Monkeys-Sprockets

# config/initializers/monkeys.rb

# for example we will modify 'videojs-youtube' src file
# on the line 624 there a video quality changing attempt
# but it will not work, unless a video is stopped before quality change\

Monkeys::Changer.tap do | change |

  # first we find a file by a full sprockets path
  # path, that you currently use in a //= require directive
  # but with full extensions
  # in this case it's just '.js'

  # sprockets will throw an error, if it couldn't find the file

  change.file 'videojs-youtube/src/youtube.js' do | file |

    # then we get the 624 line
    # and checking that it has expected content
    # if not - error will be thrown
    # you can check content by string or regexp

    file.line 624, 'self.ytplayer.setPlaybackQuality(quality);' do | line |

      # block will receive line content
      # and returned value ( a string or an array )
      # will be used instead of this line

      [
        'var xy334 = self.paused();',
        'self.ytplayer.stopVideo();',
        line,
        'if ( ! xy334 ) self.ytplayer.playVideo();'
      ]

    end

  end

end