JavaScript - Test for Element in Viewport

viewport-test.js

// Check to see if an element is in the viewport
  function elementInViewport(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 &&
      left >= window.pageXOffset &&
      (top + height) <= (window.pageYOffset + window.innerHeight) &&
      (left + width) <= (window.pageXOffset + window.innerWidth)
    );
  }