[, yxvm, zzozybj, booifcbcs, ...
^^

This, means there is actually an empty string at the first element. And as a matter of fact...:

Empty string values should be ignored.

(unmeaningful message, I need to track a bug...)

and? Don't you see your problem? You printed the input, so you should see what you forgot, considering the requirements.

Sample tests are just "sample". If you wanna know the input, print it to the console (System.out.println(Arrays.toString(...));)

input

...?

Anyway, you're wrong about that, because you're not following this requirement:

formatWords(['ninja', '', 'ronin']) // should return "ninja and ronin"

I just did the java version without troubles. Do you have an example of something going wrong? (meaning: input, acutal and expected outputs, what's wrong about the expected one?)