Why are my tests occasionally passing and occasionally failing? (Jest, Mongoose, MongoDB) - mongodb

Question

I have a setup file for my tests that looks like this:
const mongoose = require('mongoose');
mongoose.set('useCreateIndex', true);
mongoose.promise = global.Promise;
async function removeAllCollections() {
const collections = Object.keys(mongoose.connection.collections);
for (const collectionName of collections) {
const collection = mongoose.connection.collections[collectionName];
await collection.deleteMany();
}
}
async function dropAllCollections() {
const collections = Object.keys(mongoose.connection.collections);
for (const collectionName of collections) {
const collection = mongoose.connection.collections[collectionName];
try {
await collection.drop();
} catch (error) {
// Sometimes this error happens, but you can safely ignore it
if (error.message === 'ns not found') return;
// This error occurs when you use it.todo. You can
// safely ignore this error too
if (error.message.includes('a background operation is currently running'))
return;
console.log(error.message);
}
}
}
export default function setupDB(databaseName) {
// Connect to Mongoose
beforeAll(async () => {
const url = `mongodb://127.0.0.1/${databaseName}`;
await mongoose.connect(
url,
{
useNewUrlParser: true,
useCreateIndex: true,
useUnifiedTopology: true
},
err => {
if (err) {
console.error(err);
process.exit(1);
}
}
);
});
// Cleans up database between each test
afterEach(async () => {
await removeAllCollections();
});
// Disconnect Mongoose
afterAll(async () => {
await dropAllCollections();
await mongoose.connection.close();
});
}
I am then writing tests like this:
import User from 'db/models/User';
import setupDB from 'utils/setupDbForTesting';
setupDB('mongoose_bcrypt_test');
it('correctly hashes and salts passwords', async done => {
// create a user a new user
const newUser = new User({
username: 'jmar777',
password: 'Password123'
});
await newUser.save(function (err) {
if (err) {
console.log(err);
}
});
const user = await User.findOne({ username: 'jmar777' });
user.comparePassword('Password123', function (err, isMatch) {
if (err) throw err;
expect(isMatch).toBeTruthy();
});
user.comparePassword('123Password', function (err, isMatch) {
if (err) throw err;
expect(isMatch).toBeFalsy();
});
done();
});
However, every other time I run these tests, they pass (or fail) so for every time T that the tests pass, T + 1 they will fail. My question is - why?
The tests fail because user (in the callback for User.findOne) returns null, even though the user has been saved.
I think the issue lies in the tearing down of the database, but I really can't see any problems. Any help would be appreciated, thanks.