function areDownKeys() {
  for (var key in downKeys) {
    if (downKeys.hasOwnProperty(key) && downKeys[key] === true) {
      return true;
    }
  }
  return false;
}
The system variable mouseY always contains the current vertical position
of the mouse, relative to (0, 0) of the canvas. If touch is
used instead of mouse input, mouseY will hold the y value of the most
recent touch point.

@property {Number} mouseY
@readOnly

@example
<div>
<code>
// Move the mouse across the canvas
function draw() {
  background(244, 248, 252);
  line(0, mouseY, 100, mouseY);
}
</code>
</div>

@alt
vertical black line moves up and down with mouse y-position