verify jsonwebtoken function

function verifyToken (req, res, next) {
    var token = req.headers.Authorization || req.headers.authorization || "";

    if (!token) {
      res.status(401).json({
        success: false,
        message: "please authenticate."
      });
    } else if (token) {
      jwt.verify(token, process.env.JWT_SECRET, (err, decoded) => {
        if (err) {
          res.status(500).json({
            success: false,
            message: "server error",
            error: err
          });
        } else if (decoded) {
          req.user = decoded;
          next();
        }
      });
    }
  },