FetchEvent

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

Browser compatibility

[1] Service workers (and Push) have been disabled in the Firefox 45 and 52 Extended Support Releases (ESR.)

See also

• Promise
• Fetch API