authentication.js

var passport = require('passport');
var FacebookStrategy = require('passport-facebook').Strategy;
var User = require('./models').User;
// var config = require('./oauth.js')

var getUser = function(token, refreshToken, profile, done) {
  
  User.findOne({ 'facebook.id' : profile.id }, function(err, user) {
    var saveUser = function(err) {
      if (err) {throw err; } 
      return done(null, user);              
    };

    // if there is an error, stop everything and return that error
    if (err) { return done(err); }

    // if the user is found, then log them in
    if (user) {
      user.facebook.token = token;
      user.save(saveUser);
    } else {
      // if there is no user found with that facebook id, create them
      var newUser = new User();
      
      // set all of the facebook information in our user model
      newUser.first_name = profile.name.givenName;
      newUser.last_name = profile.name.familyName;
      newUser.image = "https://graph.facebook.com/" + profile.id + "/picture";
      newUser.facebook.id    = profile.id; // set the users facebook id                 
      newUser.facebook.token = token; // we will save the token that facebook provides to the user                  

      // save our user to the database
      newUser.save(saveUser);
    } 
  });
};

module.exports = passport.use(new FacebookStrategy({
   clientID: process.env.clientID || config.clientID,
   clientSecret: process.env.clientSecret || config.clientSecret,
   callbackURL: process.env.callbackURL || config.callbackURL
  }, getUser)
);