pass javascript value to Stash variable

[fenster12]

I am wanting to pass a JS value to a Stash variable but the following didn't work. Is this possble?

<script> var w = window.innerWidth; var x = document.getElementById("demo"); x.innerHTML = w ; </script>

{exp:stash:set name="foo" id="demo"}{/exp:stash:set}

{exp:stash:get name="foo"}