A Mocha test to find out what's the problem with Rewire and Express apps.

app.js

var express = require('express');
var app = express();

function sayHello(req, res) {
  res.send(200, 'Hello!');
  res.end();
}

// all environments
app.set('port', process.env.PORT || 3000);
app.use(express.errorHandler());
app.use(app.router);

app.get('/', sayHello);

module.exports = app;

package.json

{
  "name": "rewire-mocha-express",
  "version": "0.0.1",
  "private": true,
  "scripts": {
    "start": "node app.js"
  },
  "dependencies": {
    "express": "3.2.5",
    "jade": "*",
    "superagent": "*",
    "rewire": "*"
  }
}

test-rewire.js

var fs = require('fs')
  , request = require('superagent')
  , assert = require('assert')
  , http = require('http');


describe('App', function() {

  var base = 'http://localhost:3000';

  it('should say Hello', function(done) {
    var app = require('../app.js');
    var server = http.createServer(app);
    server.listen(3000);
    var path = '/';
    request.get(base + path).redirects(0).end(function(res) {
      assert.equal(res.status, 200);
      assert.equal(res.text, 'Hello!');
      server.close();
      done();
    });
  });
  
  it('should say Rewired', function(done) {
    var rewire = require('rewire');
    var app = rewire('../app.js');
    app.__set__('sayHello', function(req, res) { 
      res.send(200, 'Rewired!');
      res.end();
    });
    var server = http.createServer(app);
    server.listen(3000);
    var path = '/';
    request.get(base + path).redirects(0).end(function(res) {
      assert.equal(res.status, 200);
      assert.equal(res.text, 'Rewired!');
      server.close();
      done();
    });
  });
  
});