This is an error prone code

still-error-prone-code.js

// ...

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

function saveUser(users, updatedUser) {
  return users.map(user => (user.id === updatedUser.id ? updatedUser : user));
}

// ...
const user = { ...users.find(userByMatcher({ key: 'id', value: 2 })) };
const updatedUser = updateUserName(user, { lastName: 'new last name' });
// no need to call `saveUser`, user has already been saved :(
// check users
console.log(users);