// Oberklasse
class Shape {
    getType() {
        return "Shape";
    }
}

// Unterklasse erbt von Oberklasse
class Rectangle extends Shape {
    constructor(a, b) {
        super();

        this.a = a;
        this.b = b;
    }
}

const r = new Rectangle(2, 3);

console.log(r.getType());

// Implementierung per Prototype weiterhin möglich, falls man so will
Rectangle.prototype.getType = function() {
    return "Rectangle";
}

console.log(r.getType());