A service worker lets a web app intercept requests and serve saved responses when the network is unavailable. Browser storage holds those responses and, when needed, structured app data. Together they can keep planned parts of an app usable offline—but only after the relevant resources or data have been saved, and browser storage is not guaranteed to last forever.
How an offline web app works
A service worker is an event-driven script associated with a site origin and path. It runs separately from the page’s main thread. After the browser installs and activates it, pages within its registered scope can be controlled by the worker. Requests from those pages then trigger fetch events, where the worker can choose an available cached response, request a current response from the network, or provide a fallback. See MDN’s Service Worker API and guide to using service workers.
Install: save what the app needs to start
The install event is a common place to put an app shell—such as its core HTML, CSS, JavaScript, and essential interface assets—into Cache Storage. Use event.waitUntil() for asynchronous setup so the browser knows installation work is still in progress. If required resources fail to save, the app should handle that failure rather than assuming offline setup succeeded.
Activate: maintain the current version
Activation is commonly used to remove caches belonging to earlier worker versions. Cache names or another explicit versioning strategy help distinguish old resources from the current set. Cleanup needs care: deleting a cache too soon can leave an already-open page or an older controlled client without resources it still expects.
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Fetch: choose a response for each request
When a controlled page makes a request, the worker’s fetch handler can return a cached response, try the network, or apply a fallback suited to that request. Offline support is therefore a planned behavior, not a switch that makes every page and feature work without connectivity. A resource or data record that was never saved cannot be served from the browser while offline.
Choose storage for the data you need offline
| Need | Suitable mechanism | What it does | Important limitation |
|---|---|---|---|
| App shell, assets, or requestable content | Cache Storage | Stores Request/Response pairs that a service worker can return for matching requests. | The app manages updates, versions, and cleanup; entries do not refresh or expire automatically. |
| Structured records, locally available content, or queued state | IndexedDB | Stores structured client-side records and supports working with them without a network. | Quota and eviction behavior vary by browser; handle failed writes. |
| Small, simple string settings | Web Storage: localStorage or sessionStorage | Provides convenient key/value strings to page scripts. | Synchronous, small-capacity, and unavailable inside service workers. |
| More durable treatment for origin data | Storage API persistence request | Lets an app request persistent storage treatment from the browser. | Approval and behavior vary; users can still clear site data. |
Cache Storage is an application-managed set of request-and-response pairs, not the browser’s ordinary HTTP cache. Saving a response there does not mean it will automatically be refreshed when the server changes it, and HTTP cache headers do not make Cache Storage entries expire automatically. The app must define how it updates and removes them. MDN explains the Cache interface and caching for progressive web apps.
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IndexedDB is the better fit when offline behavior needs records rather than a response for a URL—for example, locally stored content or actions queued for later synchronization. It is asynchronous and available to service workers. MDN describes its key characteristics and basic terminology.
For scale, MDN gives a Web Storage maximum of 10 MiB per origin in total: up to 5 MiB for localStorage and 5 MiB for sessionStorage. That figure applies to Web Storage only; it is not a quota for Cache Storage or IndexedDB. Those quotas are browser-specific.
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What happens to saved data over time?
Browser storage is not an unconditional backup. Best-effort storage is the default and may be evicted to reclaim space; users can also clear site data. An app can request persistent storage with navigator.storage.persist(), but browsers differ in how they handle the request, and persistence does not prevent users from clearing data.
Use navigator.storage.estimate() to inspect approximate origin storage use rather than assuming a fixed large quota. Catch write failures, including QuotaExceededError, and decide whether to remove nonessential data, limit new offline content, or tell the user that offline availability is reduced. Do not rely on browser storage as the only copy of irreplaceable user data. MDN details storage quotas and eviction criteria.
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Requirements and reliability checks
- Serve the deployed app through HTTPS. Service workers require a secure context; HTTPS is the typical production requirement. See MDN’s service worker guide.
- Keep the scope intentional. A worker controls pages and their requests within its registered scope, not every page on the web origin by default.
- Wait for essential setup. Use
event.waitUntil()to keep install or activation work alive until its asynchronous operations finish. - Plan cache compatibility. Version cache contents and remove old caches only when they are no longer needed by active pages.
- Test distinct offline states. Check first visit, successful installation, reload while offline, a cache update, and behavior when a resource is missing or storage writes fail. These cases expose gaps that a simple “works offline” check can miss.
Private-browsing behavior, quotas, persistence approval, and eviction policies vary by browser and can change. There is no single quota figure that can safely be applied to every browser or to every storage API.
What offline support does—and does not—promise
A service worker supplies the request-handling logic; Cache Storage and IndexedDB supply different kinds of local data. An app can use them to make selected screens, assets, and records available without a connection, provided it saved what those features need and can still access the saved data. It cannot guarantee that all features work offline, that cached responses stay current without an update strategy, or that the browser will retain locally stored data indefinitely.
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