Smooth Scroll Javascript

var btnAnchor2 =  document.querySelectorAll('main .btn_item a[href*="#"]');
Array.prototype.slice.call(btnAnchor2).forEach(function(ele, index) {
  ele.addEventListener('click', function(e) {
    var headerWrap = document.querySelector('header .primary-section');
    var href = this.getAttribute("href");
    var getHash = href.split('#')[1];
    var elem = document.querySelector("#"+getHash);
    var headerHeight = headerWrap.offsetHeight;
    var newOff =  elem.offsetTop - headerHeight;
    console.log(headerHeight + 'header-height ');
    console.log( newOff +' off');

    smoothScrollBackToTop(e, newOff);
    e.stopPropagation();
    e.preventDefault();
  });
});

function smoothScrollBackToTop(e, targetPos=0) {
  e.preventDefault();
  var targetPosition = targetPos;
  var startPosition = window.pageYOffset;
  var distance = targetPosition - startPosition;
  var duration = 750;
  var start = null;
  window.requestAnimationFrame(step);
  function step(timestamp) {
    if (!start) start = timestamp;
    var progress = timestamp - start;
    window.scrollTo(0, easeInOutCubic(progress, startPosition, distance, duration));
    if (progress < duration) window.requestAnimationFrame(step);
  }
}
function easeInOutCubic(t, b, c, d) {
  t /= d/2;
  if (t < 1) return c/2*t*t*t + b;
  t -= 2;
  return c/2*(t*t*t + 2) + b;
};