thats why you should use tail recurrsion, carrying the accumulator, then any number the function will hold

Right. My bad!

From the description: "[...] return a string representation [...]"

I understand why you'd rather have a different return type (me too), but these are the requirements.

(coffeescript): The test cases should be actually undefined and 0, not "undefined" and "0" as strings.

There seems to be something wrong with the odd test data, it's expecting "���" which obviously does not match the two characters that it should be getting (sorry, I don't know why my browser is not displaying the characters correctly):

returns correct board for odd dimensions  
✔   Test Passed: Value == ■ □ ■
□ ■ □
■ □ ■  
✘   Expected: ■ □ ■ □ ■ □ ■
□ ■ □ ■ □ ■ □
■ □ ■ □ ■ □ ■
□ ■ □ ■ □ ■ □
■ □ ■ □ ■ □ ■
□ ■ □ ■ □ ■ □
■ □ ■ ��� ■ □ ■, instead got: ■ □ ■ □ ■ □ ■
□ ■ □ ■ □ ■ □
■ □ ■ □ ■ □ ■
□ ■ □ ■ □ ■ □
■ □ ■ □ ■ □ ■
□ ■ □ ■ □ ■ □
■ □ ■ □ ■ □ ■ 

Uhh. Please ignore this.