That's an edge case and your first assumption is wrong. You're also missing some other part of the instructions, this one:

whether the two arrays have the "same" elements, with the same multiplicities.

Think about what that means.

What would you return for this input? a = [4] b = [2, 2, 2]

Sorry I'm being a little vague, but I'm giving you hints so you can solve the kata yourself.

You're confused by this:

If a or b are empty then the result is self-evident.

What should the expected value be if both are empty? Use console.log to print the input values.