why promises are great

new-passport.js

import User from '../models/user'
import { userQueries } from './queries'

let LocalStrategy = require('passport-local').Strategy

export default function (passport) {
    passport.serializeUser(serializeUser)
    passport.deserializeUser(deserializeUser)
    passport.use('register', getStrategy(register))
    passport.use('login', getStrategy(login))
}

function deserializeUser (id, done) {
    User.findById(id, done)
}

function getStrategy (done) {
    return new LocalStrategy({
        usernameField : 'email'
        , passwordField : 'password'
        , passReqToCallback : true
    }, done)
}

function login (req, email, password, done) {
    userQueries.findBy.email(email)
    .then((user) => {
        if (!user) {
            throw new Error('User not found in database.')
        } else {
            return user.validPassword(password)
        }
    }).then((state) => {
        if (state.correctPassword) {
            return Promise.resolve(state.user)
        } else {
            throw new Error('Oops! Wrong password.')
        }
    }).then((user) => {
        return done(null, user)
    }).catch((err) => {
        return done(null, false, req.flash('message', err.message || err))
    })
}

function register (req, email, password, done) {
    userQueries.findBy.email(email)
    .then((user) => {
        if (user) {
            throw new Error('That email is already in use.')
        } else if (email.indexOf('@') === -1) {
            throw new Error('Invalid email.')
        } else {
            return new User().setUp(email, password).save()
        }
    }).then((newUser) => {
        return done(null, newUser)
    }).catch((err) => {
        return done(null, false, req.flash('message', err.message || err))
    })
}

function serializeUser (user, done) {
    done(null, user._id)
}

old-passport.js

import User from '../models/user'

let LocalStrategy = require("passport-local").Strategy

export default function (passport) {
    passport.serializeUser(serializeUser)
    passport.deserializeUser(deserializeUser)
    passport.use("register", getStrategy(register))
    passport.use("login", getStrategy(login))
}

function deserializeUser (id, done) {
    User.findById(id, done)
}

function getStrategy (done) {
    return new LocalStrategy({
        usernameField : "email"
        , passwordField : "password"
        , passReqToCallback : true
    }, done)
}

function login (req, email, password, done) {
    User.findOne({ "email" :  email.toLowerCase().trim() }, function (err, user) {
        if (err) {
            return done(err)
        } else if (!user) {
            return done(null, false, req.flash("message", "User not found in database."))
        } else {
            user.validPassword(password, function (err, correct) {
                if (err) {
                    return done(null, false, req.flash("message", err))
                } else if (!correct) {
                    return done(null, false, req.flash("message", "Oops! Wrong password."))
                } else {
                    return done(null, user)
                }
            })
        }
    })
}

function register (req, email, password, done) {
    email = email.trim().toLowerCase()
    User.findOne({ "email" :  email }, function (err, user) {
        if (err) {
            return done(err)
        } else if (user) {
            return done(null, false, req.flash("message", "That email is already in use."))
        } else if (email.indexOf("@") < 0) {
            return done(null, false, req.flash("message", "Not a valid email address."))
        } else {
            let newUser = new User()
            newUser.setUp(email, password, function (err) {
                if (err) {
                    return done(err, false, req.flash("Something went wrong with your request. Please try again."))
                } else {
                    newUser.save(function (err) {
                        if (err) {
                            return done(null, false, req.flash("message", "Something went wrong. Please try again."))
                        } else {
                            return done(err, newUser, req.flash("message", err))
                        }
                    })
                }
            })
        }
    })
}

function serializeUser (user, done) {
    done(null, user._id)
}