How to Add hook_form_alter() in Drupal 8

Discover how to add hook_form_alter() in Drupal 8. Our Drupal Support team is ready to assist.

How to Add hook_form_alter() in Drupal 8

One of the most powerful features of Drupal is its hook system. It allows developers to extend and alter existing behavior without hacking core code. Among these, `hook_form_alter()` and its variants are widely used to modify form behavior, add or remove elements, apply custom validation, and more.

Today, we will walk through the order of form alteration hooks, practical examples of how to use them, and debugging tips to make the form customizations effective and efficient.

Understanding the Order of Form Alter Hooks

When a form is created in Drupal, various modules and themes can implement hooks to alter it. These hooks are called in a specific order:

• hook_form_alter()
• hook_form_BASE_FORM_ID_alter()
• hook_form_FORM_ID_alter()

These hooks are called within each module, and for each module, the more general hooks are invoked first, followed by the more specific ones. For example:

• hook_form_alter() is called for all forms.
• hook_form_BASE_FORM_ID_alter() targets forms that share a base form ID.
• hook_form_FORM_ID_alter() is used to target a specific form.

The complete calling order is:

1. All hook implementations from Module A
2. Then from Module B
3. Followed by any base theme(s)
4. And finally, the current theme

Furthermore, module execution order is based on system weight and module name.

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A Common Use Case: Altering the Node Form

A frequent scenario is adding or modifying fields on the node form. We can access the node object like this:

$form['#node'];Copy Code

To alter the node form in the custom module, use:

use Drupal\Core\Form\FormStateInterface;
/
* Implements hook_form_alter().
*/
function yourmodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
if ($form_id == 'node_article_edit_form' || $form_id == 'node_article_form') {
// Modify the node article form here.
}
}
Copy Code

Also, always clear the cache after writing hook functions to make Drupal recognize them.

Use the `drush cr` command (if you’re using Drush 8+) or go to Manage > Configuration > Performance > Clear All Caches.

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Debugging Tips: Is Your Hook Being Called?

To verify if the hook is working, we can use the Devel module along with Kint, a debugging tool that prints structured variable output.

Install and enable both modules, then in the hook, use:

function yourmodule_form_alter(&$form, &$form_state, $form_id) {
kint($form); // This prints all form variables.
}
Copy Code

To target a specific form like `node-question-form`:

function yourmodule_form_node_question_form_alter(&$form, &$form_state, $form_id) {
kint($form); // Debug the specific form.
}
Copy Code

Alternatively, we can use `echo` or `print_r()` and check the browser’s source code using `Ctrl+F` to locate the debug text.

Tip: Learn how to troubleshoot AJAX errors on file upload in Drupal.

Practical Example: Forcing Sort Order in an Exposed Views Form

Here’s how to control the sort direction of a Views exposed filter based on the user’s selection:

function MYTHEME_form_alter(&$form, &$form_state, $form_id) {
if ($form_id !== 'views_exposed_form' || $form['#id'] !== 'views-exposed-form-search-custom-page-1') {
return;
}
$user_input = $form_state->getUserInput();
if (empty($user_input['sort_by'])) {
return;
}
if ($user_input['sort_by'] == 'relevance') {
$user_input['sort_order'] = 'ASC';
} elseif ($user_input['sort_by'] == 'created') {
$user_input['sort_order'] = 'DESC';
}
$form_state->setUserInput($user_input);
}
Copy Code

Replace `”views-exposed-form-search-custom-page-1″` with the actual form ID, and ensure that `”relevance”` and `”created”` match the sort field machine names.

Altering Forms in Custom Themes

If we are working within a custom theme, we can use the following format in your `THEMENAME.theme` file:

function bootstrap_ga_form_user_login_form_alter(&$form, &$form_state, $form_id) {
$form['#validate'][] = '_bootstrap_ga_form_user_login_form_validate';
}
Copy Code

In this case, `bootstrap_ga` is the name of the custom theme. This example adds custom validation to the user login form.

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Conclusion

Modifying forms in Drupal using hook functions is a core part of custom module and theme development.

In short, our Support Engineers demonstrated how to add hook_form_alter() in Drupal 8.