wow, this is so clean code!
I'm from Indonesian

clearly didn't read the description

Mutating an argument is the worst thing you can do.

I also thought of using a switch, but as you can see by the comments here, using switch(true) and then doing evaluations in each case is kind of an anti-pattern that can be hard to read and debug.
I would avoid and look for one of the other conditional options in this board.

This is what I did. Awesome job!

wow, so good

good blesss.

you can do this if you have exactly one parameter.

cool , I did this the same way but put the 'list' parameter into parenthesis,dindn't know I could do it without it.

good solving:)

same sol ^^

no substring ? wot´s this dark magic ?

So clever.

To further clarify for those still confused on why passing in switch(value) doesn't work, it's because it's comparing a Boolean to a numerical one, which will always be false. Say our average is 95 and we're assigning that to a variable avg. If we try to pass in avg as the argument to the switch statement's parameter, this is essentially what is happening:

switch(avg) {              //avg == 95
   case (avg >= 90):         //comparing 95 to true, which is false
     return 'A';

(avg >= 90) is returning a Boolean. We'll never reach a switch statement that evaluates to true if we're passing in our avg variable as the argument. I hope this helps!