When i use this function, result is undefined. WHy?

exports.user_verification = function(username, password, callback) {
    
    var queryString_user_veryfication = "SELECT * FROM users WHERE (login = '"+ username +"' AND password = '" + password + "')";
    
    connection.query(queryString_user_veryfication, function(err, rows, fields) {
        if (err) throw err;
        
        if(rows.length === 0){
            return = false;
        } else {
            return = rows;
        }
    });
}

I was reading a little bit about this problem but i don't know how to use callback to this case...

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