var workbook = wijmo.grid.xlsx.FlexGridXlsxConverter.saveAsync(theGrid, {
      includeColumnHeaders: true,
      includeStyles: true,
    },
   );
     // Call the save() method of the workbook and refer to JSZip 2.5.0.
    wijmo.xlsx.useJSZip(jszip25);
    let base64 = workbook.save();
    console.log(base64)
    // Sent this base64 string to the server and verify.
    //
    // save the book when needed by calling useJSZip() method to refer to the JSZip 3.10.1.
     wijmo.xlsx.useJSZip.useJSZip(jszip25);
    workbook.saveAsync('PivotGrid.xlsx');