Boa tarde!

Segui os passos descritos, mas estou encontrando o seguinte erro:

TypeError: Cannot read property 'emit' of undefined
    at C:\node\casadocodigo\app\routes\promocoes.js:15:22
    at Layer.handle [as handle_request] (C:\node\casadocodigo\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\node\casadocodigo\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (C:\node\casadocodigo\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (C:\node\casadocodigo\node_modules\express\lib\router\layer.js:95:5)
    at C:\node\casadocodigo\node_modules\express\lib\router\index.js:281:22
    at Function.process_params (C:\node\casadocodigo\node_modules\express\lib\router\index.js:335:12)
    at next (C:\node\casadocodigo\node_modules\express\lib\router\index.js:275:10)
    at C:\node\casadocodigo\node_modules\express-validator\lib\express_validator.js:250:5
    at Layer.handle [as handle_request] (C:\node\casadocodigo\node_modules\express\lib\router\layer.js:95:5)

app/views/home/index.ejs

</footer>
<script src="/socket.io/socket.io.js"></script>
<script>
    const socket = io();

    socket.on('novaPromocao', function(data){
        alert('Livro em promoção ' + data.livro.id);
    });
</script>
</body>
</html>

app/routes/promocoes.js

module.exports = function(app){
    app.get('/promocoes', function(req, res){
        var connection = app.infra.connectionFactory();
        var produtosDAO = new app.infra.produtosDAO(connection);
        produtosDAO.lista(function(err, results){
            res.render('promocoes/form',{
                lista:results
            });
        });
        connection.end();
    });

    app.post('/promocoes',function(req, res){
        var promocao = req.body;
        app.get('io').emit('novaPromocao', promocao);
        res.redirect('promocoes');
    });
}

app.js

var app = require('./config/express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.set('io'.io);

app.listen(3000, function(){
    console.log('Servidor rodando');
})

Obrigado.