ES6 destructure named args into properties of constructed object (this).

run.js

'use strict';
class Person {
  constructor(arg = {
    firstName: 'John',
    lastName: 'Doe'
  }) {
    ({
      firstName: this.firstName = '<FIRSTNAME_UNKNOWN>',
      lastName: this.lastName = '<LASTNAME_UNKNOWN>'
    } = arg);
  }
  printName() {
    console.log(this.lastName, ',', this.firstName);
  }
}

var person;
person = new Person({
  firstName: 'Alex'
})
person.printName();
person = new Person;
person.printName();