler: 'CacheFirst',
            options: {
              cacheName: 'image-cache',
              expiration: {
                maxEntries: 50,
                maxAgeSeconds: 60 * 60 * 24 * 30 // 30 days
              }
            }
          }
        ]
      }
    })
  ]
};

Workbox Manual Service Worker

// sw.js
import { precacheAndRoute } from 'workbox-precaching';
import { registerRoute } from 'workbox-routing';
import { CacheFirst, NetworkFirst, StaleWhileRevalidate } from 'workbox-strategies';
import { ExpirationPlugin } from 'workbox-expiration';
import { CacheableResponsePlugin } from 'workbox-cacheable-response';

// Precache static assets (generated by build tool)
precacheAndRoute(self.__WB_MANIFEST);

// Cache images
registerRoute(
  ({ request }) => request.destination === 'image',
  new CacheFirst({
    cacheName: 'images',
    plugins: [
      new CacheableResponsePlugin({ statuses: [0, 200] }),
      new ExpirationPlugin({
        maxEntries: 60,
        maxAgeSeconds: 30 * 24 * 60 * 60 // 30 days
      })
    ]
  })
);

// Cache API responses
registerRoute(
  ({ url }) => url.pathname.startsWith('/api/'),
  new NetworkFirst({
    cacheName: 'api-responses',
    plugins: [
      new CacheableResponsePlugin({ statuses: [0, 200] }),
      new ExpirationPlugin({
        maxEntries: 100,
        maxAgeSeconds: 24 * 60 * 60 // 24 hours
      })
    ]
  })
);

// Cache page navigations
registerRoute(
  ({ request }) => request.mode === 'navigate',
  new NetworkFirst({
    cacheName: 'pages',
    plugins: [
      new CacheableResponsePlugin({ statuses: [0, 200] })
    ]
  })
);

---

Offline Experience

Offline Page

<!-- offline.html -->
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Offline - App Name</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      display: flex;
      align-items: center;
      justify-content: center;
      min-height: 100vh;
      margin: 0;
      background: #f5f5f5;
    }
    .offline-content {
      text-align: center;
      padding: 2rem;
    }
    .offline-icon { font-size: 4rem; }
    h1 { color: #333; }
    p { color: #666; }
    button {
      background: #3367D6;
      color: white;
      border: none;
      padding: 0.75rem 1.5rem;
      border-radius: 4px;
      cursor: pointer;
      font-size: 1rem;
    }
  </style>
</head>
<body>
  <div class="offline-content">
    <div class="offline-icon">📡</div>
    <h1>You're offline</h1>
    <p>Check your connection and try again.</p>
    <button onclick="location.reload()">Retry</button>
  </div>
</body>
</html>

Offline Detection

// Online/offline status handling
function updateOnlineStatus() {
  const status = navigator.onLine ? 'online' : 'offline';
  document.body.dataset.connectionStatus = status;

  if (!navigator.onLine) {
    showNotification('You are offline. Some features may be unavailable.');
  }
}

window.addEventListener('online', updateOnlineStatus);
window.addEventListener('offline', updateOnlineStatus);
updateOnlineStatus();

Background Sync (Queue Offline Actions)

// sw.js with Workbox
import { BackgroundSyncPlugin } from 'workbox-background-sync';
import { registerRoute } from 'workbox-routing';
import { NetworkOnly } from 'workbox-strategies';

const bgSyncPlugin = new BackgroundSyncPlugin('formQueue', {
  maxRetentionTime: 24 * 60 // Retry for 24 hours
});

registerRoute(
  ({ url }) => url.pathname === '/api/submit',
  new NetworkOnly({
    plugins: [bgSyncPlugin]
  }),
  'POST'
);

// main.js - Queue form submission
async function submitForm(data) {
  try {
    const response = await fetch('/api/submit', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(data)
    });
    return response.json();
  } catch (error) {
    // Will be retried by background sync when online
    showNotification('Saved offline. Will sync when connected.');
  }
}

---

App-Like Features

Install Prompt

let deferredPrompt;

window.addEventListener('beforeinstallprompt', (e) => {
  e.preventDefault();
  deferredPrompt = e;
  showInstallButton();
});

async function installApp() {
  if (!deferredPrompt) return;

  deferredPrompt.prompt();
  const { outcome } = await deferredPrompt.userChoice;

  console.log(`User ${outcome === 'accepted' ? 'accepted' : 'dismissed'} install`);
  deferredPrompt = null;
  hideInstallButton();
}

window.addEventListener('appinstalled', () => {
  console.log('App installed');
  deferredPrompt = null;
});

Detecting Standalone Mode

// Check if running as installed PWA
function isInstalledPWA() {
  return window.matchMedia('(display-mode: standalone)').matches ||
         window.navigator.standalone === true; // iOS
}

// Listen for display mode changes
window.matchMedia('(display-mode: standalone)')
  .addEventListener('change', (e) => {
    console.log('Display mode:', e.matches ? 'standalone' : 'browser');
  });

Push Notifications

// Request permission
async function requestNotificationPermission() {
  const permission = await Notification.requestPermission();
  if (permission === 'granted') {
    await subscribeToPush();
  }
  return permission;
}

// Subscribe to push
async function subscribeToPush() {
  const registration = await navigator.serviceWorker.ready;
  const subscription = await registration.pushManager.subscribe({
    userVisibleOnly: true,
    applicationServerKey: urlBase64ToUint8Array(VAPID_PUBLIC_KEY)
  });

  // Send subscription to server
  await fetch('/api/push/subscribe', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(subscription)
  });
}

// sw.js - Handle push events
self.addEventListener('push', (event) => {
  const data = event.data.json();
  event.waitUntil(
    self.registration.showNotification(data.title, {
      body: data.body,
      icon: '/icons/icon-192.png',
      badge: '/icons/badge-72.png',
      data: { url: data.url }
    })
  );
});

// Handle notification click
self.addEventListener('notificationclick', (event) => {
  event.notification.close();
  event.waitUntil(
    clients.openWindow(event.notification.data.url)
  );
});

Share Target

// sw.js - Handle share target
self.addEventListener('fetch', (event) => {
  if (event.request.url.endsWith('/share') &&
      event.request.method === 'POST') {
    event.respondWith((async () => {
      const formData = await event.request.formData();
      const title = formData.get('title');
      const text = formData.get('text');
      const url = formData.get('url');

      // Store or process shared content
      // Redirect to app with shared data
      return Response.redirect(`/?shared=true&title=${encodeURIComponent(title)}`);
    })());
  }
});

---

Performance Optimization

Critical Rendering Path

<!-- Inline critical CSS -->
<style>
  /* Critical above-the-fold styles */
</style>

<!-- Preload important resources -->
<link rel="preload" href="/fonts/main.woff2" as="font" type="font/woff2" crossorigin>
<link rel="preload" href="/scripts/app.js" as="script">

<!-- Defer non-critical CSS -->
<link rel="stylesheet" href="/styles/main.css" media="print" onload="this.media='all'">
<noscript><link rel="stylesheet" href="/styles/main.css"></noscript>

Image Optimization

<!-- Responsive images -->
<img
  src="/images/hero-800.webp"
  srcset="
    /images/hero-400.webp 400w,
    /images/hero-800.webp 800w,
    /images/hero-1200.webp 1200w
  "
  sizes="(max-width: 600px) 400px, (max-width: 1200px) 800px, 1200px"
  alt="Hero image"
  loading="lazy"
  decoding="async"
>

<!-- Modern formats with fallback -->
<picture>
  <source srcset="/images/hero.avif" type="image/avif">
  <source srcset="/images/hero.webp" type="image/webp">
  <img src="/images/hero.jpg" alt="Hero image" loading="lazy">
</picture>

Code Splitting

// Dynamic imports for route-based splitting
const routes = {
  '/': () => import('./pages/Home.js'),
  '/about': () => import('./pages/About.js'),
  '/settings': () => import('./pages/Settings.js')
};

async function loadPage(path) {
  const loader = routes[path];
  if (loader) {
    const module = await loader();
    return module.default;
  }
}

---

Testing PWA

Lighthouse Audit

# Run Lighthouse from CLI
npx lighthouse https://your-app.com --view

# Key metrics to check:
# - PWA badge (installable, offline-ready)
# - Performance score
# - Best practices
# - Accessibility

Manual Testing Checklist

• Installability

  • Install prompt appears on desktop Chrome
  • Can be added to home screen on mobile
  • App opens in standalone mode after install

• Offline Support

  • App loads when offline (airplane mode)
  • Cached pages display correctly
  • Offline fallback page shows for uncached routes
  • Background sync works when coming back online

• Performance

  • First Contentful Paint < 1.8s
  • Largest Contentful Paint < 2.5s
  • Time to Interactive < 3.8s
  • Cumulative Layout Shift < 0.1

• Service Worker

  • SW registers successfully
  • Static assets cached on install
  • SW updates correctly (new version)
  • No stale cache issues

• Manifest

  • All required fields present
  • Icons display correctly
  • Theme color applied
  • Splash screen shows on launch

Testing Service Worker Updates

// Force update check
if ('serviceWorker' in navigator) {
  navigator.serviceWorker.ready.then((registration) => {
    registration.update();
  });
}

// Listen for updates
navigator.serviceWorker.addEventListener('controllerchange', () => {
  // New service worker activated
  window.location.reload();
});

---

Project Structure

project/
├── public/
│   ├── manifest.json           # Web app manifest
│   ├── sw.js                   # Service worker (if not bundled)
│   ├── offline.html            # Offline fallback page
│   ├── robots.txt
│   └── icons/
│       ├── icon-72.png
│       ├── icon-96.png
│       ├── icon-128.png
│       ├── icon-144.png
│       ├── icon-152.png
│       ├── icon-192.png
│       ├── icon-384.png
│       ├── icon-512.png
│       ├── icon-maskable.png   # For adaptive icons
│       ├── apple-touch-icon.png
│       └── favicon.ico
├── src/
│   ├── sw.js                   # Service worker source (if bundled)
│   ├── pwa/
│   │   ├── install.js          # Install prompt handling
│   │   ├── offline.js          # Offline detection
│   │   └── push.js             # Push notification handling
│   └── ...
└── tests/
    └── pwa/
        ├── manifest.test.js
        ├── sw.test.js
        └── offline.test.js

---

Common Mistakes

Mistake	Fix
Missing maskable icon	Add icon with "purpose": "maskable"
No offline fallback	Create offline.html and cache it
Cache never expires	Use ExpirationPlugin with Workbox
SW caches too aggressively	Use appropriate strategies per resource type
No update mechanism	Implement skipWaiting() + reload prompt
Broken install prompt	Ensure manifest meets all criteria
No HTTPS in production	Configure SSL certificate
Large cache size	Set maxEntries and maxAgeSeconds
Stale API responses	Use NetworkFirst for dynamic data
Missing start_url tracking	Add query param: /?source=pwa

---

PWA Development Checklist

Before Launch

• HTTPS configured (production)
• Manifest complete with all required fields
• Icons in all required sizes (192, 512, maskable)
• Service worker registered and working
• Offline page created and cached
• Cache strategies defined for all resource types
• Install prompt handling implemented
• Lighthouse PWA audit passes

After Launch

• Monitor cache sizes
• Test SW updates don't break app
• Track PWA installs via analytics
• Test on multiple devices/browsers
• Monitor Core Web Vitals
• Set up push notification flow (if needed)

---

Framework-Specific Guides

Next.js

npm install next-pwa

// next.config.js
const withPWA = require('next-pwa')({
  dest: 'public',
  disable: process.env.NODE_ENV === 'development'
});

module.exports = withPWA({
  // Your Next.js config
});

Create React App

# CRA 4+ has PWA support built-in
npx create-react-app my-pwa --template cra-template-pwa

Vite (Any Framework)

npm install vite-plugin-pwa -D

See Workbox with Vite section above for configuration.

---

Quick Reference

Caching Strategy Cheat Sheet

Static Assets (CSS, JS, images)     → Cache First
API Responses                        → Network First
User-generated content              → Stale While Revalidate
Analytics, non-cacheable            → Network Only
Offline-only assets                 → Cache Only

Manifest Minimum Requirements

{
  "name": "App Name",
  "short_name": "App",
  "start_url": "/",
  "display": "standalone",
  "icons": [
    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" }
  ]
}

Service Worker Lifecycle

1. Register → 2. Install → 3. Activate → 4. Fetch
     ↓              ↓            ↓           ↓
  Load app    Cache assets  Clean old   Serve requests
                            caches      from cache/network