streamSave.js

...
else if (output === 's3Stream') { // as one file, node.js streaming, save as new line delimited JSON.

    const uploadStream = ({ Bucket, Key }) => {
        const pass = new stream.PassThrough();
        return {
            writeStream: pass,
            promise: s3.upload({ Bucket, Key, Body: pass }).promise(),
        };
    };

    const { writeStream, promise } = uploadStream({ Bucket: bucket, Key: s3key });
    pr(`saving to aws s3 cp s3://${bucket}/${s3key} ./tmp/${s3key}`);

    s1.stream({ highWaterMark: BATCH_SIZE }) // stream() is just to wrap into pipeable object, and not to enable 'result' events, they are emitted anyway.
        // eslint-disable-next-line func-names
        .pipe(through2.obj(function(row, enc, next) {
            this.push(`${JSON.stringify(row)}\n`);
            next();
        },
        ))
        .pipe(writeStream)
        .on('close', () => {
            console.log('upload finished');
        });

    promise.then(() => {
        console.log('upload completed successfully');
        resolve(`saved to aws s3 cp s3://${bucket}/${s3key} ./tmp/${s3key}`);
    }).catch((err) => {
        console.log('upload failed.', err.message);
    });

}