Hide one form element until another is selected

Question

Is there any way to disable the field 'model_article_nr' until 'model' is selected

this is what i have and as it is now i can select 'model_article_nr' before i have choosen a model which leads to wrong article nummer if i change model afterwords, because when no model is selected the first model is standard in my '$article_list[$curent_model]' and beeing displayed as options in 'model_article_number'

$model_list = array();
$article_list = get_model_article_list();

foreach($article_list as $key => $value) {
        if ( empty($curent_model) ) $curent_model = $key;
        $model_list[$key] = $key;
    }

$form['model'] = array(
        '#type' => 'select',
        '#title' => t('Model'),
        '#title_display' => 'invisible',
        '#empty_option' => t('Select Model'),
        '#options' => $model_list,
        '#ajax' => array(
            'callback' => 'form_model_callback',
            'wrapper' => 'article-number-ajax-wrapper',
        ),
        '#prefix' => '<div class="clearfix"></div><div class="form-helper-container">',
        '#suffix' => '<button type="button" class="btn tooltip-btn" data-toggle="modal" data-target="#model-help"><span class="glyphicon glyphicon-question-sign"></span></button></div>',
    );

    $form['model_article_nr'] = array(
        '#type' => 'select',
        '#title' => t('Article Number'),
        '#title_display' => 'invisible',
        '#attributes' => array('placeholder' => t('Article Number'), 'class' => array('space-bottom')),
        '#empty_option' => t('Select Article Number'),
        '#validated' => TRUE,
        '#options' => $article_list[$curent_model],
        '#prefix' => "<div id='article-number-ajax-wrapper'>",
        '#suffix' => "</div>",
    );

And my callback

function form_model_callback($form, &$form_state) {
    return $form['model_article_nr'];
}

Answer 1

As @leymannx stated you can achieve this using #states. Here is a simple example:

 $form['show_textbox'] = array(
  '#type' => 'checkbox',
  '#title' => t('Show text Box 1'),
        '#attributes' => [
        'id' => 'show_textbox',
        'name' => 'field_show_textbox',
      ],
);

    $form['text_box1'] = [
      '#type' => 'textfield',
      '#size' => '60',
      '#placeholder' => 'Enter Name',
      '#attributes' => [
        'id' => 'text_box1',
        'autocorrect' => 'none',
        'class' => 'text_box1',
        'autocapitalize' => 'none',
        'spellcheck' => 'false',
      ],
      '#states' => [
        //show this textfield only if the radio button 'show_textbox1' is selected
        'visible' => [
                ':input[name="field_show_textbox"]' => array('checked' => TRUE),
        ],
      ],
    ];

You can always change the condition.

Answer 2

Thank you Hunde Tekle and leymannx it works :D

$form['model'] = array(
        '#type' => 'select',
        '#title' => t('Model'),
        '#title_display' => 'invisible',
        //'#attributes' => array('placeholder' => t('Model')),
        '#empty_option' => t('Select Model'),
        '#options' => $model_list,
        //'#default_value' => $curent_model,
        //'#disabled' => TRUE,
        '#ajax' => array(
            'callback' => 'form_model_callback',
            'wrapper' => 'article-number-ajax-wrapper',
        ),
        '#prefix' => '<div class="clearfix"></div><div class="form-helper-container">',
        '#suffix' => '<button type="button" class="btn tooltip-btn" data-toggle="modal" data-target="#model-help"><span class="glyphicon glyphicon-question-sign"></span></button></div>',
    );

    $form['model_article_nr'] = array(
        '#type' => 'select',
        '#title' => t('Article Number'),
        '#title_display' => 'invisible',
        '#attributes' => array('placeholder' => t('Article Number'), 'class' => array('space-bottom')),
        '#empty_option' => t('Select Article Number'),
            '#states' => array(
            'visible' => array(
              array(
                'select[name="model"]' => array('!value' => ''),
              ),
            ),
        ),
        '#validated' => TRUE,
        '#options' => $article_list[$curent_model],
        '#prefix' => "<div id='article-number-ajax-wrapper'>",
        '#suffix' => "</div>",
    );