callback.js

app.post('/',function (req, res, next) {
    var body = req.body;
    if (!body.name || !body.email || !body.password) {
      return res.status(400).send("Missing username or email or password")
    };
    // case #1- if user was registered (socials platform) before -> just add a password;
    User.findOneAndUpdate({
        email: body.email 
    }, {
        $set: {password: body.password}
    }, (err, user) => {
        if (!user) {
            // case #2 - user has never been registering before -> create new one;
            User.create(body, (err, user) => {              
                if (err && err.name === 'ValidationError') {
                    return res.status(400).send(err)
                } else if (err) {
                    return res.status(500).send(err)
                }
                return res.send({ user: user })
            });
        } else {
            return res.send({ user: body })
        }
    });
});