interface IMessageService {
    sendMessage(): void
}

// a service that 
// implements IMessageService
class EmailService implements IMessageService {
    sendMessage(): void {
        console.log("Email sent!")
    }
}

// another service that
// implements IMessageService
class SMSService implements IMessageService {
    sendMessage(): void {
        console.log("SMS sent!")
    }
}

// client service that depends on
// an instance of IMessageService
// through constructor injection 
class NotifierService {
    constructor(private message: IMessageService) {}

    sendMessage(): void {
        this.message.sendMessage()
    }
}

// assembler/injection code
// manual wiring

const email = new EmailService()
const emailNotifier = new NotifierService(email)

const sms = new SMSService()
const smsNotifier = new NotifierService(sms)

for (const notifier of [emailNotifier, smsNotifier]) {
    notifier.sendMessage()
}