// parent constructor
function CarMaker() {}


// a method of the parent
CarMaker.prototype.drive = function () {
  console.log( "Vroom, I have " + this.doors + " doors" );
};

// the static factory method
CarMaker.factory = function (type) {
  var constr = type, newcar;

  // error if the constructor doesn't exist
  if (typeof CarMaker[constr] !== "function") {
    throw {
      name: "Error",
      message: constr + " doesn't exist"
    };
  }

  // at this point the constructor is known to exist
  // let's have it inherit the parent but only once
  if (typeof CarMaker[constr].prototype.drive !== "function") {
    CarMaker[constr].prototype = new CarMaker();
  }

  // create a new instance
  newcar = new CarMaker[constr]();

  // optionally call some methods and then return...
  return newcar;
};


// define specific car makers
CarMaker.Compact = function () {
  this.doors = 4;
};

CarMaker.Convertible = function () {
  this.doors = 2;
};

CarMaker.SUV = function () {
  this.doors = 24;
};


var corolla   = CarMaker.factory('Compact');
var solstice = CarMaker.factory('Convertible');
var cherokee = CarMaker.factory('SUV');

corolla.drive();  // "Vroom, I have 4 doors"
solstice.drive(); // "Vroom, I have 2 doors"
cherokee.drive(); // "Vroom, I have 17 doors"