Nodejs数据库同步查询

var connection = mysql.createConnection({
 host: 'localhost',
 user: 'root',
 password: '密码',
 database: '数据库'
 });
connection.connect();
exports.selectByUsername = selectByUsername;
function selectByUsername(username){
var promise = new Promise(function(resolve){
 var sql = "SELECT * FROM user WHERE username = ?";
 var sqlParams = [username];
 connection.query(sql,sqlParams,function(err,result){
 if(err){
 return console.log(err.message);
 }
 console.log("------------------------开始查询---------------------");
 console.log(result);
 var str = JSON.stringify(result);
 var json = JSON.parse(str);
 console.log("------------------------查询结束---------------------");
 resolve(str);
 });
 // connection.end();
});
promise.then(function(value){
 console.log(value);
 return value;
});
return promise;
}

 var promise =exports.selectByUsername(req.body.username);;
promise.then(function(value){
 console.log(value);
 if(value!=null){
// req.session.user = value;
// req.session.save();
 res.end(value);
 }else{
 res.end(null);
 }
});