js-004.js

class Foo {
  constructor() {
    this.constructedAt = (new Date()).toISOString();
  }
  static getInstance() {
    if (typeof this.instance === 'undefined') {
      this.instance = new Foo();
    }
    return this.instance;
  }
  getValue() {
    return this.constructedAt;
  }
}

setTimeout(function () {
  const foo1 = Foo.getInstance();
  console.log(foo1);
}, 1000);


setTimeout(function () {
  const foo2 = Foo.getInstance();
  console.log(foo2);
}, 2000);