Use-cases where creating arrays in Liquid can prove useful

array-constructor.liquid

{% comment %}
  In this case, the merchant, for UX testing would like subscription products to be on top of the cart drawer, followed by the one-time purchase items.
  Usually, we would have two forloops inside the markup, with their distinctive conditions in order to sort the different products.
  Instead, we will prepare all the necessary data beforehand and loop over the more explicit arrays in the markup.
  For performance, in place of two for loops over the items, we only have one. In more critical use-cases, several ms are gained.
  For readability and maintenability, we have a DRY declaration at defined location which makes it more maintainable and readable.
  
  Note: The arrays are constructed using a for loop, a common pattern that works pretty well !
{% endcomment %}

{% liquid
  assign subscription_item_list = ''
  assign one_time_item_list = ''
  
  for item in cart.items
    if item.selling_plan_allocation
      if subscription_item_list == blank
        assign subscription_item_list = item | uniq
      else
        assign subscription_item_list = item | uniq | concat: subscription_item_list
      endif
    else
      if one_time_item_list == blank
        assign one_time_item_list = item | uniq
      else
        assign one_time_item_list = item | uniq | concat: one_time_item_list
      endif
    endif
  endfor
  
  assign subscription_item_list = subscription_item_list | reverse
  assign one_time_item_list = one_time_item_list | reverse
%}

{% # Later in the document %}
{% for subscription_item in subscription_item_list %}
  {% # Subscription items markup is handled here %}
{% endfor %}
{% for one_time_item in one_time_item_list %}
  {% # One time items markup is handled here %}
{% endfor %}

////////////////////////////////////
{% comment %}
  In this case, I want to have a generic snippet for images.
  Instead of writing a complex if condition looping over all format types at multiple times, I can create a list that will leverage the "contains" keyword
  For performance, at any time I use my list, performance is gained over a complicated if condition.
  For readability and maintainability, editing/extending the list is pretty easy and preparing the list beforehand makes it DRY. 
{% endcomment %}

{% liquid
  # We pass a media element (image, video ...) to the snippet
  
  if media.media_type == 'image'
    assign media_source = 'image_object'
  else
    # Easy to maintain format list
    assign image_format_list = '.jpg' | uniq
    assign image_format_list = '.png' | uniq | concat: image_format_list
    assign image_format_list = '.webp' | uniq | concat: image_format_list
    # ...
    assign image_format_list = image_format_list | reverse
    
    for image_format in image_format_list
      if media contains image_format
        assign media_source = 'image_url'
        break
      endif
    endfor
  endif
  
  if media_source == null
    assign media_source = 'not_image'
  endif
  
  # Now, I have sorted which elements are images or not, links or references. Perfect to setup my markup later
%}