The test that you're failing is when both arrays are empty.

If you want to post code, mark it as a spoiler. The author of the Kata will be able to read it.

Your solution doesn't respect the same multiplicity condition. That means with a test like this: comp([2, 2, 3], [4, 9, 9]) or comp([2, 3], [4, 9, 9]) you code will return true.
You can check the input using console.log in javascript.

If the input array is empty or null, return an empty array.

So, the check for an empty array is missed.