User.getUser no es una función en Nodejs y Mongodb todo proviene de findOne función

Estoy codificando Nodejs y obtener este problema con la función de autenticación como esta. Todo el problema proviene de la función User.getUser y proviene de findOne. Pero no sé cómo arreglarlo. Además, tengo código con mangosta y jwt. El error es : [ingrese la descripción de la imagen aquí][1] ¿Cómo puedo solucionar con la función findOne. Gracias.

const express = require('express');
const dotenv = require('dotenv');
const morgan = require('morgan');
const bodyParser = require('body-parser');
const User=require('../models/UserModels');
const bcrypt=require('bcrypt');
const mongoose = require('mongoose');
const session = require('express-session');
const MongoStore = require('connect-mongo')(session);
//var passport = require('../server1/database/config/passport');
var passportJWT = require('passport-jwt');
var ExtractJwt = passportJWT.ExtractJwt;
var JwtStrategy = passportJWT.Strategy;
var jwtOptions = {};
var jwt = require('jsonwebtoken');
var config=require('./config/main');
const randtoken = require('rand-token');
//const database=require('../database/index.js')
//const blockchain = require("../blockchain/index");

// jwtOptions.jwtFromRequest = ExtractJwt.fromAuthHeaderAsBearerToken();
jwtOptions.secretOrKey = 'thisisanexamplesecret';

const PORT = process.env.PORT || 3000;
const db = mongoose.connection;

// app.set("view engine", "ejs");
// app.set("views", "./views");
// app.use(express.static("public"));
// app.use(morgan("dev"));
// app.use(bodyParser.json());
// app.use(cookieParser());
// app.use(bodyParser.urlencoded({ extended: true }));


dotenv.config();

//connect to db
mongoose.set('useCreateIndex', true);
mongoose.connect(process.env.DB_URL,{ useNewUrlParser: true, useUnifiedTopology: true }).then(() => console.log("Success."));
db.on('error',(err)=>{
    console.log("Error: "+ err.message);

});

// function thêm mới 1 user
const createUser = async({email, password,card_name,role}) => {
    return await User.create({ email, password,card_name,role });
};

// function lấy ra danh sách users
const Users = async() => {
    return await User.find();
};

// function lấy ra 1 users
const getUser = async obj => {
    return await User.findOne({
        where: obj,
    });
};

function Authentication(req,res,onSuccess){
    var {email,password}=req.body;
    var usermain=User.getUser({email:req.body.email},function(err,result,fields){
        if(err){
            onSuccess({success:false,error:err});
            return;
        }
        if(!result[0]){
            onSuccess({ success: false, error: "Authentication failed. User not found." });
            return;
        }
        else{
            if(email&&password){
                // we get the user with the name and save the resolved promise returned
                var user=User.getUser({email:req.body.email})

                if(!user){
                    res.status(401).json({msg:'No such user found', user});
                    onSuccess({ success: false, error: "Authentication failed. User not found." });
                    return;
                }
                hash=bcrypt.hash(''+password,10);

                if(bcrypt.compare(hash,user.password)){
                    // from now on we’ll identify the user by the id and the id is
                    // the only personalized value that goes into our token

                    var payload={email:user.email};
                    var token=jwt.sign(payload,jwtOptions.secretOrKey);
                    res.json({msg: 'ok', token: token});
                    onSuccess({ success: true, token: token});
                    return;
                }else{
                    res.status(401).json({ msg: 'Password is incorrect' });
                    onSuccess({ success: false, error: "Authentication failed. User not found." });
                    return;
                }
            }
        }
    })
}

module.exports={
    Authentication
};```


  [1]: https://i.stack.imgur.com/94EI0.png