Shopify: Reloads the page with the default variant if no variant is selected

reload-with-default-variant.liquid

{%- comment -%}
  Reloads the page with the default variant if no variant is selected
    - product: Product object

  use: {% render 'reload-with-default-variant', product: product %}
{%- endcomment -%}
{%- liquid
  # Reload and select 12 ounce variant if no variant is selected
  assign on_option_value = '12 ounce'
  assign reload_with_selected_variant_id = 0

  if product.selected_variant == null
    for value in product.options_by_name['Weight'].values
      if value.name == on_option_value
        assign reload_with_selected_variant_id = value.variant.id
        break
      endif
    endfor
  endif
-%}
<script type="text/javascript">
  const selectedVariantId = {{ reload_with_selected_variant_id }};
  if (selectedVariantId) {
    const url = new URL(window.location.href);
    url.searchParams.set('variant', selectedVariantId);
    window.location.href = url.href;
  }
</script>