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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →An offline-capable web app uses a service worker to intercept requests and browser storage to keep the responses or data it needs. When the network is unavailable, the worker can return a suitable cached response—but only for resources the app has deliberately stored. Browser storage can also be cleared or evicted, so offline support is a planned capability, not a guarantee that every page or user record will always be available.
How a service worker makes a web app work offline
A service worker is an event-driven worker associated with a website’s origin and a path scope. It runs in a separate execution context from the page. Once active and controlling a page, it can intercept eligible page navigations and resource requests, then decide whether to return a network response, a locally cached response, or a fallback. See MDN’s Service Worker API and guide to using service workers.
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The lifecycle provides places to prepare and maintain offline resources. During installation, an app commonly stores the files required for its offline shell. During activation, it can remove caches belonging to older worker versions. Asynchronous setup and cleanup should be tied to the lifecycle event with event.waitUntil() so the browser knows the work must finish before the event completes.
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Service workers require a secure context, typically HTTPS in production. Their scope also matters: a worker controls pages and requests within its registered scope, not automatically every path on a site. Plan the registration path and scope around the pages the app must support offline. MDN documents the secure-context requirement in its service worker guide.
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Choose storage by what you need to keep
| Need | Suitable mechanism | What it stores | Important limitation |
|---|---|---|---|
| App shell, scripts, styles, images, or fetched content | Cache Storage, managed by the service worker | Request/Response pairs | The app must refresh, version, and remove entries; cached entries do not expire automatically. |
| Structured records, offline content collections, or queued state | IndexedDB | Structured client-side data, with support for richer querying | Quota and eviction behavior depend on the browser; it is not a guaranteed backup. |
| Small, simple string settings | Web Storage: localStorage or sessionStorage | Key/value strings | Synchronous and small; Web Storage is unavailable inside service workers. |
| Request less aggressive eviction for important local data | Storage API persistence request | Does not store data itself; requests persistent treatment for the origin’s storage | The browser may deny the request, and the user can still clear site data. |
Cache Storage is an application-managed collection of request/response pairs, not the browser’s ordinary HTTP cache. A cache entry will not update merely because the server now has a newer file, and HTTP cache headers do not make Cache Storage expire automatically. Your application needs an explicit update policy: name caches by version or otherwise track compatibility, add current resources, and remove old entries only when they are no longer needed. MDN explains the Cache API and caching for progressive web apps.
IndexedDB is the better fit for structured records that an app needs to read, update, or query while offline. A service worker can use IndexedDB, but it cannot use synchronous Web Storage APIs such as localStorage or sessionStorage. MDN’s overview of IndexedDB key characteristics and terminology describes its role as a database for client-side data.
Rank #2
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Plan cache updates and offline fallbacks
Choose what must be available offline rather than attempting to cache everything. A compact offline shell can let the app open and explain its status even when a particular content request cannot be served. For data-dependent screens, decide whether the app should show the last stored result, a clear unavailable message, or locally queued changes. The worker’s fetch handler is where the app applies a response strategy suited to each request.
- During installation: cache the essential files needed to render the offline shell. Use
event.waitUntil()for the asynchronous cache work; if required setup fails, the worker should not be treated as fully installed. - During activation: remove obsolete versioned caches only when doing so will not break pages still relying on them. Ensure the new worker and its cached resources are compatible.
- For each fetch: decide whether the request should use the network, a cached response, or a deliberate fallback. A cached response is possible only if a suitable entry was previously stored.
- When data changes: define how fresh responses replace older cache entries and how structured records in IndexedDB are reconciled with the server once connectivity returns.
MDN’s offline service worker tutorial demonstrates caching an application for offline use. The exact strategy for freshness and fallback depends on the app’s content and the consequences of showing stale information.
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Understand storage limits and data loss
Browser-managed storage is best-effort by default: browsers can evict stored data to reclaim space, and users can clear site data. Quotas and eviction rules differ by browser and may change, so there is no single reliable Cache Storage or IndexedDB capacity to assume. An origin can inspect approximate usage with navigator.storage.estimate() and request persistent storage with navigator.storage.persist(), but browsers handle persistence requests differently and may not grant them. MDN covers these details in Storage quotas and eviction criteria.
MDN states a maximum of 10 MiB of Web Storage per origin in total: up to 5 MiB for localStorage and 5 MiB for sessionStorage. This figure is for Web Storage only; it is not a quota for Cache Storage or IndexedDB. When writing to browser storage, handle failures such as QuotaExceededError, and decide which content can be removed or how the app should explain reduced offline availability.
Rank #4
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Do not treat browser storage as the sole copy of irreplaceable user data. For data that must survive device loss or clearing site data, the app needs an appropriate server-side or export backup as well as its offline copy. Private-browsing behavior can also differ, so test the behavior that matters for your supported browsers rather than assuming local data will persist indefinitely.
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Validate the offline experience before release
- Test a first visit with a network connection, then confirm that required installation and caching work completes.
- Reload while offline and verify the expected shell, cached resources, and unavailable-content messages.
- Test an update where the service worker and cached files change, checking that an active page is not left using incompatible resources.
- Exercise missing-resource and storage-write failures, including a quota failure, and confirm the app degrades clearly rather than appearing to save data it could not store.
- Check the intended service worker scope and secure deployment context for the pages the app must control.
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