Getting the available template variables in a custom module I'm making a custom module that will hook into the default theme and override a template file. Like you can do in a custom theme. At this moment I can already use the template that was made in the custom module with the hook_theme() function, it shows in my page. But the available variables are not showing in here. (for example: content, attributes, ...).

Probably because the module does not recognise these variables? How do I pass these variables to the template in my custom module. I read about the function hook_preprocess_HOOK() but no luck so far.

This is the implementation of hook_theme().

/**
 * Implements hook_theme()
 */
function episode_data_theme($existing, $type, $theme, $path) {
        return [
        'paragraph__episode_content_question_slider' =&gt; ['variables' =&gt; [] ],
    ];
}


This is the content of the template. 

{#
/**
 * @file
 * Default theme implementation to display a paragraph.
 *
 * Available variables:
 * - paragraph: Full paragraph entity.
 *   Only method names starting with "get", "has", or "is" and a few common
 *   methods such as "id", "label", and "bundle" are available. For example:
 *   - paragraph.getCreatedTime() will return the paragraph creation timestamp.
 *   - paragraph.id(): The paragraph ID.
 *   - paragraph.bundle(): The type of the paragraph, for example, "image" or "text".
 *   - paragraph.getOwnerId(): The user ID of the paragraph author.
 *   See Drupal\paragraphs\Entity\Paragraph for a full list of public properties
 *   and methods for the paragraph object.
 * - content: All paragraph items. Use {{ content }} to print them all,
 *   or print a subset such as {{ content.field_example }}. Use
 *   {{ content|without('field_example') }} to temporarily suppress the printing
 *   of a given child element.
 * - attributes: HTML attributes for the containing element.
 *   The attributes.class element may contain one or more of the following
 *   classes:
 *   - paragraphs: The current template type (also known as a "theming hook").
 *   - paragraphs--type-[type]: The current paragraphs type. For example, if the paragraph is an
 *     "Image" it would result in "paragraphs--type--image". Note that the machine
 *     name will often be in a short form of the human readable label.
 *   - paragraphs--view-mode--[view_mode]: The View Mode of the paragraph; for example, a
 *     preview would result in: "paragraphs--view-mode--preview", and
 *     default: "paragraphs--view-mode--default".
 * - view_mode: View mode; for example, "preview" or "full".
 * - logged_in: Flag for authenticated user status. Will be true when the
 *   current user is a logged-in member.
 * - is_admin: Flag for admin user status. Will be true when the current user
 *   is an administrator.
 *
 * @see template_preprocess_paragraph()
 *
 * @ingroup themeable
 */
#}
{% block paragraph %}
    &lt;div{{ attributes }}&gt;
        {% block content %}
            {% set sliderDescription = content.field_slider_description|render|striptags %}
            {{ content.field_available_for }}
            &lt;div {{ attributes }}  class="content-block content-block-slider"&gt;
                {{ content.field_slider_question }}
                &lt;div class="field_slider_description"&gt;{{ sliderDescription|length &gt; 200 ? sliderDescription|slice(0, 200) ~ '...' : sliderDescription }}&lt;/div&gt;
                &lt;a class="got-it" href="#content-block-{{ paragraph.id() }}" data-id="{{ paragraph.id() }}"&gt;{{ 'Got it'|trans }}&lt;/a&gt;
            &lt;/div&gt;
            &lt;div id="content-block-{{ paragraph.id() }}" class="modal-dialog"&gt;
                &lt;div&gt;
                    &lt;a href="#close" data-id="{{ paragraph.id() }}" id="{{ paragraph.id() }}" class="content-block-done close-modal-dialog"&gt;&lt;/a&gt;
                    {{ content.field_slider_question }}
                    {{ content.field_slider_description }}
                &lt;/div&gt;
            &lt;/div&gt;
        {% endblock %}
    &lt;/div&gt;
{% endblock paragraph %}


This is the debug output I get on the page.
According to your code, you are creating a custom template for a paragraph.

As you are trying to access variables, that will be added by the paragraph module, you'll have to tell the theming system that it is a paragraph.

You can do so by adding the 'paragraph' base hook. This way the template will get the variables provided by the paragraph (pre-)processing:

/**
 * Implements hook_theme()
 */
function episode_data_theme($existing, $type, $theme, $path) {
  return [
    'paragraph__episode_content_question_slider' =&gt; [
      'base hook' =&gt; 'paragraph',
    ],
  ];
}