use the backtics
for best practice

I had the same issue with my JS solution. I was missing a step.

The instructions cover this case, where the string length is already less than or equal to 3 characters:

e.g. trim("He", 1) should return "H..."

But not this case, where the act of trimming reduces the string length to less than/equal to 3 characters:

e.g. trim("Elephant", 2) should return "El..."

That part was not clear to me until the final test cases (the basic tests also didn't cover the case).

I think the following statement could be clarified to indicate that the string length referred to is not just that of the original input, but also of the trimmed fragment:

"If the string is smaller than or equal to 3 characters then the length of the dots is not added to the string length."