Dependency Inversion Principle

We need to create loosely coupled Modules / Classes by removing reductant dependency. This is known as Dependency Inversion Principle.

Our Store is directly dependent on Payment method. If we want to change Stripe to Paypal, there will be lot of code change directly in Store.

class Store {
  constructor(user) {
    this.stripe = new Stripe(user);
    // this.paypal = new Paypal()
    // this.user = user
  }

  purchaseBike(quantity) {
    this.stripe.makePayment(200 * quantity * 100);
    // this.paypal.makePayment(this.user, 200 * quantity)
  }

  purchaseHelmet(quantity) {
    this.stripe.makePayment(15 * quantity * 100);
    // this.paypal.makePayment(this.user, 15 * quantity)
  }
}

class Stripe {
  constructor(user) {
    this.user = user;
  }

  makePayment(amountInCents) {
    console.log(
      this.user,
      "made payment of",
      amountInCents / 100,
      "dollar with stripe"
    );
  }
}

class Paypal {
  makePayment(user, amountInDollar) {
    console.log(user, "made payment of", amountInDollar, "dollar with paypal");
  }
}

const store = new Store("Walmart");
store.purchaseBike(5); // Walmart made payment of 1000 with stripe
store.purchaseHelmet(2); // Walmart made payment of 30 with stripe

To fix this issue we need a intermediate wrapper to wrap both paypal and stripe so that our Store remains unaffected.

class Store {
  constructor(paymentProcessor) {
    this.paymentProcessor = paymentProcessor;
  }

  purchaseBike(quantity) {
    this.paymentProcessor.pay(200 * quantity);
  }

  purchaseHelmet(quantity) {
    this.paymentProcessor.pay(15 * quantity);
  }
}

class StripePaymentProcessor {
  constructor(user) {
    this.stripe = new Stripe(user);
  }

  pay(amountInCents) {
    this.stripe.makePayment(amountInCents * 100);
  }
}

class Stripe {
  constructor(user) {
    this.user = user;
  }

  makePayment(amountInCents) {
    console.log(
      this.user,
      "made payment of",
      amountInCents / 100,
      "dollar with stripe"
    );
  }
}

class PaypalPaymentProcessor {
  constructor(user) {
    this.paypal = new Paypal();
    this.user = user;
  }

  pay(amountInDollar) {
    this.paypal.makePayment(this.user, amountInDollar);
  }
}

class Paypal {
  makePayment(user, amountInDollar) {
    console.log(user, "made payment of", amountInDollar, "dollar with paypal");
  }
}

const store1 = new Store(new StripePaymentProcessor("Walmart"));
store1.purchaseBike(5);
store1.purchaseHelmet(2);
// Walmart made payment of 1000 dollar with stripe
// Walmart made payment of 30 dollar with stripe

const store2 = new Store(new PaypalPaymentProcessor("Target"));
store2.purchaseBike(5);
store2.purchaseHelmet(2);
// Target made payment of 1000 dollar with paypal
// Target made payment of 30 dollar with paypal