Ag (aka The Silver Searcher) is an amazing piece of software. It allows you to define file types (like Ack) and comes prepackeged with some file types.
ag --ruby --js -l
Ag has the added benefit over Ack, that it ignores gitignored files, so you only get the files that matter (and not stuff from node_modules etc).
To see what filetypes Ag supports:
The list is pretty extensive! Unlike Ack however, there is currently no way to add new file types or extend the list.