如何使用javascript中的命令模式

var Stock = function(){
  this.name = 'baidu';
  this.quantity = 100;
  this.buy = function(){
    console.log('購買了：' + this.quantity + '股' + this.name);
  }
  this.sell = function(){
    console.log('賣出了：' + this.quantity + '股' + this.name);
  }
}
 
var BuyStock = function(stock){
  this.stock = stock;
 
  this.execute = function(){
    this.stock.buy();
  }
}
 
var SellStock = function(stock){
  this.stock = stock;
 
  this.execute = function(){
    this.stock.sell();
  }
}
 
var Broker = function(){
  this.orderList = [];
 
  this.takeOrder = function(order){
    this.orderList.push(order);
  }
  this.placeOrders = function(){
    this.orderList.map(function(item){
      item.execute();
    })
    this.orderList = [];
  }
}
 
var stock = new Stock();
var buyStock = new BuyStock(stock);
var sellStock = new SellStock(stock);
 
var broker = new Broker();
broker.takeOrder(buyStock);
broker.takeOrder(sellStock);
broker.placeOrders();
// 購買了：100股baidu
// 賣出了：100股baidu