I get undefined as a start value in function getSerie.

I get undefiend as a start value every test in JS

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