如何找到具有構造函數參數等於輸入的Class的特定新對象

Question

我有一個名為Country的類，帶有4個構造函數參數。

然后，我使用指定的值從該類創建一些新的國家/地區。

我的問題是，如何創建一個方法，可以找到並返回一個this.value等於方法輸入的對象？

class Country {

  constructor(name, area, population, topdomain) {
    this.name = name;
    this.area = area;
    this.population = population;
    this.topdomain = topdomain;
  }

  static findCountry = domain => {
    /*Here is where the magic should happen. 
      If domain is in any of the countries below, then it should return the country name.
     */
  }
}

norway = new Country("Norway", 323802, 5320045, ".no");
sweden = new Country("Sweden", 450295, 9960487, ".se");
russia = new Country("Russia", 17098242, 142257519, ".ru");
china = new Country("China", 9596960, 1379302771, ".cn");

這個函數應該返回“Norway”：

Country.findCountry(".no");

Answer 1

要使其工作，該類必須保留所有已創建實例的列表。 由於JS沒有弱引用，這意味着沒有任何實例可以被垃圾收集（所以要小心）：

 static instances = [];

 constructor(/*...*/) {
   /*...*/
   Country.instances.push(this);
 }

 static findCountry = domain => {
  return this.instances.find(country => country.domain === domain);
 }

Answer 2

不要來這里要求別人寫你的代碼;）

class Country {
    constructor(name, area, population, topdomain) {
        this.name = name;
        this.area = area;
        this.population = population;
        this.topdomain = topdomain;
        Country._ALL.push(this);
    }
    static findBy(key, value) {
        let output = [];
        for ( let i in Country._ALL) {
            let c = Country._ALL[i];
            if (c.hasOwnProperty(key) && c[key] === value)
                output.push(c);
        }
        return output;
    }
}
Country._ALL = [];

警告！ ES6類不支持靜態變量，如static variable = []; 如果你想在ES6中使用靜態類變量，你必須使用ClassName.variable = []; 之后類的聲明。

Answer 3

你的類不知道你在某處實例化的4個對象。 您需要將它們放在一個集合（例如數組）中，然后在搜索方法中顯式引用該集合：

class Country {
  constructor(name, area, population, topdomain) {
    this.name = name;
    this.area = area;
    this.population = population;
    this.topdomain = topdomain;
  }

  static findCountry(domain) {
    return (knownCountries.find(country => country.topdomain == domain) || {}).name;
//          ^^^^^^^^^^^^^^
  }
}

const norway = new Country("Norway", 323802, 5320045, ".no");
const sweden = new Country("Sweden", 450295, 9960487, ".se");
const russia = new Country("Russia", 17098242, 142257519, ".ru");
const china = new Country("China", 9596960, 1379302771, ".cn");

const knownCountries = [norway, sweden, russia, china];
//    ^^^^^^^^^^^^^^