// ...

fastify.get('/', { schema: REQUEST_SCHEMA }, async (req, reply) => {
  reply.header('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
  reply.header('Content-Disposition', 'attachment; filename=export.xlsx');

  const workbook = new ExcelJS.stream.xlsx.WorkbookWriter({
    stream: reply.raw,
    useStyles: true,
    useSharedStrings: true,
  });
  const worksheet = workbook.addWorksheet('Export');

  worksheet.columns = ['1', '2', '3'];

  const row = ['1', '2', '3'];

  worksheet.addRow(row).commit();

  await workbook.commit();
});

// ...

// ...

fastify.get('/', { schema: REQUEST_SCHEMA }, async (req, reply) => {
  reply.header('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
  reply.header('Content-Disposition', 'attachment; filename=export.xlsx');

  const passThrough = new PassThrough({ highWaterMark: 64 * 1024 });

  const workbook = new ExcelJS.stream.xlsx.WorkbookWriter({
    stream: passThrough,
    useStyles: true,
    useSharedStrings: true,
  });
  const worksheet = workbook.addWorksheet('Export');

  worksheet.columns = ['1', '2', '3'];

  const row = ['1', '2', '3'];

  reply.send(passThrough);

  worksheet.addRow(row).commit();

  await workbook.commit();
});

// ...