[app.js]
const http = require('http');
const fs = require('fs');
const db = require('./db');

const hostname = '127.0.0.1';

const port = 3000;

const server = http.createServer((req,res) => {
  res.statusCode = 200;
  res.setHeader('Content-type','text/html');
  res.write('your domain is '+ hostname);
  res.end();
});

server.listen(port, hostname, () => {
  console.log('this project is at '+ port + ' and hostname is '+ hostname);


  // this is called a 'callback pattern'
  var callback = function(err, data) {
    if(err) throw err;
    else console.log(data);
  };

  // note how the callback is the last argument passed to the function
  db.statement('SELECT * FROM draft_tbl where id = 3', callback);

});

[db.js]

// move this outside of your function because you don't need to recreate the
// connection each time you run a query
const mysql = require('mysql');
var con = mysql.createConnection({
  host: "127.0.0.1",
  user: "root",
  password: "",
  database: "mydbname"
});

// and run this immediately because you want to know if it fails *before* you
// run any queries
con.connect(function(err) {
  if (err) throw err;
  else console.log("Actually db is connected!");
});

var statement = function (query_arg, done) {

  // this function also uses a callback, which is a sign that the function is
  // asynchronous
  con.query(query_arg, function (error, rows, fields) {
    if (error){
      done(error);
    } else {

      console.log(rows); // this console.log working directly. show true result of query

      // here we call the "callback" with the rows returned as the second argument
      done(null, rows);
    }
  });
}