factory.js

function CarMaker(){}

CarMaker.prototype.drive = function(){
    return "Vroom, I have " + this.doors + " doors";
};

CarMaker.factory = function(type){
    var constr = type, newcar;

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

    if(typeof CarMaker[constr].prototype.drive !== "function"){
        CarMaker[constr].prototype = new CarMaker();
    }

    newcar = CarMaker[constr]();

    return newcar;
};

CarMaker.Compact = function(){
    this.doors = 4;
};

var car = CarMaker.new("Compact");
car.drive();