Shopify Select First Variant On Stock When Product Page Loads

product.liquid

{%- comment -%}
  If the product first varaiant inventory is 0 then find the variant with inventory and load the product page 
{%- endcomment -%}

{%- assign current_variant = product.selected_or_first_available_variant -%}
{%- assign availableVariantId = '' -%}
{%- assign availableVariantFound = false -%}

{%- if current_variant.inventory_quantity <= 0 -%}
    {%- for variant in product.variants -%}
      {%- if variant.inventory_quantity > 0 and availableVariantId == '' -%}
        {%- assign availableVariantId = variant.id -%}
        {%- assign availableVariantFound = true -%}
        {% break %}
      {%- endif -%}
    {%- endfor -%}
 {% endif %} 

 {% if availableVariantFound %}
    <script>
    window.location.href = '/products/{{ product.handle }}?variant={{ availableVariantId }}';
  </script>

{% endif %} 

you the best!