Function Is target String found in an Array of Strings in CoffeeScript

If you want to make it "more like CoffeeScript", use array comprehensions:

isFoundIn = (searchTerm, arrayToSearch) ->
  return true for item in arrayToSearch when item is searchTerm
  false

Expanding on my comment above:

Your isFoundIn function can be rewritten as simply

isFoundIn = (term, array) -> array.indexOf(term) isnt -1

indexOf uses the strict === comparison, which is what CoffeeScript also uses for is - i.e. it's equivalent to your code.

One could extend the native JS Array prototype with a include function to more closely mimic Ruby. In CoffeeScript, it'd be written as

Array::include = (term) -> @indexOf(term) isnt -1

Then you can do [1, 2, 3].include(3) and such. But extending native prototypes is generally frowned upon, and I can't recommend it. Tempting though.

As for your original function, you could just do an explicit return right away if the searchTerm is found - no need to loop through the rest of the array

    isFoundIn = (searchTerm, arrayToSearch) ->
      for a in arrayToSearch
        return true if searchTerm is a
      false

The point is moot, though, as indexOf does it all for you

Addendum: I do like the ? hack you used to fully match Ruby's method name, but it is still just that: A hack. You're effectively adding undefined as a possible return value for what should be a straight boolean. It'll also absorb errors (as that's its purpose) where e.g. the object responds to indexOf just fine, but - for whatever reason - hasn't been extended with the include function. So it's easy to get false negatives (unless you explicitly start checking for undefined and branch if that's the case and... well, the code fogs up quick)

So again, definite points for creativity - wish I'd thought of it - but I personally wouldn't go near it in production code

Any reason to not use item in ary syntax? it compiles down to a slightly convoluted usage of .indexOf >=0 but it is the coffeescript way.