Oh it's Huffman encoding that is like that, I understand now.

The test is "singleton frequency encode". I get "Expected: null, instead got: ''". The input is encode([['a', 1]], "");

Javascript

Some tests don't make sense. It expects an empty string to be returned when passing an empty string, except if the frequencies have 1 key. Why? Decoding or encoding an empty string should return an empty string.

That is much better, I don't know why I didn't see that.

Yes I know another way to solve it. I just don't understand why pop fails to remove the last item.

Anyone want to explain why pop works for test cases and not on random tests?