midi -> nodejs -> socket.io -> browser

index.html

<script src="/socket.io/socket.io.js"></script>
<script>
var socket = io.connect('http://localhost');
socket.on('midi', function (data) {
    console.log(data);
});
</script>

index.js

var app = require('express')()
    server = require('http').createServer(app),
    io = require('socket.io').listen(server),
    midi = require('midi'),
    input = new midi.input();

input.on('message', function(deltaTime, message) {
    console.log('m:' + message + ' d:' + deltaTime);
    io.sockets.emit('midi', message); 
}); 

input.openPort(0);

server.listen(8000);

app.get('/', function (req, res) {
  res.sendfile(__dirname + '/index.html');
});