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

User.prototype.getFullName = function() {
  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