TypeScript Classes

TypeScript Classes: OOP, Access Modifiers, & Implements

TypeScript brings full object-oriented programming (OOP) capabilities to JavaScript classes, adding access modifiers (public, private, protected), readonly fields, parameter properties, abstract classes, and interface implementation.

Access Modifiers Summary

• public: Accessible from anywhere (default).
• private (# or private): Accessible only within the declaring class.
• protected: Accessible within the declaring class and its subclasses.
• readonly: Field can only be assigned during declaration or inside the constructor.

Practical Code Example

interface Printable {
  printSummary(): void;
}

abstract class BaseEntity {
  constructor(public readonly id: string, public readonly createdAt: Date) {}
  abstract getEntityType(): string;
}

class UserAccount extends BaseEntity implements Printable {
  private _hashedPasswordHash: string;
  protected loginCount: number = 0;

  // Parameter Properties shortcut in constructor
  constructor(
    id: string,
    public username: string,
    public email: string,
    passwordHash: string
  ) {
    super(id, new Date());
    this._hashedPasswordHash = passwordHash;
  }

  public getEntityType(): string {
    return "UserAccount";
  }

  public recordLogin(): void {
    this.loginCount++;
    console.log(`User ${this.username} logged in. Total logins: ${this.loginCount}`);
  }

  public printSummary(): void {
    console.log(`[${this.getEntityType()}] ID: ${this.id} | User: ${this.username} (${this.email})`);
  }
}

const user = new UserAccount("usr_001", "alice_dev", "[email protected]", "secret_hash");
user.recordLogin();
user.printSummary();

Best Practices & Gotchas

• Use Constructor Parameter Properties: constructor(public name: string) eliminates redundant class field assignments.
• Prefer JS Hard Private #field for True Enclosure: TypeScript private keyword is stripped at compile-time, while ES #field enforces runtime privacy.
• Use abstract Classes for Framework Blueprints: Define common base logic while enforcing abstract method overrides in derived classes.

Self-Check Challenge

Create a class Car with a private field speed and a public method accelerate(amount: number).