Hi John, the library Khan Academy uses is called Processing.js, and is more geared for drawing graphics, with Javascript concepts, than anything. If you'd like to run other code like c# or pure JS, I suggest you find a free code editor like Sublime Text or Atom, they are both really nice editors, but don't come with live output... you would need to refresh pages when using them.

Something you may not know is that the webpage environment does support pure Javascript and is updated live from a <script> tag, here's the Intro course, assuming you know about html/css and such.

If you like the live output and still want to use Processing.js off KA, you can follow these steps from our good friend, Peter.

Hope this helps you out! :)

These pages may help, but only for *offline* ProcessingJS:

https://www.khanacademy.org/computer-programming/processingjs-inside-webpages-template/5157014494511104 

https://github.com/Team-Code/KA_Offline 

https://khanacademy.zendesk.com/hc/en-us/articles/203326990-How-do-I-program-offline-outside-of-KA-