Code snippet that can be used to Conditionally hide/enable a form field programmatically in Drupal 8.
if you want to visually hide/show a form field, based on a specific value from another field, Form API #states property allows to easily show or hide, enable or disable, require or collapse form fields based on values selected or entered in other fields.
Conditional Fields Example
enable the radios only if the custom cms textbox is empty
$form['cms'] = [
'#type' => 'radios',
'#title' => $this->t('Select a cms'),
'#options' => [
'drupal' => $this->t('Drupal'),
'wordpress' => $this->t('Wordpress'),
'joomla' => $this->t('Joomla'),
'other' => $this->t('Other'),
],
'#attributes' => [
'name' => 'field_select_cms',
],
'#states' => [
'enabled' => [
':input[name="field_other_cms"]' => ['value' => ''],
],
],
];
show this textfield only if the radio 'other' is selected
$form['other_cms'] = [
'#type' => 'textfield',
'#placeholder' => 'Enter your cms',
'#attributes' => [
'name' => 'field_other_cms',
],
'#states' => [
'visible' => [
':input[name="field_select_cms"]' => ['value' => 'other'],
],
],
];
