jQuery filter to detect if a element or part of it is appearing in the window (viewport)

appearing-filter.js

jQuery.expr[':'].appearing = function(elem) {
  var $window = $(window),
      windowViewTop = $window.scrollTop(),
      windowViewBottom = windowViewTop + $window.height(),
      elemTop = $(elem).offset().top,
      elemBottom = elemTop + $(elem).height(),

      isAppearingFully = ((elemTop >= windowViewTop) && (elemBottom <= windowViewBottom)),
      isAppearingBottom = ((windowViewTop > elemTop) && (windowViewTop < elemBottom)),
      isAppearingTop = ((windowViewBottom > elemTop) && (windowViewTop < elemBottom));

  return isAppearingFully || isAppearingBottom || isAppearingTop;
};

A little improvement:

var isAppearingFully = ((elementTop >= windowViewTop) && (elementBottom <= windowViewBottom));

Nice improvement.
Updated :D

There's an error in line 9, it should be
#9: isAppearingFully = ((elemTop >= windowViewTop) && (elemBottom <= windowViewBottom)),

I've fixed it. Thanks @jhuesos