Class

TypeScript · Reference cheat sheet

📖 Overview

TypeScript classes add parameter properties, visibility modifiers, readonly, abstract, and typed implements/extends. Emit targets and useDefineForClassFields affect field initialization semantics in TS 5.x.

🧩 Core concepts

• Visibility — public (default), protected, private (compile-time); #private is true runtime privacy.
• Parameter properties — constructor(private name: string) declares and assigns in one step.
• readonly / static / abstract — immutability, class-level members, base-only APIs.
• implements — structural check against an interface; does not generate runtime artifacts.
• override — marks intentional overrides (noImplicitOverride recommended).
• Generic classes — class Box<T> \{ ... \}.

💡 Examples

interface Identifiable {
  id: string;
}

abstract class Entity implements Identifiable {
  constructor(public readonly id: string) {}
  abstract summarize(): string;
}

class User extends Entity {
  #passwordHash: string;

  constructor(id: string, public name: string, passwordHash: string) {
    super(id);
    this.#passwordHash = passwordHash;
  }

  override summarize(): string {
    return `${this.id}:${this.name}`;
  }

  static fromJson(json: { id: string; name: string }): User {
    return new User(json.id, json.name, "");
  }
}

class Repo<T extends Identifiable> {
  constructor(private items: T[] = []) {}
  get(id: string): T | undefined {
    return this.items.find((i) => i.id === id);
  }
}

⚠️ Pitfalls

• private is erased at compile time — use #fields for runtime encapsulation.
• Class fields vs constructor assignment differ under useDefineForClassFields / target ES2022+.
• Implementing an interface does not copy default values or methods — only checks shape.
• Arrow-function class fields are not on the prototype (affects inheritance/this binding).

🔗 Related

• Object types
• Decorators
• Generic types
• Type guards
• Module