Gist for the article where we take a look at PassportJS and how easy it is to use the it's Facebook Strategy to authenticate Users for your app.

passpostJS-FacebookStrategy.js

const passport = require('passport');
const Strategy = require('passport-facebook').Strategy;
require('dotenv').config();

// Configure Passport authenticated session persistence.
passport.serializeUser(function(user, cb) {
  cb(null, user);
});
    
passport.deserializeUser(function(obj, cb) {
  cb(null, obj);
});

  
// Configure the Facebook strategy for use by Passport.
passport.use(new Strategy({
    clientID: process.env['FACEBOOK_CLIENT_ID'],
    clientSecret: process.env['FACEBOOK_CLIENT_SECRET'],
    callbackURL: 'http://localhost:3000/user/return',
    profile: ['id', 'displayName']
  },
  function(accessToken, refreshToken, profile, done) {
    //Check the DB to find a User with the profile.id
    User.findOne({ facebook_id: profile.id }, function(err, user) {
      if(err) {
        console.log(err);  // handle errors!
      }
      
      if (user) {
        done(null, user); //Login if User already exists
      } else { //else create a new User
        user = new User({
          facebook_id: profile.id, //pass in the id and displayName params from Facebook
          name: profile.displayName
        });
        user.save(function(err) { //Save User if there are no errors else redirect to login.
          if(err) {
            console.log(err);  // handle errors!
          } else {
            console.log("saving user ...");
            done(null, user);
          }
        });
      }
    });
  }
));

// Initialize Passport and restore authentication state, if any, from the
// session.
router.use(passport.initialize());
router.use(passport.session());

// Define routes.
router.get('/home',
  (req, res) => {
    res.json({ user: user });
  });

router.get('/login', (req, res) => { res.json({msg: "login failed"}); });

router.get('/login/facebook', passport.authenticate('facebook'));

router.get('/return', 
  passport.authenticate('facebook', { failureRedirect: '/login' }),
  function(req, res) {
    res.redirect('/api/v1/user/home');
});