const server = app.listen();
console.log(`Server listening on: ${server.address().port}`)

// .... 

const start = async () => {
    await config.load();
    await DB.connect(dbOptions);
    router(app);
    app.use((err, req, res, next) => { // error handler
        // ...
    });

    const server = http.createServer(app);
    // proposal:
    return server.listen(process.env.PORT || 3000); // if .listen would be a promise resolving to server
    // or await server.listen(...); return server; else

    // currently I have to do:
    // return new Promise((res, rej) => server.listen(process.env.PORT || 3000, err => err ? rej(err) : res()));

    // or using express's app.listen, which wraps createServer and listen:
    // return new Promise((res, rej) => app.listen(process.env.PORT || 3000, function (err) {
    //      if (err) return rej(err);
    //      res(this) // this is node's http Server
    // }));

    // using util.promisify doesn't look much better:
    // return promisify(server.listen).bind(server)(process.env.PORT || 3000);
    // or
    // return promisify(cb => server.listen(process.env.PORT || 3000, cb))();
};

start()
        .then(server => console.log(`Server listening on: ${server.address().port}`))
        .catch(err => {
           // ...
        });