TypeScript Class Patterns

Use abstract classes, private fields, access modifiers, and implements vs extends correctly

When to Use

• Modeling domain entities with encapsulation and inheritance
• Defining abstract base classes with enforced method contracts
• Choosing between

  implements

  (interface conformance) and

  extends

  (inheritance)
• Using access modifiers and private fields for encapsulation

Instructions

1. Basic class with typed properties:

class User {
  readonly id: string;
  name: string;
  private email: string;

  constructor(id: string, name: string, email: string) {
    this.id = id;
    this.name = name;
    this.email = email;
  }

  getEmail(): string {
    return this.email;
  }
}

2. Parameter properties — shorthand for constructor assignment:

class User {
  constructor(
    readonly id: string,
    public name: string,
    private email: string
  ) {}
}
// Equivalent to the longer form above

3. Access modifiers:

   • public

     (default) — accessible everywhere

   • private

     — accessible only within the class (TypeScript-only enforcement)

   • protected

     — accessible within the class and subclasses

   • #field

     — true private (JavaScript runtime enforcement, not just TypeScript)

class Account {
  #balance: number; // True private — not accessible even via type assertions

  constructor(initial: number) {
    this.#balance = initial;
  }

  get balance(): number {
    return this.#balance;
  }
}

4. Abstract classes — define contracts with partial implementations:

abstract class Shape {
  abstract area(): number;
  abstract perimeter(): number;

  describe(): string {
    return `Area: ${this.area()}, Perimeter: ${this.perimeter()}`;
  }
}

class Circle extends Shape {
  constructor(private radius: number) {
    super();
  }

  area(): number {
    return Math.PI * this.radius ** 2;
  }
  perimeter(): number {
    return 2 * Math.PI * this.radius;
  }
}

5. implements

   for interface conformance:

interface Serializable {
  serialize(): string;
  deserialize(data: string): void;
}

class Config implements Serializable {
  constructor(private data: Record<string, string>) {}

  serialize(): string {
    return JSON.stringify(this.data);
  }
  deserialize(data: string): void {
    this.data = JSON.parse(data);
  }
}

6. extends

   vs

   implements

   :

   • extends

     — single inheritance, inherits implementation

   • implements

     — multiple interface conformance, no implementation inherited

interface Loggable {
  log(message: string): void;
}
interface Cacheable {
  cache(): void;
}

class Service extends BaseService implements Loggable, Cacheable {
  log(message: string): void {
    /* ... */
  }
  cache(): void {
    /* ... */
  }
}

7. Static members:

class IdGenerator {
  private static counter = 0;

  static next(): string {
    return `id_${++IdGenerator.counter}`;
  }
}

8. Generic classes:

class Repository<T extends { id: string }> {
  private items = new Map<string, T>();

  save(item: T): void {
    this.items.set(item.id, item);
  }
  find(id: string): T | undefined {
    return this.items.get(id);
  }
  findAll(): T[] {
    return [...this.items.values()];
  }
}

const userRepo = new Repository<User>();

9. Prefer composition over inheritance when the relationship is "has-a" not "is-a":

// Instead of: class UserService extends DatabaseService extends LoggingService
class UserService {
  constructor(
    private db: DatabaseService,
    private logger: LoggingService
  ) {}
}

Details

TypeScript classes compile to JavaScript classes (ES2015+) or constructor functions (ES5 target). They add type annotations, access modifiers, and abstract members on top of standard JavaScript class syntax.

private

#private

• private

  — TypeScript compile-time only. The property is still accessible at runtime via

  (obj as any).field

  or

  obj['field']

• #private

  — JavaScript runtime enforcement. The property is truly inaccessible outside the class. Cannot be accessed even through reflection

Abstract classes vs interfaces:

• Abstract classes can contain implemented methods (shared logic) and abstract methods (contracts)
• Interfaces contain only type signatures (no implementation)
• A class can extend only one abstract class but implement many interfaces
• Use abstract classes when you need shared behavior; use interfaces when you need only a contract

override

class Animal {
  move(): void {
    /* ... */
  }
}

class Dog extends Animal {
  override move(): void {
    /* ... */
  } // Error if parent method does not exist
}

Enable

noImplicitOverride

override

Trade-offs:

• Classes provide clear encapsulation boundaries — but can lead to deep inheritance hierarchies
• Abstract classes enforce contracts AND share code — but restrict you to single inheritance
• Parameter properties reduce boilerplate — but can be surprising to developers from other languages

• #private

  fields are truly private — but cannot be accessed in tests, which some teams dislike

Source

https://typescriptlang.org/docs/handbook/2/classes.html

Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

Harness Integration

• Type: knowledge — this skill is a reference document, not a procedural workflow.
• No tools or state — consumed as context by other skills and agents.

Success Criteria

• The patterns described in this document are applied correctly in the implementation.
• Edge cases and anti-patterns listed in this document are avoided.