I like my solution more because it is immutable

There is nothing in this kata that should make it worth kyu 5.

However it's not useful to complain

I disagree. It is useful to complain because a) it's a signal that something is wrong which is affecting the user experience, b) the comments will be useful when organizing future cleanup efforts, should it eventually become easier to rerank and c) they provide opportunity for discussion about matters that affect site quality.

If someone takes the time to leave a comment about a ranking, then it probably reeled their expectations enough to be worth a look; not only is this useful for people who are less sensitive to challenge gradations at the lower ranks, but it can also be reassuring to less experienced warriors who would otherwise be put off.

do you have a right example how to solve it with closures?

I'm sorry .reset() got you confused. You choose to focus on the wrong things. The template is quite straightforward:

DocumentParser.prototype.parse = function()
{
  // TODO: Code Here!
};

it means if next exists, then add '- ${next}'
lets say for exapmple when byDay.prev and byDay.cur are not equal
next is just '' empty stroke, so we dont add - ${next}

seems like this merits a spoiler

Don't give free hints please. I marked your comment with a spoiler flag.

Note: I didn't solve this kata, and that's really the last kind of things I enjoy finding in Discourse page.