With the release of Rails 6.1, the class_names view helper was introduced. It is one of those helpers I use a lot.
Before you had to write some hard to read code:
<div class="<%= item.for_sale? ? 'active' : '' %>">
But with the class_names helper, you can do:
<div class="<%= class_names(active: item.for_sale?) %>">
Both output the following HTML (assuming item.for_sale? returns true):
<div class="active">
More advanced is the following:
class_names("item", { active: item.for_sale?, "out-of-stock": item.out_of_stock? })
Here item is applied in all cases. And active / out-of-stock only when the respective conditionals are met.
The class_names helper is also supported in tag-helpers. So this works just the same:
<%= tag.div class: class_names(active: item.for_sale?) %>
Or even shorter, like this:
<%= tag.div class: [active: item.for_sale?] %>
It’s one of those little, unknown features in Rails that helps you write better views and components. No wonder it is used in almost all Rails Designer’s components. 😅