Welcome back! We are continuing our journey through Behavioral Patterns in software design. In previous lessons, we explored the Command and Observer patterns, focusing on object communication and state changes. Now, we will learn how to implement the Strategy Pattern in TypeScript. We will break down the pattern into manageable parts and illustrate its practical application through a clear example.

Consider a scenario in which you have a ShoppingCart class that can handle payments through different methods, such as credit cards or PayPal. Using the Strategy Pattern, we can encapsulate these payment methods into separate classes and have the ShoppingCart class use any of these strategies interchangeably.

First, we define an interface that all payment strategies will follow. In TypeScript, interfaces provide a clear contract for classes to implement, ensuring type safety and consistency.

Here, the PaymentStrategy interface declares a pay method that accepts an amount of type number and returns nothing (void). Any payment strategy must implement this method.

Next, we implement concrete strategies that encapsulate different payment methods. Here, we define two strategies: CreditCardStrategy and PayPalStrategy. Each class implements the PaymentStrategy interface and provides its own logic for the pay method.

In the CreditCardStrategy class, we implement the pay method to handle credit card transactions. This class requires a card number upon initialization, which is stored as a private property.

Similarly, the PayPalStrategy class implements the pay method for PayPal transactions. It requires an email address to initialize, which is stored as a private property.

The ShoppingCart class is our context class that will use any given payment strategy. This class keeps a reference to a PaymentStrategy object and can switch strategies at runtime. TypeScript's type system ensures that only valid strategies can be assigned.

In the ShoppingCart class, the setPaymentStrategy method allows us to set the payment strategy, and the checkout method uses the selected strategy to make a payment. The strategy property is typed as PaymentStrategy | null to indicate that it may not always be set.

Below is the complete code for our example, integrating all the parts we've discussed:

Understanding the Strategy Pattern is crucial because it promotes flexibility and reusability in your code. Instead of hardcoding multiple algorithms within a class, you can encapsulate them into separate strategy classes. This makes your code more maintainable and scalable.

Consider a real-world example: an e-commerce platform. One customer might prefer to pay using a credit card, while another might choose PayPal. With the Strategy Pattern, you can easily switch payment methods without altering the underlying business logic of the shopping cart.

By mastering the Strategy Pattern, you'll be equipped to build systems that can adapt to varying requirements with minimal changes to the codebase.