NodeJS Express enumerate files in a folder

Sorry, I'm new with NodeJS Express. I want to get all files from a folder. I tried the following but don't work, test is always empty string:

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

router.get('/', function(req, res, next) {
    var test = '';

    const testFolder = './public/images';

    fs.readdir(testFolder, (err, files) => {
      test = files[0];
    })

    res.render('index', { title: 'Express', file: test });
});

Basically, I want to get all filenames from public images folder.

via jomsk1e