How to get the root domain on which cookies can be written

getRootDomain.js

function getRootDomain() {
        var split = window.location.hostname.split('.');
        var position = split.length - 1;
        while (position >= 0) {
                var currentDomain = split.slice(position, split.length).join('.');
                document.cookie = 'unique=unique;domain=' + currentDomain + ';';
                if (document.cookie.indexOf('unique') > -1) {
                        return currentDomain;
                }
                position -= 1;
        }
        // Cookies cannot be read
        return window.location.hostname;
}