Example using promises.

example-async.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);
  });
}

let getUserFromEmailAsync = (email) => {
  return Promise(function(resolve, reject) {
    getUserFromEmail(email, (e, user) => {
      if (e) {
        reject(e);
      } else {
        resolve(user);
      }
    });
  });
}

console.log('Fetching user');
getUserFromEmailAsync('user@email.com')
  .then((user) => {
    console.log(user.email) // user@email.com
  })
  .catch((reason) => {
    console.log('Error occurred');
  });