Getting the width of the current window

Does anyone know how to find the width of the browser window that the page is in. i.e., when a page is loaded I want to intialise a variable with the value of the width of the windowit's in.

Thanks

<script type="text/javascript">
window.onload = function()
{
var iWidth = (document.all)?document.body.clientWidth:window.innerWidth;

alert(iWidth);
}
</script>

Thanks.