在 Javascript 中，每一个类形成一个单独的文件，然后相互引用

1、参考：在 重构：改善既有代码的设计 (第2版) 中，构筑测试体系 – 待测试的示例代码的编写：https://www.shuijingwanwq.com/2023/01/28/7368/ 。现在存在的问题是，多个类存在于同一个文件中。决定拆分开，每一个类形成一个单独的文件。

2、新建类文件 /modules/producer.js，将类定义迁移至文件中

class Producer {
    constructor(aProvince, data) {
        this._province = aProvince;
        this._cost = data.cost;
        this._name = data.name;
        this._production = data.production || 0;
    }

    get name() {
        return this._name;
    }

    get cost() {
        return this._cost;
    }

    set cost(arg) {
        this._cost = parseInt(arg);
    }

    get production() {
        return this._production;
    }

    set production(amountStr) {
        const amount = parseInt(amountStr);
        const newProduction = Number.isNaN(amount) ? 0 : amount;
        this._province.totalProduction += newProduction - this._production;
        this._production = newProduction;
    }
}

export {Producer};

3、新建类文件 /modules/province.js，将类定义迁移至文件中。导入类 Producer

import {Producer} from './producer.js';

class Province {
    constructor(doc) {
        this._name = doc.name;
        this._producers = [];
        this._totalProduction = 0;
        this._demand = doc.demand;
        this._price = doc.price;
        doc.producers.forEach(d => this.addProducer(new Producer(this, d)));
    }

    addProducer(arg) {
        this._producers.push(arg);
        this._totalProduction += arg.production;
    }

    get name() {
        return this._name;
    }

    get producers() {
        return this._producers.slice();
    }

    get totalProduction() {
        return this._totalProduction;
    }

    set totalProduction(arg) {
        this._totalProduction = arg;
    }

    get demand() {
        return this._demand;
    }

    set demand(arg) {
        this._demand = parseInt(arg);
    }

    get price() {
        return this._price;
    }

    set price(arg) {
        this._price = parseInt(arg);
    }

    get shortfall() {
        return this._demand - this.totalProduction;
    }

    get profit() {
        return this.demandValue - this.demandCost;
    }

    get demandCost() {
        let remainingDemand = this.demand;
        let result = 0;
        this.producers
            .sort((a, b) => a.cost - b.cost)
            .forEach(p => {
                const contribution = Math.min(remainingDemand, p.production);
                remainingDemand -= contribution;
                result += contribution * p.cost;
            });
        return result;
    }

    get demandValue() {
        return this.satisfiedDemand * this.price;
    }

    get satisfiedDemand() {
        return Math.min(this._demand, this.totalProduction);
    }
}

export {Province};

4、编辑 /productionPlan.js，删除两个类相关的代码，然后导入类 Province

import {Province} from './modules/province.js';

function sampleProvinceData() {
    return {
        name: "Asia",
        producers: [{name: "Byzantium", cost: 10, production: 9}, {
            name: "Attalia",
            cost: 12,
            production: 10
        }, {name: "Sinope", cost: 10, production: 6},],
        demand: 30,
        price: 20
    };
}

const asia = new Province(sampleProvinceData());
console.log(asia.shortfall);

5、再次运行代码(建议强制刷新浏览器)，在控制台的输出无变化，符合预期。如图1

图1

6、最终的代码结构如下所示。如图2

图2