Call ServerSide function from Javascript

aspx file:

<script type="text/javascript"> function firePostBack() {  <%= ClientScript.GetPostBackEventReference(this, "CallServerSideFunction1") %>; } </script>

aspx.cs file:

private void Page_Load(object sender, System.EventArgs e)
{
 if ( this.IsPostBack )
 {
  string eventArgument = (this.Request["__EVENTARGUMENT"] == null) ? string.Empty : this.Request["__EVENTARGUMENT"];

  if ( eventArgument == "CallServerSideFunction1" )
   ServerSideFunction1();
 
 }
}
private void ServerSideFunction1()
{
 this.Response.Write("ServerSideFunction1 called");
}