Understanding vhost in Express Node.js

Question

I am trying to understand how vhost actually works in Express JS. Here's a working code sample (forgot where I pulled this from):

// -- inside index.js --
var EXPRESS = require('express');
var app = EXPRESS.createServer();

app.use(EXPRESS.vhost('dev.example.com', require('./dev').app));

app.listen(8080);


// -- inside dev.js --
var EXPRESS = require('express');
var app = exports.app = EXPRESS.createServer();

app.get('/', function(req, res)
{
    // Handle request...
});

Now, my question is, why do we call createServer() twice? Why does this even work? Is vhost internally "merging" the two servers together?

Answer 1

Node.js is event-driven, and when a request comes in, the request event is raised on a http.Server. So basically, express.vhost (or really, connect.vhost) is a middleware function which raises the request event on another instance of a http.Server:

function vhost(req, res, next){
    if (!req.headers.host) return next();
    var host = req.headers.host.split(':')[0];
    if (req.subdomains = regexp.exec(host)) {
      req.subdomains = req.subdomains[0].split('.').slice(0, -1);
      server.emit('request', req, res);
    } else {
      next();
    }
  };