node js verifyAdmin function

function isAdmin (req, res, next) {
    const id = req.user.userId;

    User.findOne({
      _id: id
    }, (err, user) => {
      if (err) {
        res.status(500).json({
          success: false,
          message: "server error",
          error: err
        });
      } else if (!user) {
        res.status(400).json({
          success: false,
          message: "user not found",
          error: err
        });
      } else if (!user.isAdmin) {
        res.status(403).json({
          success: false,
          message: "unauthorized",
          error: err
        });
      } else if (user && user.isAdmin) {
        req.user.isAdmin = user.isAdmin;
        next();
      }
    });
  }