index.js

const path = require('path')
const fs = require('fs')

module.exports = function (dir, callback) {
  fs.readdir(dir, (err, files) => {
    if (err) return callback(err)

    function checkRandom () {
      if (!files.length) {
        // callback with an empty string to indicate there are no files
        return callback(null, undefined)
      }
      const randomIndex = Math.floor(Math.random() * files.length)
      const file = files[randomIndex]
      fs.stat(path.join(dir, file), (err, stats) => {
        if (err) return callback(err)
        if (stats.isFile()) {
          return callback(null, file)
        }
        // remove this file from the array because for some reason it's not a file
        files.splice(randomIndex, 1)

        // try another random one
        checkRandom()
      })
    }
    checkRandom()
  })
}