check if element is in viewport

element_in_viewport.js

function elementInViewport2(el) {
    var top = el.offsetTop;
    var left = el.offsetLeft;
    var width = el.offsetWidth;
    var height = el.offsetHeight;

    while(el.offsetParent) {
      el = el.offsetParent;
      top += el.offsetTop;
      left += el.offsetLeft;
    }

    return (
      top < (window.pageYOffset + window.innerHeight) &&
      left < (window.pageXOffset + window.innerWidth) &&
      (top + height) > window.pageYOffset &&
      (left + width) > window.pageXOffset
    );
}