A progressive web app can save selected files and data on a device for faster responses or offline use. That caching is separate from installing the website as an app: a service worker may be installed by the browser after a visit even if you never choose an install option. The app’s code determines what gets cached and when; neither offline support nor user installation happens automatically.
How a PWA caches resources
A service worker is a script that runs separately from a page’s main JavaScript thread. When a page within its scope makes a request, the worker can intercept the fetch and decide what response to provide. The Fetch API represents requests and responses; the Cache API stores request-and-response pairs so they can be looked up later. The worker does not cache resources by default—the application must implement that behavior.
As an Amazon Associate I earn from qualifying purchases.
Resources can enter a cache at different times:
- Precached: The worker saves selected files during its browser-level
installevent, often for an app shell or other resources needed to start the interface. - Cached during a fetch: The worker saves a response as it handles a request, making it available for a later visit.
- Saved on request: The app can let a user explicitly download or save content for later use.
These are implementation choices, not properties every PWA shares. A service worker can return a cached response, fetch a current response from the network, or generate a response locally. See MDN’s PWA caching guide and web.dev’s service worker guide.
Which caching strategy fits each resource?
Choose a policy according to how quickly a resource must load, how current it must be, and what should happen when the network is unavailable. The same app can use different policies for different resource types.
#1 Best Overall
| Strategy | Typical behavior | Strength | Trade-off |
|---|---|---|---|
| Cache-first | Check the cache and use a match before trying the network. | Can respond quickly and work offline when the required response is cached. | Cached content may be stale unless the app updates or replaces it appropriately. |
| Network-first | Try the network, then use a cached response if the network request fails. | Usually favors current server content while providing an offline fallback when one exists. | Can be slower on an unreliable connection; a request with no usable cache fallback still fails unless the app supplies another response. |
For example, an app might treat a stable interface shell differently from frequently changing account or inventory data. This is a design choice, not a universal recipe. Decide what should happen when the app is offline, when a cached item is outdated, and when neither the cache nor network has the requested resource. MDN discusses these trade-offs in its guide to offline and background operation.
What caching does—and does not—guarantee offline
A service worker can only intercept requests for clients within its scope; the worker script’s location affects that scope. On a first visit, the page’s requests go to the server before a newly registered worker controls the page. Offline behavior may therefore differ on that first load from later visits, after the worker has taken control and any required resources have been cached.
Rank #2
- HTML CSS Design and Build Web Sites
- Comes with secure packaging
- It can be a gift option
Offline support also depends on the app’s chosen resources and fallbacks. Caching an app shell does not automatically make every page, image, or data request available without a connection. A worker can be stopped while idle and restarted for a later event, and an updated worker may wait until pages controlled by the old worker are closed. Account for those lifecycle details when designing updates and fallbacks; MDN’s offline operation guide covers them.
The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Service-worker installation is not PWA installation
The word “install” refers to two different things here. A browser installs a service worker as part of its technical lifecycle, commonly after a person visits a site and the site registers the worker. That process can happen whether or not the visitor adds the site to their device as an app. By contrast, PWA installation is a user-facing browser or platform action that makes the site available in an app-like form.
Rank #3
- Brand: Wiley
- Set of 2 Volumes
- A handy two-book set that uniquely combines related technologies Highly visual format and accessible language makes these books highly effective learning tools Perfect for beginning web designers and front-end developers
Neither event guarantees the other: a service worker does not itself make a site installable, and a user’s installation choice is not what causes the worker’s install event. Installability depends on browser and platform support and the site’s implementation. MDN’s installability guide and web.dev’s installation guide describe platform-specific behavior, which can change. Check current documentation for the browser, operating system, and audience before giving device-specific install steps.
Quick Recap
How to plan and verify a caching setup
- List the resources and their needs. Separate stable files, changing data, and content users might deliberately save. Note which must work offline and how fresh each needs to be.
- Choose a policy for each resource class. Decide whether to prefer a cached response or a network response, and define a fallback for failure.
- Check the worker’s scope and first-visit behavior. Confirm which pages it controls and test the initial visit separately from later visits.
- Test offline and update cases. Verify expected behavior when a resource is cached, missing, stale, or unavailable from the network; also verify what happens when a new worker is waiting to activate.
- Check installation separately. Test the user-facing install flow on each target browser and operating system rather than treating successful caching as proof that the site can be installed.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
