is element in viewport

isInViewport

function isElementInViewport (el) {

  //special bonus for those using jQuery
  if (typeof jQuery === "function" && el instanceof jQuery) {
    el = el[0]
  }

  var rect = el.getBoundingClientRect()

  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /*or $(window).height() */
    rect.right <= (window.innerWidth || document.documentElement.clientWidth) /*or $(window).width() */
  )
}