Smart, that added to my knowledge, thank you.

Well, this is smart. I do understand that the letter is picked from the outer string on the bases of the index of the innter string. My question, "is there a website where these techniques are explained"?

this is because you create a variable within your function with the same name as the function and declared this variable without let, which puts it in the global scope and overwrites the function itself

Using javascript gives me error "bmi is not a function".