Check if the user is scrolled to the bottom of the page.

scroll-offset.js

window.onscroll = function() {
  var d = document.documentElement;
  var offset = d.scrollTop + window.innerHeight;
  var height = d.offsetHeight;

  console.log('offset = ' + offset);
  console.log('height = ' + height);

  if (offset === height) {
    console.log('At the bottom');
  }
};