Not a kata issue, OP got confused between classes and arguments, closing.

The circle arg is a Circle object. Take a look at the test section where the object is instantiated.

This not an issue, but I cannot close it.

Are you sure of this? if (arr.length === 1) return arr[0][0];
b is constant in your code, you should be using the current ball position in your loop.

What do you think one ? false : true; does?
Use console.log(str) before the return line.