1---
2title: Prefer Constructor Injection
3impact: CRITICAL
4impactDescription: Required for proper DI and testing
5tags: dependency-injection, constructor, testing
6---
7 
8## Prefer Constructor Injection
9 
10Always use constructor injection over property injection. Constructor injection makes dependencies explicit, enables TypeScript type checking, ensures dependencies are available when the class is instantiated, and improves testability. This is required for proper DI, testing, and TypeScript support.
11 
12**Incorrect (property injection with hidden dependencies):**
13 
14```typescript
15// Property injection - avoid unless necessary
16@Injectable()
17export class UsersService {
18  @Inject()
19  private userRepo: UserRepository; // Hidden dependency
20 
21  @Inject('CONFIG')
22  private config: ConfigType; // Also hidden
23 
24  async findAll() {
25    return this.userRepo.find();
26  }
27}
28 
29// Problems:
30// 1. Dependencies not visible in constructor
31// 2. Service can be instantiated without dependencies in tests
32// 3. TypeScript can't enforce dependency types at instantiation
33```
34 
35**Correct (constructor injection with explicit dependencies):**
36 
37```typescript
38// Constructor injection - explicit and testable
39@Injectable()
40export class UsersService {
41  constructor(
42    private readonly userRepo: UserRepository,
43    @Inject('CONFIG') private readonly config: ConfigType,
44  ) {}
45 
46  async findAll(): Promise<User[]> {
47    return this.userRepo.find();
48  }
49}
50 
51// Testing is straightforward
52describe('UsersService', () => {
53  let service: UsersService;
54  let mockRepo: jest.Mocked<UserRepository>;
55 
56  beforeEach(() => {
57    mockRepo = {
58      find: jest.fn(),
59      save: jest.fn(),
60    } as any;
61 
62    service = new UsersService(mockRepo, { dbUrl: 'test' });
63  });
64 
65  it('should find all users', async () => {
66    mockRepo.find.mockResolvedValue([{ id: '1', name: 'Test' }]);
67    const result = await service.findAll();
68    expect(result).toHaveLength(1);
69  });
70});
71 
72// Only use property injection for optional dependencies
73@Injectable()
74export class LoggingService {
75  @Optional()
76  @Inject('ANALYTICS')
77  private analytics?: AnalyticsService;
78 
79  log(message: string) {
80    console.log(message);
81    this.analytics?.track('log', message); // Optional enhancement
82  }
83}
84```
85 
86Reference: [NestJS Providers](https://docs.nestjs.com/providers)
87