Extracting the user ID from the first-party Snowplow cookie

duid.js

/*
 * Function to extract the Snowplow user ID from the first-party cookie set by the Snowplow JavaScript Tracker
 *
 * @param string cookieName (optional) The value used for "cookieName" in the tracker constructor argmap
 *                                     (leave blank if you did not set a custom cookie name)
 *
 * @return string or bool The ID string if the cookie exists or false if the cookie has not been set yet
 */
function getSnowplowDuid(cookieName) {
    cookieName = cookieName || '_sp_';
    var c = document.cookie.split(';');
    for (var i = 0; i < c.length; i++) {
        if (c[i].substr(0, 6) === cookieName + 'id') {
            return c[i].split('=')[1].split('.')[0];
        }
    }
    return false;
}