console.log('progress at 0 %') // at the top
  // code to update the progress bar

  console.log('progress at 23 %') // Somewhere else
  // code to update the progress bar

var appScript = document.createElement('script')
var xhr = new XMLHttpRequest();
xhr.addEventListener('progress', function (e) {
    var percent = e.loaded / e.total
    console.log('loaded', percent)
    // update loader
})
xhr.addEventListener('load', function () {
    appScript.innerHTML = this.responseText
    document.body.appendChild(appScript)
    // ^ at this point the app javascript will run
})
xhr.open('GET', '/js/app.js')
xhr.send()