person.js

function Person(firstName: string, lastName: string, age: number) {
  if (!this) return new Person(firstName, lastName, age); // if no new, then new to have type
  return Object.freeze(Object.assign(this || {}, { // assign is use of enhance this, to have type
    copy(obj) {
      const values = { firstName, lastName, age, ...obj };
      return new Person(values.firstName, values.lastName, values.age);
    },
    get firstName() {
      return firstName;
    },
    get lastName() {
      return lastName;
    },
    get age() {
      return age;
    },
    toString() {
      return `Person(${firstName}, ${lastName}, ${age})`;
    }
  }));
}

const person1 = Person('John', 'DOE', 42)
const person2 = new Person('Jane', 'DOE', 42);
const person3 = person1
  .copy({ firstName: 'Billy' })
  .copy({ age: '28' });

console.log(person1.toString());
console.log(person2.toString());
console.log(person3.toString());

person1.hello = 'hello'; // will not work
person1.firstName = 'Eugene'; // will not work