The problem is you're using for...in, read this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in and in the tests, the array prototype is modified.

You, and the kata's author/translator used a solution var without declaring it (with var or let), that's the problem. I've fixed it in the testing part, I suggest you do that also in your code as good practice.

Can't reproduce it, was it in javascript? Post your code as stated here: https://docs.codewars.com/training/troubleshooting#post-discourse

I have an issue that seems related to the tests and not my code. I get this error if i return the correct string in attempt:

It should work for random tests too - Expected: ['Marge,', 'Barney,', 'Moe,', 'Lisa', '& Ned'], instead got: 'Marge, Barney, Moe, Lisa & Ned'

If I return the array instead of a string, all the random tests pass, but then i fail the basic tests which correctly ask for a string in return, am I missing something or it is just maybe a newly introduced bug with the tests?

No idea without seeing your code, read this: https://docs.codewars.com/training/troubleshooting#post-discourse

Test Results:
Test Passed: Value == 'Bart, Lisa, Maggie, Homer & Marge'
Test Passed: Value == 'Bart, Lisa & Maggie'
Test Passed: Value == 'Bart & Lisa'
Test Passed: Value == 'Bart'
Test Passed: Value == ''

However when I attempt:
Must work with many names - Expected: 'Bart, Lisa, Maggie, Homer & Marge', instead got: 'Bart, Lisa, Maggie, Homer & Marge, '
Must work with many names - Expected: 'Bart, Lisa & Maggie', instead got: 'Bart, Lisa & Maggie, '
Must work with two names - Expected: 'Bart & Lisa', instead got: 'Bart & Lisa, '
Wrong output for a single name - Expected: 'Bart', instead got: 'Bart, '
Must work with no names - Expected: '', instead got: ', '

How did the tests work but 0/45 additional tests not work (especially ones that are the same)? When I run in code editor I get back the correct results I received in the test cases.