IE8 doesn't have window.innerWidth or window.innerHeight So here is two functions to help with that.

inner.js

/**
*	Get window width
**/
var getWindowWidth = function() {
  return window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;	
};

/**
*	Get window height
**/
var getWindowHeight = function() {
  return window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;
};