user login functiom

const User = require("../models/User");
const bcrypt = require('bcrypt');
const jwtAuth = require('../utils/jwtAuth');

function loginUser (req, res){
    User.findOne({
        email: req.body.email
      })
      .exec((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 does not exist."
          });
        } else if (user) {
          const plainPassword = req.body.password;

          bcrypt.compare(plainPassword, user.password, (err, match) => {
            if (err) {
              res.status(400).json({
                success: false,
                message: "bcrypt password compare error",
                error: err
              });
            } else if (match) {
              const token = jwtAuth.createToken(user._id, process.env.JWT_SECRET);
              user.password = undefined;

              res.status(200).json({
                success: true,
                message: "user login successfull :)",
                user,
                token
              });
            } else if (!match) {
              res.status(400).json({
                success: false,
                message: "invalid password",
              });
            }
          });
        }
      })
  },