repeatedly converting the input string to lowercase
oops, I see that this has already been mentioned

Nice comment

loved this one, ty!

I think he noticed that but he just sacrificed performance for compactness

Brilliant!

Damn thats clever, mine is more or less the same but I didn't think to use the test method which could've shrunk my code down into a single line like yours, nice :)

I see a lot of coding solutions that are just a few lines and involve lots of complex thinking and computing that others who look at the code may struggle to understand (including me), but this isn't that complex at all in my opinion, you're simply just creating an array of letters from the alphabet, then checking if each letter is included in the input string using the every() method, I think its very efficient if anything. I think even someone using another language could probably understand whats going on since even the method names used are very intuitive.