I am trying to save different ID per tab and getting different errors and I don't know the cause

Question

I am trying to save information on ViewState to have different ID per tab but I am getting this error and can't figure out what to be the cause of it for a few days now. When I try to run it without debugging Compiler Error Message: CS1026:) expected.

And on the other hand when I do try to debug it is giving me another error CS1525 Invalid expression term ';'. please could anyone help me figure out what may be the cause this error?

var makat;
window.onload = () => {
<% 
if(ViewState["IsPostBack"] != null && ViewState["IsPostBack"].ToString() != "true") 
{
ViewState["IsPostBack"] = "true";
ViewState["makat"] = %> sessionStorage.getItem("latestMakat") <% ;
} %>

makat = <%=ViewState["makat"].ToString();
%> }