class User {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  getFullName() {
    console.log(this.firstName + " " + this.lastName);
  }
}

var user = new User("Caio", "de Lima Granero");
user.getFullName();   //"Caio de Lima Granero"

console.log(user instanceof User);  // true
console.log(user instanceof Object);  // true
console.log(typeof(User));  // function