https://github.com/mozilla/pdf.js/blob/0df1a5661958df0c4ed2f73b0f3266d825c5c280/src/display/api.js#L710-L719
The return type signature `@returns {Promise<{num: number, gen: number}>}` is mismatched with its definition. (should be `@returns {Promise<number>}`)
setVerbosityLevel(params.verbosity);
  if (!worker) {
    const workerParams = {
      verbosity: params.verbosity,
      port: GlobalWorkerOptions.workerPort,
    };
    
    
    worker = workerParams.port
      ? PDFWorker.fromPort(workerParams)
      : new PDFWorker(workerParams);
    task._worker = worker;
  }
  const docId = task.docId;
  worker.promise
    .then(function () {
      if (task.destroyed) {
        throw new Error("Loading aborted");
      }
      const workerIdPromise = _fetchDocument(
        worker,
        params,
        rangeTransport,
        docId
      );
      const networkStreamPromise = new Promise(function (resolve) {
        let networkStream;
        if (rangeTransport) {
          networkStream = new PDFDataTransportStream(