The Portage tree has recently added the package dev-lang/coffee-script. However, The cake command, the default build tool of CoffeeScript, is included in the coffee-script package, but not installed in src_install() with 'doexe' and 'dosym'.
Sorry about that, I mistakenly thought that cake required underscore.js. It should be fixed in the -r1 (just committed). *coffee-script-1.9.3-r1 (10 Jul 2015) 10 Jul 2015; Michael Orlitzky <mjo@gentoo.org> +coffee-script-1.9.3-r1.ebuild, -coffee-script-1.9.3.ebuild: Revbump to install cake, bug #554308 thanks to Peter Cai.