      connection.connect( function () {         // insert user data with IP, location --- has got a status.         let values = [];         if (mine == null){             mine = "local server";         }         values.push(mine);         values.push('location');         var sql = "INSERT INTO user_list (IP_address, location) VALUES (?)";         connection.query(sql, [values], function (err, res){             if (err) return callback(err);              // control chatting connection between users             connection.query("SELECT IP_address FROM user_list WHERE status = ? AND location = ?", [0, "location"], function (err, res){                 if (err) return callback(err);                  