js-003.js

class Foo {
  constructor() {
    this.constructedAt = (new Date()).toDateString();
  }
  static getInstance() {
    this.foo = 1;
    this.instance = this;
    return this.instance;
  }
  getValue() {
    return this.constructedAt;
  }
}
const foo1 = Foo.getInstance();
const foo2 = Foo.getInstance();
console.log(foo1, foo2);
console.log(foo1 === foo2, foo1.getValue());