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>}`)
return this._transport.getStats();
  }
  /**
   * Cleans up resources allocated by the document on both the main and worker
   * threads.
   *
   * NOTE: Do not, under any circumstances, call this method when rendering is
   * currently ongoing since that may lead to rendering errors.
   *
   * @param {boolean} [keepLoadedFonts] - Let fonts remain attached to the DOM.
   *   NOTE: This will increase persistent memory usage, hence don't use this
   *   option unless absolutely necessary. The default value is `false`.
   * @returns {Promise} A promise that is resolved when clean-up has finished.
   */
  cleanup(keepLoadedFonts = false) {