xhr.addEventListener('load', function(){
  var result = JSON.parse(xhr.responseText);
  var resultDiv = document.querySelector(".result");

  if(result.email) {
    // if email exists, redirect to /main page (login successful)
    window.location.href="/main";
  }
  // if no result, print message and link to Sign up
  else if (xhr.status === 401) resultDiv.innerHTML = result +
    "<div class='signup'><a href='/join'>Sign up</a></div>"
  else resultDiv.innerHTML = result;
  console.log(xhr.responseText);

router.get('/', function(req,res){
  console.log('main.js is loaded', req.user)
  // onle can access when session value exists (login)
  var id = req.user;
  // if no id value, redirect to login page.
  if(!id) res.render('login.ejs');
  // if id value exists, render main.ejs page.
  res.render('main.ejs', {'id' : id})
});

<div><a href="/logout">logout</a></div>

var logout = require('./logout/index')

router.use('/logout',logout)

var express = require('express')
var app = express()
var router = express.Router();

router.get('/', function(req, res){
  // session expire
  req.logout();
  // redirect to login page
  res.redirect('/login');
});

module.exports = router;