A service worker can make a website’s app shell available without a network connection by caching its files and deciding how later requests are answered. Register it from a secure page, cache the files needed to render a useful offline view, and add request strategies that match each resource. The initial visit must finish registering and caching while the device is online; a service worker cannot make a first visit offline work.
What a service worker does—and what it does not
A service worker is a separate, asynchronous worker that can intercept network requests and use the Cache API to return saved responses. MDN describes service workers as “proxy servers that sit between web applications, the browser, and the network (when available)” in its Service Worker API overview. It has no access to the page’s DOM, so it cannot directly update visible page elements. The page and worker communicate through browser APIs and messages when needed.
Offline support is not automatic just because a worker is registered. You choose which requests to save, when to use saved responses, and what to show when neither the network nor a cache has the answer. A cached page can still display information but cannot perform server-dependent actions unless the site has separately implemented offline data storage and synchronization.
Before you start: use a secure context and choose the worker’s scope
Service workers are available only in secure contexts. Production sites need HTTPS; browsers treat localhost as secure for development. An ordinary HTTP deployment on a public host will not be able to use the API. See MDN’s secure-context guidance.
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Put the worker file at a URL whose directory covers the pages it should control. A worker at /sw.js can normally control pages throughout the site; a worker in a subdirectory is limited to that directory and its descendants unless the server sets an appropriate scope header. Register the root worker from page JavaScript:
if ('serviceWorker' in navigator) {
window.addEventListener('load', () => {
navigator.serviceWorker.register('/sw.js')
.catch((error) => console.error('Service worker registration failed:', error));
});
}The leading slash makes the registration URL root-relative. If the site is hosted under a path rather than at the domain root, adjust the worker URL and cached paths to match that deployment.
Precache a small, useful app shell during installation
Registration causes the browser to download and evaluate the worker, then run its lifecycle. The first lifecycle event is install, as MDN notes in Using Service Workers. This is the normal place to cache the minimum set of files needed for a meaningful offline screen: the page or app shell, its CSS and JavaScript, essential images, and an offline fallback document.
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Give the cache a versioned name so a later release can use a new cache without mixing incompatible files with the previous version. event.waitUntil() tells the browser that installation depends on the asynchronous cache operation completing; if a required file cannot be cached, installation can fail rather than leave an incomplete shell.
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const CACHE_NAME = 'site-shell-v1';
const APP_SHELL = [
'/',
'/index.html',
'/styles.css',
'/app.js',
'/images/logo.png',
'/offline.html'
];
self.addEventListener('install', (event) => {
event.waitUntil(
caches.open(CACHE_NAME).then((cache) => cache.addAll(APP_SHELL))
);
});Every path in the list must actually exist and be accessible to the page’s origin. Keep this list deliberately small: adding large images or files that the offline screen does not need increases storage use and can make installation more fragile.
Choose a request strategy that fits each resource
Use a fetch handler to intercept requests and call event.respondWith() with the response to return. Static assets that change only when you deploy are often a good fit for cache-first. Frequently changing pages or API responses are often better served network-first, with a cached fallback where it makes sense.
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|---|---|---|---|---|
| Cache-first | May serve an older cached copy until the cache is updated. | Strong when the requested resource was cached earlier. | Can avoid a network request when a cached response exists. | Versioned CSS, JavaScript, logos, and other stable assets. |
| Network-first | Uses a current network response when one is available. | Works offline only if a suitable cached response exists as fallback. | Usually waits for the network before falling back; uses the network when online. | Content or API data where freshness matters more than instant cache delivery. |
Cache-first for static files
This example checks the cache first, then fetches a missing resource and saves a successful response. It leaves failed requests and unsuccessful HTTP responses uncached.
self.addEventListener('fetch', (event) => {
const request = event.request;
const url = new URL(request.url);
if (request.method !== 'GET' || url.origin !== self.location.origin) {
return;
}
event.respondWith((async () => {
const cached = await caches.match(request);
if (cached) return cached;
const response = await fetch(request);
if (response.ok) {
const cache = await caches.open(CACHE_NAME);
await cache.put(request, response.clone());
}
return response;
})());
});Cache-first deliberately trades freshness for availability and speed. If an asset changes, use a new cache version and ensure the page references the corresponding assets; otherwise the worker may keep returning an older cached file.
Network-first for changing pages or data
For requests where current content matters, try the network and save a successful response for later. If the request fails, use a cached response; if there is none, return a clear fallback rather than an unhandled network error.
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async function networkFirst(request) {
const cache = await caches.open('site-runtime-v1');
try {
const response = await fetch(request);
if (response.ok) {
await cache.put(request, response.clone());
}
return response;
} catch (error) {
const cached = await cache.match(request);
if (cached) return cached;
return new Response('You are offline, and this content is not available yet.', {
status: 503,
headers: { 'Content-Type': 'text/plain; charset=utf-8' }
});
}
}
self.addEventListener('fetch', (event) => {
const request = event.request;
const url = new URL(request.url);
if (request.method !== 'GET' || url.origin !== self.location.origin) {
return;
}
if (url.pathname.startsWith('/api/')) {
event.respondWith(networkFirst(request));
}
});These are starting patterns, not a universal routing policy. Decide deliberately which URLs belong in each strategy and how the page should handle a fallback. Do not cache every response indiscriminately: API data can become stale, and large or user-specific responses may be inappropriate to save.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep app-shell caching and runtime caching separate
Precached files and runtime-cached responses serve different purposes. The shell is a known, small set of assets saved during installation; runtime caching accumulates responses as people use the site. Keeping them in separate named caches makes it easier to update the shell without removing useful runtime data.
| Cache type | When it is populated | Typical contents | Update and failure considerations |
|---|---|---|---|
| Precached app shell | During the worker’s install event. | Essential HTML, CSS, JavaScript, images, and an offline page. | Update the versioned cache when deploying changed assets; missing required files can prevent installation from completing. |
| Runtime cache | As requests occur after installation. | Selected pages or GET responses, such as API data chosen for offline fallback. | Can grow and become stale; cache selectively and define what to do when a resource has never been requested online. |
The Cache API stores request-response pairs, but it is not a complete offline data system. For structured data that must be available offline, IndexedDB is available in the worker context. The worker cannot use synchronous Web Storage such as localStorage.
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Handle updates without leaving users on mixed versions
When the worker script changes, the browser can install a new worker in the background while the previous worker continues controlling open pages. Normally the new worker waits to activate until pages controlled by the old worker are gone. This protects an open page from suddenly receiving a different set of cached files, but it means an update may not take over immediately.
During activation, delete obsolete caches owned by this site. Match cache names precisely so cleanup does not remove data belonging to unrelated features or libraries:
const CURRENT_CACHES = ['site-shell-v2', 'site-runtime-v1'];
self.addEventListener('activate', (event) => {
event.waitUntil((async () => {
const names = await caches.keys();
await Promise.all(
names
.filter((name) => name.startsWith('site-') && !CURRENT_CACHES.includes(name))
.map((name) => caches.delete(name))
);
})());
});skipWaiting() and clients.claim() can make a new worker activate and control pages sooner, but using them without considering the page’s state can mix old page code with new worker behavior. Use them only when the update experience is designed for that transition—for example, with a deliberate reload or update notice—and test it with open tabs.
Test the offline experience in the right order
Offline behavior depends on a successful online visit first. Test from a clean state so an old cache or already-installed worker does not disguise a setup problem.
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- In browser developer tools, clear the site’s storage and reload the page.
- Check that registration succeeds, the worker installs, and the app-shell cache contains every listed file.
- Reload while still online, then switch the developer tools’ network setting to offline and reload again.
- Visit a cached route and an uncached route; verify the cached content or intended fallback appears rather than a blank page.
- Change the worker and cache version, reload, and inspect the worker lifecycle and Cache Storage to confirm the new shell is installed and obsolete caches are removed.
If registration works on localhost but not on a public HTTP URL, the likely issue is the secure-context requirement: production needs HTTPS. If the worker registers but the page is not controlled, check that the worker file’s directory and registered URL cover the page. If installation fails, confirm that every precached path resolves successfully and that the worker can finish its waitUntil() work.
Know the limits of an offline site
A cached page can render offline, but a form submission, purchase, login, or other server operation still needs a network unless you build additional offline queuing and synchronization. That design must address retries, duplicate submissions, data freshness, and conflicts; a simple service worker cache does not solve those problems. Make the interface distinguish saved content from actions that require a connection, and provide a useful fallback for requests that have never been cached.
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