FetchObserver.state 只读
Returns a FetchState enum value indicating the current state of the fetch request.

Event handlers

Invoked when a statechange event fires, i.e. when the state of the fetch request changes.
Invoked when a requestprogress event fires, i.e. when the request progresses.
Invoked when a responseprogress event fires, i.e. when the download of the response progresses.


The FetchSignal interface inherits methods from its parent interface, EventTarget.


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.

Note that these event handlers are not yet supported anywhere.

var controller = new FetchController();
var signal = controller.signal;

downloadBtn.addEventListener('click', function() {
  fetch(url, {
    observe(observer) {
      observer.onresponseprogress = function(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() {

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


Not part of a specification yet.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help! (en-US)

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support


未实现 未实现[1] 未实现


Feature Android Android Webview Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile Chrome for Android
Basic support 未实现 未实现 未实现 未实现[1] 未实现 未实现 未实现 未实现

[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