Problems with query and express in nodeJS

I'm developing a web page using express for the server with nodeJS. I'm with the sign up page, and I'm trying to validate user inserted data, but when I make the query I get an error.

auth.js

const express = require('express');
const router = express.Router();
const { bd } = require('../database');
const help_functions = require('../lib/common');

router.post('/signup', async (req,res) => {
    const fullname = req.body['fullname'];
    const email = req.body['email'];
    const username = req.body['username'];
    const password = req.body['password'];
    const password_repeat = req.body['password_repeat'];
    var validation_msg = help_functions.validateSignUp(fullname, email, username, password, password_repeat);
    validation_msg = await help_functions.checkRepeatedUserEmail(email);
});

database.js

const mysql = require('mysql');
const { promisify } = require('util');

const database =  { // Database credentials }
const bd = mysql.createPool(database);
bd.getConnection((err,connection) => {
    if (err) {
        if (err.code === 'PROTOCOL_CONNECTION_LOST') {
            console.error('Database connection failed !');
        }
        if (err.code === 'ER_CON_COUNT_ERROR') {
            console.error('Database has too many connections !');
        }
        if (err.code === 'ECONNREFUSED') {
            console.error('Database connection was refused !');
        }
    }
    if (connection) {
        connection.release();
        console.log('Database is connected !');
        return;
    }
});
bd.query = promisify(bd.query);
module.exports = bd;

common.js

const { bd } = require('../database');
const helper_functions = {}

helper_functions.validateSignUp = (fullname, email, username, password, password_repeat) => {
    if (fullname === '' || email === '' || username === '' || password === '' || password_repeat === '') {
        return 'All the fields had to be completed!';
    }
    if (!(password.length >= 8 && (/\d/g.test(password) && (/[A-Z]/.test(password)))) ) {
        return 'The password needs to contain at least one capital letter, a number and 8 digits!';
    }
    if(password != password_repeat) {
        return 'Both passwords had to be the same!';
    }
    return 'Validated!';
}
helper_functions.checkRepeatedUserEmail = async (email) => {
    const user = await bd.query('SELECT * FROM users WHERE email = ?', [email]);
    if (user.length) {
        return 'This email is used, please change it!';
    } else {
        return 'Validated!';
    }
}
module.exports = helper_functions;

The error says the next text:

(node:14616) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'query' of undefined at Object.helper_functions.checkRepeatedUserEmail (proyect_path/src/lib/common.js:19:27) ..................

(node:14616) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag --unhandled-rejections=strict (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2) (node:14616) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Anyone knows what's happening?? Thanks for reading!

You are exposing the database as the default export in database.js:

module.exports = bd;

But you are importing it as if it was exported with the name db:

const { bd } = require('../database');

Either change the export in database.js to:

module.exports = {
    bd: bd
};

Or import it in the common.js file as:

const bd = require('../database');