Can someone explain how this MongoClient.connect code work?

Question

Can someone please explain the concept here. I've put three console.log in the code in order to find out how this code is running. However, I don't understand why contestCount++ is being called four times before it moves on to client 2. (By the way, there are four documents in the contests collection) I expect the code goes all the way from top to bottom then loops back to the top again. Thank you

MongoClient.connect(
  config.mongodbUri,
  { useUnifiedTopology: true },
  (err, client) => {
    assert.equal(null, err);

    let contestCount = 0;
    /* client 1 */
    client  
      .db('test')
      .collection('contests')
      .find({})
      .each((err, contest) => {
        assert.equal(null, err);
        if (!contest) {
          return;
        }

        contestCount++;
        /* console.log */
        console.log(`contestCountPlus: ${contestCount}`);

        /* client 2 */
        client
          .db('test')
          .collection('names')
          .find({ id: { $in: contest.nameIds } })
          .project({ _id: 1 })
          .toArray()
          .then((_ids) => {
            const newIds = _ids.map((o) => o._id);

            /* client 3 */
            client
              .db('test')
              .collection('contests')
              .updateOne({ id: contest.id }, { $set: { nameIds: newIds } })
              .then(() => {
                contestCount--;
                /* console.log */
                console.log(`contestCountMinus: ${contestCount}`);
                if (contestCount === 0) {
                  /* console.log */
                  console.log(`contestCountFinal: ${contestCount}`);
                  client.close();
                }
              });
          })
          .catch(console.error);
      });
  }
);

Here's what I get from the terminal

Stevens-MBP:learn-fullstack-javascript stevenlai$ babel-node updateTestData.js
contestCountPlus: 1
contestCountPlus: 2
contestCountPlus: 3
contestCountPlus: 4
contestCountMinus: 3
contestCountMinus: 2
contestCountMinus: 1
contestCountMinus: 0
contestCountFinal: 0

Answer 1

All of the client operations are asynchronous. In your code you are initiating the decrements when you are processing each of the documents in the outer loop, but this isn't the order in which decrements are actually executed. In your case first all documents in the outer loop are processed and the decrements are "queued", then each of the decrements gets processed.

This is in general how node works, nothing here is mongodb-specific.