finally.js

// ...

function updateUserName(user, { firstName, lastName }) {
  const { name } = user;
  const newName = {
    firstName: firstName || name.firstName,
    lastName: lastName || name.lastName,
  };
  return { ...user, name: newName };
}

const updatedUser = updateUserName(user, { lastName: 'new last name' });
// finally we need to call `saveUser` to actually update our data store, check `users` before
console.log(users);
users = saveUser(users, updatedUser)
// now check again