Connect flash where is the session object from

I am using in my express app the message alert middleware connect-flash. I can find this middleware on github https://github.com/jaredhanson/connect-flash.
When i look at connect-flash source code, i really don't know, where the this.session object come from. Consider the connect-flash source code:

module.exports = function flash(options) {
  options = options || {};
  var safe = (options.unsafe === undefined) ? true : !options.unsafe;

  return function(req, res, next) {
    if (req.flash && safe) { return next(); }
    req.flash = _flash;
    next();
  }
}

function _flash(type, msg) {
  if (this.session === undefined) throw Error('req.flash() requires sessions');
  var msgs = this.session.flash = this.session.flash || {};
  if (type && msg) {
    // util.format is available in Node.js 0.6+
    if (arguments.length > 2 && format) {
      var args = Array.prototype.slice.call(arguments, 1);
      msg = format.apply(undefined, args);
    } else if (isArray(msg)) {
      msg.forEach(function(val){
        (msgs[type] = msgs[type] || []).push(val);
      });
      return msgs[type].length;
    }
    return (msgs[type] = msgs[type] || []).push(msg);
  } else if (type) {
    var arr = msgs[type];
    delete msgs[type];
    return arr || [];
  } else {
    this.session.flash = {};
    return msgs;
  }
}

To implement in express, i have to include within the app.configure block. Consider the code

app.configure(function () {
        //Other middleware
        app.use(function (req, res, next) {
            console.log(this.session);
            next();
        });
        app.use(flash());

Look at my custom middleware, when i display this.session object, i got "undefined". Why is this.session in connect-flash working, i get overthere the session object, but not in my middleware. The callback pattern for create a middleware are exactly the same

(function (req, res, next) {
        //Code
         next();
}

The session object is added by the session middleware. If req.session is undefined you either don't have the session middleware defined or you defined it after the middleware where you are expecting it.

Defined

• session middleware
• custom middleware: req.session is defined

Undefined

• custom middleware: req.session is undefined because the object gets added later.
• session middleware

So my guess is that you defined the flash middleware after express.session but your custom middleware is defined before express.session.

Since express is just an array of functions (middleware) that does stuff to the request and returns a response means that the order in which you define those functions matters.