Is there a client side script equivalent for PHP's $_SERVER['HTTPS']?

I would like to change a url link to https or http depending on which type of protocol is already in a given person's browser addressbar. Is there a way to insert server side code into a netsuite web store page and have it execute much like this statement would?

This is the PHP equivalent of what I want:
Input:
http://domain.com/index/ is open in my browser window.

PHP code:
if(isset($_SERVER['HTTPS']) and $_SERVER['HTTPS'] == 'on'){
$prefix = "https://";
} else {
$prefix = "http://";
}

If there's no easy way to do this on the server-side (by inserting the code directly into the web page in with the html and having it parse correctly), then is there a way to pass this data from Cookies to a server-side script?