app.js

var express = require('express')
  , http = require('http')
  , path = require('path');

var app = express();

app.configure(function(){
  app.set('port', process.env.PORT || 3000);
  app.set('views', __dirname + '/views');
  app.set('view engine', 'jade');
  app.use(express.favicon());
  app.use(express.logger('dev'));
  app.use(express.bodyParser());
  app.use(express.methodOverride());

  app.param("hash",function(req, res, next, id){
    req.hash_id = id+2;
    return next();
  });
   
  app.use(function(req, res, next){
    //console.log(req.params.hash); not present
    console.log(req.hash_id); // undefined
    return next();
  });

  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));
});

app.configure('development', function(){
  app.use(express.errorHandler());
});

app.all("/test/:hash",function(req,res){
  console.log(req.params.hash); // original
  console.log(req.hash_id); // edited by params
  res.send("hello world");
});

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