I had the same problem. The tests feed back the output from your encode method to your decode method. So take a look at your encode method, especially the usage of javascript bitwise operations if you have them, like people have discussed below.

How come NOT ASCII characters are passed to a decoder method? Range 65485-65535. Isn't it about converting non readable binary data to a readable or at least pronouncable text and vice versa?

Does not test if function supports getting the value at all
Does not test if prototype methods/properties are ignored

Finally finished the kata.
See my open issue for comments.

Tests are not testing the rights things in the right time and mislead to wrong assumption.

It is not clarrified that function must return the modified list. It is not clarified that the function should modify the list given as parameter.

It could be interesting to have a quick discussion on parameters passed by reference and by value in JS.

It could be more natural to add a function to the array prototype instead of creating a function.

i believe the test should not expect the array to be returned