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

const doc = await PDFNet.PDFDoc.createFromURL(filename);

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

// Export annotations from FDF to XFDF.
const xfdf_data = doc_fields.saveAsXFDFAsString();
saveAs(new Blob([xfdf_data], {type: 'application/xml'}), output_xfdf_filename);

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)