Sinon stub out module's function

Imagine I have a file here:

// a.js
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 db-connection.js looks like this:

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)
  }
}

and I have a test to stub out find() method to return at least true (in order to test out if the a.js's return value is true. How can I do that?

The unit test strategy is to stub the DB connection, we don't need to connect the real mongo DB server. So that we can run test cases in an environment isolated from the external environment.

You can use stub.get(getterFn) replaces a new getter for dbConnection.db getter.

Since the MongoDB client initialization happens when you require the db-connection module. You should provide a correct URL for MongoClient from the environment variable before requiring a and db-connection modules. Otherwise, the MongoDB nodejs driver will throw an invalid url error.

E.g.

a.js:

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

module.exports = function () {
  const dbClient = dbConnection.db;
  const docs = dbClient.collection('test').find();

  if (!docs) {
    return true;
  }
};

db-connection.js:

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

const client = new MongoClient(url, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

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

module.exports = {
  init,
  client,
  get db() {
    return client.db(dbName);
  },
};

a.test.js:

const sinon = require('sinon');

describe('a', () => {
  afterEach(() => {
    sinon.restore();
  });
  it('should find some docs', () => {
    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 actual = a();
    sinon.assert.match(actual, true);
    sinon.assert.calledWithExactly(dbStub.collection, 'test');
    sinon.assert.calledOnce(dbStub.find);
  });
});

Test result:

  a
    ✓ should find some docs (776ms)


  1 passing (779ms)

------------------|---------|----------|---------|---------|-------------------
File              | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
------------------|---------|----------|---------|---------|-------------------
All files         |      75 |       50 |      25 |      75 |                   
 a.js             |     100 |       50 |     100 |     100 | 7                 
 db-connection.js |      60 |      100 |       0 |      60 | 11-13,21          
------------------|---------|----------|---------|---------|-------------------