dandySingelton.js

function Person(name) {
  console.log("old constructor");

  let obj;

  // Another time just return obj above
  Person = function Person() {
    console.log("new constructor");
    return obj;
  };

  // set proper prototype (new instance)
  Person.prototype = this;

  // Call the old constructor from line 1
  obj = new Person(name);

  // set new Constructor from line 7
  obj.constructor = Person;

  // Do some logic
  this.name = name;

  // First time return instance by old constructor
  return obj;
}

var test = new Person("Dandy boy");
var test2 = new Person("Dandy horse");

Person.prototype.getName = function () {
  console.log(this.name);
};

test.getName();
test2.getName();

console.log(test instanceof Person);
console.log(test2 instanceof Person);
console.log(test === test2);