Extract device ID from Google Analytics _ga cookie

gistfile1.js

/**
 * Get a browser cookie
 *
 * @param string cname = The cookie name
 * @return string = The cookie value or an empty string ("") if the cookie doesn't exist
 */
getCookie = function(cname) {
    var name = cname + "=";
    var decodedCookie = decodeURIComponent(document.cookie);
    var ca = decodedCookie.split(";");
    for (var i = 0; i < ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0) == " ") {
            c = c.substring(1);
        }
        if (c.indexOf(name) == 0) {
            return c.substring(name.length, c.length);
        }
    }
    return "";
};

/**
 * Get the device id from the Google Analytics cookie.
 */
getGoogleAnalyticsDeviceId = function() {
    var gaCookie = getCookie("_ga");

    if (gaCookie === "") {
        return null;
    }

    // Parse the _ga cookie value to the right format.
    var lastElements = gaCookie.split(".").slice(-2);

    if (lastElements.length == 2) {
        return lastElements.join(".");
    }

    return null;
};

var deviceId = getGoogleAnalyticsDeviceId();