Frontend testing: what and how to test, and what tool to use?

Question

I have been writing tests for my Ruby code for a while, but as a frontend developer I am obviously interested in bring this into the code I write for my frontend code. There is

Answer 1

There are lots of options and tools for that. But their choice depends on whether you have a web UI or it's a desktop app?

Supposing from the tools you've mentioned it's Web UI. I would suggest Selenium (aka WebDriver): http://seleniumhq.org/docs/

There is a variety of languages it supports (Ruby is in the list). It can be run against a variety of browsers, ad it's quite easy to use with lots of tutorials and tips available. Oh, and it's free, of course :)