When to Use Composition vs Inheritance

Inheritance (is-a)

1. Create a Logger base class with log(message: string): void that prints exactly Log: {message}
2. Create a FileLogger that extends Logger and overrides log to print exactly File Log: {message}
3. Create a ConsoleLogger that extends Logger and overrides log to print exactly Console Log: {message}

Composition (has-a)

4. Create an EmailService that has a logger (composition), not is a logger:
   • Accept a Logger in the constructor
   • Keep that logger as private class state (use #)
   • sendEmail(to: string, subject: string): void must produce the send-action message Sending email to ${to}: ${subject} through the injected logger and on the console (both)
5. Export all four: export { Logger, FileLogger, ConsoleLogger, EmailService }

Fixtures and success criteria

const fileLogger = new FileLogger()
const consoleLogger = new ConsoleLogger()
const emailService = new EmailService(fileLogger)
emailService.sendEmail('user@example.com', 'Welcome')

• fileLogger instanceof Logger and consoleLogger instanceof Logger
• fileLogger.log('hi') prints File Log: hi
• consoleLogger.log('hi') prints Console Log: hi
• new Logger().log('hi') prints Log: hi
• EmailService can be constructed with either logger subclass
• After sendEmail('user@example.com', 'Welcome') with a FileLogger, console output includes both:
  • File Log: Sending email to user@example.com: Welcome
  • Sending email to user@example.com: Welcome