FormData does't include the button Javascript

I'm having a problem with FormData, it was working a couple days ago but now it doesn't work, it submits all the inputs except the submit button. Here's my login form.

<form action="" method="post" name="login_user">
    <label for="username">Username/e-mail <span class="error" id="user-error"></span></label>
    <input type="text" name="username" id="username" required>
    <br>
    <label for="passwd">Password <span class="error" id="passwd-error"></span></label>
    <input type="password" name="passwd" id="passwd" required>
    <br>
    <p><input type="checkbox" checked name="remember"> Remember me <span class="forgot"><a href="<?php echo SITE_URL; ?>/reset">Forgotten password</a></span> </p>
    <input type="submit" id="login" value="Login" name="login">
    <p>Don't have an account? <a href="<?php echo SITE_URL; ?>/register/">Sign up</a></p>
</form>

JS for login, uses MVC.

//ajax login
var login = document.forms.namedItem('login_user');
if (login) {
    login.addEventListener('submit', function (e) {
        if (validatePassword('passwd', 'passwd-error')) {
            var data = new FormData(login);
            var userlogin = new XMLHttpRequest();
            var btn = document.getElementById('login');
            btn.value = 'Login, Please wait...';

            userlogin.open('POST', url + 'login/login_user_ajax', true);
            userlogin.onload = function (event) {
                if (userlogin.status == 200) {
                    var result = JSON.parse(userlogin.responseText);
                    if (result.results == 'success.') {
                        alert('logged in'); //change this later
                    } else {
                        document.getElementById('cred-error').innerHTML = result.results;
                    }
                    btn.value = 'Login';
                }
            };
            userlogin.send(data);
        }
        e.preventDefault();
    }, false);

The login method in my controller, the button is not detected.

public function login_user_ajax() {
    $this->login_user(1);
}

private function login_user($ajax  = '')
{
    if (filter_has_var(INPUT_POST, 'login')) {
        $new_user = $this->model('User');
        $new_user->setDb(Controller::getDb());
        $new_user->set_site(SITE_URL);
        $user = trim(filter_input(INPUT_POST, 'username', FILTER_SANITIZE_STRING));
        $passwd = trim(filter_input(INPUT_POST, 'passwd', FILTER_SANITIZE_STRING));
        if ($new_user->login($user, $passwd)) {
            if ($ajax) {
                $site_data['results'] = 'success.';
                echo json_encode($site_data);
                return true;
            }else {
                $this->redirect(SITE_URL . '/edit');
            }
        } else {
            if (!$ajax){
            return 'The username or password is incorrect.';
            }else {
                $site_data['results'] = 'The username or password is incorrect.';
                echo json_encode($site_data);
                return false;
            }
        }
    }else {
        print_r($_POST);
    }
}

I get this when I print_r $_POST, with no button.