vertical scroll position

scroll-pos.js

/**
 * Returns a number between 0 and 100 relative to scroll position.
 * 
 * If target scroll container has top and bottom margins, results may be different
 * than expected.
 * @param {*} scrollContainer target scroll container, default is document.body.
 * @returns number between 0 & 100.
 */
function getVerticalScrollPercentage( scrollContainer = document.body ){
   const p = scrollContainer.parentNode;
   const percentage = (scrollContainer.scrollTop || p.scrollTop) / (p.scrollHeight - p.clientHeight ) * 100;
   return Math.round(percentage);
}