Karma with Rails

lib_tasks_karma.rb

namespace :karma do
  task start: :environment do
    with_tmp_config :start
  end

  task run: :environment do
    with_tmp_config :start, "--single-run"
  end

  private

  def with_tmp_config(command, args = nil)
    Tempfile.open("karma_unit.js", Rails.root.join("tmp")) do |f|
      f.write unit_js(application_spec_files)
      f.flush

      system "./node_modules/.bin/karma #{command} #{f.path} #{args}"
    end
  end

  def application_spec_files
    assets = Rails.application.assets
    assets.append_path Rails.root.join("spec/javascripts/support/karma")
    assets.find_asset("application_spec.js").to_a.map { |e| e.pathname.to_s }
  end

  def unit_js(files)
    unit_js = File.open("spec/javascripts/support/karma/config/unit.js", "r").read
    unit_js.gsub "APPLICATION_SPEC", "\"#{files.join("\",\n\"")}\""
  end
end