Simple and small vanilla js script for animating scroll to top

scroll-top.js

(function() {
  'use strict';
  var toTopLink = document.querySelector('.js-scroll');
  var scrollToTop = function(scrollDuration) {
    var scrollStep = -window.scrollY / (scrollDuration / 15),
        scrollInterval = setInterval(function() {
        if ( window.scrollY !== 0 ) {
            window.scrollBy( 0, scrollStep );
        }
        else {
          clearInterval(scrollInterval);
        } 
    },15);
  };

  // Click event listener for back to top button
  if (toTopLink) {
    toTopLink.addEventListener('click', function(e) {
      e.preventDefault();
      scrollToTop(500);
    });
  }
})();

// EaseInOut
// time < 0.5 ? 4 * time * time * time : (time - 1) * (2 * time - 2) * (2 * time - 2) + 1;