Strategy Pattern In JavaScript

A modular checkout processor that delegates payment execution to interchangeable strategy classes (Credit Card, PayPal, Crypto). This allows the application to swap complex payment and verification algorithms dynamically at runtime without altering the core checkout logic.

// Strategy Pattern

interface IPaymentStrategy {
  pay(amount: number): void;
}

class CreditCardPayment implements IPaymentStrategy {
  private cardNumber: string;

  constructor(cardNumber: string) {
    this.cardNumber = cardNumber;
  }

  public pay(amount: number): void {
    console.log(
      `Processing $${amount} via Credit Card ending in ${this.cardNumber.slice(-4)}`,
    );
  }
}

class PayPalPayment implements IPaymentStrategy {
  private emailAddress: string;

  constructor(emailAddress: string) {
    this.emailAddress = emailAddress;
  }

  public pay(amount: number): void {
    console.log(
      `Processing $${amount} via PayPal account: ${this.emailAddress}`,
    );
  }
}

class CryptoPayment implements IPaymentStrategy {
  private walletAddress: string;

  constructor(walletAddress: string) {
    this.walletAddress = walletAddress;
  }

  public pay(amount: number): void {
    console.log(
      `Processing $${amount} via Crypto Wallet: ${this.walletAddress}`,
    );
  }
}

class CheckoutProcessor {
  private paymentStrategy: IPaymentStrategy;

  constructor(strategy: IPaymentStrategy) {
    this.paymentStrategy = strategy;
  }

  public setPaymentStrategy(strategy: IPaymentStrategy): void {
    this.paymentStrategy = strategy;
    console.log("--> Payment strategy updated.");
  }
  public processOrder(amount: number): void {
    console.log("Starting checkout process...");
    this.paymentStrategy.pay(amount);
    console.log("Checkout complete!\n");
  }
}

const creditCardMethod = new CreditCardPayment("4111222233334444");
const checkout = new CheckoutProcessor(creditCardMethod);

checkout.processOrder(150.0);

const paypalMethod = new PayPalPayment("user@exaple.com");
checkout.setPaymentStrategy(paypalMethod);

checkout.processOrder(45.5);