CODESAMPLE

Dependency Injection - JavaScript

Dependency Injection JavaScript

Dependency Injection (DI) is a design pattern where a class receives its dependencies from external sources rather than creating them itself. This promotes loose coupling, making code more testable, reusable, and maintainable. Our JavaScript example uses constructor injection – dependencies are provided through the class constructor. A UserService class depends on a UserRepository to fetch user data. Instead of UserService instantiating UserRepository, we pass an instance of it in when creating UserService. This is idiomatic JavaScript as it leverages the flexibility of passing functions as arguments and doesn’t enforce strict class hierarchies, favoring composition. It is well-suited for testing, allowing us to mock the UserRepository easily.

// UserRepository interface/abstract class (defines what a repository should do)
class UserRepository {
  getUserById(id) {
    throw new Error("Method 'getUserById' must be implemented.");
  }
}

// Concrete implementation of UserRepository (e.g., using a database)
class DatabaseUserRepository extends UserRepository {
  constructor() {
    super();
    // Simulate database connection
    this.users = {
      1: { id: 1, name: "Alice" },
      2: { id: 2, name: "Bob" },
    };
  }

  getUserById(id) {
    return this.users[id];
  }
}

// UserService class - depends on UserRepository
class UserService {
  constructor(userRepository) {
    this.userRepository = userRepository;
  }

  getUserName(id) {
    const user = this.userRepository.getUserById(id);
    if (user) {
      return user.name;
    } else {
      return "User not found";
    }
  }
}

// Usage: Inject the dependency
const userRepository = new DatabaseUserRepository();
const userService = new UserService(userRepository);

console.log(userService.getUserName(1)); // Output: Alice
console.log(userService.getUserName(3)); // Output: User not found

// For testing, you can easily inject a mock UserRepository:
class MockUserRepository extends UserRepository {
  getUserById(id) {
    return { id: id, name: "Mock User" };
  }
}

const mockRepository = new MockUserRepository();
const userServiceWithMock = new UserService(mockRepository);
console.log(userServiceWithMock.getUserName(5)); // Output: Mock User