index.js

var GAP = 10;
// [TODO]: DRY
var scrollToTop = function(scrollDuration) {
  var scrollHeight = window.pageYOffset;
  var scrollStep = Math.PI / (scrollDuration / 15);
  var cosParameter = scrollHeight / 2;
  var scrollCount = 0;
  var scrollMargin;
  var scrollPosition;

  var scrollInterval = setInterval(function() {
    if (window.pageYOffset != 0) {
      scrollCount = scrollCount + 1;
      scrollMargin =
        cosParameter - cosParameter * Math.cos(scrollCount * scrollStep);

      if (scrollHeight - scrollMargin > scrollPosition) {
        clearInterval(scrollInterval);
        scrollToTop(scrollDuration / scrollCount);
      }

      scrollPosition = scrollHeight - scrollMargin;

      window.scrollTo(0, scrollPosition);
    } else {
      clearInterval(scrollInterval);
    }
  }, 15);
};

var scrollToElement = function(element, interval, options) {
  var offset = getElementOffset(element);
  var initialOffset = window.pageYOffset;

  var scrollHeight = offset - initialOffset;

  var scrollStep = Math.PI / (interval / 15);
  var cosParameter = scrollHeight / 2;
  var scrollCount = 0;
  var scrollMargin;
  var previousScrollMargin;
  var signChanged = false;
  var gap = options && options.gap ? options.gap : GAP;

  var scrollInterval = setInterval(function() {
    // Elelement we want to scroll to below current scroll position
    if (
      (window.pageYOffset < offset - gap || window.pageYOffset > offset) &&
      !signChanged
    ) {
      scrollCount = scrollCount + 1;
      scrollMargin =
        cosParameter - cosParameter * Math.cos(scrollCount * scrollStep);

      if (previousScrollMargin) {
        signChanged = Math.abs(scrollMargin) < Math.abs(previousScrollMargin);
      }

      window.scrollTo(0, initialOffset + scrollMargin - gap);
      previousScrollMargin = scrollMargin;
    } else {
      clearInterval(scrollInterval);
    }
  }, 15);
};

var getElementOffset = function(element) {
  return window.pageYOffset + element.getBoundingClientRect().top;
};

module.exports = {
  scrollToTop: scrollToTop,
  scrollTo: scrollToElement
};