Streaming from ps-promise to json2csv to express response

downloadroute.js

// This code queries postgresql and streams the results into a csv
// Works like a charm for a download button, no hangup for large files!

const Json2csvTransform = require('json2csv').Transform;
const QueryStream = require('pg-query-stream');
const JSONStream = require('JSONStream');

router.get('/download.csv', async (req, res) => {
  const { query } = req;

  const SQL = 'SELECT * FROM table';

  const qs = new QueryStream(SQL);

  const transformOpts = { highWaterMark: 16384, encoding: 'utf-8' };
  const json2csv = new Json2csvTransform({}, transformOpts);

  // Set approrpiate download headers
  res.setHeader('Content-disposition', 'attachment; filename=projects.csv');
  res.writeHead(200, { 'Content-Type': 'text/csv' });

  // Flush the headers before we start pushing the CSV content
  res.flushHeaders();

  db.stream(qs, (s) => {
    // initiate streaming into the console:
    s.pipe(JSONStream.stringify()).pipe(json2csv).pipe(res);
  })
    .then((data) => {
      console.log(
        'Total rows processed:', data.processed,
        'Duration in milliseconds:', data.duration,
      );
    })
    .catch((error) => {
      console.log('ERROR:', error);
    });
});