How to detect screen size using java script and assign to a usable variable?

var viewportwidth = 800, winh;
var viewportheight = 600, winw;
// the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight
if (typeof window.screen.width != 'undefined')
{
      viewportwidth = window.innerWidth
      viewportheight = window.screen.height
}
// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)

else if (typeof document.documentElement != 'undefined'
     && typeof document.documentElement.clientWidth !=
     'undefined' && document.documentElement.clientWidth != 0)
{
       viewportwidth = document.documentElement.clientWidth,
       viewportheight = document.documentElement.clientHeight
}
// older versions of IE
else
{
       viewportwidth = document.getElementsByTagName('body')[0].clientWidth,
       viewportheight = document.getElementsByTagName('body')[0].clientHeight
}
winw = (65/100)*viewportwidth;

now this variable winw can be used as per needs!