Design Patterns - lesson 7 | HackIndia Skills

Design Patterns - lesson 7 | HackIndia Skills | HackIndia

Example

Before: hard-coded

This checkout service creates its own gateway. Now every test touches the real gateway shape, and you cannot run a clean offline demo.

class RazorpayGateway {
  async charge(amountRupees, userId) {
    // call payment provider here
    return { ok: true, paymentId: "pay_123" };
  }
}

class CheckoutService {
  constructor() {
    this.gateway = new RazorpayGateway();
  }

  async buyCourse(userId, amountRupees) {
    const result = await this.gateway.charge(amountRupees, userId);
    if (!result.ok) throw new Error("Payment failed");
    return result.paymentId;
  }
}

module.exports = { CheckoutService };

Example

After: injected

Now CheckoutService depends on a role, not a concrete class. The app wires the real gateway. Tests can pass a fake.

class CheckoutService {
  constructor(paymentGateway) {
    this.paymentGateway = paymentGateway;
  }

  async buyCourse(userId, amountRupees) {
    const result = await this.paymentGateway.charge(amountRupees, userId);
    if (!result.ok) throw new Error("Payment failed");
    return result.paymentId;
  }
}

class RazorpayGateway {
  async charge(amountRupees, userId) {
    return { ok: true, paymentId: "pay_123" };
  }
}

class FakePaymentGateway {
  async charge(amountRupees, userId) {
    return { ok: true, paymentId: "fake_payment" };
  }
}

// app.js: production wiring
const checkout = new CheckoutService(new RazorpayGateway());

// checkout.test.js: test wiring
const testCheckout = new CheckoutService(new FakePaymentGateway());

module.exports = { CheckoutService, RazorpayGateway, FakePaymentGateway };

Wire code using dependency injection

One idea

Where DI fits

Common mistake

Before: hard-coded

After: injected

Refactor safely

Use AI carefully

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Quick check