是否可以检测何时在JavaScript中创建ES6类的第一个实例？

Question

这是我的JavaScript类：

class Animal{
  constructor(name, sound){
    this.name = name;
    this.sound = sound;
  }
  speak(){
     console.log(this.name + `${this.sound}`);
  }
}

我想在创建Animal的第一个实例时执行一些代码。我的意思是：

let dog1 = new Animal('n1', 's1'); //first instance - run my code
let dog2 = new Animal('n2', 'n2');// second instance - do nothing

有可能吗？当然无需更改上面代码中的Animal类。仅使用其构造函数。

Answer 1

只需在构造函数中进行检查：

let haveMadeFirstInstance = false;
class Animal{
  constructor(name, sound){
    this.name = name;
    this.sound = sound;
    if (!haveMadeFirstInstance) {
      console.log('First instance - running some code!');
      haveMadeFirstInstance = true;
    }
  }
  speak(){
     console.log(this.name + `${this.sound}`);
  }
}

console.log('About to create dog1');
let dog1 = new Animal('n1', 's1');
console.log('dog1 has been created');
let dog2 = new Animal('n2', 'n2');
console.log('dog2 has been created');

如果要封装自定义代码，请随时将类放入IIFE：

const Animal = (() => {
  let haveMadeFirstInstance = false;
  return class Animal{
    constructor(name, sound){
      this.name = name;
      this.sound = sound;
      if (!haveMadeFirstInstance) {
        console.log('First instance - running some code!');
        haveMadeFirstInstance = true;
      }
    }
    speak(){
       console.log(this.name + `${this.sound}`);
    }
  }
})();

console.log('About to create dog1');
let dog1 = new Animal('n1', 's1');
console.log('dog1 has been created');
let dog2 = new Animal('n2', 'n2');
console.log('dog2 has been created');

如果您根本无法修改原始类，并且您也无法控制何时创建，那么不行，您要执行的操作就不可能。

Answer 2

您可以使用这样的静态属性：

class Animal{

  constructor(name, sound){
    if(!Animal.instance) {
      console.log('First instance');
      this.name = name;
      this.sound = sound;
      Animal.instance = this;
    }
  }
  speak(){
     console.log(this.name + `${this.sound}`);
  }
}

let dog1 = new Animal('n1', 's1'); //first instance - run my code
let dog2 = new Animal('n2', 'n2');// second instance - do nothing

console.log('dog1: ', dog1);
console.log('dog2: ', dog2);