server.js

app.post('/oauth/signin', [
    oauthLogger,
    jsonParser,
    function(req, res) {
        if(!req.body) {
            res.status(400).send({
                error: {
                    status_code: 400,
                    status: 'Bad Request',
                    message: 'Missing required body'
                }
            }).end();

            return;
        }

        if(!req.body.provider || !req.body.code) {
            res.status(400).send({
                error: {
                    status_code: 400,
                    status: 'Bad Request',
                    message: 'Missing required parameter'
                }
            }).end();

            return;
        }

        OAuth.auth(req.body.provider, req.session, {
            code: req.body.code
        })
        .then(function(requestObject) {
            return requestObject.me();
        })
        .then(function(me) {
            if(!me.raw.id) {
                res.status(500).send({
                    error: {
                        status_code: 500,
                        status: 'Internal Server Error',
                        message: 'Missing required id'
                    }
                }).end();

                return;
            }

            req.session.oauth.provider = req.body.provider;
            req.session.oauth.id = me.raw.id;

            res.status(200).send({
                email: me.email,
                avatar: me.avatar
            }).end();
        })
        .fail(function(err) {
            res.status(503).send({
                error: {
                    status_code: 503,
                    status: 'Service Unavailable',
                    message: err
                }
            }).end();
        });
    }
]);