[pyrotechnique]

Both.

Since CoffeeScript itself is written in CoffeeScript it was quite easy for the team to create a distribution of the compiler that runs in the browser.

You can find it in the extras folder of the source code repository.

Usually though, people opt to pre-compile their CoffeeScript on the server, pack, optimise and minify it before serving it to clients.

At Feisty, we write CoffeeScript that is intended to be executed on both the server and client.

To achieve this, we wrote "requisition" - https://github.com/feisty/requisition - a server-side packager and client-side loader - via require() - for CommonJS Modules written in CoffeeScript.

For a simpler, ad-hoc solution check out my tutorial project "Walk the Line" - http://github.com/pyrotechnick/walk_the_line