C'est magnifique !

Solved finally :)
Thanks a lot !!!

perfect

Thanks !!! Apparently I misunderstood something. Now it is clear the error message format: 'expected Actual to equal Expected'. I'll go through my code one more time.

I doubt that, you're looking forward in the string for repetitions, but what if there was already a duplicate? Look at moonmen, at the first m, you look forward for another and find it, the same with the first o, but what happens when you reach the second one? There are no more forward, the duplicate is behind. The error message format is expected Actual to equal Expected, the first value is what your function wrongly returns.

Yes, the only non-repeated letter there is 'e' and I run my code on WebStorm and it gave 'e'. But according to the error message 'o' was expected. I cannot understand why.

The only non-repeated letter there is e, isn't it? The error message means your code returned o instead of e.

JS: is it correct, the test ? Why 'o' is expected?
'moonmen' expected 'o' to equal 'e'

very clever

very nice

simple and nice solution