The respondWith()
method of FetchEvent
prevents the browser's default fetch handling, and allows you to provide a promise for a Response
yourself.
In most cases you can provide any response that the receiver understands. For example, if an <img>
initiates the request, the response body needs to be image data. For security reasons, there are a few global rules:
From Firefox 59 onwards, when a service worker provides a Response
to FetchEvent.respondWith()
, the Response.url
value will be propagated to the intercepted network request as the final resolved URL. If the Response.url
value is the empty string, then the FetchEvent.request.url
is used as the final URL.
In the past the FetchEvent.request.url
was used as the final URL in all cases. The provided Response.url
was effectively ignored.
This means, for example, if a service worker intercepts a stylesheet or worker script, then the provided Response.url
will be used to resolve any relative @import
or importScripts()
subresource loads (Firefox bug 1222008).
For most types of network request this change has no impact because you can't observe the final URL. There are a few, though, where it does matter:
- If a
fetch()
is intercepted, then you can observe the final URL on the result's Response.url
. - If a worker script is intercepted, then the final URL is used to set
self.location
and used as the base URL for relative URLs in the worker script. - If a stylesheet is intercepted, then the final URL is used as the base URL for resolving relative
@import
loads.
Note that navigation requests for Windows
and iframes
do NOT use the final URL. The way the HTML specification handles redirects for navigations ends up using the request URL for the resulting Window.location
. This means sites can still provide an "alternate" view of a web page when offline without changing the user-visible URL.
This fetch event tries to return a response from the cache API, falling back to the network otherwise.
addEventListener("fetch", (event) => {
event.respondWith(
(async () => {
const cachedResponse = await caches.match(event.request);
if (cachedResponse) return cachedResponse;
return fetch(event.request);
})(),
);
});