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.

i think var will make counter accessible via constructor, and there was no test for this case

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!

any source?

How does the instances have access to the counter variable? It's not passed as a method (e.g this.counter), it's not passed as a prototype either (e.g Counter.prototype.counter).

So how? Closure? That would make sense if we call it directly on the Counter function constructor, not its instances.

Because we are looking for one of the case condition results to be true :) Like, we have 87 as avg, so case (avg >= 90) gives you false.. next gives you true, so it will be used.

why we need swith(true) not switch(value)？ I just don't understand

You could also try the filter method or recursion to solve this :)

yeah, i thought about regex before anything, than i tried the for loop.

It's also good to expand on your knowledge and practice all methods of solving as well, regex being one of them even if they are slower :)

regex can be 87% slower than own implementation using for loop. shouldnt matter tho. gg

The instructions are a little unclear and should be elaborated.