Definition

Cache-first strategy

Short definition

Cache-first is a fetch-handling strategy: event.respondWith(caches.match(event.request).then((cached) => cached ?? fetch(event.request))). It serves cached content instantly and skips the network entirely when a match exists, making it ideal for versioned, immutable assets such as hashed JS/CSS bundles or app-shell icons that never change once deployed.

Reviewed July 30, 2026

Because a cache hit never touches the network, cache-first should only be used for content that is safe to serve stale indefinitely — using it for an API endpoint that returns live data would show users outdated information with no way to detect it.

A common refinement adds a background refresh after serving from cache, which is effectively the stale-while-revalidate pattern rather than pure cache-first.

Cache-first assets are typically populated during the install event via precaching, so the first request after each deploy is a guaranteed cache hit rather than a race with the network.

The Workbox library provides this exact strategy out of the box as workbox.strategies.CacheFirst, including built-in cache expiration plugins for size and age limits.

Related terms

Network-first strategy

Network-first is a fetch-handling strategy: event.respondWith(fetch(event.request).then((res) => { cache.put(event.request, res.clone()); return res; }).catch(() => caches.match(event.request))). It prioritises up-to-date data and only serves a cached copy if the network request fails outright, making it the standard choice for API calls and frequently changing pages.

Stale-while-revalidate

Stale-while-revalidate is a fetch-handling strategy that serves whatever is already in Cache Storage without waiting on the network, then kicks off a background fetch to refresh that cache entry for the next request. It gives cache-first speed with eventual freshness, at the cost of occasionally showing content that is one request-cycle out of date.

Precaching

Precaching means fetching and storing a known list of build-time assets (HTML shell, CSS, JS bundles, key icons) in Cache Storage inside the install event handler, so they are guaranteed to be available offline from the very first load rather than being cached only after a user happens to request them.

Runtime caching

Runtime caching means storing a response in Cache Storage the first time a matching request is actually made by the user, inside the fetch event handler, as opposed to precaching, which loads a known asset list upfront during install. It is the natural fit for content whose full URL set is not known ahead of time, such as user-generated images or paginated API results.

Workbox

Workbox is a collection of JavaScript libraries and build tools (workbox-precaching, workbox-routing, workbox-strategies, workbox-expiration, and more) maintained by the Chrome team that implement the recurring service worker patterns — precache manifests, route matching, cache-first/network-first/stale-while-revalidate strategies — so teams do not hand-roll them from scratch.

Push API

The Push API lets a service worker subscribe to a browser-run push service and receive encrypted messages from a server at any time, including when no tab or window for the site is open. Subscribing is done with registration.pushManager.subscribe({ userVisibleOnly: true, applicationServerKey }), and incoming messages arrive in the worker's push event.