Your reduce tests are incomplete.

[WORMSS]

You are treating null and undefined initalValues wrong.
They are perfectly valid initalValues but you are discarding them.

The real reduce returns undefined321

[3, 2, 1].reduce((p, c) => String(p)+String(c), undefined) // undefined321

but your code produces the wrong results of 321

reduce([3, 2, 1], (p, c) => String(p)+String(c), undefined) // 321

[ishmaell]

@WORMSS This was a great observation.