Simple jQuery function that determines whether DOM element is in the browser's viewport.

inViewport.js

// Custom jQuery function.
// Determines if element is withing the viewport.
$.fn.inViewport = function() {
  $el = this;
  var elTop         = $el.offset().top;
  var elBot         = elTop + $el.outerHeight(true);
  var winScrollTop  = $(window).scrollTop();
  var winHeight     = $(window).height();
  var inViewport = winScrollTop <= elBot && (winScrollTop + winHeight) >= elTop;
  return inViewport;
};