Figure this out

/**
 * Registers a function used to serialize user objects into the session.
 *
 * Examples:
 *
 *     passport.serializeUser(function(user, done) {
 *       done(null, user.id);
 *     });
 *
 * @api public
 */
Authenticator.prototype.serializeUser = function(fn, req, done) {
  if (typeof fn === 'function') {
    return this._serializers.push(fn);
  }

  // private implementation that traverses the chain of serializers, attempting
  // to serialize a user
  var user = fn;

  // For backwards compatibility
  if (typeof req === 'function') {
    done = req;
    req = undefined;
  }

  var stack = this._serializers;
  (function pass(i, err, obj) {
    // serializers use 'pass' as an error to skip processing
    if ('pass' === err) {
      err = undefined;
    }
    // an error or serialized object was obtained, done
    if (err || obj || obj === 0) { return done(err, obj); }

    var layer = stack[i];
    if (!layer) {
      return done(new Error('Failed to serialize user into session'));
    }


    function serialized(e, o) {
      pass(i + 1, e, o);
    }

    try {
      var arity = layer.length;
      if (arity == 3) {
        layer(req, user, serialized);
      } else {
        layer(user, serialized);
      }
    } catch(e) {
      return done(e);
    }
  })(0);
};