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>}`)
}
      let fetched = false;
      sink.onPull = () => {
        if (fetched) {
          sink.close();
          return;
        }
        fetched = true;
        this.CMapReaderFactory.fetch(data)
          .then(function (builtInCMap) {
            sink.enqueue(builtInCMap, 1, [builtInCMap.cMapData.buffer]);
          })
          .catch(function (reason) {
            sink.error(reason);
          });
      };
    });
  }
  _onUnsupportedFeature({ featureId }) {
    if (this.destroyed) {
      return; 
    }
    if (this.loadingTask.onUnsupportedFeature) {
      this.loadingTask.onUnsupportedFeature(featureId);
    }
  }
  getData() {
    return this.messageHandler.sendWithPromise("GetData", null);
  }
  getPage(pageNumber) {