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.messageHandler.sendWithPromise("GetPageMode", null);
  }
  getViewerPreferences() {
    return this.messageHandler.sendWithPromise("GetViewerPreferences", null);
  }
  getOpenAction() {
    return this.messageHandler.sendWithPromise("GetOpenAction", null);
  }
  getAttachments() {
    return this.messageHandler.sendWithPromise("GetAttachments", null);
  }
  getJavaScript() {
    return this.messageHandler.sendWithPromise("GetJavaScript", null);
  }
  getDocJSActions() {
    return this.messageHandler.sendWithPromise("GetDocJSActions", null);
  }
  getPageJSActions(pageIndex) {
    return this.messageHandler.sendWithPromise("GetPageJSActions", {
      pageIndex,
    });
  }
  getPageXfa(pageIndex) {
    return this.messageHandler.sendWithPromise("GetPageXfa", {
      pageIndex,
    });
  }
  getStructTree(pageIndex) {