Change url in browser and render tempate by express js

my question is how to update ejs template and url path when we have GETrequest from the client. Now I'm doing it in that way:

Server routing:

router.get('/', function(req, res, next) {
  res.redirect('/cat/');
});

router.get('/cat', function(req, res, next) {
  var files = require('../lib/treeBuilder').renderDir('./');
  res.render('index', { title: 'FileBrowser', dir: '/', files: files });
});


router.get('/cat/:file*', function(req, res, next) {
  var path = req.path.slice(4);//get path without 'cat'
  console.log('IN "/:file" >' + path);
  var files = require('../lib/treeBuilder').renderDir('./' + path);
  res.render('index', { title: 'FileBrowser', dir: path, files: files });
});

Client js

_sendXMLHTTPRequest(folder) {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', `${window.location.pathname}${folder}`, true);

    xhr.onreadystatechange = function() {
      if (this.readyState != 4) return;

      if (this.status != 200){
        return alert('Error xhr!');
      }
    }
    //xhr.send();//it must be correct way
    window.location.href += `${folder}/`;// it's my way
  }

I'm doing something like file browser - we have some list of dirs on /cat/ page - user clicks on one of them - it sends a request to server to show it. Click on lib dir - go to cat/lib/ - render template.

As you can see, I manually change window.location.href instead of make xhr.send() - it's all because of xhr.send() doesen't changes URL, so my EJS template doesen't update. Thanks for help.