Nicepage smoothscroll & back to top

<script>
window.onload = function() {

///////////////////
// smooth scroll //
///////////////////

    document.body.addEventListener('click', e => {

      const href = e.target.href;
      if (!href) return;

      const id = href.split('#').pop();
      const target = document.getElementById(id);

      if (!target) return;
      e.preventDefault();
      history.pushState({}, document.title, href);
      target.scrollIntoView({
        behavior: 'smooth',
        block: 'start'
      });

    });

/////////////////
// back to top //
/////////////////

    var btn = $('#bttbutton');

    $(window).scroll(function() {
      if ($(window).scrollTop() > 300) {
        btn.addClass('show');
      } else {
        btn.removeClass('show');
      }
    });

    btn.on('click', function(e) {
      e.preventDefault();
      $('html, body').animate({scrollTop:0}, '300');
    });


}
</script>