工厂模式定义：实例化对象，用工厂方法代替new操作。

function CarMaker () {
}
CarMaker.prototype.drive = function () {
  console.log("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 = new CarMaker[constr]();
  return newcar;
}
CarMaker.Compact = function(){
  this.doors = 4;
}
CarMaker.Convertible = function(){
  this.doors = 2;
}
CarMaker.SUV = function(){
  this.doors = 3;
}
var corolla = CarMaker.factory('Compact');
var solstice = CarMaker.factory('Convertible');
var cherokee = CarMaker.factory('SUV');
corolla.drive();
solstice.drive();
cherokee.drive();

内置的Object也表现了工厂的行为：

var o = new Object(),
    n = new Object(1),
    s = Object("s"),
    b = Object(true);
console.log(o.constructor);
console.log(n.constructor);
console.log(s.constructor);
console.log(b.constructor);