Using bcrypt on Node.js results in validation failed - node.js

Question

I am writing code in Node.js to encrypt passwords using bcrypt.
However, if you use bcrypt, you will get an ValidationError: User validation failed: password: Cast to String failed for value "Promise { <pending> }" at path "password"
I do not get this error if I save it as plain text without encryption.
Is there a secret of bcrypt I do not know?
bcrypt (not working)
const bcrypt = require('bcrypt');
sign_up = (req, res, next) => {
const { email, password } = req.body;
const User = User.findOne({ email: email });
if (exUser) {
return res.send('exist user');
}
const hash = bcrypt.hash(password, 8);
const user = new User({
email: email,
password: hash
});
user.save((err) => {
if (err) {
return next(err);
}
res.send('signup success');
});
};
no bcrypt (working)
sign_up = (req, res, next) => {
const { email, password } = req.body;
const User = User.findOne({ email: email });
if (exUser) {
return res.send('exist user');
}
const user = new User({
email: email,
password: password
});
user.save((err) => {
if (err) {
return next(err);
}
res.send('signup success');
});
};

Answer 1

To elaborate on Chris's comment:
It appears that bcrypt.hash is asynchronous, and is returning a Promise.
To fix this, I would recommend using an async function and awaiting the result. MDN page
This may require a newer version of NodeJS than what you are running.
const bcrypt = require('bcrypt');
// Async function allows us to use await
sign_up = async (req, res, next) => {
const { email, password } = req.body;
const User = User.findOne({ email: email });
if (exUser) {
return res.send('exist user');
}
// We await the result of the hash function
const hash = await bcrypt.hash(password, 8);
const user = new User({
email: email,
password: hash
});
user.save((err) => {
if (err) {
return next(err);
}
res.send('signup success');
});
};
Do not use the bcrypt.hashSync function, as while it is running your server will not be able to do anything else.