Extend Rspec to include additional matcher.

Assets be_undefined_in_js (Rspec)

describe "Assets", type: :feature, js: true do
  describe "application.js" do
    it "does not include page-specific content" do
      visit root_path

      expect("d3").to be_undefined_in_js
      expect("moment").to be_undefined_in_js
    end
  end

  private

  RSpec::Matchers.define :be_undefined_in_js do
    match do |actual|
      evaluate_script("typeof #{actual} === 'undefined'") == true
    end

    failure_message do |actual|
      "expected JS not to include #{actual}"
    end
  end
end