javascript - How to calculate the actual screen distance jQuery? -


i want place ad in page column. when scroll down, want ad follow way, position in vertical middle of page. using script right can set distance cannot set exact 50% distance.

i have tired $window.height()/2 or $document.height()/2, calculate table height instead of actual screen height. how solve this? thanks!

<html> <head> <script type='text/javascript' src='http://code.jquery.com/jquery-1.9.1.js'></script> <script type="text/javascript"> $(function () {  var $sidebar = $("#sidebar"),     $window = $(window);  $window.scroll(function () {     if ($window.scrolltop() > $window.height() / 2) {          $sidebar.css('position', 'absolute');         $sidebar.css('top', $window.scrolltop() + $window.height() / 2);         $sidebar.slidedown(500);     } else {         $sidebar.css('position', 'auto');         $sidebar.css('top', 'auto');     } });  }); </script> </head>  <body>  <br><br> <table bgcolor="#cccccc" width="800" height="3000" align="center" border="1"> <tr> <td width="150" valign="top"> <div style="width:100px;height:100;background:white;"></div> <br> <div style="width:100px;height:100;background:blue;"></div> <br> <div style="width:100px;height:100;background:yellow;"></div> <br> <div style="width:100px;height:100;background:pink;"></div> <br> <div style="width:100px;height:100;background:maroon;"></div> <br> <!-- ad --> <div style="width:100px;height:100;background:red;" id="sidebar">test</div>  <br>  </td> <td width="500"></td> <td width="150"></td> </tr> </table>  </body> </html> 

from jquery document, $(window).height(); should return height of browser viewport.

i represent demo based on html , js. looks site :)

see it: http://jsfiddle.net/frfsp/1/


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