toLowerCase() doesn't return new String? Func Prog JS Convert Strings to URL Slugs

Trying the functional programming portion of the JS Certificate.

function urlSlug(title) {
  return title.toLowerCase().split(/\s+/).filter(String).join("-");

This is not working. Keeps telling me:

The globalTitle variable should not change.

But doesn’t toLowerCase() give back a new string?

Hi @kacalica, I just tried your solution in Chrome (70.0) and it passes. The function does return a string, and I’m not getting this error.