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FetchEvent

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.

Constructor

FetchEvent.FetchEvent()
Creates a new FetchEvent object.

Properties

Inherits properties from its ancestor, Event.

FetchEvent.isReload Read only
Returns a 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
Returns the Request that triggered the event handler.
FetchEvent.clientId Read only
Returns the id of the client that the current service worker is controlling.

Deprecated properties

FetchEvent.client Read only
Returns the Client that the current service worker is controlling.

Methods

Inherits methods from its parent, ExtendableEvent.

FetchEvent.respondWith()
Resolves by returning a 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.

Examples

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;
      });
    })
  );
});

Specifications

Specification Status Comment
Service Workers
The definition of 'FetchEvent' in that specification.
Working Draft Initial definition.

Browser compatibility

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.)

See also

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, jpmedley, teoli, fscholz, Meggin
 Last updated by: chrisdavidmills,