Working with legacy code can be difficult, particularly if it is disorganized or has developed technical debt over time. On the other hand, legacy code can be made considerably more scalable, maintainable, and readable by adding design patterns. Using real-world TypeScript examples, we will examine recommended practices for applying design patterns in legacy code in this post.

1. Assess the Current State of the Code

Before introducing design patterns, it's essential to thoroughly assess the current state of your legacy code. Identify pain points, such as areas with frequent bugs, complex methods, or tightly coupled components. This assessment will guide you in choosing the right design patterns to apply.

Example: Analyzing Legacy Code

Suppose you have a legacy class responsible for handling multiple payment methods, but it has grown cumbersome and difficult to maintain:

class PaymentProcessor {
    processPayment(paymentMethod: string, amount: number): void {
        if (paymentMethod === "creditCard") {
            // Process credit card payment
        } else if (paymentMethod === "paypal") {
            // Process PayPal payment
        } else if (paymentMethod === "bankTransfer") {
            // Process bank transfer
        } else {
            throw new Error("Unsupported payment method");
        }
    }
}

2. Refactor Incrementally

When working with legacy code, refactor incrementally to avoid introducing new bugs or breaking existing functionality. Apply design patterns in small steps, and ensure each change is well-tested.

Example: Introducing the Strategy Pattern

The PaymentProcessor class can be refactored using the Strategy pattern, which allows you to define a family of algorithms and make them interchangeable.

interface PaymentStrategy {
    process(amount: number): void;
}

class CreditCardPayment implements PaymentStrategy {
    process(amount: number): void {
        console.log(`Processing credit card payment of ${amount}`);
    }
}

class PayPalPayment implements PaymentStrategy {
    process(amount: number): void {
        console.log(`Processing PayPal payment of ${amount}`);
    }
}

class BankTransferPayment implements PaymentStrategy {
    process(amount: number): void {
        console.log(`Processing bank transfer of ${amount}`);
    }
}

class PaymentProcessor {
    private paymentStrategy: PaymentStrategy;

    constructor(paymentStrategy: PaymentStrategy) {
        this.paymentStrategy = paymentStrategy;
    }

    processPayment(amount: number): void {
        this.paymentStrategy.process(amount);
    }
}

// Usage
const paymentProcessor = new PaymentProcessor(new CreditCardPayment());
paymentProcessor.processPayment(100);

3. Prioritize Decoupling and Modularity

Legacy code often suffers from tight coupling, making it difficult to introduce changes without affecting other parts of the system. Design patterns like the Adapter, Facade, and Dependency Injection can help decouple components and promote modularity.

Example: Using the Adapter Pattern

If your legacy code relies on an outdated API, you can use the Adapter pattern to introduce a new API without altering the existing codebase.

// Legacy API
class LegacyPaymentGateway {
    makePayment(amount: number): void {
        console.log(`Payment of ${amount} made using legacy gateway`);
    }
}

// New API interface
interface PaymentGateway {
    processPayment(amount: number): void;
}

// Adapter for the legacy API
class LegacyPaymentAdapter implements PaymentGateway {
    private legacyPaymentGateway: LegacyPaymentGateway;

    constructor(legacyPaymentGateway: LegacyPaymentGateway) {
        this.legacyPaymentGateway = legacyPaymentGateway;
    }

    processPayment(amount: number): void {
        this.legacyPaymentGateway.makePayment(amount);
    }
}

// Usage
const legacyGateway = new LegacyPaymentGateway();
const adapter = new LegacyPaymentAdapter(legacyGateway);
adapter.processPayment(150);

4. Implement Automated Tests
When refactoring legacy code and implementing design patterns, automated testing is crucial. Ensure that you write unit tests to validate each refactoring step. This reduces the risk of introducing bugs and ensures that your refactoring maintain the expected behavior.

Example: Unit Tests with Jest

You can use Jest to create unit tests for the refactored PaymentProcessor class:

import { PaymentProcessor, CreditCardPayment } from './payment-processor';

describe('PaymentProcessor', () => {
    it('should process credit card payment', () => {
        const paymentProcessor = new PaymentProcessor(new CreditCardPayment());
        const consoleSpy = jest.spyOn(console, 'log');

        paymentProcessor.processPayment(100);

        expect(consoleSpy).toHaveBeenCalledWith('Processing credit card payment of 100');
    });
});

5. Document Your Changes

As you introduce design patterns into legacy code, document your changes thoroughly. This includes not only code comments but also updating any existing documentation to reflect the new structure and design. Good documentation helps onboard new developers and assists future maintenance.

Example: Documentation Comment

/**
 * PaymentProcessor class uses the Strategy pattern to process payments
 * based on the provided payment strategy (e.g., CreditCardPayment, PayPalPayment).
 */
class PaymentProcessor {
    //...
}

6. Involve the Team

Refactoring legacy code is a collaborative effort. Involve your team in the process to ensure that everyone is on the same page. Conduct code reviews and discuss design decisions to gain collective insights and ensure consistency across the codebase.

Conclusion

Refactoring legacy code with design patterns is a powerful way to improve the maintainability, scalability, and readability of your codebase. By assessing the current state, refactoring incrementally, focusing on decoupling, implementing automated tests, documenting changes, and involving your team, you can successfully introduce design patterns into legacy code. These practices, supported by TypeScript examples, can transform your legacy system into a more modern and robust application.