fix: adding back the service worker

It was mistakenly removed in 4c71c4a

demo/src/service-worker.ts

@@ -0,0 +1,65 @@
+/// <reference types="@sveltejs/kit" />
+/// <reference no-default-lib="true"/>
+/// <reference lib="webworker" />
+
+declare const self: ServiceWorkerGlobalScope;
+
+import {build, files, prerendered, version} from '$service-worker';
+
+const CACHE = `cache-${version}`;
+
+const ASSETS = [...build.map((file) => file.replace(/\/index\.html$/, '/')), ...prerendered, ...files];
+
+const splitAssets = (assets: string[]) => {
+	const withHash: string[] = [];
+	const withoutHash: string[] = [];
+	const hashRegExp = /[.-]\w{8,}\.\w{2,4}$/;
+	for (const asset of assets) {
+		(hashRegExp.test(asset) ? withHash : withoutHash).push(asset);
+	}
+	return {withHash, withoutHash};
+};
+
+self.addEventListener('install', (event) => {
+	event.waitUntil(
+		(async () => {
+			const cache = await caches.open(CACHE);
+			const {withHash, withoutHash} = splitAssets(ASSETS);
+			const missingWithHash: string[] = [];
+			await Promise.all(
+				withHash.map(async (url) => {
+					const response = await caches.match(url);
+					if (response?.ok) {
+						cache.put(url, response);
+					} else {
+						missingWithHash.push(url);
+					}
+				})
+			);
+			await cache.addAll([...missingWithHash, ...withoutHash]);
+			await self.skipWaiting();
+		})()
+	);
+});
+
+self.addEventListener('activate', (event) => {
+	event.waitUntil(
+		(async () => {
+			await self.clients.claim();
+			await Promise.all((await caches.keys()).filter((key) => key !== CACHE).map((key) => caches.delete(key)));
+		})()
+	);
+});
+
+self.addEventListener('fetch', (event) => {
+	const url = new URL(event.request.url);
+	if (event.request.method === 'GET' && ASSETS.includes(url.pathname)) {
+		event.respondWith(
+			(async () => {
+				const cache = await caches.open(CACHE);
+				const response = await cache.match(url.pathname);
+				return response!;
+			})()
+		);
+	}
+});