Why should you care?

Unit tests miss wiring bugs — wrong middleware order, missing auth on route.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

import request from 'supertest';
import { app } from '../src/app';
import { MongoMemoryServer } from 'mongodb-memory-server';

let mongod: MongoMemoryServer;

beforeAll(async () => {
  mongod = await MongoMemoryServer.create();
  process.env.MONGO_URI = mongod.getUri();
  await connectDb();
});

it('POST /api/transfers returns 201 with valid JWT', async () => {
  const token = signTestToken({ id: userId, roles: ['teller'] });
  const res = await request(app)
    .post('/api/transfers')
    .set('Authorization', `Bearer ${token}`)
    .send({ fromAccountId, toAccountId, amountCents: 5000 });
  expect(res.status).toBe(201);
  expect(res.body.status).toBe('posted');
});

What happened?

• MongoMemoryServer spins ephemeral DB.
• supertest calls Express without network port.
• Full stack from HTTP through Mongoose validates integration.

Practice next

1. Setup test DB before suite; teardown after.
2. Factory helpers seed users and accounts.
3. Test auth, validation, and happy path CRUD.
4. Add test for 403 when role missing on transfer.
5. Run same tests against Docker compose in CI.

Remember

Integration = HTTP + DB + middleware together. Memory Mongo keeps CI portable. Catch wiring bugs unit tests miss.

Release gate

Refactor auth middleware accidentally skips /transfers.

Outcome: Integration test 401/403 expectations fail; deploy blocked.