I think what you see is "honors". Honors give you ranks and privileges. See:
https://github.com/Codewars/codewars.com/wiki/Honor-&-Ranks
https://github.com/Codewars/codewars.com/wiki/Privileges

Badges are already earned when you get enough honors. They are shown in the "Profile badges" section accessible by pushing the "View Profile Badges" button in your profile page.

I suggest a test to add. Motivation: if an HTML input string already contains "&", you don't want to transform it into "&";
describe("Solution", function(){
it("@amp;", function(){
Test.assertEquals(htmlspecialchars("&"), "&", "the & in the already correct substrings should not change");
});
});

OK the answer to my own question is: yes, strings in JS and TS are immutable, that is, once created, their internal state cannot be changed.
Functions that operate on a string in fact return new strings.
My solution would work even if I do not create a new string to compute the result.