example without using promises

example.js

let getUserFromEmail = (email, cb) => {
  $service.findUser({ email }, (err, user) => {
    if(err) {
      return cb(err);
    } else if(user == null) {
      return cb(new Error('The user was not found'));
    }
    console.log('User found');
    return cb(undefined, user);
  });
}

console.log('Fetching user');
getUserFromEmail('user@email.com', (e, user) => {
  if (e) {
    console.log('Error occurred');
  } else {
    console.log(user.email); // user@email.com
  }
});