Inheritance in TypeScript

class Car{
speed: String;
    setSpeed(speed: string) {
        this.speed = speed;
        alert("Car speed changed");
    }
}

class Ferrari extends Car{
    speed: String;
    setSpeed(speed: string) {
        this.speed = speed;
        alert("Farrari speed changed");
    }
}

var car = new Ferrari();
car.setSpeed('');