Expected and actual params in the fixed tests were flipped. Fixed.

There is a bug in the test cases which will return NaN if your solution is returning non-numbers.

But the test cases are locked so it can't be changed anymore. It also doesn't affect correct solutions. Which means you need to try harder ;-)

Not an issue.

Not an issue. The function parameter names are arbitrary anyway, and there is no ambiguity there as to which one is which.

If begin value is greater than the end, function should returns 0

Not an issue, a question.

Not an issue, this is how the test framework works.

Javascript. What are the context for NaN outputs. The instruction clearly states to find the sum of the sequence and if begin is greater than end sum should be 0. However, some of the expected outputs are asking for NaN. I already have a code written which give the sum of the sequence and if begin is greater than end sum is 0.