SOLID Principles

Five principles for object-oriented design that lead to maintainable, extensible software.

S - Single Responsibility Principle

A class should have only one reason to change.

// BAD - multiple responsibilities
class UserService {
  createUser(data: UserData) { /* ... */ }
  sendEmail(user: User, message: string) { /* ... */ }
  generateReport(users: User[]) { /* ... */ }
  validateEmail(email: string) { /* ... */ }
}