ObserverCallback

The ObserverCallback interface of the Fetch API provides a callback function that runs when the fetch request it is assocated with runs. This provides a FetchObserver object that can be used to retrieve information concerning the status of a fetch request.

The ObserverCallback is specified as the value of the fetch request init object's observe property (see WindowOrWorkerGlobalScope.fetch()).

Methods

ObserverCallback.handleEvent()

Runs when the associated fetch request runs, providing a FetchObserver object to observe the fetch.

Examples

In the following snippet, we create a new FetchController object, get its signal, and then give the signal to the fetch request via the signal parameter of its init object so the controller can control it. Later on we specify an event listener on a cancel button so that when the button is clicked, we abort the fetch request using FetchController.abort().

We also specify an observe property inside the fetch request init object — this contains a ObserverCallback object, the sole purpose of which is to provide a callback function that runs when the fetch request runs. This returns a FetchObserver object that can be used to retrieve information concerning the status of a fetch request.

Here we use FetchController.responseprogress and FetchController.onstatechange event handlers to respectively fill up a progress bar as more of the reponse downloads, and to determine when the download has completed and display a message to let the user know.

var controller = new FetchController();
var signal = controller.signal;
downloadBtn.addEventListener('click', function() {
  fetch(url, {
    signal,
    observe(observer) {
      observer.onresponseprogress = function(e) {
        console.log(e);
        progress.max = e.total;
        progress.value = e.loaded;
      }
      observer.onstatechange = function() {
        if (observer.state === 'complete') {
          reports.textContent = 'Download complete';
        }
      }
    }
  }).then( ... ) // do something with the response
});
cancelBtn.addEventListener('click', function() {
  controller.abort();
});

You can find a work-in-progress demo showing usage of FetchObserver on GitHub (see the source code and the live example).

Specifications

Not part of a specification yet.

Browser compatibility

[1] Hidden behind a preference in 55+ Nightly. In about:config, you need to create two new boolean prefs — dom.fetchObserver.enabled and dom.fetchController.enabled — and set the values of both to true.

See also

• ServiceWorker API
• HTTP access control (CORS)
• HTTP