The result value of mysql running on node is not output

var http = require('http');
var url = require('url');

var db_config = require('mysql.js');

let mysqld = {} // mysql result
app.get('/server', function(req, res){

   let num = req.query.pID;
   let sql = 'select * from wt_serials where id=?';

   let params = [num];
   let conn = db_config.init();

   db_config.connect(conn);

   conn.query(sql, params, function(err, rows, fields){
      mysqld = JSON.stringify(rows);
      console.log('1 '+mysqld); // result is corrent
    });

    console.log('2 '+mysqld);  // result is [object Object]
    rss.json(mysqld);  // empty
}