const file1 = 'one.csv';
const file2 = 'two.csv';
const stream = fs.createReadStream(file1);
const stream2 = fs.createReadStream(file2);
const fileData1 = [];
const fileData2 = [];

const file1Promise = new Promise((resolve) => {
  csv
      .parseFile(file1, {headers: true})
      .on('data', function(data) {
        fileData1.push(data);
      })
      .on('end', function() {
        console.log('done');
        resolve();
      });
});

const file2Promise = new Promise((resolve) => {
  csv
      .parseFile(file2, {headers: true})
      .on('data', function(data) {
        fileData2.push(data);
      })
      .on('end', function() {
        console.log('done');
        resolve();
      });
});

Promise.all([
  file1Promise,
  file2Promise,
])
    .then(() => {
      const fileData3 = fileData1.concat(fileData2);
      console.log(fileData3);

      const csvStream = csv.format({headers: true});
      const writableStream = fs.createWriteStream('outputfile.csv');

      writableStream.on('finish', function() {
        console.log('DONE!');
      });

      csvStream.pipe(writableStream);
      fileData3.forEach((data) => {
        csvStream.write(data);
      });
      csvStream.end();
    });