Guide on how to utilize jest for mocking MongoDB manager in typescript

Question

Guide on how to utilize jest for mocking MongoDB manager in typescript

Is there a way to return a mongodb.connection.db() type value and mock its collection for testing purposes? I have implemented a mongoClient connection and use its db() function. If everything is set up correctly, I should be able to access the collections but I am finding it difficult to write tests due to the inability to mock it.

dbManager.ts

import { MongoClient } from 'mongodb';

class DBManager {
  private connection?: MongoClient;
  private mongoUrl = null;

  constructor(url: string) {
    this.mongoUrl = url;
  }

  get db() {
    return this.connection!.db();
  }

  async start() {
    if (!this.connection) {
      this.connection = await MongoClient.connect(this.mongoUrl);
    }
  }
}

export default DBManager;

index.ts

 const dbManager = new DBManager(url);
  await dbManager.start();
  const db = dbManager.db;

  if (db) {
    const collection = db.collection(collectionName);
  }

index.spec.ts

  const dbManager = new DBManager( 'mongoUrl');
      jest.spyOn(dbManager, 'start').mockResolvedValue();
      jest.spyOn(dbManager, 'db', 'get').mockImplementation();

typescript unit-testing jestjs mocking ts-jest

Answer 1

Answer №1

The reason behind the failure of the mock is due to creating a new instance of DBManager in the test case, and jest.spyOn() only spies on the methods of this specific instance. The tested code contains another instance of DBManager, which still calls its original unspied method.

To resolve this issue, you should spy on the DBManager.prototype.start() and DBManager.prototype.get() methods.

Additionally, remember to restore mocks back to their original values in the afterEach hook to ensure that the mocked object from the test case does not impact other test cases.

For example:

DBManager.ts:

import { MongoClient } from 'mongodb';

class DBManager {
  private connection?: MongoClient;
  private mongoUrl: string = '';

  constructor(url: string) {
    this.mongoUrl = url;
  }

  get db() {
    return this.connection!.db();
  }

  async start() {
    if (!this.connection) {
      this.connection = await MongoClient.connect(this.mongoUrl);
    }
  }
}

export default DBManager;

index.ts:

import DBManager from './dbManager';

export async function main() {
  const url = 'mongodb://localhost:27017';
  const collectionName = 'user';
  const dbManager = new DBManager(url);
  await dbManager.start();
  const db = dbManager.db;

  if (db) {
    const collection = db.collection(collectionName);
  }
}

index.test.ts:

import { main } from './';
import DBManager from './dbManager';
import { Db } from 'mongodb';

describe('69011729', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  test('should pass', async () => {
    const mDB = ({
      collection: jest.fn(),
    } as unknown) as Db;
    jest.spyOn(DBManager.prototype, 'start').mockResolvedValue();
    jest.spyOn(DBManager.prototype, 'db', 'get').mockReturnValueOnce(mDB);
    await main();
    expect(DBManager.prototype.start).toBeCalledTimes(1);
    expect(mDB.collection).toBeCalledWith('user');
  });

  test('should restore original methods', () => {
    expect(jest.isMockFunction(DBManager.prototype.start)).toBeFalsy();
  });
});

Test result:

 PASS  examples/69011729/index.test.ts (10.734 s)
  69011729
    ✓ should pass (4 ms)
    ✓ should restore original methods

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        11.516 s
Ran all test suites related to changed files.

Answer 2

The reason behind the failure of the mock is due to creating a new instance of DBManager in the test case, and jest.spyOn() only spies on the methods of this specific instance. The tested code contains another instance of DBManager, which still calls its original unspied method.

To resolve this issue, you should spy on the DBManager.prototype.start() and DBManager.prototype.get() methods.

Additionally, remember to restore mocks back to their original values in the afterEach hook to ensure that the mocked object from the test case does not impact other test cases.

For example:

DBManager.ts:

import { MongoClient } from 'mongodb';

class DBManager {
  private connection?: MongoClient;
  private mongoUrl: string = '';

  constructor(url: string) {
    this.mongoUrl = url;
  }

  get db() {
    return this.connection!.db();
  }

  async start() {
    if (!this.connection) {
      this.connection = await MongoClient.connect(this.mongoUrl);
    }
  }
}

export default DBManager;

index.ts:

import DBManager from './dbManager';

export async function main() {
  const url = 'mongodb://localhost:27017';
  const collectionName = 'user';
  const dbManager = new DBManager(url);
  await dbManager.start();
  const db = dbManager.db;

  if (db) {
    const collection = db.collection(collectionName);
  }
}

index.test.ts:

import { main } from './';
import DBManager from './dbManager';
import { Db } from 'mongodb';

describe('69011729', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  test('should pass', async () => {
    const mDB = ({
      collection: jest.fn(),
    } as unknown) as Db;
    jest.spyOn(DBManager.prototype, 'start').mockResolvedValue();
    jest.spyOn(DBManager.prototype, 'db', 'get').mockReturnValueOnce(mDB);
    await main();
    expect(DBManager.prototype.start).toBeCalledTimes(1);
    expect(mDB.collection).toBeCalledWith('user');
  });

  test('should restore original methods', () => {
    expect(jest.isMockFunction(DBManager.prototype.start)).toBeFalsy();
  });
});

Test result:

 PASS  examples/69011729/index.test.ts (10.734 s)
  69011729
    ✓ should pass (4 ms)
    ✓ should restore original methods

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        11.516 s
Ran all test suites related to changed files.

Guide on how to utilize jest for mocking MongoDB manager in typescript

Answer №1

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