This is MRuby compiled from C to JavaScript using Emscripten. It's running in your browser without any plugins.
This code is mirrored from an original implementation here. Parts of this code were borrowed from a Lua interpreter running in a browser. The source code can be found here. The irb-style console below is using jq-console. Lastly, I encourage you to check out Xuejie Xiao's blog!