const sql = require('../../node_modules/mysql');

module.exports =
    {
        connect_to_db: function (sql_query)
        {
            let con = sql.createConnection({
                host: "localhost",
                user: config.server_username,
                password: config.server_password,
                database: config.database_name
            });

            con.connect((err)=> {
                if (err){
                    console.log("Problem connecting to the DB!");
                    return;
                }
                console.log("Connected to the DB!");
            });

            con.query(sql_query, (err, result) => {
                if (err) throw err;
                console.log('Data received from the DB');
                console.log(result);
                return result;
            });

            con.end((err) => {});


        }
    };

const connect_to_DB = require('DB_Connection');
let sql_query = "SELECT * FROM table";
                    database_results.push(connect_to_DB.connect_to_db(sql_query));

                    console.log(database_results);

[ undefined ]

Connected to the DB!
Data received from the DB
[ RowDataPacket {
    mail_id: ,
    from: ,
    to: ',
    subject: ,
    message: ,
    date:,
    read_date:  } ]

Process finished with exit code 0

const sql = require('../../node_modules/mysql');

module.exports =
    {
        connect_to_db: function (sql_query)
        {
            return new Promise((resolve, reject) => {
                (async () => {
                    let con = sql.createConnection({
                        host: "localhost",
                        user: config.server_username,
                        password: config.server_password,
                        database: config.database_name
                    });

                    con.connect((err)=> {
                        if (err){
                            console.log("Problem connecting to the DB!");
                            return;
                        }
                        console.log("Connected to the DB!");
                    });

                    con.query(sql_query, (err, result) => {
                        if (err) throw err;
                        console.log('Data received from the DB');
                        console.log(result);
                        resolve();
                        return result;
                    });

                    con.end((err) => {});

                })();
            });
        }
    };

[ Promise { <pending> } ]