saveLocal.js

const mysql = require('mysql2/promise');
const csvstringify = require('csv-stringify');

...
const outputStream = fs.createWriteStream('output.csv', { encoding: 'utf8' }); // writable stream to save query results locally.
const s1 = connection.connection.query(sql); // establish a connection
s1.stream({ highWaterMark: BATCH_SIZE }) // readable stream from MySQL db. stream() is just to wrap into pipeable object, and not to enable 'result' events, they are emitted anyway.
    .pipe(csvstringify({ header: true }))
    .pipe(outputStream)
    .on('finish', () => { resolve('saved data locally'); });