SOLID principles

The five principles behind maintainable object-oriented code, each with the smell that tells you it is being broken.

Single Responsibility Principle

Also called SRP

What: A class or module should have one reason to change: it answers to one actor or concern.
Why: Changes stay local; a tax rule change should not risk the PDF layout.
Example: Smell: an Invoice class that calculates totals, renders HTML and emails itself. Split into calculator, renderer and sender.

Open/Closed Principle

Also called OCP

What: Code should be open for extension but closed for modification: add behaviour by adding new code, not by editing working code.
Why: Every edit to a tested class risks breaking existing behaviour.
Example: Smell: a switch on payment type that grows with each new method. Fix: a Payment interface with one class per method.

Liskov Substitution Principle

Also called LSP

What: Any subtype must be usable wherever its base type is expected, without surprising the caller.
Why: Polymorphism only works if subclasses keep the promises of their parent.
Example: Smell: a Square subclass of Rectangle whose setWidth also changes the height, breaking code that sets both.

Interface Segregation Principle

Also called ISP

What: Many small, focused interfaces beat one large one; no class should be forced to implement methods it does not use.
Why: Fat interfaces create empty or throwing methods and couple clients to things they never call.
Example: Smell: a Worker interface with code(), test() and deploy(), where a tester implements deploy() by throwing.

Dependency Inversion Principle

Also called DIP

What: High-level policy should depend on abstractions, and low-level details should implement them; neither depends directly on the other.
Why: Business logic can then be tested with fakes and moved to a new database or vendor without rewrites.

TypeScript

// Policy owns the interface
interface OrderRepo { save(o: Order): Promise<void>; }

class PlaceOrder {
  constructor(private repo: OrderRepo) {}
}
// Details implement it
class MongoOrderRepo implements OrderRepo { /* ... */ }

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