bus.js

var events = require('events');

module.exports = new events.EventEmitter();

client.js

...

socket.on('arduino', function(data) {
	console.log('arduino data has arrived');
});

server.js

var bus = require('./bus');

...

app.get('/arduino',  function(req, res) {
	bus.emit('arduino');
	res.send("OK");
});

var server = http.createServer(app).listen(app.get('port'), function(){
  console.log("Express server listening on port "+ app.get('port') +" in "+ app.get('env') +" mode.");
});

...

bus.on('arduino', function() {
	console.log("arduino");
	io.sockets.emit('arduino');
});