Function to determine if a jQuery or window element is in view vertically

isInViewVertically.js

// rectangle.bottom shows the absolute position in pixels of the bottom of the rectangle from the top of the entire DOM.
// as you scroll down the page, this number gets smaller, and becomes negative when above the visible window rectangle.height
// shows the height in pixels of the rectangle.
// the difference between them should never exceed the height of the visible window

function isInViewVertically(element) {
  if (element instanceof jQuery) element = element[0]

  var rectangle = element.getBoundingClientRect();
  var windowHeight = window.innerHeight;

  return Math.abs(rectangle.bottom - rectangle.height) <= windowHeight;
}