Unit Testing With Jest

Nest's Test.createTestingModule builds a TS test container where you can override any provider with a mock.

4 min read Level 2/5 #nestjs#testing#jest

What you'll learn

Use Test.createTestingModule and compile()
Override providers with useValue or useFactory mocks
Assert on the unit under test with Jest

Nest ships a tiny testing helper that builds a real DI container with the providers you list — no Nest application, no HTTP server. That makes unit tests fast and isolated.

The Pattern

import { Test } from '@nestjs/testing';
import { UsersService } from './users.service';
import { UsersRepository } from './users.repository';

describe('UsersService', () => {
  let service: UsersService;
  let repo: jest.Mocked<UsersRepository>;

  beforeEach(async () => {
    repo = { findAll: jest.fn(), findOne: jest.fn() } as any;

    const module = await Test.createTestingModule({
      providers: [
        UsersService,
        { provide: UsersRepository, useValue: repo },
      ],
    }).compile();

    service = module.get(UsersService);
  });

  it('returns all users', async () => {
    repo.findAll.mockResolvedValue([{ id: 1, name: 'Ada' }]);
    await expect(service.findAll()).resolves.toHaveLength(1);
    expect(repo.findAll).toHaveBeenCalledTimes(1);
  });
});

useValue is the simplest mock — a plain object. Use useFactory if the mock needs to depend on other test fixtures.

Why Not New The Class Directly?

You could write new UsersService(repo) and skip Nest entirely — and for truly trivial classes that is fine. The moment a class injects more than one dep, the testing module saves you from re-typing constructor signatures on every test.

Auto-Mocking Big Dependencies

For services with many collaborators, define a factory:

const mockRepo = () => ({
  findAll: jest.fn(),
  findOne: jest.fn(),
  save: jest.fn(),
});

const module = await Test.createTestingModule({
  providers: [
    UsersService,
    { provide: UsersRepository, useFactory: mockRepo },
  ],
}).compile();

What To Cover

Branching logic inside services.
Edge cases in pipes and guards.
Custom decorators and reflection helpers.

Skip unit tests for thin controllers — e2e tests cover them better.

End-to-End Testing →