gistfile1.txt

// Sign in with Twitter.
passport.use(
  new TwitterStrategy(
    secrets.twitter, function(req, accessToken, tokenSecret, profile, done) {
      if (req.user) {
        User.findOne({ twitter: profile.id }, function(err, existingUser) {
          if (err) { return done(err); }

          if (existingUser) {
            req.flash('errors', {
              msg: [
                'There is already a Twitter account that belongs to you. ',
                'Sign in with that account or delete it, then link it with ',
                'your current account.'
              ].join('')
            });
            done();
          } else {
            User.findById(req.user.id, function(err, user) {
              if (err) { return done(err); }

              user.twitter = profile.id;
              user.tokens.push({
                kind: 'twitter',
                accessToken: accessToken,
                tokenSecret: tokenSecret
              });
              user.profile.name = user.profile.name || profile.displayName;
              user.profile.username = user.profile.username || profile.username;

              user.profile.location =
                user.profile.location || profile._json.location;

              user.profile.picture =
                user.profile.picture || profile._json.profile_image_url_https;

              user.profile.username = profile.username;
              user.save(function(err) {
                req.flash('info', { msg: 'Twitter account has been linked.' });
                done(null, user);
              });
            });
          }
        });

      } else {
        User.findOne({ twitter: profile.id }, function(err, existingUser) {
          if (err) { return done(err); }
          var user = existingUser || new User();
          if (req.user.id == user.id) {
            user.twitter = profile.id;
            user.email = user.email || '';
            user.tokens.push({
              kind: 'twitter',
              accessToken: accessToken,
              tokenSecret: tokenSecret
            });
            user.profile.name = user.profile.name || profile.displayName;
            user.profile.username = user.profile.username || profile.username;

            user.profile.location =
                user.profile.location || profile._json.location;
            user.profile.picture =
                user.profile.picture || profile._json.profile_image_url_https;

            user.save(function (err) {
              if (err) {
                return done(err);
              }
              done(null, user);
            });
          } else {
            { return done("Login failed - please try again"); }
          }
        });
      }
    })
);