Progressive Web App (PWA) Development

Progressive Web Apps combine the best of web and mobile apps. IdleGames' primary PWA requirement is enabling the app to run completely offline—games and features work without any internet connection.

This skill provides guidance for building robust offline-capable PWAs with service workers, precaching strategies, and manifest configuration—based on IdleGames' production PWA implementation.

Core PWA Components

Offline-First as a Primary Requirement

IdleGames is designed to work completely offline. Users can:

•Launch the app with no internet connection
•Play all games without connecting to the network
•Save progress locally (localStorage/IndexedDB)
•Get notifications from service worker even when offline
•Have a full app experience without ever connecting

This drives all architectural decisions:

•Precaching: All HTML/CSS/JS must be cached before first use
•No external dependencies: All third-party libraries (Three.js, Transformers.js) are vendored
•Local storage: Game saves stored in localStorage, not sent to server
•Service worker durability: SW must handle full offline scenarios indefinitely

The service worker is the critical component—if it fails, the entire offline experience breaks. This is why IdleGames invests heavily in:

•Robust SW registration with retries
•URL normalization to prevent blank screens
•Navigation preload for fast cold starts
•Cache versioning to handle updates safely

1. Web App Manifest

Create a manifest.webmanifest file with proper icons and display settings:

json

{
  "name": "IdleGames",
  "short_name": "IdleGames",
  "id": "/idlegames",
  "description": "A curated collection of relaxing idle and puzzle games, plus useful web tools.",
  "start_url": "./index.html",
  "scope": "./",
  "handle_links": "preferred",
  "display": "standalone",
  "background_color": "#0a0a0a",
  "theme_color": "#667eea",
  "icons": [
    {
      "src": "assets/appicons/idle-games-512x512px.png",
      "sizes": "512x512",
      "type": "image/png",
      "purpose": "any maskable"
    },
    {
      "src": "assets/appicons/idle-games-512x512px.png",
      "sizes": "192x192",
      "type": "image/png"
    }
  ]
}

Key properties:

•id: Unique identifier, must start with /
•handle_links: "preferred": Allows opening links within scope in the app
•purpose: "maskable": Supports adaptive icon displays on different platforms
•Use real image paths (not relative to web root)

2. Service Worker Registration with Retry Logic

IdleGames uses a robust registration with cache-busting and retry logic:

javascript

// assets/js/pwa.js - Production PWA Registration
const versionModulePromise = import(`./version.js?cache-bust=${Date.now().toString(36)}`);
const SW_PATH = './sw.js';
const MAX_SW_RETRIES = 10;
const BASE_RETRY_DELAY_MS = 15000;

if ('serviceWorker' in navigator) {
  window.addEventListener('load', async () => {
    try {
      const { APP_VERSION: moduleVersion } = await versionModulePromise;
      
      // Build versioned SW URL with cache-busting parameter
      const baseSwUrl = new URL(SW_PATH, window.location.href);
      const versionedSwUrl = new URL(baseSwUrl.toString());
      versionedSwUrl.searchParams.set('version', moduleVersion);

      // Try registration with exponential backoff retry
      const ensureServiceWorkerRegistration = async (attempt = 0) => {
        try {
          // Check if SW script exists before registering
          const response = await fetch(versionedSwUrl.toString(), { 
            method: 'HEAD', 
            cache: 'no-store' 
          });
          
          if (!response.ok) throw new Error(`SW fetch returned ${response.status}`);

          const registration = await navigator.serviceWorker.register(
            versionedSwUrl.toString(),
            {
              scope: './',
              updateViaCache: 'none'  // Always fetch fresh SW
            }
          );

          console.info(`IdleGames ${moduleVersion} ready. Service worker scope:`, registration.scope);
          return registration;
          
        } catch (error) {
          if (attempt >= MAX_SW_RETRIES) {
            console.error('Service worker registration failed after retries:', error);
            return null;
          }
          
          const delay = Math.min(120000, BASE_RETRY_DELAY_MS * (attempt + 1));
          console.info(`SW registration retry ${attempt + 1}/${MAX_SW_RETRIES} in ${Math.round(delay / 1000)}s`);
          await new Promise(resolve => setTimeout(resolve, delay));
          return ensureServiceWorkerRegistration(attempt + 1);
        }
      };

      const registration = await ensureServiceWorkerRegistration();
      if (!registration) return;

      // Handle SW updates
      const trackInstalling = (worker) => {
        if (!worker) return;
        worker.addEventListener('statechange', () => {
          if (worker.state === 'installed' && navigator.serviceWorker.controller) {
            // New SW installed, trigger update
            worker.postMessage({ type: 'idle-games-skip-waiting' });
          }
        });
      };

      if (registration.installing) trackInstalling(registration.installing);
      registration.addEventListener('updatefound', () => {
        trackInstalling(registration.installing);
      });

      // Listen for version updates from SW
      navigator.serviceWorker.addEventListener('message', (event) => {
        if (event.data?.type === 'idle-games-sw-version') {
          const incoming = event.data.appVersion;
          localStorage.setItem('idle-games-sw-version', incoming);
          showUpdateNotification('Idle Games updated');
        }
      });

      // Reload when new SW takes control
      let refreshing = false;
      navigator.serviceWorker.addEventListener('controllerchange', () => {
        if (!refreshing) {
          refreshing = true;
          window.location.reload();
        }
      });

    } catch (error) {
      console.error('Service worker setup failed:', error);
    }
  });
}

Key patterns:

•Cache-busting query params: Version URLs to force fresh fetches
•Retry with exponential backoff: Handle deployment timing issues
•updateViaCache: 'none': Never serve stale SW files
•HEAD fetch check: Verify SW exists before registration
•Auto-reload: Reload page when new SW activates

3. Service Worker Caching Strategy

IdleGames uses a sophisticated hybrid approach:

javascript

// scripts/sw.template.js - Production Service Worker
const APP_VERSION = '__APP_VERSION__';
const CACHE_NAME = `IdleGames-v${APP_VERSION}`;
const PRECACHE_URLS = __PRECACHE_LIST__;

// Install: Precache essential assets
self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(CACHE_NAME).then((cache) => 
      cache.addAll(PRECACHE_URLS)
    )
  );
  self.skipWaiting(); // Activate immediately
});

// Activate: Clean old caches and enable navigation preload
self.addEventListener('activate', (event) => {
  event.waitUntil((async () => {
    // Enable navigation preload for faster cold starts
    if (self.registration.navigationPreload) {
      await self.registration.navigationPreload.enable();
    }
    
    // Delete old caches
    const keys = await caches.keys();
    await Promise.all(
      keys
        .filter((key) => key !== CACHE_NAME)
        .map((key) => caches.delete(key))
    );
    
    await self.clients.claim();
    
    // Notify all clients of new version
    const clients = await self.clients.matchAll({ type: 'window' });
    clients.forEach((client) => {
      client.postMessage({ type: 'idle-games-sw-version', appVersion: APP_VERSION });
    });
  })());
});

// Fetch: Navigation vs. Asset strategy
self.addEventListener('fetch', (event) => {
  if (event.request.method !== 'GET') return;

  const requestURL = new URL(event.request.url);
  
  // Skip own service worker
  if (requestURL.pathname === '/sw.js' || requestURL.pathname === '/sw.js/') return;
  
  // Same-origin only
  if (requestURL.origin !== self.location.origin) return;

  // **CRITICAL: Normalize directory URLs to index.html**
  let normalizedRequest = event.request;
  if (requestURL.pathname.endsWith('/')) {
    const indexUrl = new URL(requestURL.href);
    indexUrl.pathname = requestURL.pathname + 'index.html';
    normalizedRequest = new Request(indexUrl.href, {
      method: event.request.method,
      headers: event.request.headers,
      mode: event.request.mode === 'navigate' ? 'same-origin' : event.request.mode,
      credentials: event.request.credentials,
      redirect: event.request.redirect
    });
  }

  const isNavigation = event.request.mode === 'navigate';
  
  // Navigation: Cache-first with network race
  if (isNavigation) {
    event.respondWith(handleNavigation(isNavigation, event.preloadResponse, normalizedRequest));
  } else {
    // Assets: Network-first with cache fallback
    event.respondWith(handleAsset(normalizedRequest));
  }
});

async function handleNavigation(isNav, preloadResponse, request) {
  const cache = await caches.open(CACHE_NAME);
  
  // Try preload response first (already requested in parallel)
  if (preloadResponse) {
    try {
      const response = await preloadResponse;
      if (response?.ok) {
        cache.put(request, response.clone());
        return response;
      }
    } catch (e) {
      // Preload failed, continue to cache
    }
  }

  const cached = await cache.match(request, { ignoreSearch: true });
  
  // Race network against 2s timeout
  const networkPromise = fetch(request, { cache: 'no-store' })
    .then(async (response) => {
      if (response?.ok) {
        await cache.put(request, response.clone());
      }
      return response;
    })
    .catch(() => null);

  if (cached) {
    // Return cached if network is slow, else wait for network
    const timeoutPromise = new Promise((resolve) => 
      setTimeout(() => resolve(null), 2000)
    );
    const networkResult = await Promise.race([networkPromise, timeoutPromise]);
    return networkResult?.ok ? networkResult : cached;
  }

  // No cache: must wait for network
  const networkResult = await networkPromise;
  if (networkResult) return networkResult;

  // Last resort: index.html fallback
  const fallback = await cache.match('./index.html');
  if (fallback) return fallback;

  throw new Error('No cached content available');
}

async function handleAsset(request) {
  try {
    const networkResponse = await fetch(request, { cache: 'no-store' });
    if (networkResponse?.ok) {
      const cache = await caches.open(CACHE_NAME);
      await cache.put(request, networkResponse.clone());
    }
    return networkResponse;
  } catch (error) {
    const cache = await caches.open(CACHE_NAME);
    const cached = await cache.match(request, { ignoreSearch: true });
    if (cached) return cached;
    throw error;
  }
}

Strategy breakdown:

•
Navigation requests: Use preload response (parallel fetch) + cache race (2s timeout)
- •Prioritizes cached version for instant load (offline-first)
- •Tries network in parallel but doesn't wait if cache exists
- •Falls back to index.html if all else fails
•
Asset requests: Network-first with cache fallback
- •Updates assets when online
- •Uses cache immediately when offline
•Directory normalization: /idlegames/ → /idlegames/index.html (prevents blank screens)
•Navigation preload: Enabled for faster cold starts
•Precache busting: Version in cache name forces full refresh on updates

Critical for offline scenarios:

•Navigation race timeout (2s) ensures offline users see cached content immediately
•Fallback to index.html prevents blank screens when network fails
•Assets cache gracefully if network unavailable
•The app remains fully functional with zero network connectivity

4. URL Normalization (CRITICAL for Offline)

This is the most important PWA feature—it prevents blank screens for offline users who bookmarked the directory URL:

javascript

// In service worker fetch handler
const requestURL = new URL(event.request.url);

// Normalize /idlegames/ to /idlegames/index.html
if (requestURL.pathname.endsWith('/')) {
  const indexUrl = new URL(requestURL.href);
  indexUrl.pathname = requestURL.pathname + 'index.html';
  normalizedRequest = new Request(indexUrl.href, {
    method: event.request.method,
    headers: event.request.headers,
    // Navigate requests need mode: 'same-origin' for Request constructor
    mode: event.request.mode === 'navigate' ? 'same-origin' : event.request.mode,
    credentials: event.request.credentials,
    redirect: event.request.redirect
  });
}

Why this matters in offline scenarios:

•Users bookmark/share: https://example.com/app/ (with trailing slash)
•Manifest specifies start_url: "./index.html"
•When launching offline, device may request the bookmarked directory URL
•If SW can't find it in cache, user sees blank screen while offline
•The SW can't reload to fix this—it completely controls responses
•Without normalization, offline users are stranded with broken app

Real-world example:

•Day 1: User visits with internet, app caches successfully
•Day 2: User goes to subway (no wifi), taps app icon on home screen
•Device requests /idlegames/ (the bookmarked URL)
•If SW doesn't normalize, it looks for /idlegames/ in cache (not found)
•Result: Blank screen, user can't play games

Testing:

bash

# Test with trailing slash (the dangerous case)
http://localhost:4173/              # Must work offline
http://localhost:4173/index.html    # Must also work offline

5. Manifest Icons Setup

Properly sized and formatted icons for all platforms:

json

{
  "icons": [
    {
      "src": "assets/appicons/idle-games-512x512px.png",
      "sizes": "512x512",
      "type": "image/png",
      "purpose": "any maskable"  // Supports adaptive icons
    },
    {
      "src": "assets/appicons/idle-games-512x512px.png",
      "sizes": "192x192",
      "type": "image/png"
    }
  ]
}

Icon requirements:

•512x512: Primary icon for splash screens and app stores
•192x192: Home screen icon (Android)
•purpose: maskable: Allows OS to apply adaptive icon masks
•Use PNG format for transparency support
•Test on actual devices (simulator may not match)

HTML meta tag for iOS:

html

<link rel="apple-touch-icon" href="assets/appicons/idle-games-180x180.png">
<link rel="manifest" href="manifest.webmanifest">
<meta name="theme-color" content="#667eea">

Best Practices

1. Versioning and Cache Busting

Generate version from package.json:

javascript

// scripts/build-sw.mjs
const packageJson = JSON.parse(await fs.readFile('./package.json', 'utf8'));
const appVersion = packageJson.version;

// Generate version module
const versionModule = `export const APP_VERSION = '${appVersion}';`;
await fs.writeFile('assets/js/version.js', versionModule);

// Replace in SW template
const swContent = swTemplate
  .replaceAll('__APP_VERSION__', appVersion)
  .replace('__PRECACHE_LIST__', JSON.stringify(precacheList));

await fs.writeFile('dist/sw.js', swContent);

Use version in cache name and URLs:

javascript

const CACHE_NAME = `IdleGames-v${APP_VERSION}`;  // Old caches auto-deleted

// Version query param for cache busting
const versionedSwUrl = new URL('./sw.js');
versionedSwUrl.searchParams.set('version', resolvedVersion);

2. Precaching Strategy for Offline-First

Offline-first requirement changes precaching approach:

For an app that must work completely offline, precaching is not optional—it's fundamental. IdleGames precaches all code and assets needed for the full experience:

javascript

const PRECACHE_EXTENSIONS = new Set([
  '.html', '.css', '.js', '.json', '.webmanifest',
  '.png', '.svg', '.ico', '.woff2'
  // All essential static assets
]);

async function collectPrecache(dir) {
  const entries = await fs.readdir(dir, { withFileTypes: true });
  for (const entry of entries) {
    if (entry.isDirectory()) {
      await collectPrecache(path.join(dir, entry.name));
    } else {
      const ext = path.extname(entry.name).toLowerCase();
      if (PRECACHE_EXTENSIONS.has(ext)) {
        precacheList.push(entry.name);
      }
    }
  }
}

What must be precached for offline:

•✅ All HTML pages (index.html, game pages, etc.)
•✅ All JavaScript (game code, vendor libs like Three.js)
•✅ All CSS stylesheets
•✅ All images, SVGs, icons
•✅ Fonts (critical for display)
•✅ Manifest and metadata

What you can skip precaching:

•❌ Large video/audio files (users download on demand)
•❌ User-specific data (fetch on startup if online)
•❌ Dynamic content (rare for offline apps)
•❌ External resources (by definition, not available offline)

Size considerations:

•IdleGames' precache is intentionally comprehensive
•Precache size is a one-time install cost
•Users accept larger initial downloads for offline reliability
•The benefit (works anywhere, anytime) justifies the size

3. Navigation Preload for Cold Starts

Navigation preload allows the browser to fetch the next page while the SW is starting:

javascript

// In SW activate
if (self.registration.navigationPreload) {
  await self.registration.navigationPreload.enable();
}

// In SW fetch
self.addEventListener('fetch', (event) => {
  if (event.request.mode === 'navigate') {
    event.respondWith(
      // Use preloaded response (already fetching in parallel)
      event.preloadResponse
        .then(response => response || fetchWithFallback(event.request))
        .catch(() => fetchWithFallback(event.request))
    );
  }
});

4. Handling Slow Networks

Use a race between network and cache with timeout:

javascript

// Return cached immediately if network is slow
const cached = await cache.match(request);
const networkPromise = fetch(request, { cache: 'no-store' });
const timeoutPromise = new Promise(resolve => 
  setTimeout(() => resolve(null), 2000)  // 2s timeout
);

if (cached) {
  const networkResult = await Promise.race([networkPromise, timeoutPromise]);
  return networkResult?.ok ? networkResult : cached;
}

5. Update Notifications

Show subtle notification when app updates:

javascript

// Show 5-second transient notification
function showTransientNotification(message, duration = 5000) {
  const el = document.createElement('div');
  el.textContent = message;
  el.style.position = 'fixed';
  el.style.bottom = '20px';
  el.style.background = 'rgba(0,0,0,0.85)';
  el.style.color = '#fff';
  el.style.padding = '10px 16px';
  el.style.borderRadius = '8px';
  el.style.zIndex = '100000';
  
  document.body.appendChild(el);
  
  setTimeout(() => {
    el.style.transition = 'opacity 300ms ease';
    el.style.opacity = '0';
    setTimeout(() => el.remove(), 350);
  }, duration);
}

// Show when version changes
const swVer = localStorage.getItem('idle-games-sw-version');
const lastNotified = localStorage.getItem('idle-games-last-notified-version');
if (swVer && swVer !== lastNotified) {
  showTransientNotification('App updated');
  localStorage.setItem('idle-games-last-notified-version', swVer);
}

Testing Checklist

Critical: Offline Testing is Non-Optional

Every deployment must verify offline functionality works completely:

Local Testing:

bash

npm run build    # Build to dist/
npm run serve    # Start server on http://localhost:4173

Chrome DevTools (offline mode):

•Open DevTools → Network tab
•Check Offline checkbox (top-left corner)
•Reload page—should load fully from cache
•All game features should work without network
•Uncheck Offline—should reconnect seamlessly

Chrome DevTools Inspection:

•Application tab → Service Workers: Verify registration
•Application tab → Cache Storage: Inspect IdleGames-vX.X.X cache
•Network tab: Set throttling (slow 3G) to test race conditions
•Verify precache list includes all game files

Real Device Testing (Essential):

code

✓ Install app on home screen (varies by OS)
✓ Close browser completely (kill background process)
✓ Reopen PWA from home screen icon
✓ Works offline (airplane mode or disable wifi)
✓ Directory URL loads: https://example.com/idlegames/
✓ All games playable offline
✓ Game saves persist offline
✓ Multiple game sessions work offline
✓ URL normalization works (/idlegames/ = /idlegames/index.html)

✓ Handles slow networks gracefully ✓ Update notification appears ✓ New version auto-activates after reload

code


**Mobile-Specific:**
- iOS: Safari → Share → Add to Home Screen
- Android Chrome: Menu → Install app
- Test on low-memory devices (older phones)

## Debugging Service Worker Issues

**Offline-First Debugging Approach:**

For an offline-first app, the SW is the entire system. If it fails, users have zero access to the app. Debug strategically:

**Critical problems (offline breaks):**

1. **Blank screen when offline**
   - Users can't access app at all when internet is unavailable
   - Check URL normalization is working (trailing slash handling)
   - Verify precache list includes `./index.html` and all game HTML files
   - Check SW has correct PRECACHE_LIST injected
   - Test: Enable offline mode in DevTools → page should load instantly

2. **Missing game files offline**
   - Some games work, others show errors when offline
   - Check precache includes all `.js` and `.html` for that game
   - Verify vendor libraries (Three.js, Transformers.js) are in cache
   - Inspect: DevTools → Application → Cache Storage → check what's cached

3. **Old version still showing**
   - Users get outdated game after update
   - Clear browser cache and localStorage
   - Verify SW is registered with new version
   - Check CACHE_NAME includes version number
   - Ensure version.js was regenerated at build time

4. **Can't play offline but online works**
   - This indicates precache is broken or incomplete
   - Verify all assets in precache list are actually in dist/
   - Check precache extensions list includes needed file types
   - Enable offline mode → should still load instantly

3. **Network request not caching**
   - SW might not intercept third-party requests
   - Check origin matches (CORS issues)
   - Verify response.ok check (4xx/5xx responses skipped)

4. **Slow cold starts**
   - Enable navigation preload
   - Check precache list size
   - Profile with DevTools Performance tab

**Debug console logs:**
```javascript
// In SW
console.info(`IdleGames ${APP_VERSION} ready`);
console.info(`Caching ${PRECACHE_URLS.length} files`);

// In app
console.info(`Service worker registration successful`);
console.info(`App version: ${APP_VERSION}`);

Production Rollout

Offline-First Deployment Checklist:

Before deploying any version, offline functionality must be verified:

•Update version in package.json
•Run npm run build (regenerates version.js and precache list)
•Enable offline mode in DevTools → Test all games work
•Test with slow 3G throttling → Should still load from cache
•Test directory URL offline: http://localhost:4173/ (not index.html)
•Verify all game saves persist across sessions offline
•Commit and push to main branch
•GitHub Actions deployment
•On production: Enable offline mode again → Verify one more time

Critical offline verification before deployment:

code

✓ App loads completely offline
✓ All games are playable offline  
✓ Game saves work offline
✓ Directory URLs work offline (/idlegames/)
✓ Precache includes all HTML, JS, CSS, assets
✓ No "Resource not found" errors in offline mode
✓ Performance acceptable on slow networks

Rollback if needed: If offline breaks after deployment:

•Users are stranded—cannot access app without internet
•Immediately revert commit with broken version
•Fix issue locally
•Bump package.json version again
•Redeploy emergency fix

Why this is critical:

•Users may rely on offline functionality (commuting, areas without coverage)
•Broken SW prevents even online access until fixed
•Once deployed, users get broken version automatically via SW
•Testing is the only defense—you cannot hotpatch deployed SWs