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>}`)
/**
     * @param {PDFWorkerParameters} params - Worker initialization parameters.
     */
    constructor({
      name = null,
      port = null,
      verbosity = getVerbosityLevel(),
    } = {}) {
      if (port && pdfWorkerPorts.has(port)) {
        throw new Error("Cannot use more than one PDFWorker per port");
      }
      this.name = name;
      this.destroyed = false;
      this.postMessageTransfers = true;
      this.verbosity = verbosity;
      this._readyCapability = createPromiseCapability();
      this._port = null;
      this._webWorker = null;
      this._messageHandler = null;
      if (port) {
        pdfWorkerPorts.set(port, this);
        this._initializeFromPort(port);
        return;
      }
      this._initialize();
    }
    get promise() {