service-worker.js

import { timestamp, files, shell } from '@sapper/service-worker';

const ASSETS = `cache${timestamp}`;

// `shell` is an array of all the files generated by the bundler,
// `files` is an array of everything in the `static` directory
const to_cache = shell.concat(files);
const staticAssets = new Set(to_cache);

self.addEventListener('install', event => {
	event.waitUntil(
		caches
			.open(ASSETS)
			.then(cache => cache.addAll(to_cache))
			.then(() => {
				self.skipWaiting();
			})
	);
});

self.addEventListener('activate', event => {
	event.waitUntil(
		caches.keys().then(async keys => {
			// delete old caches
			for (const key of keys) {
				if (key !== ASSETS) await caches.delete(key);
			}

			self.clients.claim();
		})
	);
});


/**
 * Fetch the asset from the network and store it in the cache. 
 * Fall back to the cache if the user is offline.
 */
async function fetchAndCache(request) {
	const cache = await caches.open(`offline${timestamp}`)

	try {
		const response = await fetch(request);
		cache.put(request, response.clone());
		return response;
	} catch (err) {
		const response = await cache.match(request);
		if (response) return response;

		throw err;
	}
}

self.addEventListener('fetch', event => {
	if (event.request.method !== 'GET' || event.request.headers.has('range')) return;

	const url = new URL(event.request.url);

	// don't try to handle e.g. data: URIs
	const isHttp = url.protocol.startsWith('http');
	const isDevServerRequest = url.hostname === self.location.hostname && url.port !== self.location.port;
	const isStaticAsset = url.host === self.location.host && staticAssets.has(url.pathname);
	const skipBecauseUncached = event.request.cache === 'only-if-cached' && !isStaticAsset;

	if (isHttp && !isDevServerRequest && !skipBecauseUncached) {
		event.respondWith(
			(async () => {
				// always serve static files and bundler-generated assets from cache.
				// if your application has other URLs with data that will never change,
				// set this variable to true for them and they will only be fetched once.
				const cachedAsset = isStaticAsset && await caches.match(event.request);

				// for pages, you might want to serve a shell `service-worker-index.html` file,
				// which Sapper has generated for you. It's not right for every
				// app, but if it's right for yours then uncomment this section
				/*
				if (!cachedAsset && url.origin === self.origin && routes.find(route => route.pattern.test(url.pathname))) {
					return caches.match('/service-worker-index.html');
				}
				*/

				return cachedAsset || fetchAndCache(event.request);
			})()
		);
	}
});