Does Twig have equivalent 'append'?

Question

{% block page %}
   <ul class="gallery">
     {% set foo = 0 %}
     {% for products in prodArr %}
       {% if foo % 4 == 0 %}
         <ul class="slides">
           <li>Product</li>
         </ul>
       {% else %}
         <li>Another Product<li> // this
       {% endif %}
     {% set foo = foo + 1 %}
     {% endfor %}
   </ul>
{% endblock %}

In other case I want that li will append to ul with class slides. Is this possible?

I want that my HTML will be look like this:

<ul class="gallery">
  <ul class="slides">
    <li></li>
    <li></li>
    <li></li>
    <li></li>
  </ul>
  <ul class="slides">
    <li></li>
    <li></li>
    <li></li>
    <li></li>
  </ul>
</ul>

Answer 1

Twig has embed and include, but in your case you can just refactor your code like this:

// UPDATED:
<ul class="gallery">
    {% for chunk in products|batch(4) %}
        <ul class="slides">
            {% for product in chunk %}
                <li>{{ product.title }}</li> // assuming you have title property
            {% endfor %}
        </ul>
    {% endfor %}
</ul>

P.S. Unfortunately I can not test it but you will get the general idea ;)