coffeescript-cookbook/coffeescript-cookbook.github.com — pull request #89 by nepsilon (2013-09-05 11:16:00+00:00)
Title: Bring CoffeeScript idiom to conditional statement.

An advantage of CoffeeScript compared to JavaScript is its ability to
check for array membership with the `in` keyword.
A feature it borrowed from Python.

I thought it could be a good idea to use it whenever possible, as it draws a clear difference between the 2 languages, showcasing a convenient CoffeeScript feature.