Testing JS actions in a Rails controller, the response is always empty

having a views:

# views/users/index.html.erb
<%= link_to 'Destroy', user_destroy_path(user), remote: :true %>

having a destroy action:

# controllers/users_controller.rb
respond_to :js

def destroy
   @user = user.find(params.require(:id))
   @user.destroy
end

which renders:

# views/users/destroy.js.erb
$('#some_div').html('Bye Bye' + user.name);

I want to write controller tests test for it

# spec/controllers/users_controller_spec.rb
describe UsersController do 
  it 'says goodbye' do
     delete :destroy, id: user
     expect(response.body).to include('Bye Bye #{user.name}')
  end 
end

but "response.body" is always empty string. Why? How to examine content of js response?

You need to tell Rspec to render views. It doesn't by default.

require "spec_helper"

describe UsersController, type: :controller do
  render_views

  it 'says goodbye' do
    delete :destroy, id: user
    expect(response.body).to include("Bye Bye #{user.name}")
  end
end

However, seeing as you are relying on JavaScript, you will not be able to find the text as this will just return HTML.

In order to get this working, you need to use an actual browser, so this means a feature spec using something like PhantomJS (see the poltergeist gem for capybara), or moving to Cucumber.

Your test would then assert things from the user's point of view, something like this:

it 'says goodbye', js: true do
  visit home_page
  click_button 'Log out'
  expect(page).to have_content("Bye Bye #{user.name}")
end