twig

{% extends "field--no-container.html.twig" %}
{% block template %}
  <div class="toc-content">
    {{ parent() }}
  </div>
{% endblock %}

{#
/**
 * @file
 * Theme override for a field.
 *
 * To override output, copy the "field.html.twig" from the templates directory
 * to your theme's directory and customize it, just like customizing other
 * Drupal templates such as page.html.twig or node.html.twig.
 *
 * Instead of overriding the theming for all fields, you can also just override
 * theming for a subset of fields using
 * @link themeable Theme hook suggestions. @endlink For example,
 * here are some theme hook suggestions that can be used for a field_foo field
 * on an article node type:
 * - field--node--field-foo--article.html.twig
 * - field--node--field-foo.html.twig
 * - field--node--article.html.twig
 * - field--field-foo.html.twig
 * - field--text-with-summary.html.twig
 * - field.html.twig
 *
 * Available variables:
 * - attributes: HTML attributes for the containing element.
 * - label_hidden: Whether to show the field label or not.
 * - title_attributes: HTML attributes for the title.
 * - label: The label for the field.
 * - multiple: TRUE if a field can contain multiple items.
 * - items: List of all the field items. Each item contains:
 *   - attributes: List of HTML attributes for each item.
 *   - content: The field item's content.
 * - entity_type: The entity type to which the field belongs.
 * - field_name: The name of the field.
 * - field_type: The type of the field.
 * - label_display: The display settings for the label.
 *
 *
 * @see template_preprocess_field()
 */
#}
{% block template %}
  {% for item in items %}
    {{ item.content }}
  {% endfor %}
{% endblock %}

{# Checks if a URL exists and passes it to the component. Otherwise, sets it to false. Works with primitive data types. #}
{% set link = url ? { url: url } : false %}

{# Checks if the media object is not empty and passes it to the component. Otherwise, sets it to false. Suitable for complex data types like objects. #}
{% set media = media is not empty ? { media: media } : false %}

<!DOCTYPE html>
<html{{ html_attributes }}>
  <head>
    <head-placeholder token="{{ placeholder_token|raw }}">
    <meta name="theme-color" content="#ef9222">
    <title>{{ head_title|safe_join(' | ') }}</title>
    <css-placeholder token="{{ placeholder_token|raw }}">
    <js-placeholder token="{{ placeholder_token|raw }}">
  </head>
  <body{{ attributes }}>
    {#...#}
  </body>
</html>

{# The elements array we want to add a '0' to at the beginning. #}
{% set elements = ['1', '2', '3'] %}
{# Working: Merge elements to a new array and override the variable. The result will be ['0', '1', '2', '3']. #}
{% set elements = ['0']|merge(elements) %}
{# Not working: The result will be ['1', '2', '3', '0']. #}
{% set elements = elements|merge(['0']) %}

parameters:
  twig.config:
    debug: true
    auto_reload: null
    cache: false