To extract data from PDF to FDF, then export FDF as XFDF

async function main() {
  const doc = await PDFNet.PDFDoc.createFromURL(filename);

  // Extract annotations to FDF.
  // Optionally use e_both to extract both forms and annotations
  const doc_fields = await doc.fdfExtract(PDFNet.PDFDoc.ExtractFlag.e_annots_only); //PDFNet.PDFDoc.ExtractFlag.e_forms_only

  // Export annotations from FDF to XFDF.
  const xfdf_data = await doc_fields.saveAsXFDFAsString();

  //optionally save the blob to a file or upload to a server
  const blob = new Blob([xfdf_data], { type: 'application/xml' });
}
PDFNet.runWithCleanup(main);

PDF Form Fill and Form Data Extraction
Full sample code which illustrates basic FDF merge/extract functionality and full support for FDF (Forms Data Format)