Абстрактные классы

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

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

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

const c: Shape = new Circle(5);
console.log(c.describe());