async function main(db) {
  const cursor = await this.db.collection('testCollection').aggregate([{"$count": "documentCount"}]);

  cursor.forEach(
    (doc) => {
      console.log('doc:', doc); // document is logged twice
    },
    (err) => { // handle error }
  );  // OR cursor.each((err, doc) => {    if (err) { // handle error }       if (doc !== null) {      console.log('doc:', doc); // document is logged twice     }  }); 

}