I can't help but wonder whether or not this is really a 6 kyu(I think it is harder than the 6 kyus i've completed), but my main criticism is that the sample tests aren't verbose enough to catch issues that would arise from not handling parsing lowercase / uppercase properly.

A good learning lesson that sometimes the first function you find in a reference list is not always the most efficient one for the job (or that there are sometimes subtle differences). indexOf is definitely the better choice here.