Design Patterns - lesson 6 | HackIndia Skills

Design Patterns - lesson 6 | HackIndia Skills | HackIndia

Example

Small TypeScript example

Here the app wants a simple PaymentGateway. Razorpay or any other provider can have its own response shape behind an adapter.

The app service does not know provider fields. It only knows pay().

type PaymentResult = {
  ok: boolean;
  referenceId: string;
};

interface PaymentGateway {
  pay(amountRupees: number): Promise<PaymentResult>;
}

class RazorpayAdapter implements PaymentGateway {
  constructor(private razorpayClient: any) {}

  async pay(amountRupees: number): Promise<PaymentResult> {
    const response = await this.razorpayClient.createPayment({
      amount: amountRupees * 100
    });

    return {
      ok: response.status === "captured",
      referenceId: response.id
    };
  }
}

class CheckoutService {
  constructor(private payments: PaymentGateway) {}

  async checkout(totalRupees: number) {
    const result = await this.payments.pay(totalRupees);
    if (!result.ok) throw new Error("Payment failed");
    return result.referenceId;
  }
}

Example

Repository shape

A repository should sound like your product, not your database tables. Keep query details out of routes and UI components.

Start with the methods your feature needs today. Do not build a full data layer for imaginary screens.

type Project = {
  id: string;
  title: string;
  ownerId: string;
};

interface ProjectRepository {
  findByOwner(ownerId: string): Promise<Project[]>;
  save(project: Project): Promise<void>;
}

class ApiProjectRepository implements ProjectRepository {
  constructor(private baseUrl: string) {}

  async findByOwner(ownerId: string): Promise<Project[]> {
    const res = await fetch(`${this.baseUrl}/projects?ownerId=${ownerId}`);
    return res.json();
  }

  async save(project: Project): Promise<void> {
    await fetch(`${this.baseUrl}/projects`, {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify(project)
    });
  }
}

Bridge boundaries with adapter and repository

Boundary rule

Why this matters

Adapter versus repository

Small TypeScript example

Repository shape

Common mistake

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