I don't understand an answer to a question amongst the Javascript challenges. Hoping someone can explain?

Question

What is the output of this code?
var x = 0;
var arr = [4,9,2];
arr[x] = (++x) -1;
document.write(arr[x]);

Apparently the answer is 9. I don't understand why it's not 4.

Answer 1

The position 0 got replaced by 0
meanwhile, x was incremented so x is 1, because of the ++x
then it writes what is at the position 1 which is 9

I hope this helps :)

Answer 2

Thank you.
For some reason I just couldn't make sense of the code :)

Looks like I need to refresh my memory on arrays.