(function() {

    const section = document.querySelectorAll(".section");
    let sections = {};
    let i = 0;

    sectionArray =  Array.from(section);
    sectionArray.forEach(function(element) {
      sections[element.id] = element.offsetTop - 75;
    });

    window.onscroll = function() {
      let scrollPosition = document.documentElement.scrollTop || document.body.scrollTop;

      for (i in sections) {
        if (sections[i] <= scrollPosition) {
          document.querySelector('.active').setAttribute('class', ' ');
          document.querySelector('a[href*=' + i + ']').setAttribute('class', 'active');
        }
      }
    };
})();