viewportSize.js

var viewportwidth;
var viewportheight;

// the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight

if (typeof window.innerWidth != 'undefined') {
  viewportwidth = window.innerWidth,
  viewportheight = window.innerHeight
}

// 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
}