feat: implement PaymentMethod interface with CreditCard and PayPal classes, add processPayment function, and update README instructions

Author: kentcdodds

exercises/02.interfaces-and-classes/02.problem.programming-to-abstractions/README.mdx

@@ -6,11 +6,12 @@ concrete classes.
 
 🐨 Open <InlineFile file="index.ts" /> and:
 
-1. Use the `PaymentMethod` interface from the previous exercise
-2. Create a `PaymentProcessor` class with a method that accepts `PaymentMethod`
-3. Write a function that processes payments using the interface type
-4. Test with different payment method implementations
+1. Create a `processPayment` function that accepts a `PaymentMethod` parameter
+2. The function should call `method.pay(amount)` and return the result
+3. Test it with both `CreditCard` and `PayPal` instances
 
-💰 Use the interface type for parameters so different implementations work.
+💰 Use the interface type (`PaymentMethod`) for the parameter instead of a
+concrete class type. This way, the function works with any class that
+implements the interface.
 
 📜 [TypeScript Handbook - Using Interfaces](https://www.typescriptlang.org/docs/handbook/2/objects.html#using-interfaces)

exercises/02.interfaces-and-classes/02.problem.programming-to-abstractions/index.ts

@@ -1,33 +1,46 @@
 // Programming to Abstractions
 
-// 🐨 Define the PaymentMethod interface:
-// - Method: pay(amount: number) returns string
+interface PaymentMethod {
+	pay(amount: number): string
+}
 
-// 🐨 Create a CreditCard class implementing PaymentMethod:
-// - Field: cardNumber (string)
-// - Constructor that takes cardNumber
-// - Implement pay(amount: number) returns "Paid $${amount} with credit card ${cardNumber}"
+class CreditCard implements PaymentMethod {
+	cardNumber: string
 
-// Test CreditCard
-// const creditCard = new CreditCard('1234-5678-9012-3456')
-// console.log(creditCard)
+	constructor(cardNumber: string) {
+		this.cardNumber = cardNumber
+	}
 
-// 🐨 Create a PayPal class implementing PaymentMethod:
-// - Field: email (string)
-// - Constructor that takes email
-// - Implement pay(amount: number) returns "Paid $${amount} with PayPal ${email}"
+	pay(amount: number): string {
+		return `Paid $${amount} with credit card ${this.cardNumber}`
+	}
+}
 
-// Test PayPal
-// const paypal = new PayPal('user@example.com')
-// console.log(paypal)
+class PayPal implements PaymentMethod {
+	email: string
+
+	constructor(email: string) {
+		this.email = email
+	}
+
+	pay(amount: number): string {
+		return `Paid $${amount} with PayPal ${this.email}`
+	}
+}
 
 // 🐨 Create a processPayment function:
 // - Parameters: method (PaymentMethod), amount (number)
 // - Returns: string
 // - Calls method.pay(amount)
 
 // Test processPayment
+// const creditCard = new CreditCard('1234-5678-9012-3456')
+// const paypal = new PayPal('user@example.com')
 // console.log(processPayment(creditCard, 100))
 // console.log(processPayment(paypal, 50))
 
-// export { CreditCard, PayPal, processPayment }
+export {
+	CreditCard,
+	PayPal,
+	// processPayment
+}