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


serviceWorkerGlobalScope.onfetch = function(fetchEvent) { ... };


This code snippet is from the service worker prefetch 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 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

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See also