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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 onfetch property of the ServiceWorkerGlobalScope interface is an event handler fired whenever a fetch event occurs (usually when the ServiceWorkerGlobalScope.fetch method is called.)


ServiceWorkerGlobalScope.onfetch = function(FetchEvent) { ... }


This code snippet is from the service worker fetch sample (see prefetch example live.) The ServiceWorkerGlobalScope.onfetch event handler listens for the fetch event. When fired, the code returns a promise that resolves to the first matching request in the Cache object. If no match is found, the code fetches a response from the network.

The code also handles exceptions thrown from the ServiceWorkerGlobalScope.fetch operation. Note that an HTTP error response (e.g., 404) will not trigger an exception. It will return 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);

    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 'Event Handlers' 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 ? No support
Feature Android Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support ? 44.0 (44.0) (Yes)[3] No support ? No support 40.0

[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, Sebastianz, kscarfone, jpmedley
 Last updated by: chrisdavidmills,