exports.getUser = function (email, password, callback) {
    var values = [email, password];
    var sql = 'SELECT * FROM users WHERE email = ? AND password = ?';
    db.query(sql, values, function (err, results) {
        callback(err, results);
    });
 }

service.getUser("email", "abc123", function (err, results) {
    if (err) throw err;
    if(results) ....
});