index.js

class User {
  constructor(firstName, lastName) {
    this._firstName = firstName;
    this._lastName = lastName;
  }
  
  get fullName() {
    console.log(this._firstName + " " + this._lastName);  
  }
  
  set firstName(name) {
    this._firstName = name;
  }

  set lastName(name) { }
}

const user = new User("Jon", "Snow")
user.fullName; // Jon Snow
user.firstName = "Joon"