What is the Strategy pattern, and how would you apply it to support multiple payment providers in a Node.js checkout service?

The Strategy pattern defines a family of interchangeable algorithms behind a common interface, letting the calling code select and swap the concrete implementation at runtime without changing its own logic -- for a checkout service supporting Stripe, PayPal, and bank transfers, each provider implements the same processPayment() interface, and the checkout flow simply calls whichever strategy was selected based on the customer's chosen payment method.

class StripeStrategy { async pay(amount) { return stripeClient.charge(amount); } }
class PayPalStrategy { async pay(amount) { return paypalClient.createPayment(amount); } }

class Checkout {
  constructor(paymentStrategy) { this.strategy = paymentStrategy; }
  async completeOrder(amount) { return this.strategy.pay(amount); }
}

new Checkout(new StripeStrategy()).completeOrder(4999);