Cannot change variable values from a callback scope

Question

I designed a function that connects to MYSQL and calls the callback parameter with the 'connection' object. I'm calling this function(lets call it A) inside another function (lets call it B), and, in the callback I'm sending to function A, I'm trying to modify a value from function B, but it doesn't work. Simply the value remains the same.

The code:

Function A:

function databaseConnection(callback) {
    // MYSQL Connection Details
    const connectionDetails = {
        host: 'localhost',
        user: 'root',
        password: 'Mobileapp!?',
        database: 'social-app'
    }
    //

    // Configuring the connection
    const connection = mysql.createConnection(connectionDetails);
    connection.connect();
    //
    // Calling the user callback
    callback(connection);
    //

    // Ending database connection
    connection.end();
    //
}

Function B:

function isLoginDataValid(username, password) {
    let valid = true;

    // Starts the database connection and execute the given callback with the supplied object 'connection'
    databaseConnection(connection => {
        connection.query(`SELECT id from users where username = '${username}' and password = '${password}'`, (error, results, fields) => {
            if (!error && !(results.length))
                valid = false;
        })
    })
    //
    return valid;
}

Thank you!.