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 (
        typeof Worker !== "undefined" &&
        !isWorkerDisabled &&
        !getMainThreadWorkerMessageHandler()
      ) {
        let workerSrc = getWorkerSrc();
        try {
          
          
          if (
            typeof PDFJSDev !== "undefined" &&
            PDFJSDev.test("GENERIC") &&
            !isSameOrigin(window.location.href, workerSrc)
          ) {
            workerSrc = createCDNWrapper(
              new URL(workerSrc, window.location).href
            );
          }