scroll.js

scroll.js

function currentYPosition() {
  if (self.pageYOffset) return self.pageYOffset;
  if (document.documentElement && document.documentElement.scrollTop) return document.documentElement.scrollTop;
  if (document.body.scrollTop) return document.body.scrollTop;
  return 0;
}

function elmYPosition(eID) {
  let elm = document.getElementById(eID);
  let y = elm.offsetTop;
  let node = elm;
  while (node.offsetParent && node.offsetParent != document.body) {
    node = node.offsetParent;
    y += node.offsetTop;
  }
  return y;
}

export default function smoothScroll(eID) {
  let startY = currentYPosition();
  let stopY = elmYPosition(eID);
  let distance = stopY > startY ? stopY - startY : startY - stopY;
  let speed = Math.round(distance / 10);
  let speedTimeout = 250;
  if (speed >= 100) speed = 100;
  let step = Math.round(distance / 25);
  let leapY = stopY > startY ? startY + step : startY - step;
  let timer = 0;
  if (stopY > startY) {
    for (let i = startY; i < stopY; i += step) {
      setTimeout('window.scrollTo(0, ' + leapY + ')', timer * speed);
      leapY += step;
      if (leapY > stopY) leapY = stopY;
      timer++;
    }
    return;
  }
  for (let i = startY; i > stopY; i -= step) {
    setTimeout('window.scrollTo(0, ' + (leapY) + ')', timer * speed);
    leapY -= step;
    if (leapY < stopY){
      leapY = stopY;
    } 
    timer++;
  }
}