Terminal emulator?

I hope I am not asking too much here but I have always wondered...

How does PythonAnywhere create terminal windows in a browser? They seem be fully functional and really push the commands on the server.

Is there some open source Javascript project you use or is this proprietary code? Thanks for any leads.

It's an open source project -- well, actually two of them :-) Normally we use hterm, but for some accounts we use xterm.js.

Nicely implemented! :)