Single Responsibility Principle

// ❌ Three reasons to change: order logic, email, and invoicing
class OrderService {
  placeOrder(order: Order): void {
    // ... validate and save order
    this.db.save(order);

    // Sending email has nothing to do with placing an order
    const html = `<h1>Thanks for your order #${order.id}</h1>`;
    this.mailer.send({ to: order.email, subject: "Order confirmed", html });

    // PDF generation is a third concern
    const pdf = this.pdfGen.render(order);
    this.storage.upload(`invoices/${order.id}.pdf`, pdf);
  }
}

// ✓ One reason to change per class
class OrderService {
  constructor(
    private repo: OrderRepository,
    private emailer: OrderEmailer,
    private invoicing: InvoiceService,
  ) {}

  async placeOrder(order: Order): Promise<void> {
    await this.repo.save(order);          // one concern
    await this.emailer.confirm(order);    // delegates, doesn't own
    await this.invoicing.generate(order); // delegates, doesn't own
  }
}

class OrderEmailer {
  async confirm(order: Order): Promise<void> { /* only email logic */ }
}

class InvoiceService {
  async generate(order: Order): Promise<void> { /* only PDF logic */ }
}