# Service workers

Service workers act as proxy servers that handle network requests inside your app. This makes it possible to make your app work offline, but even if you don't need offline support (or can't realistically implement it because of the type of app you're building), it's often worth using service workers to speed up navigation by precaching your built JS and CSS.

In SvelteKit, if you have a `src/service-worker.js` file (or `src/service-worker/index.js`) it will be bundled and automatically registered. You can change the [location of your service worker](configuration#files) if you need to.

You can [disable automatic registration](configuration#serviceWorker) if you need to register the service worker with your own logic or use another solution. The default registration looks something like this:

```js
if ('serviceWorker' in navigator) {
	addEventListener('load', function () {
		navigator.serviceWorker.register('./path/to/service-worker.js');
	});
}
```

## Inside the service worker

Inside the service worker you have access to the [`$service-worker` module]($service-worker), which provides you with the paths to all static assets, build files and prerendered pages. You're also provided with an app version string, which you can use for creating a unique cache name, and the deployment's `base` path. If your Vite config specifies `define` (used for global variable replacements), this will be applied to service workers as well as your server/client builds.

The following example caches the built app and any files in `static` eagerly, and caches all other requests as they happen. This would make each page work offline once visited.

```js
/// file: src/service-worker.js
// Disables access to DOM typings like `HTMLElement` which are not available
// inside a service worker and instantiates the correct globals
/// <reference no-default-lib="true"/>
/// <reference lib="esnext" />
/// <reference lib="webworker" />

// Ensures that the `$service-worker` import has proper type definitions
/// <reference types="@sveltejs/kit" />

// Only necessary if you have an import from `$env/static/public`
/// <reference types="../.svelte-kit/ambient.d.ts" />

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

// This gives `self` the correct types
const self = /** @type {ServiceWorkerGlobalScope} */ (/** @type {unknown} */ (globalThis.self));

// Create a unique cache name for this deployment
const CACHE = `cache-${version}`;

const ASSETS = [
	...build, // the app itself
	...files  // everything in `static`
];

self.addEventListener('install', (event) => {
	// Create a new cache and add all files to it
	async function addFilesToCache() {
		const cache = await caches.open(CACHE);
		await cache.addAll(ASSETS);
	}

	event.waitUntil(addFilesToCache());
});

self.addEventListener('activate', (event) => {
	// Remove previous cached data from disk
	async function deleteOldCaches() {
		for (const key of await caches.keys()) {
			if (key !== CACHE) await caches.delete(key);
		}
	}

	event.waitUntil(deleteOldCaches());
});

self.addEventListener('fetch', (event) => {
	// ignore POST requests etc
	if (event.request.method !== 'GET') return;

	async function respond() {
		const url = new URL(event.request.url);
		const cache = await caches.open(CACHE);

		// `build`/`files` can always be served from the cache
		if (ASSETS.includes(url.pathname)) {
			const response = await cache.match(url.pathname);

			if (response) {
				return response;
			}
		}

		// for everything else, try the network first, but
		// fall back to the cache if we're offline
		try {
			const response = await fetch(event.request);

			// if we're offline, fetch can return a value that is not a Response
			// instead of throwing - and we can't pass this non-Response to respondWith
			if (!(response instanceof Response)) {
				throw new Error('invalid response from fetch');
			}

			if (response.status === 200) {
				cache.put(event.request, response.clone());
			}

			return response;
		} catch (err) {
			const response = await cache.match(event.request);

			if (response) {
				return response;
			}

			// if there's no cache, then just error out
			// as there is nothing we can do to respond to this request
			throw err;
		}
	}

	event.respondWith(respond());
});
```

> [!NOTE] Be careful when caching! In some cases, stale data might be worse than data that's unavailable while offline. Since browsers will empty caches if they get too full, you should also be careful about caching large assets like video files.

## During development

The service worker is bundled for production, but not during development. For that reason, only browsers that support [modules in service workers](https://web.dev/es-modules-in-sw) will be able to use them at dev time. If you are manually registering your service worker, you will need to pass the `{ type: 'module' }` option in development:

```js
import { dev } from '$app/environment';

navigator.serviceWorker.register('/service-worker.js', {
	type: dev ? 'module' : 'classic'
});
```

> [!NOTE] `build` and `prerendered` are empty arrays during development

## Other solutions

SvelteKit's service worker implementation is designed to be easy to work with and is probably a good solution for most users. However, outside of SvelteKit, many PWA applications leverage the [Workbox](https://web.dev/learn/pwa/workbox) library. If you're used to using Workbox you may prefer [Vite PWA plugin](https://vite-pwa-org.netlify.app/frameworks/sveltekit.html).

## References

For more general information on service workers, we recommend [the MDN web docs](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers).