The problem isn't because you modify the string, you didn't. The problem is you're creating a standalone function instead of adding a method to the String prototype.
You can take a look at this: MDN: Object Prototypes

toString not tostring

sweeeeeet, I'll update this with some markdown magic.

Danke!

Cheers tu6619, just fixed it :)

Which words did you spot? was it 'love'? I might need to be a bit more specific in the description

Well spotted, thanks!

Thanks Thomas - I will sort that out.

hey! i think they are strings and the reason they don't have '' around them is because they are single words, unlike the others. I wondered the same and this is the conclusion i came to. It's just the way the function outputs the resulting object

You didn't call lower()

I'm not sure what you mean by not responding. When I try to submit your solution it passes three tests before failing (as you expected.) Could it have been a problem with the site?

1. Any return will exit the current function. Which means that your first return is the culprit.
2. Your first return line is completely off, since you're missing the () after the function's name.
3. The actual error should be "but got function...". No idea why it said "undefined" instead.

Since you've asked many questions on many JS katas lately, I recommend you to take a look at the Mozilla JS guide.

Just get 'undefined'...

Which means that there's at least path where the function doesn't return anything. Hint: does your if ... else if take care of everything?