db.js

const mongoose = require('mongoose');
const { MongoMemoryServer } = require('mongodb-memory-server');

let mongo = undefined;

module.exports.setUp = async () => {
    mongo = await MongoMemoryServer.create();
    const url = mongo.getUri();

    await mongoose.connect(url, {
      useNewUrlParser: true,
  });
};

module.exports.dropDatabase = async () => {
    if (mongo) {
        await mongoose.connection.dropDatabase();
        await mongoose.connection.close();
        await mongo.stop();
    }
};

module.exports.dropCollections = async () => {
    if (mongo) {
        const collections = mongoose.connection.collections;

        for (const key in collections) {
            const collection = collections[key];
            await collection.deleteMany();
        }
    }
};