function uuid(len) {
    var s = [];
    var hexDigits = "0123456789abcdefghijklmnopqrstuvwxyz";
    for (var i = 0; i < len; i++) {
      s[i] = hexDigits.substr(Math.floor(Math.random() * 36), 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;
}