smoothScroll.js

function smoothScroll (scroller, endY) {
  if (typeof scroller === 'number') {
    endY = scroller;
    scroller = window;
  }
  const startY = scroller === window ? scroller.pageYOffset : scroller.scrollTop;
  const maxY = (scroller === window) ?
    document.documentElement.scrollHeight - document.documentElement.clientHeight : 
    scroller.scrollHeight - scroller.clientHeight;
  endY = Math.min(endY, maxY);
  const dir = (endY - startY) / Math.abs(endY - startY);

  const setY = scroller === window ? function (y) { scroller.scrollTo(0, y) } : function (y) { scroller.scrollTop = y }

  let y = startY

  const doScroll = function () {
    const distance = Math.abs(endY - y);
    if ( distance > 1.5 ) {
      y += dir * Math.ceil(Math.min( distance / 3, 2000 ));
      setY(y);
      requestAnimationFrame(doScroll);
    } else {
      setY(endY);
    }
  }
  requestAnimationFrame(doScroll);
}