Singleton - JS Implementation with tests

function Person() {

  if(typeof Person.instance === 'object')
    return Person.instance;

  Person.instance = this;

  return this;
}

let person1 = new Person();
let person2 = new Person();

/* test to see whether we have access to the instance through the window */
/* by comparing the window accessible instance to the reference variables */
console.log(window.Person.instance === person2); //true
console.log(window.Person.instance === person1); //true

//references to the same instance, not just copies of the same object template 
person1 === person2; //true

source: https://github.com/fbeline/Design-Patterns-JS/blob/master/src/creational/singleton/singleton.js