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>}`)
if (!isWorkerDisabled) {
        warn("Setting up fake worker.");
        isWorkerDisabled = true;
      }
      setupFakeWorkerGlobal()
        .then(WorkerMessageHandler => {
          if (this.destroyed) {
            this._readyCapability.reject(new Error("Worker was destroyed"));
            return;
          }
          const port = new LoopbackPort();
          this._port = port;
          
          const id = "fake" + nextFakeWorkerId++;
          
          
          const workerHandler = new MessageHandler(id + "_worker", id, port);
          WorkerMessageHandler.setup(workerHandler, port);
          const messageHandler = new MessageHandler(id, id + "_worker", port);
          this._messageHandler = messageHandler;
          this._readyCapability.resolve();
          
          messageHandler.send("configure", {
            verbosity: this.verbosity,