The commonly accepted practices are not that good

Isn’t it frustrating that there are no elegant ways to conditionally append CSS class names to the :class option of a link_to helper call?

The code to do the conditionals is ugly and clutters up the views, especially when there’s more than one.

Everyone suggests a slightly different syntax, but which one is best? They all look messy and require putting logic in the view layer.

For such a common task, why isn’t there a better solution?

Turning to React for inspiration

Facebook’s React front-end framework has an add-on called classNames (formerly known as classSet) which offers a solution to this problem.

Given a hash with key/value pairs consisting of CSS class name strings and boolean values, the classNames function returns a string of classes where the boolean value evaluated to true.

classNames({ nav: true, active: false, hidden: someTruthyVar })
// => "nav hidden"

Wouldn’t it be nice to have a similar method in Rails?

Finally, a better solution for Rails

The css-class-string gem provides just such a helper for our Rails views.

Here’s an example of a link_to which requires multiple conditional classes to be appended:

<%= link_to "Hello", root_path, class: "nav #{'active' if @some_boolean} #{@another_boolean ? 'hidden' : 'visible'" %>

Now here is the same example using the class_string helper method:

<%= link_to "Hello", root_path, class: class_string("nav", active: @some_boolean, [:hidden, :visible] => @another_boolean) %>

Although the code is about the same length, removing the string interpolation makes it easier to read, and there is no more logic inside the view which is a better practice.

The same helper can be used in any Rails tag helper which expects a :class option (e.g. content_for).

Don’t want to use the gem? Implement it yourself

The css-class-string gem has some useful advanced features such as using bare arguments for default classes and arrays for “one-or-the-other” classes, however if you don’t want to add another gem to your Gemfile then you can create a very basic version of the helper yourself.

# Conditionally joins a set of CSS class names into a single string
# @param css_map [Hash] a mapping of CSS class names to boolean values
# @return [String] a combined string of CSS classes where the value was true
def class_string(css_map)
  classes = []

  css_map.each do |css, bool|
    classes << css if bool
  classes.join(" ")

Or if you prefer one-liners:

def class_string(css_map)
  css_map.find_all(&:last).map(&:first).join(" ")

I’ll be adding css-class-string or a helper like above to all of my projects moving forward.