NODE.js with socket and Mysql

Question

I have a problem with Node, I believe it must be something related to callback s, but I’m not sure, but anyway, my problem is basically in pulling data from a database Mysql, this data will be stored in a variable called msg. My complete code is this:

        const express = require('express');
        const path = require('path');
        const app = express();
        const server = require('http').createServer( app );
        const io = require('socket.io')(server);
        const mysql = require('mysql');

        // conexao com o banco de dados 
        var con = mysql.createConnection({
        host: "localhost",
        user: "root",
        password: "",
        database: "dados_users"
        });


        app.use(express.static(path.join(__dirname,'public')));
        app.set('views',path.join(__dirname,'public'));


        app.engine('html',require('ejs').renderFile);
        app.set('view engine','html');

        app.use('/', ( request , res ) => {
            res.render('index.html');
        });

        let msg =[];  

        io.on('connection', socket => {

        //este select deve retornar as mensagens que estão no banco para a 
        //variavel msg
        // con.connect(function(err) {
        //     con.query('SELECT * FROM dados', function(err, rows, fields) {
        //         if (err) throw err;
        //           msg = rows;
        //     });
        // });

        socket.emit('msg_existentes',msg);    //retorna dados earmazenados na variavel msg 
        socket.on('recebe_do_front', data => {



        console.log(msg);

        // con.connect(function(err) {
        //     var sql = "INSERT INTO dados (nome, msg) VALUES ('"+data.nome+"', '"+data.msg+"')";
        //     con.query(sql, function (err, result) {
        //     if (err) throw err;
        //         console.log("1 record inserted");
        //     });
        // });

        msg.push( data );     //manter msg na section da pagina
        socket.broadcast.emit('envia_para_usuario', data);   //retorna msg do canudo 
    });
});
server.listen(3001);

The function that is not doing the part of it is the function of [ select ], this below:

   //este select deve retornar as mensagens que estão no banco para a 
  //variavel msg
  con.connect(function(err) {
       con.query('SELECT * FROM dados', function(err, rows, fields) {
           if (err) throw err;
                 msg = rows;
       });
    });

then that is, when the program is run, it brings the empty msg variable. Thank you very much!

Answer 1

Study asynchrony on Ode and its relationship with callbacks. The code is not running in the sequence you are thinking of.

let msg =[];  

io.on('connection', socket => {

    con.connect(function(err) {

        con.query('SELECT * FROM dados', function(err, rows, fields) {
            if (err) throw err;
            // [1]
            msg = rows;
            socket.emit('msg_existentes',msg);
        });

    });

});