check the given input variables in getSerie
One is an optional agrument

I get undefined as a start value in function getSerie.

I get undefiend as a start value every test in JS

PureScript Translation Kumited - please accept :D

I didn't say that, read the instructions (until you get them) and the sample tests. Do you know what common denominator is?

(207,390)(261,3930)(9,12) // all denominators are different here

You don't need to reduce the fractions after you got the common denominator, the denominator as I already said before, has to be the same for all fractions.

You have to produce a result in the form

(N_1, D) ... (N_n, D)

So you want to say that 2 is same denominator as 12? Seriously?

The output fractions should have the same denominator, as seen in the instructions.

convertFracs [(1, 2), (1, 3), (1, 4)] shouldBe [(6, 12), (4, 12), (3, 12)]

in which D is as small as possible and

N_1/D == numer_1/denom_1

My code generate (3,6) for pare (1,2) and here D is the smallest, but test code generate (6,12). And such situation in each test.
One more example.
Expected: '(18078,34060)(2262,34060)(25545,34060)', instead got: '(207,390)(261,3930)(9,12)'
But my D is smaller than test D and my answer still correct because
18078/34060 = 207/390
2262/34060 = 261/3930
25545/34060 = 9/12