Classes in TypeScript

Use TypeScript's class features: access modifiers, parameter properties, readonly, and abstract classes for shared base behavior.

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Duration

⏱ About 1.5 hours

Level

📊 Intermediate

Prerequisite

🎯 ts-02

OUTCOME

Model real domain objects with class hierarchies

What you'll learn

1Use access modifiers (public, private, protected)
2Take advantage of parameter properties
3Mark immutable fields with readonly
4Define abstract base classes

1. Basic Class

typescript

class BankAccount {
  private balance: number = 0;

  constructor(public readonly owner: string) {}

  deposit(amount: number): void {
    if (amount <= 0) throw new Error("amount must be positive");
    this.balance += amount;
  }

  getBalance(): number {
    return this.balance;
  }
}

const acct = new BankAccount("Alice");
acct.deposit(100);
// acct.balance = 0;   // error: private

public readonly owner: string is a parameter property — it both accepts the arg and assigns this.owner in one place.

2. Inheritance & abstract

typescript

abstract class Shape {
  abstract area(): number;
  describe(): string { return `area=${this.area()}`; }
}

class Circle extends Shape {
  constructor(private r: number) { super(); }
  area() { return Math.PI * this.r ** 2; }
}

class Rect extends Shape {
  constructor(private w: number, private h: number) { super(); }
  area() { return this.w * this.h; }
}

const shapes: Shape[] = [new Circle(5), new Rect(3, 4)];
shapes.forEach(s => console.log(s.describe()));

3. implements an Interface

typescript

interface Serializable { toJSON(): string }

class Note implements Serializable {
  constructor(public title: string, public body: string) {}
  toJSON() { return JSON.stringify(this); }
}

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗