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>}`)
this._messageHandler = messageHandler;
              this._port = worker;
              this._webWorker = worker;
              if (!data.supportTransfers) {
                this.postMessageTransfers = false;
              }
              this._readyCapability.resolve();
              
              messageHandler.send("configure", {
                verbosity: this.verbosity,
              });
            } else {
              this._setupFakeWorker();
              messageHandler.destroy();
              worker.terminate();
            }
          });
          messageHandler.on("ready", data => {
            worker.removeEventListener("error", onWorkerError);
            if (this.destroyed) {
              terminateEarly();
              return; 
            }
            try {
              sendTest();
            } catch (e) {
              
              this._setupFakeWorker();
            }