jQuery method to test if any part of a given element is visible in the viewport.

isonscreen.js

/**
 * jQuery method to test if any part of a given element is visible in the viewport.
 * Source: http://upshots.org/javascript/jquery-test-if-element-is-in-viewport-visible-on-screen
 **/

$.fn.isOnScreen = function () {
  var win = $(window);  

  var viewport = {
    top: win.scrollTop(),
    left: win.scrollLeft()
  };
  viewport.right = viewport.left + win.width();
  viewport.bottom = viewport.top + win.height();
  
  var bounds = this.offset();
  bounds.right = bounds.left + this.outerWidth();
  bounds.bottom = bounds.top + this.outerHeight();
  
  return (!(viewport.right < bounds.left || viewport.left > bounds.right ||
      viewport.bottom < bounds.top || viewport.top > bounds.bottom));
};