Oh I'm dumb. I forgot that methods only refer to functions, not variables.

Javascript sample tests will pass returning any non falsy value (confusing people about those passing and the other tests don't). Don't use test.expect, use test.assertDeepEquals.

Tests are wrong in this way, the sample tests use test.expect and that's no way to compare two arrays (try returning [1] or any non falsy value and it'll pass all sample tests).
What you've copied there won't work either (note that you're passing 3 arguments to your function).
The tests on attempt use another way that makes test.expect to work, there is a function that compares each element of the expected and actual array with each other.
test.assertDeepEquals should be used.

The function name that is being called in sample tests (difference) is not the same that function name in final submit (array_diff) - Rust

Sample tests != test suite. Your code is wrong.

When I run the sample tests everything passes, but the final submit does not. Even thought I am runing the same tests at the final submit.