Dependency Injection

Types

Constructor Injection

Most common way of doing Dependency Injection.

class UserService {
  constructor(private userRepository: UserRepository) {}
 
  getUser(id: string) {
    return this.userRepository.getUserById(id);
  }
}
 
class UserRepository {
  getUserById(id: string) {
    // Retrieve user from the database
  }
}
 
const userService = new UserService(new UserRepository());

Property Injection

Dependencies are injected through public properties of a class.

class AuthService {
  private _userRepository!: UserRepository;
 
  set userRepository(userRepository: UserRepository) {
    this._userRepository = userRepository;
  }
 
  login(username: string, password: string) {
    // Perform authentication using the injected UserRepository
  }
}
 
const authService = new AuthService();
authService.userRepository = new UserRepository();

While possible, this does make things harder to track. I also dislike objects that are unusable after they have been constructed.