Build Progressive Web Apps (PWAs) with offline support, installability, and caching strategies. Trigger whenever the user mentions PWA, service workers, web app manifests, Workbox, 'add to home screen', or wants their web app to work offline, feel native, or be installable.
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:
manifest.json) — Makes the app installable and defines its appearance on device home screens.sw.js) — A background script that intercepts network requests, manages caches, and enables offline functionality.Every PWA implementation must include these files at minimum:
index.html — Links manifest, registers service workermanifest.json — Full app metadata and icon setsw.js — Service worker with install, activate, and fetch handlersapp.js — Main app logic with SW registration and install prompt handlingoffline.html — Fallback page shown when navigation fails offline (required — missing file will cause install to fail)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.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>
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;
});
sw.js)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
];
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();
});
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();
});
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;
}
beforeinstallprompt — users must install via the Share → "Add to Home Screen" menu manually.apple-mobile-web-app-* meta tags (shown in index.html above) for proper iOS integration.https:// origins. http://localhost is the only exception for development.mkcert or ngrok if you need HTTPS locally with a custom hostname.CACHE_VERSION in sw.js when deploying new assets. This ensures activate clears old caches and users get fresh files.200 status.staleWhileRevalidate for cross-origin resources, or use a library like Workbox which handles this safely.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());
manifest.json has name, short_name, start_url, display, icons (192 + 512)purpose: "any maskable"sw.js registers without errors in DevTools → Application → Service Workersoffline.html fallback is cached and served when navigation fails offline