How to validate and pretty-print JSON in a Drupal Form without third-party

JSON textareas can be validated and pretty-printed relatively simple in Drupal forms without third-party needs!

I made a first shot in the Posthog Drupal module (posthog_js submodule settings form).

Here are the Form class snippets:

/**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state): array {
    $form['init_config_json'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Initialization configuration (JSON)'),
      '#description' => $this->t("Posthog initialization JSON. Add values to override <a href=\"https://posthog.com/docs/libraries/js#config\" target=\"_blank\">the defaults</a>") . $accessJsSettingsWarning,
      '#default_value' => $this->config('posthog_js.settings')->get('init_config_json') ?? "{}",
      '#placeholder' => "{\n  // autocapture: true,\n}",
      '#disabled' => !$accessJsSettings,
    ];
    
    return parent::buildForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);

    try {
      // Validate JSON:
      $initConfigJson = trim($form_state->getValue('init_config_json') ?? '');
      if (!empty($initConfigJson)) {
        $form_state->setValue('init_config_json', json_encode(json_decode($initConfigJson ?: '{}', FALSE, 5, JSON_THROW_ON_ERROR | JSON_INVALID_UTF8_SUBSTITUTE), JSON_FORCE_OBJECT | JSON_INVALID_UTF8_SUBSTITUTE | JSON_PRETTY_PRINT | JSON_THROW_ON_ERROR));
      }
    }
    catch (\JsonException $e) {
      $form_state->setErrorByName('init_config_json', $this->t('Invalid JSON'));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state): void {
    $this->config('posthog_js.settings')
      ->set('init_config_json', $form_state->getValue('init_config_json'))
      ->save();
    parent::submitForm($form, $form_state);
  }

(https://git.drupalcode.org/project/posthog/-/blob/1.x/modules/posthog_j…)

Feel free to further improve it :) And buy me a coffee, if you liked it and it saved you time! ;)