Simple SOLID in TypeScript

This repository contains clear and practical examples of the SOLID principles implemented in TypeScript.

Each principle includes:

• ❌ An incorrect implementation (violating the principle)
• ✅ A corrected version (applying the principle)

The goal is to help developers understand clean architecture and object-oriented design in a simple, hands-on way.

---

What are the SOLID Principles?

SOLID is an acronym for five fundamental principles of object-oriented programming that promote better software design:

Principle	Meaning
S	Single Responsibility Principle (SRP) – A class should have one, and only one, reason to change.
O	Open/Closed Principle (OCP) – Software should be open for extension, but closed for modification.
L	Liskov Substitution Principle (LSP) – Subtypes must be substitutable for their base types.
I	Interface Segregation Principle (ISP) – Clients shouldn't be forced to depend on methods they don’t use.
D	Dependency Inversion Principle (DIP) – High-level modules shouldn't depend on low-level modules, but on abstractions.

---

Examples

Each principle has its own file with before/after code examples.

1. Single Responsibility Principle (SRP)

File: 01-single-responsability-principle.ts

• ❌ UserController handles too many responsibilities
• ✅ Separated into UserService, UserNotificationService, and UserController

---

2. Open/Closed Principle (OCP)

File: 02-open-close-principle.ts

• ❌ Modifying a PaymentService to support new payment types
• ✅ Using interfaces for easy extensibility without changing existing code

---

3. Liskov Substitution Principle (LSP)

File: 03-liskov-substitution-principle.ts

• ❌ A child class (SoldProperty) breaks the behavior of its parent
• ✅ Refactored using clear interfaces for safer substitutions

---

4. Interface Segregation Principle (ISP)

File: 04-interface-segregation-principle.ts

• ❌ A large interface forces unnecessary implementations
• ✅ Smaller interfaces (BasicProperty, Reformable) for more control

---

5. Dependency Inversion Principle (DIP)

File: 05-dependency-inversion-principle.ts

• ❌ High-level InvestorNotifier depends directly on SendGridClient
• ✅ Introduced EmailService interface to decouple layers

---

Contributing

Contributions are welcome! If you want to:

• Add more real-world examples
• Improve explanations
• Translate into another language

Feel free to open an issue or pull request

---

License

This project is licensed under the MIT License.