Using AJAX in a Drupal form

The Form API has a mechanism to perform AJAX requests without writing any JavaScript. In this example, we will create a counter with an increment and decrement button.

How to do it…

1. Create a file named CounterForm.php in the src/Form directory for your module to hold the CounterForm form class.
2. We will define the CounterForm class with a form ID of mymodule_counter_form:

   <?php

   namespace Drupal\mymodule\Form;

   use Drupal\Core\Form\FormBase;

   use Drupal\Core\Form\FormStateInterface;

   class CounterForm extends FormBase {

     public function getFormId() {

       return 'mymodule_counter_form';

     }

     public function buildForm(array $form,

       FormStateInterface $form_state) {

       return $form;

     }

     public function submitForm(array &$form,

       FormStateInterface $form_state) {

     }

   }

3. In the buildForm method...