The tests trivially pass as written. They should look like this (note I've added an =)

(deftest string-to-number-test
(is (= (string-to-number "1234") 1234))
(is (= (string-to-number "605") 605))
(is (= (string-to-number "1405") 1405))
(is (= (string-to-number "-7") -7))
)

The example Clojure tests are misleading. They will pass with the most trivial implementation (as long as the functions are defined).

(deftest example-tests
(is (head [5 1]) 6)
(is (tail [1]) [])
(is (init [1 5 7 9]) [1 5 7])
(is (last_ [7 2]) 2)
)

They should be

(deftest example-tests-fixed
(is (= (head [5 1]) 6))
(is (= (tail [1]) []))
(is (= (init [1 5 7 9]) [1 5 7]))
(is (= (last_ [7 2]) 2))
)

Update: I've confirmed that this is solvable in JS. However, I still don't see how this can be solved in Coffeescript due to differences in the language. I'll explain more in my next post and mark it as a spoiler.

@osuushi Thanks for your question. I tried the Coffeescript solution. I also double checked it was not working at http://coffeescript.org/. I must be missing something! Can I send you a gist of what I tried?

I'm so confused. I struggled with this for a long time and finally gave up and looked at the answers. I must be missing something, but the solutions don't even work for me! I've copy/pasted in the top solution and the first test expectation and it fails. I don't want to say too much, but the failing assertion concerns ==.