It appears to me that the Node.js driver waits for connectTimeoutMS milliseconds until it releases all resources after MongoClient.close() is called, which resolves the Promise immediately. I would expect that when all connections are closed, also all other resources (socket connections, timeouts...?) are released.

This is a problem when testing with Jest, especially in a CI environment such as Bitbucket Pipelines.

To be clear, I can confirm that the actual MongoDB connection(s) are indeed closed as soon as I call the close method and the Promise returned by close resolves pretty much immediately. It's Jest that keeps waiting, because it notices that not all resources are freed.

I figured out that Jest not exiting after a test involving MongoDB is related to the time specified in connection option connectTimeoutMS, which is by default 6 minutes. If I set it to a lower value such as 5000ms or 500ms, that will be the time the exit is delayed for. Although I don't know what is going on under the hood, I get the feeling that something is not quite right there and relying on a connection timeout points to a deeper problem.

Example test file:

const { MongoClient } = require('mongodb');
let client;
let db;
beforeAll(async () => {
    client = await MongoClient.connect('mongodb://localhost:27017', {
        useNewUrlParser: true,
        socketTimeoutMS: 5000, // <-- this is how long the exit is delayed
    });
    db = await client.db('testdb');
});
afterAll(async () => {
    await client.close();
});
it('should insert and find document', async () => {
    const collection = db.collection('files');
    await collection.insert({name: 'Jest'});
    const doc = await collection.findOne({name: 'Jest'});
    expect(doc._id).toBeDefined();
    expect(doc.name).toBe('Jest');
});

I am not sure if I'm doing something wrong or if this is a bug. I have also posted a question to StackOverflow. Thanks!