I've found that you can only retrieve that using the RequestParameter() AMPScript function.

Here's an example:

<script type="text/javascript" runat="server">

  Platform.Load("core", "1.1.1");  

  // other code

</script>
%%[
  set @oneClickUnsubscribe = RequestParameter("oneClickUnsubscribe")
]%%
<script type="text/javascript" runat="server">

  var oneClickUnsubscribe = Variable.GetValue("@oneClickUnsubscribe");

  // other code

</script>

I've found that you can only retrieve that using the RequestParameter() AMPScript function.

Here's an example:

<script type="text/javascript" runat="server">

Platform.Load("core", "1.1.1");  

// other code
  
</script>
%%[
set @oneClickUnsubscribe = RequestParameter("oneClickUnsubscribe")
]%%
<script type="text/javascript" runat="server">
  
var oneClickUnsubscribe = Variable.GetValue("@oneClickUnsubscribe");
  
// other code

</script>