W3School TIY Editor

abstract class Polygon { public abstract getArea(): number; public toString(): string { return `Polygon[area=${this.getArea()}]`; } } class Rectangle extends Polygon { public constructor(protected readonly width: number, protected readonly height: number) { super(); } public getArea(): number { return this.width * this.height; } } const myRect = new Rectangle(10,20); console.log(myRect.getArea());