I'm geting DB is not a constructor for the following code. If I move the const DB = require("./db"); inside the mysql() arrow function then it works. But I can't understand why...

const winston = require("winston");
const DB      = require("./db");

let logger  = winston.createLogger(
{
    format: winston.format.json(),
    transports: [
      new winston.transports.File({
          filename: 'logs.txt'}),

      new winston.transports.Console({
        format: winston.format.colorize()})
    ]

});


logger.mysql = (level, msg, filename, stack)=>
{

    //Create the database object.
    const db = new DB();

    //Open the connection.
    db.open().then((result)=>
    {
        db.conn.query(`INSERT INTO t_logs (m_level, m_msg, m_filename, m_stack) 
        VALUES (?, ?, ?, ?);`, [level, msg, filename, stack], (err)=>
        {
            //Something went wrong.
            if (err)
                logger.error(err.stack);

            //Close the connection.
            db.close().catch((e)=>logger.error(e.stack));
        });
    })
    .catch((e)=>
    {
        logger.error(e.stack);
    });
};


module.exports = logger;