Instance Methods is not working in Sequelize - node.js

Question

I can't seem to set the User model properly. I've implemented some instance methods that don't seem to be working
var UserDetails = mysequelize.sequelize.define('user_tb', {
id: {
autoIncrement: true,
type: mysequelize.sequelize.Sequelize.INTEGER,
allowNull: false,
},
username: {
type: mysequelize.sequelize.Sequelize.STRING,
primaryKey: true,
unique: true,
},
hierarchyid: {
type: mysequelize.sequelize.Sequelize.STRING
},
password: {
type: mysequelize.sequelize.Sequelize.STRING,
validate: {
len: {
args: [6, 15],
msg: "Please enter a password with at least 6 chars but no more than 15"
}
},
allowNull: false
},
customerid: {
type: mysequelize.sequelize.Sequelize.INTEGER
},
statususer: {
type: mysequelize.sequelize.Sequelize.STRING,
allowNull: false
},
userid: {
unique: true,
type: mysequelize.sequelize.Sequelize.STRING,
allowNull: false
},
authtoken: {
unique: true,
type: mysequelize.sequelize.Sequelize.STRING,
allowNull: false
}
},
{
tableName: 'user_tb',
timestamps: false,
freezeTableName: true
});
UserDetails.prototype.toJSON = function(){
var user = this;
var userObject = user.toObject();
return _.pick(userObject,['userid','password']);
};
UserDetails.prototype.findByCredentials = function(userid, password)
{
console.log('Sunddep');
var User = this;
// return User.fin
User.findOne({userid}).then((user)=> {
if(!user)
{
return Promise.reject();
}
return new Promise((resolve,reject) => {
bcrypt.compare(password,user.password,(err,res) => {
if(res)
{
resolve(user);
}
else{
reject();
}
})
})
});
}
UserDetails.prototype.generateAuthToken = function()
{
var user = this;
var access = 'authtoken';
var token = jwt.sign({userid:
user.userid.toHexString(),access},process.env.JWT_SECRET).toString();
user.build({
access: token
});
user.save().then(() =>{
return token;
});
}
module.exports = {UserDetails}
server.js
app.post('/user/login', (req, res) => {
console.log(req.body);
var body = _.pick(req.body, ['userId', 'password']);
user.findByCredentials(body.userId, body.password).then(() => {
res.send('Sundeep');
},
(e) => {
sendData: ({
wsState: '0',
messageCode: 'WS000001',
message: 'No user find with this Id',
userData: []
});
res.status(400).send(sendData)
});
});
Hi, I am getting error while calling instance method from other class. Can any one tell me how can i achieve it
UserDetails.prototype.findByCredentials = function(userid, password) {.....}
But while run the server.js file i getting error like UserDetails.findByCredentials is not a function while calling from other class
Thanks for help in advance.