ModalForm.php

<?php

/**
 * @file
 * Contains Drupal\modal_test\Controller\ModalForm.
 */

namespace Drupal\modal_test\Controller;


use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\CloseModalDialogCommand;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

class ModalForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'modal_test_context_configure';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, $context = NULL) {
    $form['context'] = [
      '#type' => 'value',
      '#value' => $context
    ];
    $form['label'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Label'),
    ];
    $form['description'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Description'),
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Save'),
      '#ajax' => [
        'callback' => [$this, 'ajaxSubmit'],
        'event' => 'click',
      ],
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Do stuff if JavaScript is disabled.
  }

  /**
   * {@inheritdoc}
   */
  public function ajaxSubmit(array &$form, FormStateInterface $form_state) {
    // Do stuff to save via JavaScript.
    $response = new AjaxResponse();
    $response->addCommand(new CloseModalDialogCommand());
    return $response;
  }

}