factory.js

var CarMaker = function() {};

CarMaker.prototype.drive = function() {
	return 'My car has ' + this.wheels + ' wheels';
};

CarMaker.factory = function(type) {
	var car;

	if(typeof CarMaker[type] !== 'function') {
		throw new Error('The factory ' + type + ' does not exist');
	}

	car = new CarMaker[type]();
	return car;
};

// small
CarMaker.small = function() {
	this.wheels = 2;
};
CarMaker.small.prototype = Object.create(CarMaker.prototype);

// big
CarMaker.big = function() {
	this.wheels = 4;
};
CarMaker.big.prototype = Object.create(CarMaker.prototype);

// instance
var celta = CarMaker.factory('small');