js生产随机数

    var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
    var string_length = 8;
    var randomstring = '';
    for(var i = 0; i < string_length; i++) {
      var rnum = Math.floor(Math.random() * chars.length);
      randomstring += chars.substring(rnum, rnum + 1);
    }
    return  randomstring;