Related to my previous question, but this specifically pertains to how ajax works rather than what $form_state does.

I have a form class defined with the following code (an "instruction" in this code is just the name for a blank text field, and there needs to be the ability to add as many of these text fields as the user desires):

namespace Drupal\mymodule\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

class MymoduleSubmitForm extends FormBase {
 * {@inheritdoc}
public function getFormId() {
    // Unique ID of the form.
    return 'mymodule_submit_form';
 * {@inheritdoc}
public function buildForm(array $form, FormStateInterface $form_state) {
    // Create a $form API array.

    $form['#tree'] = TRUE;
    $i = 0;
    $num_instructions = $form_state->get('num_instructions');
    if (empty($num_instructions)) {
        $num_instructions = $form_state->set('num_instructions', 1);
        $num_instructions = $form_state->get('num_instructions');
    $form['instructions'] = array(
            '#type' => 'fieldset',
            '#title' => 'Instructions',
            '#prefix' => '<div id="instructions-wrapper">',
            '#suffix' => '</div>'
    for ($i = 0; $i < $num_instructions; $i++) {
      $form['instructions'][$i] = [
        '#type' => 'textfield',
        '#title' => $this->t('@num', ['@num' => $i + 1]),
    $form['add_instruction'] = array(
        '#type' => 'submit',
        '#value' => $this->t('Add step'),
        '#ajax' => array(
            'callback' => '::addInstructionCallback',
            'wrapper' => 'instructions-wrapper'
    $form['submit'] = array(
            '#type' => 'submit',
            '#value' => $this->t('Submit')
    return $form;

 * Implements callback for Ajax event
 * @param array $form
 *   From render array.
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 *   Current state of form.
 * @return array
 *   Form fields.
public function addInstructionCallback(array &$form, FormStateInterface $form_state) {
    $name_field = $form_state->get('num_instructions');
    return $form['instructions'];

 * {@inheritdoc}
public function validateForm(array &$form, FormStateInterface $form_state) {

 * {@inheritdoc}
public function submitForm(array &$form, FormStateInterface $form_state) {
    ddl(1); //ignore this for now

The form is called in mymodule.module like this:

function mymodule_node_view_alter(array &$build, Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display) {
    $current_path = \Drupal::request()->getRequestUri();
    $paths = explode('/', $current_path);
    if ($paths[1] == 'submit-set')
        $build['myform'] = \Drupal::formBuilder()->getForm('Drupal\mymodule\Form\MymoduleSubmitForm');

My problem is that my Ajax callback function addInstructionCallback does not seem to be firing at all. The indicator appears when I click the button to show that the ajax function is doing its work, and then it goes away and the form hasn't changed. I check /admin/reports/debug and it is completely empty, the ddl($form); call from within my callback function has not fired. There are no PHP error messages to be found in reports or log files either.

I am mostly following the code in this example, but I have also tried an approach using AjaxResponse like in this example and it hasn't made a difference. I have also tried changing the button to a checkbox in case it was breaking because of my unfinished submit function, but that didn't make a difference either.

What am I doing wrong with Ajax?

Update: Console is doing some weird stuff:

Uncaught AjaxError: 
An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /submit-set?ajax_form=1
StatusText: OK

ResponseText:(entire contents of my page, including theme headers and footers, followed by

{"ajaxPageState":{"theme":"bartik","libraries":"admin_toolbar\/toolbar.tree,admin_toolbar\/toolbar.tree,admin_toolbar_tools\/toolbar.icon,admin_toolbar_tools\/toolbar.icon,bartik\/global-styling,bartik\/global-styling,bartik\/messages,classy\/base,classy\/base,classy\/messages,classy\/messages,classy\/node,classy\/node,contextual\/drupal.contextual-links,contextual\/drupal.contextual-links,contextual\/drupal.contextual-toolbar,contextual\/drupal.contextual-toolbar,core\/drupal.active-link,core\/drupal.active-link,core\/html5shiv,core\/html5shiv,core\/normalize,core\/normalize,history\/mark-as-read,history\/mark-as-read,quickedit\/quickedit.inPlaceEditor.plainText,shortcut\/drupal.shortcut,shortcut\/drupal.shortcut,system\/base,system\/base,toolbar\/toolbar,toolbar\/toolbar,toolbar\/toolbar.escapeAdmin,toolbar\/toolbar.escapeAdmin,tour\/tour,tour\/tour,user\/drupal.user.icons,user\/drupal.user.icons"},"ajaxTrustedUrl":{"\/search\/node":true},"history":{"nodesToMarkAsRead":{"45":true}},"toolbar":{"breakpoints":{"toolbar.narrow":"only screen and (min-width: 16.5em)","toolbar.standard":"only screen and (min-width: 38.125em)","toolbar.wide":"only screen and (min-width: 61em)"},"subtreesHash":"Y9yCNrSN2LUELxt5wnz-3xSDjtgka559ps7ynAoGrpw"},"path":{"baseUrl":"\/","pathPrefix":"","currentPath":"node\/45","currentPathIsAdmin":false,"isFront":false,"currentLanguage":"en","currentQuery":{"_wrapper_format":"html","ajax_form":"1"}},"pluralDelimiter":"\u0003","user":{"uid":"1","permissionsHash":"d0f6778fd5e1fddf0d1d92a309af30862905f78390534ce1aa8a064cbc51b24e"}}



Drupal.Ajax.error       @   {cache}:2995
complete                @   {cache}.js:2540
t.complete              @   {cache}.js:1320
j                       @   {cache}.js:2
fireWith                @   {cache}.js:2
x                       @   {cache}.js:4
(anonymous function)    @   {cache}.js:4

Have already tried clearing cache.

Update: Modified code to match another example. Still getting the same error.

Update: Adding a console.log(status); to the called complete function above return ajax.error(xmlhttprequest, ajax.url); returns the string parsererror. Trying to log anything else related to the ajax request to console (such as the xmlhttprequest parameter) crashes the page.

  • To debug AJAX I suggest you switch to xdebug - it will be far more helpful to you than devel, as you can step through the code line by line and see what happens, see what the global/local scope has, view the call stack, etc. Particularly with AJAX callbacks, it will give you 100% verification whether or not they're being called, because the breakpoints tell all. It can be setup in Atom, VSCode, or PHPStorm.
    – Kevin
    Sep 12, 2016 at 17:27
  • Ugh..OMG... I'm soooooooo sorry for all the pain, suffering and wasted time I caused, I linked you to the wrong one, this is the correct one AjaxAddMore.php. The wrong one I linked you to is for when you select a field value, it ajax and shows another field.
    – No Sssweat
    Sep 12, 2016 at 23:47
  • Don't worry @NoSssweat, I saw that example when I was exploring the link you sent me. I just updated my code in the OP to show a closer example to that one. It is still giving me the exact same error.
    – saramm1
    Sep 13, 2016 at 13:10
  • @saramm1 Oh... phew.
    – No Sssweat
    Sep 13, 2016 at 21:03

1 Answer 1


I've figured it out. My problem was that I was inserting an Ajax-using form into a page node. I don't know why this is a problem, but I installed the Examples module and saw that the AjaxAddMore.php form was working fine as-is once installed. I replaced my custom form with the AjaxAddMore form in the setup I had (inserting a form into a page node), and it also returned the same console error I was getting before.

I looked at the fapi_example.routing.yml file in Examples, particularly the following lines:

path: 'examples/fapi_example/ajax_addmore'
  _form:  '\Drupal\fapi_example\Form\AjaxAddMore'
  _title: 'Add-more button'
  _permission: 'access content'

and set up my form to display on its own in this manner. I did not need the form to exist with any other page content, so I removed it from the page insertion setup I had before entirely.

I wouldn't call this a solution for a use case where it would be necessary to insert a form into a page node, but for my purposes the form ajax is working now.

  • I had a feeling that this was part of the problem, unfortunately, I was so busy yesterday, didn't have time to test it.
    – No Sssweat
    Sep 13, 2016 at 21:01

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