解callback嵌套

function checkPassword(username,password,callback){
        var pwdHash;
        var queryStr = 'select * from user where username = ?';
        db.query(queryStr,username,queryCallback);

        function queryCallback(err,result){
            if(err) throw err;
            pwdHash = result['password_hash'];
            hash(password,hashCallback);
        }

        function hashCallback(data){
            callback(data == pwdHash);

        }
    }