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);

}
}