Great catch. I've added fixed tests, and changed the random tests so the input can't be altered!

That's true - updated! :)

As you say, a key can never actually be a number. So hopefully the explanation is enough to resolve the issue!

Thank you for the useful feedback!

I've updated the description to specify that the the first occurrence of duplicated chars should be kept, and added the sample tests to the full test suite!

Thanks guys. I've gone with this explanation:

Whenever two keys share the same string, the keys are compared numerically and the numerically largest key will keep the string. That's why, in the example above, key "2" contains "A" and "B", as 2 > 1.

If you really want to code golf it, you could invade the global namespace and do away with let entirely!

PHP translation submitted

very neat solution!

Thanks for the feedback!

Would you mind explaining what is preffered, and why?

Nice explanation, thanks!