Test:

import javax.script.*;
import jdk.nashorn.api.scripting.JSObject;

public class Main {
  public static void main(String[] args) throws Exception {
     ScriptEngineManager m = new ScriptEngineManager();
     ScriptEngine e = m.getEngineByName("nashorn");
     JSObject jsobj = (JSObject) e.eval("({ foo: 23})");
     jsobj.eval("print(Object.keys(this))");
     jsobj.eval("print(this.foo)");
  }
}

Output expected:
foo
23

Output seen

undefined

Clearly, "global" is passed as "this" to the script evaluated by JSObject.eval.