```
function checkBoundaryCollision() {
if (x - ballRadius <= 0 || x + ballRadius >= canvasWidth) {
dx = -dx;
}
if (y - ballRadius <= 0 || y + ballRadius >= canvasHeight) {
dy = -dy;
}
}
```

Full code here: https://codepen.io/pelko/pen/gOjXdaq

```
function checkBoundaryCollision() {
if (x - ballRadius <= 0 || x + ballRadius >= canvasWidth) {
dx = -dx;
}
if (y - ballRadius <= 0 || y + ballRadius >= canvasHeight) {
dy = -dy;
}
}
```

Full code here: https://codepen.io/pelko/pen/gOjXdaq

The **x** and **y** values appear to be the co-ordinate positions of the centre of the ball (i.e. where the **x** and **y** axes of the red cross meet, **x** being horizontal and **y** being vertical).

So, the code you posted detects whether the edges of the ball meet (or cross) the boundaries of the canvas, by adding/subtracting the radius of the ball from the position of its centre.

If so, the direction of the ball in either axis (**dx**, **dy**) will change accordingly, causing the ball to ‘bounce’ off of the boundary in the opposite direction.

I only gave this a cursory glance but that what it seems to me… please correct me if I’m wrong!

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