efficient-update.js

function completedChallenge(req, res, next) {
  // build user update data, no longer mutates user object
  const { updateData, alreadyCompleted, points } = buildUpdateData(req);
  // calls an update data using user instance id and sends only updateData
  return user.update$(updateData)
    .subscribe(
      count => log('%s documents updated', count),
      next,
      function() {
        return res.json({
          points,
          alreadyCompleted
        });
      }
    );
  }
}