| name | progressive-web-app |
| description | 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. |
| risk | safe |
| source | community |
| date_added | 2026-03-17 |
| tags | ["pwa","web-dev","service-worker","frontend","offline","caching"] |
| tools | ["gemini","cursor","claude"] |
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:
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>
<link rel="manifest" href="/manifest.json">
<meta name="theme-color" content="#0055ff">
<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>
<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)
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);
}
});
}
let deferredPrompt;
const installBtn = document.getElementById('install-btn');
window.addEventListener('beforeinstallprompt', (e) => {
e.preventDefault();
deferredPrompt = e;
if (installBtn) installBtn.hidden = false;
});
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;
});
}
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}`;
const APP_SHELL = [
'/',
'/index.html',
'/styles.css',
'/app.js',
'/assets/icons/icon-192x192.png',
'/offline.html',
];
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);
})
);
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);
})
);
})
);
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);
if (request.method !== 'GET' || url.origin !== location.origin) return;
if (url.pathname.match(/\.(css|js|png|jpg|svg|woff2)$/)) {
event.respondWith(cacheFirst(request));
return;
}
if (request.headers.get('Accept')?.includes('text/html')) {
event.respondWith(networkFirst(request));
return;
}
if (url.pathname.startsWith('/api/')) {
event.respondWith(staleWhileRevalidate(request));
return;
}
});
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 {
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.
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 || []);
registerRoute(({ request }) => request.destination === 'image', new CacheFirst());
registerRoute(({ request }) => request.mode === 'navigate', new NetworkFirst());
registerRoute(({ request }) => request.destination === 'script', new StaleWhileRevalidate());
Checklist Before Shipping
Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.
