Nodejs module export, wait for function to finish - node.js

Question

I am developping an app using node js
in my export_db.js where I export the connection and query function
When I call the function, inside the export_db the output is really a string filled with information, but in my main.js the output is undefined, as if the function wasn't finished and the code continue to run before the results comes in.
How can I force it to wait ?
File export_db.js
var con = mysql.createConnection({
....
});
con.connect(function(err) {if (err) throw err;});
con.CustomQuery = function(SQLQuery){
..DO Stuff
return stringoutput="";
con.query(SQLQuery, function (err, result,fields) {
if (err) throw err;
var arr = result;
//Do Stuff Transform result into a String
stringoutput=result as string
});
return string output;
});
module.exports = con;
File import_db.js
var db = require('../db/export_db_connection');
//DO Stuff
Queryresult = db.CustomQuery(SQLQuery);
bot.reply(Queryresult) // Send the String to the bot to get a response message
//DO Stuffs

Answer 1

Since your code is fundamentally asynchronous in nature (you have to wait for it to be ready). It might be better to change your _db.js to export a factory function which returns a promise which resolves with an instance of the connection when it is available.
// _db.js
function connect() {
return new Promise(function (resolve, reject) {
var con = mysql.createConnection({
//....
});
con.CustomQuery = function (SQLQuery) {
return new Promise(function(resolve, reject){
con.query(SQLQuery, function(err, result, fields){
if(err) return reject(err)
// var str = convert result to string here
resolve(str);
})
})
};
con.connect(function (err) {
if (err) return reject(err);
resolve(con)
});
})
}
let conn = null;
function getConnection() {
if (!conn) {
conn = connect();
}
return conn;
}
module.exports = getConnection;
And then, when you want to use the connection:
var getConnection = require('/path/to/_db.js');
getConnection()
.then(function (conn) {
return conn.CustomQuery(sqlQuery)
})
.then(function(str){
// Query result is available here
console.log(str);
})
You can also do this without Promises using callbacks
// _db.js
function connect(cb) {
var con = mysql.createConnection({
//....
});
con.CustomQuery = function (SQLQuery) {
//..DO Stuff
// return stringoutput="";
};
con.connect(function (err) {
if (err) return cb(err);
cb(null, con)
});
})
}
let conn = null;
function getConnection(cb) {
if (!conn) {
return connect(function(err, con){
if(err) return cb(err);
conn = con;
cb(null, conn);
});
}
cb(null, conn);
}
module.exports = getConnection;
And then, when you want to use the connection:
var getConnection = require('/path/to/_db.js');
getConnection(function (err, conn) {
if(err){
// handle errors
}
QueryResult = conn.CustomQuery(SQLQuery);
})