Fixed.

Added tests with 0s

Sometimes the tests generate no zeroes which allows solutions to do incorrect short-circuit evaluation.

That's actually an issue.

The user can modify the input.

Right, fixed!

JohanWiltink's suggestion is a good improvement since it corrects the confusion around the second and third parameter.

And my confusion came from the examples where a third parameter isn't included AND one array is shorter than the other one, in which case the padding is done using 0s. I can see how that was a misinterpretation on my behalf.

third parameter is missing in javascript sample code

"If one array is shorter than the other, use the optional third parameter (defaulted to 0) to or the unmatched elements."

If one array is shorter than the other, use the optional second parametero (defaulted to 0) to OR the unmatched elements.

This should say the third parameter.

I assumed it had to be used once, and then (if necessary) pad with 0s.

And what gave you that idea?

If one array is shorter than the other, use the optional second parameter (defaulted to 0) to OR the unmatched elements.

Could you suggest a better wording?

Bit of criticism: it wasn't made clear (to me, at least) that when a third parameter is supplied, then the padding of whichever array/list is shorted should be done using that third parameter. I assumed it had to be used once, and then (if necessary) pad with 0s.

5. Second reinforcement -> "cccaa"

Nope, it will be "ccbaa" because the third b hasn't been "used" yet