This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.
The parameter passed into the ServiceWorkerGlobalScope.onfetch
handler, FetchEvent
represents a fetch action that is dispatched on the ServiceWorkerGlobalScope
of a ServiceWorker
. It contains information about the request and resulting response, and provides the FetchEvent.respondWith()
method, which allows us to provide an arbitrary response back to the controlled page.
FetchEvent.FetchEvent()
FetchEvent
object.Inherits properties from its ancestor, Event
.
FetchEvent.isReload
Read only
Boolean
that is true
if the event was dispatched with the user's intention for the page to reload, and false
otherwise. Typically, pressing the refresh button in a browser is a reload, while clicking a link and pressing the back button is not.FetchEvent.request
Read only
Request
that triggered the event handler.FetchEvent.clientId
Read only
FetchEvent.client
Read only
Client
that the current service worker is controlling.Inherits methods from its parent, ExtendableEvent
.
FetchEvent.respondWith()
Response
or a network error to Fetch
.ExtendableEvent.waitUntil()
Extends the lifetime of the event. It is intended to be called in the install
EventHandler
for the installing
worker and on the active
EventHandler
for the active
worker.
This code snippet is from the service worker fetch sample (run the fetch sample live). In an earlier part of the code, an InstallEvent
controls caching of a number of resources. The ServiceWorkerGlobalScope.onfetch
event handler then listens for the fetch
event. When fired, FetchEvent.respondWith()
returns a promise back to the controlled page. This promise resolves to the first matching URL request in the Cache
object. If no match is found (i.e. that resource wasn't cached in the install phase), the code fetches a response from the network.
The code also handles exceptions thrown from the ServiceWorkerGlobalScope.fetch()
operation. Note that a HTTP error response (e.g., 404) doesn't trigger an exception. It returns a normal response object that has the appropriate error code set.
self.addEventListener('fetch', function(event) { console.log('Handling fetch event for', event.request.url); event.respondWith( caches.match(event.request).then(function(response) { if (response) { console.log('Found response in cache:', response); return response; } console.log('No response found in cache. About to fetch from network...'); return fetch(event.request).then(function(response) { console.log('Response from network is:', response); return response; }).catch(function(error) { console.error('Fetching failed:', error); throw error; }); }) ); });
Specification | Status | Comment |
---|---|---|
Service Workers The definition of 'FetchEvent' in that specification. | Working Draft | Initial definition. |
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | 40.0 | 44.0 (44.0)[1] | No support | 24 | No support |
Feature | Android | Android Webview | Firefox Mobile (Gecko) | Firefox OS | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|---|
Basic support | No support | ? | 44.0 (44.0) | (Yes) | No support | ? | No support | 44.0 |
[1] Service workers (and Push) have been disabled in the Firefox 45 Extended Support Release (ESR.)
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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/FetchEvent