How do you implement dependency injection in Node.js?

Question

Accepted Answer

**Dependency injection** — pass dependencies instead of creating them inside.

**Constructor injection:**
```javascript
class UserService {
    constructor(userRepo, emailService) {
        this.userRepo = userRepo;
        this.emailService = emailService;
    }

async createUser(data) {
        const user = await this.userRepo.create(data);
        await this.emailService.send(user.email, "Welcome!");
        return user;
    }
}

// Composition root
const userRepo = new UserRepository(db);
const emailService = new EmailService(smtp);
const userService = new UserService(userRepo, emailService);
```

**Using InversifyJS (IoC container):**
```javascript
const { Container, injectable, inject } = require("inversify");

@injectable()
class UserService {
    @inject("UserRepository") userRepo;
    @inject("EmailService") emailService;
}

const container = new Container();
container.bind("UserService").to(UserService);
container.bind("UserRepository").to(UserRepository);

const service = container.get("UserService");
```

**Benefits:**
- Testable (easy to mock).
- Loose coupling.
- Swappable implementations.

Node.js: How do you implement dependency injection in Node.js?