I have had about 12 interns spanning 3 years of starting up. I usually gave them to learn any of the base MVC architecture oriented JS Framework. KrakenJs or Express or Sails. The already packaged and ready to use framework approach coupled with easy to understand MVC pattern is a real starter for beginners. They can experiment right away, build a very small application like a blog or resume or to-do list. So start there.
My 2 Paise.