A StreamFilter is an object you can use to monitor and modify HTTP responses.

To create a StreamFilter, call webRequest.filterResponseData(), passing it the ID of the web request that you want to filter.

You can imagine the stream filter sitting between the networking stack and browser's rendering engine. The filter is passed HTTP response data as it is received from the network, and can examine and modify the data before passing it along to the rendering engine, where it will be parsed and rendered.

The filter generates four different events:

  • onstart when the filter is about to start receiving response data.
  • ondata when some response data has been received by the filter and is available to be examined or modified.
  • onstop when the filter has finished receiving response data.
  • onerror if an error has occurred in initializing and operating the filter.

You can listen to each event by assigning a listener function to its attribute:

filter.onstart = event => {

Note that the request is blocked during the execution of any event listeners.

The filter provides a write() function. At any time from the onstart event onwards you can use this function to write data to the output stream.

If you assign listeners to any of the filter's events, then all the response data passed to the rendering engine will be supplied through calls you make to write(): so if you add a listener but don't call write(), then the rendered page will be blank.

Once you have finished interacting with the response you call either of the following:

  • disconnect(): This disconnects the filter from the request, so the rest of the response is processed normally.
  • close(): This closes the request, so no additional response data will be processed.

The filter also provides functions to suspend() and resume() the request.


Closes the request.
Disconnects the filter from the request.
Resumes processing of the request.
Suspends processing of the request.
Writes some data to the output stream.


Event handler which is called when incoming data is available.
Event handler which is called when an error has occurred.
Event handler which is called when the stream is about to start receiving data.
Event handler which is called when the stream has no more data to deliver and has closed.
When webRequest.StreamFilter.onerror is called, this will describe the error.
Describes the current status of the stream.

Browser compatibility

ChromeEdgeFirefoxFirefox for AndroidOpera
Basic support No No5757 No
close No No5757 No
disconnect No No5757 No
error No No5757 No
ondata No No5757 No
onerror No No5757 No
onstart No No5757 No
onstop No No5757 No
resume No No5757 No
status No No5757 No
suspend No No5757 No
write No No5757 No


This code listens for onstart, ondata, and onstop. It simply logs those events, and the response data itself:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);

  filter.onstart = event => {
  filter.ondata = event => {

  filter.onstop = event => {

  //return {}; // not needed

  {urls: ["https://example.org/"], types: ["main_frame"]},

Document Tags and Contributors

Contributors to this page: kernp, Watz, wbamberg
Last updated by: kernp,