Reading progress bar

Reading progress bar

if ($('.post').length) {
    let winHeight = $(window).height(),
      docHeight = $('.post').height(),
      postOffset = $('.post').offset().top,
      progressBar = $('.progress-container'),
      max, value;

    max = docHeight - winHeight;
    progressBar.attr('max', max);

    $(document).on('scroll', function() {
      value = $(window).scrollTop() - postOffset;
      if (value >= 0) {
        progressBar.show();
        let width = (value / max) * 100;
        width = width + '%';
        $('.progress-bar').css({'width': width});
      } else {
        progressBar.hide();
      }
    });
  }