In Rails, how do you functional test a Javascript response format?

Question

If your controller action looks like this:

respond_to do |format|
  format.html { raise \'Unsupported\' }
  format.js # index.js.erb
end

and yo

Answer 1

RSpec 3.7 and Rails 5.x solution:

A few of these answers were a little outdated in my case so I decided to provide an answer for those running Rails 5 and RSpec 3.7:

it "should render js" do
  get :index, xhr: true

  expect(response.content_type).to eq('text/javascript')
end

Very similar to Steve's answer with a few adjustments. The first being xhr is passed as a boolean key/pair. Second is I now use expect due to should receiving deprecation warnings if used. Comparing the content_type of the response to be equal to text/javascript worked for me.