typescript 53 lines · 7 steps

Unit testing a NestJS service with mocked providers

Wiring fake repositories and gateways into a NestJS test module to verify a service in isolation.

Explained by highlit

NestJS Intermediate dependency injection mocking unit testing test doubles provider overrides

1import { Test, TestingModule } from '@nestjs/testing';
2import { getRepositoryToken } from '@nestjs/typeorm';
3import { Repository } from 'typeorm';
4 
5import { UsersService } from './users.service';
6import { PaymentGateway } from '../payments/payment.gateway';
7import { User } from './entities/user.entity';
8 
9describe('UsersService', () => {
10  let service: UsersService;
11  let paymentGateway: jest.Mocked<PaymentGateway>;
12 
13  const userRepository = {
14    findOne: jest.fn(),
15    save: jest.fn(),
16    create: jest.fn((dto) => dto),
17  };
18 
19  beforeEach(async () => {
20    const module: TestingModule = await Test.createTestingModule({
21      providers: [
22        UsersService,
23        {
24          provide: getRepositoryToken(User),
25          useValue: userRepository,
26        },
27        {
28          provide: PaymentGateway,
29          useValue: {
30            createCustomer: jest.fn().mockResolvedValue({ id: 'cus_123' }),
31            charge: jest.fn(),
32          },
33        },
34      ],
35    }).compile();
36 
37    service = module.get(UsersService);
38    paymentGateway = module.get(PaymentGateway);
39  });
40 
41  it('provisions a payment customer when registering', async () => {
42    userRepository.findOne.mockResolvedValue(null);
43    userRepository.save.mockImplementation(async (u: User) => ({ id: 1, ...u }));
44 
45    const user = await service.register({
46      email: 'ada@example.com',
47      password: 'hunter2',
48    });
49 
50    expect(paymentGateway.createCustomer).toHaveBeenCalledWith('ada@example.com');
51    expect(user.stripeCustomerId).toBe('cus_123');
52  });
53});

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Overriding providers with useValue lets you swap real dependencies for controllable test doubles.
2getRepositoryToken resolves the injection token TypeORM uses so you can mock a repository cleanly.
3Asserting on mocked collaborators verifies a service's side effects without hitting a real database or API.

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