<!--First the html-->
<script type="text/javascript">
function updateProgress(percentage) {
document.getElementById('ProgressBar').style.width = percentage+"%";
}
</script>
<!--Define your function or call it from a .js file-->
<!--On the C# Code behind-->
string updateProgress = "18%";
ClientScript.RegisterStartupScript(this.GetType(), "updateProgress", "updateProgress('" + updateProgress + "');", true);
<!--You can pass C# variables as parameters to the js function this way: '" + param + "'-->