Dependency Injection

Dependency Injection Jgql1

👨‍💼 Now let's see the power of composition! We can swap different logger implementations without changing EmailService. This is dependency injection—a key benefit of composition.

Logger and EmailService from the previous step are already provided. EmailService.sendEmail(to, subject) produces Sending email to ${to}: ${subject} through whatever logger you inject (and also on the console).

🐨 Open

index.ts

and:

Create and export a MockLogger that extends Logger:
- Override log(message) to store each message in order (do not print)
- Expose getLogs(): Array<string> that returns the stored messages
Create and export a SilentLogger that extends Logger:
- Override log(message) to do nothing (no print, no storage, no throw)
Demonstrate that the existing EmailService works with both loggers
Export: export { Logger, EmailService, MockLogger, SilentLogger }

Fixtures and success criteria

const mockLogger = new MockLogger()
mockLogger.log('Test message 1')
mockLogger.log('Test message 2')

const emailService = new EmailService(mockLogger)
emailService.sendEmail('test@example.com', 'Test Subject')

const silentLogger = new SilentLogger()
const silentEmail = new EmailService(silentLogger)
silentEmail.sendEmail('user@example.com', 'Welcome')

mockLogger.getLogs() starts as ['Test message 1', 'Test message 2'] before sendEmail
After sendEmail('test@example.com', 'Test Subject'), getLogs() includes 'Sending email to test@example.com: Test Subject'
silentLogger instanceof Logger is true
Using SilentLogger with EmailService does not throw
The same EmailService API works unchanged with either logger

💰 This is the power of dependency injection: EmailService doesn't care which logger it gets—it just uses whatever logger is provided. This makes testing easy and allows you to swap implementations at runtime.

📜 Dependency Injection

04. Composition vs Inheritance/02. Dependency Injection (💪 problem)

Dependency Injection

Fixtures and success criteria