FetchEvent
This is the event type for fetch
events dispatched on the service worker global scope. It contains information about the fetch, including the request and how the receiver will treat the response. It provides the event.respondWith()
method, which allows us to provide a response to this fetch.
Constructor
FetchEvent()
-
Creates a new
FetchEvent
object. This constructor is not typically used. The browser creates these objects and provides them tofetch
event callbacks.
Instance properties
Inherits properties from its ancestor, Event
.
FetchEvent.clientId
Read onlyFetchEvent.handled
Read only-
A promise that is pending while the event has not been handled, and fulfilled once it has.
FetchEvent.preloadResponse
Read only-
A
Promise
for aResponse
, orundefined
if this fetch is not a navigation, or navigation preload is not enabled. FetchEvent.replacesClientId
Read only-
The
id
of theclient
that is being replaced during a page navigation. FetchEvent.resultingClientId
Read only-
The
id
of theclient
that replaces the previous client during a page navigation. FetchEvent.request
Read only-
The
Request
the browser intends to make.
Instance methods
Inherits methods from its parent, ExtendableEvent
.
FetchEvent.respondWith()
-
Prevent the browser's default fetch handling, and provide (a promise for) a response yourself.
ExtendableEvent.waitUntil()
-
Extends the lifetime of the event. Used to notify the browser of tasks that extend beyond the returning of a response, such as streaming and caching.
Examples
This fetch event uses the browser default for non-GET requests. For GET requests it tries to return a match in the cache, and falls back to the network. If it finds a match in the cache, it asynchronously updates the cache for next time.
js
self.addEventListener("fetch", (event) => {
// Let the browser do its default thing
// for non-GET requests.
if (event.request.method !== "GET") return;
// Prevent the default, and handle the request ourselves.
event.respondWith(
(async () => {
// Try to get the response from a cache.
const cache = await caches.open("dynamic-v1");
const cachedResponse = await cache.match(event.request);
if (cachedResponse) {
// If we found a match in the cache, return it, but also
// update the entry in the cache in the background.
event.waitUntil(cache.add(event.request));
return cachedResponse;
}
// If we didn't find a match in the cache, use the network.
return fetch(event.request);
})()
);
});
Specifications
Specification |
---|
Service Workers # fetchevent-interface |
Browser compatibility
BCD tables only load in the browser