Check custom protocol exist or not

Question

I have to identify whether custom protocol is installed or not, For that i tried protocolCheck.js and also i tried

try {
      window.location = "custom:";
    } catch (err) {
      if (err.toString().search("NS_ERROR_UNKNOWN_PROTOCOL") != -1) {
        alert("No protocol registered");
   }
 

With this i can able to detect and launch custom protocol but My problem is, I have to only detect whether custom protocol is register or not. I don't want to launch or don't want to appear browser dialog.

Can anyone help me to get this.