phoroth 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:
- HTTPS — Required in production for service workers to register (localhost is exempt for development).
- Web App Manifest (
manifest.json) — Makes the app installable and defines its appearance on device home screens. - 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:standalonehides browser UI;minimal-uishows minimal controls;browseris standard tab.purpose: "maskable"on icons enables adaptive icons on Android (safe zone matters — keep content in center 80%).screenshotsis 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 inindex.htmlabove) 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://localhostis the only exception for development. - Use a tool like
mkcertorngrokif you need HTTPS locally with a custom hostname.
Cache-Busting on Deploy
- Always increment
CACHE_VERSIONinsw.jswhen 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
200status. - Prefer
staleWhileRevalidatefor 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.jsonhasname,short_name,start_url,display,icons(192 + 512) - Icons have
purpose: "any maskable" -
sw.jsregisters without errors in DevTools → Application → Service Workers - App shell loads from cache when network is throttled to "Offline" in DevTools
-
offline.htmlfallback 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)
Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for enprojectnment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.