故事的开头是这样子的…
在很久很久以前，有一个学生叫小明
有一天，老师对小明，小明啊，你去给班上买一个篮球，一个足球，一套积木和一套乐高

class Good {
    constructor(name, price) {
        this.name = name;
        this.price = price;
    }
    showInfo() {
        console.log(`商品名：${this.name} 价格：￥${this.price}`);
    }
}

class Shop {
    constructor(customer) {
        this.customer = customer;
    }
    sell(good) {
        this.customer.buy(good);
        this.customer.total += good.price;
    }
}

class ToyShop extends Shop{
    constructor(customer) {
        super(customer);
    }
    sell(name) {
        switch(name) {
            case '乐高': super.sell(new Good('乐高', 3600));
            case '积木': super.sell(new Good('积木', 288));
        }
    }
}

class SportShop extends Shop{
    constructor(customer) {
        super(customer);
    }
    sell(name) {
        switch(name) {
            case '足球': super.sell(new Good('足球', 80));
            case '篮球': super.sell(new Good('篮球', 65));
        }
    }
}

class Student {
    constructor(name) {
        this.name = name;
        this.goodList = [];
        this.total = 0;
    }
    buy(good) {
        this.goodList.push(good);
    }
    getTotal() {
       console.log(`${this.name}一共花费了￥${this.total}`);
    }
}

于是，小明拿着钱，走在大街上…

let xiaoming = new Student('小明');

小明进了球具店，买了足球和篮球

let sportShop = new SportShop(xiaoming);
sportShop.sell('足球');
sportShop.sell('篮球');

小明进了文具店，买了乐高和积木

let toyShop = new ToyShop(xiaoming);
toyShop.sell('乐高');
toyShop.sell('积木');

小明结束了采购

xiaoming.getTotal();

小明并不关心每次购买的过程是怎样的：商家库存少了一个商品，商家盈利了多少钱，小明账单上多了多少钱
小明只关注一件事情：商家卖出了一件商品给我