FetchEvent.respondWith()

The respondWith() method of the FetchEvent interface is intended for containing code that generates custom responses to the requests coming from the controlled page. This code will resolve by returning a Response or network error to Fetch.

Renderer-side security checks about tainting for cross-origin content are tied to the transparency (or opacity) of the Response body, not URLs. If the request is a top-level navigation and the return value is a Response whose type attribute is opaque (i.e. an opaque response body), a network error is returned to Fetch. The final URL of all successful (non network-error) responses is the requested URL.

Syntax

FetchEvent.respondWith(
  //Code that resolves to a Response or a network error.
​)

Returns

Void.

Parameters

Any custom response-generating code.

Examples

This code snippet is from the service worker fetch sample (run the fetch sample live.) The ServiceWorkerGlobalScope.onfetch event handler listens for the fetch event. When fired, FetchEvent.respondWith(any value) 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, 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);

  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 Extended Support Release (ESR.)

See also

• Using Service Workers
• Service workers basic code example
• Is ServiceWorker ready?
• Promise
• Using web workers
• Fetch API