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Service Workers and Offline HTML Caching

Answer​

Service Workers are JavaScript scripts that run in the background, separate from web pages, enabling features like offline support, push notifications, and background sync.

Service Worker Lifecycle​

Registering a Service Worker​

// In your main JavaScript file
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.log("SW registration failed:", error);
		}
	});
}

Basic Service Worker (sw.js)​

const CACHE_NAME = "v1";
const urlsToCache = [
	"/",
	"/index.html",
	"/styles.css",
	"/app.js",
	"/images/logo.png",
	"/offline.html",
];

// Install event - cache resources
self.addEventListener("install", (event) => {
	event.waitUntil(
		caches.open(CACHE_NAME).then((cache) => {
			console.log("Caching app shell");
			return cache.addAll(urlsToCache);
		})
	);
	// Activate immediately
	self.skipWaiting();
});

// Activate event - clean old caches
self.addEventListener("activate", (event) => {
	event.waitUntil(
		caches.keys().then((cacheNames) => {
			return Promise.all(
				cacheNames
					.filter((name) => name !== CACHE_NAME)
					.map((name) => caches.delete(name))
			);
		})
	);
	// Take control immediately
	self.clients.claim();
});

// Fetch event - serve from cache
self.addEventListener("fetch", (event) => {
	event.respondWith(
		caches.match(event.request).then((response) => {
			return response || fetch(event.request);
		})
	);
});

Caching Strategies​

Cache First Strategy​

// Best for: CSS, JS, images, fonts
self.addEventListener("fetch", (event) => {
	event.respondWith(
		caches.match(event.request).then((cachedResponse) => {
			if (cachedResponse) {
				return cachedResponse;
			}

			return fetch(event.request).then((networkResponse) => {
				return caches.open(CACHE_NAME).then((cache) => {
					cache.put(event.request, networkResponse.clone());
					return networkResponse;
				});
			});
		})
	);
});

Network First Strategy​

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

Stale While Revalidate​

// Best for: User data, social feeds
self.addEventListener("fetch", (event) => {
	event.respondWith(
		caches.open(CACHE_NAME).then((cache) => {
			return cache.match(event.request).then((cachedResponse) => {
				const fetchPromise = fetch(event.request).then((networkResponse) => {
					cache.put(event.request, networkResponse.clone());
					return networkResponse;
				});

				return cachedResponse || fetchPromise;
			});
		})
	);
});

Offline Fallback Page​

// sw.js
self.addEventListener("fetch", (event) => {
	if (event.request.mode === "navigate") {
		event.respondWith(
			fetch(event.request).catch(() => {
				return caches.match("/offline.html");
			})
		);
	}
});
<!-- offline.html -->
<!DOCTYPE html>
<html lang="en">
	<head>
		<meta charset="UTF-8" />
		<title>Offline - Site Name</title>
		<style>
			body {
				display: flex;
				justify-content: center;
				align-items: center;
				min-height: 100vh;
				font-family: system-ui;
			}
			.offline-message {
				text-align: center;
			}
		</style>
	</head>
	<body>
		<div class="offline-message">
			<h1>You're Offline</h1>
			<p>Please check your connection and try again.</p>
			<button onclick="location.reload()">Retry</button>
		</div>
	</body>
</html>

Update Service Worker​

// Register with update check
const registration = await navigator.serviceWorker.register("/sw.js");

// Check for updates every hour
setInterval(() => {
	registration.update();
}, 60 * 60 * 1000);

// Listen for new service worker
registration.addEventListener("updatefound", () => {
	const newWorker = registration.installing;

	newWorker.addEventListener("statechange", () => {
		if (newWorker.state === "installed") {
			if (navigator.serviceWorker.controller) {
				// New content available
				showUpdateNotification();
			}
		}
	});
});

Debugging​

// Check registration status
navigator.serviceWorker.getRegistration().then((reg) => {
	console.log("SW state:", reg?.active?.state);
});

// Clear all caches
caches.keys().then((names) => {
	names.forEach((name) => caches.delete(name));
});

// Unregister service worker
navigator.serviceWorker.getRegistrations().then((regs) => {
	regs.forEach((reg) => reg.unregister());
});

Key Points​

  • Service Workers run in background, separate from page
  • Must be served over HTTPS (except localhost)
  • Control caching strategies for offline support
  • Lifecycle: install → waiting → activate
  • Use skipWaiting() for immediate activation
  • Multiple caching strategies for different content types