/design-patterns

Design Patterns

Suggests design patterns for your code and checks if it follows SOLID principles.

Usage

/design-patterns [analysis_target] [options]

Options

• --suggest: Suggest applicable patterns (default)
• --analyze: Analyze existing pattern usage
• --refactor: Generate refactoring proposals
• --solid: Check compliance with SOLID principles
• --anti-patterns: Detect anti-patterns

Basic Examples

# Analyze patterns for entire project
/design-patterns

# Suggest patterns for specific file
/design-patterns src/services/user.js --suggest

# Check SOLID principles
/design-patterns --solid

# Detect anti-patterns
/design-patterns --anti-patterns

Pattern Categories

1. Creational Patterns

• Factory Pattern: Abstracts object creation
• Builder Pattern: Step-by-step construction of complex objects
• Singleton Pattern: Ensures only one instance exists
• Prototype Pattern: Creates object clones

2. Structural Patterns

• Adapter Pattern: Converts interfaces
• Decorator Pattern: Dynamically adds functionality
• Facade Pattern: Simplifies complex subsystems
• Proxy Pattern: Controls access to objects

3. Behavioral Patterns

• Observer Pattern: Implements event notifications
• Strategy Pattern: Switches algorithms
• Command Pattern: Encapsulates operations
• Iterator Pattern: Traverses collections

SOLID Principles We Check

S - Single Responsibility (one class, one job)
O - Open/Closed (open for extension, closed for modification)
L - Liskov Substitution (subtypes should be replaceable)
I - Interface Segregation (don't force unused methods)
D - Dependency Inversion (depend on abstractions, not details)

Output Example

Design Pattern Analysis Report
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Currently Used Patterns
├─ Observer Pattern: EventEmitter (12 instances)
├─ Factory Pattern: UserFactory (3 instances)
├─ Singleton Pattern: DatabaseConnection (1 instance)
└─ Strategy Pattern: PaymentProcessor (5 instances)

Recommended Patterns
├─ [HIGH] Repository Pattern
│  └─ Where: src/models/*.js
│  └─ Why: Separate data access from business logic
│  └─ Example:
│      class UserRepository {
│        async findById(id) { ... }
│        async save(user) { ... }
│      }
│
├─ [MED] Command Pattern
│  └─ Where: src/api/handlers/*.js
│  └─ Why: Standardize how requests are handled
│
└─ [LOW] Decorator Pattern
   └─ Where: src/middleware/*.js
   └─ Why: Better way to combine features

SOLID Violations Found
├─ [S] UserService: Does too much (auth AND authorization)
├─ [O] PaymentGateway: Must change code to add payment types
├─ [D] EmailService: Depends on specific classes, not interfaces
└─ [I] IDataStore: Has methods nobody uses

How to Fix
1. Split UserService into AuthService and AuthorizationService
2. Add a PaymentStrategy interface for new payment types
3. Create an EmailService interface
4. Break up IDataStore into smaller interfaces

Advanced Usage Examples

# See what happens if you use a pattern
/design-patterns --impact-analysis Repository

# Get example code for a pattern
/design-patterns --generate Factory --for src/models/Product.js

# Find patterns that work well together
/design-patterns --combine --context "API with caching"

# Check your architecture
/design-patterns --architecture MVC

Example: Before and After

Before (Problem Code)

class OrderService {
  processOrder(order, paymentType) {
    if (paymentType === "credit") {
      // Credit card processing
    } else if (paymentType === "paypal") {
      // PayPal processing
    }
    // Other payment methods...
  }
}

After (Applying Strategy Pattern)

// Strategy interface
class PaymentStrategy {
  process(amount) {
    throw new Error("Must implement process method");
  }
}

// Concrete strategies
class CreditCardPayment extends PaymentStrategy {
  process(amount) {
    /* Implementation */
  }
}

// Context
class OrderService {
  constructor(paymentStrategy) {
    this.paymentStrategy = paymentStrategy;
  }

  processOrder(order) {
    this.paymentStrategy.process(order.total);
  }
}

Anti-Patterns We Find

• God Object: Classes that do everything
• Spaghetti Code: Tangled mess of control flow
• Copy-Paste Programming: Same code everywhere
• Magic Numbers: Random numbers with no explanation
• Callback Hell: Callbacks inside callbacks inside callbacks

Best Practices

1. Go slow: Add patterns one at a time
2. Need first: Only use patterns to solve real problems
3. Talk it out: Get team buy-in before big changes
4. Write it down: Document why you chose each pattern