there should be one more test case in javascript i.e '1' because my solution pass all your test cases but i separately test my solution against that input('1') and my solution fails.

It is bad practice to return different data types.

Getting undefined when running the tests, even after pasting in several solutions which are supposed to work. Every test turns out undefined, not sure if something's wrong.

Something is weird with this one.

My tests (test.assert_equals(f(6), 21) ; test.assert_equals(f('6'), None)) are passing, yet when submitting I'm getting an error "✘ n must be an integer: 6 should equal None"

Even if the code works, when submitting it gives this error

Traceback:
in
in f
TypeError: Can't convert 'int' object to str implicitly

The implementation is faulty in Python. It tries to concatenate the returned integer with a string (I think).

Maybe a test case can be added to test "float that is integer", such as:
test.assert_equals(f(15.0), 120)

Why does it give me this error "n must be an integer: 6 should equal None"? 6 is not an integer?!?

[quote]Due to another of his misbehaved, the primary school's teacher of the young Gauß, Herr J.G. Büttner, to keep the bored and unruly young schoolboy Karl Friedrich Gauss busy for a good long time, while he teaching arithmetic to his mates, assigned him the problem of adding up all the whole numbers from 1 through a given number n.[endquote]

Would be better written as:
Due to the young Gauß, Herr J.G. Büttner's misbehavior while his teacher was teaching arithmetic to his classmates, his primary school teacher assigned him the problem of adding up all whole numbers from 1 to a given number n in order to keep him busy for a long time.

(Did someone use google translate on a german paragraph?)