webRequest.StreamFilter.onstop

An event handler that will be called when the stream has no more data to deliver. In the event handler you can still call filter functions such as write(), disconnect(), or close().

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This example will append "extra stuff" to the response:

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

  filter.ondata = event => {
    // pass through all the response data
    filter.write(event.data);
  }

  filter.onstop = event => {
    filter.write(encoder.encode("extra stuff"));
    filter.disconnect();
  }
}

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

Here's another version of the example above:

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

  let data = [];
  filter.ondata = event => {
    data.push(event.data);
  };

  filter.onstop = event => {
    for (let buffer of data) {
      filter.write(buffer);
    }
    filter.write(encoder.encode("extra stuff"));
    filter.close();
  };
}

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