How to call a fetch request and wait for it's answer inside a onBeforeRequest in a web extensio-CodePudding

I'm trying to write a web extension that stops the requests from a url list provided locally, fetches the URL's response, analyzes it in a certain way and based on the analysis results, blocks or doesn't block the request. Is that even possible? The browser doesn't matter. If it's possible, could you provide some examples?

I tried doing it with Chrome extensions, but it seems like it's not possible. I heard it's possible on mozilla though

CodePudding user response：

I think that this is only possible using the old webRequestBlocking API which Chrome is removing as a part of Manifest v3. Fortunately, Firefox is planning to continue supporting blocking web requests even as they transition to manifest v3 (read more here).

In terms of implementation, I would highly recommend referring to the MDN documentation for webRequest, in particular their section on modifying responses and their documentation for the filterResponseData method.

Mozilla have also provided a great example project that demonstrates how to achieve something very close to what I think you want to do.

Below I've modified their background.js code slightly so it is a little closer to what you want to do:

function listener(details) {
  if (mySpecialUrls.indexOf(details.url) === -1) {
    // Ignore this url, it's not on our list.
    return {};
  }

  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();
  }

  // This is a BlockingResponse object, you can set parameters here to e.g. cancel the request if you want to.
  // See: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/BlockingResponse#type
  return {};
}

browser.webRequest.onBeforeRequest.addListener(
  listener,
  // 'main_frame' means this will only affect requests for the main frame of the browser (e.g. the HTML for a page rather than the images, CSS, etc. that are loaded afterwards). You might want to look into whether you want to expand this.
  {urls: ["*://*/*"], types: ["main_frame"]},
  ["blocking"]
);