index.js
function onRequestResponse (options, error, response, body) {
  const callback = options.callback;

  // Encoding is null so body should be a buffer object
  if (error || !body || !body.toString) {
    // Pure request error (bad connection, wrong url, etc)
    return callback(new RequestError(error, options, response));
  }

  response.responseStartTime = Date.now();
  response.isCloudflare = /^cloudflare/i.test('' + response.caseless.get('server'));
  response.isHTML = /text\/html/i.test('' + response.caseless.get('content-type'));

  // If body isn't a buffer, this is a custom response body.
  if (!Buffer.isBuffer(body)) {
    return callback(null, response, body);
  }

  // Decompress brotli compressed responses
  if (/\bbr\b/i.test('' + response.caseless.get('content-encoding'))) {
    if (!brotli.isAvailable) {
      const cause = 'Received a Brotli compressed response. Please install brotli';
      return callback(new RequestError(cause, options, response));
    }

    response.body = body = brotli.decompress(body);
  }

  if (response.isCloudflare && response.isHTML) {
    onCloudflareResponse(options, response, body);
  } else {
    onRequestComplete(options, response, body);
  }
}
function onRequestResponse (options, error, response, body) {
  const callback = options.callback;

  // Encoding is null so body should be a buffer object
  if (error || !body || !body.toString) {
    // Pure request error (bad connection, wrong url, etc)
    return callback(new RequestError(error, options, response));
  }

  response.responseStartTime = Date.now();
  response.isCloudflare = /^cloudflare/i.test('' + response.caseless.get('server'));
  response.isHTML = /text\/html/i.test('' + response.caseless.get('content-type'));

  // If body isn't a buffer, this is a custom response body.
  if (!Buffer.isBuffer(body)) {
    return callback(null, response, body);
  }

  // Decompress brotli compressed responses
  if (/\bbr\b/i.test('' + response.caseless.get('content-encoding'))) {
    if (!brotli.isAvailable) {
      const cause = 'Received a Brotli compressed response. Please install brotli';
      return callback(new RequestError(cause, options, response));
    }

    response.body = body = brotli.decompress(body);
  }

  if (response.isCloudflare && response.isHTML) {
    onCloudflareResponse(options, response, body);
  } else {
    onRequestComplete(options, response, body);
  }
}
The argument convention is options first where possible, options always before response, and body always after response.