function Sale(price) {
  this.price = price || 100;
  this.decorators_list = [];
}
Sale.prototype.getPrice = function () {
  var price = this.price,
      i,
      max = this.decorators_list.length,
      name;
  for (i = 0; i < max; i+= 1) {
    name = this.decorators_list[i];
    price = Sale.decorators[name].getPrice(price);
  }
  return price;
};

Sale.decorators = {}; // 装饰器

Sale.decorators.fedtax = {
  getPrice: function (price) {
    return price + price * 5 / 100;
  }
};

Sale.decorators.quebec = {
  getPrice: function (price) {
    return price + price * 7.5 / 100;
  }
};

Sale.prototype.decorate = function (decorator) {
  this.decorators_list.push(decorator);
};

var sale = new Sale();
console.log(sale.getPrice());   //100
sale.decorate('fedtax');
console.log('after fedtax:' + sale.getPrice()); //after fedtax:105