Create my own session store for Express 4.

app.js

var express = require('express');
var cookieParser = require('cookie-parser');
var session = require('express-session');
var MyMemoryStore = require('./MyMemoryStore')(session);
var app = express();

...

app.use(cookieParser());
app.use(session({
  store: new MyMemoryStore({ debug: true }),
  secret: process.env.SESSION_SECRET || 'session secret'
}));

...

MyMemoryStore.js

module.exports = function (session) {
  var Store = session.Store;

  function MyMemoryStore(options) {
    Store.call(this, options || {});
    this.debug = options && options.debug;
    this.sessions = {};
  }

  MyMemoryStore.prototype.__proto__ = Store.prototype;

  MyMemoryStore.prototype.get = function (sid, fn) {
    var sess = this.sessions[sid];
    if (this.debug) console.log('MyMemoryStore.get: sid = ' + sid + ', sess = ' + sess);
    if (sess) {
      sess = JSON.parse(sess);
      setImmediate(fn, null, sess);
    } else {
      setImmediate(fn);
    }
  }

  MyMemoryStore.prototype.set = function (sid, sess, fn) {
    sess = JSON.stringify(sess);
    if (this.debug) console.log('MyMemoryStore.set: sid = ' + sid + ', sess = ' + sess);
    this.sessions[sid] = sess;
    fn && setImmediate(fn);
  }

  MyMemoryStore.prototype.destroy = function (sid, fn) {
    if (this.debug) console.log('MyMemoryStore.destroy: sid = ' + sid);
    delete this.sessions[sid];
    fn && setImmediate(fn);
  }

  return MyMemoryStore;
};