How To Mock MongoDB ( mongoClient ) With Jest - node.js

Question

I have a DBManager class to connect to mongoClient
import { MongoClient } from 'mongodb';
class DBManager {
private url = process.env.MONGODB_URL;
private _connection: MongoClient;
constructor() {
this._connection = null;
}
get connection() {
return this._connection;
}
async start() {
if (!this._connection) {
this._connection = await MongoClient.connect(this.url);
}
}
}
export default new DBManager();
and I call this class like this
await DBManager.start();
const db = DBManager.connection.db();
I get this error when I try to mock:
Received: [TypeError: db_manager_1.default.connection.db is not a function]
this is how to mock method i use:
DBManager.start = jest.fn().mockResolvedValue(() => ({
connection: jest.fn().mockReturnThis(),
db: jest.fn().mockResolvedValue({success: true})
}));
thanks..

Answer 1

You can use a real MongoDB server to use in tests with the package mongodb-memory-server.
So in your case, you just need to do:
import { MongoMemoryServer } from '../index';
describe('Single MongoMemoryServer', () => {
let con;
let mongoServer;
beforeAll(async () => {
mongoServer = await MongoMemoryServer.create();
process.env.MONGODB_URL = mongoServer.getUri();
});
afterAll(async () => {
if (con) {
await con.close();
}
if (mongoServer) {
await mongoServer.stop();
}
});
it('DBManager connection', async () => {
await DBManager.start();
const db = DBManager.connection.db();
// ...
});

Answer 2

You can use jest.spyOn(object, methodName, accessType?) to mock DBManager.start() method and DBMananger.connection getter.
E.g.
dbManager.ts:
import { MongoClient } from 'mongodb';
class DBManager {
private url = process.env.MONGODB_URL || '';
private _connection: MongoClient | null;
constructor() {
this._connection = null;
}
get connection() {
return this._connection;
}
async start() {
if (!this._connection) {
this._connection = await MongoClient.connect(this.url);
}
}
}
export default new DBManager();
main.ts:
import DBManager from './dbManager';
export async function main() {
await DBManager.start();
const db = DBManager.connection!.db();
db.collection('users');
}
main.test.ts:
import { main } from './main';
import DBManager from './dbManager';
import { Db, MongoClient } from 'mongodb';
describe('68888424', () => {
afterEach(() => {
jest.restoreAllMocks();
});
test('should pass', async () => {
const mockDbInstance = ({
collection: jest.fn(),
} as unknown) as Db;
const mockDb = jest.fn(() => mockDbInstance);
jest.spyOn(DBManager, 'start').mockResolvedValueOnce();
jest.spyOn(DBManager, 'connection', 'get').mockReturnValue(({ db: mockDb } as unknown) as MongoClient);
await main();
expect(DBManager.start).toBeCalledTimes(1);
expect(DBManager.connection!.db).toBeCalledTimes(1);
expect(mockDbInstance.collection).toBeCalledWith('users');
});
});
test result:
PASS examples/68888424/main.test.ts (8.621 s)
68888424
✓ should pass (5 ms)
--------------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
--------------|---------|----------|---------|---------|-------------------
All files | 75 | 50 | 50 | 75 |
dbManager.ts | 57.14 | 50 | 33.33 | 57.14 | 12-17
main.ts | 100 | 100 | 100 | 100 |
--------------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 9.532 s