designing-oop-typescript

name	designing-oop-typescript
description	Designs classes and class hierarchies in TypeScript using SOLID, encapsulation, and immutability. Use when creating new domain entities, application services, or refactoring procedural code into OOP.
license	MIT

Designing OOP TypeScript

When to use

• Creating new domain entities or value objects
• Refactoring procedural code into OOP
• Designing class hierarchies with SOLID principles

Core rules

1. Classes over functions: all behavior encapsulated in classes
2. Explicit access modifiers: public, protected, private always written
3. SRP: one responsibility per class, split when growing
4. Encapsulation: no public mutable fields, expose intent via methods
5. Immutability: value objects are readonly, entities use private state
6. SOLID: Single responsibility, Open/closed, Liskov, Interface segregation, Dependency inversion

Reference shape (TypeScript)

Domain Entity

export class User {
  private constructor(
    private readonly _id: string,
    private _email: string,
    private _isActive: boolean
  ) {}

  static create(id: string, email: string): User {
    return new User(id, email, true);
  }

  get id(): string { return this._id; }
  get email(): string { return this._email; }

  deactivate(): void {
    if (!this._isActive) throw new BusinessError('ALREADY_INACTIVE', 'User already inactive');
    this._isActive = false;
  }
}

Value Object

export class Email {
  private constructor(private readonly value: string) {}

  static create(email: string): Email {
    if (!PATTERNS.EMAIL.test(email)) throw new ValidationError([{ field: 'email', message: 'Invalid' }]);
    return new Email(email.toLowerCase());
  }

  equals(other: Email): boolean {
    return this.value === other.value;
  }
}

Examples — Do

class Order {
  private items: OrderItem[] = [];
  addItem(item: OrderItem): void { this.items.push(item); }
  get total(): number { return this.items.reduce((sum, i) => sum + i.price, 0); }
}

Examples — Don't

// ❌ Function module, no encapsulation
export function createUser(id: string, email: string) { return { id, email }; }
// ❌ Public mutable fields
class User { id: string; email: string; }

Checklist

• Classes with explicit access modifiers
• No public mutable fields
• Value objects are immutable
• Entities encapsulate behavior
• SOLID principles applied

See reference/oop-patterns.md for full patterns.