Create custom validation/submit handler to an existing form in Drupal 8

In this post, I'll show you how to create custom validation/submit handler to an existing form in Drupal 8.

Custom validation handler

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter() for the FORM_ID() form.
 */
function mymodule_form_FORM_ID_alter(&$form, FormStateInterface $form_state, $form_id) {
    $form['#validate'][] = '_mymodule_form_FORM_ID_validate';
}

/**
 * Validates submission values in the FORM_ID() form.
 */
function _mymodule_form_FORM_ID_validate(array &$form, FormStateInterface $form_state) {
    // Validation code here
}

Custom submit handler

use Drupal\Core\Form\FormStateInterface;

/**
 * @param $form
 * @param FormStateInterface $form_state
 * @param $form_id
 */
function mymodule_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
    foreach (array_keys($form['actions']) as $action) {
        if ($action != 'preview' && isset($form['actions'][$action]['#type']) && $form['actions'][$action]['#type'] === 'submit') {
            $form['actions'][$action]['#submit'][] = 'mymodule_form_submit';
        }
    }
}

/**
 * @param array $form
 * @param FormStateInterface $form_state
 */
function mymodule_form_submit(array $form, FormStateInterface $form_state){
    // submit code here

}

Example: how to create custom validation/submit handler on user registration form.

/**
 * Implements hook_form_form_id_form_alter().
 */
function mymodule_form_user_register_form_alter(&$form, &$form_state, $form_id) {
    $form['#validate'][] = '_mymodule_user_register_validate';
    $form['#submit'][] = '_mymodule_user_register_form_submit';


}

/**
 * @param $form
 * @param FormStateInterface $form_state
 */
function _mymodule_user_register_form_submit($form, FormStateInterface $form_state)
{
// submit code here
}

/**
 * Custom validation function.
 *
 * Checks if the domain in the email address is on a list of allowed domains.
 */
function _mymodule_user_register_validate(&$form, &$form_state) {
 // Validation code here
 
}