如何从一个文件中导出字符串，并在NODE.js中的另一个文件上要求(）导出的字符串

我的目录结构如下：

collegesapp
├── node_modules
│   ├── express
│   ├── connect
│   ├── jade
│   └── passport
├── routes
│   └── routes.js
├── views
│   ├── index.jade
│   └── signin.jade
├── app.js
└── package.json

exports.serialized_user = serialized_user; //in app.js file *The variable serialized_user returns "Alex"
var serialized_user = require('../app.js').serialized_user; //in routes.js file

因此，当我将console.log(typeof(serialized_user))添加到我的routes.js文件并运行应用程序时，我最终会在控制台中看到undefined而不是"string"；当我将console.log(serialized_user)添加到routes.js文件并运行该应用程序后，我最终在控制台中什么也看不到而不是"Alex"。

你认为我的问题是什么？提前感谢：）

这是我的整个应用程序：

//app.js文件

var express=require（'express'）；

var connect = require('connect');
var passport = require('passport');
var localStrategy = require('passport-local').Strategy;
var mongoose = require('mongoose');
var mongodb = require('mongodb');
var bodyParser = require('body-parser');
var methodOverride = require('method-override');;
var serveStatic = require('serve-static');
var errorHandler = require('errorhandler');
var routes = require('./routes/routes.js');
var user_model = require('./models/user_model.js');
var app = express();
//configuration
app.set('views', __dirname + '/views');
app.set('view engine', 'jade');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: false}));
app.use(methodOverride());
app.use(express.Router());
app.use(serveStatic(__dirname + '/public_assets'));
app.use(passport.initialize());
app.use(passport.session());
if('development' == app.get('env')){
    app.use(errorHandler({dumpExceptions:true,showStack:true}));
}
if('production' == app.get('env')){
    app.use(errorHandler());
}
//user authentication on sign in
passport.use('signin_local_strategy', new localStrategy(
    function(username,password,done){
        user_model.findOne({username: username}, function(err, user){
            if(err)
                {
                    return(done(err));
                }
            if(!user)
                {
                    return(done(null,false));
                }
            if(user.password != password)
                {
                    return(done(null,false));
                }
            return(done(null,user));
        });
    }
));
var serialized_username;
passport.serializeUser(function(user, done){
    done(null, user.id);
    serialized_user = user.username;
    console.log(serialized_user);   //<<--- returns "Alex"
    console.log(typeof(serialized_user)); //<<--- returns string
});
exports.serialized_user = serialized_user;
passport.deserializeUser(function(done,user){
    user_model.findById(function(id, err){
        done(err,user);
    })
});

//routes
app.get('/', routes.indexResponseHandler);
app.get('/signin', routes.signinResponseHandler);
app.get('/signup', routes.signupResponseHandler);
app.get('/profile', routes.userProfileResponseHandler);
app.get('/failure', routes.signinErrorResponseHandler);
app.post('/signin',
    passport.authenticate('signin_local_strategy', { successRedirect:'/profile', 
                                                     failureRedirect: '/failure'})
    );
app.get('/db', routes.dbQuery);
app.listen(3000);

//routes.js

var user_model = require('../models/user_model.js');
var serialized_user = require('../app.js').serialized_user;

exports.indexResponseHandler = function (req, res){
    res.render('index', {title: "College Clubs MN"});
}
exports.signinResponseHandler = function (req, res){
    res.render('signin', {title:"College Clubs MN | Sign in"});
}
exports.signupResponseHandler = function(req, res){
    res.render('signup', {title: "College Clubs MN | Sign Up"});
}
exports.userProfileResponseHandler = function(req, res){
    console.log(typeof((serialized_user))); //logs undefined but was supposed to logstring 
    res.send(typeof((serialized_user))); //<<-- shows undefined but was supposed to return string 
}
exports.signinErrorResponseHandler = function(req, res){
    res.send("Faild to Sign in ! :(");
}

//user_models.js

var mongoose = require('mongoose');
var schema = mongoose.Schema;
mongoose.connect(--);//connected to db 
var userSchema = new schema({
    username : String,
    password : String,
    hometown : String,
    age : Number
}, {collection : 'user'});
var user_model = mongoose.model('user', userSchema);
module.exports = user_model;