FetchObserver.onrequestprogress

The onrequestprogress property of the FetchObserver interface is an event handler invoked when a requestprogress event fires, i.e. when the request progresses.

Syntax

observer.onrequestprogress = function() { ... };

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 the FetchController.onrequestprogress event handler to fill up a progress bar as more of the request progresses.

Note that this event handler is not yet supported anywhere.

var controller = new FetchController();
var signal = controller.signal;
downloadBtn.addEventListener('click', function() {
  fetch(url, {
    signal,
    observe(observer) {
      observer.onrequestprogress = function(e) {
        progress.max = e.total;
        progress.value = e.loaded;
      }
    }
  }).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

See also

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