How to calculate vector in JS?

So, i am trying to make simple js game, where you have to shoot some objects. Direction of the bullet should change on wheel (as my arrow). But i dont understand how to do it. Please help

The direction is controlled in the constructor for Ball with:

                this.dx = 2;
                this.dy = -2;

You will need to pass the values when you instantiate the ball. You will have to do a little trig based on the angle set by the wheel.