Edit in JSFiddle

function Vehicle() {
    this.registration = Math.random();
}

function LandVehicle(){}
function Car(){}
function SportsCar(){}
function RacingCar(){Vehicle.call(this)}

LandVehicle.prototype = new Vehicle(); 
Car.prototype         = new LandVehicle(); 
SportsCar.prototype   = new Car();
RacingCar.prototype   = new SportsCar();

//now we are free to set other prototypes . 
Vehicle.prototype.make  = null;
Vehicle.prototype.model = null; 
Vehicle.prototype.registration; 

//formula1 cars
var f1 = new RacingCar();
var f2 = new RacingCar();

//returns two different numbers
console.log( f1.registration, f2.registration )