Do not ask for rounding. This is documented.

No random tests

Returning inconsistent datatypes is not a best practice. Either ask for null if there is no match, or ask for throwing an Error. ( The former option seems better. )

null is not an array either, but it is the best encoding of an option type ( which it does not have ) JS has.

The only info you have is that 7 years later the older sibling will be ..

".. is that in 7 years the older .."

.. the young one.

".. the younger one."

It very much looks like you didn't have describe, it and require("chai") in your test suite. Please get with the times and build your test suite as documented.

You also need a lot more tests.

Needs random tests