login-example-co.js

var co = require('co')

app.post('/',function (req, res, next) {
    var body = req.body;

    if (!body.name || !body.email || !body.password) {
        return res.status(400).send("Missing a username or email or password")
    };

    co(function *(){
        var user = yield User.findOneAndUpdate({
            email: body.email 
        }, {
            $set: {password: body.password}
        });
        if (!user) {
            user = yield User.create(body);
        }
        res.send({ user: body })
    }).catch((err) => {
        if (err && err.name === 'ValidationError') {
            return res.status(400).send(err)
        } else if (err) {
            return res.status(500).send(err)
        }
        res.send({ user: user })
    });
});