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Check the requirements before registering
- Use a secure context. Service workers require HTTPS on a public site. Browsers treat
localhostas secure for local development. See MDN’s Service Worker API overview. - Keep the worker same-origin. The worker script must be served from the same origin as the page registering it. Check the final deployed URL rather than assuming where a build tool placed the file.
- Choose the intended scope. Scope determines which pages the worker can control. By default, it covers the worker script’s directory and its descendants.
- Use trusted worker code. A service worker can intercept requests within its scope. Do not derive its script URL from untrusted input, and configure Content Security Policy to restrict worker sources where applicable.
Register the service worker
Put the registration code in a page script that runs in the browser. Feature-detect support and handle a rejected registration promise:
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/sw.js', { scope: '/' })
.catch((error) => console.error('Service worker registration failed:', error));
}Here, /sw.js is a root-relative URL on the page’s origin. The explicit / scope requests control across paths on that origin, subject to the browser’s scope rules and the worker response headers. If the worker is at /app/sw.js, its default scope is normally /app/ and below; it cannot simply claim / unless the response authorizes a broader scope with Service-Worker-Allowed. Review MDN’s register() reference for scope and registration details.
Registration returns a promise for a service worker registration. A fulfilled promise means the browser accepted registration; it does not mean that offline behavior is implemented or that every page is already controlled.
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Implement the worker’s install and activate steps
Create the worker file at the URL used in registration. Use its install event to prepare resources the site must have available offline. Put asynchronous cache work inside event.waitUntil(), so the browser can treat a failed required cache operation as a failed installation.
self.addEventListener('install', (event) => {
event.waitUntil(
caches.open('site-v1').then((cache) => {
return cache.addAll(['/offline.html', '/styles.css', '/app.js']);
})
);
});This is an example of the install pattern, not a complete offline strategy: the paths must exist on your site, and this code does not define how later fetches are served. Decide which assets are essential enough to precache; other requests may need a fetch-time network or cache strategy.
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Use activate to remove caches that are definitely obsolete. Be conservative: pages still controlled by an older worker may depend on its cache while they remain open. The MDN guide to using service workers covers the install, activate, and caching lifecycle.
Understand first install and updates
First installation
The browser downloads, installs, and activates the worker. A page that was already open when the first worker activated generally is not controlled retroactively; reload it to enter the controlled lifecycle. Pages opened afterward within scope can be controlled.
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Replacing an existing worker
When a changed worker is installed, the current worker normally remains active until its clients are gone. The new worker then activates. This default avoids abruptly switching an open page to a worker with different behavior.
Taking over immediately
skipWaiting() can request that a waiting worker activate sooner; clients.claim() can let an active worker take control of existing pages in scope. These choices can speed up deployment, but an already-open page may then send requests to new worker code or caches. Use them only when the page and worker versions remain compatible, or coordinate the transition in your application.
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Choose scope, caching, and update behavior deliberately
| Decision | Practical choice | Trade-off |
|---|---|---|
| Scope | Use the narrowest path that contains the feature; request a broader scope only when the site needs it. | A worker in a subdirectory is limited by default. Broader scope may require the Service-Worker-Allowed response header. |
| Update takeover | Let the new worker wait for old clients to close, or deliberately request earlier activation with skipWaiting() and, where appropriate, clients.claim(). |
Faster takeover can expose open pages to changed worker behavior. Check compatibility before switching. |
| Cache policy | Precache only resources essential to the offline experience; define fetch-time behavior for other requests. | Deleting a cache too early can break pages still using the previous worker. Remove only caches known to be obsolete. |
Troubleshoot registration failures
If registration fails, inspect the browser console and verify the deployed script URL, origin, secure context, and requested scope. Common causes include:
- The site is not on HTTPS and is not running on
localhost. - The worker file is missing, served from a different origin, or not at the URL passed to
register(). - The requested scope reaches above the worker script’s directory without an appropriate
Service-Worker-Allowedresponse header. - Browser settings or a Content Security Policy prevent the worker from loading.
Keep the registration rejection handler while diagnosing the problem; its error can help identify a path, scope, or security-context failure. Browser policy and settings can also affect whether registration succeeds. See MDN’s registration troubleshooting and security notes.
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