service-worker.ts

/// <reference lib="webworker" />

import {
	build,
	files,
	version
} from '$service-worker';

const worker =
	self as unknown as ServiceWorkerGlobalScope;
const FILES = `cache${version}`;

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

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

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

			worker.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: Request) {
	const cache = await caches.open(
		`offline${version}`
	);

	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;
	}
}

worker.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;
	// ... adding a `not-same-origin` check for `requests`
	// ... more info -> https://stackoverflow.com/questions/48463483/what-causes-a-failed-to-execute-fetch-on-serviceworkerglobalscope-only-if
	const skipBecauseNotSameOrigin =
		event.request.mode !== 'same-origin';

	if (
		isHttp &&
		!isDevServerRequest &&
		!skipBecauseUncached &&
		!skipBecauseNotSameOrigin
	) {
		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));

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