filter function should only return a boolean value.

Your solution should support any alphabet provided to the constructor, and should leave characters that are not in the specified alphabet in situ.

Edit, also the description for the test case specifically says: Testing with Latin lowercase alphabet

I think there is test case error, please help to check:
encodeCODEWARS //this is my console log information which contains current funtion name and the str.
Expected: CODEWARS, instead got: XLWVDZIH

For decode(CODEWARS), the expected should be 'XLWVDZIH'.