webRequest.filterResponseData()

Use this function to create a webRequest.StreamFilter object for a request. The stream filter gives the web extension full control over the stream, with the ability to monitor and modify the response. It's the extension's responsibility to write and close or disconnect the stream, as the default behavior is to keep the request open without a response.

You typically call this function from a webRequest event listener.

Firefox uses an optimized byte cache for script requests. This optimized byte cache overrides the normal request caching. Data from this cache is not available in a form useful to extensions. If your extension needs to filter scripts, create your filter in webRequest.onBeforeRequest. Doing this ensures that the filter is created prior to the attempt to load from cache, thereby avoiding the optimized cache.

To use this API you must have the "webRequestBlocking" API permission, as well as the normal permissions needed for the event listener (the "webRequest" permission and the host permission for the host).

requestId

string. ID of the request to filter. You can get this from the details object that is passed into any webRequest event listeners.

A webRequest.StreamFilter object that you can use to monitor and modify the response.

This example shows a minimal implementation that passes through the stream data and closes the filter stream when the stream finishes receiving data. The code would be called from a webRequest event listener and the event listener provides details.

let filter = browser.webRequest.filterResponseData(details.requestId);
filter.ondata = event => {
  console.log(`filter.ondata received ${event.data.byteLength} bytes`);
  filter.write(event.data);
};
filter.onstop = event => {
  // The extension should always call filter.close() or filter.disconnect()
  // after creating the StreamFilter, otherwise the response is kept alive forever.
  filter.close();
};

This example, taken from the http-response example extension, creates a filter in webRequest.onBeforeRequest and uses it, to modify the first chunk of the response:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  filter.ondata = event => {
    let str = decoder.decode(event.data, {stream: true});
    // Just change any instance of Example in the HTTP response
    // to WebExtension Example.
    str = str.replace(/Example/g, 'WebExtension Example');
    filter.write(encoder.encode(str));
    filter.disconnect();
  }

  return {};
}

browser.webRequest.onBeforeRequest.addListener(
  listener,
  {urls: ["https://example.com/*"], types: ["main_frame"]},
  ["blocking"]
);

• http-response