Sinon stub out module's function from a middleware

Based on this question, I need to also make a test for a middleware which also uses the db-connection.js file. The middleware file will look like this:

const dbConnection = require('./db-connection.js')

module.exports = function (...args) {
   return async function (req, res, next) {
      // somethin' somethin' ...
      const dbClient = dbConnection.db
      const docs = await dbClient.collection('test').find()
 
      if (!docs) {
         return next(Boom.forbidden())
      }
   }
}

, the database connection file do not change, which is:

const MongoClient = require('mongodb').MongoClient
const dbName = 'test'
const url = process.env.MONGO_URL

const client = new MongoClient(url, { useNewUrlParser: true,
  useUnifiedTopology: true,
  bufferMaxEntries: 0 // dont buffer querys when not connected
})

const init = () => {
  return client.connect().then(() => {
    logger.info(`mongdb db:${dbName} connected`)

    const db = client.db(dbName)
  })
}

/**
 * @type {Connection}
 */
module.exports = {
  init,
  client,
  get db () {
    return client.db(dbName)
  }
}

How the middleware works is by passing list of strings (that strings is roles), I have to query to the database and check whether there is a record of each roles. If the record exists, I will return next(), while if the record does not exist, I will return next(Boom.forbidden()) (next function with a 403 status code from Boom module).

Given the details above, how does one make a test to test out the return value of the middleware if the record exists or not? This means I have to assert the next() and next(Boom.forbidden) to be exact.

Based on the answer. You can create stubs for the req, res objects, and next function.

E.g.(Doesn't run, but it should work.)

const sinon = require('sinon');

describe('a', () => {
  afterEach(() => {
    sinon.restore();
  });
  it('should find some docs', async () => {
    process.env.MONGO_URL = 'mongodb://localhost:27017';
    const a = require('./a');
    const dbConnection = require('./db-connection.js');

    const dbStub = {
      collection: sinon.stub().returnsThis(),
      find: sinon.stub(),
    };
    sinon.stub(dbConnection, 'db').get(() => dbStub);
    const req = {};
    const res = {};
    const next = sinon.stub();
    const actual = await a()(req, res, next);
    sinon.assert.match(actual, true);
    sinon.assert.calledWithExactly(dbStub.collection, 'test');
    sinon.assert.calledOnce(dbStub.find);
    sinon.assert.calledOnce(next);
  });
});