How Do I Get innerWidth in Internet explorer 8

The innerWidth is supported by IE9 not IE8, you can do this insteaad:

var width = window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;

The above line will get you the width from IE as well as other standard-compliant browsers.

If you use jQuery, $(window).innerWidth() will give you desired result in all browsers too.