config.js

let mongoServer;
  beforeAll(async () => {
    mongoServer = new MongoMemoryServer();
    const URI = await mongoServer.getUri();

    mongoose.connect(URI, {
      useNewUrlParser: true,
      useCreateIndex: true,
      useUnifiedTopology: true,
    });
  });

  afterAll(async done => {
    mongoose.disconnect(done);
    await mongoServer.stop();
  });

  afterEach(async () => {
    const collections = await mongoose.connection.db.collections();

    for (let collection of collections) {
      await collection.deleteMany();
    }
  });