I want to retrieve the SubscriberKey from my list of subscribers

But I'm not retrieving anything

follow the code :

    <script runat="server" language="javascript">
Platform.Load("Core","1");
 var jsonpost = 'SubscriberKey: obj[cols[0]], Lists: [{ ID: xxxx,Status: "Active" }]'

   var json = Platform.Function.ParseJSON(jsonpost)

Write("<br>jsonpost: "+jsonpost);
Write("<br>parsed and stringified json: "+Stringify(json));
  
var SubscriberKey = jsonpost.SubscriberKey[0].obj

Write("<br>SubscriberKey: "+ json.SubscriberKey[0].obj)



</script>

web page response :

jsonpost: SubscriberKey: obj[cols[0]], Lists: [{ ID: xxxx,Status: "Active" }]
parsed and stringified json: "SubscriberKey: obj[cols[0]], Lists: [{ ID: xxxx,Status: \"Active\" }]"
SubscriberKey: undefined

What's wrong in my code ? Thanks in advance