Express and node send error messages to handlebars view - node.js

Question

How do I output validation errors to the view for the email unique: true option? In my handlebars view I am getting errors passed from var errors = req.validationErrors(); and displaying it in the view which is working. but the email validation for uniqueness is going to user.save((err) and is just being sent to the console. The validation is working correctly, no user is created if there is a duplicate email. I'm just trying to send the error message to the view. Can I do a req.check for unique: true ?
const UserSchema = new Schema({
email: { type: String, index: { unique: true } },
password: { type: String, required: true }
});
my createUser.handlebars view
{{#if errors}}
<section class="errors">
<ul>
{{#each errors}}
<li>{{this.msg}}</li>
{{/each}}
</ul>
</section>
{{/if}}
the createUser function
module.exports.createUser =
(req, res, next) => {
let user = new User({
email: req.body.email,
password: req.body.password
});
req.check('email', 'Invalid email').isEmail();
req.check('password', "Passwords must match").isLength({min: 6}).equals(req.body.passwordConfirmation);
var errors = req.validationErrors(); // save any error messages
// check if there are any errors
if (errors) {
req.session.errors = errors;
req.session.success = false;
return res.redirect('/users/new');
} else {
req.session.success = true;
}
// save the user
user.save((err) => {
if (err) {
console.log("Error : %s ", err);
}
// set the session messages back to null and redirect
req.session.success = null;
req.session.errors = null;
res.redirect('/');
});
};

Answer 1

If I'm not wrong, you're using express-validator for validation. The issue is you're mixing two things: form validation and database constraints.
Uniqueness is not an intrinsic property of a data item and it cannot be checked independently of the entire set.
You can only check for uniqueness using a call to the database. If your database schema defines this attribute/column as unique, then trying to store it will throw an error (at least in SQL-based databases and MongoDB).

Answer 2

You can create your custom validation check like this
check('email').custom(value => {
return userController.findByEmail(value).then( user => {
if(user){
return Promise.reject('this email already exists')
}
})
})
i write this in Router and define function in Controller
async findByEmail(email){
const candidate = await User.findOne({ where: {username}})
if(candidate){
return true
}else{
return false
}
}