[TazeTSchnitzel]

Oh, you, Tom Scott, made it? :D I'm a big fan.

A few suggestions, then:

1. Don't use i as the name of the parameter. A lot of people use that as the name of loop variables, and it means breaking habit or wasting time changing the name.

2. For test 3, try some multiple-extension files to make sure they did it right (some.file.multiple.extensions, should return "extensions")

3. For test 5, it says "integers". Try some non-integers during testing, to make sure that it's only checking for integers.

4. For test 5, it says "arrays". Try some objects to make sure they're not being lazy and using typeof thing === "object"

Anyway, thanks for making this. I love it!