For anyone else who had no idea what this was, opal[0] is a ruby-to-js transpiler, and AFAIK this is just an example of it being used to make a hello world app.
Looks cool and all, but what are the benefits of using this over say normal js? I guess if you want to use other libraries, you still need to include them somehow in the build step or can opal take care of that as well?
I would guess this is for people who know Ruby, and for some reason need a Node.js stack but don't want to bother learning Node.js.