function guid() {

    var s = [];

    var hexDigits = "0123456789abcdef";

    for (var i = 0; i < 36; i++) {

        s[i] = hexDigits.substr(Math.floor(Math.random() * 0x10), 1);

    }

    s[14] = "4";  // bits 12-15 of the time_hi_and_version field to 0010

    s[19] = hexDigits.substr((s[19] & 0x3) | 0x8, 1);  // bits 6-7 of the clock_seq_hi_and_reserved to 01

    s[8] = s[13] = s[18] = s[23] = "-";

    var uuid = s.join("");

    return uuid;

}