Callback function not returning?

Question

I'm learning Node.js and I'm trying to make a site where people can review movies. Everything works so I'm adding a feature that grabs the poster. I have this callback setup but I can't seem to figure out why it isn't working. It logs to the console fine, but nothing returns to the server.

function getPoster(title) {
    var dataret;
    imdbId(title, function (err, imdb_id) {
        if (!err) {
            console.log(imdb_id);
            imdb(imdb_id, (err, data) => {
                if (err) {
                    console.log(err.stack);
                }
                if (data) {
                    console.log(data);
                }
                dataret = data;
            });
        }
    });
    return dataret;
}

app.get('/getposter/:id', function (req, res) {
    // IT WORKS UP TO HERE
    res.setHeader('Content-Type', 'application/json');
    res.status(200).send({ json: JSON.stringify(getPoster(req.params.id)) });
});