loader.html

<script>
  var btn = document.getElementById('myButton');
  btn.addEventListener('click', loadProgress);
  
  function loadProgress() {
    console.log('Button clicked!');
    var progressBar = document.getElementsByClassName('progress-bar')[0];
    var interval = 30;
    var currentValue = 0;
    for (var i = 0; i < 100; i += interval) {
      console.log('Setting interval', i)
      setTimeout(function() {
        currentValue += i;
        console.log('Setting progress width to:', currentValue, '%');
        progressBar.setAttribute('style', 'width: ' + currentValue + '%');
      }, i);
    }
  }
</script>