class Singleton {
  constructor(x){
    const instance = this.constructor.instance;
    if(instance) {
      return instance; 
    }
    this.constructor.instance = this;
  }
}

Monostate 单态

class ChiefExecutiveOfficer {
  get name(){
    return ChiefExecutiveOffice._name;
  }
  set name(value){
      ChiefExecutiveOffice._name = value;
  }
  get age(){
    return ChiefExecutiveOffice._age;
  }
  set age(value){
      ChiefExecutiveOffice._age = value;
  }
  toString(){
      return `CEO的名字是${this.name} ` + `ta的年龄是${this.age}岁`;
  }
}
ChiefExecutiveOfficer._name = undefined;
ChiefExecutiveOfficer._age = undefined;