What will happen if an error is passed to the callback in this code?

What will happen if an error is passed to the callback in this code?

function getUser(callback) {
  callback(new Error('User not found'), null);
}
getUser((err, user) => {
  if (err) {
    console.error(err.message);
  } else {
    console.log(user);
  }
});