The Dropbox blog post addresses some of the common complaints about CoffeeScript, including performance and debugging. According to the blog post: “The size of the compressed bundle didn’t change significantly pre- and post-coffee transformation, so our users shouldn’t notice anything different. The site performs and behaves as before.” They also conducted extensive testing of the code changes during the week to make sure nothing broke.
The team used js2coffee, an automatic conversion tool, to speed up the process.
Here’s a before and after example from Dropbox: