Question [Help] How can I show an exit popup in a specific case?

Hi, I want to show an exit popup by using onbeforeunload like below.

Code (JavaScript):

1. <script>
2.     window.onbeforeunload = function () {
3.         if (specific case) return "show popup"; // show popup and then close window.
4.         else return "";  // just close window
5.     }
6. </script>

But the specific case what I want is a global variable(bool) on a C# script (This script is a singleton instance).
How can I get this C# script variable state in the index.html?

 

I solved this problem after reading this thread.
https://forum.unity.com/threads/call-javascript-function-in-index-html-from-webgl.625480/

I replaced the cs global variable into my index.html var.
And when I want to change a state of this variable, I called a .jslib method at a c# script.