Rake task build application.js assets

karma.rake

require 'sprockets'
require 'pry'
require 'fileutils'

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

  task :run => :environment do
    fail unless with_tmp_config(:start, "--single-run")
  end

  private

  def with_tmp_config(command, args = nil)
    Tempfile.open(['karma', '.config.js.coffee'], Rails.root.join('tmp')) do |f|
      build_js do |app_js_file|
        f.write new_karma_config_file(app_js_file)
        f.flush

        return system("karma #{command} #{f.path} #{args}")
      end
    end
  end

  def build_js
    sprocket =  Sprockets::Environment.new
    sprocket.append_path('app/assets/javascripts')
    sprocket.append_path('vendor/assets/bower_components')

    Tempfile.open('application.js', Rails.root.join('tmp')) do |appjs_file|
      puts 'building app.js..'
      appjs_file.puts(sprocket.find_asset('application.js'))
      puts 'app.js built, booting karma..'
      yield appjs_file
      appjs_file.close
    end
  end

  def new_karma_config_file(app_js)
    File.open('config/karma.conf.js.coffee', 'r').read
      .gsub(/'http.*application\.js'/, "'" + app_js.path + "'")
  end
end