I use the API Form of Drupal 8 to create my form. I had an ajax callback on a field:

$form['association']['numero'] = array(
        '#type' => 'textfield',
        '#title' => t('foo'),
        '#size' => 10,
        '#maxlength' => 10,
        '#suffix' => '<div id="ajax_error"></div>',
        '#ajax' => [
            'callback' => '::fetchApiData',
            'event' => 'change',
            'prevent' => 'click',

with my callback function :

public function fetchApiData(array $form, FormStateInterface $form_state)
    return 'foo';

It's all working fine, but i want to trigger the callback when the user writes 10 characters precisely. Does anyone has a solution for that?

1 Answer 1


You can do this:

1) Decide on an event name. In this code, the event name will be 'overminlength'

2) In the form element being defined using the Form API, set your #ajax as normal, but set the #ajax 'event' to the event name chosen in step 1:

'#ajax' => [
  'wrapper' => 'some_wrapper',
  'callback' => 'some_callback',
  // Set the custom event here:
  'event' => 'overminlength',

3) Create a custom JS file and attach it to the field

'#attached' => [
  'library' => ['mymdodule/mylibrary'],

4) Add a $.keyup() listener to the field.

  • In the callback, count the number of characters in the field
  • If it's more than X characters, call $.trigger(), and trigger the event from step 1

    (function($, Drupal) {
      "use strict";
      function fieldwatcher(context) {
        $(context).find("#edit-myfield").once("fieldwatcher").each(function() {
          $(this).keyup(function() {
            // Change '3' to the number of characters after which you want to
            // trigger the ajax.
            if ($(this).val().length > 3) {
      Drupal.behaviors.testScript = {
        attach: function(context) {
    }(jQuery, Drupal));
  • Hello Jaypan and thank you for your response. I don't really understand your point, can you please explain me more ? Regards,
    – lchabrand
    Commented Apr 24, 2018 at 5:43
  • Well, my answer is all jQuery and JavaScript based. If you don't understand how to work with those, the answer I gave above isn't going to help you, and I can't really teach JS and jQuery on Drupal answers. If you are looking for a method of doing what you want purely through the Form API, the answer is that it cannot be done. The solution I gave above would be a custom one that requires custom JS coding.
    – Jaypan
    Commented Apr 24, 2018 at 5:47
  • Ho okay, yes i can do that with javascript but my question was more about using the Form API, but it seems we actually can't.
    – lchabrand
    Commented Apr 24, 2018 at 6:00
  • 2
    That's correct. The Form API is great, but this is not functionality it provides by default. It may make for an interesting add-on however.
    – Jaypan
    Commented Apr 24, 2018 at 6:14
  • I've actually tested my theory, and it works. I've re-written my post above to show specific instructions.
    – Jaypan
    Commented Apr 27, 2018 at 2:04

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