SOLID Principles

Guidelines for reviewing and refactoring TypeScript code.

Single Responsibility Principle (SRP)

A module/class/function should have only one reason to change.

Violation Signs

• Class or module handling multiple domains (e.g., user data + API calls + caching)
• Function with multiple // Step X comments doing unrelated things
• Hook managing unrelated state pieces
• File imports from many unrelated domains

When NOT to Apply

Avoid over-splitting trivial logic. A simple utility function doing 2-3 closely related operations is fine.

→ See srp-example.md for before/after code.

Open/Closed Principle (OCP)

Modules should be open for extension, closed for modification.

Violation Signs

• Switch/if-else chains on type discriminators that grow with new features
• Modifying existing functions when adding new variants
• type === 'X' checks scattered across codebase
• Adding new feature requires touching many existing files

When NOT to Apply

Don't create abstraction layers for 2-3 stable variants that rarely change. OCP adds indirection — apply when you genuinely expect extension.

→ See ocp-example.md for before/after code.

Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types without altering correctness.

Violation Signs

• Subclass throws exceptions the parent doesn't declare
• Overridden method ignores or reinterprets parent's parameters
• Consumer code checks instanceof before calling methods
• Subclass with empty/no-op method implementations

When NOT to Apply

LSP applies to inheritance hierarchies. For simple data objects or composition-based designs, focus on interface contracts instead.

→ See lsp-example.md for before/after code.

Interface Segregation Principle (ISP)

Clients should not depend on interfaces they don't use.

Violation Signs

• Implementing class has methods throwing NotImplemented
• Interface with 10+ methods where most implementers use only a few
• Props interface with many optional fields, most unused per component
• Mocking pain: test needs to mock methods unrelated to test case

When NOT to Apply

Don't split interfaces that are naturally cohesive and always used together. 2-3 tightly coupled methods belong in one interface.

→ See isp-example.md for before/after code.

Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Violation Signs

• Direct instantiation of dependencies (new ConcreteService())
• Importing concrete implementations in domain/usecase layer
• Hard to test without real database/API/filesystem
• Framework imports in business logic

When NOT to Apply

Don't abstract stable, unlikely-to-change utilities (e.g., date formatting). DIP adds indirection — apply for infrastructure boundaries (DB, API, storage, analytics).

→ See dip-example.md for before/after code.

Review Checklist

Refactoring Priority

1. DIP violations — hardest to test, fix first
2. SRP violations — cascading impact, fix early
3. ISP violations — testing pain, medium priority
4. OCP violations — refactor when adding new variants
5. LSP violations — fix when inheritance hierarchy grows