app/overrides/add_javascript_to_product_show.rb
Deface::Override.new(
virtual_path: 'spree/products/show',
name: 'add_javascript_to_product_show',
insert_before: %Q(erb[silent]:contains(" @body_id = 'product-details' ")),
text: "<%= javascript_include_tag 'spree/frontend/favoritable_product.js' %>"
)