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.

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

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