Mohssine Kissane - Software engineer

Technical debt costs companies $85 billion annually. SOLID principles prevent it.

What: Five object-oriented design principles that create flexible, maintainable, and scalable codebases:

S - Single Responsibility Principle Every class should have one reason to change.

code

// Violation: Multiple responsibilities
class UserManager {
  createUser(data) { /* validation + DB + email */ }
}

// Correct: Separated concerns
class UserValidator { validate(data) {} }
class UserRepository { save(user) {} }
class EmailService { sendWelcome(user) {} }

O - Open/Closed Principle Open for extension, closed for modification.

code

// Extensible payment system
class PaymentProcessor {
  process(amount) { throw new Error('Implement in subclass'); }
}

class StripeProcessor extends PaymentProcessor {
  process(amount) { /* Stripe logic */ }
}

class PayPalProcessor extends PaymentProcessor {
  process(amount) { /* PayPal logic */ }
}

L - Liskov Substitution Principle Subtypes must be substitutable for their base types without breaking functionality.

I - Interface Segregation Principle No client should depend on methods it doesn't use. Split large interfaces into smaller, specific ones.

D - Dependency Inversion Principle Depend on abstractions, not concrete implementations.

code

// Wrong: Depends on concrete class
class OrderService {
  constructor() {
    this.emailer = new EmailService();
  }
}

// Right: Depends on abstraction
class OrderService {
  constructor(notificationService) {
    this.notifier = notificationService; // Can be email, SMS, push
  }
}

Implementation priority: Start with Single Responsibility and Dependency Inversion—they deliver the highest immediate ROI.