user_promise.js

module.exports.getUserByUsername = function (username, callback) {
  const query = { username };

  return new Promise((resolve, reject) => {
    User.findOne(query, (error, user) => error ? reject(error) : resolve(user));
  });
};

userPromise.js

const UNKNOWN_MESSAGE = 'Unknown User';
const INVALID_MESSAGE = 'Invalid password';

passport.use(new LocalStrategy(((username, password, done) => {
  let userStore;

  User.getUserByUsername(username).then((user) => {
    if (!user) {
      return done(null, false, { message: UNKNOWN_MESSAGE });
    }
    
    userStore = user

    return User.comparePassword(password, user.password);
  }).then((isMatch) => {
    if (isMatch) {
      return done(null, userStore);
    }

    return done(null, false, { message: INVALID_MESSAGE });
  }).catch(({ message }) => console.error(message));
})));